Heartworms are long, spaghetti-like worms that inhabit the pulmonary arteries and the right side of the heart. The worms can interfere with the heart's pumping action, irritate the heart and blood vessels, and lead to death if not treated.
Heartworm infection, which is caused by the parasite Dirofilaria immitis, affects dogs, cats, and wild members of the Canidae family, such as foxes and coyotes. It's also been diagnosed in horses, otters, orangutans, and even people.
Infected dogs can harbor one to 250 worms. Heartworms live in dogs for about five to seven years. The female worm produces immature worms called microfilariae, which circulate in the dog's blood, often causing inflammation in the various tissues they pass through.
A mosquito transmits heartworm microfilariae when it takes a blood meal from an infected dog. In the mosquito, the microfilariae mature into infective larvae in two to two and a half weeks. When the mosquito bites another dog, larvae enter the skin and eventually migrate to the heart and pulmonary arteries where they mature into adult heartworms.
Risk factors and detection
Heartworm infection affects dogs of all ages, but clinical signs typically appear in 3- to 8-year-old dogs. Outdoor dogs and those not on heartworm preventives are at increased risk.
Signs of heartworm infection include:
- a persistent cough
- breathing difficulty
- vomiting
- exercise intolerance
- fainting
- weight loss
- abdominal enlargement.
Call your veterinarian if you notice any signs of infection. Routine examinations and yearly heartworm screenings help your veterinarian detect and treat infection early. Your dog's doctor will perform a complete physical examination and diagnostic tests, including blood tests, X-rays, and an ultrasound, to detect infection and determine its severity. The good news is that many heartworm preventives protect dogs from infection.
Prevention
Your doctor can recommend a monthly heartworm preventive for dogs. The oral preventives include ivermectin (Heartgard), moxidectin (Proheart), and milbemycin (Interceptor and Sentinel). Selamectin (Revolution) is a new topical preventive. Another oral medication, diethlycarbamazine (Filaribits) or DEC, is administered daily. Dogs must test negative for microfilariae before taking DEC to avoid severe reactions.
Many dog owners choose more convenient, monthly preventives, rather than a daily dose of DEC.
Treatments
Before starting heartworm treatment, your veterinarian will conduct a complete evaluation to identify any heartworm-related complications, such as heart, kidney, or liver damage. This may involve blood and urine tests, X-rays, and an ultrasound. These diagnostics help your veterinarian evaluate treatment risks and choose the most appropriate therapy.
Treating dogs
When treating dogs, your veterinarian's first goal is to remove the adult heartworms with adulticide medications, usually melarsomine or thiacetarsamide. These drugs kill but don't remove the worms. Your dog's immune system will handle that task, which occurs primarily in the lungs as the worms decay. This gradual absorption process takes four to eight weeks.
Any activity that raises blood pressure, such as exercise or barking, could dislodge a mass of dead worms. This can result in a pulmonary thromboembolism (PTE), a condition in which the clot lodges in the blood vessels of the lungs. PTE can cause death, so it's critical that you severely restrict your dog's activities during heartworm treatment.
Veterinarians occasionally choose surgery to remove worm masses blocking the blood flow into the heart or to reduce large worm numbers before beginning adulticide treatment.
In the treatment's second stage, which usually occurs about three to four weeks after adulticide therapy, your veterinarian will use ivermectin or milbemycin to kill the remaining microfilariae circulating in the bloodstream. Finally, your dog will begin taking a monthly heartworm preventive.
Prognosis
Because heartworm disease can cause complications throughout the body—congestive heart failure and PTE—your dog may need additional treatments to address these problems.
Your dog has an excellent chance for full recovery when you and your veterinarian detect heartworm disease early. Prognosis is guarded for dogs with large worm burdens or those suffering additional complications, such as congestive heart failure or damage to the liver or kidneys. Your best bet: Practice heartworm prevention.
