21 – Fighting against worm resistance

Life on earth didn’t get to where it is without learning a trick or two along the way.

Nature provides plenty of evidence showing that the ability to adapt is a valuable evolutionary trait.

Most of us will be aware of concerns about the resistance of bacteria to antibiotics and the conscious effort of doctors to prescribe them only when absolutely necessary.

It should therefore come as no surprise that parasites have not taken the arrival of drenches lying down.

They, too, have been gradually adapting in this long-running battle, not just in horses but in all manner of species where drenches are used, such as sheep and cattle.

Resistance is simple enough to understand.

Organisms enjoy genetic diversity. Among the countless billions of any one particular species of equine parasite, there will be a small number whose genes make them more resilient to a particular drench that the others.

If they survive a drenching they get to breed and pass on that genetic advantage. The numbers of those with this advantage grow while those with no special resistance diminish.

Eventually, the drenches we use become less effective because more resistant worms are surviving. We know that we can slow this process by using different drench families, which means that we can kill some of the resistant worms with a different chemical, but it is little more than a delaying tactic.

That said, drench rotation is important. You want to avoid the temptation of sticking solely with the likes of ivermectin or moxidectin because they prove so effective. A slow-growing worm resistance problem is much better than a fast-growing worm resistance problem.

Resistant specimens need not even exist in a given population. Mutations occur from time to time in a population and, when we talk parasites, we’re talking countless billions. Most mutations in an organism are bad and usually prove fatal.

Just occasionally, mutations occur that deliver some benefit to an organism and, if that benefit happens to be resistance to a particular drench, there is every likelihood the parasite will breed on this desirable trait (well, it’s desirable from a parasite’s point of view!).

Unfortunately, worm resistance is a bit like earning compound interest at the bank. Numbers grow slowly to start with, but with each generation there are more resistant offspring and finally – from a horse owner’s point of view – you’ll be noticing through faecal egg counts that drenching results are not as good as they once were.

Worm resistance is a global problem with a regional twist. Patterns of resistance vary from country to country, region to region, and even between properties, depending very much upon their horse history.

Fortunately, faecal egg counts can provide us with some indication of whether resistance is becoming a problem on our own property. It’s most accurate in terms of strongyles because their numbers are that much greater.

We gain this measurement through what is called a faecal egg-count reduction test. This rather grand name simply describes before and after faecal egg-count tests where we compare the results to gain a measure of effectiveness.

Firstly, we need to perform an egg count before drenching to provide us with a basis for comparison. Naturally, the results are really only useful if the drench you use contains one active ingredient, otherwise how will we determine how a particular drench family is performing?

We then drench the horse and conduct a follow-up egg count around 12 days later. Then, it’s a simple matter of comparing results.

The first question is: what did you drench with? Each drench family will deliver a different result so this an important part of the equation.

If you used a drench from the benzimidazole or tetrahydropyrimidine families, you would hope to see a 90% reduction in the egg count. Anything less than that and there is a reasonable chance that resistant parasites are present. If the reduction is 80 per cent or worse, you can bet your bottom dollar you have a resistance problem – at least to that particular family.

Ivermectin, moxidectin and abamectin remain highly effective drenches so you would hope to see an egg reduction of at least 98 per cent. Below that, and you would have to suspect a resistance problem. In fact, small strongyle resistance to this family of drench (the macrocyclic lactones) has only recently been shown in a study published early in 2008 based on research conducted on 250 horses on a central Kentucky farm.

In this particular case, we can come up with a collective result for all horses on your property.

While you will be wanting to record the data for each particular horse, you can add the totals across all horses tested and make the comparison. In other words, if you tested eight horses, add up the total egg count for the eight animals taken before the drenching, and add up the follow-up test results to make a direct comparison.

The more horses you test, the more accurate the result.

Remember, too, the multiplying factor involved in egg counts. If, for example, one of your horses return an egg count of 75 per gram of faeces before drenching (meaning you saw three eggs within the grid on the slide) and saw just one in the follow-up (for a count of 25), the numbers are simply not statistically significant.

By way of statistical comparison, the more horses the better for a faecal egg count reduction test.

» Next: Cardinal drenching sins

 

First published on Horsetalk.co.nz in February, 2009