By Alan Green and Sophy Jesty
There are many diseases that we have successfully created effective prevention for. One of the more devastating and common diseases, especially here in the Lowcountry is heartworm disease. This month, our esteemed colleague, CVRC board certified cardiologist Dr. Sophie Jesty, describes the disease, its transmission and methods of prevention.
Heartworm disease is a parasitic infection seen in dogs much more commonly than in cats. Mosquitoes carry the immature or larval form of the worm, so animals are infected through a mosquito bite. Up to 20 percent of mosquitoes carry immature heartworms. This is why heartworm disease is much more common in the South, including the Lowcountry, even in indoor-only pets. In this region, more than 50 cases of heartworm disease are diagnosed each year in the average veterinary clinic. Heartworms cannot mature in people, so although humans get bitten by mosquitoes, we do not get the disease.
When a dog is infected by a mosquito, the immature worms eventually migrate into the arteries in the lungs. It takes six-to-nine months for the worms to reach adulthood inside the body. If the adult heartworms reproduce, the immature worms in the bloodstream (microfilariae) can be picked up by a mosquito feeding on the dog and the cycle starts again. Heartworm disease is not directly contagious between dogs (even if they mix blood) because some phases of heartworm maturation must occur within the mosquito. Nonetheless, a heartworm-positive dog puts other dogs at risk because mosquitos can carry the disease between dogs.
The adult heartworms, which can measure up to one inch in length, cause incredible damage to the blood vessels in the lungs and most of the symptoms seen with heartworm disease are because of lung disease. Common symptoms include fatigue with exercise, trouble breathing, cough and fainting. Nonspecific signs include decreased appetite and weight loss. Because the right side of the heart has to pump blood into the lungs, the increase in pressure in the lungs caused by the adult heartworms can also cause right-sided heart failure, which manifests as abdominal distention. At its worst, heartworm disease can cause total collapse of multiple organ systems and death of the animal.
Testing for heartworm disease starts with a couple of blood tests; one to look for adult heartworms (an antigen test) and one to look for the offspring of heartworms (microfilarial test). If your dog tests positive, additional testing will likely be recommended, such as chest X-rays and an echo (ultrasound of the heart). Treatment for heartworm disease requires a medication containing arsenic (Immiticide) and therefore treatment can be risky for the dog (although less risky than not treating a heartworm positive dog). If there are a large number of heartworms, they can readily be seen in the right side of the heart on echocardiography (ultrasound). In these cases, surgical removal of the heartworms through the jugular vein (a large vein in the neck) under general anesthesia is recommended.
Luckily for dog lovers, heartworm disease is highly preventable and therefore you should make sure your dog is on heartworm prophylaxis year round. There are many good heartworm preventative products, so speak with your veterinarian about which is best for your dog. Your dog should be tested for heartworm disease before starting preventative and yearly thereafter to ensure that they remain heartworm negative. There has been a concern in the last few years that heartworms are becoming resistant to our preventative medications, but this is actually unusual. Late or skipped doses of the preventative is the far more common reason for a dog “on heartworm preventative” testing positive for heartworms. So be sure to give your dogs the monthly preventative on time every single month! There is also an injectable form of the preventative that last six months; this may be used in adult dogs.