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Chapter 8.2



Part II. Okinawa and Other Islands of the Ryukyus


    This report is concerned with the history of malaria and other insect-borne diseases on Okinawa and adjacent islands in the Ryukyu Islands Command from the initial landings on 1 April 1945 until late in the fall of that year, after the end of the war.

    Malaria was a potential threat which was prevented from developing during the combat phase and was subsequently held to a minor outbreak among natives in military government camps and to a small number of cases among adjacent troop units. This was accomplished by careful planning and prompt initiation of antimosquito measures. Airplane spraying of ricefield areas, even under combat conditions, was brought to the highest peak of efficiency obtained during the war in the Pacific and was carefully coordinated with ground control activities. An essential feature was the development of a plant for large-scale mixing of DDT with diesel oil.

29 See footnotes 24 and 25, p. 472.
30Anderson, W. L., and Wing, W. M.: Tsutsugamushi Disease (Scrub Typhus) a Clinical Study of Forty-nine Cases. War Med. 8 163-166, September 1945.


    It was evident within 30 days after the initial landings that personnel, equipment, and supplies would be more than adequate to control malaria. A considerable amount of effort was then diverted to control breeding of flies, especially during the combat phase. It is believed that these efforts were a factor in the absence of diarrhea and dysentery which had plagued all previous combat operations in the tropics and subtropics. It should be noted, however, that fly control would have availed little had it not been for the outstanding work of the sanitary engineers in providing a safe water supply.

    A localized but sharp outbreak of Japanese B encephalitis occurred among natives concurrently with the epidemic of malaria already noted, and 38 cases of encephalitis developed in troops bivouacked near these natives. Intense antimosquito activities were immediately initiated in the epidemic area, and the outbreak was rapidly brought under control.

    The antimalaria organization and the control activities were similar to those already described (p. 401) with certain notable exceptions which are to be discussed in detail.


    The military invasion and the occupation of the Ryukyus was the first step in the plan of the United States to penetrate the inner ring of Japanese defense. In addition to cutting the enemy's air communications through the Ryukyus and flanking his sea communications to the south, Okinawa furnished airbases within medium bomber range of Japan and an advanced naval base. The plan was to use it as a supporting position for the invasion of Kyûshû and Honshu. The calendar of operations was as follows:

    Assault landing, Kerama Islands, 26 March, secured 29 March 1945.
    Assault landing, Okinawa Island, 1 April, secured 30 June 1945.
    Assault landing, Ii-shima Island, 16 April, secured 21 April 1945.

    This campaign, as is well known, was marked by some of the bitterest fighting and by the heaviest Allied casualties of the war in the Pacific. Severe combat on Okinawa was concentrated in the southern half and especially in the southern one-third of the island.

    Before the military occupation of Okinawa, it was known, through Japanese reports, that malaria was endemic in the Ryukyu Islands. The extent of the problem and degree of endemicity was unknown.

    Some of the troops, ground and air force and Navy, taking part in the invasion and occupation, had been seeded earlier with malaria in the Philippines, New Guinea, and the Solomon Islands, although there were no heavily seeded units in the operation. Most of the units participating had had minimal or no exposure to malaria earlier. Table 68 indicates the strength of the Tenth U.S. Army.

    By 18 April, the entire northern peninsula of Okinawa had been captured (although guerrilla forces still operated), and the battle lines in the south had settled down along a line across the island from Naha and Shuri


to south of Yonabaru. The entire central part of the island and the north, freed of all except occasional guerrilla actions, was available for rear-area installations. Military government installations and native camps were located in the north of the island, and airfields, dumps, hospitals, and camps were set up in the flatter central region. 31

TABLE 68.- Strength of Tenth U.S. Army and marine and naval forces, in the Ryukyus campaign, 30 April-30 June 1945



    The Ryukyu Islands lie southwest of Japan proper, northeast of Formosa and the Philippines, and west of the Bonins. The islands, peaks of submerged mountains, stretch in an arc about 790 miles long between Kyûshû and Formosa, and form a boundary between the East China Sea and the Pacific Ocean. The archipelago consists of about 140 islands, only about 30 of which are large enough to support substantial populations.

    Okinawa is the largest of the islands in the Ryukyus. Extending generally north and south, it is 60 miles long and from 2 to 18 miles wide, with an area of 485 square miles. The terrain in northern Okinawa, the two-thirds of the island above the Ishikawa Isthmus, is extremely rugged and mountainous. A central ridge, with elevations of 1,000 feet or more, runs through the length of the region; the ridge is bordered on the east and west by terraces which are dissected by ravines and watercourses, and it ends at the coast in steep cliffs. About 80 percent of the area is covered by pine forests interspersed with dense undergrowth. Narrow coastal roads go up the east and the west coast, with several narrow mountainous cross-communicating roads. The Motobu Peninsula, which is nearly square in shape and juts to the west, also has a mountainous and difficult terrain. Two mountain tracts separated by a central valley run east and west the length of the peninsula. About 31/2 miles off the northwest end of the Motobu Peninsula is the small

31 Appleman, Roy E., Burns, James M., Gugeler, Russell A., and Stevens, John: Okinawa: The Last Battle. United States Army in World War II. The War in the Pacific. Washington: U.S. Government Printing Office, 1948.


flat-topped island of Ii-shima, with a sharp pinnacle about 500 feet high at the eastern end.

