Bull. Org. mond. Sante 1968, 39, 219-230 Bull. Wid Hlth Org.

Ecological Considerations in Scrub Typhus* 2. Vector Species

ROBERT TRAUB1 & CHARLES L. WISSEMAN JR2

Certain features, characteristic of outbreaks of scrub typhus, can be explained by the behaviour of the chigger vectors which are remarkably hardy and can survive weeks of freezing or immersion in water. The established vectors are all species of the genus Lepto- trombidium (), i.e., L. (L.) akamushi (Brumpt, 1910), L. (L.) deliense (Walch, 1922), L. (L.) pallidum (Nagayo et al., 1919), and L. (L.) scutellare (Nagayo et al., 1921). These chiggers (i.e. larval ) infest a broad variety of birds and mam- mals, and tend to be found in clusters on certain specific sites on the host. However, the precise site for any species ofmite varies with the host, and it is believed that the grooming habits of the infested account for this " site preference ". The degree of infestation cannot validly be ascribed to the size of the host. L. (L.) pavlovskyi (Schluger, 1948), L. (L.) orientale (Schluger, 1948), L. (L.) arenicola Traub, 1960, and L. (L.) tosa (Sasa & Kowashima, 1951) are regarded as probable vectors. Other species, some belonging to other genera, are under suspicion in this regard. L. (L.) subintermedium (Jameson & Toshioka, 1954) and certain other chiggers were found in " ecological islands" in all montane habitats studied in West Pakistan, despite the intervening high mountains, broad rivers and belts ofsemi-desert. The infection- rate of Rickettsia tsutsugamushi in vector spe'cies in nature is believed to be low, and chiggers may be serving as reservoirs of infection and not just as vectors.

Recent field- and laboratory-studies on Lepto- chiggers is presented first because so many attributes trombidium (L.) deliense (Walch, 1922), L. (L.) of scrub typhus infection are related to the habits of akamushi (Brumpt, 1910) and other chiggers known these mites. The subject has been well reviewed by as, or suspected of being, vectors of scrub typhus Sasa (1961) while many additional details have been have added to our knowledge of the ecological provided by Audy (1961) and Neal & Barnett (1961). factors involved in the transmission of this infection, The following notes are based primarily upon and have provided background information useful studies on the bionomics or culture of L. akamushi in developing methods of control. In discussing this and L. deliense carried out under prevailing condi- subject, therefore, some of the points made in Part 1 tions in Malaya or in our laboratory in Baltimore at of this series of papers (Traub & Wisseman, 1968a) about 27°C and 85 % relative humidity. dealing with the natural history of scrub typhus are The main points about the life-cycle are that trom- considered in more detail as a basis for the discussion biculid mites are parasitic in only one stage-the larva ofarea control in Part 3 (Traub & Wisseman, 1968b). or chigger-and that the chigger normally feeds on its A brief summary of the life-cycle and behaviour of host only once, and subsequently is non-parasitic. It is axiomatic, therefore, that any infection transmit- * From the Department of Microbiology, University of ted by the chigger must have been derived from the Maryland School of Medicine, Baltimore, Md., USA. Aspects of the research and field studies were supported by female parent (by transovarian transmission). the Army Medical Research and Development Command, The eggs are laid singly and loosely in the super- Washington, D.C. (Contract No. DA-49-193-MD-2277) of the soil at the rate of about 1-5 a under the auspices of the Commission on Rickettsial Diseases ficial layers of the US Armed Forces Epidemiological Board. Robert day; they hatch in about 2 weeks. The 6-legged Traub was awarded NIH Career Development Grant larvae are generally ready to feed within 1 or 2 days No. AI-14829. l Colonel, US Army (Retired) and Research Professor. of emergence and await a host while perched on bits ' Professor and Head of Department. of debris near the ground. The unfed chiggers are

