Heartworm in Cats

By Jean Hofve, DVM

In a seemingly diabolical plot, veterinarians and pharmaceutical companies have teamed up in a marketing campaign to frighten guardians into giving year-round heartworm preventatives to their cats. They say they’re doing this to improve protection for individual pets, but the facts say they have other motives.

With few exceptions, heartworms (Dirofilaria immitis) are a completely seasonal problem, so there is no reason to give heartworm medicine to ANY pet year-round–except to make money for those who make and sell it! Those financially-motivated folks say the number of cases will rise unless everybody gives the medications. They rationalize this by citing statistics on how most people don’t use the products, and proclaiming that the number of unprotected dogs will surely cause even more disease. However, despite years of many animals being given unnecessary drugs and many more who aren’t, the prevalance of heartworm has not really changed.

Heartworms are transmitted by mosquitoes. Heartworm larvae, called microfilaria, live in the blood and are sucked up by the bug. Once inside the mosquito, they must further develop before they can infect another dog. For that to occur, outside temperatures must remain above 57 degrees F, day and night, for a certain period of time (but at least 8 days).

The warmer the temperature, the faster the larvae will mature. If the temperature drops below the critical level, larval development will stop, but the larvae don’t die—development will re-start at the same point when the weather warms back up. Larvae reach their infective stage in 8 to 30 days (the latter being the insect’s entire lifespan–if the larvae haven’t matured by then, they will die along with the mosquito).

It should be obvious that during seasons and in areas where there are no mosquitoes, there is no risk of heartworm. Evidently that little fact escaped the attention of the veterinarian who prescribed heartworm protection—in December–for a puppy living high in the Colorado mountains. At that altitude, temperatures are never warm enough for heartworms!

On this map,  heartworm risk is shown by the month when the use of heartworm preventatives should begin. In most states, protection should be continued through November or December. In the Michigan UP, preventatives are suggested from August through October. Within 150 miles of the Gulf Coast and other areas in pink, prevention is recommended from April through January. In the red areas of southern Texas and Florida, year-round preventatives may be needed. Local conditions may vary from year to year. Global warming, hurricanes/flooding, and other factors may increase the mosquito population and thus influence heartworm risk. (This map is a very loose approximation only, and is not intended to be used exclusively to determine risk.)

When an infected mosquito bites a dog or cat, the microfilaria are deposited on the skin, where they then crawl into the bite wound and enter the bloodstream. Inside the body, they grow and progress through other larval forms. In dogs, the heartworm’s natural host, larvae migrate to the heart and eventually develop into adult worms. In cats, full-grown worms can develop (but not reproduce). A cat cannot transmit the disease. Adult heartworms are over a foot long when grown (in 6-8 months), but it takes only 1 or 2 to fill up a cat’s tiny heart and cause serious problems. However, in 80% of infections, the cat’s own immune system kills the larvae at an earlier stage, and clears the infection.

However, heartworms don’t have to be full-grown to cause problems. In cats, a respiratory condition can develop. Not-quite-full-grown microfilaria can get stuck in the capillary beds of the lungs and cause significant inflammation and damage. This uniquely feline condition is called Heartworm Associated Respiratory Disease, or HARD. Symptoms are similar to asthma; and it’s possible that some asthmatic cats are misdiagnosed with an immune-mediated disease instead of a parasite. Even so, about half of infected cats never develop any signs of heartworm disease at all. And while the disease causes serious damage to the lungs, much of the damage may be reversible by the body’s normal healing processes. Chronic and sequential infections have not been studied.

Indoor cats are, of course, less likely to be bitten by a mosquito, but anyone who’s ever been around them knows that the little buggers can be quite persistent, so it isn’t impossible. In one study, 25% of heartworm-positive cats were reported to be indoors-only. Outdoor cats are at higher risk not only for heartworm but also for feline leukemia, feline immunodeficiency virus (FIV or feline AIDS), and all the other parasites, injuries, and diseases common to outdoor cats.

