A mosquito-transmitted threat to pets: Canine heartworm
The vet’s office
By Dr. Roger Krogstad, DVM
Wildwood Animal Hospital and Clinic LLC
Mosquitoes are in the news with the summer Olympics approaching and concerns about Zika virus. In our own neighborhood, a mosquito-transmitted disease continues to be a threat to our pets but receives little national attention: canine heartworm.
Rarely seen before the late 1970s, this tropical disease came to the Midwest as motor homes became prevalent and more dogs accompanied vacationers. The large mosquito population here allows an infected dog to spread the disease to an entire neighborhood in one season. In my last year in veterinary school, any coughing dog with the address of “Top of Hollow Road” was considered heartworm positive until proven otherwise.
The mosquito is not the reservoir but the transmitter of infection. Ten days of incubation in the mosquito is necessary for the microfilaria to be infectious.
What could be the source of heartworm disease? Infected but untreated neighborhood dogs are likely the source. There is some evidence that rural dogs may get infected from foxes or coyotes. Also of concern are Southern dogs re-homed to the Midwest that are in the early, nondetectable stages of the disease.
As the name “heartworm” implies, this parasite ends its tissue migration in the chambers of the pet’s heart. Symptoms can depend on the number of worms and the pet’s immune response to the adult worm and the microfilaria offspring. Many dogs have no outward symptoms early on but still have circulating microfilaria. Other dogs may show respiratory symptoms as worms extend into the pulmonary arteries or kidney disease due to the antibody-microfilaria complexes that damage the kidneys’ filtering mechanisms. The major symptom of the heartworm infection in cats is sudden death from pulmonary embolism.
In the early 1980s, daily medication was given as a preventative. A major risk was the medical side effect of the sudden death of the adults and microfilaria. Today’s preventatives are much less likely to cause sudden death. The preventatives, when appropriately given, approach 100 percent protection.
A Wisconsin misconception is a nine-month mosquito season. Mosquitoes can live year-round in your home, and there is even a “snowmelt” mosquito that hatches in the cold weather. It is recommended that the heartworm preventative be given all year.
A six-month injection has taken the worry out of remembering to give the monthly oral medication. Both methods of prevention are extremely effective and if purchased through your veterinarian are accompanied by a manufacturer guarantee should your pet become infected. Be careful with online products. Some could be less effective and generally have no manufacturer guarantee.
Treatment of heartworm disease can be costly and risky as the adult worms can become trapped in the circulatory system ending up in the lungs upon death. A strict protocol is followed to minimize risks.
There are a number of options for heartworm protection for your pets. Ask your veterinarian or trained staff for recommendations.
Wildwood Animal Hospital and Clinic LLC is located at 210 Airpark Road in Marshfield and online at