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Chapter 6, Part 3

Medical Science Publication No. 4, Volume II



Our knowledge of arthropod-borne diseases has been considerably enhanced since the end of World War II, and to a great extent this has been due to Department of the Army research and experiences in the Pacific area. Mosquito-borne diseases are being reviewed by others and hence the current discussion is limited to a review of the advances during the past 8 years in the field of mites and fleas as vectors of disease. An attempt is also made to indicate how the Army might profit as a result of lessons learned during combat operations in a relatively little-known region of the world.

Since modern control measures for trombiculid mites (chiggers), laelaptid mites and fleas are basically identical, this subject will be discussed as an entity after consideration of such diseases as hemorrhagic fever and scrub typhus.

A. Hemorrhagic Fever

Among the diseases known or believed to be arthropod-borne in Korea, hemorrhagic fever was second only to malaria in importance. In the absence of a known susceptible laboratory host, the vector of Korea hemorrhagic fever cannot be definitely determined. However, Russian workers studying this disease in Siberia (1, 2), Japanese scientists observing the same clinical entity in Manchuria (3, 4) and American investigators in Korea (5) independently concluded that this is a disease in which rodents are the probable reservoirs and ectoparasites of these mammals the probable vectors. While chiggers, mites, fleas and ticks fall into this category, epidemiological and entomological data discussed elsewhere (6) indicate that chiggers are the most likely vectors.

Practically all of the cases of hemorrhagic fever have occurred north of the Seoul area, and of these the great majority were north of the 38th parallel (7). The infection invariably seemed to have been contracted in rural areas, especially where troops had been in close contact with the outdoors. The bulk of cases have occurred dur-

*Presented 29 April 1954, to the Course on Recent Advances in Medicine and Surgery, Army Medical Service Graduate School, Walter Reed Army Medical Center, Washington,
D. C.


ing two seasons of the year: in the late spring-early summer period and again in the fall, although cases did occur throughout the year. The Japanese and Russian workers also have reported two peaks of seasonal incidence: May-June and October-November. Even during the spring and fall, the periods when the disease may be considered to be epidemic, the distribution of cases has been extremely spotty, 90 percent of the cases in United Nations troops occurring as isolated events.

As a general rule only one soldier in a company has become ill at any single period. However, the remaining one-tenth of the cases have occurred in small isolated outbreaks, in which perhaps 10 percent of the company apparently contracted hemorrhagic fever almost simultaneously. It is significant that in these outbreaks cases were not evenly distributed throughout the company, but only certain platoons or even tentmates were affected. The soldiers almost invariably reported an absence of insect bites, while ticks, fleas and other biting arthropods were seldom if ever noted. For these reasons, the following criteria have been attributed to the vectors (6): (1) wide distribution in rural areas; (2) activity throughout the year; (3) greatest peak of activity in the spring and fall, or most numerous at these times; (4) limited range of movement; (5) extremely spotty distribution where ever it does occur; (6) availability to transmit the disease for only a very brief period; (7) ability to transmit the infection without leaving a scar, bite or other clue as to the mode of transmission.

Chiggers meet these criteria particularly well. Thus, a minimum of seven species of trombiculid mites are common in the endemic areas of Korea and at least some of these can be found at all times of the year. Chiggers are wingless, crawling arthropods, with a very limited range of movement, as indicated by the fact that they can be extremely abundant in one area and quite scarce in a similar environment a few feet away (8). Many specimens can be found in a particular localized patch of ground for a period of a few days and then apparently disappear because of transformation to a nonparasitic stage. Parasitic larvae may not reappear until months or even a year later. Moreover, chiggers are tiny and are easily overlooked, and some of the Korean species are closely related to forms such as Trombicula (Leptotrombidium) akamushi, the vector of scrub typhus, which do not cause any appreciable itching at the site of attachment (6).

Since there is a characteristically greater increase in number of cases of hemorrhagic fever in the spring and fall months, it is reasonable to expect that the vectors of this disease would show similar peaks of abundance. As indicated in figure 1, in Korea chiggers have been found to be far more abundant in the spring and fall than at other times of the year. The rapid rise in the chigger population



preceded the increase in cases in 1952 and 1953 by a period of 2 to 4 weeks, and the decline in the numbers of chiggers was followed by a decrease of cases after a somewhat similar interval of time. It is reiterated that there is as yet no evidence of a casual relationship. Not all species of chiggers exhibit the same seasonal trends. For example, some are most common during the summer, others during the fall. Some species can be found only during certain times of the year. Trombicula (L.) pallida is the chigger which apparently exhibits a seasonal pattern of abundance which most closely fits that of the case incidence of hemorrhagic fever. T. (L.) orientalis also is very abundant during the epidemic peaks.

