Heartworm Disease In Cats

August 18, 2023

Understanding Heartworm Disease in Felines:

Cat heartworm disease stems from a parasite named Dirofilaria immitis, which mosquitoes transmit. Although cats can contract heartworm, they aren't the ideal hosts for the parasite. In fact, in the same regions, cats are less frequently affected by heartworm than dogs, with figures suggesting only 5-20% become infected.

In felines, merely a quarter of the heartworms mature into adulthood. Furthermore, cats usually carry a minimal number of these worms, often between one to four. Of these, fewer than 20% will evolve into microfilaria, which is the infectious stage of the heartworm. Moreover, cats exhibit a higher tendency for irregular heartworm movement, which is seen when these worms don't make their way to the pulmonary arteries as expected, but instead wander into other parts of the body, including various cavities, blood vessels, or even the central nervous system.

The progression of heartworms involves a series of intricate stages, transitioning from immature to mature forms:

Initially, when a mosquito feeds on a dog with heartworm disease, it picks up microfilaria.
After being consumed by a mosquito, within 10-14 days, the microfilaria undergoes a series of changes, evolving into three distinct larval stages: L1, L2, and L3.
After its third transformation, the L3 larva can infect both dogs and cats.
The mosquito then bites a cat, transmitting the infectious L3 larva into the bite via its saliva.
For 3-4 days, the L3 resides in the cat's tissue, eventually transforming into L4.
The L4 stage persists in the tissues for about two months.
Subsequently, the L4 transitions to L5, its pre-adult stage. This form enters the bloodstream from the tissues.
Arriving in the pulmonary arteries, which carry deoxygenated blood from the heart to the lungs for oxygen replenishment, the L5 matures over the next 4-6 months.
Upon full maturation, the heartworm can, under certain conditions, release microfilaria into the bloodstream, paving the way for the next cycle.

The American Heartworm Society categorizes heartworm affliction in felines into two primary phases:

In stage 1, the young L5 worms reach the pulmonary arteries, where many perish. This results in an intense, immediate inflammatory reaction. This reaction is frequently mistaken for conditions like asthma or other lung diseases. This primary inflammatory condition is a heartworm-associated respiratory disease (HARD). As the surviving worms evolve, the inflammatory reaction diminishes, partly because the mature worms actively inhibit the host's immune response.

Stage 2 emerges when the mature heartworms succumb, triggering an extremely inflammatory, anaphylactic reaction that can often be deadly. The demise of these worms initiates a potent inflammation in the lungs. Considering the minuscule and constricted nature of a cat's blood vessels, dead adult heartworm can easily lead to an embolism. Felines that endure the death of adult heartworm will inevitably suffer from irreversible lung damage and persistent respiratory ailments.

Signs Of Heartworm Affliction In Felines

The manifestations of heartworm disease in cats can significantly differ in intensity. Some felines might not display any noticeable symptoms. The most apparent problems arise when the adult heartworm unexpectedly expires, leading to sudden complications or abnormal worm movement, causing tissue harm. When symptoms do become evident, veterinarians often observe the following:

Fatigue
Persistent cough
Reduced hunger
Shedding weight
Reduced stamina during activities
Nausea and throwing up
Struggling to breathe
Rapid respiratory rate
Breathing with the mouth open
Neurological irregularities
Audible heart irregularities
Abrupt fatality

Cause Of Heartworm

Affliction in Felines Heartworm disease in cats stems from the parasite named Dirofilaria immitis, which is spread via mosquitoes. Regions with warmer climates, particularly in the southern parts of the United States, provide ideal conditions for mosquito breeding. As a result, these areas see a heightened prevalence of heartworm cases in cats. While cats outdoors are more prone to this disease, even those indoors aren't entirely safe, as mosquitoes can occasionally find their way inside homes.

Diagnosing Heartworm Disease In Cats By Veterinarians

Diagnosing heartworm disease in cats is more challenging than in dogs. Cats often yield false negatives in microfilaria tests as they rarely have these circulating.

Antibody blood tests can detect early-stage infections about two months post-infection. Still, they don't confirm adult infections and might remain positive even after the infection clears.

Antigen blood tests, typically used for dogs, only detect female heartworms. This test might overlook some infections because cats can have low and single-gender worm counts.

Chest X-rays can hint at heartworm disease, showcasing symptoms like enlarged vessels, lung inflammation, and edema in nearly half the cases.

Echocardiography allows visualization of live worms in the heart and measures pulmonary pressures, assisting in distinguishing from other cardiac diseases.

While no test is infallible, vets combine physical exams with various tests to gauge heartworm disease's extent in cats.

Managing Heartworm Disease In Cats

Heartworm treatment in cats is challenging. Many therapies, such as melarsomine used for dogs, are toxic to cats. The American Heartworm Society advises against treatments that exterminate adult heartworms in felines.

Cats without symptoms may be observed for a potential spontaneous cure, where their immune system naturally clears the infection. The drug ivermectin can reduce worm count over prolonged use (2+ years), but risks include allergic reactions.

Veterinarians might recommend steroids like prednisolone to curb the cat's intense inflammatory reactions. During respiratory distress, cats need supportive care. The antibiotic doxycycline might be given to counteract the bacteria Wolbachia within heartworms. Bronchodilators such as terbutaline can aid respiratory issues.

Given the treatment challenges, surgical removal of heartworms is often preferred. Invasive surgeries, where worms are extracted from the heart, carry risks, including death.

For cats with acute respiratory issues, immediate stabilization with oxygen therapy, IV fluids, and medications is essential. Any sign of respiratory distress in a cat, like open-mouth breathing, warrants emergency vet attention.

Preventing Heartworm In Cats

Heartworm is a daunting yet avoidable disease. Several monthly or bimonthly preventatives are available for cats, combatting intestinal worms and ticks.

Kittens can begin these preventatives from 8 weeks onwards and sustain them throughout their life. Even predominantly indoor cats need protection, as mosquitos can intrude indoors. Key heartworm preventive medications are:

Managing Heartworm In Cats

Cats testing positive for heartworm should undergo serologic tests, echocardiograms, and chest x-rays every 6-12 months, irrespective of symptoms or treatment. Comprehensive testing typically involves both antibody and antigen assessments.

Cats are only considered fully recovered from heartworm when blood tests turn negative and clinical and x-ray symptoms have subsided. On average, cats with heartworm have a survival rate of about four years.

Effective management for heartworm-positive cats entails vigilant monitoring, consultations with veterinarians, and awareness of the disease's implications. The American Heartworm Society offers detailed resources on this condition in cats.