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.