Ticks and fleas may be virtually eliminated as important vectors of hemorrhagic fever in Korea because of their scarcity and because their presence could not have been consistently overlooked by troops (6). The laelapitid mites, however, merit consideration, particularly since the Japanese in Manchuria in one instance had isolated the etiological agent of hemorrhagic fever from wild-caught Laelaps jettmari (3, 9). While these mites must be considered as potential vectors of hemorrhagic fever, data from Korea fail to indicate a correlation between the abundance of any laelaptid mite and the spring and fall peaks of hemorrhagic fever. It may very well be that the above single isolation represents the harboring of the agent by an ectoparasite which has fed on the body fluids of an infected rodent, analogous to the isolation of plague from the lice of rodents in the western United States. These laelaptids range over much of Asia


and Europe, and in Korea they were just as common in military camps which were free of infection as in the endemic area. It is considered possible that these mites might serve as vectors at times when troops are in close contact with rodents and their nests, such as during construction of a bunker. Such a mode of transmission might well account for occasional cases of hemorrhagic fever, but it seems an unlikely explanation for the overall epidemiological pattern.

B. Scrub Typhus

The treatment of scrub typhus is no longer a problem, as has been amply demonstrated by Army research teams in Malaya (10, 11), and with the availability of the newer repellents and insecticides, control of the vector is practicable, as will be shown below. Nevertheless, scrub typhus must still be reckoned with as a potent military factor in the Pacific. Hyperendemic foci in scrub terrain are well known in tropical areas and during the summer in parts of Japan, where T. (L.) akamushi and T. (L.) deliensis are the vectors. It has recently been demonstrated that in addition to such foci, infection of Rickettsia tsutsugamushi is widespread in many types of small mammals and in other chiggers in geographical areas and in terrain formerly considered free of scrub typhus. For example, it is relatively easy to isolate R. tsutsugamushi from the tissues of the Korean mice, Apodemus and Microtus (12), but there have been only five confirmed cases of scrub typhus in United Nations soldiers (13). The established vectors of scrub typhus are not known to occur in Korea, or else are so rare that they have not yet been collected, but forms closely related to T. akamushi are plentiful. It is believed that the absence or dearth of primary vectors is responsible for the low incidence of cases.

Scrub typhus also unexpectedly turned up in U. S. troops on maneuvers in the Mt. Fuji area in Japan in 1948, a region far removed from the classical scrub typhus areas of Japan and one that had been considered free of infection (14). Further, the outbreak occurred in the fall, and T. (L.) akamushi is found in Japan only during the summer. While the vector in the Mt. Fuji area has not been definitely determined, R. tsutugamushi had been isolated from local T. (L.) palpalis. The occurrence of so-called "winter scrub typhus" on small islands off southern Honshu is also pertinent (15). Here the classical vectors are also absent and T. scutellaris is under suspicion.

In accord with the current concept of widespread latent infection is the demonstration of infection of R. tsutsugamushi in the Malayan jungle in nonvector chiggers such as Euschöngastia audyi and in three species of rats which are true inhabitants of the primary forest (16). In this regard it is important to note that the vector chiggers are also


more widely distributed than is generally appreciated. T. (L.) akamushi and T. (L.) deliensis have recently been collected in the mountain forests of Malaya and Borneo, even as high as 7,000 feet altitude and hence in literally temperate climates. R. tsutsugamushi has been isolated from these vector chiggers collected in the Malayan mountains (17) as well as from every other region where isolation attempts have been made. The infection rate in these vector species may be quite high-Rickettsiae have twice been isolated from pools containing as few as five T. (L.) deliensis (18) and hence even if these vector chiggers are relatively uncommon in the extremes of their geographic range, they may be able to present a scrub typhus problem.

There is a definite connection between the weather conditions and the abundance of vector chiggers. In southeast Asia, T. (L.) akamushi and T. (L.) deliensis became relatively scarce during dry spells, and the incidence in experimental volunteers in Malaya (18) and in natural cases in Malaya and Burma dropped accordingly (19, 20).

C. Miscellaneous Notes Regarding Mites and Diseases in the Pacific

Although the Asian trombiculid fauna is very rich, only two species have been definitely incriminated as vectors of disease. These are Trombicula (L.) akamushi and T. (L). deliensis, in the case of scrub typhus. As indicated above, other species are under suspicion in this regard. Rickettsia tsutsugamushi has been isolated from Euschöngastia audyi and T. (L.) palpalis (16, 21), while Rickettsia mooseri, the causative organism of murine typhus, has been isolated from Euschöngastia indica in Indonesia (22). It is not known if these species bite man or serve as vectors. The isolation of Rickettsia akari, the causative organism of this disease, from the tissues of a Korean rodent is of interest. No cases of this disease have yet been observed in United Nations forces in the Pacific, and the vector laelaptid mite, Allodermanyssus sanguineus, has not been found there, although it has been collected in such widely separated areas as the deserts of Egypt and the western United States (23).