2217 -219- 220 R. TRAUB & C. L. WISSEMAN JR orange-yellow in colour and so small as to be barely EPIDEMIOLOGICAL FEATURES OF SCRUB TYPHUS visible to the naked eye. They become very active at the close approach of a warm-blooded host and, if The most characteristic features of an outbreak contact is made, they swarm on to the host and, of scrub typhus are: (1) the obvious association sooner or later, settle down, usually in a sheltered with certain types of terrain; (2) the marked locali- and characteristic locus, where they attach to the skin zation of many cases within certain small foci; (generally in a cluster of siblings) and feed. The (3) the large percentage of susceptible people, who larvae become greatly distended as they engorge, may be infected simultaneously following exposure and finally their volume is many times greater than for relatively short periods; (4) the absence of a originally. After about 36-72 hours, the larvae history of bites or attack by . Thus, disengage and drop off the host on to the ground. a brigade-sized unit of the British Army exposed in Here they enter a superficially quiescent stage an infective area in Ceylon for only 4 days during within 3 days and after 7-10, or more, days in this the Second World War sustained 756 cases of scrub nymphophane stage, 8-legged, reddish, pilose typhus (Audy, 1949b), and these were all admitted nymphs emerge. The nymphs crawl actively about in to hospital within a 1-week period (Sayers & Hill, or on the soil, and feed upon the fluid contents of the 1948). In New Guinea, one-third of the US forces eggs of arthropods or recently dead, soft-bodied invading Owi Island were ill with this disease insects. After a week or two of active life, the within 3 weeks of landing (Philip, 1949). Because nymphs become quiescent and enter the imago- of the relatively high death-rate and the lack of chrysalis stage, from which the adults emerge 1 or 2 specific therapy at that time, by virtue of the spotty weeks later as sexually mature, enlarged replicas of distribution of cases in " islands of infection" the nymphs, with similar feeding habits and general whereby all cases might occur in one platoon but behaviour. Males produce stalked spermatophores not in its neighbours (Philip, 1948), and because of within a day, and these are collected by the females, the absence of any apparent means of transmission, which may be equally young, in their genital open- scrub typhus was a fearsome and mysterious thing ings. In 2 weeks, the females start to lay eggs and to the soldiers who "were often more afraid of may continue for periods of 3-5 months, averaging clumps of grass than they were of the enemy " about 350-450 eggs each. The males continue to (Audy, 1949b). However, all these epidemiological produce spermatophores all their lives. The entire characteristics can be explained on the basis of the life-cycle thus lasts usually about 2-3 months; in the behaviour of the trombiculid vectors. laboratory, adults of both sexes can live for 15 months or more. However, L. deliense and L. DISTRIBUTION AND BEHAVIOUR PATTERNS akamushi are species associated in the main OF CHIGGERS with hot, humid weather, and their development proceeds more rapidly than that of montane The close association of L. deliense with woody species now under study. The latter may need terrain or ground that has been cleared of forest 8-10 months to complete the entire cycle in the within a few years, and of L. akamushi with grassy laboratory (at 220C).1 fields, has already been discussed (Traub & Wisse- Reproduction in the absence of males has been man, 1968a; Traub et al., 1967). The islands of observed (Kaufmann & Traub, 1966) in L. (L.) are- infection were shown to be " islands" (Philip nicola Traub, 1960, a species closely related to et al., 1949; Audy, 1961) where vector chiggers L. deliense. This is the first record ofparthenogenesis could be plentiful in one spot but apparently absent in the family and it is not yet known in another a few feet away in the same habitat. if it is a regular phenomenon and whether it occurs The precise explanation for this phenomenon in other species. awaited the observation of Gentry et al., 1963 that Further details about the behaviour of these and unfed L. deliense and L. akamushi tend to aggregate other chiggers are given after a discussion of some closely in clusters on twigs and debris a few inches pertinent epidemiological factors. above the ground and await a host. Under favour- able conditions in endemic foci, vector chiggers may 1 Subsequent generations of the montane species Lepto- become so abundant that wild rats may be infested trombidium (L.) subintermedium (Jameson & Toshioka, 1000 1954) have adapted to laboratory conditions, and now the with mites per animal (Traub & Frick, 1950; life-cycle is completed in 2-3 months at room temperature. Traub & Dowling, 1961), while Tupaia may have ECOLOGICAL CONSIDERATIONS IN SCRUB TYPHUS. 2 221 as many as 5000 1 and birds 11 000 (Audy, 1956c). batches of L. akamushi to Washington by airmail Accordingly, it is easy to understand how groups from Malaya simply by submerging engorged of men living and working in close contact with the chiggers in vials of water and shipping them in infested ground could be struck by such devastating ordinary mailing-tubes. Despite the 10-14 days outbreaks. Unengorged Leptotrombidium are tiny, spent in transit, the mortality of the mites was low and generally do not cause appreciable itching, and a colony was soon established in Washington. even at the site of attachment (Philip et al., 1949), Hsu, Su & Chen (1958) in 1958 stated that engorged and therefore it is not surprising that the vectors chiggers maintained within or on top of water will escaped the notice of their victims. nevertheless develop into nymphs and mature nor- Even the so-called seasonal incidence of scrub mally. These observations are in accord with the war- typhus can be associated with the habits of the time findings of the United States of America Typhus vector. In monsoon countries, the bulk of the Commission in Burma,2 where an island which had cases occur in the rainy season, although a limited been completely submerged in the middle of a river number are contracted in the dry season (Traub, during the monsoon season was noted, nevertheless, 1949; Traub & Frick, 1950) and in Malaya, during to harbour unfed chiggers shortly after the waters experiments on chemoprophylaxis, it was noted that had subsided and grass had begun to grow. the incidence of scrub typhus in volunteers was At the other extreme, species of Leptotrombidium much higher during the rains than immediately (Trombiculindus) have been found to be present only after one of the characteristic, but irregular, dry during the dry season in Assam and Burma (Traub spells (Traub & Frick, 1950). As has been pointed & Evans, 1951; Traub & Nadchatram, 1967b) and out, L. deliense and L. akamushi are much more we have data indicating that in Iran even such truly abundant in warm, wet climates and moist habitats desert rodents as Calomyscus are infested with a than during dry spells or in xeric areas (Traub & minimum of 11 species representing 5 genera (includ- Wisseman, 1968a; Audy, 1961; Traub & Frick, 1950). ing Leptotrombidium). However, the possibility One reason why the possibility of chigger-borne exists that these desert species, and those reported infection has often been overlooked is that these in xeric terrain in Pakistan (Traub & Nadchatram, mites (and scrub typhus) can occur in geographic 1967a), may be limited to local oasis areas or to areas and habitats where their presence is not activity immediately following the infrequent periods expected, such