Canine Heartworm Disease

Canine Heartworm Disease

Canine Heartworm Disease is a serious and potentially fatal disease caused by the parasite Dirofilaria Immitis. The disease can infect over 30 species, including humans, however dogs are the definitive host.

The most common way this disease is transmitted from one animal to the next is through mosquitoes. A mosquito carrying infective heartworm larvae bites a dog and transmits the infection to them. The larvae grow, develop, and migrate in the body over a period of 6 to 7 months, in which time they become sexually mature male and female worms. this is the prepatent period. The worms then reside in the heart, lungs, and associated blood vessels. The worms begin to mate and release microfilaria into the blood stream. When a mosquito bites an infected dog it takes in some of the microfilaria in the blood. After 10 to 30 days there is larvae in the mosquito’s salivary gland which can then be passed on to the next dog the mosquito bites.

Canine Heartworm Disease can also be transmitted to puppies through the placenta of an infected mother. However in this case the puppies will only be carriers, but this makes them at risk of severe reactions when starting canine heartworm prevention. Another way larvae can be passed is through blood transfusions. To prevent this all donors must be cleared of heartworm disease before donating.

Dogs infected with Canine Heartworm Disease can have from 1 to 250 worms living in them for 5 to 7 years. The organs usually being affected the greatest are the heart and lungs and with each case the way it affects them is different. Some dogs only have an acute case; this is when no abnormal clinical signs are observed. The next level of the disease would be just a mild one were a cough is present. Dogs with a cough, exercise intolerance, and abnormal lung sounds would have a moderate form. The last and most severe form is when the dog is experiencing the previously mentioned clinical signs and/or, dyspnea, hepatomegaly (liver enlargement), syncope, ascites (fluid in abdomen), abnormal heart sounds and possibly death.

Diagnosing the disease can be relatively simple, but this depends on accurate patient history and ones ability to recognize varied clinical signs and then perform diagnostic procedures. These diagnostic procedures may include antigen testing, x-rays, ultrasonography, angiography, a complete blood count, the Knott’s or Filter test, and in the worst case necropsy.

Of these, antigen testing is the preferred method for both heartworm diagnosis and screening. However the test can only detect circulating microfilaria so if the animal was infected in the last 6 to 7 months the results would be negative. On the other hand if the animal was treated for heartworms in the past 16 weeks the results of an antigen test would be positive. For these reasons the diagnosis of Canine Heartworm should not be left entirely to the results of one time antigen testing.

The treatment for Canine Heartworm Disease is usually successful, with the goal being to kill all adult worms and microfilaria. There are two FDA approved adulticides for killing adult worms. The first is Immiticide (melarsomine