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ASK THE VETS: Heartworm disease risk is high; stick with your pet's medications

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Dr. Michael J. Watts

Dr. Michael J. Watts, a Culpeper veterinarian, has written more than 800 Star-Exponent columns over the past 16 years.

MY friend’s dog was on the same kind of heartworm medicine that I give to my dog. Her dog died last week in the emergency clinic from a disease called IMHA. The emergency doctor told her that it could have been related to the heartworm medicine she gave two days before or the Lyme disease vaccine that was given a week earlier. Should I change types of heartworm medication?

Please do not change heartworm medications, or worse yet stop giving heartworm preventives, because of this story. IMHA, or immune mediated hemolytic anemia, is an immune system malfunction where the body attacks and destroys its own red blood cells.

We do not understand enough about the immune system to know for sure why it occasionally malfunctions. We do know that there is a genetic component of the IMHA disease. Certain breeds and certain lines can carry the gene. This is the only certain cause in all cases. But we also know that in susceptible animals any stimulation of the immune system can trigger an attack.

The immune system is stimulated by thousands of different challenges every day. Bacteria in the mouth or skin stimulate the immune system. Viruses and pollens in the air stimulate it. Chemicals and detergents in the environment cause immune stimulation. Bug bites and protein in food can set off an immune reaction.

Vaccines are designed to stimulate an immune reaction, so they indeed can trigger a crisis in patients with IMHA. Any medication has the potential to stimulate the immune system if an animal is sensitive to it. When a patient with IMHA has a crisis event, it is usually impossible to tell exactly what set it off—unless it is associated with a vaccine or known insect bite.

While the details will have to wait for a future column, I also believe strongly in vaccination for Lyme disease for almost all dogs. I recommend the newer recombinant-DNA vaccines that are very gentle on the immune system compared to the older “whole cell” products that are more likely to cause a negative reaction.

A pet with a history or family history of IMHA should probably limit vaccines as much as possible. Talk to your veterinarian about the most appropriate program for your individual pet.

Heartworm medication is frequently blamed for IMHA reactions by pet owners, but there is no scientific evidence that the preventives currently on the market stimulate the immune system any more than other medications and chemicals pets are exposed to. It turns out that roughly one in five dogs having an IMHA crisis had a heartworm preventive in the past week.

Many people take the easy answer and suggest this is evidence of a link. But correlation does not mean causation. The same percentage of dogs who get hit by cars also have had a heartworm pill in the past week. Why? Because most dogs get these medicines once a month, not because the pills cause car accidents!

The main point here is that, unless your pet is directly related to your friend’s, you should not change your pet’s regimen based on her unfortunate circumstances. The risk of heartworm disease is very high in Virginia. Four to six thousand dogs are diagnosed with the disease every year in our state.

The preventives also reduce the incidence of intestinal parasites, which infect as many as one in three dogs and pose a very real risk to human health. Almost every dog in Virginia should receive a dose monthly.

We also see more and more Lyme disease every year. I see dogs get sick and even die from Lyme disease much, much more frequently than I see IMHA.

Dr. Michael J. Watts owns Clevengers Corner Veterinary Care in Amissville.

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