Spring is here (finally!), and with it comes the heartworm season. Most dog owners are already familiar with this deadly but preventable disease, but some points are worth repeating.

Heartworm disease is a parasitic condition affecting mainly dogs, and is transmitted by mosquitoes. The female mosquito feeds on a heartworm -infected dog, and ingests the immature form of the parasite (called a "microfilaria"), which circulates within the bloodstream. This microfilaria undergoes several stages of maturation within the mosquito to become a larva, which is then injected into the tissue of another dog as the mosquito feeds again. The larva migrates through the tissues of the newly-infected dog over the next few months, and finally settles in the vessels around the heart. The mature worm can reach 14 inches in length, and some dogs can be infected with hundreds of worms. Obviously, these worms can have a serious negative effect on heart function. As the worms reach adulthood, they produce microfilariae which circulate throughout the bloodstream, awaiting a mosquito to begin the entire cycle again.

Heart failure is a common result of heartworm infection. Exercise intolerance, fluid accumulation within the lungs, coughing and sudden death can all result from heartworm disease. Treatment is available if the disease is not severe, but is expensive, and involves killing the adult worms by using drugs which may have serious side effects. The dying worms may also cause a problem by migrating from the vessels around the heart to the lungs and other organs. Veterinary monitoring is required during treatment and for months afterwards to identify any complications.

Preventing heartworm disease is far easier and less expensive than its treatment. Prevention is available as pills or topical liquids given once monthly. Many forms of heartworm prevention also treat intestinal parasites, such as hookworms and roundworms, and some are combined with medications for flea control. It is important to ensure that dogs are not infected with heartworm prior to beginning preventative medication to avoid serious complications. This involves a simple blood test, usually performed between April and June in Southwestern Ontario. Medication is then prescribed from June until November, which is the period during which mosquitoes are able to transmit heartworm infection. Periodic heartworm testing is required during a dog's life to ensure infection has not taken place, and prevention is then continued annually. Talk to your veterinarian about heartworm testing and which prevention is best for your dog.