Heartworm Disease In Cats | PetMD

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 heartworms, they are a fairly resistant, imperfect host—meaning they aren’t ideal for mosquitoes to feed on them. 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. Typically, cats also have a low worm burden (usually one to four worms compared to hundreds in dogs), 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 is when heartworms fail to migrate to the pulmonary arteries and instead travel to other body cavities, blood vessels, or the central nervous system.  

 

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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 two 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.  

Stages of Heartworm Disease in Cats 

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.   

For PetMD's complete guide on heartworm disease in cats, click here. 

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