[Britster]

Interesting.... kind of scary!

Resistance to Heartworm Preventatives | petMD

I was skimming through some journals yesterday and came across an incredibly disturbing case report. The paper, titled "Macrocyclic lactone resistance in Dirofilaria immitis," appeared in the September 27, 2011 issue of Veterinary Parasitology.

First, some translation: Macrocyclic lactones are the drugs used to prevent heartworm infections and kill baby heartworms (i.e., microfilariae) in the blood stream during heartworm treatment. Ivermectin and milbemycin oxime are macrocyclic lactones. Dirofilaria immitis is the scientific name for heartworms.

And now for some history. Rumors of a small number of dogs coming down with heartworm infections while on heartworm preventative have been circulating for years, but these cases have been incredibly difficult to confirm. It is next to impossible to determine whether a dog received and absorbed his heartworm prevention every month all year round, particularly since the lapse in question had to have occurred at least six months previously. (It takes six months for the juvenile heartworms passed through the bite of an infected mosquito to mature into the adults that are responsible for a positive heartworm test and most clinical signs of heartworm disease.)

Veterinarians have been starting to suspect that some populations of heartworms were in the process of developing resistance to heartworm preventatives, but we haven’t had any definitive proof … until now.

Read more: Resistance to Heartworm Preventatives | petMD