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Medical Science Publication No. 4, Volume II



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

Since modern control measures for trombiculid mites (chiggers), laelaptidmites and fleas are basically identical, this subject will be discussedas an entity after consideration of such diseases as hemorrhagic feverand 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 absenceof a known susceptible laboratory host, the vector of Korea hemorrhagicfever cannot be definitely determined. However, Russian workers studyingthis disease in Siberia (1, 2), Japanese scientists observing thesame clinical entity in Manchuria (3, 4) and American investigatorsin Korea (5) independently concluded that this is a disease in whichrodents are the probable reservoirs and ectoparasites of these mammalsthe probable vectors. While chiggers, mites, fleas and ticks fall intothis 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 northof the Seoul area, and of these the great majority were north of the 38thparallel (7). The infection invariably seemed to have been contractedin rural areas, especially where troops had been in close contact withthe 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 periodand again in the fall, although cases did occur throughout the year. TheJapanese and Russian workers also have reported two peaks of seasonal incidence:May-June and October-November. Even during the spring and fall, the periodswhen the disease may be considered to be epidemic, the distribution ofcases has been extremely spotty, 90 percent of the cases in United Nationstroops occurring as isolated events.

As a general rule only one soldier in a company has become ill at anysingle period. However, the remaining one-tenth of the cases have occurredin small isolated outbreaks, in which perhaps 10 percent of the companyapparently contracted hemorrhagic fever almost simultaneously. It is significantthat in these outbreaks cases were not evenly distributed throughout thecompany, but only certain platoons or even tentmates were affected. Thesoldiers almost invariably reported an absence of insect bites, while ticks,fleas and other biting arthropods were seldom if ever noted. For thesereasons, 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 numerousat these times; (4) limited range of movement; (5) extremely spotty distributionwhere ever it does occur; (6) availability to transmit the disease foronly a very brief period; (7) ability to transmit the infection withoutleaving a scar, bite or other clue as to the mode of transmission.

Chiggers meet these criteria particularly well. Thus, a minimum of sevenspecies of trombiculid mites are common in the endemic areas of Korea andat least some of these can be found at all times of the year. Chiggersare wingless, crawling arthropods, with a very limited range of movement,as indicated by the fact that they can be extremely abundant in one areaand quite scarce in a similar environment a few feet away (8). Manyspecimens can be found in a particular localized patch of ground for aperiod of a few days and then apparently disappear because of transformationto a nonparasitic stage. Parasitic larvae may not reappear until monthsor 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 notcause any appreciable itching at the site of attachment (6).

Since there is a characteristically greater increase in number of casesof hemorrhagic fever in the spring and fall months, it is reasonable toexpect 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 moreabundant in the spring and fall than at other times of the year. The rapidrise in the chigger population



preceded the increase in cases in 1952 and 1953 by a period of 2 to4 weeks, and the decline in the numbers of chiggers was followed by a decreaseof cases after a somewhat similar interval of time. It is reiterated thatthere is as yet no evidence of a casual relationship. Not all species ofchiggers exhibit the same seasonal trends. For example, some are most commonduring the summer, others during the fall. Some species can be found onlyduring certain times of the year. Trombicula (L.) pallida is thechigger which apparently exhibits a seasonal pattern of abundance whichmost 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 ofhemorrhagic fever in Korea because of their scarcity and because theirpresence could not have been consistently overlooked by troops (6).The laelapitid mites, however, merit consideration, particularly sincethe Japanese in Manchuria in one instance had isolated the etiologicalagent of hemorrhagic fever from wild-caught Laelaps jettmari (3,9). While these mites must be considered as potential vectors of hemorrhagicfever, data from Korea fail to indicate a correlation between the abundanceof any laelaptid mite and the spring and fall peaks of hemorrhagic fever.It may very well be that the above single isolation represents the harboringof the agent by an ectoparasite which has fed on the body fluids of aninfected rodent, analogous to the isolation of plague from the lice ofrodents in the western United States. These laelaptids range over muchof Asia


and Europe, and in Korea they were just as common in military campswhich were free of infection as in the endemic area. It is considered possiblethat these mites might serve as vectors at times when troops are in closecontact with rodents and their nests, such as during construction of abunker. Such a mode of transmission might well account for occasional casesof hemorrhagic fever, but it seems an unlikely explanation for the overallepidemiological pattern.

B. Scrub Typhus

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

Scrub typhus also unexpectedly turned up in U. S. troops on maneuversin the Mt. Fuji area in Japan in 1948, a region far removed from the classicalscrub 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 vectorin the Mt. Fuji area has not been definitely determined, R. tsutugamushihad been isolated from local T. (L.) palpalis. The occurrenceof so-called "winter scrub typhus" on small islands off southernHonshu is also pertinent (15). Here the classical vectors are alsoabsent and T. scutellaris is under suspicion.

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


more widely distributed than is generally appreciated. T. (L.) akamushiand T. (L.) deliensis have recently been collected in the mountainforests of Malaya and Borneo, even as high as 7,000 feet altitude and hencein literally temperate climates. R. tsutsugamushi has been isolatedfrom 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-Rickettsiaehave twice been isolated from pools containing as few as five T. (L.)deliensis (18) and hence even if these vector chiggers are relativelyuncommon in the extremes of their geographic range, they may be able topresent a scrub typhus problem.

