Heartworm Disease in Cats

Year-round preventatives are the best way to avoid heartworm infection in cats.
Year-round preventatives are the best way to avoid heartworm infection in cats.

Table of Contents:

  1. Heartworm Life Cycle
  2. Common Heartworm Symptoms in Cats
  3. Diagnosis
  4. Treatment of Heartworm Disease in Cats
  5. Prevention in Cats

Heartworm disease is a global issue caused by a worm known as Dirofilaria immitis. It is most prevalent in regions with high humidity and a correspondingly high mosquito population.

Heartworm Life Cycle

To understand heartworm disease, you must first understand the life cycle of the worm. The disease itself needs an intermediate host, which is typically a mosquito. Replication and transmission to other hosts occurs within the mosquito.

This process begins with a mosquito feeding on an infected host, which is typically a domestic or wild canid. When a mosquito bites an infected host, they ingest some microfilaria. Microfilaria is the worm circulating within the bloodstream. These microfilaria replicate and advance in the intermediate host, which is a process that can’t be completed without the mosquito.

The heartworm develops into the first three larval stages (L1, L2, and L3) in the body cavity of the mosquito. The L3 heartworm can move into the mouth of the mosquito and be transmitted to definitive hosts. Canines tend to be the preferred hosts, but felines and ferrets are also able to develop heartworm disease. The maturation time frame is dependent on temperature and humidity. At 80 °F with 80% humidity, the process takes between 10 – 14 days. This process takes longer in cooler and less humid environments. In the new host, the L3 molts to larval stage 4 (L4). This can occur between 3 – 12 days.

Stages of Heartworm Development

The final stages of development in the definitive hosts are as follows:

  • 50 – 70 days post infection
    • L4 to juvenile/immature adults
    • Migration through the body
  • 67 – 120 days post infection
    • Juvenile worms migrate to the pulmonary system
    • Average 1 – 1.5” in length
  • 120 days post infection
    • Sexual maturity
  • 6 – 9 months post infection
    • Microfilaria are visible in the bloodstream

Adult worms can be found in pulmonary arteries, the right ventricle of the heart, and artium.

Though dogs are usually the preferred hosts, cats can get infected. Dogs are preferred due to the higher ability of the L3 to be able to continue to mature when compared to other species. In dogs, about 56% of L3 are able to continue the maturation process, while only about 6% mature in cats. Cats tend to have lower worm burdens than dogs, with infected cats having only 1 – 3 worms. Adult heartworms tend to have a shorter life span in cats as well, living only 2 – 3 years, as opposed to 5 – 7 years.

Common Heartworm Symptoms in Cats

Clinical signs in cats with heartworm disease can also be more subtle than in dogs. These clinical signs are often similar to those seen with asthma or bronchitis.

Common symptoms include:

  • Coughing
    Asthma-like episodes
    Weight loss
    Vomiting
    Intermittent tachypnea

Cats also have damage to the type II alveolar cells, as opposed to type I damage seen in dogs. This damage causes hyperplasia (thickening) of the cells, which leads to impairment of oxygenation across the cells. Due to thickening of the alveolar cells, cats tend to show signs of lower airway disease that mimic asthma and bronchitis. Clinical signs are more subtle in cats and can be intermittent depending on the location and severity of cell injury. Cats are less likely to develop pulmonary hypertension or congestive heart failure compared to dogs, however, they’re more likely to die of heartworm disease due to compromised oxygenation.

Cats can develop heartworm-associated respiratory disease (HARD) syndrome, which is thought to be secondary to juvenile worms dying when they reach the pulmonary vasculature. Clinical signs of HARD include coughing, tachypnea, or trouble breathing that will resolve or can be intermittent. Occasionally, HARD syndrome can be fatal in cats.

Diagnosis

Diagnosis in cats can be difficult and standard antigen testing is not always reliable. The antigen test requires at least one mature female worm to be present. With cats having a low-worm burden, there may only be male worms present, which prevents the antigen test from working. Cats with clinical signs should be tested for heartworm disease, especially in endemic areas, and both antibody and antigen testing should be conducted.

Antibody testing can detect both male and female worms, and cats can harbor antibodies to heartworms for months even if they do not have active infections. A negative antibody test in cats indicates a greater than 90% probability of the absence of an infection. Antibody tests can also help to identify at risk cats. Antigen testing, on the other hand, should not be used in cats due to low sensitivity.

Diagnosis can also be made in cats with an echocardiogram and visualization of worms in the heart and pulmonary artery. Echocardiogram tends to be the most reliable test to look for active infections in cats.

Additional diagnostic options include:

  • Complete blood count (CBC). Some cats may have elevations in eosinophils and basophils with heartworm infections.
  • Serum chemistry panel. This evaluates organ function for the liver and kidneys, as well as electrolyte and protein levels.
  • Chest radiographs. Changes are less prominent than in dogs, but radiographs can help to rule out other underlying disease processes like asthma, bronchitis, primary cardiac disease, and neoplasia.

Treatment of Heartworm Disease in Cats

Treatment is usually focused on management of clinical signs with corticosteroids, bronchodilators, and oxygen support in times of crisis. Due to the short life span of adult worms, spontaneous recovery is possible if reinfection is prevented. There is no approved adulticide for cats. Melarsomine is not recommended for treatment in cats, as data has shown it to be toxic at low doses. If worms are present on echocardiograms in a cat, surgical removal of the worm is the preferred treatment option.

Prevention in Cats

All cats, even indoor cats, should receive monthly heartworm preventatives in endemic areas. Unfortunately, mosquitos can get indoors and cause an infection. Year-round treatment is recommended in endemic climates, as mosquitoes can transmit the disease year round. Monthly heartworm preventatives can also help treat gastrointestinal parasites, just like preventatives used in dogs. Consult with your veterinarian to determine if your cat is at risk and review monthly preventative options.

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