On the Heartworm Society’s Map of Heartworm incidence page 4, the spread and increase in heartworm disease since 2001 is pretty shocking.
If you live in the Southeast below the 37th parallel (the North Carolina/Virginia border), and in the areas highlighted as high incidence, or in the Gulf Coast states, your dog needs to be on monthly heartworm prevention year round.
Except for the subtropical South, heartworm transmission does have seasonal parameters. We need to be aware when those vulnerable periods occur and be very conscientious about using heartworm preventative. In the northern half of the country the highest transmission period is May through October. In regions where the heartworm transmission occurs more than ½ the year, you will want to give year round preventative.
I listed the options for prevention in a previous blog, but the preventative Ivermectin has an added benefit of being effective against late precardiac larvae and young (less than 7 months post infection) adult heartworms.
Heartworm prevention drugs are good for one month and according to the Heartworm Society “remain high for at least an additional month.” However, they add a disclaimer that the manufacturers added this longer efficacy for inadvertent delays or unplanned breaches in the monthly regimen. There are some who suggest that monthly heartworm can be administered every 45 days and still be effective. I know I have used that protocol myself. However, during the peak months of May through October or in any of the high incidence areas, I think it’s clear that monthly is safest, to avoid risk.
A study from Washington State University by Dr. Karen Mealey revealed that Collies, Australian Shepherds, Shelties, Border Collies, and Old English Sheepdogs and other collie related dogs can carry the MDR1 mutant gene, which makes them more sensitive to some drugs. You can get your dog tested to see if he/she carries this gene. Some veterinarians I spoke to said that the dosage in Heartgard and the generic Ivermectin brands are a low enough dose to be safe, with one cautioning me that Ivermectin, even at a low dose, is still going through the blood brain barrier. If your dog is tested to be free of the MDR1 gene, then there is no problem, otherwise the decision to give your dog Heartworm preventative with Ivermectin is a personal one after you’ve discussed it with your vet.
For alternative preventative suggestions I turned to Dr. Richard Pitcairn and his book Natural Health for Dogs and Cats. He points out that wild animals are surprisingly resistant to the parasite and if they do contract heartworm disease, they tend to get a light infestation and then become immune. He attributes this to differences between wild and domestic animals in exercise, raw diet, no drugs or toxic products, and immune systems able to resist the parasite. He advocates if we maximize our dogs’ health the ability to resist disease will be much better.
With global warming, rising temperatures, and in some places wetter climates, Dr. Pitcairn says despite effective preventatives heartworm disease is on the rise, and although he hates the thought of continually poisoning our dogs with these drugs, for now our only option is to use monthly preventative to safeguard our dogs.
If your vet is willing to write you a prescription, you can find competitive prices online at many of the pet supply websites. My favorites are KV Vet and Omaha Vaccine, but there are many other vendors listed if you google heartworm prevention. Most folks like the convenience of one stop shopping at their veterinarian, but if your budget is tight, it’s worth the effort to ask your vet if he/she will write you a prescription and shop around. Some vets will match the price you find online.
Illustration: Two Boys (detail) by Frank W. Benson, American 1862-1951