Don't Let Them Worm into Your Heart

Heartworms (Dirofilaria immitis) are a species of parasitic roundworms that primarily infect dogs, but can also infect cats, wild canids, and, in rare cases, even humans. They are found in every state in the country and spread from one host to the next through mosquito bites, which deposit larval heartworms into the new host when they feed.  Since their lifecycle depends on mosquitos, infection rates are on the rise as human activities, such as irrigation and urban development, create new habitats suitable for mosquitos. Heartworm infections develop slowly, with symptoms often presenting themselves only months past since the initial transmission.

The first symptoms are innocuous enough, just a slight cough. As the worms grow and lodge themselves in the arteries leading into the heart and lungs, the coughing becomes worse and persistent.  The dog will exhibit difficulty breathing and a reluctance to exercise, which gets worse as time goes on. Eventually, a severe infection of heartworms can cause heart failure and death. Luckily, there are preventative medications that can be given to dogs which kill heartworm larvae before they have a chance to mature.  However, the American Heartworm Society recommends that, in addition to giving dogs preventative medications, dogs should also be tested once a year for the presence of heartworms. There are two main tests used to detect the presence of heartworms: antigen testing and microfilariae (the larval heartworms) detection.

Both methods have been proven to be effective at detecting heartworm infections, but each has advantages and disadvantages. Antigen testing looks for proteins produced by adult female heartworms in the dog’s blood using an enzyme-linked immunosorbent assay (ELISA) or an immunochromatographic test system.  The greatest advantage of this type of test is that the antigens produced by female heartworms is totally unique to the heartworms, which means antigen testing produces very few false positives. However, antigen testing will not reveal the presence of any microfilariae or an infection that consists entirely of male heartworms. The sensitivity of antigen tests also varies slightly with the number of adult female heartworms present. For those reasons, the American Heartworm Society also recommends that antigen tests are supplemented by microfilaria detection.

The two main methods of microfilariae detection are the modified Knott’s methods and the filtration method. The Knott’s method involves concentrating a blood sample and then staining it to find the microfilariae under a microscope.  The filtration method works by concentrating a sample of blood through an 8.0 micron clear polycarbonate filter and examining the captured microfilariae under a microscope.  The optical clarity of polycarbonate filters makes them ideal for viewing directly with a microscope since it can done without having to transfer the sample to another surfaces or slide. One of the disadvantages of microfilariae detection is that heartworm microfilariae can be difficult to distinguish from the microfilariae of other parasites that may be present in the sample and require a different treatment.

You can learn more about heartworms and how to protect your pet from them at the American Heartworm Society’s website. Sources Carithers, Doug, ed. Current Canine Guidelines for the Prevention, Diagnosis and Management of Heartworm Infection in Dogs. Wilmington, DE: American Heartworm Society, 2014. Print. "Techniques." 2010. Web. 28 July 2015.