    The southern third of Okinawa, south of Ishikawa, is rolling hilly country, lower than the north, but broken by terraces, steep natural escarpments, and ravines. The section is almost entirely under cultivation and contains three-fourths of the population of the island and the large towns of Naha, Shuri, Itoman, and Yonabaru. Rice paddies fill the lowlands near the coasts. The roads are more numerous than in the north but usually little more than country lanes. Drainage is generally poor, and heavy rains turn the area into a quagmire of deep claylike mud.

    Shortly after the occupation, the Yontan and Katena airfields were taken over, the naval base established in Nakagusuka (Buckner) Bay, and that portion of the island between Ishikawa to the north and Adaniya to the south became the center of the base military activities of the island. Most of the unloading was done on the western beaches, and most of the quartermaster areas were there. Unloading of naval supplies proceeded on the beaches on the east coast.

Climatic Features

    Although located within the Temperate Zone, the Ryukyu Islands have a subtropical marine climate with a relatively high and equable temperature throughout the year. This is due in large part to the influence of the Kuroshio which arises in the equatorial ocean currents north of the Philippines and flows northeastward among the islands of the Ryukyu Archipelago. Ranging from 50 to 15 0 F. warmer than the other ocean waters about it, the Kuroshio serves to intensify the heat and humidity of the sultry monsoon winds from the south in the summer and to warm and moisten the cold monsoon winds from the north in the winter. The result is both an increase in year-round temperature and humidity and a modification of the climate extremes that would otherwise prevail. At Naha. which is located approximately midway in the Ryukyu chain, the mean annual temperature is 75 0F. January and February are the coldest months, averaging 610 F. in Naha. Between March and May, the temperature rises rapidly, culminating in the very warm season from June to September, when the temperature hovers about 810 F. In the last quarter of the year, it drops swiftly, and December averages only two to three degrees warmer than the two coldest months which follow. Extreme temperature variations are seldom experienced (table 69).

    The mean annual moisture content of the air around the Ryukyu Islands is excessive, averaging 76 percent or more. Humidity is highest during the April-September period, when it averages about 80 percent. Even during the winter, the minimum mean compiled for any month at any station was 73 percent. Diurnal humidity variation is never very great; in summer, it amounts


to only about 20 percent and in winter to about 14 percent. The diurnal maximums occur between 5 a.m. and 6 a.m. and the minimums between 1 p.m. and 3 p.m.

TABLE 69.- Air temperatures (in degrees Fahrenheit), Naha, Okinawa, during a 30-year observation period

    Rainfall is heavy at Naha, the mean annual precipitation being 82.8 inches, with at least 4 inches falling in every month of the year. There is considerable variation in the amount of rainfall from year to year. The wettest year ever recorded at Naha had 113 inches; the driest only 42 inches.

    The summer months receive the greatest amount of precipitation, and the winter months, from November through February, the least. The amount of rainfall in any single month, however, varies considerably from year to year. At Naha, the least precipitation ever recorded for a single month was 0.6 inch, the greatest 28.0 inches. The maximum recorded fall for 1 day at Naha was 16.8 inches. Such extremes of precipitation, however, are usually associated with typhoons. In contrast to the seasonal and monthly variation in amount of rainfall, the number of rainy days is fairly uniform throughout the year. Table 70 presents data on rainfall (in inches) and on the number of rainy days (with at least 0.04 inch of rain) for Naha, observations ranging from 30 to 40 years, as indicated in parentheses. Observations made at 13 minor weather stations on Okinawa indicated that the average annual precipitation varied from about 65 to 90 inches at different locations. The prevailing winds are easterly with a mean annual velocity of 8 miles per hour (range from 6 to 10).

    The islands lie within the typhoon belt of the East China Sea. From 12 to 45 typhoons affect the Okinawa region each year, and from 3 to 6 can be


expected to cross directly over the area. The months of greatest typhoon frequency are from May to October. 32

TABLE 70.- Rainfall (measurement in inches) and rainy days, Naha, Okinawa

Native Population

    Okinawa, previously semi-independent and paying tribute to China, was annexed by Japan in 1879, and the people were integrated into the Japanese governmental, economic, and cultural structure. The racial origins of the Okinawans are similar to, but not identical with, those of the Japanese, and the Okinawan stock and culture has been subject to considerable Chinese influence. While the Okinawans generally resemble the Japanese in physique, they differ appreciably in their language, the native Luchuan tongue.

    The standard of living of the Okinawan people is low; the Japanese made no attempt to raise it, regarding the Okinawans as inferior rustics. Most of the inhabitants subsist on small-scale agriculture. Every foot of usable land, even extending far up the narrow valleys, dissecting the northern peninsula, is planted, and terracing is extensive. Crops are sugarcane, sweet potatoes, rice, and soybeans. Livestock (cattle, horses, goats) were apparently abundant before military occupation. In 1940, the human population of the island was 435,000.