as high in the Himalayas in subarctic of rain. Even the classical vectors are more hardy conditions, in semi-desert (Traub et al., 1967; Traub or adaptable than is generally appreciated. L. deliense, & Wisseman, 1968a) or in bitterly cold mid-winter a species which can survive the Japanese winters, has conditions in Korea (Traub, 1954; Jackson et al., been found in every type of habitat in Malaysia 1957) or in a locality hundreds of miles from the which is associated with the contemporary or recent nearest known locus (Nur Ahmad & Burney, 1962; presence of trees or shrubs. It has thus been found Traub et al., 1967). In nature, then (although not in a focus deemed to be endemic in primary jungle always in the laboratory) chiggers as a group are (Traub, 1962) and in swamp-forests,3 and high in surprisingly hardy. We have noted that certain mountains under temperate conditions (Traub & temperate and alpine species, while still attached to Audy, 1954), but not on sandy beaches, where L. are- the ears of the host, can withstand storage in solid nicola, a sibling species, replaces it (Traub, 1960). carbon dioxide or deep-frozen at a temperature On the other hand, certain chiggers are quite of -20°C for a period of several weeks and appear restricted regarding the habitat in which the egg or perfectly normal and active when thawed; Shoshina the post-larval stages develop, while others are (1965) reported regular metamorphic development remarkably specialized in the parasitic stage. For in engorged larval Neotrombicula zachvatkini (Schlu- example, L. arenicola is found only on sandy ger, 1948) (referred to as ) after freezing. beaches, and Ascoshoengastia (Laurentella) indica The tolerance of larval trombiculids to immersion (Hirst, 1915) (formerly referred as Neoschoengastia in water is also extraordinary. In 1955 we sent 2 Traub, R., unpublished data on files of the India-Burma I Traub, R., unpublished data in reports of US Army Field Party, USA Typhus Commission, Washington, D.C., Medical Research Unit (Malaya) to Research and Develop- USA, 1944-45. ment Board, Office of the Surgeon General, Department of 3Traub, R., unpublished data in reports of the US the Army, Washington, D.C., USA; and unpublished data Army Medical Research Unit (Malaya), to the Research and on files of the India -Burma Field Party, USA Typhus Com- Development Board, Office of the Surgeon General, Depart- mission, Washington, D.C., USA, 1944-45. ment of the Army, Washington, D.C., USA. 222 R. TRAUB & C. L. WISSEMAN JR or Euschoengastia), like most or all other members Pakistan (Traub et al., 1967) and noted elsewhere of the subgenus, spends its non-parasitic stages in (Traub & Wisseman, 1968a; Harrison & Audy, 1954; the nests of arboreal rodents-in rat nests, in this Tamiya, 1962). However, as Audy (1956a) has instance. It is interesting to note that A. indica is pointed out, in addition to these " ecological host common in purely urban parts of Singapore, infest- preferences" (Traub & Morrow, 1955), there are ing rats living many floors above ground level,1 just principal hosts as well as casual ones. Certain as it is the dominant chigger on rats dwelling in trombiculids, moreover, are quite host-specific, and trees on oil-palm estates, or on roof-rats in towns such " physiological host preference " is exhibited by and villages in Malaya (Audy, 1954, 1956b). It is many Gahrliepia (Gahrliepia) Oudemans, 1912, on either the only, or else the most common, species rats (Traub & Morrow, 1955) and by some Gahr- on cosmopolitan rats in urban areas like Bangkok, liepia (Walchia) Ewing, 1931, on rats and shrews Rangoon, Hong Kong, Manila and cities in India (Traub & Evans, 1957). The classical vectors of and Indonesia, etc. (Audy, 1954, 1956b; Gispen, scrub typhus infest a great variety of hosts (Harrison 1950). We have found it abundantly on rats in & Audy, 1954; Audy, 1956a) and the same is true the heart of Lahore where, as elsewhere, it was for such probable vectors as L. (L.) pallidum (Nagayo often found embedded in mucus and wax deep in et al., 1919), L. (L.) orientale (Schluger, 1948) (Traub the external auditory canal, as described by Philip et al., 1954; Tamiya, 1962; Tarasevic, 1964), and & Woodward (1946). It seems probable that other for L. (L.) scutellare (Nagayo et al., 1921) (Tamiya, trombiculids breed in the nest of the host, for it has 1962). Other chiggers suspected as transmitting recently been demonstrated that many species, for- this infection are treated later, but in this discussion merly deemed rare on the basis of existing collec- of hosts of chiggers some new data, based upon our tions, are actually common in the burrows of the studies in Pakistan, are relevant (see Fig. 1 and 2). hosts or in holes in the ground (Kundin et al., 1966). As can be seen from the figures, species of Lepto- Hypodermal parasitism is an example of speciali- trombidium (Leptotrombidium) were in the majority zation that has apparently developed independently in each area and habitat, and they readily parasitized in a number of species infesting rats, i.e., Gahrliepia the local hosts, whether rodent or insectivore. penetrans Traub & Morrow, 1955 (Sabah) and Virtually all the other species of chiggers had an Trombicula (T.) hypodermata Nadchatram & Traub, equally broad host-range, but it is apparent that 1966 (West Pakistan); the same is true for the trait some hosts were more favoured by certain chiggers of intranasal infestation, occurring in all known than others. Thus, Microtrombicula were more Doloisia Oudemans, 1910, certain Ascoschoengastia prevalent on Alticola voles and on Rattus than on (Laurentella) Audy, 1956, L. (L.) spicata Traub, Apodemus or shrews, and were not taken on ham- 1960, etc. Very little is known about the possible sters, while Schoengastiella were most apt to be role of hypodermal or intranasal chiggers in the found on shrews. transmission of infection, or about their life-history As pointed out by Audy (1956a), most species of (other than that they spend weeks or months in the chiggers " have a favoured site of attachment on the parasitic stage, unlike typical chiggers which gener- host ", but it is important to note that the " same ally engorge in a few days or within 2 weeks.) species frequently prefers different sites on different hosts ". These statements have been borne out by HOST PREFERENCE AND FEEDING PATTERNS our studies, not only in South-East Asia, but also OF CHIGGERS in West Pakistan, Korea, New Guinea and elsewhere, It has frequently been observed that many species and are worth emphasizing. L. deliense, for example, of trombiculids are apt to infest nearly every kind when infesting rats, is the most likely species to of mammal or bird encountered in the microhabitat occur in clusters in the ear fossae of rats in all these (Wharton, 1946; Wharton & Fuller, 1952; Audy, areas, but on the erinaceid gymnuran Hyolomys 1 956a), and this wide variety of hosts probably suillus Muller, 1839 (Malaya and Borneo), it is accounts for the broad range of found found in close groups on the hairless rump and naturally infected with scrub typhus rickettsiae in relatively nude patches on the belly and the inguinal folds. On tree-shrews (Tupaia; range: Assam to 1 Traub, R., unpublished data in reports of the US the Philippines), there are generally huge clusters Army Medical Research Unit (Malaya), to the Research and Development Board, Office of the Surgeon General, Depart- of L. deliense on the midline of the belly (sometimes ment of the Army, Washington, D.C., USA. totalling 3000 or more chiggers) and in the inguinal FIG. 1 THE MOST PREVALENT CHIGGERS FOUND ON ALTICOLA VOLES AND ON APODEMUS IN CERTAIN MOUNTAINOUS AREAS IN WEST PAKISTAN

