Onchocerca is a genus of parasitic, thread-like roundworms that belong to the group of Filarioidea, also called filariae. This genus is found mainly in tropical and subtropical regions of the world, including parts of Europe and the USA. Some species with veterinary importance are:
- Onchocerca dukei, Onchocerca ochengi and Onchocerca armillata in cattle in Africa.
- Onchocerca gutturosa in cattle in Australia, North Africa and Europe.
- Onchocerca gibsoni in cattle in Australia, Asia, Southern Africa and North America.
- Onchocerca lupi in dogs, worldwide
- Onchocerca cervicalis in horses worldwide
- Onchocerca reticulata in horses worldwide
- Onchocerca raillieti in horses in Africa
Incidence varies strongly regionally and seasonally, and depends on the abundance and activity of the intermediate hosts. A study in Portugal and Greece showed that 8.4% of the sampled dogs were infected with Onchocerca lupi, although most of them were asymptomatic.
Onchocerca lupi affects occasionally dogs in North America and Europe, mainly in rural regions, and very occasionally humans as well.
Onchocerca volvulus is a human parasite of this genus, the causative agent of river blindness in numerous tropical countries.
The disease caused by Onchocerca worms is called onchocerciasis.
Are cattle, dogs or horses infected with Onchocerca worms contagious for humans?
- Usually No. The main reason is that these worms do not lay eggs or larvae that contaminate their environment or output (droppings, urine, exudates, etc.) and the infective larvae need to spend some time inside an intermediate host (see life cycle below) to become infective for other mammals. Additionally most Onchocerca species that are parasitic for livestock or dogs are not human parasites. BUT, a few cases of human infections with Onchocerca lupi have been reported in the last years.
Final location of Onchocerca worms
Predilection sites of adult Onchocerca depend on each species:
- Onchocerca dukei subcutaneous, muscular and perimuscular tissues of cattle.
- Onchocerca ochengi causes nodules in the flanks, udders and scrotum of cattle.
- Onchocerca armillata in the tunica media (middle layer) of the aorta of cattle.
- Onchocerca gutturosa in the nuchal ligament and in the connective tissue around the spleen and the rumen of cattle.
- Onchocerca gibsoni causes subcutaneous nodules in the chest and the hind legs of cattle.
- Onchocerca lupi in the sclerotic coat of the eyes of dogs.
- Onchocerca cervicalis in the ligamentum nuchae of horses.
- Onchocerca reticulata in the connective tissue of the flexor and suspensory ligaments of the fetlock, mainly in the forelimbs of horses.
- Onchocerca raillieti in the cervical ligament (adults), cysts in the penis and in the connective tissue around the muscles.
However, there can be regional differences and the predilection sites depend also on the behavior of the intermediate hosts (see life cycle below). Microfilariae can be found transiently in the blood.
Anatomy of Onchocerca worms
Adult Onchocerca worms can be up to 50 cm long, depending on the species, but rather thin. Males are always significantly smaller than females. As other roundworms, the body of Onchocerca worms is covered with a cuticle, which is flexible but rather tough. In this genus the cuticle is transversally striated forming structures like rings. 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 unequal spicules for attaching to the female during copulation.
Adult females are viviparous, i.e. they do not lay eggs but already hatched larvae (microfilariae) that are not longer than 0.5 mm.
The time between infection and release of infective larvae (prepatent period) is quite long. Depending on the species it may take up to one year.
Onchocerca worms have an indirect life cycle with cattle, dogs and other mammals as final hosts and several bloodsucking insects as intermediate hosts, which are species specific. Midges of the genus Culicoides are intermediate hosts of Onchocerca gibsoni, and black flies of the genus Simulium are intermediate hosts of Onchocerca gutturosa and Onchocerca dukei. Adult female worms in final hosts release larvae that reach the blood stream. The bloodsucking insect gets the microfilariae with its blood meal on an infected host. Microfilariae mature inside the insect, which transmits them further to another final host. However, the life cycles of some species have not been completely elucidated.
Harm caused by Onchocerca infections, symptoms and diagnosis
Usually, most Onchocerca species that affect cattle do not cause clinical signs. Nodules may be palpable in the predilection sites of some species. Economic damage is mainly the consequence of carcass rejection at slaughter due to the repugnant nodules.
Dogs infected with Onchocerca lupi may develop nodules as large as a bean in the sclerotic coat of the eye. Untreated infections can cause blindness.
In horses, Onchocerca cervicalis and Onchocerca raillieti are non-pathogenic, but microfilariae can cause skin irritation. Onchocerca reticulata may cause chronic inflammation leading to calcification and lameness.
Diagnosis depends on each species. Subcutaneous nodules can be identified through palpation, those in deeper organs are usually detected only after slaughtering. In horses, a full skin biopsy (>6 mm) properly macerated and stained may reveal microfilariae under the microscope.
Prevention and control of Onchocerca infections
There is little experience on how to prevent infections with these worms. Whatever reduces the population of midges and black flies - the intermediate hosts - will reduce the incidence in cattle or dogs. Protecting cattle against these vectors (e.g. with pour-ons containing synthetic pyrethroids) may reduce the number of infected cattle in a herd or the number of worms in an infected animal.
Normally the use of anthelmintics is not indicated on livestock. No anthelmintic controls the adult worms. Nevertheless it is known that some macrocyclic lactones (e.g. ivermectin), levamisole, and diethylcarbamazine, a piperazine derivative, are effective against the larvae (microfilariae).
There are so far no true vaccines against Onchocerca worms. To learn more about vaccines against parasites of livestock and pets click here.
Biological control of Onchocerca 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 Onchocerca worms to anthelmintics
So far there are no reports on resistance of Onchocerca worms to anthelmintics.
This means that if an anthelmintic fails to achieve the expected efficacy, chance is very high that it is not due to resistance bu either the product was unsuited for the control of Onchocerca worms, or it was used incorrectly.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Onchocerca control.