    At the beginning of the military operations, the natives took refuge in caves and in the mountains; as soon as possible they were concentrated in AMG

32 Further geographic and climatologic details can be secured from the Civil Affairs Handbook, Ryukyu Islands, Office of the Chief of Naval Operations. U.S. Navy Department, 15 Nov. 1944.


(American military government) camps in central and northern Okinawa. Some few natives were employed in the established military areas as laborers during the latter part of time campaign, but on the whole there was little association with troops, with the exception of the personnel of the AMG units and hospitals stationed directly in the native camps. The largest concentrations of natives after occupation was the northeast coast, with a series of camps from Kin to Sedake, and another large concentration on the northwest coast in the Gaira-Hentona region at the beginning of the Motobu Peninsula. The bulk of troops was far separated from these areas, in the central and southern part of the island.

    According to reports of several native doctors interviewed during the early part of the campaign, malaria was unknown among the people in the southern part of the island, but occasional cases were seen each year in the north. Extensive examinations of natives carried out by malaria survey teams confirmed this impression--only 9 blood smears were positive in more than 2,000 examinations. A different situation was found during the subsequent malaria epidemic in northern Okinawa (table 71).

TABLE 71.- Results of blood examinations of natives during malaria outbreak in northern Okinawa, August-September 1945

Mosquito Vectors

    The anopheline species present on Okinawa are limited to Anopheles (Anopheles) sinensis Wiedeman 1828, and Anopheles (Anopheles) saperoi Bohart and Ingram 1946.

    Anopheles sinensis (fig. 55) was found to be widely distributed and abundant in the ricefield areas--in the south, center, and north of the island. A. saperoi 33 was found only in limited numbers in northern Okinawa, and, although captured biting man, was apparently so rare that it could be dropped from consideration as an effective malaria vector. The sole vector, by exclusion, was therefore A. sinensis.

    Anopheles sinensis was commonly encountered breeding in flooded rice paddies, before the rice was high, and there can be no doubt that this con-

33 (1) Bohart, R. M., and Ingram, R. L.: Four New Species of Mosquitoes from Okinawa. J. Wash. Acad. Sc. 36: 46-52, 1946. (2) Bohart, R. M., and Ingram, R. L.: Mosquitoes of Okinawa and Islands in the Central Pacific. Washington, D.C.: Bureau of Medicine and Surgery, U.S. Navy Department, 1946.


FIGURE 55.- Anopheles sinensis.


stituted the most favored breeding area. Widespread antilarval operations were begun promptly so that it was not possible to follow the aquatic phase closely.

    Numerous observations were made on the habits of the adults of this species. Despite careful search of hundreds of dwellings, and of caves which would appear to be ideal refuges, adults were rarely encountered in such localities during the daylight hours, and there can exist no doubt that this species does not seek human habitation for refuge. Outdoor refuges were never encountered, although in some areas, mosquito densities, as determined by night captures, were high.

    These mosquitoes became very active at dusk. If one stood outdoors, or in the doorway of a house, dozens of mosquitoes would land and attempt to bite within a short period of time. Even indoors, mosquitoes would be found to have entered freely and to be attempting to feed. This peak of activity lasted for less than an hour, and no observations are available as to whether activity was sustained through the night. Searching of houses shortly after the peak of evening activity revealed few or no mosquitoes resting within, suggesting that the mosquitoes leave after feeding.

    Mosquitoes fed readily on man but were also observed and captured feeding on horses. No comparative studies of host preferences were made. In September, during the malaria outbreak in northern Okinawa, 72 A. sinensis were dissected but none contained Plasmodium.

Effects of Combat and Occupation on Malaria Potentialities of Area

    Antimosquito operations were sufficiently effective that an outbreak of malaria was possible only under unusual circumstances. The required conditions developed around AMG camps in the areas shown on map 27 and resulted in a sharp epidemic of malaria among natives quartered in and near these camps, described as follows:

    Good mosquito breeding places were adjacent to several of these camps. Although all of the AMG camps maintained antilarval crews to apply larvicidal oil to their immediate camp areas, this work was not extended far afield, or up to the narrow terraced valleys inland from the camps, because of the ever-present danger of sniper action, long after the island was declared secure. Airplane spraying was largely limited to the areas of troop concentration in the southern half of the island during May and June and was infrequent over the northern half. Moreover, the mountainous terrain in the north forced the planes to fly high and so decreased efficiency. Consequently, in many regions of northern Okinawa, anopheline populations had the opportunity to and did maintain themselves in considerable numbers. Moreover, the native livestock, probably the most important source of blood meals for the mosquitoes under normal conditions, was virtually eliminated. After several days' search in the Sedake area in the month of September, it was finally possible to find one horse to be used as mosquito "bait." Several horses


were found and utilized in Japanese B encephalitis studies. Cows and goats were rarely seen.

    It appears that the imbalance in the malaria transmission picture, occurring as the result of military occupation, was caused by the destruction of the usual animal blood sources, deviating the mosquitoes to humans, and, at the same time, the placing of humans in much more intimate contact with the mosquitoes, in the AMG camps, than would ordinarily be the case.