DIR STATE SWAT KAGHAN VALLEY

6 000 ft-8 000 ft 8 300 ft - I OOO ft 6 000ft - 8 900 ft 7 000 ft. 9 500 ft 9 500 ft - 12 000 ft (1 830m-2440m) (2450m-3050m) (1 830m-2470m) (2130m-2800m) (2900m-3670m)

LEPTOTROMBIDIUM (L) DI HUMERALE 77%1717* 17r LEPTOTROMBIDIUM (L) IRREGULARE I9 2% 9 3% LEPTOTROMBIDIUM (L) 36% RUPESTRE 80% -82%97%82% 80% -.--'zz~ .iii9 LEPTOTROMBIDIUM (L) 56% 41% SUBINTERMIDIUM 6% J _ J% LEPTOTROMBIDIUM (L) TITHWALENSE 3% 7% 0 0 2% | I%_ 8% |2% 1% LEPTOTROMBIDIUM (L) (OTHER) 2% _2%/ 2% ASCOSCHOENGASTIA (L) SPECIES (3) 0 1% 5% HELENICULA SPECIES (3) 1% 2% 0 3% MICROTROMBICULA 58% 48% NEW SPECIES (?) | e n 3% 3% NEOTROMBICULA AUTUMNALIS 9% 12% O 0 19%|16% NEOTROMBICULA KASKMIRENSIS 5% 2% 1~ 0 0 11% NEOTROMBICULA NEW SPECIES "B" 6% 4% 3% 0 NEOTROMBICULA NEW SPECIES "C" 13% 7% NEOTROMBICULA (OTHER) 1% 1% 0 5% 2%

(SCHOENGASTIELLA) 43% 19% 0 KALRATA (SCHOENGASTIELLA) (OT HE R) (3) 1% 5% 1% 4 * 5% 1% 0 SHUNSENNIA WISSEMANI 0 9% 3% 0 18% 18%

(OTHER) 1% 0 | 3% | % 2% 1%

No. of Chiggers examined 206 120 | 220 348 852 212 296 222 | 100 329 ±100% ±100% ±100% ±100% ±100% ±100% ±l00%o ±100% 100% ±100%

- CHI GGERS FROM APODEMUS W CCHIGGERS FROM ALTICOLA *0 - LESS THAN 1% WHO 80681 224 R. TRAUB & C. L. WISSEMAN JR

FIG. 2 THE MOST PREVALENT CHIGGERS FOUND ON CRICETULUS HAMSTERS, RATTUS AND SHREWS IN CERTAIN AREAS IN WEST PAKISTAN

' |SiALKOT | LAHORE | MALAKAND KAGHAN GILGIT AGENCY QUETTA PLAINS PLAINS Desert and L Mowuntain des*ert Mountain semi-desert Forest etc. ond ravines forest Hill semi-desert 3000ft.4200t 8U00ftr.9S50ft S000It.10 00 ft 9500 It 6 000f.SO00 ft (914m*1225)m (2440m-2900m) (2440m.30S0m7 (2990m) (11 830 m-2 440 m)

LEPTOTIOM5lD)UM 5% DE LIENSE

LgPtOI5OMASIDIUM (L) OSINUMIIAL( 5 LEPTOTEOMEIDIUM (IL ...J-.._.------~~~~82°w GLI5ICOL(NS 14% 1i910% 2%. -40006-~~ ~ ~ ~ ~ -78% LEPTOtROMIIDIUM (LI 40% *8% JAVEWICK5EMIP1A 55hl U

ILIPTOtIOMIIDIUM. (LI RUPEStII .t 72% LEIFOI5OM5IDIUMA (LI SUEINIE*MIDIUM 21% 29%

LEPIOIOMOUM(Li._._ 1IINWALINSE 9%

LIPIOT5OM5IDIUM (I) (Of Hell *A_)_ __, _, _ _ t% ACOMIATACA5US SPECIES . . _ ' _ . _ _ 3____1%

MIC5OT5OMASCULA 47% 28% sPECIES (4) __

NEOI5ROMBI1CULA SPECIES '(4)lo

(SCNOENOAS*SELLA) 36% 21% 18%

ISCNIENOAStIILLA) 34% 47% 39% 39% g toutLIOULA 1~~~2%Jr...... R.e5aIg 1mi ...... L._____6 %a

