The most obvious signs of heartworm infection are a chronic soft, dry cough, shortness of breath, low stamina, weakness, listlessness and nervousness. Because heartworms impair circulation and interfere with the delivery of oxygen to the body, these signs are most obvious after exercise. A dog with advanced heartworm infestation may even faint after or during vigorous exercising.

A vet may notice other signs of heartworm infestation on examination. He or she may hear abnormal heart and lung sounds, or note congestive heart failure symptoms. The abdomen and legs may swell from fluid accumulation, and the dog may lose weight or be anemic. Infected dogs may die suddenly during exercise or excitement from congestive heart failure.

How Heartworm Spreads

Heartworm is spread from one infected dog to another by mosquitoes. The cycle starts when a mosquito takes a blood meal from an infected dog. The blood that the mosquito ingests is infested with microfilaria – immature heartworms. That microfilaria will live in the mosquito's digestive system over the next two to three weeks, maturing there. When they are mature, they move from the digestive system to the mouth parts of the mosquito. The next time that the mosquito takes a blood meal, the microfilaria – which are now called infective larvae – are deposited on the skin of the animal. From there, they burrow into the skin until they reach a blood vessel and are carried through the bloodstream to the heart where they settle in to mature into adults. Within two to three months, the larva will be a sexually mature adult, and start producing microfilaria which is released into the bloodstream to start the cycle all over again.

Heartworm Prevention – What dogs should be protected

Because heartworm is spread by mosquitoes, it is most common in states where the mosquito population is high – but it has been reported in all 50 states. In states that have a mosquito problem year round, all dogs should be treated with heartworm preventative each month. In other states, dogs should be tested for heartworm at the start of mosquito season and if they are not infected, should be treated with heartworm preventative monthly through mosquito season.

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