Heartworm Disease in Cats

What Is Heartworm Disease in Cats?

Heartworm disease is caused by a parasite, Dirofilaria immitis, which is spread by mosquitos. While cats are susceptible to heartworm, they are fairly resistant, imperfect hosts. Cats acquire heartworm far less than dogs in the same geographic location, with only 5-20% affected. 

Only around 25% of heartworms reach adulthood in cats. Cats also typically have a low worm burden (usually one to four worms), and less than 20% of those worms will produce microfilaria, the infective form of heartworm. Cats also have higher rates of aberrant heartworm migration, which occurs when heartworms fail to migrate to the pulmonary arteries and instead travel to other body cavities, blood vessels, or the central nervous system. 

The life cycle of heartworms is complex and involves many larval, or immature, phases: 

1. First, a mosquito acquires microfilaria when it bites a dog infected with heartworm disease.  

2. Once ingested by a mosquito, the microfilaria will molt, or transform, over the next 10-14 days into three different forms of larva: L1, L2, and L3.  

3. After the third molt, the L3 larva can now infect new dogs and cats. 

4. The mosquito bites a cat and transfers the infective L3 through saliva into the bite wound. 

5. The L3 stays within the cat’s tissues for 3-4 days until it molts again to L4. 

6. L4 remains in the tissues for approximately 2 months. 

7. Finally, L4 molts to L5, the immature adult, which leaves the tissues via the bloodstream.  

8. L5 lands in the pulmonary arteries—the vessels responsible for bringing oxygen-poor blood from the heart to the lungs to re-oxygenate.  

9. L5 continues to develop in the pulmonary artery for another 4-6 months until, finally, it becomes an adult heartworm. The adult is capable, in some worms, of releasing microfilaria into the bloodstream to start the cycle all over again. 

The American Heartworm Society divides heartworm disease in cats into two stages: 

• Stage 1 occurs when immature L5 worms arrive in the pulmonary arteries, many of them dying. This causes a severe, acute inflammatory reaction (often misdiagnosed as asthma or other respiratory diseases) as the vessels and heart respond to the parasite. Veterinarians and researchers call this acute inflammatory process heartworm-associated respiratory disease (HARD). As the living worms mature, the inflammatory response lessens, in part from the adult worms actively suppressing the immune system. 

• Stage 2 occurs when adult heartworms die, invoking a highly inflammatory, anaphylactic, and often fatal response. The dead worms start off a strong pulmonary inflammatory cascade, and a cat’s blood vessels are small and narrow. When an adult heartworm dies, it easily causes an embolism. A cat who survives adult heartworm death will have permanent lung damage and chronic respiratory disease.  

Symptoms of Heartworm Disease in Cats

Clinical signs of heartworm disease vary greatly in severity. Some cats may not have any clinical signs at all. Most of the noticeable issues come from the adult heartworm suddenly dying and causing acute issues within the cat or abnormal worm migration causing tissue damage. When cats do show signs of illness, veterinarians most commonly see the following:  

• Lethargy 

• Coughing 

• Decreased appetite 

• Weight loss 

• Exercise intolerance 

• Vomiting 

• Difficulty breathing 

• Increased breathing 

• Open-mouth breathing 

• Neurologic abnormalities 

• Heart murmur 

• Sudden death 

Causes of Heartworm Disease in Cats

The parasite Dirofilaria immitis is the cause of heartworm disease. It is transmitted by mosquitos. Warm areas, especially in the southern United States, allow efficient mosquito reproduction and, therefore, southern states have a higher percentage of heartworm in cats. Outdoor cats are more susceptible, but even indoor cats can acquire heartworm if a mosquito ventures inside. 

How Veterinarians Diagnose Heartworm Disease in Cats

Heartworm disease in cats is often difficult to diagnose, especially compared to dogs. 

• Tests for microfilaria may give frequent false negatives, since cats rarely have circulating microfilaria.  

