Verminous Hemorrhagic Dermatitis
Also known as
Parafilariasis, summer bleeding disease, verminous nodules, summer wound
Verminous hemorrhagic dermatitis is a filariasis (tropical disease caused by thread-like parasitic round worms) of cattle. The worms create a bleeding nodule in the skin of affected animals.
The parasitic worms are spread by fly vectors of the genus Musca (which includes houseflies). The flies feed on secretions and discharges from the animal’s body, and ingest microfilariae (the early stages of young worms) when feeding on the skin lesions.
A similar disease, sometimes called Cascado, occurs in Asia (Indonesia, Malaysia) and is caused by Stephanofilaria species.
Distribution of this disease is dependent upon temperature and rainfall patterns, and subsequent multiplication of the flies.
In Germany, the verminous nodules start appearing in May and June, 2 to 4 weeks after cattle go to pasture. The nodules may heal spontaneously in October and November.
In Morocco, the disease appears in years when heavy rainfalls occur in March and April. Numerous cases are then observed in April and May, in regions surrounding the ponds. Some cases heal spontaneously, but others progress to become wider subcutaneous lesions which require veterinary treatment. Severity and aggressive nature of these skin lesions suggests that the causative agent may be slightly different from the one in Western Europe.
Parafilaria infections are not highly pathogenic for cattle or horses but the worms living in the skin can cause “summer bleeding” with subcutaneous edema (swelling under the skin). Secondary infections with bacteria can become a problem in some cases. The biggest economic losses result from having to trim off some of the meat--or even full carcass rejection--at slaughter or damage to the hides.
Diagnosis is based on the characteristic nodules, confirmed by detection of eggs or microfilariae in samples from the seeping nodules, observed under the microscope, or by samples sent to a lab for an ELISA test.
- Nodules in the skin that often bleed
Parafilaria is a genus of filarial parasitic roundworms found in the skin of domestic and wild animals worldwide. Parafilaria bovicola infects mainly cattle, buffaloes and other bovines. It is found worldwide but most abundant in Africa, Asia and certain European countries including Russia, Scandinavia, and the Mediterranean region. Parafilaria multipapillosa is a related species that affects horses, donkeys and mules and is common in Eastern Europe. These worms do not affect sheep, pigs, poultry, dogs or cats.
Incidence varies considerably by region and season and is highly dependent on numbers of vector flies. Studies in South Africa and Belgium reported up to 50% and 15% of the cattle in a herd to be infected, respectively.
Adult Parafilaria are slender whitish-colored worms that grow up to 6 centimeters (2.36 inches) long, with female worms longer than males. The body of these worms is covered with a tough, flexible cuticle. These worms have a tubular digestive system with 2 openings--mouth and anus. They have a nervous system but no circulatory system. The tiny eggs have a thin shell and contain a fully developed larva (microfilaria).
The main intermediate hosts of Parafilaria bovicola are fly species of the genus Musca (such as Musca domestica which is the common housefly).
These flies become infected with microfilariae when feeding on wounds caused by the worms in the skin of infected hosts. Microfilariae develop inside the flies for a few weeks and become larvae. The flies re-infect their hosts while feeding on eye tears or exudate from skin wounds.
As the fly feeds, the larvae inside the fly penetrates into the animal’s skin and migrate to other locations of the body surface such as the neck, shoulders, rump, loins, etc., where they complete their development and become adults. The adult worms create the skin nodules. To lay eggs, the female worms make a hole in the nodule, and this results in “summer bleeding”.
The prepatent period (the time between infection and first eggs shed) is 7 to 10 months.
No vaccine is available against Parafilaria worms.
Parafilaria infections can be prevented or reduced by controlling vector flies, but Muscid flies are not easy to control—since they feed on many other hosts and organic waste, manure, etc. Treating cattle with insecticides is often insufficient to reduce the population of these flies, which means there will always be enough flies to transmit disease. Manure removal in cattle facilities can help reduce the fly population.
Many of the traditional anthelmintics (benzimidazoles, levamisole, salicylanilides, tetrahydropyrimidines, etc.) are not effective against these worms, since these worms are in the skin and not the digestive tract of the host.
Several macrocyclic lactones (such as doramectin, eprinomectin, ivermectin, moxidectin) are effective against adult worms that are consuming blood, but may not completely control migrating larvae in the skin.
This means that after the treatment new nodules may appear due to surviving larvae. Nitroxinil (a different type of treatment for internal parasites) has been reported to be effective against these worms.
Ivermectin is generally effective for individual treatment, but surgical excision of the nodules, although very bloody, is more efficient to get rid of the adult worms.
There are a no reports on resistance of Parafilaria worms to the proper types of anthelmintics, so if a deworming drug fails to achieve expected efficacy, it is likely that either the product was unsuited for the control of these worms, or was used incorrectly.