Monday, August 7, 2006

Testing for Anthelmintic Resistance

DrenchRite® is an in vitro test that involves hatching out worm larvae from a composite fecal sample and testing the larvae for vulnerability to the different anthelmintics. The DrenchRite® test is commonly used in Australia and is available through the diagnostic lab at the University of Georgia College of Veterinary Medicine.

On a Maryland farm
A DrenchRite® test was recently conducted on a composite fecal sample from a sheep farm in central Maryland. Test results showed high resistance to the benzimidazole group of anthelmintics (Valbazen and SafeGuard). Drug efficacy for this class of drugs was predicted to be only 6 percent. Resistance to Levamisole (Levasol, Tramisol, Prohibit) was moderately high. Drug efficacy for levamisole was predicted to be 66 percent for the barber pole worm (Haemonchus contortis) and only 51 percent for Trichostrongylus.

Suprisingly, barber pole worms on this farm were still sensitive to Ivermectin. However, Trichostrongylus worms were resistant to Ivermectin. All stomach worms were sensitive to moxidectin (Cydectin, Quest). The DrenchRite® test does not directly test for moxidectin resistance, but resistance can be determined based on ivermectin doses.

Limit use of moxidectin
Based on the results of this test, this producer should not rely on the benzimidazoles for treatment of stomach worms. These products should still be effective against tapeworms, though tapeworms tend to be non-pathogenic in most flocks. To preserve the effectiveness of moxidectin, this producer should limit its use to clinically parasitized animals (e.g. FAMACHA© scores 4 or 5).

The DrenchRite® assay offers a viable alternative to the laborious task of performing multiple fecal egg counts. However, testing is currently on hold, as the lab in Georgia seeks a new lab technician.

DrenchRite® Larval Development Assay

1 comment:

Anonymous said...

Hi Susan,

I select heavily for worm resistance in my breeding program and rarely worm anymore. Last winter I noticed that some of my does (myotonic goats) whose tissues had been the color of ripe tomatoes were pale. They also had high FECs. It turned out they were positive for Johnes Disease.

I'm thinking that JD weakened my stock's ability to tolerate worms. Any thoughts?