Infection of horses and cattle by Parafilaria nematodes occurs outside of North America and is characterized by bleeding caused by subcutaneous and intramuscular parasites. Examination of blood can show parasite eggs and microfilariae. Treatment and prevention consists of anthelmintics and fly control.
Parafilaria bovicola Infection in Cattle
Parafilaria bovicola, a filarial parasite of cattle, causes subcutaneous lesions that resemble bruising. It has also been reported in water buffalo (Bubalus bubalis). The worm is whitish; adult females are 50–65 mm long, and males are 30–35 mm.
P bovicola is found in Asia (the Philippines, Japan, Russia, Pakistan, India), Europe (Bulgaria, Greece, Romania, France, Italy, Germany, Austria, Sweden), and Africa (Morocco, Tunisia, Rwanda, Burundi, South Africa, Namibia, Botswana, Zimbabwe). A specimen was recovered in Canada from a bull imported from France; however, P bovicola does not appear to have established itself on the American continents and has not been reported in Australia.
Parafilaria infection has been identified as a source of considerable economic loss to the beef industries of South Africa and Sweden, despite their climatic differences. In southern Africa, it is found primarily in range cattle in savanna areas. In Sweden, it has emerged as a problem in cattle following spring turnout to pasture after winter housing.
Clinical Findings
The only external signs of Parafilaria infection in cattle are focal cutaneous hemorrhages (“bleeding spots”) that can ooze for hours before clotting and drying in matted hair. Bleeding spots are induced by the female worm, which causes the formation of a small nodule, perforates the skin, and oviposits in 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, Parafilaria-induced bleeding spots are markedly seasonal, occurring most commonly in spring and early summer. Most bleeding spots appear along an infected animal's dorsum, particularly in the forequarters.
The invertebrate hosts of Parafilaria are face flies of the genus Musca (subgen Eumusca), which ingest the eggs when feeding at bleeding spots. Various Musca species have been identified as hosts in different geographical regions: M autumnalis in Sweden, M lusoria and M xanthomelas in South Africa, and M vitripennis in Asia. Parafilaria develop over 10–12 days into infective third-stage larvae within these flies. Transmission to cattle probably occurs when the flies feed on wounds, Parafilaria bleeding spots, or ocular secretions.
Because of seasonal bleeding and cutaneous nodules, severe P bovicola infections can impair the productivity of working bullocks (oxen or bulls); however, the main impact of Parafilaria in beef-producing countries is damage to subcutaneous tissues. Carcasses of infected animals display irregular, edematous, greenish-yellow lesions that resemble bruising. These are usually superficial; occasionally, however, underlying muscles are extensively involved. Lesions are the most severe during spring and summer.
Trimmed carcasses are often visibly damaged and consequently downgraded. In severe cases, the carcass can be condemned. Lesions are more common and severe in bulls than in steers, which in turn are less severely affected than female animals.
Diagnosis
Examination of blood
Histological examination or ELISA
Seasonal bleeding spots caused by Parafilaria are sometimes confused with wounds 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. Characteristic Parafilaria eggs are found on microscopic examination of the sediment.
Carcass lesions due to Parafilaria 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 they are often difficult to find because of their color and the accompanying inflammatory reaction. Affected tissues can be incubated in warm saline solution to facilitate recovery of parasites. An ELISA for the detection of antibodies against P bovicola is available.
Treatment and Prevention
Ivermectin
Fly control
Ivermectin (200 mcg/kg, SC, once) or, in countries where it is available, nitroxynil (20 mg/kg, SC, once) decreases 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 can induce fresh lesions as they mature.
In trials in Sweden, the use of pyrethroid-impregnated ear tags controlled flies well and decreased parafilarial lesions at slaughter by 75% (1). Ear tagging of all cattle in a given area resulted in total control of Parafilaria. The use of residually active, synthetic pyrethroid dips has also effectively decreased transmission of the parasite.
It might be possible to screen imported animals with an ELISA to prevent spread of Parafilaria infection to unaffected countries or, in conjunction with the administration of residual insecticides and effective anthelmintics, to eradicate new foci of infection.
Parafilaria multipapillosa Infection in Horses
P multipapillosa is found in subcutaneous tissues of horses in various parts of the world; it is especially common in the Russian steppes and eastern Europe. It is similar in size, appearance, life cycle, and development to P bovicola. Bloodsucking Haematobia spp (horn flies) are thought to be the invertebrate hosts.
In spring and summer, P multipapillosa causes skin nodules on horses, particularly on the head and upper forequarters. These nodules bleed transiently but often profusely (“summer bleeding”) and then resolve; other hemorrhaging nodules develop as the parasite moves to a different site on its host. Occasionally, nodules suppurate. The nodules and bleeding are unsightly and interfere with harnesses of working horses; generally, however, they are of little consequence. These clinical signs are pathognomonic.
No satisfactory treatment of P multipapillosa infection has been reported; however, fly control can decrease the incidence.
Key Points
Parafilaria-associated dermatitis and bleeding is due to a skin-associated nematode.
The disease occurs in summer or autumn and consists of skin lesions that ooze blood.
Treatment with anthelmintics and fly control are the primary methods of decreasing disease.
For More Information
Nielsen MK, Reinemeyer CR. Handbook of Equine Parasite Control. 2nd ed. John Wiley & Sons; 2018.
Hedjazi M, Mirzayans A. Equine parafilariosis in Teheran area (Iran). Clinical aspects and treatment. Rev Med Vet (Toulouse). 1978;129:1685-1695.
References
Hamel D, Visser M, Mayr S, Tauchmann O, Silaghi C, Rehbein S. Bovine parafilariosis—new autochthonous cases from Germany and summary of recent reports from Europe. Vet Parasitol Reg Stud Reports. 2022;28:100678. doi:10.1016/j.vprsr.2021.100678