Because research on mite-borne diseases has suffered through our relative lack of knowledge of the classification and habits of the chiggers and laelaptid mites of Asia, Army-sponsored projects are under way to correct these deficiencies (24-27).

D. Fleas

Fleas proved to be a very minor problem in the Pacific, despite the ever-present hazard of an outbreak of flea-borne plague culminating in a massive epidemic of pneumonic plague in Korea. While such fleas as Pulex irritans and Ctenocephalides felis are often readily found among the civilian population in Korea, infestations in troops have been extremely rare, no doubt as a result of the liberal use of


DDT powder in clothes and bedding. DDT-resistant fleas have not yet been encountered in the field. Rodent control and insect control activities in camp sites have kept the flea population to a minimum, and probably are largely responsible for the fact that no cases of murine typhus have occurred in U. S. troops in Korea. Even the fleas of wild rodents are scarce in Korea throughout the year. A preliminary account of the fleas of Japan and Korea has recently been prepared under Army auspices (28).

E. Control of Mites and Fleas

Adequate measures exist for the control of chiggers, laelaptid mites and fleas by means of repellents and residual insecticides.

1. Repellents. During the latter days of World War II, clothing impregnants were developed which are credited with having greatly reduced the scrub typhus incidence (29, 30). These compounds were either dibutyl phthalate or dimethyl phthalate, or preparations containing dibutyl phthalate and benzyl benzoate. In each instance the impregnant contained an emulsifying agent.

The value of these early-type clothing impregnants in preventing scrub typhus is demonstrated by the experiences of the
U. S. Armed Forces on maneuvers in the Mt. Fuji-Gotemba area of Japan (14). Although a case had been reported in the area in 1934, no outbreaks of the disease had ever occurred there, and everyone naturally came to feel that scrub typhus simply did not exist in this region. The vector chiggers were likewise unknown on Mt. Fuji. Thousands of U. S. troops had maneuvered in that area in the postwar years without incident. However, in the fall of 1948, a total of 24 cases of scrub typhus occurred in troops on maneuvers, resulting in an attack rate of 2.4 percent. Because there was no reason to suspect the presence of scrub typhus, none of the troops had used repellent or impregnated clothing. Subsequent to this attack, troops deployed on Mt. Fuji religiously used impregnated clothing and no cases have been reported in U. S. Army personnel since the original 1948 outbreak. However, in the fall of 1953, the Third Marine Division went on maneuvers in this same region. Medical officers of the Marine Division had been informed of the Army experiences with scrub typhus on Mt. Fuji in 1948 and the fact that repellent had been used each year since then. Moreover, repellent was offered to them by the Army (31). This information apparently did not reach the line officers in the field, and the Marines operated in this endemic area without impregnated clothing. A total of 79 cases of scrub typhus followed the exposure of the Marines on Mt. Fuji (32, 33).


The new standard Army clothing impregnant, designated as M-1960,* was developed to meet the need for protection against a broad spectrum of arthropod vectors of diseases. M-1960 is highly effective against chiggers, laelaptid mites, fleas, ticks, mosquitoes and leeches. Had this compound been available in quantity in 1950, it would have been issued to troops in Korea as a routine protective measure against mosquito-borne diseases. The troops might also have been somewhat protected against hemorrhagic fever. In contrast to M-1960, the older-type impregnants were developed for protection against scrub typhus chiggers for issue on an emergency basis as needed. However, hemorrhagic fever had never been reported in Korea, although it is of interest that it was listed among "Diseases of Special Military Importance" in the TB MED, Medical and Sanitary Data on Korea (34), where hemorrhagic fever is cited as a disease of unknown etiology and vectorship.

Because of the unavailability of M-1960, and because of enormous logistical difficulties, a proper repellent program against this disease was not in effect until the summer of 1952, a year after the outbreak of the disease (35). Despite the best efforts of the Eighth Army medical officers, the first mass shipments of repellents to Korea consisted largely of an obsolete item, i. e., dimethyl phthalate in a form lacking an emulsifying agent, even though the superior items previously mentioned had been made standard items of issue. The obsolete type of repellent necessitated the use of vast quantities of laundry soap in order to prepare the dimethyl phthalate for clothing impregnation, and hence there soon was a marked shortage of soap for this purpose.

