April is National Heartworm Month and a great time to review your heartworm treatment protocols. We asked Area Medical Director Dr. Kelly Dunham to review best practices for feline heartworm; for a more general take on heartworm treatment (not species-specific), please click here.
It’s finally spring, which means warmer weather and more time outside with pets. It also, unfortunately, means greater risk for heartworm.
The American Heartworm Society has great resources on heartworm for vets and pet owners alike. They report that as of 2019, no state is heartworm free … which means no matter where you live, prevention and treatment should be top of mind.
Heartworm disease affects dogs, cats and ferrets, as well as wildlife such as wolves, coyotes, foxes, sea lions and sometimes even humans (although this is super uncommon).
Cats are not a typical host for heartworms, and most heartworms in cats do not survive to the adult stage. At first glance, this seems great, but not only can this cause significant delays in diagnosis, but the immature heartworms can still cause significant damage.
Where about 60 adult heart worms may develop in a dog, a cat with the same infection load may only develop 3-10 adult heartworms. Of these cases, 30% are same sex infections. This is still considered to be a heavy infection in a cat.
In cats, there is even greater risk from smaller number of heart worms as infection can cause sudden death. Clinical signs and risk of sudden death are most likely during the time when heartworms reach the pulmonary vasculature (approximately 3-4 months after infection) and during the death of adult heartworms in the system.
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Feline heartworm diagnosis
Heartworm disease is often misdiagnosed in cats due to the limitation in testing.
Cats can develop heartworm-associated respiratory disease (HARD) which can present similar to asthma or allergic bronchitis. Although respiratory signs are most common, other clinical signs can include GI upset, neurologic changes, or generalized ADR which are typically transient.
If a cat survives the initial arrival of heartworms to the pulmonary vasculature they are likely to stabilize due to suppression of immune function from the live heartworms.
However, when the adult heartworms start to die (after approximately 2-4 years) the resulting pulmonary inflammation and thromboembolism can cause an acute fatal lung injury.
This process is different from caval syndrome which is seen in dogs. In dogs, caval syndrome results from the sudden death of large numbers of adult heart worms. In cats acute lung injury and death can result from the death of even 1 adult heartworm.
Challenges to testing and diagnosing feline heartworm
Testing and diagnosis can be very challenging in cats. Microfilariae are not often seen during test, so performing more specified testing such as a modified knotts test can help increase changes of an accurate diagnosis.
While antigen testing is part of the gold standard for testing in dogs it is not reliable in cats due to the common occurrence of immature or same sex infections. Heated sample tests can be done through the reference lab, just like with the dog, but is not combatable for use within hospital snap tests.
This will still have the risk of false negative to sue a degree, that a negative test should be considered No Antigen Detected, and positives can be considered true positives.
Imaging such as radiographs and ultrasound can also be helpful at identifying physiologic changes such as enlargement to the main lobar and peripheral pulmonary arteries and in some cases presence of adult worms. Even this can be tricky to see, as it can taper or normalize over time in cats.
Feline heartworm treatment
So, what do we do? We do not currently have any established effective treatment protocol for cats.
Unfortunately, Melarsomine is toxic to cats and cannot be given as part of adulticide treatment in cats. Ivermectin given monthly for 2 years had show to reduce worm burdens by 65%. However, in cats, who usually only have 1-3 worms to begin with it is not the amount of worms that causes the problem, but the potential anaphylaxis which can occur during their death.
Due to the risks associated and difficulty of killing adult heartworm in cats, surgical intervention is currently our only form of actual treatment. IT is recommended to locate the adult worms via ultrasound prior to planning surgical removal to ensure they are reachable, and not every worm is always able to be reached and removed.
The first step is always to make sure the pet is stabilized first. This sometimes includes fluid and/or oxygen therapy, glucocorticoids, and bronchodilators.
For all affected cats, we can give a course of prednisone, particularly in cats with radiographic changes, or clinical signs to reduce inflammation and the risks of sudden death. The current recommendation is to start at 2mg/kg/day and by 2 weeks slowly taper to 0.5mg/kg/q48 for 2 weeks before discontinuing.
If clinical signs recur, treatment with pred can be repeated. As in dogs NSAIDS should be avoided and can even worsen parenchymal pulmonary disease.
Feline heartworm prevention
The moral of the story, is prevention! Prevention should be started in all cats at 8 weeks old, and continued monthly for life in all cats living anywhere that dogs can have heartworm (which is everywhere).
While testing is recommended prior to starting a monthly preventative, these medications can be started and continued without prior testing due to the limitations of testing in cats. Additionally the low levels of microfilaria in cats make it unlikely to cause an anaphylactic reaction.
The best course of action when dealing with feline heartworm is prevention. Treating and diagnosing feline heartworm can be tricky due to testing limitations and a lack of established treatment protocols. For more resources, visit the American Heartworm Society.
Dr. Kelly Dunham is IndeVets Area Medical Director for greater New York.