Filaria in the Dog

Risk factors

The cardiopulmonary filariosis in the dog does not seem to have a predilection of age or race; however it seems that male dogs are more frequently affected than females.

Of course, large dogs that are generally used to living outdoors are more at risk than small dogs that live in the house. Moreover, contrary to what one thinks, the length of the animal's hair does not seem to have much influence.


As anticipated, before the filaria parasites reach sexual maturity, and settle at the cardiac level (in the heart) and pulmonary arteries, several months pass.

Then, before the adult parasites reach such a number as to cause obvious symptoms in the dog, a further period of time passes. Therefore, when the owner sees signs of illness in his animal, it has been infested for several months or even years.

The dogs affected by Filaria are usually divided into 4 symptomatic classes (based on the signs they show), depending on the severity of the infestation (which corresponds to the number of adult parasites present in the pulmonary arteries and in the heart, therefore at the time of the invasion of the Filaria in the animal):

  • Class 1 ( subclinical or mild form ), the dog has no changes in health status, looks good, shows no signs and symptoms of disease, but the laboratory test is positive for the antigen research test (elements of the parasite they are sought after to identify it) of Filaria (in practice, parasites are present but still do not cause so great damage as to compromise the health of the dog);
  • Class 2 ( moderate form ), the Filaria parasites have reached, inside the pulmonary arteries and the right heart (that part of the heart that receives venous blood and the "pump" to the lungs), such size and number as to cause symptoms such as dog fatigue, occasional cough, poor performance with dyspnea (breathing difficulty) under exertion (eg after a run or after playing), possible heart murmurs (changes in the function of heart valves) and possible weight loss;
  • Class 3 ( severe form ), Filaria has parasitized the animal for so long that it has reached such dimensions and numbers as to cause numerous damages to the heart and to the pulmonary arteries, which can occur with: a poor physical condition of the dog (the which also appears to be thinner), dyspnea or tachypnea (increased breath rate), cough, anemia (decrease in red blood cells, as Filaria feeds on blood cells), abdominal volume increase, right heart failure (poor function), epistaxis (loss of blood from the nose), pulmonary changes and thromboembolic phenomena (fragments of parasites, together with thrombi - or blood clots caused by the parasites themselves - can cause sudden occlusion of blood vessels);
  • Class 4 ( vena cava syndrome ), the filaria parasites are so large and numerous that they occupy not only the pulmonary arteries and the right heart, but to go up into the right atrium up to the vena cava (large vessel that carries blood to the heart) obstructing it and seriously compromising the life of the animal.


We have seen that this disease needs two protagonists to manifest itself: the parasitic Filaria and the mosquito ( Culex, Aedes and Anopheles ). For lack of one of the two, therefore, our dog cannot get sick.

In this regard, to reduce the possibility that the intermediate host (mosquito) comes into contact with our animal (therefore, by pricking it it can transmit Filaria), repellent substances can be used and insecticides can be used regularly.

Dogs should also be kept in closed places (especially at night when there are mosquitoes) and, if possible, use mosquito nets.


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