Heartworm Tests in Dogs and Cats
Short answer
Heartworm is checked with antigen kits that find proteins from adult female worms, and with microscope or RT-PCR tests for the tiny young worms in blood. Cats are tested differently from dogs.
By Pashupatinath Vet Lab Share
Heartworm tests look for the worm in different ways. Antigen kits find proteins from adult female worms, while a microscope test or RT-PCR (real-time PCR) looks for the tiny young worms in the blood. Dogs and cats are tested differently, and a negative result does not always rule heartworm out.
How does a pet get heartworm?
Heartworm is a long, thin worm called Dirofilaria immitis. A mosquito that bites an infected animal picks up tiny young worms, called microfilariae, from its blood. When that mosquito bites another dog or cat, it passes on larvae that slowly grow into adult worms. The adults settle in the heart and in the blood vessels that run to the lungs.
Many pets show nothing in the early stage. Later a dog may cough, tire quickly on walks or breathe with effort. A cat may cough, breathe fast or vomit, and some cats collapse suddenly. Any pet with breathing trouble needs a vet quickly. Ask your vet how to protect your pet from mosquito-borne worms where you live, and mention any travel, because the risk differs from place to place.
How does an antigen kit work in dogs?
An antigen is a protein that belongs to a germ or worm. The Canine Heartworm Antigen (Rapid Test) looks in a dog's serum or blood for antigen released by adult female heartworms. It is quick and simple, which is why vets often start with it.
The kit needs mature female worms to work. A dog infected only recently may test negative because the worms have not grown up yet, so your vet may repeat the kit later or add another test. A positive result is not a full answer either. The vet will want to know how the heart and lungs are doing before planning anything.
Why are cats tested differently?
A cat is not a small dog. A cat usually carries only a few heartworms, and microfilariae are uncommon in a cat's blood. With so few adult worms, an antigen kit can miss an infection that is really there.
The Feline Heartworm Ag (Rapid Test) is made for cats and uses blood or serum. A positive result is taken seriously, but a negative one does not close the matter. Because the picture is less clear in cats, a vet usually weighs the result against the cat's signs and may add pictures of the chest and heart.
What do the microfilaria smear and RT-PCR add?
The Microfilaria Detection (Knott's Method) hunts for the young worms directly. Fresh blood in an EDTA tube is treated so that the red cells break up, then spun so that any larvae settle at the bottom, and the sediment is checked under a microscope.
Not every infected dog has microfilariae in its blood, so a negative smear does not rule out adult worms. Larvae of some other worms can also look alike under the microscope, which is why a vet sometimes wants a second method.
The Dirofilaria immitis (Heartworm) RT-PCR uses EDTA blood. RT-PCR copies tiny traces of genetic material so that they can be detected, and here it is mainly used to confirm heartworm or to identify microfilariae.
What does a negative result mean?
A negative result means the test did not find what it looks for. It does not always mean the pet is free of heartworm. Timing, a low number of worms and the way cats are infected can all lead to a negative kit. If your pet still has a cough or breathing trouble, the vet may repeat the test later, add a microfilaria test or RT-PCR, or look at the heart and lungs in other ways.
A registered veterinarian reads every result together with the pet's signs and history. Please do not start any heartworm medicine on your own. Treatment is the vet's decision, and it depends on careful checks first.
Read next
Cat owners can read Toxoplasma and Bartonella Tests for Cats to see how antibody tests and RT-PCR differ for two germs that can also reach people. For readers who also keep farm animals, Copper, Zinc and Selenium: Trace Mineral Tests for Dairy Herds shows why some samples need special tubes.
Testing in Bhopal
Call or message +91 88175 01118 to book a heartworm check. If you live in Bhopal, the blood sample can be collected free from your door or clinic. From other districts, a courier, a bus parcel or your own vet can bring it to C-200, C Sector, Shahpura, and the lab confirms the method, sample needs and reporting time when you book.
This article is general information and does not replace a veterinarian’s advice. Please talk to a registered veterinarian about your animal’s signs, tests and treatment.