Heartworm testing in dogs – Pet tip 121

Late spring and early summer is when many dogs get tested for heartworm by their veterinarians. Why this time of year? The organism that causes the disease, Dirofilaria immitis, is transmitted by mosquito bites. There are three life stages of the heartworm worth knowing. The first is the adult heartworm itself, which lives in the heart and pulmonary artery and causes clinical signs. Adults produce microfilariae (tiny baby worms) which live in peripheral blood. They are picked up by a mosquito that bites an infected animal, and inside that mosquito they mature to stage 3 larvae over the course of about 3 weeks during warm temperatures. An infective mosquito injects these stage 3 larvae into the dog’s blood as it bites. It takes 6-7 months for the larvae to develop into adult worms which shed a protein that is picked up on the heartworm test. So if the transmission season – the time of year when infective mosquitoes can bite – ends in October, it will take till late spring for a dog infected at the end of the season to show positive on a test.

Dogs are tested before being put on preventative medication over the summer season. Preventative medication is given once a month and its purpose is to destroy the stage 3 larvae before they have a chance to become adult worms, thus preventing heartworm disease in dogs bitten by infective mosquitoes. But if a dog has adult worms, giving it preventative medication carries the risk of destroying these worms too quickly. Fragments of worms can plug up blood vessels in the lungs, which is an emergency and can cause rapid death.

Testing for heartworm, as for any other infection, is not a black-and-white matter: a positive test result does not necessarily mean infection, while a negative test result does not always mean absence of infection. What does it mean if a dog tests positive on a heartworm test? The test used in most clinics tests for heartworm antigen – a protein produced by adult female worms. The most obvious answer is that the dog is carrying worms, at least some of which are female. This sounds like the worst case scenario, but the good news is that most dogs infected with heartworm do not show clinical signs – the worm burden is rarely high enough to damage a heart chamber or the artery running from the heart to the lungs, the two places where worms are usually found. Another possibility is that the worms that shed the protein have already been destroyed by the dog’s immune system, so the dog is in fact negative for disease even if it is positive on the test. Finally, the result could be a so-called false positive: a protein closely resembling the one shed by worms was picked up on the test. This can happen if the dog has a concurrent disease which produces various proteins some of which “fool” the test. Either way, a positive test result will be followed up by another test to find out if the dog is actually carrying worms. If it is, X-rays of the chest will be taken to see if there are changes in the heart and lungs – signs that the worms are in fact causing heartworm disease. Most importantly, your veterinarian will look for and ask you about clinical signs of heartworm disease in your dog: getting out of breath, coughing, becoming tired after short walks.

What about a negative result on a heartworm test? Recall that it takes 6-7 months for stage 3 larvae injected by a mosquito to develop into adults. If a dog has been tested during this time and the result is negative, it may be because the worms are not yet “old” enough to produce the protein picked up on the test. Another possibility is that the dog is carrying only male adult worms, or a very small number of worms who do not produce enough protein to trigger a positive test result. So if a negative test result does not always mean absence of worms, how seriously should we take it? The value of a heartworm test depends on how likely it is that your dog is infected. In most of Canada heartworm disease has a low prevalence, meaning that only a small percentage of dogs are infected. If a dog tested negative before its first preventative treatment, has been receiving this treatment regularly throughout the transmission season every year, lives in an area where heartworm disease is uncommon, and shows no signs of heartworm disease, yearly testing for heartworm is not informative since a positive test result is most likely to be false-positive. (Such testing may still be required before preventative medication is given, due to the risks of inadvertently and rapidly killing adult worms.) It makes sense, however, to test a dog that has either never been treated with preventative medication, or has lapsed on its monthly treatment, or has travelled to an area where heartworm disease is common. In that case, a positive test result is more likely to be a true positive.

By Veronica Gventsadze – Pets.ca writer