Three reasons accounted for this regrettable situation. Under the system then in vogue in the Quartermaster Corps, several varieties of repellents bore the same stock numbers and, even though a specific compound was ordered, any one of three to five compounds might be shipped to fulfill the order. The second factor was the attitude often encountered in the lower supply echelons-reasons of economy dictated the issuing of the oldest item first. The third and most important reasons was that we were fighting a fluid and defensive war on an emergency basis and it was impossible for us to be properly prepared.

M-1960 is now available in quantity in Korea, but it is very difficult to properly evaluate the efficacy of the repellent program against hemorrhagic fever in Korea, although some data indicate that the results have been favorable. In order to ensure proper employment of clothing impregnants on a mass basis, the impregnation should be made a part of the Quartermaster laundry process. Since many units

*M-1960 consists of equal parts of N-butylacetanilide, 2-butyl-2-ethyl-1, 3-propanediol and benzyl benzoate, with 10 percent emulsifying agent (5).


in forward areas in Korea choose to rely upon native help for laundry services, a marked variation exists as to the extent and degree of impregnation. Discipline has admittedly been poor in certain units in this respect. Nevertheless, in the spring of 1953 there were only approximately one-fourth as many hemorrhagic fever cases in the U. S. Army forces as there had been in the previous spring (36). It is quite possible that this reduction to a large extent is due to the efficacy of clothing impregnation, but many other factors enter into this, and adequate data are lacking. Among these factors may be mentioned the numbers and location of troops in the endemic areas during both these periods.

Under certain conditions the use of clothing impregnants should be coupled with repellents which are applied to the skin. New compounds have been developed for this purpose and are generally available. One is a new compound known as "repellent, insect, formula M-2020" (containing dimethyl phthalate-40 percent; 2-ethyl-1; 3 hexanediol-30 percent; and dimethyl carbate-30 percent), and this offers broad coverage against a variety of medically important arthropods, including chiggers, laelaptid mites, and fleas.

2. Insecticides. A joint U. S. Army-British Colonial Office Medical Research Unit demonstrated in North Borneo in 1953 that the insecticide dieldrin provides a practical and efficient means for control of the mite vectors of scrub typhus through disinfestation of terrain (37). Dieldrin, which is a standard issue item in the Army, when applied at the rate of 21/4 pounds of the toxicant per acre, reduced the chigger population to the point where, more than 5 weeks after application, there were nearly 75 times as many chiggers on rats trapped in the untreated area as in the dieldrin-treated plot. Lindane, another item of issue, is also effective in this regard.

In California, in a research project financed by the Office of The Surgeon General, it was shown that dieldrin is highly effective for the control of fleas on wild rodents (38). In treated plots, the flea index was less than one a few days after application, whereas there were over 100 fleas per ground squirrel in the area treated with DDT and over 170 in the control section. DDT proved to be a good residual insecticide with the passage of time, and by 200 days after application was nearly as effective as the other insecticides tested. However, after 270 days, dieldrin was far superior to DDT, and in the dieldrin area there were less than one-tenth as many fleas on ground squirrels as in the control section. None of the compounds tested were found to be effective by the 336th day.

On the basis of the above experiments, it can be recommended that dieldrin be applied to camp sites, bivouac areas, and other regions of troop concentration in areas where it is suspected that diseases may


occur which are transmitted by chiggers, mites, fleas or ticks. It is quite probable that lindane could be used as a fairly effective substitute for dieldrin.


Adequate measures are available to Army personnel for the prevention of mite- and flea-borne diseases. These consist of the use of clothing impregnants such as M-1960, skin repellants such as M-2020, and area control of vector arthropods by means of the insecticides dieldrin and lindane. Where troops have been exposed to areas harboring chigger- or mite-borne diseases, outbreaks of such diseases as scrub typhus have occurred if troops were not properly protected by repellants. Such difficulties have arisen at times by necessity through enforced exposure of men on an emergency basis, but also because the officers involved were not fully cognizant of earlier Army experience with the problem.


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11. Smadel, J. E., Jackson, E. B., and Ley, H. L., Jr.: Terramycin as a Rickettsiostatic Agent and its Usefulness in Patients with Scrub Typhus. Ann. N. Y. Acad. Sci. 53: 375-384, 1950.

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14. Thompson, A. H.: A New Endemic Area of Scrub Thyphus in Japan. Bull. U. S. Army Med. Dept. 9 (11) : 871-879, 1943.

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37. Traub, R., Newson, H. D., Walton, B. C., and Audy, J. R.: Efficacy of the Insecticides Dieldrin and Aldrin in Area Control of the Chigger Vectors of Scrub Typhus. Jour. Econ. Ent. (In press).

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