Signs of anthelmintic resistance in tapeworms?

Horses frequently carry tapeworm infections, particularly Anoplocephala perfoliata. While these parasites can be linked to colic, most infected horses generally tolerate them well and show no adverse signs.. 

Tapeworms are commonly found in horses grazing on pasture because the intermediate host, an oribatid mite, resides in the pasture environment. In contrast, tapeworms are seldom observed in horses in dry and arid conditions.

 

Two drugs, praziquantel and pyrantel, are commonly employed for tapeworm control and are generally acknowledged as effective in managing these parasites.

 

While the issue of anthelmintic resistance has garnered widespread attention concerning roundworms, it has not received as much consideration in relation to tapeworms. Anthelmintic resistance in equine tapeworms, as in other parasites, poses a significant challenge in the management of horse health. 

 

In contrast to roundworms, which can be easily diagnosed and assessed through a faecal egg count, tapeworms present a greater challenge as they excrete eggs sporadically. This erratic pattern not only complicates diagnosis but also poses challenges in evaluating the response to treatment and identifying anthelmintic resistance.

 

In a report from North America, Martin K. Nielsen, affiliated with the M.H. Gluck Equine Research Center in the Department of Veterinary Science at the University of Kentucky in Lexington, USA, sheds light on an apparent treatment failure involving praziquantel and pyrantel pamoate against tapeworms. The report relates to animals on a Thoroughbred stud farm in Central Kentucky in 2023. The findings have been published in the "International Journal for Parasitology: Drugs and Drug Resistance."

 

Fifty-six young horses were initially dewormed using a combination of ivermectin and praziquantel, followed by a treatment with pyrantel pamoate. The effectiveness of the deworming was assessed by checking the number of parasite eggs in their faeces on the day of treatment and again 14 days later.

 

Two groups of female horses, consisting of 39 and 45 individuals, were also given the ivermectin/praziquantel treatment, and their faeces were examined before and after treatment.

 

In the yearlings, the overall effectiveness against tapeworms, measured by FECR levels, was 23.5% for praziquantel and 50.9% for pyrantel pamoate. 

 

Praziquantel successfully removed tapeworm eggs in three out of 17 yearlings, but five other yearlings changed from having no tapeworm eggs to having them after treatment. 

 

Unfortunately, pyrantel pamoate did not eliminate tapeworm eggs in any of the 14 yearlings that tested positive for tapeworms.

 

Among the tested mares, nine out of 84 were found to have tapeworm eggs, and after praziquantel treatment, seven of them still tested positive for tapeworm eggs.

 

The results also showed that the ivermectin and pyrantel pamoate treatment was not very effective against roundworm (strongylid)  parasites in the young horses. The average reduction in the number of parasite eggs in their faeces was 75.6% or less, and the upper limit of the 95% credible interval was below 90% in all cases.

 

Nielsen highlights that the results differ significantly from the initial field efficacy studies conducted for both active compounds, raising concerns about the potential development of anthelmintic resistance.

 

He suggests the need for continued research and advancements in parasite management strategies to develop more sustainable approaches in addressing equine tapeworm infections.

 

 

 

For more details, see:

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Apparent treatment failure of praziquantel and pyrantel pamoate against anoplocephalid tapeworms

M K Nielsen 

Int J Parasitol Drugs Drug Resist. 2023 Aug:22:96-101.

 doi: 10.1016/j.ijpddr.2023.06.002