Upstate New York vets report seeing more cases of drug-resistant heartworm
By Jolene Cleaver
Have you heard about drug-resistant heartworm disease and its impact on dogs?
Plainly, it’s a condition where some heartworm microfilariae are no longer susceptible to the active ingredients in heartworm preventives.
While experts say the issue emerged two decades ago in the Mississippi Delta region, a quick Google search for “drug resistant heartworms” produces scant reporting. Check it out, here: https://tinyurl.com/2xm3bbam
While there is an apparent lack of recent reporting on the issue, the Upstate New York veterinary community is seeing cases of drug-resistant heartworm (also known as Dirofilaria immitis) and its range expansion to the northern parts of the United States.
When contacted for this story, Mohawk Valley veterinarians acknowledged that they are aware of the strain and its migration north as infected animals travel, often after being adopted.
Among them, Dr. Nickel’s Animal Hospital in Whitesboro pointed to the guidance from the American Heartworm Society and veterinary science programs at universities for the latest information.
Diving in
As with most healthcare issues, there is ongoing study and searches for advanced therapies as medicine advances.
“Research exploring apparent resistance of populations of heartworms to medications is only in preliminary stages and much work remains before the issue is fully understood,” notes information from the American Heartworm Society.
Information from the heartworm society adds that year-round preventive medication is the core of heartworm disease prevention. However, more than 56% of dogs seen by veterinarians in the southern U.S. never receive a single dose of preventive.
Further, it is advised that preventive medications must be given every 30 days (with the exception of injectable moxidectin, which must be administered every 180 days).
As with most courses of medication, delays in administration will reduce product efficacy.
‘Cornell would be your best bet’ for raw data, local vets advise
There, Manigandan Lejeune, an associate professor of practice at the Department of Population Medicine & Diagnostic Sciences College of Veterinary Medicine at Cornell University, explained the nuts and bolts of canine heartworm infection.
The life cycle of heartworm involves various species of mosquitoes as the vectors that allow the baby heartworm (microfilaria) taken during a blood meal from an infected dog, to develop into an infective larva 3 (L3) that is then transmitted to a susceptible dog.
Lejeune adds that once transmitted, the L3 takes three to four days to grow as larva 4 (L4). The monthly heartworm preventatives (mostly macrocyclic lactone drugs) administered to dogs are aimed to kill L3 and up to 30-day-old L4 when these drugs are known to be 100% efficacious.
While non compliance with monthly preventives administered to dogs is noted as the major reason for the ‘lack of efficacy’ of macrocyclic lactones, drug-resistant strains of heartworm indeed occur. Unfortunately, commercial diagnostic labs lack the appropriate diagnostic tools to detect drug resistance or drug-resistant strains circulating in the United States, he said.
“Resistance to monthly preventives means the heartworm disease can spread to new geographical areas and establish wherever the right species of mosquito vectors occur. This indicates huge epidemiological risks. However, I do not see any impact on how heartworm disease is treated currently. Macrocyclic lactones are not effective against the mature and adult stages of heartworm. Melarsomine is the best adulticide drug and there is no known resistance to this drug. Though the Melarsomine treatment protocol is tedious and expensive, there is no effective alternative,” Lejeune added.
Elsewhere across the state, another veterinary science program, at Long Island University, weighed in.
“… I do find in my experience animals that are tested negative on original intake end up being positive within one year of adoption. I find that this is likely all due to timing of the heartworm lifecycle. Dogs that are adopted out with a negative heartworm test should be re-tested again in six months, while continuing to take monthly oral macrocytic lactone,” said vet Victoria Bove, an assistant professor of clinical skills at Long Island University College of Veterinary Medicine.
When it comes to educating student veterinarians, Bove added, “We follow the American Heartworm Society guidelines for treatment (three injections) and in my clinical experience, I have not had any dogs be resistant to this treatment, when followed accordingly.”
Compliance begins in the veterinary clinic
The American Heartworm Society offers guidance to pet owners on effective treatment of heartworm for pups that notes:
• Clinic staff must provide specific administration instructions or treat the pet in the hospital.
• Client education is critical in order to ensure accurate product use.
• Compliance also includes timely and appropriate administration or application of the medication in the proper weight range. Loss of an oral product prior to absorption can occur with vomiting; and swimming or bathing immediately after application may affect the absorption of a topical.
• Review of patient history and age at initial preventive dose will also illustrate potential loopholes in prevention protocols.
For specific information relating to resistant strains of heartworm disease, visit: https://tinyurl.com/4r73hmjr