Veterinary Importance

Louse flies directly affect their hosts by feeding on blood. Sometimes heavily infested animals become emaciated and susceptible to secondary infections. Juvenile birds and mammals are often more heavily infested with hip-poboscids than are older animals of the same species. The body conditions of wintering hosts—birds or mammals—may be worsened by infestation of these parasites. In addition to the discomfort their biting causes, louse flies can be annoying to their hosts simply by crawling about on the body. Louse flies also serve as vectors of pathogens and parasites (Table I) and as disseminators of certain ectoparasitic arthropods. These include mammalian trypanosomes and filarial worms, avian try-panosomes, haemosporinan blood protozoans, lice, and mites.

Baker (1967) published a review of the role played by the Hippoboscidae as vectors of endoparasites. All known hippoboscid vectors of parasitic protozoans are members of two subfamilies: the Ornithomyinae on birds and the Lipopteninae on mammals. Dipetalonema dracuncu-loides, a parasitic filarial nematode of dogs and hyenas in the Old World, undergoes cyclical development in the dog fly (H. longipennis), which is thought to be its vector. Hippoboscids may transmit filariae of other mammals, particularly those of camels and lemurs, as well as ostriches and other birds. According to Pfadt and Roberts

(1978), the role of hippoboscids as vectors of pathogens is probably much greater than presently known. This may be true of streblids and nycteribiids as well.

The sheep ked transmits Trypanosoma melophagium, a nonpathogenic flagellate protozoan of sheep present wherever ked-infested sheep are found. Although it is distributed worldwide, this flagellate protozoan is rarely observed because it is present in relatively small numbers. The trypanosomes are ingested by the keds while they feed on sheep blood. The immature forms of the parasite develop in the posterior midgut of the sheep ked, while infective forms develop in the hindgut and are voided with the feces. They normally do not cross the gut wall into the hemolymph. Flagellates gain entry into the sheep when the keds or their feces are ingested. L. capreoli, an ectoparasite of domestic goats and the chamois goat in the Old World, transmits T. theodori in a similar manner, as does Ornithomya avicularia, which transmits T. avium, found in corvid birds. P. canariensis and Stilbometopa impressa are possible vectors of other avian trypanosomes of pigeons (Columbidae) and quail (Phasianidae).

Several hippoboscid flies have been identified as vectors of Haemoproteus species, haemosporidian blood parasites that cause bird malarias. The importance of hippoboscids in the natural transmission of most species of Haemoproteus is unknown. It is generally assumed that individual species of Haemoproteus are transmitted by either hippoboscid flies or Culicoides species (Ceratopogonidae), but not both.

Development of Haemoproteus in a hippoboscid vector is similar to that of the mosquito-borne malarial parasites in the genus Plasmodium. After microgamete production and fertilization of the macrogamete in the hippoboscid midgut, the zygote develops into a motile ookinete which penetrates and encysts on the outside of the wall of the stomach. The oocyst enlarges and its contents differentiate into sporozoites. The enlarged oocyst bursts to release the sporozoites, some of which enter the salivary glands to be introduced into the next host on which the hippoboscid feeds (Baker, 1967).

The best known Haemoproteus species is Haemoproteus columbae, which is parasitic in erythrocytes and visceral endothelial cells of the domestic pigeon (C. livia). It is transmitted by the pigeon fly, P. canariensis. Infections can result in anemia and unthriftiness in pigeons and cause economic losses to pigeon breeders in the form of nestling mortality. Several other species of Haemoproteus are transmitted by hippoboscid flies to a variety of avian hosts (Table I). Proven and presumed vectors of avian haemoproteids include species of Pseudolynchia, Stilbometopa, Icosta, Ornithomya, and other hippoboscid genera (Baker, 1967; Pfadt and Roberts, 1978).

Although they are neither mechanical nor biological vectors of any important diseases of sheep, sheep keds have

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