Equine onchocerciasis.

This research investigated the presence of Onchocerca cervicalis, a specific type of parasitic worm, in horses. The study found that this worm, traditionally associated with bovine infections, was apparently common in horses, signaling a lower host specificity than previously thought. The infection patterns were also observed.

Study Design and Methodology

• As part of a broader study into bovine onchocerciasis (a worm infection in cattle), twenty horse carcasses were examined for Onchocerca cervicalis infections, focusing on the ligamentum nucliae, a region in the horse’s neck.
• Limbs of the horses were examined for nodules (indicating infection), and 19 fore or hindquarters were selected randomly for detailed examination and potential worm infection, using the method for cattle described by Ottley and Moorhouse in 1978.
• The horses varied in age and health condition. Many were being euthanized due to old age, and all originated from south-east Queensland, either from Brisbane or along the Condamine River.

Results and Findings

• Infections with Onchocerca species were found only along the ligamentum nuchae in the horse’s neck. Some horses were infected with Onchocerca cervicalis alone, some with another species, Onchocerca guirurosa, some carried mixed infections, and some were completely free of infection.
• The indicators used to identify the specimens matched the descriptions provided by Bain in his 1975 study.
• It was observed that all the female Onchocerca guirurosa were found on the part of the ligament called lamellar. Most were found alone, except for one found in association with a male. These infections did not cause significant visible damage.
• Onchocerca cervicalis was found in the thoracic expansion of the funicular part of the ligamentum nuchae. These infections did cause palpable damage to the ligament or the supra-spinous bursa in some horses. Lesions contained more than one female worm, and caused visible calcifications and caseous foci (cheese-like areas), and associated fibrosis (thickening and scarring) and vascularisation (formation of blood vessels) around the bursa.

Implications and Further Questions

• The finding that Onchocerca guirurosa, a nominally bovine (cattle-specific) parasite, is apparently common in horses may help explain conflicting observations in the literature regarding the damage attributed to Onchocerca cervicalis.
• Researchers stressed the importance of accurate worm identification before conducting epidemiological or pathological studies because host specificity (the range of hosts a parasite can infect) seems to be lower than previously thought.
• The occurrence of infections in a variety of hosts likely depends on both the host’s ability to tolerate the parasite and on the biting behavior of the insect vector (the insect that transmits the infection).