Q:  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.  An article on the internet left her believing that it could have been related to the heartworm medicine she gave two days before or a vaccine that was given a week earlier.  Should I change types of heartworm medication?               

               A:  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 disease.  Certain breeds and certain lines can carry the gene.  Genetics is the only certain cause in all cases.  However, 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.  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.

               Heartworm medication is frequently blamed for these reactions, but there is little to 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 eight dogs having an IMHA crisis had a heartworm preventive in the past week.  Without additional medical knowledge, many people suggest this is evidence of a link.  However, roughly the same percentage of dogs who get hit by cars have had a heartworm pill in the past week.  Do heartworm pills cause car accidents?  Of course not.  Most dogs get these medicines once a month, so any random group is likely to have a significant proportion dosed recently.  

               The question is could the heartworm preventive have triggered the IMHA event?  Could the vaccine have played a role?  The answer has to be yes in both cases.  However, the question for the rest of us should be did the medicine or the vaccine cause the IMHA.  The answer here is clearly no.  The disease is caused by defective DNA in the dog.  The disease is triggered by drugs, vaccines, insects, allergies, etc.   

               No reasonable person would blame peanuts for “causing” allergies or suggest nobody should eat a certain brand of peanut butter because an individual with a food allergy died after eating a peanut butter sandwich.  If a person has multiple sensitivities to food proteins, there are certain foods, like peanuts, that should be avoided.   If there are lines/breeds of dogs with known IMHA risks, or individuals with the disease, then the selection of drugs and vaccines should be made very carefully. 

               However, 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.