GINIsA. (OTNIt S * 8% - 10% Is%3% 1% __. __ _ D __2_ __ _ T _ 4__ |_.__.__-_- -. iN.aCbisi ses"|amie330 200|:1337 1601r294 2001|170 _ ISO15 207 Lx2 I 160 1 4S7 --1 *109% . * *s tlOn^ *_ t1iv l_ A25wn-gww :1wnT1%% titlwj

FROM RATTUS 0 0 CHIGGERS FROM CRICETUWS MIGRATORIUS C HIOCU5FROMUSIRfWS CHICGERS Less than WHO 80682 ECOLOGICAL CONSIDERATIONS IN SCRUB TYPHUS. 2 225 region, but the ears are frequently unaffected. On ground. However, this cannot be an important con- Crocidura shrews, the thighs and perineal area are the sideration since the chiggers, at least in the labora- usual sites of attachment for these chiggers, while tory, wander about the host, sometimes for hours, on Macacus monkeys in India they occur in odd- before attaching. Moreover, small animals, like looking orange patches on the eyelids and eyebrows. shrews and Hylomys, would then bear large patches The reasons for such variation in site of attachment of L. deliense on their backs or ears, instead of on in one species are unknown, but it appears to us the legs and perineum where they characteristically most likely that they are correlated with the par- occur. ticular grooming habits of the host rather than The size of the host, however, has been considered indicating any " preference" on the part of the to be associated with the numbers of chiggers per chiggers. Our reasons are as follows: house-mice individual. Gentry et al. (1963) point out that Rattus (Mus) are well known for their self-cleaning activ- argentiventer, " whose back is 3.5 inches high ... ities and for the fact that under natural conditions would be expected to encounter (clusters of) L. aka- they are rarely infested by chiggers. (When held mushi frequently on the ears, sides and back... while in restraint, house-mice are excellent laboratory- Rattus exulans, which reaches approximately two hosts for chiggers, and this suggests that the inches, may less frequently contact these mites. odour of the mouse is not the determining factor.) This smaller species living in the same habitat nor- Rats cannot effectively scratch or clean their ear mally carried about one-tenth the number of larvae conchae or perineum, or the midline of the dorsum as the larger rat ". The authors might have added (where the burrowing T. hypodermata is almost that a third rat in the same habitat, R. jalorensis, invariably found) and they are often seen to have is intermediate both in size and in the number of large clusters of chiggers in protected sites, chiggers carried. Mohr (1961) also believes that the such as under flaps of skin torn in fights or under size of the host, along with its home-range, are the mats of hair. The foreign matter so common in the major factors in the ectoparasite load borne; this fur oftree-shrews indicates that these animals are not view is based upon his own studies in New Guinea adept at cleaning their long hair, and captive speci- and the USA, and upon calculations made on the mens spend little time in grooming themselves. basis of the published data of others, including those The attachment sites of other kinds of chiggers working in Malaya. Thus, he noted that in one also support this hypothesis. Thus, Gahrliepia laci- Malayan area, the chigger index (i.e., the average niata Traub & Morrow, 1955, are regularly found number per host) for R. whiteheadi (the smallest on the muzzles of rats, at the base of the vibrissae rat) was 0.6; for R. exulans, 1.0; R. jalorensis, 15 or stout hairs where they are relatively secure from and R. argentiventer, 78. Mohr also stated, " Al- dislodgement (Traub & Morrow, 1955). Burrowing though the observed (home) ranges were similar for chiggers must be especially annoying to the host, exulans and argentiventer,' the former bore only 28 % judging from the scar tissue around the attachment as many chiggers. Since the ventral surface of area and the large plugs of mucus and skin in or exulans is 29 % of that of argentiventer, much of the near the apertures of craters which had been for- difference in infestation may be the result of size". merly occupied by mites. However, such hypodermal Despite Mohr's careful calculations and interesting chiggers are generally attached on areas the host approach and theories, he did not have sufficient cannot reach readily or would not scratch vigorously, information about the areas where he himself did i.e., eyelids, lips, perineum, etc. Those chiggers that not collect, and Gentry et al. (1963) were apparently are found in relatively accessible areas, such as the also somewhat misled regarding the correlation chin, legs, ear fringe, etc., are those found in numbers between size of host and degree of infestation. in rodent burrows as unfed larvae, indicating that While it may appear logical that a large animal they hatch in the nest and may attach while the host has more surface area on which to harbour chiggers is sleeping. (or fleas, etc.) than a small animal has, and that a mammal covering a large territory would be exposed HOST SIZE AND CHIGGER INFESTATION to more chiggers than one with a limited range, It might be thought that the size of the host there are many examples to the contrary, indicating determines the site of attachment of the chigger, 1 Surprisingly enough, Gentry (1965), using radio- since size affects the area of contact between hosts tracking devices, found that the small species, R. jalorensis, and a cluster of unfed chiggers on debris near the had the larger territory. 226 R. TRAUB & C. L. WISSEMAN JR that other important factors are at work. Thus, physical and chemical conditions of the micro- the mouse-sized Hylomys (Malaya and Sabah) is habitats of both mites and vertebrates, the effect of usually heavily infested with chiggers (Traub & the season, atmospheric conditions, etc. In short, Evans, 1957), an index of 250 chiggers per individual much more field and experimental data are needed. being regarded as minimal in Sabah (Traub & Morrow, 1955). The Korean vole, Microtus fortis CHIGGERS AS VECTORS OF SCRUB TYPHUS pelliceus Thomas, 1911, is certainly no greater in L. akamushi and L. deliense are the- best-studied body surface area than R. jalorensis (and has a vectors of scrub typhus but space does not permit limited home-range), and yet an index in excess of a detailed consideration of their roles in this regard. 1000 chiggers per mouse has been reported (Traub Reviews of the subject and references may be et al., 1954a). Even the example of the small-sized obtained from Audy (1958, 1961), Burgdorfer & R. whiteheadi mentioned by Mohr does not agree Varma (1967), Hsu et al. (1958), Mackie et al. (1946), with the theory of association between size of host Tamiya (1962), and Tarasevic (1960). On morpho- and numbers of chiggers, for if he had considered logical grounds, doubt has been expressed that the intranasal chiggers as well as those on the body, two names represent distinct species, as noted by the index for R. whiteheadi would be well over 100 Audy (1958). Thus, Philip & Kohls (1948) regarded or 200, since that species is one of the best hosts L. deliense as a subspecies of L. akamushi. However, for intranasal trombiculids (Audy, 1956a). Further, the taxonomic character of the relative length of many species of giant rats in Malaya and Borneo coxa II, and other morphological distinctions living in the same habitat as R. whiteheadi are rarely reported by Traub & Audy (1954) as means of infested by such intranasal chiggers. Conversely, separating the two species, have apparently proven large mammals, which range over large territories, reliable, at least in South-East Asia. Further, in frequently have very low chigger-infestation rates. Malayan fields, engorged larvae of L. deliense are We have data from Assam, North Burma, Malaya, consistently a lighter yellow-orange in colour in life Thailand and West Pakistan, showing that the index than L. akamushi, and there are distinctions in on 5 tigers, 6 leopards, 4 deer, 12 civets, and habitat as well which separate the two (Hubert & 6 weasels was generally less than 20 chiggers each. Baker, 1963; Gentry et al., 1963), provided one Since the carnivores and ungulates usually were duly considers the time-factor and the changes fairly well parasitized by ticks, which climb shrubs wrought by ecological succession (Traub & Wisse- and bushes to await their prey, it may be that the man, 1968a). The problem may have been com- clusters of chiggers close to the ground had little pounded by the introduction of L. akamushi into chance of coming in contact with these hosts unless certain areas where L. deliense occurred (Audy, 1958), the hosts lay down. The main stimulus activating but the real confusion may be nomenclatorial not chiggers coming into proximity with a host is the morphological. Thus, the species which Western carbon dioxide expired by the host, and not move- workers call L. deliense occurs in the classical scrub ment or warmth (Sasa et al., 1957; Gentry et al., typhus area in Niigata Prefecture in Japan (Philip, 1963; Audy, 1961), but there is no reason to assume 1947b) along with true L. akamushi, whereas the that a relatively fast-moving, remote source of Japanese literature seems to mention only " L. aka- carbon dioxide (i.e., a carnivore) is a more effective mushi ". While there seem to be 2 valid species stimulus than a slower one (such as a rat) moving represented, both names apparently have been near the level of the ground. Here again, it would applied at different times to each of the species. seem that a cluster of chiggers 2 or 3 inches from the There is obviously great need for full agreement as ground, as reported by Gentry et al. (1963) would to the definition of the two species and consistency be more likely to make contact with a rat the size regarding the name applied to each. of R. argentiventer than with a larger mammal. Fortunately, the confusion does not extend to the However, the abundance of vectors and other vectorial behaviour of the two species. Each has chiggers on such small creatures as mice, shrews repeatedly been found naturally infected in nature, and Hylomys indicates that other factors are also as demonstrated by inoculating engorged larvae, responsible. Among the items to be considered with collected from wild-caught hosts, into susceptible respect to infestation rates are the behaviour of the laboratory animals and isolating Rickettsia tsutsu- host and the habits of the chiggers (in specific areas, gamushi as a result. Although the criteria cited by since these may vary from place to place), the some workers (Traub et al., 1954b; Traub et al., ECOLOGICAL CONSIDERATIONS IN SCRUB TYPHUS. 2 227