    It is important to note, in connection with the habits of A. sinensis (entering houses to bite when necessary but feeding by preference outdoors and practically never remaining in houses as refuges), that all the native dwellings and structures in AMG camps and outside had been sprayed with a residual deposit of DDT in diesel oil before the epidemic began and one or more times during the course of the epidemic. The interior house walls were usually of unpainted wood paneling. No tests were made to determine how long the DDT residual deposits retained activity against mosquitoes.


    There was no malaria in troops on Okinawa during the combat phase, 1 April-30 June 1945. In mid-July, an extensive outbreak of malaria began among the natives on northern Okinawa, and a few cases occurred among troops who were stationed nearby (map 28). 34 Only a few troop units were involved, and reports were fragmentary. The 27th Infantry Division reported 94 cases of malaria in August, most of them new infections. Company I, 5th Regiment, 1st Marine Division, spent 10 days at Kayô (near Sedake) in August and reported 48 new cases of malaria in 205 men. The 3d Battalion of the 147th Infantry stationed at Nago reported 24 cases of primary P. vivax malaria soon after its arrival early in September. None of these units was taking suppressive Atabrine. By mid-September, the epidemic had waned.

    This outbreak was limited to troops in the northern part of the island; there was no malaria among the bulk of the troops who were stationed in the southern half of the island, although they were in proximity to many square miles of rice paddies and other potential breeding areas. The successful prevention of malaria among troops except for these minor outbreaks was due to early and intensive antimalaria work, especially airplane spraying. The cool weather during April, when the night temperature frequently fell to about 60 0 F., slowed down larval development at a time when control personnel were scarce. By the first of May, a regular schedule of air and ground application of DDT was in operation and practically eliminated mosquito production in the occupied portion of the southern half of the island.

    The outbreak among the natives in the AMG camps on northern Okinawa was due to the unusual circumstances already described (p. 483). Malaria ap-

34 Downs, W. G.: Malaria on Okinawa. Bull U.S. Army M. Depart. 9:652-655, August 1949.


MAP 28.- Okinawa Islands, showing localization of malaria outbreak of 1945 .

peared in July and rapidly assumed epidemic proportions until brought to an end in September. It is estimated that from 2,000 to 3,000 cases occurred among the 20,000 to 30,000 natives who were crowded in the areas shown on map 28. All the cases observed were of P. vivax. In 600 slides examined from sick natives at Sedake, 90 percent were positive for malaria.

    This epidemic of malaria was concurrent with an outbreak of about 160 cases of Japanese B encephalitis which occurred among natives and troops on northern Okinawa and on a small offshore island (p. 494).


    It was soon apparent that airplane application of DDT and a small amount of antimalaria work by ground personnel would be sufficient to control malaria transmission in the areas of heavy troop concentration. Therefore, a large amount of work was devoted to the elimination of fly populations, especially during the combat phases and until bodies were buried. These activities have already been described in this chapter, but it is pertinent to note here that this was the first campaign in the experience of these authors in which there was virtually no dysentery or diarrhea among troops during the combat phase.


At Theater Level

    Preinvasion planning was carried out at Navy and Army headquarters in Hawaii. Theater directives were prepared which were similar in essential respects to those already quoted for the South Pacific Area.

    The staff officers who were responsible for developing the theater directives at headquarters in Hawaii were then assigned on temporary duty to fill the positions in preventive medicine in the Surgeon's Office of the Ryukyu Islands Command.

    Arrangements were made for airplane spraying of the occupied area from D+2 to D+ 5 from a carrier based plane under the direction of Lt. John D. Maple. This activity was then transferred to Yontan airfield Forman which a C-47 was used for spraying; about D + 7 this plane crashed and several aboard were killed, including Lieutenant Maple. The malaria control and survey personnel attached to the Army and Marine divisions taking part in this operation were shipped to arrive with their respective headquarters units. The majority of the Army malaria survey and control detachments and the medical sanitary companies were assigned to the Ryukyu Islands Command and were shipped to arrive in echelons from D+15 to D+180. Antimalaria supplies, especially DDT and equipment for its application, were also shipped in echelons.

At Local or Island Level

    A team of five Army and two Navy officers, all of whom had worked together for long periods in the South Pacific Malaria Control Organization formed the nucleus of the Preventive Medicine Section of the Surgeon's Office of the Ryukyu Islands Command and coordinated malaria control activities for the Army, Navy, Marines, and military government units. These individuals went ashore with the Ryukyu Islands Command headquarters on D + 4.

    The following directives provide authority for and information about the insect and rodent program, of which malaria control was an integral part:

    Operational Directive No. 4A, Headquarters, Tenth Army, dated 6 May 1945


    Operational Directive No. 7, Headquarters, Ryukyu Islands Command, dated 16 May 1945.

    Memorandum No. 26, Headquarters, Ryukyu Islands Command, dated 1 June 1945.

    The details of this program were explained in Medical Bulletin No. 6, Headquarters, Ryukyu Islands Command, dated 14 July 1945. The Preventive Medicine Section of the Ryukyu Islands Command was made responsible for the coordination of all antimalaria and other insect control activities. All malaria control personnel were ordered to communicate with the Preventive Medicine Section as soon after arrival on the island as feasible.

