Thank God it’s Spring. It’s such a lovely season, and I don’t know anyone who isn’t relieved and grateful for its arrival. Bugs will arrive too, so April is a good month to be sure that your pets are protected against heartworms. Heartworms are circulatory parasites. They don’t live in the gastrointestinal tract as do other familiar worms. As the name implies, they colonize the heart and great vessels of the lungs. Their young microfilaria, swim freely in the bloodstream. Although previously thought to be a disease of dogs, it is now known that heartworms can live in other species, including man and cats. The signs of heartworm disease (HWD) differ according to the species affected.
Heartworms are transmitted by mosquitoes. If a heartworm positive mosquito bites a dog, then microscopic larvae penetrate the skin and begin their migration to the heart. They burrow through tissues, undergoing maturation through five larval stages, finally reaching adulthood six to eight months post inoculation. Larvae tunnel under layers in blood vessels until, upon reaching the great vessels in the chest, they emerge into the artery’s lumen and attach. Adult worms are approximately eight inches long. If an infection is bisexual, then the adults will reproduce, generating microfilaria (baby worms). These will grow and attach to the lining of more blood vessels and the cycle continues until arteries are clogged with worms. Because of their length, adult worms actually extend into the chambers of the heart. Depending on the number of mosquito bites, the prevalence of male and female adults and the ages of the parasites, the worm burden may be light or heavy and therefore, the disease may be mild or severe. Clinical signs run the gamut from mild exercise intolerance to coughing and difficulty breathing. So in the dog, it’s the obstruction of blood flow from the physical presence of worms which causes illness from heartworm disease.
In the cat, heartworm disease is an immune problem. Cats are not the appropriate hosts for Dirofilaria, the causative agent of HWD, and so the parasite does not know where to migrate in the cat’s body. Feline heartworms undergo tissue migration, but often do not make it to the heart and lungs. Occasionally they go astray, causing localized inflammation but more often are attacked by a cat’s immune defenses. This sets up a chain of reactions which culminate in damage to blood vessels and a heightened immune response. Basically, the immune system reacts badly to the presence of the parasite. Cats can vomit, cough or collapse from heartworm disease. Sudden death is not uncommon in cats with HWD.
Heartworm disease is not treatable in cats. Veterinarians simply must try to control symptoms while waiting for the worms to die. In the cat, this can take up to two years. Dogs can be given a series of injections to kill adult worms, then one month later, a follow up shot to kill circulating microfilaria. Because the therapeutic level of adulticide medicine is not much different from the toxic level, vets much prefer to prevent HWD than to treat it.
Hence, in both species, the emphasis with heartworm disease is prevention. Dogs have long been put on monthly oral dewormers, but now some spot-on topical flea control products include heartworms in their spectrum of action. It is now recommended to put cats on monthly dewormers. As with dogs, the preventative can be administered as a chewable treat or applied topically. Either way, please be sure to put both cats and dogs on heartworm prevention year round. Since mosquitoes are the vectors for heartworms, and this has been such a rainy year, be especially vigilant to keep your pets protected.