1967) have not always been met, it is believed that nated on laboratory and epidemiological grounds such infection in these chiggers has been adequately namely, L. arenicola, on sandy beaches in Malaya demonstrated in the following countries, as indicated (Traub, 1960, 1962); L. pavlovskyi (Schluger, 1948) below. in Siberia (Primorye), where transtadial transmission Leptotrombidium deliense. (1) Burma: Mackie et al. to the nymphal strain was also demonstrated (1946); Audy et al. (1947); Audy (1949a). (Kudrjasova & Tarasevic, 1964). L. tosa (Sasa & (2) China: Yu & Lin (1957); Yu & Wu (1959). Kowashina, 1951) is regarded as a vector in Kochi (3) India: Mackie et al. (1946); Krishnan et al. Prefecture, Japan, in the summer (Tamiya, 1962). (1949); Kalra (1959). (4) Malaya: Philip, Traub The evidence is not nearly as strong for chiggers & Smadel (1949); Audy & Harrison (1951). in other genera. Gispen (1950) failed to isolate (5) New Guinea: Kohls et al. (1945); Philip & R. tsutsugamushi from Ascoschoengastia indica from Kohls (1948). (6) Philippines: Philip, Woodward rats in an endemic region in Batavia (Djakarta) & Sullivan (1946). (7) Thailand: Trishnanda despite repeated attempts (although he did recover et al. (1966). the rickettsiae of murine typhus in the process) and On several occasions, offspring of naturally stated " this species should be excluded as a vector infected L. deliense have been shown to harbour of... scrub typhus '. However, Tarasevic (1960) rickettsiae (transovarian transmission) (Mackie et al., cites A. indica as a vector in Sumatra, and Yu & Lin 1946; Krishnan et al., 1949; Yu & Wu, 1959). (1957) reported this species to be present in a pool Strongly presumptive evidence for this species serv- of other chiggers shown to be positive. Kalra (1959) ing as a vector in Pakistan has been obtained isolated R. tsutsugamushi from a pool ofmites believed (Traub et al., 1967; Nur Ahmed & Bumey, 1962), to consist solely of Gahrliepia (Schoengastiella) ligula and it is suspected of transmitting scrub typhus in (Radford, 1946) but the presence of other species Australia (Southcott, 1947; Barrow, Domrow & in the inoculum could not be ruled out. This species Derrick, 1963; Cook et al., 1967), but to date is quite common in a variety of habitats in endemic there have been no isolations of rickettsiae from areas, some of which are mentioned below, and mites effected in that country. merits further investigation. Other species appearing as components in pools of inocula yielding strains Leptotrombidium akamushi. (1) Japan: Miyajima have been cited in the first article in this series & Okumura (1917); Kawamura (1926); Philip (Traub & Wisseman, 1968a). (1947a); Sasa (1954); Tamiya (1962). (2) New It has been pointed out (Traub et al., 1967; Guinea: Blake et al. (1945); Kohls et al. (1945). Traub & Wisseman, 1968a) that scrub typhus infec- (3) Philippines: Philip et al. (1946). tion is present in semi-desert, mountain desert and Transovarian transmission in L. akamushi has alpine habitats in West Pakistan in the absence of been accomplished in Japan by the above means or known vectors. In those studies, only a few strains by exposing laboratory animals in endemic foci, of rickettsiae were isolated from mites, in contrast where they were infested by chiggers (Miyajima & to rodent sources; hence, pending further field- and Okumua, 1917; Kawamura, 1926; Tamiya, 1962). laboratory-studies, we can only rely upon ecological There are good reasons for regarding L. akamushi data of th- type shown in Fig. 1 and 2 in a search as an important vector in Malaya (Philip et al., 1949; for clues to the local vector. Natural scrub typhus Audy & Harrison, 1951). Gispen (1950) felt the infection has been demonstrated in rodents in all same was true in Batavia (now Djakarta), but lacked the localities cited in these figures, save for Dir and experimental data. Quetta, which have not been surveyed adequately. In Japan, L.pallidum and L. scutellare are undoubt- From the charts, it can be seen that L. deliense has edly major vectors; the former in the winter and been found in only one area, the Sialkot plains, early spring in the Miura and Izu peninsulae; which was the site ofseveral outbreaks ofscrub typhus the latter in the fall and winter in the Mt Fuji (Nur Ahmad & Burney, 1962; Traub et al., 1967). area and certain islands (Asanuma, 1962; Kumada, L. subintermedium (Jameson & Toshioka, 1954) is 1959; Tamiya, 1962). L. pallidum has also been one of the most abundant chiggers in the mountains, dezmed a vector in Korea (Jackson et al., 1957) and and is the dominant species infesting mice and voles, is considered one in the Primorye region as well and accordingly rates consideration as a potential (Tarasevic et al., 1964; Kudrjasova et al., 1967). vector. Colonies of this species, and of G. (S.) ligula, Other species of Leptotrombidium have been incrimi- a common chigger in the foothills and plains, have 228 R. TRAUB & C. L. WISSEMAN JR been established and are under study in our labora- cept of the chigger serving as a reservoir of infection tory. L. (L.) rupestre Traub & Nadchatram, 1967, and not just as a vector (Philip & Burgdorfer, 1961) L. (L.) irregulare Traub & Nadchatram, 