    The Preventive Medicine Section of the Island Command Surgeon's Office was responsible for the allocation of antimalaria supplies. Knapsack sprayers and dusters were in limited supply but were obtainable at engineer dumps when requisitions were approved by Preventive Medicine Section. Other supplies were obtainable from the Quartermaster dumps, with the following allowances:

    5 percent DDT in kerosene.- 60 gallons per 1,000 men per month.
    Aerosol, 1 pound dispenser. - 225 per 1,000 men per month.
    DDT dusting powder (larvicide) . - 50 pounds per 1,000 men per month.

    Insect repellent (2-ounce bottle) . - 2,000 bottles per 1,000 men per month as authorized by Circular No. 91, Headquarters, U.S. Army Forces, Pacific Ocean Areas, 12 June 1945. The Preventive Medicine Section established a mixing station for DDT and diesel oil at Yontan airfield and assumed responsibility for the allocation of all of this material on quota basis.

    At the time of termination of hostilities, 30 June 1945, three Army malaria control detachments, seven Army malaria survey detachments, three Army medical sanitary companies, and eight Navy units (including two rodent control units) had arrived and were at work on Okinawa or adjacent islands.

    The Preventive Medicine Section of the Island Command Surgeon's Office, Tenth U.S. Army, maintained direct control over the malaria survey and control units attached to the Army divisions; the surgeon of the naval base maintained direct control over the Navy epidemic disease control units attached to the naval base and to AMG; the Air Force surgeon maintained direct control over the malaria survey and control personnel attached to the Air Force; and the Surgeon, III Amphibious Corps, Fleet Marine Force, maintained control over the two Navy epidemic disease control units attached to the two Marine divisions.

    The activities of all were coordinated into the overall control program directed from the Island Command Surgeon's Office. Survey and control groups were assigned as teams and lived and worked together. Each team was assigned an area of the island as its responsibility. Maps were prepared and kept up to date showing the areas of responsibility of each team. Following cessation of hostilities, Army and Navy activities were sharply separated. A line of demarcation was drawn roughly down the center of the island; all


activities to the east of the line were placed under Navy supervision and all activities to the west, under Army supervision. In spite of this geographic separation, cooperation continued to be close and effective.

    One of the factors in maintaining coordinated efforts among the various malaria control detachments was a series of monthly meetings, begun in April 1945, of officers from all preventive medicine units on the island. Attendance was voluntary and excellent. A résumé of the disease picture as known, pertinent data on change in assignment of areas for control, on control methods, availability of supplies, and similar subjects were presented, followed by the presentation of results of investigations and operations conducted by the various units.

    Unit antimalaria details or sanitary squads were organized and worked in the same manner as previously described (p. 446).

Malaria Control Labor

    Skilled labor and supervisory personnel were provided by the malaria survey and control detachments and similar units noted previously. Medical sanitary companies and troop unit antimalaria details (sanitary squads) provided the bulk of unskilled labor. Natives were little used for control operations except around AMG camps. The wide use of airplane spraying greatly reduced the need for unskilled labor.

Airplane Spraying Arrangements

    Preinvasion plans for airplane spraying called for daily operations beginning with the initial flights by carrier based TBF's and continued by land based C-47's as soon as the tactical situation permitted. This schedule was interrupted by the crash of the only C-47 available at the time after 2 days spraying from Yontan airfield. Airplane spraying was not resumed until May 1945, when one C-47 from the 9th Troop Carrier Squadron became available. Subsequently, two C-47's from the 316th Troop Carrier Squadron became available in June 1945.

    During the combat operations on Okinawa, airplane spraying was under the operational control of the Island Command entomologist, Capt. Paul W. Oman, and the Command sanitary engineer, Capt. Eugene O. Harrison, SnC. The entomologist was responsible for selecting the area to be sprayed on the basis of survey information obtained from various malaria survey units, scheduling the flights, briefing the pilot and navigator as to the limits of the area to be covered, and coordinating with the Island Command Artillery control officer. The last precaution was necessary to assure that gun batteries would post a lookout for the low-flying plane during the hours when flights were scheduled. The engineer was responsible for mixing the necessary quantity of DDT solution and for loading the plane at Yontan airfield, where a


hardstand had been constructed for that purpose. The efficient operation of the airplane spraying program during the early part of the campaign was largely due to the efforts of Captain Harrison and his crew, who were able to reduce the loading time to a few minutes, thus saving valuable flying time during daylight hours.

Malaria Control Battalion

    Early in the Okinawa campaign, the personnel concerned with the survey and control aspects of the preventive medicine program established a headquarters near Katena airfield, some 3 miles removed from Island Command Headquarters. As the number of malaria survey and control detachments engaged in work on the island grew, it became apparent that there was needed some adequately organized and staffed headquarters that could coordinate the work of the various units, handle supply matters pertaining to their activities, and centralize the administrative functions. As a result, shortly after the island was declared secure, a malaria control battalion was activated.

