Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart in dogs, cats and other species of mammals. Heartworm infection is found in all 50 states. In the past, the incidence of disease in Oregon has been low with some areas having a higher number of reported cases. However, with the influx of animals with potential heartworm disease from the Southern U.S. after hurricane damage, it is anticipated that we may see a rise in heartworm infection in the Northwest. Protecting your animal is important. We follow the guidelines of the American Heartworm Society. For more information please see the website for heartwormsociety.org.
Transmission of heartworms is by mosquitoes. An infected animal that has adult worms is the source of infection. Adult worms produce offspring that circulate in the blood, and are then transmitted to mosquitoes once they bite the infected animal. These offspring, microfilaria, undergo development to an infective larval stage within 14 days in the mosquito, and can then be transmitted to another animal when the infected mosquito bites again. The infective heartworm larvae in the mosquito's mouthparts enter through skin of the animal through the mosquito bite wound. In the dog, the larvae progress in their development to an adult form of the worm over a 7 month period, and live in the heart, lung and associated blood vessels, where they continue the life cycle and cause extensive injury. Cats are less susceptible to infection. In the cat the larvae molt as well but fewer worms survive to adulthood.
Clinical signs of infection depend on the number of adult heartworms present and level of activity of the pet. Dogs with low number of adult worms and are not exercised strenuously or those with an early infection may not show signs of heartworm disease. As the disease progresses one will see coughing, exercise intolerance, abnormal lung sounds, difficult breathing, liver enlargement, fainting, fluid accumulation in the abdomen, abnormal heart sounds, or death. Most cats have no symptoms or may exhibit signs, such as vomiting, coughing, open mouth breathing, lethargy, lack of appetite, or weight loss.
Diagnosis of heartworm disease is based on history, clinical signs, blood tests, x-rays, ultrasound, or necropsy. Screening blood tests will detect infections with one or more mature female heartworms that are at least 7 or 8 months old. Therefore, we do not test pets for heartworm until they are at least 7 months of age. Diagnosing heartworm disease in the cat is more difficult than in a dog. Blood tests are not as sensitive and will not detect male only infections. X-rays and ultrasound are the most useful tests.
Treatment of infected dogs is usually successful; however, patients with severe disease have a higher complication rate and may die. Medication is given to kill all adult and immature worms. Currently, there are no products to treat cats with heartworm infection and supportive care is given.
Prevention is much safer than treating the disease. Effective, safe monthly preventative medication protects your pet from heartworm, and also treats for roundworms and hookworms. The American Heartworm Society recommends year round administration of preventive, even in seasonal areas to make sure the medication has been given properly by the pet owner. Before starting medication, all pets should be tested for heartworms. Annual testing for heartworm is now highly recommended. Even though preventives are essentially 100% effective, animals on heartworm preventive occasionally test positive for heartworms. This is usually due to owner compliance failure, travel or relocation of the animal to an area of active heartworm transmission, or unknown prior infection. Annual testing gives you peace of mind that your pet is free of heartworms, and in cases where the animal is infected, it assures you of early diagnosis of infection and maximal benefits from heartworm disease treatment.