Chiggers Trombiculid Mites
Common names: Chiggers, red bugs, harvest mites, itch mites, scrub mites, bichos colorados, bêtes rouges, rouget, herbstmilben, akamushi, tsutsugamushi, kedani, (Wharton and Fuller, 1952), heel-bugs, black soil itch mites, lepte automnale, aoutat, and scrub itch mites, duck-shooters itch mites (Prescott, 1984)
HISTORY: Trombiculid mites (chiggers) are pestiferous mites that attack man, his domestic animals and wild animals. Knowledge of chiggers probably extends back to prehistoric times. Even today primitive peoples have descriptive names for these parasites. Reference to chiggers in the Western hemisphere does not occur prior to 1733, however there are references to this mite as early as the sixth century in China. In 1758, Linnaeus described a single species of chigger Acarusbatatas (Trombiculabatatas). Most of the information concerning chiggers and the problems they cause in man was garnered during and following World War II. Wharton and Fuller (1952) noted the cat has served as host to 8 species of chiggers: Trombicula (Eutrombicula) alfreddugesi, Trombicula (Leptotrombidium) akamushi, Trombicula (Leptotrombidium) legaci, Trombicula (Neotrombicula) autumnalis, Trombicula (Neotrombicula) shannoni, Euschöngastiawestraliense, Acomatacarus (Acomatacrus) adelaideae, and Acomatacarus (Acomatacrus) australiensis. Walchiaamericana has also been implicated as a parasite of cats (Lowenstine et al., 1979). Prescott (1984) notes five species that have been described on cats: Bryobiapraetiosa, Leeuwenhoekiaaustraliensis, Leeuwenhoekiaadelaidiae, Schongastiaphillipinensis, and Schongastiawestraliensis.
GEOGRAPHIC DISTRIBUTION: Over 10,000 species of chiggers have been recorded from throughout the world. It is indeed fortunate that not all chiggers produce itch reactions in man and his domestic animals (Harwood and James, 1979). Virtually every continent serves as habitat to chiggers who feed avidly on vertebrate hosts.
LOCATION IN HOST: In animals other than man, cutaneous lesions tend to be restricted to areas that come in contact with the ground, e.g., the head, ears, limbs, interdigital areas and ventrum. Other areas on the body of the host may also be infested (Greene et al., 1985). In cats, the external ear is usually involved; an acute inflammatory disease of the ear canal may also occur (Bullmore et al., 1976).
IDENTIFICATION: The six-legged chigger larva is 200 to 400µ in diameter. The body of the larval chigger is more or less rounded. The genera of veterinary importance possess a single dorsal scutum and four to six setae (hairs). The palpal tibia has three setae and a terminal claw. The palpal tarsus usually possesses four to seven setae. The palpal tarsus articulates and opposes the tibial claw in a thumb-like manner. The dorsal body setae posterior to the scutum are on the venter between and posterior to the coxae. Each coxae bear at least one plumose seta (Harwood and James, 1979).
The nonparasitic adult chiggers are about 1 mm long. They may be oval to figure-eight-shaped. They are covered with tiny, dense hairs giving them a velvet-like appearance. The color of the adult is usually bright red (Harwood and James, 1979).
LIFE CYCLE: The number of generations per year will vary from species to species and with the local climatic conditions. The eggs of the chigger are spherical. After one week, the egg shell splits, revealing the maturing larva or deutovum. The larval chigger is six-legged and is the only stage that is parasitic on vertebrates. Larvae are parasitic on a wide variety of vertebrates, animals, birds, reptiles and man. The most common hosts are small rodents. The larval mite attaches to the host. Its salivary secretions form a tube (stylostome) through which the larvae suck up liquefied host tissues. Once it is replete, the engorged larva drops off the host and passes into a quiescent prenymphal stage. The nymphal and adult chiggers are eight-legged are non-parasitic. The nymph passes into a quiescent pre-adult stage before molting to the adult. The normal food of the adult chigger is insect eggs and inactive soil invertebrates. Female chiggers are inseminated by males. There is no evidence to support parthenogenesis. One adult female mite often produces many ova. As a result, the emerging larvae are amassed together in one site in their woodland habitat. Any unwitting mammalian (feline, human or other) host that enters this site can be parasitized by many mites at a single instance (Harwood and James, 1979).