1967 and is intriguing, but the problem is complicated by the L. (L.) dihumerale Traub & Nadchatram, 1967, also occurrence of strains of low virulence of the type merit investigation. L. (L.) jayewickremei (Womers- reported by Taraseviw (1966a) and the possibility ley, 1952) is the most abundant chigger in the that these or even avirulent strains may become lowland and xeric hills, and may prove to be the activated or reactivated when generations of mites vector there. The genus Microtrombicula Ewing, feed on mammalian hosts. Tarasevic (1966a) won- 1950, constitutes one of the major faunal elements, dered whether isolation of a particularly virulent but very little is known of the habits and bionomics strain from engorged larvae which had been retained of the species, and the same is true regarding more than 30 days in the laboratory, indicated Helenicula Audy, 1954. " reactivation ". Traub & Frick (1950) reported The figures also show that despite the intervening failure to recover rickettsiae from limited samples high mountains, broad rivers and extensive belts of of inocula containing nymphs, adults or unfed semi-desert, certain species like L. subintermedium larvae of L. deliense and L. akamushi from hyper- were found in all montane habitats, lending support endemic areas where strains had been readily isolated to the " ecological islands " and " oasis scrub from engorged larvae. Kawamura (1926) also failed, typhus" hypotheses (Traub & Wisseman, 1968a; using large numbers of adults. However, Audy Traub et al., 1967). (1958, 1961) mentions data suggesting "that eggs and adults may harbour rickettsiae in a virulent DISCUSSION phase", and Yu & Wu (1959) claim isolation of In discussing the rate of infection in chiggers in R. tsutsugamushi from nymphal and adult L. deliense, nature, it is necessary to realize the drawbacks in but details are lacking. A nymphal strain was our techniques. Thus, an estimate based upon presumably isolated from L. pavlovskyi in the Sibe- animal-inoculation may or may not reflect what rian focus (Kudrjasova & Tarasevic, 1964). The happens when that species bites man. In an effort scarcity of such reports indicates the need for further to get more meaningful data, the US Army Medical research on this aspect. Research Unit (Malaya) is currently undertaking an The possibility that chiggers serve as a reservoir extensive programme in which laboratory-reared L. of infection as well as a vector is indicated by deliense or L. akamushi individually feed on white Tamiya (1962), who in referring to the studies of mice in attempts to induce infections. The individual his colleagues and those of Fukuzumi (1953) stated, chiggers are being reared to follow the course of " . . . the negative results in experiments on trans- infection through the various stages and through mission of the rickettsiae to the trombiculid mite subsequent generations and the results are awaited strongly suggest that the rodent does not serve as a with interest. Audy (1958), using preliminary data reservoir of rickettsiae for infecting the trombiculid based on other types of studies, believed that the mite ". Since chiggers do not feed on the host's infection rate in L. deliense chiggers in the field blood, per se, but rather upon serum and the pro- is low, ranging from 0.1 % to 10%, depending upon ducts of lysis of tissues at the site of the bite, there the focus. Traub & Frick (1950) reported isolation may only be a slight possibility of the mite acquiring of rickettsiae from pools containing as few as 5 or rickettsiae while feeding on a host suffering from 10 mites, but these were from localized hyperinfec- rickettsaemia. However, blood extravasated from the tive areas. In general, in most investigations on capillaries is ingested at times and this may account scrub typhus, regardless of locality, the inocula for the demonstration of R. tsutsugamushi in the have usually consisted of pools of 100-5000 chiggers American pest-chigger, Eutrombicula alfreddugesi each, and even where L. deliense was common in (Oudemans, 1910) after feeding upon experimentally known endemic foci, the isolation rate was no higher infected hosts with rickettsaemia (Philip, 1949; than 39% (Traub, 1949). On the basis of work Audy, 1961). carried out in his laboratory, and by Krishnan Little precise information is available on the fate et al. (1949), Audy (1958, 1961) felt that transovarian and behaviour of the rickettsiae within the tissues of transmission to subsequent generations " may not be the various stages of the mite-vectors (Burgdorfer & very efficient " in L. deliense which had fed on normal Varma, 1967; Audy, 1961) and it is not known if mice. Further studies would be desirable. This con- infection can be transmitted to the host via contact ECOLOGICAL CONSIDERATIONS IN SCRUB TYPHUS. 2 229 with the excrement of the chigger (Rehacek, 1965) the use of the fluorescent antibody technique and the as it can in the case of epidemic typhus and the electronmicroscope-including the newly developed louse. Here too there are tremendous opportunities scanning-type. This article is replete with such for sound research, as there are in studies entailing challenges and it is hoped they will be met.

