Equine tapeworm monitoring remains a challenge – parasitologist

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The distinctive D-shaped egg of the tapeworm, Anoplocephala perfoliata. The dark circles in the picture are air bubbles.
The distinctive D-shaped egg of the tapeworm, Anoplocephala perfoliata. The dark circles in the picture are air bubbles. © Martin Krarup Nielsen

Challenges remain in using some form of tapeworm monitoring in a surveillance-based parasite control strategy, a scientific review has found.

Parasitologists around the world have been promoting surveillance-based worm control in horses as a better alternative than calendar-based drenching, which has been popular for decades.

But Assistant Professor Martin Nielsen found strengths and weaknesses in the range of diagnostic tests  available to clinicians for the most common tapeworm, Anoplocephala perfoliata.

“Egg-counting techniques have been modified to achieve acceptable to good diagnostic performance but the trade-off is often a more time-consuming method,”  Nielsen reported.

The equine parasitologist, in a review of scientific literature covering equine tapeworms published in Equine Veterinary Education, noted the commercial availability of several enzyme-linked immunosorbent assays (ELISAs) for tapeworms.

“These can generate useful information about tapeworm exposure on the herd level but are less reliable for individual diagnosis.”

Unfortunately, none of the available diagnostic techniques were useful for evaluating the effectiveness of any worming treatment given to horses, said Nielsen, who is with the M.H. Gluck Equine Research Center at the University of Kentucky.

He noted that equine tapeworm infection had gained increasing attention over the past few decades, with most published research focused mainly on most common of the the tapeworm species, A. perfoliata. He noted some new information had also been generated for the other two species, A. magna and Anoplocephaloides mamillana, previously known as Paranoplocephala mamillana.

A. perfoliata is by far the most prevalent, while the other two species are reported only sporadically. Only A. perfoliata has so far been reported to cause ulcers and other problems around their attachment site.

Nielsen said the large body of scientific evidence supported an association between A. perfoliata infection and certain types of equine colic, although somke discrepancy existed between studies.

Available evidence suggested that A. perfoliata was common on different continents and in different climates around the world, with its prevalence varying from 20 percent to 80 percent.

“Thus, the parasite should be expected to be present in most equine establishments but not necessarily in every single horse.”

It is theoretically feasible to target only the infected animals in a selective treatment approach similar to what is recommended for strongyle control, but available evidence suggested that only the laborious and time-consuming manure-based techniques were suitable for this purpose.

He said that while a proportion of horses in a given herd may not necessarily need treatment, it currently seemed to be a reasonable precautionary procedure to treat these rather than leave them untreated.

Only two drugs are available for the treatment of equine tapeworms, pyrantel pamoate and praziquantel. To date, drug resistance has not been reported in any of the equine tapeworms.

Treating for tapeworms more frequently than once or twice a year should only be justified by diagnostic evidence of a tapeworm problem, he said.

It should, he said, be carried out with caution as resistance was just as biologically feasible in  tapeworms as has been experienced with strongyles and ascarids in horses.

“Therefore, one or two targeted tapeworm treatments each year delivered to all horses on a given farm is a pragmatic decision.”

He said the timing of such treatments would depend on the parasite transmission dynamics on the given equine operation but in temperate climates, tapeworm burdens generally rose over the course of the grazing season, making the autumn or early winter an appropriate time for treatments.

“However, this may be substantially different for warmer climates where the summers may be too dry and hot for tapeworm transmission.”

Nielsen said, in terms of gathering evidence on which to base treatment, the manured-based techniques for tapeworm egg detection may ultimately be too laborious and time-consuming for large-scale surveillance in veterinary practice, but they remained useful diagnostic techniques for individual horses.

However, when properly used, the currently available diagnostic tools can guide veterinarians to make strategic decisions regarding tapeworm control.

Tapeworm surveillance and control should be considered as part of the overall parasite control strategy, Nielsen said.

He noted that most of the tapeworm research had focused on development and validation of diagnostic techniques for tapeworm detection and the role of these parasites in equine gastrointestinal disease.

“A good amount of useful scientific evidence regarding equine tapeworms has been produced in the last couple of decades,” Nielsen concluded,  “but several issues still remain to be resolved.”

Crucial among these was the apparent inability of any tests to evaluate the effectiveness of the tapeworm treatment. The quest for new and improved diagnostic methods should therefore be continued, he said.

Equine tapeworm infections: Disease, diagnosis and control
M.K. Nielsen
Equine Veterinary Education
DOI: 10.1111/eve.12394
The abstract can be read here.

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