Heartworm disease, more prevalent in dogs than cats, have few causes and symptoms and can be fatal if untreated. Complications can be avoided with preventive medication and simple precautions.
Heartworm, or Dirofilaria immitis, is a potentially dangerous parasitic worm spread by mosquitoes. Dogs are the most common hosts for heartworm, although it is also found in cats and other animals, including humans. It commonly lives in the right side of the heart as well as the lungs and the pulmonary arteries. In cats, heartworm is usually only found in the lungs. Cats are initially more resistant to heartworm than dogs. Outdoor male pets are much more likely to be infected than indoor pets.
The life cycle of a heartworm in a dog is six to seven months; in a cat it is eight months. Larvae from an infected female mosquito are deposited in the skin usually where the coat is thinnest. They burrow into the animal and change in form, moving into the veins and eventually the heart. In three to four months adult worms emerge. They can survive for about five years in a dog's heart and reach lengths of twelve inches. In cats, worms are smaller and live for only two or three years. Male and female worms mate and produce millions of offspring called microfilaria that live in the small blood vessels of up to 90% of infected dogs for as long as seven years, causing lung and liver problems from blocked blood flow. Single-sex heartworm infections are more common in cats; microfilaria are seen in less than 20%, and they are only present for about a month. The microfilaria are then swallowed by biting mosquitoes, and develop into infective larvae in the mosquito in ten to forty-eight days, depending on the climate. The larvae move into the mouth parts of the mosquito, where they can infect a new host--a cat or dog--when the mosquito bites the animal.
The disease may be advanced by the time an animal shows signs of infection. It is diagnosed mostly in three-year-old to eight-year-old dogs; but in some regions, dogs as young as one-year-old can be infected. Cats can harbor a heartworm infection at any age from nine months to seventeen years. Some cats seem to be able to get rid of the infection spontaneously. Dogs have been found to have as many as 40 to 250 heartworms; cats rarely have more than ten, and usually only one or two. The severity of the disease depends on the number of worms, their location, the duration of infection, and the dog's immune response. If there are only a few worms present, there may be no signs of heartworm disease. In cats, just a few worms can produce fatal illness, and sometimes infected cats die suddenly with no time for diagnosis or treatment. There may be no symptoms at all, or they may exhibit many of the same signs of infection as dogs. These include cough, shortness of breath, fainting after exercise, tiring easily, weight loss and loss of appetite, and listlessness and nervousness. There may be anemia, jaundice, poor coat condition, swelling of the abdomen, and bloody sputum and stool. Labored breathing at rest, prominent ribs, and chest bulging are signs of progressive disease. In advanced stages, heart failure and pulmonary clotting can cause collapse and death. Cats may also suffer from convulsions, diarrhea and vomiting, rapid heartbeat, and even blindness.
For dogs, an antigen detection test is widely used, although it is not considered useful for cats because it is not sensitive enough. Another type of blood test looks for microfilaria, but it is more commonly used in dogs because the presence of microfilaria in cats is so transient. Antibody detection tests are more accurate in detecting the presence of heartworm in cats. Veterinarians can also perform a quick blood smear in their office to determine the presence of microfilaria. If they are found in the blood, then they are also in the heart.
However, none of the heartworm tests are foolproof, for a number of reasons. To complicate matters, a harmless worm that lives under the skin called Dipetalonema reconditum is sometimes mistaken for heartworm because it also releases microfilaria into the bloodstream.
Diagnosis of heartworm infection is more complex in cats than in dogs. But in both cats and dogs, it depends on a good patient history, the presence of various symptoms, and the use of diagnostic procedures such as radiology, angiography and echocardiography, microfilarial detection, and various laboratory tests. Blood counts and tests for kidney and liver function may also indicate the presence of heartworms.
Heartworm treatment is complex and often dangerous. The animal must be healthy enough to begin with, so any organ problems should be treated first. Treatment of severe heartworm disease can lead to complications and death. In cats, the choice is to let the adult worms die without treatment over a period of years--during which time there is a risk that the cat may become quite ill and even die--or to get rid of the worms right away, but end up with serious complications from all the worms dying at once.
