One season of pasture exposure fails to induce a protective resistance to cyathostomes but increases numbers of hypobiotic third-stage larvae.

The research article investigates whether resistance to a common type of parasitic worm, known as cyathostome, could be developed by horses after a single season of exposure to a pasture contaminated with the parasite. Despite their exposure, it was observed that these horses did not develop significant resistance, however, the numbers of dormant-stage larvae inside the horses increased.

Study Design and Methodology

• The study involved a total of seventeen ponies, all of whom were parasite-naive, ranging from 2-3 years of age. These ponies were divided into three groups for the study. The groups consisted of ponies exposed to cyathostome-infected pastures for four months, ponies that grazed on clean pastures, and a group of ponies that were kept in a barn without exposure to any parasites.
• After the exposure period, all animals were dewormed using ivermectin and oxibendazole, which are medications used to eliminate most types of worms, including cyathostomes. The efficiency of the deworming was confirmed via necropsies performed on five ponies seventeen days after the treatment.

Findings and observations

• The remaining horses were then given an oral dose of 100,000 cyathostome-infective larvae three weeks post-deworming to test their resistance to re-infection. A follow-up necropsy was performed seven weeks after this challenge.
• No significant difference amongst the groups was found in respect to the total burdens of cyathostomes. However, it was observed that the horses which were exposed to the worm-infected pastures had a significantly higher proportion of dormant, or ‘hypobiotic’, early stage larvae known as EL3. This state of dormancy is known as hypobiosis.
• The researchers suggest that this increase in hypobiotic larvae supports the hypothesis that an acquired resistance against cyathostomes could potentially promote a state of dormancy in the larvae. Furthermore, there was a notable increase in the weight of the gut tissue samples which were taken from the cecum and ventral colon, areas where significant numbers of the worms are harbored, in the exposed horses. This was associated with the increase in dormant larvae.

Conclusions

• The failure to develop a significant resistance to the worms after one season of exposure shows that more prolonged or multiple exposures might be necessary to develop immunity to the parasites.
• The increase in dormant larvae also provides implication that previous exposures to the parasite may trigger an adaptive response in the worms, leading to increased dormancy or hypobiosis.