Heartworm Disease Overview

Heartworm disease [HWD] is a mosquito-borne parasitic disease caused by the parasite Dirofilaria Immitus. Suitable mosquito vectors [transmitters] exist worldwide, as do suitable hosts in the form of domestic and wild canines and felines. Serious endemic problems exist historically and mainly in the regions of the “Pacific Ring of Fire”. However, with the increased mobility of our pets worldwide we now find HWD throughout the USA, Mexico, Australia, the Caribbean, South America, Southern Europe, Asia and Japan. In Canada, HWD has been recorded in all provinces except Saskatchewan, however is most prevalent in the Southeastern and Southwestern and provinces. All cases thus far found to be positive in Alberta have originated from, or been on holiday in, a more highly infected area than our own.

The life cycle of HWD requires the development of microfilaria in the host [dog or cat]. Thereafter, ingestion of blood by a mosquito is required. Then the microfilaria develops to the infective larval 3 stage in the mosquito. This can occur as quickly as 10 to 12 days in hot and humid climates. However, development must occur within 30 days, as that is the life span mosquito. Larval development is unable to occur below environmental temperatures of 15°C.  Thus, those of us in the cooler climates are fortunate in that even though the mosquito and infected dogs might even be present locally, the chances of disease transmission is low.

Signs of HWD are initially very difficult to assess. As the worms become increasingly established in the heart of the infected host, there will be increasing signs of pulmonary and cardiac disease, with decreased exercise tolerance. Untreated HWD can be fatal.

Control and prevention of HWD is fortunately easy. The American Heartworm Society [AHS] recommends annual antigen testing of all dogs for canine heartworm disease. For those of us in Alberta and Saskatchewan we are currently recommending that all dogs be tested about every three years, especially if they travel beyond our provincial borders. We also recommend that all dogs that travel outside of Alberta or Saskatchewan during the mosquito season be placed on HWD preventive medication, following testing for the possible presence of the disease by means of a simple blood test. Medications are administered once a month as a chew tablet or a pour-on product. As the preventive medications eliminate the infective larval 3 and 4 stages in the host, the medications used need to start within 1 -2 months of entering an infected area. We typically would continue to recommend preventive medication for at least one to two months after leaving a HWD endemic area. HWD testing to evaluate whether the dog has been infected is typically recommended at 6 to 9 months after leaving an endemic area.

In the case where pets are infected very good treatment protocols are currently available. These treatments can provide a complete cure, especially where the disease is diagnosed early.

References:

1. Bayer animal health heartworm disease newsletter

2. American Heartworm Society

3. Handbook of Small Animal Practice, 3rd edition, Rhea V. Morgan