June 1st is the start of Heartworm season in southern Ontario, although it seems the mosquitos have been biting for months.
This is the time by which we have had enough warm days for the Heartworm larvae to develop to an infective stage and be carried by the mosquitos to our dogs. Traditionally, a Heartworm preventative is given once monthly throughout the summer months to kill off any Heartworm larvae that might have been transmitted from a mosquito bite into your dog’s blood, before the larvae can form into the stringy white worms that live and breed in the blood vessels around the heart. Any dog that is bitten by a mosquito is at risk and so it is generally recommended that all dogs who spend any amount of time outside be put on preventative medication throughout the summer months as part of a Heartworm prevention plan. The other part of that prevention plan is testing. It is recommended by most veterinarians and by the manufacturers of the Heartworm medications that each dog have a test prior to receiving the medications each year, to ensure that the pet is free from Heartworm disease.
But hold on, Doc; you just told me that medication prevented infection. Why do I need a test? This question is asked of veterinarians a lot, and so I thought I would address it here.
Like most tests in medicine, the testing for Heartworm disease is designed to let us know what is going on inside your dog before we see outward signs of the disease. You go to get your cholesterol level checked so that you can find out if it is high and do something about it before plaques build up in your blood vessels and stop blood flow. The same is true of the testing for Heartworm disease- we test to find out before signs develop. Although in severe infections we see dogs who are lethargic, coughing or having difficulty breathing, for the most part, dogs with early Heartworm infections don’t show outward signs.
The testing is aimed at pre-clinical infections. Also for dogs who have high numbers of Heartworm larvae in their bloodstreams, taking preventative medications can be dangerous as killing off large numbers of larvae can cause allergic type reactions in some pets. But the medications are (in the vast majority of cases) very safe and effective, even for heartworm positive dogs.
So why do testing then?
The real reason is that dogs with Heartworm disease need special treatment. The monthly preventatives do not kill adult Heartworms. If your dog has an infection, then he needs a different course of medication and some further diagnostics and monitoring. If we don’t ever test a dog, and simply keep giving preventatives every year assuming that they are doing their job, then it is very possible that worms are growing in the heart of a dog receiving his medication every month. These are the dogs we are looking to catch with our testing. We also see some variation in the start and end dates of Heartworm season as well as the dosing of medication in pets. If a spring is particularly early or hot, or if it stays warm late into the fall, this can change the Heartworm transmission risks. And even a single missed or delayed dose of medication can put a pet at risk of contracting this disease. For these reasons we suggest that at least once yearly we check your pet for exposure even if they have been receiving their medication. If you don’t think that any of these risk factors apply to you or your pet, discuss testing with your veterinary team and see if it is right for you.
One last point; Although it is Heartworm season, we also like to focus on whole-body wellness for your pets. If we are taking a blood sample, we also have the opportunity to check for tick-borne diseases like Lyme disease, and have a look at your pet’s organ function and blood cell counts with the same sample. Often we combine these tests with Heartworm testing in a Wellness panel and this can add value to the testing by giving us a picture of what is going on inside your pet and helping identify problems before they cause illness. For all of these reasons, we think that Heartworm testing is valuable to pets and their owners, but ultimately the question of whether you need to do a Heartworm test is up to you. We are always here to help.