Feline Heartworm Disease (FHD)
Feline Heartworm Disease (FHD) is far more prevalent, incidious and destructive than previously thought. It has been found that in endemic areas FHD is as prevalent as Feline Leukemia and Feline Immunodeficiency Virus!
Feline Heartworm Disease is mosquito-borne as in the dog. Yet cats are considered to be more resistant to infection than dogs. Further, cats typically are infected with fewer worms (usually on average 1-2 worms) and unisex worm populations can occur.
The disease manifests itself differently in the cat but is just as serious. This makes antigen based tests used to identify heartworms in dogs much less effective in cats. The current antigen based tests only diagnose female adult heartworms. So a negative diagnosis with such tests does not mean that the cat is not infected. Additionally, heartworms in the cat have a shorter lifespan ( 2-3 years) and ~8 years in dogs. Further, infected cats are often asymptomatic and consequently may not be tested to begin with! Antibody tests detect male and female infections making it a more accurate diagnostic tool.
Thoracic radiographs as well as echocardiography may be warranted if additional diagnostics are indicated for a high suspicion of infection. Thoracic radiographs can be valuable to aid in accessing the severity of the disease and for monitoring the disease progression. Features of FHD can be found in about half of suspected infected cats.
Echocardiography allows visualization of the interior heart as well as the interior main pulmonary arteries. Dead as well as alive worms can be visualized using this technique.
Since heartworm disease is mosquito-borne, it is often thought that indoor dogs as well as cats were not susceptible to the disease. Yet in a recent North Carolina study, it was found that 28% of heartworm positive cats were exclusively indoors!
The name “Heartworm Disease” is often a misnomer, since the disease mostly affects the lungs and not just the heart of cats. Most recently FHD has been redefined to Heartworm Associated Respiratory Disease (HARD). Most cats tolerate their infection without clinical signs or with signs manifested only transiently. Clinical signs may include: anorexia, blindness, sudden collapse, convulsions, coughing, diarrhea, difficulty breathing, fainting, lethargy, rapid heart rate, sudden death, chronic vomiting, weight loss, allergic dermatitis, fluid in the chest and ataxia.
HARD is a disease of the lungs. Cats with HARD are often mistaken for asthma or other respiratory diseases. If a heartworm larvae is present, it can cause pathologic lesions in lung tissue. Dogs on the other hand, typically do not show signs of disease until the adult stage worms reach the pulmonary arteries.
Treatment options include: medical intervention (usually prednisone); treating symptomatically; adulticide administration (often considered a last resort as this is toxic to cats) and surgery.
Since treating HARD is not at the moment too promising, we should aim to prevent our pets from infection. This can be achieved via monthly chemoprophylaxis with Advantage Multi or Revolution (both topical products). It is recommended that a preventative be applied monthly in this part of the country. At the present, this appears to be the only way to protect our cats!