Heartworm Disease: An International Problem
BY KERRI WIEDMEYER, DVM, WVRC
Let’s Test Your Knowledge!
1. Can a dog infected with heartworm
give it to another dog in the same household?
2. Should you treat your pet with
heartworm prevention all year?
3. Can humans get heartworm?
4. When should heartworm prevention start?
Read on to find out more!
Heartworm or “Dirofilaria immitis” is parasitic worms that are transmitted by mosquitoes to certain mammals and cause severe disease. Adult heartworms are very long worms (females up to 12 inches and males up to 6 inches long) that live in the heart, lungs and blood vessels of the mammals they infect. Common mammals that can become infected include dogs, cats, wolves, coyotes, ferrets and foxes. There are several rare cases of human infections. Heartworm disease can be found on almost every continent in the world. This truly makes it an international problem. Where there are mosquitos, there are heartworms!
Transmission: The only way for heartworm to be transmitted is through a mosquito. An infected animal has immature worms, microfilaria, in the bloodstream. When a mosquito bites the animal, it sucks up the microfilaria in the blood. Once in the mosquito, the microfilaria develop over a few weeks, and when the mosquito bites the next animal, the worms infect that animal. Over the next six months, the worms in the newly-infected animal will mature into adult worms and settle in the heart and lungs. These adult worms will produce more immature worms, and the cycle will continue. The adult worms can live for several years in the host leading to years of increased spread of infection.
Heartworm Disease in Dogs
Disease: Canines are the preferred host for heartworms, and unfortunately that means the greatest amount of damage can occur in these animals. Heartworms thrive in dogs and reproduce at high numbers. Dogs can have hundreds of heartworms living in them at one time! Thus, the damage that these worms can do to the heart and lungs can quickly become irreversible and cause lifelong problems. The worms cause inflammation, scarring and obstructive problems and lead to pulmonary hypertension and congestive heart failure.
Clinical signs: Clinical signs can vary depending on how severe the worm burden is. Commonly, the first signs noticed are a cough and exercise intolerance. Signs then progress to coughing up blood, lethargy, difficulty breathing, ascites (fluid in the abdomen) secondary to heart failure and then caval syndrome. Caval syndrome occurs when the amount of heartworms is so numerous that normal blood flow cannot occur in the heart. This leads to a series of problems including anemia, liver and kidney failure and potential death.
Testing: A simple blood test can be performed that detects antigens to adult female worms. This test can be performed in any dog older than seven months as it takes the worms six months to become adults.
Treatment: If a dog is positive for heartworm disease, more testing will likely be warranted to see how severely the dog is infected. Dogs that are severely infected will need to be stabilized prior to treating the heartworms themselves. Treatment then consists of a series of injections, antibiotics and commonly steroids. Treatment is not only painful and expensive but has risks as well. During the course of treatment, which is typically over several months, dogs have to be strictly exercise restricted. Exercise can lead to the heart and lungs working harder, which can cause the worms to act as emboli, thus stopping blood flow to organs, causing organ failure and potentially sudden death.
Prognosis: Dogs that have low worm burdens or minor symptoms typically have a good prognosis with treatment. Dogs with large worm burdens can also successfully be treated but may have more complications and are more at risk for unsuccessful recovery.
Heartworm Disease in Cats
Disease: Cats are not the primary host of heartworms, and thus the disease process is very different. Heartworms are much less likely to make it to the adult stage in cats. While fewer adult worms means less disease, it also means many cats that are infected with heartworms will not show up positive on tests. The immature worms can still cause significant lung disease and, unfortunately, cats are more likely than dogs to die from a heartworm infection.
Clinical signs: Cats infected with heartworms will commonly cough, have respiratory changes, lethargy, weight loss and decreased appetite. However, if a cat has an adult worm and that worm dies, the body’s reaction to that worm can cause respiratory distress, shock or sudden death.
Testing: Testing for heartworm disease in cats is more challenging as the commonly-used antigen test is only positive with adult worms, and cats will often only be infected with immature worms. An antibody test would come back positive with immature worms but is much less commonly performed unless clinical signs are present.
Treatment: Unfortunately, the medication used to treat dogs for heartworm disease is not safe for cats. Treatment is generally supportive care and can include hospitalization and sometimes surgical removal of the adult worms, if possible. This is why heartworm prevention is so very important.
Prognosis: Cats with heartworm disease can survive with treatment, but prognosis varies depending on the severity of disease at the time of treatment.
Prevention: PREVENTION IS KEY! The good news is that heartworm disease is completely preventable! Heartworm preventative is recommended year-round as it only takes one bite from one mosquito to infect your dog or cat. There are many forms of heartworm prevention at this time such as topical treatments, chewable pills and injectable medication. These products do have to be prescribed by a veterinarian as there is currently no “holistic” or “natural” heartworm preventative. An added benefit of many of the heartworm preventative medications is that they will kill other parasites such as fleas, ticks and intestinal parasites. These preventatives are generally safe and inexpensive in comparison to the cost and severity of disease your pet could have if they become infected with heartworms.