CLINICAL PRESENTATION AND PATHOGENESIS: Chigger larvae do not burrow into the skin as commonly believed (Fig. 5-25). Neither do they feed primarily on blood. Their food consists of the serous components of tissue. A few red blood cells may be ingested during feeding. Chiggers attach firmly to the definitive host and inject a digestive fluid that produces cellular autolysis. The cytoplasm becomes disorganized and the nuclei fragmented. The host's skin becomes hardened and a tube called a stylostome forms at the chigger's attachment site. When the mite is satiated, it loosens and falls to the ground. The action of the digestive fluid causes the attachment site to itch after a few hours (Harwood and James, 1979). The digestive secretions may produce an intense pruritus which may result in self excoriation (Prescott, 1984).
Bullmore et al. (1976) described a case of chigger infestation in a cat which presented with dry, crusty lesions on the anterior margins of both ears. Chiggers (Trombiculaalfreddugesi) appeared as multiple orange-red dots within the crusty areas. Lowenstine et al. (1979) presented a similar case of chiggers in a cat that presented with anorexia and reluctance to move. The cat was alert, affectionate and well nourished but demonstrated a diffuse nodular thickening of the skin. When the cat walked, it would periodically lift one of its hind limbs and shake it as if it had stepped in something. The skin lesions could be readily palpated but could not be observed until the fur was parted or clipped. The skin of the ventrum and the medial aspect of all limbs and the interdigital spaces was nonpliable and thickened. The skin surface was cracked and scaly with moist serous exudate or dried yellow debris. Papules 0.1 to 0.3 cm in diameter were scattered over the sides of the trunk, behind the ears and around the vulva and anus. Some of the papules were crusted with yellow exudate; all were accompanied by a mild wheal and flare reaction. Indication of self-excoriation was minor. Greene et al. (1985) described trombiculiasis in a eight-year-old domestic shorthair cat with a history of a crusting dermatosis. This cat exhibited multifocal orange crusts on the head and on the base of the external canal of the left ear. Prescott (1984) sites Wilson-Hanson who reported a case of trombiculosis in a male cat with primary lesions on the ears, auricular and frontal regions, and in the region of the genitalia. Irritation from the chiggers or self trauma (or both) produce marked edema and urinary retention. A lesion later appeared at the commissure of the lips.
DIAGNOSIS: The diagnosis of trombiculosis is based on the presence of an orange crusting dermatosis, a history of exposure (roaming the outdoors), identification of the typical six-legged larva on skin scrapings, and histopathologic findings.
In a case described by Lowenstine et al., microscopic lesions were limited to the epidermis and dermis. This case of trombiculosis is unusual in that the chiggers burrowed deep within the epidermis. The authors were unable to determine if the mites actually burrowed this deeply or whether the skin reactions and proliferation of surface tissue buried them. Epidermal lesions were multifocal, both proliferative and degenerative, and associated with mites located either in tunnels or in regions of inflammation. Some aggregates of hair follicles had undergone acanthosis, loss of hair shafts, near or complete obliteration of follicular lumens by keratinizing epithelium, and fusion of follicles, resulting in epithelial islands that resembled squamous cell carcinoma. Dermal changes were typical of acute allergic dermatitis and were characterized by dilated capillaries, fibrocytes with plump nuclei, and eosinophils and mast cells, which were numerous adjacent to and within edematous regions.
TREATMENT: Chiggers may be physically removed, or removed by application of a 1% solution of malathion, or any other acaricide recommended for use in cats. Older remedies include potassium sulphurata and lime sulphur. Rotenone dusting powder has been use to treat for chiggers in cats. Carbaryl powder (5%) has been demonstrated to be effective (Prescott, 1984).
