Heartworm season is almost upon us here in Northern Minnesota. Heartworm disease is a serious problem which can be fatal, but is easily prevented.

Heartworms (Dirofilaria immitis) are transmitted by the mosquito. Mosquitoes start to move when temperatures are around 50 degrees. A mosquito needs to bite an infected animal and ingest the microfilariae (immature heartworm) that are circulating in the bloodstream. Inside the mosquito, the microfilaria mature and go through two life stages to become larvae. When the infected mosquito bites another dog, it releases the larvae into the bloodstream. The larvae then migrate to the heart and mature into adult heartworms, which can take approximately 6 months. Adult worms live approximately 7 years in dogs and female worms can grow up to 14 inches in length.

HW Life cycle

Heartworm Symptoms:

The most common signs of infection are cough, shortness of breath, weakness, and loss of energy or exercise intolerance. Some pets may even faint after exercise.

Live heartworms cause damage to the heart and blood vessels by irritating the inner walls and blocking blood flow. The heart may become enlarged and weak due to overworking which can lead to congestive heart failure (CHF). The blood supply is reduced to other organs including the lungs, liver, and kidneys which may lead to damage and organ failure.  Dead worms can also cause an inflammatory response which can affect the body in multiple negative ways.

A very active dog is more likely to develop severe disease than an inactive one. Some dogs may never show any signs of heartworm disease. This is why we recommend regular screenings.


Current heartworm tests are far more accurate than they have been in the past. The heartworms have to be mature and start to produce offspring before they can be detected by the test. Regular screenings are highly recommended. After a positive test, further diagnostics are generally required to determine the severity of the infection and the ability of the pet to tolerate the treatment.

Heartworm Treatment:

The earlier the disease is detected, the better chance your pet has of recovery. Veterinary hospitals will usually perform blood work and radiographs to determine the function of the organs prior to treatment which can help us predict the possibility of complications from treatment. Radiographs are also taken to help assess the severity of infection. An injectable medication is given over the course of 2 days to kill the adult heartworms. It takes approximately 30 days for the adults to die. During this time, the pet must be strictly rested as the worms are being broken down and absorbed by the body. Pets can have significant reactions to the dead worms. About one month after the initial treatment, the patient is then treated with another medication to kill the microfilariae.


Heartworm disease can be prevented with monthly prophylaxis. It is recommended to give these year round for maximum effectiveness, but should at least be given the months during mosquito season here in Northern Minnesota. Pets should be tested to make sure they are heartworm negative prior to starting preventives. Puppies should be started no later than 8 weeks of age.

There are many products on the market for heartworm prevention. Our veterinarians recommend Interceptor®, a monthly chewable treat that kills the microfilaria that may have been introduced in the bloodstream within the past month. It kills intestinal parasites such as roundworms, hookworms, and tapeworms.

Interceptor Plus

Take advantage of the pet resources Bemidji Veterinary Hospital has available to you to prevent heartworms before they become a problem for your pet. Call us today at 218-751-2753 if you have questions or would like to schedule an appointment to have your pet tested for heartworm.