Canine Heartworm Disease
What is heartworm disease?
Heartworm disease, as the name implies is caused by is a worm called Dirofilaria immitis. The heartworms are found in the right ventricle of the heart and the pulmonary arteries. The female worm may be up to a foot long; the male is about half that size. One dog may have dozens of worms. Heartworms live up to five years and during this time the females produce millions of larvae called microfilariae. These microfilariae live in the bloodstream, and may be concentrated in the spleen. The larvae go through a series of molts on their way to becoming an adult heartworm. One of the molts occurs in the salivary gland of the mosquito. Larvae cannot complete their entire life cycle in the dog; they must pass through a mosquito on their way to becoming an adult heartworm. Many species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog and ingests the microfilariae during a blood meal. The microfilariae develop in the mosquito into the infective stage. The microfilariae are now called infective larvae. At this stage of development they will grow to adulthood when they are passed to a dog. The infective larvae enter the bloodstream of the dog when the mosquito bites it. They grow to adulthood in two to three months and start reproducing, thereby completing the life cycle. It only takes one mosquito carrying larvae to infect a dog with this potentially fatal disease.
How common is heartworm disease?
Canine heartworm disease occurs worldwide. In the United States, it was once limited to the South. Due to our increased mobility, and people vacationing and wintering in infected areas the problem is now seen wherever there are mosquitoes.
My dog is never around other dogs; can it still get heartworms?
Yes, the disease is not spread directly from one dog to another. Remember, the mosquito is required for transmission. The mosquito bites an infected dog and picks up the larva. The larva molts into its infective stage and is then spread to the next dog the mosquito bites. Spread of the disease therefore coincides with the mosquito season. The longer the mosquito season, the greater incidence of heartworm disease in an area.
What effect do heartworms have on a dog?
Adult worms restrict the blood flow leaving the heart. They also interfere with the heart valves. The net effect is to reduce circulation to the vital organs. This leads to organ failure and ultimately death. Dogs infected with heartworms do not show signs right away. By the time symptoms develop, the disease is well advanced. The symptoms depend on the number of heartworms present, the duration of the infection, and the extent of damage to the heart, lungs, liver, and kidneys.
What are the signs of heartworm disease?
Most dogs with heartworm disease have a chronic, dry cough. You may also notice your pet has shortness of breath, weakness, and loss of stamina. The symptoms may be worse followingexertion or excitement. The diagnosis of heartworm disease needs to be made by your veterinarian.
How can my veterinarian tell if my dog has heartworm disease?
Most dogs with heartworm disease have a chronic, dry cough. You may also notice your pet has shortness of breath, weakness, and loss of stamina. The symptoms may be worse following exertion or excitement. The diagnosis of heartworm disease needs to be made by your veterinarian.
After examining your pet your veterinarian may find symptoms that are suggestive of heartworm disease. Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. Diagnosis of heartworm disease can be made by a blood test that can be run in the veterinarian’s office, or by a veterinary laboratory. One type of test looks for antigens (proteins) produced by the adult heartworm. This is the most accurate type of test. There can be false negative results though. This occurs when there are very few worms present. The test detects a protein produced by the female worm. If there are only males the test will be false negative. There needs to be several female worms present for the test to be positive.
Another type of blood test looks for microfilariae. These are the larval offspring of the adult worms. A blood sample is examined under the microscope for the presence of microfilariae. If microfilariae are seen, the test is positive. Approximately 20% of the dogs test negative even though they have heartworms because no larvae are present.
Complete blood counts and blood tests for kidney and liver function may suggest the presence of heartworms. X-rays of a dog with heartworms may show heart enlargement and changes in the pulmonary artery. X- rays may also reveal the condition of the other vital organs. This information allows your veterinarian to predict the possibility of complications related to treatment.
Is there treatment available?
Yes, heartworm disease is treated in two phases. After examining your dog and evaluating it’s lab work and X-rays your veterinarian can choose from several treatment options. If the problem is severe, it may be necessary to treat with antibiotics, prescription diets, and drugs to improve cardiac function prior to treatment for the heartworms. The first phase involves eliminating the adult heartworms. One method involves giving a drug called caparsolate. This compound contains arsenic, and can be quite toxic to some dogs. The goal is to give enough of the drug to kill the heartworms, but not harm the dog. Caparsolate treatment is widely used and can be safely given if the animal is healthy in all other respects. A newer drug called Immiticide is now available. This compound appears to be safer than caparsolate for some dogs. Both treatment options usually require the dog to be hospitalized for a few days. We see some dogs with advanced heartworm disease. This means that the heartworms have been present long enough to cause damage to the heart, lungs, blood vessels, kidneys, and liver. A few of these cases will be so far advanced that it will be safer to just treat the organ damage rather than risk treatment to kill the worms. Dogs in this condition are not likely to live more than a few weeks or months. The second phase involves riding the dog of the heartworm larvae. This is usually done six weeks after the pet has been treated for adult heartworms. The drugs used to kill the larvae are not nearly as dangerous as the compounds used to treat for the adult worms. The second phase is very important because a dog with heartworm larvae is the reservoir mosquitoes draw from to infect other dogs. After the second phase of treatment your veterinarian will check your dog to be sure all the adults and larvae have been eliminated.
What happens after treatment?
It is very important to let your dog rest after treatment. As the adult worms die they decompose and break up. They are pumped into to the lungs, where they lodge in the small blood vessels and are reabsorbed by the body. There is the potential for a blood clot to form while this is happening. If your dog is not kept quiet as possible the likelihood of complication increases. Heavily infected pets may cough for weeks following treatment. Your veterinarian will advise you as to what to expect following treatment. If you have any oncerns you should contact your veterinarian immediately.
After the treatment can my dog ever get heartworm disease again?
Yes, the treatment eliminates the current problem, but does nothing to prevent it from happening again. Your veterinarian can offer several very effective products to prevent heartworm disease. There are medications that can be given daily, and others that are given monthly to prevent canine heartworm disease. There is even a product that is applied topically that prevents many different parasites, including heartworms, from infecting your dog. Consult with your veterinarian to determine which is best for your pet.