EPIZOOTIOLOGY: The dermatitis produced by these parasites is usually seen from spring (Prescott, 1984) to the late summer and fall (Bullmore et al., 1976). The number of generations per year will vary with the species and local climate (Harwood and James, 1979).
As mentioned previously, a single adult female mite is capable of producing many ova. As a result, the emerging larvae are amassed together in one site in their woodland habitat. The natural hosts of the larval chiggers are in most cases small rodents. However, any unwitting mammalian (feline, human or other) host that enters this habitat may be parasitized by many mites at a single instance.
The larval chiggers typically congregate in a shaded area near the top of a blade of grass or a fallen leaf. Here they remain quiescent until they are stimulated by an air current containing carbon dioxide from an approaching vertebrate host. Some species may remain on the ground (Harwood and James, 1979).
HAZARDS TO OTHER ANIMALS: In the wild, chiggers normally feed on mammalian hosts including monotremes, marsupials, insectivores, chiropteres, primates, edentates, lagomorphs, rodents, carnivores, artiodacytls, birds, reptiles, and amphibians and even non mammalian hosts such as house flies, scorpions, and millipedes (Wharton and Fuller, 1952)! Larval mites have caused death in chickens from anemia (Prescott, 1984). The chiggers that are causing lesions on the cat will typically not then move onto the pet owner; it is more likely that both the owner and the pet will be infested at the same time.
HAZARDS TO HUMANS: Human infestations are most commonly acquired by frequenting woodland surroundings (Greene et al., 1985). On man, chigger larvae attach themselves on the part of the body constricted by clothing. With the exception of the intense pruritus produced, the public health significance of trombiculiasis is minor. The infested pet (cat or dog) rarely acts as a mechanical vector (Greene et al., 1985). Mites in the family Trombiculidae, genus Leptotrombidium, are known to be vectors of Rickettsiatsutsugamushi, the etiologic agent of scrub typhus, however this rickettsia is not a zoonotic agent (Harwood and James, 1979).
CONTROL/PREVENTION: Attack by chiggers can be almost completely prevented by using repellents. Humans often apply repellents to either skin and clothing or wear clothing impregnated with repellents (deet, dimethyl phthalate, dimethyl carbate, and ethyl hexanediol) (Harwood and James, 1979). Application of repellents to cats exposed to chigger infested areas may prove to be impractical. Cats should not be allowed to roam freely (Hendrix and Blagburn, 1983.
A more practical solution to the problem might be the use of sprays or dusts of approved acaricides applied to vegetation around premises. Controlling habitat and natural hosts of chigger mites can also reduce their numbers (Harwood and James, 1979).
Bullmore CC, Weiss ME, Phillips JT, Gebhart RN. 1976. Feline trombiculiasis. Feline Prac 6:36.
Greene RT, Scheidt VJ, and Moncol DJ. 1985. Trombiculiasis in a cat. J Am Vet Med Assoc 188:1054-1055.
Harwood RF and James MT. 1979. Helminths, Arthropods, and Protozoa of Domesticated Animals. 7th edition. Macmillan. New York. Pp. 352-357, 366-370.
Hendrix CM, Blagburn BL. 1983. Common gastrointestinal parasites. Vet Cl N Am 13:627-646.
Lowenstine LJ, Carpenter JL, Oconnor BM. 1979. Trombiculosis in a cat. J Am Vet Med Assoc 175:289-292.
Prescott CW. 1984. Parasitic Diseases of the Cat in Australia. Post-graduate Foundation in Veterinary Science. Sydney. Pp. 68-69.
Wharton GW, Fuller HS. 1952. A Manual of the Chiggers. The Entomological Society of Washington. Washington. Pp. 1-185.
Figure 5-25. Histologic section of a chigger feeding on the ear of a cat.