RESUME

Les auteurs rappellent brievement le cycle de vie et se fixant habituellement par grappes sur certaines parties le comportement des trombidides intervenant dans la du corps particulieres chez un animal donne. Chaque transmission du typhus de brousse. L'arthropode n'est espece parasitaire a ainsi ses sites d'election, variables parasite qu'au stade larvaire; la larve ne se nourrit sur suivant F'h6te et peut-etre conditionnes par les habitudes l'h6te qu'une seule fois et ne peut lui communiquer que de ce dernier en mati&e de toilette. I1 n'y a semble-t-il l'infection qui lui a e 1guee par voie transovarienne. aucun rapport entre la taille de l'animal infeste et l'inten- Certains aspects des epidemies de typhus de brousse site de l'implantation du parasite. L. (L.) 'pavloskyi, sont caracteristiques: leur association evidente avec cer- L. (L.) orientale, L. (L.) arenicola et L. (L.) tosa sont tains types de terrain; I'apparition d'un grand nombre consideres comme des vecteurs probables. On suspecte de cas dans des foyers tres limites; la forte proportion egalement le role dans la transmission du typhus de de personnes receptives a l'infection pouvant etre infec- brousse de diverses especes appartenant a d'autres genres. tees simultanement apres une breve exposition; I'absence L. (L.) subintermedium et certaines autres especes d'antecedents de piqure par arthropode. Quelques-unes ont e decouvertes dans des (ilots ecologiques # dans de ces particularites s'expliquent par le comportement toutes les regions montagneuses prospectees au Pakistan des larves qui sont remarquablement resistantes et occidental. Le taux d'infection naturelle des larves par survivent apres des semaines de refrigeration ou d'immer- Rickettsia tsutsugamushi semble peu important et on a sion dans l'eau. emis I'hypothese que les larves pourraient representer Les vecteurs certains appartiennent au genre Lepto- dans certains cas un reservoir de virus et non unique- trombidium (Leptotrombidium): il s'agit en l'occurrence ment des vecteurs. I1 faut enfin mentionner l'existence de L. (L.) akamushi, L. (L.) deliense, L. (L.) pallidum possible de souches d'arthropodes peu infectantes, mais et L. (L.) scutellare. Ils sont capables d'infester une susceptibles d'etre

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