• Antibody blood tests can detect immature infections early, even 2 months after infection. However, a positive result does not confirm an adult infection, and it can even remain positive after cats clear the infection. 

• Antigen blood tests are the gold standard of diagnosing dogs with heartworm disease but can only test for the presence of female heartworms. Because cats can have very low and single-sex (i.e. all male) worm burdens, this method of testing will also miss some feline infections. 

• Chest radiographs (x-rays) may help in diagnosing cases of feline heartworm disease. Common features of heartworms can be seen on x-rays in about half of cases. These features include the following: 

  • Enlarged vessels 

  • Lung field inflammation 

  • Lung edema 

• Echocardiography (ultrasound) of the heart is useful to visualize the actual live worms in the pulmonary artery and right side of the heart. It can also help determine if pulmonary pressures are abnormal while excluding or confirming cardiac disease. 

Positive tests, enlarged arteries on radiographs, and changes in the heart increase the level of suspicion of a heartworm infection. No test is perfect, however, and your vet will use physical exam findings in addition to multiple types of tests to determine the severity of heartworm disease in your cat. 

Treatment of Heartworm Disease in Cats

Treating heartworm disease in cats is difficult. Many common therapies—including the most common canine treatment, melarsomine—are toxic to cats. The American Heartworm Society, the leading expert for care of heartworm-infected pets, does not currently recommend using therapies that kill adult heartworms in cats. 

Cats not currently displaying any clinical signs may have the option of being monitored over time for      spontaneous cure. A spontaneous cure happens when the cat’s immune system naturally clears the heartworm infection without causing any problems and without treatment. One drug, ivermectin, can reduce worm loads when given over a long period of time (2 years or more). However, allergic shock and inflammatory reactions can still occur with both spontaneous-cure and ivermectin-treated cats.  

Veterinarians may prescribe steroids, such as prednisolone, to decrease a cat’s natural strong inflammatory response. Cats will require supportive care and treatment for acute periods of respiratory distress and shock. Your vet may also prescribe the antibiotic doxycycline to help remove the bacteria, called Wolbachia, that lives inside heartworms and increases inflammation. Bronchodilators like terbutaline, theophylline, or albuterol may also be indicated to help with respiratory distress. 

Because of the difficulty with medical management of heartworm disease in cats, the preferred method of treatment is to remove heartworms through surgery.  

A more invasive surgery can remove worms after entering the chest cavity and using forceps to directly remove the worms through the heart. However, all forms of surgery for heartworm in cats can lead to circulatory collapse and death.  

Cats in acute respiratory distress should be stabilized immediately with oxygen therapy, intravenous fluids, and intravenous steroids and bronchodilators. It is crucial to seek emergency veterinary care as soon as a cat shows any signs of respiratory distress, including open-mouth breathing.

Prevention of Heartworm Disease in Cats 

Heartworm is a scary but preventable disease. Many monthly or bimonthly preventatives exist for cats. In addition to preventing heartworm, many also prevent intestinal worm and tick activity. 

Kittens can start preventatives around 8 weeks of age and continue their entire lives. It doesn’t matter if cats spend most of their time indoors, as mosquitos can easily get inside. Common heartworm prophylactic drugs include:  

• Heartgard 

• Moxidectin 

• Milbemycin 

• Selamectin 

Recovery and Management of Heartworm Disease in Cats

All heartworm-positive cats should have serologic testing, echocardiograms, and chest radiographs performed every 6-12 months to monitor their heartworm status, whether they were treated or were asymptomatic. Ideally, veterinarians will perform both antibody and antigen tests to gather the most information.  

Veterinarians will cautiously declare a cat recovered from heartworm disease once blood tests are negative in addition to resolution of clinical and radiological signs. The median survival time for cats with heartworm disease is four years.  

Managing heartworm-positive cats for success involves close monitoring, in-depth discussions with veterinarians, and full understanding of the risks of heartworm in cats. The American Heartworm Society is a great resource and has many in-depth articles and additional information on feline heartworm disease. 

Heartworm Disease in Cats FAQs

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