Other Military Agencies Engaged in Preventive Medicine

    A highly significant undertaking in connection with the development of the preventive medicine program for Okinawa was the inclusion of a team of medical specialists, including both officers and enlisted personnel, of NAMRU No. 2, each man competent in a special field of disease investigation. The objectives of this team were to find out early and conclusively the potential danger to troops of scrub typhus, schistosomiasis, malaria, filariasis, and enteric diseases. Within a few weeks, it was shown that scrub typhus and schistosomiasis constituted no hazard and that no troop protective measures would be needed. Here it may be noted that enthusiastic but unqualified workers had claimed the presence on the island of a mite vector of scrub typhus and a snail host of schistosomiasis; both claims had been made as a result of incorrect diagnosis of the specimens and were refuted by NAMRU No. 2 investigators. The same group, cooperating with other personnel, particularly of Army and Navy survey units, helped to delineate the status of the malaria and filariasis problems and their potential hazard.

    When the Japanese B encephalitis outbreak occurred, NAMRU No. 2 specialists were on hand with laboratory personnel and equipment to aid in study of epidemiology, etiology, and transmission. Within 3 weeks of the initial landings, NAMRU No. 2 personnel, with the assistance of the Island Command entomologist, had prepared and reproduced an illustrated synoptic key to the common mosquitoes found on the island. The great value to the preventive medicine program of having fully competent experts available during the early stages of occupation of unknown territory cannot be too strongly stressed. The savings effected through discontinuance of protective measures against scrub typhus and schistosomiasis were enormous.


    Engineering work specifically aimed at malaria control was of minor significance during active operations against the enemy. Two naval construction battalions assigned groups of 50 persons to each of the two Marine divisions, to provide labor for sanitary work, and the Army Corps of Engineers assigned details to bulldoze over and clean up large areas devastated by combat. These details rendered valuable service in general sanitation and reduction of fly breeding by establishing sanitary garbage-disposal facilities and cleaning up fly-breeding areas in destroyed towns.


Training, Information, and Propaganda

    These activities were similar to those previously described (p. 451), but they adapted to the problems peculiar to the Ryukyu Islands. Training directives were written (in advance of occupation) which included information about schistosomiasis, Japanese B encephalitis, and scrub typhus in addition to the usual sections about malaria, dengue, and filariasis.

    The monthly meetings of officers from all preventive medicine units on the island have already been noted. In addition, a monthly medical bulletin, entitled "Malaria and Epidemic Disease Situation, Okinawa," was mimeographed and distributed to all preventive medicine units, Army, Navy, and Marine. This bulletin was made as complete as possible and presented details of the work conducted by all units, observations on new disease problems as these arose, and explanatory notes on new technical developments.

Survey Activities

    Initial surveys in the south central portion of the island made early in April established the fact that, while there was a high larval population of A. sinensis, most of the population consisted of early instars. Cool weather during much of April prevented any considerable emergence of adults before airplane spraying was initiated, and once this control measure was in operation the intensive surveys so necessary for satisfactory control in the islands of the South Pacific proved unnecessary for the control of Anopheles in most areas. As a result, primary survey attention was then directed toward elimination of mosquito breeding in artificial containers, determination of the incidence of different species of fleas, and detection of fly breeding. The survey program was organized on an area basis, each survey unit, both Army and Navy, having a definitely delimited area of responsibility. Since mosquito development during the early part of the season was slow, survey coverage and reports were initially established on a biweekly schedule. Special surveys of port areas were made for the detection of possible dengue fever vectors. The overall operation of the survey program was essentially the same as that reported for the South Pacific Area.


Antimosquito Operations

    Antimosquito operations were begun with the opening of time campaign and continued until the advent of cold weather in the fall.

Antilarval operations

    Antilarval work was the most important of the mosquito control activities. Every unit of company size had its antilarval detail, equipped with knapsack sprayers and supplied with 5 percent DDT in diesel oil from the central mixing station at Yontan airfield, to be described later. The mixing station prepared larvicidal solution for all Army, Navy, and Marine units on the island.

    Personnel from the malaria survey detachments made periodic surveys of the areas under their jurisdiction and when necessary indicated to unit spraying details the areas which were not being covered properly.

    Airplane spraying was done with C-47 planes equipped with two 600-gallon tanks in the cargo compartment. These tanks discharged through a 1-inch pipe which projected a short distance below the belly of the plane. The oil flowed out of the tanks by gravity and was broken up into droplets by the air current.

    Airplane spraying began on the first day of invasion (table 72). As the occupied area of the island expanded, the area covered by airplane spraying followed closely. So closely did it follow, in fact, that on more than one occasion it was discovered that the spraying was being done over enemy-held territory. Airplane spraying undoubtedly was instrumental in practically eradicating anopheline breeding from the southern part of Okinawa, where the terrain was in general low, and application of larvicide could be efficiently carried out. It is doubtful whether ground control measures would have been necessary at all in this part of the island. However, on northern Okinawa, the situation was different, The region was mountainous, with narrow deep valleys, terraced from bottom to top, with ricefields along the entire length. The planes had to fly high of necessity, and, with a work schedule necessitating flying spraying missions all day long, it was not possible to select hours of correct atmospheric conditions for spraying. Consequently, plane spraying was not adequate to cope with this situation. Ground control would have been possible, although difficult, since snipers infesting the valleys greatly hindered such work. The result was considerable production of anophelines and culicines throughout the season, and epidemics of malaria and Japanese B encephalitis occurred in northern Okinawa.

