Facts Heartworm is really not a disease, but an infection of Dirofilaria immitis, a parasitic round worm. While the name implies the worm resides in the heart, it actually starts out in the pulmonary arteries – the arteries that connect the lungs to the heart – and does massive damage to the lung tissue and vessels. Adult heartworms, which can grow to be up to 15 inches long or more, may migrate to the chambers on the right side of the heart, the vena cava, and the liver. These worms can live up to eight years inside a dog, and can be fatal if not diagnosed and treated immediately. Even with treatment, some dogs still die. These parasites irritate the lining of the dog’s blood vessels, causing injury. They obstruct the blood flow, which in turn causes pulmonary hypertension, and this can progress to thromboembolisms. Heartworms can also cause arterial inflammation, edema, autoimmune or hypersensitivity reactions, congestive heart failure, kidney damage, liver disease, irreversible fibrosis of the lungs, and Caval syndrome. When the heartworms die, they can physically block or mechanically interfere with the function and beating of the heart.

Causes Female mosquitoes are essential to the lifecycle of this parasite – without them, the parasites would never develop into larvae that can infect host animals. A mosquito becomes infected when it feeds on an animal that has microfilaria in its blood. Microfilaria are the immature version of the heartworm parasite. After they infect the mosquito, they go through several developmental stages, called L1, L2, and L3. At a point in L3, about one to two and one-half weeks after infection of the mosquito, the larvae are infective to dogs when the mosquito feeds. The larvae go from the mosquito’s mouth into the bite on the dog, and then burrow into the dog’s skin and eventually make their way to the dog’s bloodstream. They take up residence in the dog’s pulmonary arteries and continue to mature into L4 and then L5 (adults) after about six months. During this time, the dog will exhibit no signs of infection. It is only once the parasites reach maturity and begin to reproduce and release microfilaria – one mature female Dirofilaria immitis can produce up to 5000 microfilaria in just one day.

Prevention Prevention is key, because not all dogs survive treatment. Some areas of the world are affected more than others, so speak to your vet if you are not sure. In areas where heartworm is endemic, dogs should be placed on a preventative treatment plan as pups (at about 6-8 weeks of age) and kept on it for life, year-round if necessary. Before a dog receives a preventative treatment, however, it must first be tested for heartworm. Most vets will recommend you have your dog tested yearly, to be safe. Preventative treatments are easy to administer: they are either orally given or applied as liquid topically, between the dog’s shoulder blades. Most preventative treatments also prevent other parasites, like fleas, ticks, hookworms, roundworms, and whipworms. Non-chemical ways to prevent heartworm include bringing your dog in before the mosquitoes come out to feed, avoiding standing water where mosquitoes breed, and spraying the yard or area where your dog plays. These are not guarantees that your dog will not become infected, but can reduce the likelihood.

Cases of heartworm in dogs are separated into four classes:

  • Class 1, where the dog has very few or no symptoms. This occurs when the infestation is in its early stages, the dog is infected with just male or female worms and so they cannot reproduce, or the dog has been treated with a preventative that killed off the microfilaria but left the adults alive.
  • Class 2, where the dog has moderate signs of cardiac and respiratory distress.
  • Class 3, where the dog has severe signs of cardiac and respiratory distress.
  • Class 4, where the dog has Caval syndrome (also called superior vena cava syndrome), where the superior vena cava is obstructed by the parasites.

Dogs infected with heartworm may suffer from one or more of the following symptoms: fainting, abdominal distension, exercise intolerance, low energy, weight loss, abnormal jugular vein pulsation, collapse, respiratory distress, rapid breathing, enlarged spleen and/or liver, and sudden death.

Dogs at Risk Any dog that lives in an endemic area is at risk. Heartworm can strike any dog, young or old, male or female, and of any breed, although males tend to be infected about four times more often. Class 4 seems to occur more often in outdoor male dogs between the age of three and six in the spring/early summer months. Dogs used as hunting companions, or ones used as outdoor workers (herders) are at higher risk because they are outside more often.

Testing and Diagnosis Dogs are usually diagnosed with heartworm after a yearly check-up blood work test comes back positive. Others are diagnosed when their pre-treatment blood test comes back positive. If a dog shows any symptoms, it should receive a full blood workup, urinalysis, and physical exam. Chest x-rays can show the severity of the infection. The vet may also order or perform an electrocardiograph if it is suspected that the dog has an advanced case. The simplest test is a basic blood test that checks for the antigen produced by adult female heartworms. It is called ELISA (enzyme-linked immunosorbent assay and immunochromatographic test) and it is very sensitive. It will sometimes result in a false negative if the dog is infected by only males, or if there are very few female adult heartworms. Once a dog tests positive on the ELISA test, another test shows the concentration of microfilaria in the dog’s bloodstream. If this test shows positive, a positive diagnosis is made and treatment should begin. Even when this test shows negative, treatment should begin because up to one-quarter of all infected dogs do not have microfilaria in their bloodstream.

Treatment and Prognosis The goals of treatment are to kill off any adult or microfilaria in the dog, resolve complications caused by said parasites, and to prevent future infections. These treatment goals, particularly killing off the worms, can be very difficult and dangerous. Drug reactions, side effects, and where the living/dead worms end up all impact the success of treatment. Treatment depends on the severity of the infection. Any dog with severe complications will be hospitalized, placed on bed rest, and given oxygen, aspirin to prevent thromboembolisms, and steroids to reduce inflammation. The dog’s health and age, other medical conditions, owner’s financial position, and opinions towards treatment will all affect what happens next. Some heartworm medications have many side effects like liver damage, diarrhea, vomiting, kidney failure, and death. Older dogs are especially susceptible to adverse consequences. For worms lodged in the heart chamber and/or vena cava, surgery is an option – the surgeon goes down the jugular vein and pulls the worms out one by one with specialized instruments. If a dog is diagnosed early on in the infection, and have not yet started to show symptoms, their prognosis is good. A dog that suffers from heart failure, kidney damage, or liver disease, with a moderate to severe infection has a fair to guarded prognosis. A dog with Caval syndrome has a poor to grave prognosis.