Setaria species

Nematodes of the genus Setaria infect cattle, sheep, sometimes other domestic animal hosts, and many free-ranging ungulates around the world, including in Canada.  The adult parasites usually live in the peritoneal cavity, where the females produce first-stage larvae (microfilariae) that enter the blood stream.  The life cycle is indirect involving a blood feeding arthropod intermediate host in which the third-stage infective larvae develop.  Infection of the mammalian host is by inoculation of these larvae.  Setaria are usually considered to be of minimal significance in domestic animal hosts, unless they invade unusual sites.  In reindeer in Finland, however, Setaria have recently been associated with a severe peritonitis and other pathology.  Setaria is not known to be zoonotic.

Phylum: Nematoda
Class: Rhabditea
Subclass: Rhabditia
Order: Oxyurida
Suborder: Spirurina
Superfamily: Filaroidea
Faimly: Filariidae

The closest relatives of Setaria species of importance in veterinary medicine are Stephanofilaria stilesi, adults of which live in the skin of cattle, and Dipetalonema, the various species of which are found in a variety of locations in several hosts, including the subcutaneous tissues of dogs. Neither the taxonomy of the genus Setaria, nor the identities of the species in Canada, has been clearly defined.

Adult Setaria species recovered from domestic animals and wildlife in Canada are up to approximately 10 cm in length and clearly visible to the naked eye. They are usually identified on the basis of their location in the host.

Microfilariae of Setaria species are approximately 200 µm to 250 µm in length.

The various species of Setaria occur in cattle and horses, and in several free-ranging hosts, in many regions of the world. In Canada, the parasite is seen most commonly in cattle and horses.

Adult Setaria species live in the peritoneal cavity. Microfilariae produced by the females enter the bloodstream. The life cycle continues when these microfilariae are ingested by a mosquito (several species) or a horn fly (Haematobia irritans) intermediate host during blood feeding. Infective third-stage larvae develop in the intermediate hosts and infect the mammalian host during a subsequent feeding.

 

Setaria is spread among its mammalian hosts during summer, when the arthropod vectors are available. Infection is perhaps surprisingly common in domestic animals in Canada.

Adult Setaria in Canada are usually considered to be non-pathogenic. In northern Europe, however, they have been reported as the cause of severe and clinically significant peritonitis in reindeer, and larvae migrating in the central nervous system of several host species have been associated with a variety of neurological signs.

Diagnosis is usually made at post mortem, although the adult parasites are often seen during abdominal surgery. Detection of microfilariae in blood can also be attempted, but the sensitivity of this approach is not known.

There are no products approved in Canada for Setaria species and, because of the parasites low pathogenicity, treatment and control are not usually necessary in this country.

Setaria species are not known to be zoonotic.