There is a definite connection between the weather conditions and theabundance of vector chiggers. In southeast Asia, T. (L.) akamushi andT. (L.) deliensis became relatively scarce during dry spells, andthe incidence in experimental volunteers in Malaya (18) and in naturalcases 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 specieshave 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. Rickettsiatsutsugamushi has been isolated from Euschöngastia audyi andT. (L.) palpalis (16, 21), while Rickettsia mooseri, thecausative organism of murine typhus, has been isolated from Euschöngastiaindica in Indonesia (22). It is not known if these species biteman or serve as vectors. The isolation of Rickettsia akari, thecausative organism of this disease, from the tissues of a Korean rodentis of interest. No cases of this disease have yet been observed in UnitedNations forces in the Pacific, and the vector laelaptid mite, Allodermanyssussanguineus, has not been found there, although it has been collectedin such widely separated areas as the deserts of Egypt and the westernUnited States (23).

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

D. Fleas

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


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

E. Control of Mites and Fleas

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

1. Repellents. During the latter days of World War II, clothingimpregnants were developed which are credited with having greatly reducedthe scrub typhus incidence (29, 30). These compounds were eitherdibutyl phthalate or dimethyl phthalate, or preparations containing dibutylphthalate and benzyl benzoate. In each instance the impregnant containedan emulsifying agent.

The value of these early-type clothing impregnants in preventing scrubtyphus 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 ofthe disease had ever occurred there, and everyone naturally came to feelthat scrub typhus simply did not exist in this region. The vector chiggerswere likewise unknown on Mt. Fuji. Thousands of U. S. troops had maneuveredin that area in the postwar years without incident. However, in the fallof 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 reasonto suspect the presence of scrub typhus, none of the troops had used repellentor impregnated clothing. Subsequent to this attack, troops deployed onMt. Fuji religiously used impregnated clothing and no cases have been reportedin U. S. Army personnel since the original 1948 outbreak. However, in thefall of 1953, the Third Marine Division went on maneuvers in this sameregion. Medical officers of the Marine Division had been informed of theArmy experiences with scrub typhus on Mt. Fuji in 1948 and the fact thatrepellent had been used each year since then. Moreover, repellent was offeredto them by the Army (31). This information apparently did not reachthe line officers in the field, and the Marines operated in this endemicarea without impregnated clothing. A total of 79 cases of scrub typhusfollowed the exposure of the Marines on Mt. Fuji (32, 33).


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

Because of the unavailability of M-1960, and because of enormous logisticaldifficulties, a proper repellent program against this disease was not ineffect 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 anobsolete item, i. e., dimethyl phthalate in a form lacking an emulsifyingagent, even though the superior items previously mentioned had been madestandard items of issue. The obsolete type of repellent necessitated theuse of vast quantities of laundry soap in order to prepare the dimethylphthalate for clothing impregnation, and hence there soon was a markedshortage of soap for this purpose.

Three reasons accounted for this regrettable situation. Under the systemthen in vogue in the Quartermaster Corps, several varieties of repellentsbore 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 supplyechelons-reasons of economy dictated the issuing of the oldest item first.The third and most important reasons was that we were fighting a fluidand defensive war on an emergency basis and it was impossible for us tobe properly prepared.

M-1960 is now available in quantity in Korea, but it is very difficultto properly evaluate the efficacy of the repellent program against hemorrhagicfever in Korea, although some data indicate that the results have beenfavorable. In order to ensure proper employment of clothing impregnantson a mass basis, the impregnation should be made a part of the Quartermasterlaundry 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 laundryservices, 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 manyhemorrhagic fever cases in the U. S. Army forces as there had been in theprevious spring (36). It is quite possible that this reduction toa large extent is due to the efficacy of clothing impregnation, but manyother factors enter into this, and adequate data are lacking. Among thesefactors may be mentioned the numbers and location of troops in the endemicareas during both these periods.

Under certain conditions the use of clothing impregnants should be coupledwith repellents which are applied to the skin. New compounds have beendeveloped for this purpose and are generally available. One is a new compoundknown as "repellent, insect, formula M-2020" (containing dimethylphthalate-40 percent; 2-ethyl-1; 3 hexanediol-30 percent; and dimethylcarbate-30 percent), and this offers broad coverage against a variety ofmedically important arthropods, including chiggers, laelaptid mites, andfleas.

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

In California, in a research project financed by the Office of The SurgeonGeneral, it was shown that dieldrin is highly effective for the controlof fleas on wild rodents (38). In treated plots, the flea indexwas less than one a few days after application, whereas there were over100 fleas per ground squirrel in the area treated with DDT and over 170in the control section. DDT proved to be a good residual insecticide withthe passage of time, and by 200 days after application was nearly as effectiveas the other insecticides tested. However, after 270 days, dieldrin wasfar superior to DDT, and in the dieldrin area there were less than one-tenthas many fleas on ground squirrels as in the control section. None of thecompounds tested were found to be effective by the 336th day.

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


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


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


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12. Communicable Disease Division, Department of Virusand Rickettsial Diseases, AMSGS, Walter Reed Army Medical Center, QuarterlyReports, 1953.


13. Fuller, H. S., and Smadel, J. E.: Rickettsial Diseasesin Recent Advances in Medicine and Surgery, AMSGS, Walter Reed Army MedicalCenter. (In press).

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