Heartworm is a large parasite — up to 30 cm long — that lives in the heart and the main blood vessels of the lungs of infected dogs. Transmitted from dog to dog by mosquitoes, the parasite begins its growth within the mosquito and is injected into the dog’s bloodstream as the mosquito bites.
It then develops into an adult worm within the dog, migrating to the heart and lungs and, occasionally, the liver. The adult heartworms then breed and can increase to large numbers, interfering with circulation and damaging tissues and leading to heart and/or liver failure and lung disease as the lungs develop a severe reaction to the worms. Left untreated this will result in the death of your pet.
While heartworm disease is more prevalent in areas where the mosquito population is high it is not confined to these areas. It takes only one infected pet moving into a new area to introduce heartworm into a dog population previously free of the parasite. Also, because many people take their dogs with them on outings and holidays, and even for walks in different parks, the level of heartworm disease is gradually increasing.
As it is transmitted by mosquitoes, dogs do not have to come in direct contact with other dogs to catch the disease.
SEVERITY: Highly transmissible and life threatening if left untreated.
In the early stages of infection no symptoms are evident, and dogs may remain symptom free for months or even years despite carrying the parasite. The first signs may include coughing, the animals may become progressively more lethargic with exercise, and/or experience reduced appetite and weight loss. In the later stages of the infection collapsing episodes occur, as well as coughing up blood.
NB: These are all similar signs to heart disease.
Heartworm disease is diagnosed by a number of methods. A simple blood test is performed in the hospital’s pathology lab and checked under the microscope for antigens and microfilaria in the blood. If the test result is negative prevention for heartworm disease can be commenced (see ‘Prevention’ tab). If the test is positive, a second blood test is taken to confirm the result.
If this second result is also positive, confirming our diagnosis, then we will perform a series of tests to determine how advanced the infection is and how much damage the heartworm have caused so far. These include further blood tests and radiographs (x-rays).
Treatment is drug-based and varies depending on the degree of heartworm disease present in the infected dog. The drugs we use to kill the heartworm are dangerous in themselves. Some are arsenic based so great care is required. Dogs with no clinical signs have a good prognosis. Those with advanced and/or severe heartworm disease have a guarded to poor prognosis.
Fortunately, heartworm is a disease that is easily avoided with routine modern preventative medications. Prevention should begin from 6-8 weeks of age. Puppies are started on a monthly chewable treat or spot-on treatment that contains the heartworm medication. They can then stay on this medication for life without any adverse effects. Alternatively, fully grown adults can be give an annual injection that will provide them with 12 month protection against heartworm.
If you have a dog that is older than 6 months, a blood test is required before heartworm prevention can be begun.