Mosquito Season Means Heartworms ... In Cats?

Patty Khuly, DVM
Published: June 20, 2011
Mosquito Season Means Heartworms ... In Cats?

Yes, naysayers abound. They claim feline heartworm disease is an artificial construct born of a pharmaceutical industry conspiracy to fashion a market for potential feline heartworm disease sufferers — of which precious few are in evidence. Fear, say the detractors of this disease’s suggested ubiquity, is the currency of heartworm prevention’s marketeers.

In other words, detractors posit, you’re being duped when you worry about heartworm disease in your cats.

To be sure, dogs are an indisputably afflicted bunch. If you’ve ever cohabitated concurrently with dogs and mosquitoes then you know the drill: Administer a monthly heartworm preventative year-round (or for those who live in northern climes, only during the months when the ground doesn’t freeze).

But cats? Are heartworms truly worthy of the expense, bother and stress inherent to monthly prevention? Here’s what the American Heartworm Society has to say:

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%).

Zero to 25 percent. That’s a big spread. Some reports indicate that the incidence of heartworm disease in cats living in heartworm-prone areas is as high as ten percent. But it’s really hard to know if that’s true.

Partly that’s because we don’t test cats for heartworms as often as we do our dogs. This is presumably, at least to some extent, because heartworm testing for cats is more complicated than for dogs. But it’s not as if it’s rocket science, either.

So what’s my take?

If you’ve got a cat in a heartworm-prone part of the world — indoors or out — you should ideally be using a heartworm preventative. Here’s why (again, according to AHS):

[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).

So what comes after the initial phase? In some cases, sudden death is the outcome. In fact, some cats skip the whole introductory thing and go straight to gone, leaving us little time to bother with basic things like getting to a diagnosis.

It’s for this reason that I do the monthly preventative thing for my cats. And now that these products kill lots of bugs in one fell swoop, using them seems like a no-brainer. But then, I live in Florida where bugs of all stripes abound. For those living in middle zones, the choice may not be so clear. But what can I say? At the risk of ringing the drug maker’s bell, "better safe than sorry" works for me.

Dr. Patty Khuly

Pic of the day: Kitten in the Grass by Energetic Spirit

 kitten in grass, cat in summer, mosquitoes and heartworm, heartworm in cats, heartworm prevention

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