Dr. Melinda Newton: Is your pet protected?
If there were a contest for the most hated organism in human history, the mosquito would at least make it to the finals.
The annoying buzzing mosquito harbinger of diseases such as malaria, yellow fever, and West Nile virus, has another feather in its cap — heartworm.
Heart disease is something we associate with growing older, not with mosquitoes. But, for our dog and cat companions, heart disease as a result of a parasitic infection is a risk every time a mosquito bites. Once infected, the worm completes a complex lifecycle in the furry mammal host. Eventually the adult worms reside in the chambers of the heart and the large blood vessels around the heart. At this advanced stage, even with treatment, the heart damage can be irreversible, resulting in chronic disease and death.
Despite the seriousness of this infection, heartworm infestation in the early stages is silent, causing no symptoms that can be seen at home or during a physical exam. Fortunately, a simple blood test that is available at most veterinary clinics can diagnose the infection before the parasite’s presence causes clinical heart disease.
Heartworms are one of the most misunderstood parasites in the veterinary clinic. Many people are surprised to hear that mosquitos can bite our dogs and cats, even through their furry coats. Indoor pets are at risk too. How many of us have employed our ninja skills to smash that pesky mosquito that just won’t leave us alone in the house? Both the American Heartworm Society and the FDA recommend year-round heartworm prevention for indoor and outdoor pets, even for those of us who live in areas with cold winters.
Many mysteries surrounding treatment and testing recommendations make sense when we consider the lifecycle of the heartworm. Heartworm is a contagious disease of dogs, but it cannot pass directly from dog to dog. First, a mosquito must bite an infected dog. As it slurps up its meal of blood, it also ingests the baby heartworms, called microfilariae, which are floating through the infected dog’s bloodstream. Once inside the mosquito, the microfilariae molt several times before migrating to the mosquito’s mouthparts, where it can pass into an uninfected dog or cat when the mosquito feeds.
Once transferred to the skin of a new host during a mosquito bite, the worms migrate through the tissues, continue to molt and grow, and eventually end up in the heart and large vessels. This is where female worms produce new microfilariae to circulate in the bloodstream, who wait for another mosquito to bite and slurp them up. This process, from infection by the mosquito to a mature adult, takes about 6-9 months.
Like most things in medicine, an ounce of prevention is worth a pound of cure. There are several effective heartworm preventatives that are available; however all of them have one important caveat. The preventatives only kill heartworms that are in the circulating bloodstream at the earliest stages of development — approximately 1-2 months after the mosquito first infects the host. Once the immature heartworms are older, a preventative will no longer kill them, and the patient must undergo a more risky treatment to eliminate the heartworm infection.
The tests available to detect heartworm disease can only detect adult worms six months post-infection. A heartworm detection test is usually required before going on prevention for heartworms, with an exception for dogs younger than six months of age. This exception exists because even if the puppy were infected on the first day of birth, the test would be negative until it was at least six months of age. Thus, puppies younger than six months of age are started on prevention without a test. Dogs older than six months are tested before starting prevention, but likewise, any infection that was acquired in the last six months cannot be detected, and will not be treated by the prevention. A second test performed at least six months after the start of prevention will detect any of these early infections acquired in the six months prior to starting prevention.
The heartworm’s preferred host is the dog. Cats, ferrets, and other furry mammals can get heartworm disease; however, the worms do not like these hosts as well as the dog, and the developing worms tend to die at higher rates in these less-favored hosts. Infected cats with adult worms can suffer from heart disease, or have disease associated with dying maturing worms in the lungs. Preventative medication is especially important for cats because there is no treatment for cats infected with the adult worms.
Treatment for dogs is expensive and carries significant risk, including death. Testing for heartworm disease in dogs, and using preventatives in dogs and cats year-round, can save a lot of worry when the mosquitos inevitably show up as uninvited house guests.
Melinda Newton, DVM is a veterinary practitioner and freelance writer, who works with AnimalSave in Grass Valley in their spay and neuter clinic, and assists with other community outreach activities. To schedule an appointment for your dog, cat, or rabbit please call 530-477-1706. AnimalSave offers heartworm and tick-borne disease testing for dogs, and heartworm preventatives for both dogs and cats.
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