Dog Heartworm Symptoms and Treatments

Heartworm is an extremely serious infestation of parasitic worms which are passed from dog to dog via mosquitoes. Microfilariae, the fine, thread-like pre-larval heartworms are picked up from the dog's bloodstream by the mosquito through the bite wound, and 'molt' to the next stage of their life cycle using the mosquito as a host. The larvae then infect the next dog through the skin puncture when the mosquito next takes a blood meal. They migrate through the dog's circulatory system to the right ventricle and surrounding blood vessels where they settle and live for sometimes up to seven years. They can grow in length to about 5 or 7 inches, and can also migrate to other parts of the body.

The initial infection is not apparent or even detectable on blood test or X-ray and it can take a year before the signs and symptoms of heartworm start to manifest.

The first sign will probably be occasional cough, signifying that the worms have spread to the lungs and are compromising the pulmonary blood supply. As the worms multiply and grow in size they damage blood vessel linings which then become inflamed and blocked. The blood then finds alternative routes via the capillary system, which is the tiny network of single cell-walled connections between blood vessels formed when vessels are blocked or damaged. These in turn get blocked, large areas of lung tissue are now damaged and your dog will have breathing problems, pneumonia-like coughing and lung insufficiency together with a steadily increasing weakness and inability to exercise or run about.

As the right ventricle is now also filled with worms, the amount of blood it can pump is seriously restricted; additionally it cannot send the blood out through the heart valves into the major arteries because of the resistance further along the circulatory system. Your dog will be having fainting attacks and will certainly be in severe pulmonary distress. There will come a point where the heart is so packed with worms and the surrounding blood vessels so obstructed that your dog will have heart failure, and unless he has emergency treatment is likely to die within a day or two.

Prevention is always of course better than trying to cure, and your vet will advise you to have your dog blood screened and be given regular monthly medication to prevent being infected. If he is already infected but is only showing minor signs, the vet will prescribe drugs to combat the worms at both the larval and adult stages to cut the life cycle and reproduction rate.

Should your dog have become so ill that he is in distress, the vet may attempt manual removal of the worms under anaesthetic, but it is not always guaranteed to be effective particularly since the linings of the vessels will be so damaged. Your dog may be kept in the surgery during his period of treatment since he will need constant monitoring and care.

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