The Return of the Worm
Resistance to "anthelmintics" — drugs that kill parasitic worms — is becoming an ever bigger problem in veterinary medicine. I talked about new evidence for resistance to heartworm medications last week (Y’all must have been recovering from the holidays, I expected a bigger uproar to the post than it got), but I thought I’d expand the topic to include other species today.
Way back in the olden golden days when I was in vet school, anthelmintic resistance was primarily a problem for people who tended goats, and therefore it could be ignored pretty easily by the general public (and veterinary students). Goats are plagued by a type of stomach worm, called Haemonchus contortus (also known as the barber pole worm because of its striped appearance). When a goat has a large-enough parasitic load of H. contortus, it develops diarrhea and becomes anemic. The worms suck the goat’s blood through the lining of its stomach, not unlike hookworms in dogs and cats.
H. contortus has always been a problem for goat owners. When pastures become heavily contaminated, whole herds can become anemic and less productive, and many individuals will die if the infestation is severe enough. Dewormers worked initially, but they became less and less effective, until eventually there wasn’t a single, reliable one to turn to.
Now, anthelmintic resistance is being reported with increasing frequency in the cattle industry, in horses, and we have evidence of it in a heartworm infected dog. We’ve become so reliant on such a small number of drugs that their loss of efficacy is potentially catastrophic.
Obviously we need new drugs, but there are other things that the owners of grazing animals can do to limit the spread of gastrointestinal worms and anthelmintic resistance:
- Practice good pasture management. Do not overstock your fields and remove or treat feces, if possible.
- Feed off of the ground to reduce contamination of hay, grain, etc.
- Keep water supplies clean.
- Make use of fecal egg counts before and after deworming to learn which anthelmintics are still effective on your farm.
- Time your use of dewormers to the lifecycle of the parasites present in your herd.
- Use different types of dewormers (as long as each is effective) at different times of the year or use combination therapy to decrease the chance that resistance will form.
- Breed only those animals with immune systems that are effective at fighting off the worms.
I hope you dog and cat owners out there are still paying attention. This problem may be coming to a dog park or backyard near you soon enough.
Dr. Jennifer Coates