Heartworm preventative drugs do not kill adult heartworms, but they do kill microfilaria up to a certain stage of development. Currently it is believed that larvae under 6 weeks old are affected. This means that in order to prevent heartworms from reaching adulthood, the preventative can be given up to 6 weeks after the mosquito bite and still work. The recommendation is to give the drugs every 30 days, purportedly because once-a-month dosing is easier for most people to remember (and, coincidentally, it also sells more drugs).

The most common preventative drugs for heartworm are ivermectin (Heargard®) and selamectin (Revolution®). While these drugs are generally considered safe and effective at the low doses used for heartworm prevention, there are always exceptions. Toxicity associated with ivermectin include depression, ataxia (balance problems or unsteady walk), and blindness. Selamectin is also used to treat ear mites and some worms; adverse reactions include hair loss at the site of application, diarrhea, vomiting, muscle tremors, anorexia, lethargy, salivation, rapid breathing, and contact allergy.

Most veterinarians hand out heartworm preventatives like candy; but there is a serious and growing problem of resistance of heartworms to these drugs. This means that we are selecting for “superworms” that will be able to survive and grow even in animals on heartworm preventatives. As with all cases of drug resistance, the correct response is to reduce use of the drug and reserve it only for when it is absolutely necessary. Unfortunately, the veterinary profession and drug industries have decided to go for profits instead, and are continuing to call for all pets to be on medications all year round. This is bad science, and it is bad policy.

If you do use heartworm prevention products, get them from your veterinarian. Surveys have found mislabeled, expired, imported, and counterfeit products being sold from other sources. Follow dosage instructions and do not over-treat. NEVER, EVER use a heartworm product made for dogs on a cat. The components are different between dog and cat products, and dog products can kill a cat in a matter of hours. Many cats have suffered and died this way.



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12 comments for “Heartworm in Cats

  1. jhofve77
    April 17, 2013 at 10:47 am

    There is no known *reliable* alternative to heartworm medicines, but repellents can help prevent mosquito bites and thereby stifle disease transmission. However nothing is 100% reliable, not even the toxic poisons the vet recommends! Just do your best to keep your area free of mosquito breeding grounds, and stay vigilant. I recommend periodic testing for heartworm so that if microfilaria are passed to your pet, the infection is caught early enough to treat with filaricides and not have to go to the arsenic-based products.

  2. Amanda Redfern
    April 16, 2013 at 8:38 am

    Is there a known natural solution to heartworms? Not to treat them when they have already gotten inside your pet, but to help repel them. I currently use Nature’s Chemistry Natural Flea and Tick Spray which uses essential oils to help to repel these pests as well as kill some of them if they are stubborn enough to land on the animal. And don’t worry, they have one especially for cats and especially for dogs and I use both respectively. Will a spray like that help to repel the mosquitoes who might be carrying the parasite? It mentions on the bottle that it is effective on black flies and mosquitoes as well. And in my experience, the product works well. My has been tick free for over a year (which is saying something, because she had about three ticks on her in the first two weeks we had her before using this product). Do you think a repellant would help keep them safe?

  3. jhofve77
    September 16, 2011 at 8:59 am

    Because they are poisons. Here’s a detailed article on how to keep fleas away:
    The Natural Approach to Flea Control
    And if your vet is so nasty, maybe it’s time to find another vet!

  4. Lindsay
    July 21, 2011 at 10:21 am

    Good post and in general I agree with you. I try to give my pets as few meds/vaccines as possible. However, my (indoor only) cat was just diagnosed with asthma possibly from heartworm and I want to point out that there is NO treatment for heartworms in cats. The dying worms can clog arteries and/or cause an inflammatory reaction that kills 25% or so of cats! So in this case, it might be best to at least consider a preventative if you live in an area with mosquitoes. I wish I had known this info before it happened to my cat, I might have made a different decision.

