Parafilaria Infection

The only external signs of infection in cattle are focal cutaneous hemorrhages (“bleeding spots”) that may ooze for some hours before clotting and drying in the matted hair of the coat. Bleeding spots are induced by the female worm, which causes the formation of a small nodule, perforates the skin, and oviposits in the blood dripping from the central wound. The tiny eggs contain the first larval stage (microfilariae) of the parasite. In both the northern and southern hemispheres, bleeding spots are markedly seasonal, being most common in spring and early summer. Most bleeding spots occur along the dorsum of the animal, particularly in the forequarters.

The invertebrate hosts are face flies of the genus Musca (subgenus Eumusca), which ingest the eggs when feeding at the bleeding spots. M autumnalis has been identified as a host in Sweden, M lusoria and M xanthomelas in South Africa, and M vitripennis in Asia. Development to infective third-stage larvae in the fly takes 10–12 days. Transmission to cattle probably occurs when the flies feed on wounds, Parafilaria bleeding spots, or ocular secretions.

Because of seasonal bleeding and the cutaneous nodules, severe infections of P bovicola have been reported to impair the productivity of working bullocks in India; however, the major importance of Parafilaria in beef-producing countries is damage to the subcutaneous tissues. Carcasses of infected animals display irregular, edematous, greenish yellow lesions that resemble bruising. These are usually superficial but occasionally underlying muscles are extensively involved. Lesions are most severe during the spring and summer.

Trimmed carcasses are often seriously disfigured and consequently downgraded. In severe cases, the carcass may be condemned. Lesions are more common and severe in bulls than in steers, which in turn are less severely affected than female animals.

The seasonal bleeding spots are sometimes confused with those caused by thorns, wire, ticks, or biting insects. For differentiation, either fresh or dried blood should be mixed with water in a test tube and centrifuged. The characteristic eggs are found on microscopic examination of the sediment.

Carcass lesions can be differentiated from bruising by the presence of numerous eosinophils in Giemsa-stained impression smears made from the lesions. In addition, affected tissue has a characteristic, disagreeable, metallic smell.

Usually, only small numbers of worms are present in affected carcasses and are often difficult to find because of their color and the accompanying inflammatory reaction. Affected tissues can be incubated in warm saline to facilitate the recovery of parasites. An ELISA for the detection of antibodies against P bovicola is available.

Ivermectin (200 mcg/kg) or nitroxynil (20 mg/kg) given SC reduces the number and surface area of Parafilaria lesions. Animals should be treated at least 70–90 days before slaughter to provide sufficient time for lesions to resolve. The treatment-to-slaughter interval should not be >120 days, because unaffected larval forms of the parasite may induce fresh lesions as they mature.

In trials in Sweden, use of pyrethroid-impregnated ear tags gave good control of flies and reduced parafilarial lesions at slaughter by 75%. Ear tagging all cattle in an area resulted in total control of the parasite. The use of residually active, synthetic pyrethroid dips has also effectively reduced transmission.

It may be possible to screen imported animals with an ELISA to prevent spread of the disease to unaffected countries or, in conjunction with residual insecticides and effective anthelmintics, to eradicate new foci of infection.