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Thursday, March 21, 2013

NO Immunity To Heartworms

Sometimes I just don't know where misinformation comes from.  Especially when it is so far from the truth.

Recently a client brought a new dog to us that they had adopted from a local rescue group.  That group had an information sheet they sent home with all new pet parents.  That's a good idea, but it contained the following statement:   "A puppy has a natural immunity from its mother up to 6 months of age for heartworms."


This is absolutely, positively false.  And it bothers me that an animal rescue group would promote such falsehoods, essentially telling people that they didn't need to start their dogs on heartworm prevention until they were at least six months old.  Sadly I've had a few clients come in from time to time that were told by other people to wait until six months old before using prevention.


Let's briefly review heartworm disease and prevention.  Heartworm larvae are transmitted to a pet by a bite from an infected mosquito.  If the dog is not regularly on preventative medication the larvae will grow and develop into adults within the heart.  Most preventatives are monthly tablets or topicals and need to be given year-round for complete protection.  Dogs are extremely susceptible to heartworm disease and have no natural immunity whatsoever.  Even if they develop antibodies against heartworms, that typically isn't enough to fight them off.  Additionally, mosquitoes do not discriminate based on age.  They will bite a one-day old puppy just a readily as a 10 year old dog.  

A mother dog will pass on some immunity through the milk, granting temporary immunity to the puppies.  However, the mother must first have that immunity herself, so there is none passed on to prevent heartworms.  If a mother is vaccinated properly some of her antibodies will be given to the puppies.  However, those antibodies start to be cleared out of the puppy's body somewhere between six and 16 weeks old.  By four months old there are no maternal antibodies remaining, and they usually decrease even sooner.  We have to give multiple vaccines over a period of time to give the puppy its own protection after the mother's antibodies are gone.  So even if there were antibodies to heartworms passed on from the mother, they certainly wouldn't last six months.  

Part of the "six month" thing may come from the timing of testing.  Heartworms have to be in a dog's body for a minimum of 6-7 months before we can detect them.  The absolute youngest a dog should or can be tested is six months old.  Honestly, parasitologists say that a better testing age is between seven and ten months old.  Any younger than that and we will get a negative test, even if there are heartworms in the dog.  Hearing this recommendation from experts on heartworm disease is also a strong indication that heartworms certainly can be in a dog less than six months old.  I've personally seen dogs test positive at nine months old, meaning that they were infected at younger than three months old.  

So what do you do?  Start your dog on heartworm prevention around 6-8 weeks old.  Never take them off. Have them tested annually.  

I emailed the rescue group, trying to correct their misinformation.  It shouldn't be shocking that I never heard back from them, even though I offered to discuss it further.  So consider this my little way of trying to spread the right information.  Be sure to pass this on to anyone who thinks otherwise.


  1. Do you ever call the rescues to let them know they have bad information? It's amazing how much "urban legend" gets passed around when it comes to animals.


  2. Is this a rescue importing dogs from the islands? Apparently, over 50% of those dogs are heartworm positive. Some of these rescues tell the new owners the dogs tested negative prior to being imported to the U.S., which may be true if the dog is <6 months old. The new owners are then stuck with a huge veterinary bill for heartworm treatment. This has happened to three different clients of mine, who adopted from three different rescues.

  3. Ok, this is a little confusing to me as I was told by my vet that dogs need to be heart worm tested BEFORE they start any prevention because if they do have heart worms, the heart worm meds can actually be dangerous.

  4. This is a local rescue group from a small town, so they don't deal with international pets. I did email them, but never heard back.

    Yes, there is a slight chance of a reaction to heartworm prevention if a dog is positive for heartworms. That is one of the reasons we test prior to prescribing prevention, something I agree with and also do. However, since we can't detect heartworms in a dog less than 6-7 months old, and they can become infected prior to that age, we don't want to withhold prevention and potentially result in an infection. We only do prevention without testing in young dogs.

  5. Misinformation seems to be rampent in the pet world. No one ever told me that a cat could become infected with heartworms, not one of the single vets who had treated him prior finding out he was infected ever mentioned using a heartworm preventitve for my beloved Mokes. Despite the heavy infestation his heart was carrying, Mokes survived, then tragically died two years later of COPD in its earliest stage. Another condition [COPD] I was told held very few treatment options available to felines.

  6. My vet told me our 4 month old puppy couldn't have ADULT heartworms and therefore didn't need the test. She gave us heart worm prevention to start him on, which she said would get rid of any larvae he might have as well as prevent new infections(though, in his case, infection was unlikely since he was born in January and we got him in March, which in our part of the world is still way too cold for mosquitoes). Did she tell us something wrong?

  7. Your vet is correct, M.K., and that may be where some of the confusion lies. If you recall in the original blog post it takes 6-7 months from infection until we get adult heartworms that can be detected. So a 4 month old dog simply can't have adult worms and won't come up positive on a test. However, they can still be infected at that age, which means at 10 months old they could test positive, so prevention starting at 6-8 weeks old is vitally important.


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