Onchocerca spp: frequency in Thoroughbreds at necropsy in Kentucky.

This study explored the prevalence of Onchocerca spp in thoroughbred horses in Kentucky, finding it in 59% of the examined horses across a 13-month period. The frequency of infection was age-dependent, increasing to 71% to 100% in horses aged 6 to 29.

Study Process

• The study was conducted in Kentucky over a span of thirteen months from 1984 to 1985.
• 523 Thoroughbred horses aged between 1 to 29 years were examined after death (necropsy) specifically for the presence of Onchocerca species (spp), which are parasitic nematodes.
• The main focus was on the ligamentum nuchae of the horses. The ligamentum nuchae is a vital neck ligament providing support and is known to be a site for these parasites.
• Additionally, suspensory ligaments and flexor tendons from the fetlocks of thoracic limb of 100 thoroughbreds (3 to 24 years old) were also examined for Onchocerca spp, during a shorter 4-month period in 1984.

Study Findings

• The study discovered that 306 out of the 523 horses (59%) were infected with Onchocerca spp.
• The frequency of parasitic infection increased with the horses’ age. For 1-to-5-year-old horses, frequency rates were 3%, 17%, 40%, 43%, and 42%, respectively.
• It was observed that in older horses, between the ages of 6 to 29 years, the frequency of infection fluctuated between 71% and 100% with one exception: only half of the 26-year-old horses were infected.
• Interestingly, 87% of the horses in the age group of 6-29 years were found to be infected.
• Contrary to the above findings, none of the 100 thoroughbreds whose suspensory ligaments and flexor tendons were examined for Onchocerca spp were discovered to be infected.

Implications of the Research

• This study gives valuable insights into the high prevalence of Onchocerca spp in thoroughbred horses, particularly in older horses, signifying the need for increased veterinary care with age.
• The lack of Onchocerca spp in the fetlock ligaments and tendons indicates that these are not likely habitats for these parasites in these breeds.
• This research is essential for understanding the epidemiology of these parasites in horses, which in turn can guide the development of strategic control and prevention measures to enhance horse health and welfare.