Heartworm Resistance in Dogs
Heartworm disease is one of the most serious diseases that can affect many mammal species, including dogs and cats. When an animal is diagnosed with heartworms, it means that they literally have worms living in their body, which mostly attack the heart and lungs and even sometimes the blood vessels. Over time, heartworms will cause damage to all of their organs and have the ability to eventually cause heart failure, making this a potentially fatal disease.
Fortunately, heartworm disease is very preventable. The challenge for pet owners is to use heartworm preventatives on their pet consistently. Heartworm preventatives on the market have a track record of virtually 100% protection if administered regularly with no gaps.
The devil is in the gaps
Veterinarians know that the problem is in the gap—when a dog owner misses (or delays) a heartworm preventive dose. Protection lapses and the dog can become infected. When a mosquito transmits a heartworm larvae from an infected dog to an unprotected dog, it only takes about a week for the larvae to become adult heartworms. As adults, heartworms release early-stage larvae (known as microfilariae) into the dog’s circulation. The good news–heartworm doses administered without gaps in coverage stop that from happening.
In recent years, some heartworm microfilariae have become resistant to the active ingredients (based on macrocyclic lactones) in heartworm preventatives. This means that some microfilariae in the bloodstream may be able to survive and continue to multiply. If so, they can cause more harm even after the dog is back on heartworm prevention.
Veterinary medicine has been studying heartworm resistance since it was discovered about 10 years ago, but there’s still a lot to learn about heartworm resistance, such as how to identify and treat it.
A few things that studies have found out about heartworm resistance so far:
- Resistance in heart worms is hereditary
- Genetic markers can help identify what is causing macrocyclic lactone resistance
The slow kill is likely speeding up resistance
One common method of treating dogs in the early stage (Stage 1) of heartworm disease is the slow kill method. Dogs are given monthly ivermectin heartworm prevention so that worms gradually die off over a period of one to two years (while likely continuing to do some damage).
Researchers believe that the slow kill method is likely speeding the spread of heartworm resistance. That’s because the slow kill method tends to kill off the non-resistant heartworm microfilariae in the bloodstream, but is likely to make the resistant microfilariae even more resistant.
What can pet owners do
The most important thing we as pet owners can do is to be more diligent about consistency in giving our pet their heartworm prevention dose without gaps. That stops microfilariae before they start.
What can veterinarians do
- Acknowledge that heartworm resistance exists and is a cause for concern. Resistant strains are appearing outside the South, such as the highly resistant strain found in Illinois.
- Continue to help pet owners be consistent about heartworm prevention and to have their pet tested yearly. Lack of compliance by pet owners is the biggest reason dogs get heartworms.
- Don’t use preventative doses once heartworms have been detected–first kill off all microfilariae.
- If heartworms are detected, persuade the owner that clearing the dog of any heartworms upfront is the way to go. This is best to help minimize the spread of heartworm resistance.
Diagnosing and Treating Heartworm Resistance
There is currently no test that allows veterinarians to determine which heartworm microfilariae are resistant or whether a particular dog has heartworm resistance. Research is currently looking for a better way to make that determination.
Treatment of resistant strains is currently possible but expensive and labor intensive. Here at Oakland Veterinary Referral Services, we’re happy to answer any questions we can regarding heartworm prevention and resistance.