Bovine onchocercosis is an infection with the roundworm Onchocerca. At least three species infect cattle in Australia: O. gibsoni, O. gutturosa and O. lienalis. Onchocerca gibsoni has the greatest economic impact, as infections result in nodules containing long-lived, medium-sized filaroid (thread-like) adult worms. They have indirect life cycles that also involve an insect host to complete. Nodules containing the parasites are found mainly in the brisket and are identified and removed at slaughter. Inspection of carcasses by palpation and removal of the nodules is time consuming and is estimated to cost the Australian beef industry $7.7 million annually.
Infections with Onchocerca species are distributed over most of Australia with the highest prevalence being recorded in the northern part of the continent, with decreasing prevalence further south. O. gibsoni does not seem to occur in locally bred cattle from Victoria, south and south-east South Australia, and south-western Western Australia but up to 100% of cattle have been found infected in parts of the Northern Territory and northern Queensland. In Victoria, up to 10% of cattle were found infected with O. gutturosa. The prevalence of infection with Onchocerca species increases with age of cattle. The number of nodules per host is highest during summer months, coinciding with peak vector activity. (The last published studies on the distribution and prevalence of O. gibsoni are from more than 30 years ago.)
The intermediate biting insect hosts ingest microfilariae of Onchocerca species while feeding on an infected bovine host. Within the intermediate host, the microfilariae turn into infective larvae in 1-3 weeks. While the vectors are feeding on another host animal, the larvae leave the mouthparts of the insects to enter the new host. The time from infection to appearance of the adult worms is unknown, but nodules of O. gibsoni become visible after less than 6-7 months from infection.
Location in cattle
Onchocerca species are spread by biting insects, especially Culicoides species (midges), but the preferred vector varies by species. Adults of O. gibsoni generally occur in nodules (Figure 1) in the brisket. Microfilaria (earlier stages of the parasite) are found in the skin, especially in the brisket and naval area. The female worms are up to 50 cm long (commonly 14-19 cm long) and up to 600 µm wide and the male worms are up to 5.6 cm long and up to 190 µm wide. The microfilaria are 240-280 µm long.
Adults of O. gutturosa are located in ligaments and connective tissue but are easily overlooked. Adults of O. lienalis occur in the gastrosplenic ligament (in the gut).
Figure 1. Nodules of Onchocerca gibsoni recovered from the brisket of a cow. Image courtesy of Constantin Constantinoiu
Cattle infected with adult stages of Onchocerca species present in Australia generally do not show clinical signs.
In live animals, superficial nodules of O. gibsoni may be visible externally in live cattle as rounded projections or palpated in the brisket area. Microfiliaria are detectable in skin biopsies taken from various locations in affected animals, depending on which Onchocerca species is involved.
Most diagnosis occurs at slaughter. Carcasses for export are inspected according to Commonwealth meat regulations. If nodules are found the product may be rejected in some overseas markets.
Most nodules are 2.5- 3.5 cm in diameter and generally contain one female worm and one or more male worms coiled together. The mean number of nodules per brisket is generally less than 4.
No products are currently registered in Australia for control of onchocercosis or its insect vectors.
Accidentally ingested nodules of O. gibsoni do not seem to be harmful to humans if they are not infected by pathogenic bacteria. However, cases of zoonotic onchocercosis caused by O. gutturosa have been reported.