Not all horses are affected the same, probably 20 percent of horses shed 80 percent of the eggs.
"The susceptibility of horses to parasite infection appears to be controlled by genetics, and it is probably quite complex,"
says Craig Reinemeyer, DVM, PhD, President of East Tennessee Clinical Research. "You can't do anything retroactively for individual
mares, but you can increase the worm resistance of their F1 offspring by breeding them to a highly-resistant stallion."
Though the horse industry is not ready to select futurity prospects on the basis of fecal egg counts, creating a parasite
resistant horse is scientifically feasible. Once done, the animal is less likely to be affected, and therefore becomes a low
egg shedder in the herd. This strategy is currently being implemented in the sheep industry, where resistance is problematic.
"Unlike the situation with small strongyles in horses, sheep parasites will literally kill their hosts, and if you can't control
them, you will have dead animals," Reinemeyer says.
What the sheep industry is doing now is identifying 'super rams,' those that are highly genetically resistant to parasite
infection. For sheep, the heritability of this resistance to parasites is very high and does not come at the expense of commercially
desirable characteristics. This is an area of equine science that is just begging for research in the future.
For sheep, especially in Australia and New Zealand, resistance is dire. There are sheep and goat farms that have reported
total anthelmintic failure, sources say. Most of the countries where the drugs don't work at all are in South America and
"I've seen it on a goat farm in the Southern United States, too," Kaplan says.
Many sheep farms have resistance to two of the three drug classes that are available, and it is getting worse all the time,
Kaplan says. Non-chemical approaches, such as the genetic approach, i.e. breeding parasite resistant or resilient sheep and
goats, is gaining in popularity and acceptance. Those producers understand that they are getting toward the end of the line
in terms of new drugs.
"If new drugs did become available to sheep herders, and they used them the same way that they burnt out the old drugs, then
even the new drugs won't last very long," Kaplan says. "There is no doubt that there will be new drugs, but the pace of new
drug development has slowed down remarkably. Macrocyclic lactones (ML) were introduced into the marketplace 20 years ago.
There is still no new drug class for large animals. Whereas it was less than 20 years, from the introduction of the first
modern anthelmintic, thiabendazole, to the ML class, the 'champagne' of anthelmintics, it's probably unlikely that we're ever
going to see a drug class that can touch what the ML drugs can do. The ML drugs were an amazing discovery and it's unlikely
we'll ever get another drug like them, at least in our lifetimes, maybe ever."