  5. June 1, 2011 at 10:58 pm

    We have never given our dog heartworm medication, based on the advice of our holistic vets, since we live in Colorado where heartworm is not that common. If we lived in Louisiana or Florida, then we might make a different choice. We feel the deteriment of giving a pesticide (heartworm meds) to our dog outweighs the possible benefits. Instead, we test her every year so if she does ever get infected, we can catch it early before it is severe. And we would never consider giving the heartworm preventives to our cats, either. In our business, we have numerous customers with dogs that have contracted immune mediated hemolytic anemia (IMHA), a usually fatal disease, and we have observed that it usually strikes after some toxic exposure, such as vaccinations, flea and tick treatment, heartworm medication or exposure to a high level of pesticides (such as owner lives next to a golf course). Sometimes, the dog has been exposed to a combination of those things all at once. So, it is really a shame that vets do not let pet owners know that heartworm medication is not a totally harmless medication. Even if the dog or cat doesn’t show symptoms from taking it, it is adding to the total load of toxins on the body, which could have long-term consequences. That’s why we don’t give it to our pets.

  6. Gale
    May 5, 2011 at 6:23 pm

    I discussed the heartworm issue with my vet, and he said it wasn’t worth it for the cat, even though my cat does like to go outside under supervision. The incidence in the area, even for dogs, is very low. I am giving it to my dogs because they are out more and go more places than the cat.

    if they can put men on the moon, live for years in a space station and clone sheep, why can’t they sterilize mosquitos? So many diseases are spread all over the world by those beasts, there should be some further research. I know birds eat mosquitoes, but I am sure there are other things they eat.

  7. Lisa
    March 20, 2011 at 6:26 pm

    Hi, I can agree with your statement on the map’s loose approximation. I live in the upper Sacramento valley, and thanks to a persistent high pressure condition, I saw mosquitoes in February this year. Even in more normal years, the little boogers start showing up in April, not July as the map would suggest. But, since I’m a mosquito magnet, I always know when to start prevention – when I start itching.

  8. jhofve77
    January 7, 2011 at 10:40 am

    Hi Cheryl, I can’t make individual recommendations (that would be practicing without a license in your state), so I’d suggest talking with your vet about the specific risk in your area and given your particular situation. You could also call around to see, in general, what other vets in your area recommend for cats. Heartworm prevention, like vaccination, is a medical procedure, and should be tailored to the individual pet, based on a thorough analysis of risks vs. benefits.

  9. Cheryl
    January 7, 2011 at 12:39 am

    Sorry forgot to mention that we live in a development that sprays for mosquitoes. We rarely see them in our neighborhood. On those occassions when we do see them, they are weak and dying. Since they’re pesticides, I’m ultimately wondering if you think the risk of using them is smaller than the risk of not using them or vice versa. Thanks again.

  10. Cheryl
    January 7, 2011 at 12:30 am

    We live in the area of Florida that’s noted as possibly needing heartworm prevention year round. We recently adopted two kittens (now 8 months). They are indoor cats only. We had decided to forgo the heartworm prevention, as we try to avoid pesticides and most of the people w/cats we know here don’t use them, but we really want to do the right thing for them and keep vacillating. Any recommendations? Thank you.

  11. jhofve77
    December 17, 2010 at 10:36 pm

    Testing should always be done before starting a pet on heartworm preventatives.

    Cats do not have antibodies to heartworms unless they have been exposed to heartworms. However, the microfilaria (baby worms) and adult worms are so large that antibodies would not have any significant effect on them; there would be no immunity.

  12. Veronica Greenwood
    December 17, 2010 at 12:12 pm

    I took our cat off of heartworm meds because of skin reaction.

    Should the cat be tested for heartworm prior to treating with a preventative again?

    Also – I read somewhere that cats carry antibodies that fight against heartworm; how does this affect immunity of cat?

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