    The rate of use of DDT, dissolving, for airplane spraying and ground spraying was nearly 1 ton per day, and it was often necessary to mix from 7,000 to 8,000 gallons of 5 percent DDT in diesel oil daily.

    The mixing of such large amounts of DDT in diesel oil presented problems from the beginning, and, in the earlier days, it was a constant struggle to


transport drums of diesel oil to the mixing station, keep adequate stocks of DDT on hand, and mix the solution in the individual drums. Mixing was simplified by the use of a compressed air pump. A pipe with a series of holes drilled in it was inserted in the drum, and the airblast turned on, giving more rapid and better mixing than if done by hand; however, expanding operations required a much larger installation. Captain Harrison designed a large-scale mixing plant, obtained materials from many sources, and then set up and maintained a most successful operation. Two 1,100-gallon pontoon tanks were installed on a platform 8 feet high. Two 750-gallon tank trucks and a 2 l/2-ton truck, fitted with a 1,100-gallon pontoon tank were used to transport oil to the mixing station. This oil was pumped up to the pontoon tanks, where the DDT was mixed in, and stirred by an airblast from a compressed air pump. The mixed material was dispensed by gravity flow. One line went directly to the hardstand area, where the plane tanks could be filled directly.

TABLE 72.- Airplane spraying activities, Okinawa, May-July 1945

    A novel procedure was used to help dissolve the DDT in diesel oil. Large quantities of DMP had been brought to Okinawa in gallon cans, in anticipation of a scrub typhus problem which did not materialize. It was found that DDT dissolved readily in dimethyl phthalate. Enough dimethyl phthalate was added to the charge of DDT for mixing with 1,000 gallons of oil to convert it to a sludgy liquid. This was then poured into the diesel oil, the airblast was turned on, and complete mixing took place within a few minutes.

    Extensive filling and hand ditching were carried out as circumstances dictated, and with little use of heavy machinery. Apart from malaria control, extensive operations, including the use of heavy machinery, particularly bulldozers, were carried out to clean up the debris of combat from areas to be used for campsites.

Antiadult operations

    Antiadult measures were employed initially and continuously. Space spraying with a kerosene-pyrethrum spray was carried out routinely in hospitals, messhalls, and similar structures. Aerosol bombs were also used. Experimental trials were made of the Hochberg-LaMer smoke generators and of generators developed from chemical warfare smoke generators.


    Special squads from the malaria survey detachments attached to the combat divisions carried out the hazardous work of spraying all caves and shelters immediately after occupation of enemy territory by U.S. troops. Some extensive caves were sealed off by dynamiting the entrances to close them.

    Residual spraying of native dwellings with DDT and diesel oil was planned before the campaign began. No data are available on the number of native dwellings treated with residual spray in April 1945. Approximately 5,700 such dwellings were sprayed in May; 23,300 in June; and 15,100 in July. No data are available for August or September, but it is known that more native dwellings were sprayed during these months than during the month of June. It is important to note that the repeated spraying of all the dwellings in the areas of native concentration in northern Okinawa did not prevent the malaria epidemic which has been described. This epidemic progressed despite airplane and ground application of larvicides and residual spraying of houses. Two conclusions are inescapable: (1) The antilarval measures used on northern Okinawa were insufficient to handle the mosquito breeding problem, and (2) the habits of A. sinensis adults are not such as to make malaria transmission by the Species easily controllable by DDT residual applications inside dwellings.

    Additional measures of malaria control included the use of mosquito nets as soon as the tactical situation permitted. The unit malaria control officers were responsible for the enforcement of this measure. As more permanent camp areas developed, quarters were screened and mosquitoproofed.

    Insect repellents were provided in large quantities. The mosquito concentrations were low in the areas of largest troop concentrations, and demand for repellents was correspondingly small. One of the minor problems of the Quartermaster was what to do with the huge piles of cases of repellents on hand and those continually arriving.

Drug Suppression

    At the time of the invasion, all the units coming to Okinawa had been on suppressive Atabrine regimen, for at least 10 days before arrival. The regimen was the established one of 0.1 gm.. daily. The responsibility for maintaining malaria discipline within the units rested with the unit malaria control officer, who was advised and supervised in turn by the personnel of the preventive medicine unit in his area.

    It was soon evident that malaria was not going to constitute a serious problem in the areas of heavy troop concentration, and it was therefore felt that suppressive Atabrine administration was unnecessary. By the end of June 1945, not a single new case of malaria had been seen in unseeded units. The outbreak in northern Okinawa had not yet begun. In June, it was recommended that all troops, with the possible exception of some heavily seeded air


force units, be taken off suppressive Atabrine therapy, and this was accomplished by the end of August. There was no increase in reported malaria, either relapses or new cases, as a result of this, except for the troop cases which occurred in units stationed near natives in northern Okinawa, as noted previously (p. 483).


    Because there was no significant malaria hazard in the southern part of Okinawa, the efforts of the preventive medicine personnel attached to the troops were diverted to other aspects of sanitation, including fly control, the delousing of natives and prisoners of war, and the investigation of other parasitic disease problems among the natives.

