Parafilaria is a genus of filarial parasitic roundworms that is found mainly in the skin of domestic an wild animals worldwide. The most relevant species for domestic animals are:
- Parafilaria bovicola that infects mainly cattle, buffaloes and other bovines. It is found worldwide but is more abundant in Africa, Asia and certain European countries (Russia, Scandinavia, Mediterranean, etc.).
- Parafilaria multipapillosa is a related species that affects horses, donkeys and mules and is particularly abundant in Eastern Europe.
Incidence varies considerably by region and season and is highly dependent on the abundance of the vector flies (see below). Studies in South Africa and Belgium reported up to 50% and 15% of cattle in a herd to be infected, respectively.
These worms do not affect sheep, pigs, poultry, dogs or cats.
The disease caused by Parafilaria worms is called parafilariasis, also called summer bleeding disease, verminous nodules, verminous haemorrhagic dermatitis, etc.
Are cattle or horses infected with Parafilaria worms contagious for humans?
- NO: The reason is that these worms are not human parasites.
Final location of Parafilaria worms
Predilection site of adult Parafilaria worms is the skin.
Anatomy of Parafilaria worms
Adult Parafilaria are slender worms with a whitish color and up to 6 cm 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 spicules for attaching to the female during copulation.
The eggs measure ~30x45 micrometers, have a thin shell and contain a fully developed larva (microfilaria).
Parafilaria worms have indirect life cycles that are not yet completely elucidated. Main intermediate hosts of Parafilaria bovicola are fly species of the genus Musca (e.g. Musca domestica).
These flies become infected with microfilariae when they feed on the wounds that cause the worms in the skin of infected hosts. Microfilariae develop to infective larvae inside the flies in a few weeks. Such flies re-infect their hosts while feeding, often on eye tears or other exudates from skin wounds. These larvae penetrate into the skin up to the dermis and migrate further to other locations of the body surface (neck, shoulders, rump, loins, etc.), where they complete development to adults and cause the appearance of nodules. To lay eggs female worms make a hole in the nodule, which results in so-called summer bleeding.
The prepatent period (i.e. time between infection and first eggs shed) is 7 to 10 months.
Harm caused by Parafilaria worms, symptoms and diagnosis
Parafilaria infections are not highly pathogenic for cattle or horses. Nevertheless, the worms in the skin can cause hemorrhagic dermatitis ("summer bleeding") with subcutaneous edema. Secondary infections with bacteria can also occur. Main economic loss results from trimming or even full carcass rejection and/or degradation of hides after slaughter. In working horses, the sores may interfere with the harness
Diagnosis is based on the characteristic nodules and is confirmed by detection of larvated eggs or microfilariae in exudate samples observed under the microscope. Serodiagnosis (ELISA) is available in some countries.
Prevention and control of Parafilaria infections
A key preventative measure to prevent or at least reduce cattle infection with these worms is to control the vector flies. However, Muscid flies are not easy to control, because such flies can feed on many other hosts or substrates (organic waste, manure, etc.). Treating cattle with insecticides is often insufficient to reduce the population of these flies, which means that there will always be enough flies around to transmit the disease. Strict sanitation of cattle facilities (e.g. manure removal) can contribute to reducig the fly population.
Many classic anthelmintics (benzimidazoles, levamisole, salicylanilides, tetrahydropyrimidines, etc.) are not effective against these worms. Several macrocyclic lactones (e.g. doramectin, eprinomectin, ivermectin, moxidectin) are effective against adult worms, but may not completely control migrating larvae. This means that after treatment new nodules may appear due to surviving larvae. There are also reports on efficacy of nitroxinil against these worms.
So far no vaccine is available against Parafilaria worms. To learn more about vaccines against parasites of livestock and pets click here.
Biological control of Parafilaria worms (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 Parafilaria worms to anthelmintics
There are a no reports on confirmed resistance of Parafilaria worms to anthelmintics.
This means that if an anthelmintic fails to achieve the expected efficacy against Parafilaria worms it is most likely that either the product was unsuited for the control of these worms, or it was used incorrectly.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Parafilaria control.