Stephanofilaria stilesi is a filarial parasitic roundworm that infects mainly cattle and other bovines, occasionally goats.
It is found mainly in Europe, America, Siberia and Australia. Other related species (e.g. Stephanofilaria zaheeri, Stephanofilaria assamensis) are found mainly in Asia. Incidence is seasonal highly depending on the abundance of the vector flies (see below). In endemic regions it can affect most animals in a herd.
These worms do not affect sheep, pigs, dogs, cats, horses or poultry.
The disease caused by Stephanofilaria worms is called stephanofilariasis.
Are cattle infected with Stephanofilaria stilesi contagious for humans?
- NO: The reason is that these worms are not human parasites.
Final location of Stephanofilaria stilesi
Predilection site of adult Stephanofilaria stilesi is the skin.
Anatomy of Stephanofilaria stilesi
Adult Stephanofilaria stilesi are slender worms about 8 mm long, whereby females are longer than males. As in other roundworms, the body of these worms is covered with a cuticle, which is flexible but rather tough. The worms have a tubular digestive system with two openings, the mouth and the anus. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. The female ovaries are large and the uteri end in an opening called the vulva. Males have a copulatory bursa with two unequal spicules for attaching to the female during copulation.
Stephanofilaria worms are viviparous, i.e. they do not lay eggs but already hatched larvae (filariae), which in this case are very small, only 50 micrometers long.
Stephanofilaria worms have indirect life cycles that are not yet completely elucidated. Main intermediate host of Stephanofilaria stilesi in Europe, America and Australia is Haematobia irritans (horn fly, buffalo fly). Elsewhere and other Stephanofilaria species use other fly species that visit livestock (stable flies, face flies, houseflies, etc.) as intermediate hosts.
These flies become infected with microfilariae when they bite or feed on the wounds that cause the worms in the skin of cattle and other final hosts. Microfilariae develop to infective larvae inside the flies in 15 to 20 days. Such flies re-infect their hosts while feeding.
Adult worms are found in the dermis, i.e. immediately below the outer skin layer, mainly in the midventral line, between the navel and the brisket.
The prepatent period (time between infection and first microfilariae shed) is 6 to 8 weeks.
Harm caused by Stephanofilaria stilesi, symptoms and diagnosis
Stephanofilaria infections are not highly pathogenic for cattle. The worms in the skin damage the hair follicles and skin glands and cause dermatitis. Affected areas lose hair (alopecia) are irritated, itchy and covered with exudate during the first years. Such wounds can reach up to 30 cm in diameter and can affect not only the belly, but also udders and teats, scrotum, flanks and even the neck. With time the skin becomes dry and thickened. Severe infections can considerably stress affected animals.
Stephanofilariasis can be particularly annoying for dairy cattle and in endemic regions it may considerably hamper manual or mechanical milking, and make it impossible to comply with hygienic measures for milking. Damaged hides can be downgraded and even rejected at slaughter.
Diagnosis is based on the characteristic wounds and the behavior of affected animals that intensively scratch and rub against objects. It is no always easy to find microfilariae in deep skin scrappings of the wounds.
Prevention and control of Stephanofilaria stilesi infections
The key preventative measure to prevent or at least reduce cattle infection with these worms is to control the vector flies. In Europe, America and Australia this means controlling horn flies, which are the main vectors. For specific information on horn fly control click here. Where the main vectors are non-biting flies (face flies, houseflies etc.) strict sanitation of cattle facilities (e.g. manure removal) are a must to reduce the fly population.
In endemic regions with horn flies as main vectors it can be advisable to keep livestock indoors during the peak in the fly season. Horn flies will not attack cattle indoors.
Little is known about chemical control of these worms with anthelmintics. A few reports suggest that topical trichlorfon and injectable levamisole or ivermectin may control established Stephanofilaria infections. However, in most countries there are no products approved for Stephanofilaria control. This means that it must be used off-label, something veterinary doctors are allowed to prescribe in most countries.
So far no vaccine is available against Stephanofilaria stilesi. To learn more about vaccines against parasites of livestock and pets click here.
Biological control of Stephanofilaria stilesi (i.e. using its natural enemies) is so far not feasible.
You may be interested in an article in this site on medicinal plants against external and internal parasites.
Resistance of Stephanofilaria stilesi to anthelmintics
There are a no reports on confirmed resistance of Stephanofilaria stilesi to anthelmintics.
This means that if an anthelmintic fails to achieve the expected efficacy, chance is very high that it was not due to resistance but to incorrect use, or the product was unsuited for the control of these parasites. Incorrect use is the most frequent cause of failure of antiparasitic drugs.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Stephanofilaria stilesi control.