Fly Control

    It soon became evident that flyborne diseases constituted a more sermons hazard to the health of troops than did mosquitoborne diseases. By late April, most pit latrines showed considerable fly breeding, and this was supplemented by the inevitable unauthorized garbage dumps and improperly disposed fecal matter of the civil population. DDT in oil proved unsatisfactory for control of flies in latrines or in bodies. Experiments with xylene emulsion DDT showed that it would satisfactorily control flies in pit latrines, but the supply was limited. Furthermore, the use of xylene emulsion constituted a distinct hazard because of the volatility of the xylene, and, after one tightly constructed pit latrine was blown up by a flash explosion initiated by a discarded cigarette, no further consideration was given to the routine use of DDT emulsion for fly control. The problem was solved by the use of sodium arsenite, both for latrines and fly-infested bodies. 35 Because of the danger involved in the use of this poison, it was deemed advisable to control its issue and use. Accordingly, all sodium arsenite was mixed at the malaria control headquarters and issued to units, along with precautions as to its use.

    Large fly populations developed on a few occasions in small areas,, especially where combat had been severe and where large numbers of larvae were developing in unburied bodies. Airplane application of DDT destroyed the adult flies rapidly, and concurrent larvicidal work of ground crews prevented further breeding.

Epidemiologic Investigations

    Filariasis .- Filariasis (W. bancrofti) is an endemic disease of great prevalence among the Okinawans. Although elephantiasis was rarely seen, the per-

35 Stilson, A. E.: Okinawa, DDT and Water Treatment. J. Am. Water Works Assoc. 38: 625-636, 1946.


centage of natives harboring filaria on night counts was high. Blood examinations made at night during April and May on about 1,000 Okinawans showed 20 percent positive smears.

    Dissections of mosquitoes were made in order to determine the vector or vectors of filariasis. Of 211 Culex quinquefasciatus mosquitoes captured in a hospital ward for native Okinawans, where every patient had circulating microfilariae, an infection rate of 5.4 percent was found. Of 57 captured at a distance (unspecified) from native camps, none was infected.

    Simpson36 found 4 of 49 female A. sinensis and 14 of 180 female C. quinquefasciatus infected with filarial larvae and suggested that A. sinensis may be a possible vector of filariasis on Okinawa and neighboring islands.

    The danger of transmission of filariasis to troops was localized to the areas in northern Okinawa where natives were concentrated and where troop strength was relatively low and well separated from natives, except for the AMG units working directly in the native camps.

    Denguelike disease . - In late April and May, there were several thousand cases of illness, particularly among combat troops, characterized by an initial high temperature, often preceded by a chill, and leukopenia. Fever usually lasted 3 or 4 days, only occasionally with a secondary rise. Aching of back, joints, muscles, and eyeballs was a prominent feat-ure of the illness. Symptomatology and course was not unlike that seen in atypical dengue, but, in hundreds of cases, no rashes were observed. Aedes aegypti and Aedes albopictus were encountered rarely. Lacking characteristic features of dengue, and lacking the known insect vectors of dengue in numbers sufficient to maintain an epidemic of this size, it was concluded that the illness observed was not dengue. It was officially recorded as "denguelike disease" on the medical records.

    Japanese B encephalitis. - Japanese B encephalitis appeared on northern Okinawa and the small offshore islands of Heanza Shima and Hamahika in early July. A total of 38 cases with two deaths was reported among U.S. troops. At the same time, there were in all 127 cases reported among natives, with a fatality rate of 28.6 percent among the 91 cases on Okinawa, and 30.6 percent among the 36 cases on the two islands. Only a few cases were confirmed by laboratory diagnosis. The outbreak ended in late September. The efforts of preventive medicine personnel were at once directed toward limiting the extent of this epidemic as much as possible. Nothing was known of its transmission, except that a mosquito vector was suspected. Mosquito control activities on northern Okinawa, including "saturation" plane spraying of all native camps, respraying of native dwellings with residual DDT, and ground control work, were greatly intensified, malaria control personnel being moved north from southern parts of the island, and plane spraying schedules being modified

36 Simpson, T. W.: A Note on Filariasis Among the Natives of Okinawa, With Particular Reference to Possible Transmission of Wuchereria bancrofti by Anopheles hyrcanus sinensis. Am. J. Trop. Med. 31:614-616, September 1951.


to give more attention to the north. Vaccination of all military personnel on northern Okinawa was carried out, using a mouse brain vaccine previously untried on humans. There was no evidence to indicate that the outbreak was affected in any way by the preventive activities carried out, nor on the other hand was there any evidence to contradict the viewpoint that it might have been much worse had such measures not been taken. Sabin 37 reported a detailed account of the outbreak of Japanese B encephalitis on Okinawa.

37 Sabin, A. B.: Epidemic Encephalitis in Military Personnel; Isolation of Japanese B Virus on Okinawa in 1945. Serologic Diagnosis, Clinical Manifestations, Epidemic Aspects and Use of Mouse Brain Vaccine. J.A.M.A. 133: 281-293. February 1947.