Harlan Animal Hospital

Heartworms In Cats:

Heartworm infection takes place when a mosquito carrying infective, microscopic-size heartworm larvae, bites into a cat for a blood meal. The larvae then actively migrate into the new host and develop further as they travel through the subcutaneous tissue in the cat's body. At about 3-4 months, they usually settle into the arteries and blood vessels of the lungs, where they continue to develop to sexual mature male and female worms (Dirofilaria immitis). The average time from when the microscopic parasites enter the host until the females develop into mature worms and produce offspring is approximately eight months and is referred to as the prepatent period. This is about one month longer than in dogs.

 

As adults, the heartworms can mate and the females can release offspring called microfilariae (pronounced: micro-fil-ar-ee-a) into the blood stream. The cycle begins again when a mosquito takes a blood meal from the newly infected cat and draws the microfilariae into its system.

 

Cats are resistant hosts of heartworms, and microfilaremia, (the presence of heartworm offspring in the blood of the host animal), is uncommon (usually less than 20% of cases). When present, microfilaremia is inconsistent and short-lived. Some cats appear to be able to rid themselves of the infection spontaneously. It is assumed that such cats may have developed a strong immune response to the heartworms, which causes the death of the parasites. These heartworms may die as a result of an inability to thrive within a given cat's body.

 

Cats typically have fewer and smaller worms than dogs and the life span of worms is shorter, approximately two to three years, compared to five to seven years in dogs. In experimental infections of heartworm larvae in cats, the percentage of worms developing into the adult stage is low (0% to 25%) compared to dogs (40% to 90%).

 

However, heartworms do not need to develop into adults to cause significant pulmonary damage in cats, and consequences can still be very serious when cats are infected by mosquitoes carrying heartworm larvae. Newly arriving worms and the subsequent death of most of these same worms can result in acute pulmonary inflammation response and lung injury. This initial phase is often misdiagnosed as asthma or allergic bronchitis but in actuality is part of a syndrome now known as Heartworm Associated Respiratory Disease (HARD)