Heartworm and Cats

ak0402August 30, 2009

I recently received a newsletter from my vet which discussed heartworm in cats. I have never in my life treated cats for heartworm, but according to this newsletter, heartworm in cats is a newly recognized problem. The letter was quite serious, and recommended that cats get preventative heartworm treatment the way dogs do, even for indoor cats. I am unsure if this is just a scare tactic by pharmaceutical companies selling heartworm prevention who want to increase sales, or whether I should take this advice. My vet has always been a "holistic" style vet, one of the first I knew to recommend against annual vaccinations, and interested in alternative medicines. So I was surprised he was sending out this newsletter. What do you think?

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I think for an indoor cat I wouldn't bother.

    Bookmark   August 30, 2009 at 2:15PM
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I do worry about my house cats because they eat every fly, bug or spider that dares to come into the house. Also, I had mice get into my house last year and the cats killed nine of them. My vet says I can carry fleas from outside into the house on my clothes. I don't think indoor cats are totally removed from outside threats.

    Bookmark   August 30, 2009 at 5:59PM
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'My vet says I can carry fleas from outside into the house on my clothes. I don't think indoor cats are totally removed from outside threats."

that is absolutely true. in the first week when we moved to our current house our dogs became infested with fleas. we treated them and the following week they were infested all over again. we couldnt work out where the fleas were coming from until one day after my husband got out of the car and walked out of the garage to talk to me, I noticed there heaps of little black dots on his trouser cuffs, they were fleas! then I remembered the guy who lived here before had two large dogs that slept in the garage, they must have been riddled with them because as it turns at the garage was and they were hitching a ride on us.

heartworm can be transmitted by mosquitos as well.

    Bookmark   August 31, 2009 at 7:53AM
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Heartworms are ONLY carried by mosquitoes, not fleas.

Cats cannot be treated for heartworms if they get them. If they get heartworms often the first symptom the owner notices is the cat drops dead without warning. For the ones that don't have that particular symptom, they end up with severe asthma-type respiratory problems which can be life-threatening. We cannot treat cats with Immiticide like we can with dogs- you have to hope the cat outlives the worm and when the worm dies it doesn't take the cat with it.

It is true that heartworms prefer canines to felines; cats are an abberent host. Which makes diagnosis difficult as well. We detect heartworms in dogs by looking for the antigen of an adult female worm. Because cats are abberet hosts, the female worm often does not reach maturity; also the cat may be infected by a single male worm which causes all the symptoms. It is difficult for a cat to live with 14 inch long worms living in its heart. There is never microfilaria to detect as with dogs. Most cats are diagnosed on necropsy, or on echocardiogram if the vet is smart enough to suspect heartworms. Often the cat is misdiagnosed with asthma and out on prednisolone. Which is all we can do for heartworms anyway, it controls the respiratory illness.

I get mosquitoes in my house all the time. Almost 1/3rd of all cats diagnosed with heartworms is an indoor-only cat. If I had a cat, it would certainly be on heartworm prevention, year round.

    Bookmark   August 31, 2009 at 8:36PM
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hi meghane I didnt mean to imply fleas carry heartworm but my post was worded a little badly.

when I said "heartworm can be transmitted by mosquitos as well." I meant as well as contact with an infected dog although now you've got me wondering if it can be transmitted that way? or is it only by mosquitos?

    Bookmark   September 1, 2009 at 12:28AM
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I agree w/Meaghane,we get mosquitos in the house all the time.I have had to treat an abused Lab for heartworms it was horrible and very expensive.i know you were asking about a cat and as Meaghane said I would definitely have it on a prevtitive year round.Not worth the risk.


    Bookmark   September 1, 2009 at 4:24PM
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Are you sure there are no heartworm treatments for cats? My mother's cat got it (verified by ultrasound) about 15 years ago. I believe it was Tufts in Boston that was going to do the treatments. They said it was risky due to the cat's size (she was about 15 pounds). It was a long time ago, so I can't remember all the details, but I think 2/3 survived the treatment. My mother elected to not treat her because she wasn't getting worse. Eventually the coughing stopped. The vet believed she was infected with only 1 worm, as you described. She lived another 6 years and eventually died of congestive heart failure, so it did prematurely kill her. Her first symptoms, when she was first infected, was coughing and choking, very bad attacks (to the point where they rushed her to the nearest vet in the middle of the night, something I wouldn't hesitate to do, but something my parents wouldn't do unless it was literally life or death).

I asked my vet about it and he said I didn't need to treat my indoor only cats. I think next season I will revisit that with him. Hearing that 1/3 that get it are indoor cats is scary.

    Bookmark   September 1, 2009 at 9:48PM
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Meghane, thank you for your response. I wasn't aware of any of that. I have one more question: in my climate there are truly NO mosquitos from mid-October thru May. I would think I should just give the heartworm prevention in time for the summer months. How long does it take for the heartworm prevention to "kick in" after it's been administered?

    Bookmark   September 2, 2009 at 5:53PM
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OK, you *can* treat heartworms in cats if you have over $2500 for a specialist to do a "worm pull" with fluoroscopy. They use a fluoscope (full motion video x-ray, basically, not something a regular DVM has) to insert a long probe through the jugular vein in an anesthetized cat (which is risky enough considering said cat has heartworms) and they literally yank the worms out of the heart. Hopefully. I've actually seen it done a couple of times at school. Not only is the procedure itself risky, but the aftermath is equally risky because if a worm breaks off (not a rare occurence) the cat may die of anaphylaxis. And did I mention the $2500? That's *after* diagnosis...

The ONLY way to transmit heartworms is to be bitten by an infected mosquito. You can't even give an animal heartworms by blood transfusion. The lifecycle of the heartworm requires development in the mosquito- no other way to do it.

Heartworm prevention actually only prevent heartworms from completing their life cycle in the host. The mosquito injects L4 stage larvae into the tissue of the victim. The prevention kills L4 and sometimes L5 stage larvae that migrate from the tissue to the pulmonary arteries.
L6 is the adult form which lives in the pulmonary arteries.

All heartworm preventatives with the exception of ProHeart injections are only in the animals' body for a day or so. The drug kills L4 +/- L5 stage larvae, preventing the development of adult worms in the pulmonary arteries and therefore preventing symptoms. ProHeart 6 injection is basically a slow-release version of the same type drug (it's moxidectin, the same ingredient as Advantage Multi), so it is in the animal's body for the full 6 months as a slow release.

That is why we test for heartworms before putting a dog on prevention- if the dog has adult worms, heartworm prevention doesn't kill them. It also is why it takes 6 months after being bitten by an infected mosquito to make our test positive- we test for adult female worms, and can't detect L4 and L5 larvae. It takes about 6 months for the worm to develop from L4 to adult (L6) in the dog. Development to adult in the cat is rare.

Usually we say to keep an animal on HWP one month after the last mosquito is seen, and to start prevention 1 month before the first mosquito is expected. This way, we are sure to kill the L4 stage before it has a chance to develop into adult worms.

    Bookmark   September 2, 2009 at 10:05PM
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Great Information Meghane.
I didn't know all the ifo on the HW preventatives.

    Bookmark   September 6, 2009 at 10:05PM
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