In dogs, two treatment steps are involved. The first is to kill the adult worms (adulticide therapy) using one of two very potent arsenic compounds administered intravenously or intramuscularly: thiacetarsamide sodium or melarsomine dihydrochloride. If they are effective, there will be clotting of dead worms in the circulation, as well as fever, vomiting, loss of appetite, jaundice, cough, and expectoration of blood. The dog must not be allowed to exercise for two weeks following treatment because of the clotting. Cats can also be treated intravenously. The risk of complications in the cat following adulticide treatment has been found to be greater than in the dog.
Although heartworms in cats may not live as long as in dogs, severe symptoms and even death can still occur. For this reason, the antigen test should be repeated twelve weeks after a cat's adulticide therapy to determine if all heartworms have been killed.
Dogs require a second step to get rid of heartworm infection: filaricide therapy. Its purpose is to kill the microfilaria three to six weeks after the adulticide therapy. Two different drugs are available for filaricide therapy: ivermectin and milbemycin. Dogs treated with these drugs are hospitalized following treatment to guard against adverse reactions, including coma and death. Collies and Collie mixes should never be treated with ivermectin because of potential severe adverse reactions. A third drug, levamisole, can be administered orally at home, but it also has significant side effects. Three weeks after filaricide treatment, a blood test is performed. If negative, the dog is ready to start a heartworm preventive program.
Despite the toxicity and side effects of the above drugs, more then 95% of dogs with heartworms are treated successfully. In cats, the risk of adverse side effects is higher.
Heartworms are sometimes surgically removed, but this method is used only for critically ill dogs; in cats, it may be a safer choice than adulticide treatment. Sometimes the animal is so ill that treatment for organ damage is the only alternative.
Dogs and cats six months or older should be tested for heartworm before a prevention program is begun, and the tests repeated every six months or yearly, depending on the prevalence of heartworm in your area. Consult your veterinarian about this.
A number of dog heartworm preventives are on the market, and a few are now available for cats. In endemic areas where dogs serve as a reservoir for mosquitoes, cats should also be given preventive medicine. Even though heartworm disease is often self-limiting in cats, they can still suffer from heartworm-induced lung problems.
DEC (diethylcarbamazine) tablets are given on a daily basis to dogs with no microfilaria. Side effects include headaches, malaise, nausea, and weakness, but in general DEC is considered safe and reliable.
Ivermectin is the active ingredient in once-a-month heartworm preventives for dogs and cats, and they do not have to be heartworm-free to start taking it. However, severe adverse reactions have been reported in Collies and Collie mixes as well as other dogs. Milder reactions are stomach and intestinal upset, irritability, and stiffness.
Newer heartworm preventives can also control or prevent fleas, ear mites, hookworms, roundworms, and whipworms.
Although heartworm preventive medicine is very effective, it also carries significant risks. The American Veterinary Medical Association has found that 65% of all reported drug reactions and 48% of all reported deaths in pets are caused by heartworm preventives.
Heartworm is found in all fifty states, but the highest infection rates (up to 45%) are found in the coastal regions from Texas to New Jersey and along the Mississippi River and its tributaries. If you live in these areas, it's a good idea to keep your dogs and cats indoors in the late afternoons and evenings, when mosquitoes are more active.
If you live in an area where mosquitoes are present all year, start your puppy or kitten on a preventive program not later than twelve weeks of age, and, continue for its lifetime. If the mosquitoes are seasonal, start the drug one month before mosquito season and continue until two months after the first frost.
It is known that wild animals are resistant to heartworm, and suffer at most only light infestations before becoming immune. Also, up to 50% of dogs in areas with heavy heartworm infestation become immune to microfilaria after being infected, and cannot pass it on to other dogs. Most dogs do not acquire more heartworms after being infected by a few, even if infective mosquitoes continue to bite them. Therefore the extent of infestation is limited. Alternative practitioners believe that, although heartworm infection is a serious medical problem best treated with the drugs mentioned above, the health and natural resistance of an animal is important in preventing heartworm in the first place. They recommend a diet of fresh raw foods with garlic and yeast to help repel mosquitoes. Regular exercise, the limited use of drugs, and the avoidance of commercial flea products will also help prevent heartworm. Herbs, homeopathy, and acupuncture can treat many of the symptoms caused by heartworm, and a natural insect repellent consisting of eucalyptus oil and water can be efficacious. A homeopathic heartworm preventive is still in the early stages of development.