Corvidae Bparasite Maggot Larvae

H. nettionis

H. velems Hspatocystis H. brayi H. kochi

Leucocytozoon caulleryi

Crows, jays (Corvidae)

Parakeets (Psittacidae)

Finches, sparrows

(Fringillidae) Grouse (Tetraonidae)

Turkey (Meleagrididae)

Ducks, geese (Anatidae), other waterfowl Woodpeckers (Picidae)

Squirrels (Sciuridae) Monkeys (Cenopithecus)

Chickens

North America Thailand North America North America North America

Canada

North America

Malaysia Kenya

Southeast Asia, Japan

C. crepuscularis, C. sphctffnumettsis

C. stilobezzioides C. Hubeculosvts (experimental)

C. crepuscularis, C. stilobezzioides

C. sphagnumensis

C. edeni, C. arboricoilt,

C. haematopotus. C. hinmani, C. knowltoni C. dornest

C. sphagnttmensis

Culicoides spp. C. adersi

C. arakawae, C. circumscripta, C. guttifer, C. schnitzel in pheasants and chukars but apparently does not infect chickens, guinea fowl, bobwhite quail, and other gallinaceous birds.

This parasite is generally regarded as nonpathogenic. Even in cases in which large numbers of circulating red blood cells are infected with gametocytes, birds usually exhibit few signs of stress or other pathologic effects. This suggests that compensatory mechanisms are operative in which the replacement rate of erythrocytes is sufficient to maintain a stable hematocrit despite high parasitemia. In other cases, however, there is evidence to indicate that H. meleagridis does harm its avian hosts, especially domestic turkeys, Heavy infections can result in anemia, reduced weight gain and growth rates, inflammation of skeletal and cardiac muscles, lameness, damage to the spleen and liver, and a wasting condition associated with chronic infections. Young birds are particularly vulnerable.

Five Culicoides species have been identified as vectors of H. meleagridis based primarily on studies in Florida. C. edeni is regarded as the most important vector, with C. hinmani, C. arboricola, C. haematopotus, and C. knowltoni playing secondary roles in transmission. Other species, such as C. baueri, C. nanus, and C. paraensis, have been shown to support development of the parasite only to the oocyst stage. Transmission of H. meleagridis occurs throughout the year in southern Florida, whereas it is limited to the warmer months of the year throughout the rest of the United States where turkeys occur.

Leucocytozoon caulleryi

This is the only Leucocytozoon species known to be transmitted by biting midges. It has been recognized for many years as causing a serious poultry disease of chickens in Japan and Southeast Asia, where it is known as poultry leucocytozoonosis and by the earlier name Bangkok hemorrhagic disease where it occurred in Thailand. The principal vector of L. caulleryi is C. arakawae, which commonly breeds in rice paddies.

Equine Onchocerciasis

Equine onchocerciasis is caused by the filarial nematode Onchocerca cervicalis (Fig. 10.18), the most widely distributed nematode transmitted to domestic animals by biting midges. Horses are the only known host. Although it occurs worldwide, most of the problems associated with this nematode have been reported in the United States and Australia, where it commonly causes dermatitis. Various names which refer to infestations by 0. cervicalis include cutaneous equine onchocerciasis, equine ventral midline dermatitis, equine nuchal disease, and fistulous withers.

Onchocerciasis Horses
FIGURE 10.18 Onchocerca, cervicalis, histological preparation showing microfilariae in skin of infested horse. (From Monies and Vaughan, 1983.)

Prevalence of O. cervicalis is high in many regions of the United States, with up to 85% or more of older horses having been reported infected with this parasite in New York, Kentucky, and the Gulf Coast states.

Adult worms occur primarily in the nuchal ligament of the neck and between the shoulder blades, or withers. Microfilariae produced by the females move to the skin, where they are active in the dermal tissues, often eliciting a host response in the form of localized inflammation and pruritus. The highest concentrations of microfilariae tend to be along the ventral midline of the horse. High numbers of microfilariae also may occur in skin of the inner thighs, chest region, withers, and eyelids. The density of microfilariae in skin tissue varies seasonally, being highest during the spring and summer months and lowest in the winter, when they move to the deeper dermal layers. This is correlated with the seasonal activity of most biting midges.

Horses which become sensitized to O. cervicalis develop various types of skin lesions, including depigmentation, pruritus, scaling, and hair loss. This usually occurs on the face, chest, withers, and ventral midline where microfilariae are most abundant. Ocular lesions have also been reported. Diagnosis is based on clinical signs and the detection of microfilariae in skin biopsies. Treatment with ivermectin has been found to be effective in killing microfilariae, but not the adults. In most cases the skin lesions show significant improvement, or are completely resolved, within a few weeks following treatment.

Members of the Culicoides variipennis complex are the only known vector of O. cervicalis in North America. Based primarily on laboratory studies, C. victoriae, For-cipomyia townsvillensis, and the black fly Austrosimulium pestilens also have been identified as potential vectors of O. cervicalis in Australia. Since these biting midges tend to ingest very few microfilariae while feeding on an infected animal, only one or two infective third-stage larvae are typically found in field-collected flies.

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