Mosquitoes are ubiquitous creatures, and are extremely adept at finding ways to enter your home. This means that all cats and dogs are at risk of being bitten by a mosquito, which can transmit heartworms that can cause life-threatening consequences for your pet. Our team at Willow Wood Animal Hospital wants to provide information about these parasites to ensure your pet is protected.
Heartworm transmission in pets
Cats and dogs aren’t the only animals susceptible to heartworm disease. Wild canids, such as foxes, coyotes, wolves, and racoons, can also harbor the parasites. When a mosquito takes a blood meal from any infected pet or wild animal, they can ingest microfilariae (i.e., baby heartworms). After a few weeks inside the mosquito, the heartworms can be transmitted to another pet or wild animal when the mosquito takes a blood meal.
Heartworm effects in pets
Heartworms can parasitize dogs and cats, but they affect them differently, because dogs are natural hosts for the parasites, while cats are atypical hosts.
- Dogs — Once the heartworm larvae have reached the appropriate maturity level, they migrate to the blood vessels that supply the dog’s lungs. There, they can develop to adulthood, mate, and reproduce. Numerous worms can cause infection, and hundreds of worms may be found in a single dog. The larvae and the adult worms cause an inflammatory response in the lung vasculature that leads to scarring and thickening of the vessels over time. This makes the heart work harder to pump blood through this area of resistance, and eventually the heart is unable to effectively distribute blood throughout the body, leading to heart failure. Dogs typically don’t exhibit signs in the early disease stages, but as more damage occurs, signs include a persistent soft cough, lethargy, exercise intolerance, and fluid accumulation in their abdomen or chest. If the number of worms in the heart is large enough, they can block blood flow through the heart, a condition called caval syndrome, which is usually fatal. Prompt surgical removal of the worms may save the dog, but the procedure is not always successful.
- Cats — Heartworm larvae reach the cat’s lung vasculature about 75 to 90 days after infection, and the cat’s immune system reacts by mounting a strong inflammatory response that causes severe respiratory disease, resulting in difficulty breathing, wheezing, coughing, and vomiting. This condition is called heartworm associated respiratory disease (HARD). In most feline heartworm disease cases, the worms do not mature to adulthood, but one or two worms may mature fully in some cats. Since the cat’s heart is so small, only one or two worms are enough to cause a heart blockage, which can result in sudden death. The worms’ presence in the heart also increases the likelihood of clot formation. If a clot forms and travels to the base of the aorta where the vessel branches to supply the hindlimbs, a saddle thrombus can occur, resulting in sudden paralysis of one or both hind limbs.
Heartworm diagnosis in pets
Different diagnostic tests are available to test for heartworm disease in pets.
- Microfilariae testing — A direct blood smear can be evaluated under a microscope to detect microfilariae, but this method detects only infections that have about 50 microfilariae per mL of blood. The Difil and Knott’s tests are techniques that concentrate the microfilariae in the blood to make detection more accurate, but these techniques miss infected pets who do not have circulating microfilariae.
- Antigen testing — This diagnostic test is most commonly used to screen dogs for heartworm disease. They should be tested once a year, since administering heartworm medication to an infected dog can have detrimental consequences. The technology involved allows for detection of adult female heartworms, and the test is extremely sensitive, capable of detecting single worm infections. Heartworm antigen typically appears in the pet’s bloodstream about five to six months post infection.
- Antibody testing — Antibody tests detect the pet’s immune response to the parasites. These tests are especially important when diagnosing cats, since they rarely have adult worms or circulating microfilariae. Antibody tests can detect larval infections with male or female worms as early as two months post infection.
- X-ray and ultrasound — Chest X-rays and an ultrasound of your pet’s heart may also be recommended if heartworm disease is suspected or diagnosed.
Heartworm treatment in pets
No heartworm disease treatment is available for cats, so their condition must be managed using supportive care and anti-inflammatories to address the severe inflammation. While a treatment is available for heartworm disease in dogs, the process is dangerous, and requires careful management to avoid harmful side effects. Your dog’s condition is first stabilized to ensure they are healthy enough to proceed with the treatment protocol, and their activity is severely restricted, since exertion can exacerbate the damage the heartworms cause to the heart and lungs. Once treatment is initiated, your dog will receive a series of injections to gradually kill the adult worms and larval stages. Your pet still may have life-long complications caused by the parasites’ damage, despite the infection being cleared.
Year-round heartworm prevention medication is the best way to ensure your pet isn’t affected by these dangerous parasites. If your pet needs a heartworm test, contact our American Animal Hospital Association-accredited team at Willow Wood Animal Hospital to schedule an appointment.