Spectre of parasite resistance in New Zealand Thoroughbreds raised in study

New Zealand may be on the road to overt parasite resistance in the Thoroughbred industry, the findings of a soon-to-be-published preliminary study suggest.

However, the researchers stress that further work across more equine properties, in different groups of horses, and using a wider range of worming drugs should be conducted to paint a clearer picture.

Researchers Sarah Rosanowski, Charlotte Bolwell, Ian Scott, Patrick Sells and Chris Rogers set out to assess the effectiveness of ivermectin against strongyles in yearlings on the country’s Thoroughbred breeding farms.

Ivermectin is a member of the anthelmintic family known as the macrocyclic lactones. It is considered one of the key drugs in the fight against equine parasites.

The study team, whose findings are reported online in the journal Veterinary Parasitology: Regional Studies and Reports, noted that little was currently known about the resistance status of equine parasites in New Zealand.

The researchers used faecal egg count reduction tests to assess the performance of ivermectin against strongyles in yearling Thoroughbreds on six commercial stud farms in the Waikato region of the North Island.

In all, 117 yearlings were enlisted for the study, none of whom had been treated with worming drugs for at least six weeks previously. A preliminary screening strongyle faecal egg count was performed on each yearling beforehand, and they were accepted only if the count was 25 eggs per gram or greater in their faeces.

The horses were treated with ivermectin at a minimum dose rate of 200 micrograms per kg, and faeces were collected beforehand as well as on days 7, 14 and 21 for follow-up egg-count assessments. Further follow-up samples were taken on either day 28, 35 or 42, depending on the stud farm.

The average fecal egg count across the horses before dosing was found to be 609.6 eggs per gram, ranging from 100 to 2000.

It was found that 113 had a faecal egg count of zero seven days after ivermectin treatment

One horse had a positive fecal egg count 14 days after treatment and five returned a positive count 21 days after treatment.

The efficacy of ivermectin − that’s its ability to produce the desired result − ranged from 99.8% to 100% on Day 7 to between 98.5% and 100% on Day 21.

On one stud farm, the efficacy on day 14 was 94.4%.

The researchers found there was reduced efficacy – less than 90% – on three studs 28 to 42 days’ after treatment, suggesting a shortened egg reappearance period.

On one stud farm with seven horses, egg counts remained at zero up to 42 days following treatment.

The shortened egg reappearance periods on three of the farms could indicate a decline in ivermectin efficacy against the larval stages. This was a concern and current practices for parasite control in breeding and racehorses in New Zealand were arguably not sustainable, they suggested.

Discussing their findings, the researchers said the faecal egg count reduction test results showed the continued effectiveness of ivermectin against the egg-laying adult stages, but there was potential for strongyle resistance to ivermectin based on the shortened egg reappearance rates identified on three of the farms.

“A shortened egg reappearance rate is important as declining efficacy against the larval nematodes of horses is likely to presage overt resistance to ivermectin in the adult stages.

“Additionally, for studmasters attempting to implement targeted treatment regimens, where treatment is given only when faecal egg counts exceed a certain threshold, the reduced egg reappearance rate may mean that some horses may actually be treated more frequently since they have egg counts high enough to trigger retreatment after as little as 4 to 6 weeks.

“The more frequent treatment of horses was already occurring on at least one of the stud farms involved in the current study.”

The low efficacy of ivermectin observed 14 days after treatment on one stud was the result of continued egg shedding by only one animal. “It is possible that this animal may not have been adequately treated, despite yearlings on this stud farm being dosed at rates appropriate for 600kg horses (approximately 1.5 times appropriate dosing) and no reports of dosing failure in these horses.”

They observed: “This study has indicated that we may be on the road to overt resistance in the Thoroughbred industry in New Zealand, but further work across more equine properties, in different groups of horses and using other families of anthelmintics should be conducted.

“In the future, greater use may need to be made of targeted programs whereby animals are treated with anthelmintic only after a significant level of egg shedding has been demonstrated.”

Such a strategy would help protect the effectiveness of existing worming drugs. “However, as demonstrated here, the targeted strategy may not be as effective at slowing anthelmintic resistance if resistance is already developing and egg reappearance periods have significantly shortened.”

Further work is required learn more about the effectiveness of all families of worming drugs being used in horses in New Zealand, they said.

Sarah M. Rosanowski, Charlotte F. Bolwell, Ian Scott, Patrick D. Sells, Chris W. Rogers
The efficacy of Ivermectin against strongyles in yearlings on Thoroughbred breeding farms in New Zealand.
DOI: doi: 10.1016/j.vprsr.2017.02.001 LINK: http://dx.doi.org/10.1016/j.vprsr.2017.02.001

The abstract can be read here

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