Heartworms, also known as Dirofilaria immitis, may infect more than 30 species of animals although dogs are accepted as the definitive hosts for these worms.

Canine Heartworm Disease is the infection heartworm larvae by a mosquito which develop and migrate in the body over a period of several months to become sexually mature male and female worms. These worms reside in the heart, lungs and associated blood vessels of the dog and release their offspring into the blood stream. The disease is spread from dog to dog (and to cat) by mosquitoes. The mosquito bites a dog with heartworm infection, collects some of the microscopic heartworm offspring and then, after a couple of weeks, passes these on to another dog or cat.

Offspring can be detected in the blood around six to seven months after the infective larvae from the mosquito enter the dog. Male heartworms measure about four to six inches in length and females about 10-12 inches when fully grown, which is about one year after infection. In addition their heavy concentration in the heart, lungs and sometimes other organs causes serious organ damage and their numbers can physically block important arteries along with the tiny new generation of heartworms released into the circulatory system in such vast numbers that they may block smaller blood vessels too.

The onset and severity of the infection of heartworms in the dog is mainly a reflection of the number of adult heartworms present, the age of the infection and the level of activity of the dog. Until the number of mature heartworms exceeds 50 in a 25-kg dog nearly all of the heartworms will reside in the arteries of the lower lung lobes. A higher number of heartworms will result in their presence in the right chambers of the heart with the most common early pathological changes in such infections being due to the inflammation that occurs in and around the arteries of the lungs in response to the presence of the worms. This can result in an enlarged and weakened heart and possible congestive heart failure.

The active nature of the dog can influence the likelihood of developing a severe disease from a relatively small number of worms, while an inactive dog will be less inclined.
Occasionally, a dog with a large number of heartworms may not only have worms in the heart, but also in the large primary vein of the lower body between the liver and the heart. If not removed surgically, this syndrome causes sudden collapse and death within two to three days.

There are clinical symptoms associated with the infection of Heartworms with the heart and lungs being the major organs affected by the worms.

Early Infection usually shows no abnormal clinical signs, although a mild infection of Heartworms will display signs in the form of a cough. Moderate infection is displayed by a cough, and intolerance for exercise or abnormal lung sounds and a severe infection by a cough, intolerance to exercise, difficulty breathing, enlargement of the liver, temporary loss of consciousness due to poor blood flow to the brain and abnormal heart sounds.

Diagnosing canine heartworm disease depends largely on an accurate patient history and the recognition of the varied clinical signs. Several diagnostic procedures are used to determine infection including blood tests, Microfilarial (larvae) detection and differentiation, x-rays, angiography and ultrasound.

Prevention of heartworm disease is simple. There are three forms of preventative medications available for the prevention of heartworm disease: pills, topical treatments and injections. These preventative medications are only available from a veterinarian and can only be prescribed if the dog tests negative for Heartworms. Preventatives in heartworm positive dogs can cause severe reactions.

The purpose of the preventative medications is to kill the microscopic larvae that are left behind by mosquitoes when they bite the dog. Both dogs and cats can contract heartworm disease. Most commonly found in our canine friends, it is dangerous, expensive and difficult to treat. However the problem is easily avoided if one of the three forms of preventative heartworm medicines are used. Heartworm pills, topical heartworm spot on treatments and even a heartworm injection are now widely available, so keeping our pets safe should be straightforward.

Preventative heartworm medicines do not stop your pet from becoming host to larval heartworms but work to destroy the larvae before they are able to mature and do damage.

All pet medicines labelled as heartworm preventatives contain one of four active ingredients. All offer the same guaranteed prevention of the disease when used correctly. So whether you choose heartworm pills or a topical spot on product is primarily a matter of personal preference. It is critical that you consult your veterinarian to ascertain the correct method of treatment and dosage for your breed and type of dog.

Maintaining a good external parasite regime will also assist in preventing the possibility of infection by deterring mosquitoes away from the dogs. These flea and parasite treatments will reduce the chance of the pet from contracting heartworms but does not guarantee they will not get the disease. The only guaranteed method of preventing the disease is to regularly test and treat your furry friend for the larvae.

Paying careful attention to any physical of psychological changes in your dog, will allow you to detect any possible discomfort or pain the dog may be suffering which may be an indication of an infection. Any changes in energy levels and respiratory problems should be addressed by your veterinarian immediately to address the possible causes and initiate treatment.

Regular annual checkups with your veterinarian are the most certain method for prevention of internal parasitic infections, as well as other infections and possible ailments that your dog may be suffering from without displaying any symptoms and should be undertaken by any dog owner.