Heartworms in Cats
Q: Dr. Richards-
I appreciate your info about pemphigus. Some followup-"Lexy" responded very
well to a brief period of steroids. She is now on weekly gold with the steroids being tapered. All of her skin lesions are gone and she has gotten back to her job as head of the household. I would like to ask an unrelated question. I have seen some references about heartworm in cats. I always thought of this as a dog disease. None of the vets I have been to recently have brought it up, but I have never specifically asked. I live with (not own you understand) six cats.
They are mainly indoors but do have a cat door which allows free access to outside. we now live in southeast Ohio but previously lived in North Carolina. Do you think I should pursue testing and prophylaxis? Thanks for your help. I have multiple other questions but I'll try to limit them to one every other week or so.
Take care, J.S.
I am ambivalent about heartworm medication for cats. On the one hand, it is preventable disease and so I think it is a good idea to prevent it. On the other hand, it is much less common in cats than it is in dogs and despite company claims to the contrary, we have a fair number of cats who don't like the chewable pills and a lot of cat owners who don't want to administer the pills if the cat won't take them readily. I am hoping that selamectin (Revolution Rx), which is a topical (applied to the skin) product, will prove to be efficacious and safe. It is supposed to prevent heartworms and kill fleas for a month with once monthly dosing.
That will make it easier to dose all cats. The "expected" rate of heartworm disease in cats is usually about 10% the rate of dogs in an area -- so if 80% of dogs not on preventative medications would get heartworms in an area, about 8% of cats will. This is still a guess in reality, though.
At the present time the usual recommendation is just to start preventative medications without doing prior testing. This is different than the recommendations in dogs and is based on the fact that antigen tests (test directly for heartworms) aren't as accurate in cats as they are in dogs and reactions to microfilaria are theoretically unlikely since very few cats have microfilaria (baby heartworms) in their bloodstream even if they are infected with adult heartworms.
As long as the cost of the heartworm pills doesn't make other necessary care get pushed aside, I'd recommend using the preventative in areas in which heartworms occur. Especially now that there is more than one choice in preventative medications. I might not rush to be the first one in my neighborhood to try Revolution, though. I like to wait and see if there are going to be unexpected reactions, at least for a couple of months.
Mike Richards, DVM
Sudden death in cats
Q: I'd like to pose a question about our cat, Mookie, who up and died while we
were petting him this past Sunday.
I've searched the web and all pet sites about SUDDEN DEATH and have found
this "affliction" only in the context of heartworm. I wonder if you have
other ideas on this? Mookie was a healthy active 11-year-old Manx. All shots
up to date. No lethargy or appetite suppression. He was purring one moment
and the next he let out a blood-curdling scream, then died.
Thanks in advance.
Real "sudden death" is an unusual occurrence. As you noted, it can happen
in cats infected with heartworms. It is also reported to occur in cats as
the result of cardiomyopathy. This disorder can be very insidious and may
not be detected prior to death. Cats probably get aneurysms that rupture,
although I have only seen one report on these. Anything that causes
thromboembolism can cause sudden death -- this can be seen as the result of
cancers, blood clotting disorders, cardiomyopathy, trauma and other
disorders. Low serum potassium can lead to sudden death, as can high serum
potassium -- but usually there are other signs of illness making the
process seem a lot less sudden in retrospect.
I am sorry to hear about Mookie. It is unfortunate but without a necrospy
(autopsy) exam it is not possible to tell you what might have happened. It
is hard when things like this can't be resolved.
Mike Richards, DVM
Annie and Feline Heartworm Disease
Q: Dear Dr. Richards,
I have spoken with cardiologists at various teaching universities who
surgically remove heartworms from cats. Dr. Gavaghan (U.C.-Davis), Dr.
Rawlings (U. of Georgia), and Dr. Eyster (Michigan State Univ.) remove
the worms through the jugular vein. Dr. Bill Brown (Michigan Veterinary
Specialists) uses a different approach which is thoractomy and incision
into the right atrium to lift them out. All combined the number of surgeries
using both methods totals approximately 16, so there's not a lot of
experience doing this although they seem to be successful, based on
what they've told me.
The purpose for my note is to ask your advice regarding how to decide
which avenue is best for the cat's survival based on the procedures
described above or to continue treating the symptoms with Prednisone and
Aminophylline. How would you recommend that a person base this
decision (removing cost as a factor)?
In my cat's case, she was diagnosed in May '98. Her symptoms became
very serious for several days in early July - vomiting, diarrhea and
My vet injected her with Prednisone and added Aminophylline and
Furosemide to the menu. Of course, we don't know what happened, whether a worm died or not, but she suddenly stabilized and has been 100% without symptoms for the past 6 weeks.
A second question that I have for you is what your thoughts are
regarding the fact that she has no symptoms now and that I'd like to reduce the
amount of medications she is taking. We recently decided, since her lungs
sound clear now, to drop the Furosemide to once a week. I am also
considering dropping the frequency of Aminophylline (currently 25mg twice/day) to
once a day, and perhaps removing it entirely if she continues to be free of
symptoms. The reason for this is that I travel for business and, considering
my responsibility for giving her all these meds each day, have been unable
to leave her. She was once a feral cat who has become very attached to me.
No one else can get near her. It's a bit of a dilemma especially considering
the long duration of this disease. That is another reason why I am gathering
information on a surgical approach, it would bring this disease to an end,
Thank-you for your time.Nancy
It might be a good idea to consider retesting for heartworms. It would not
be unusual for a cat to have a single heartworm and the symptoms you
have seen would be consistent with death of a heartworm. We have seen the
antigen disappear from the bloodstream in as little as two months in
one cat. If there was a positive antigen test previously and it becomes
negative that would be a good sign. Ultrasonagraphy is another method
of assessing whether or not worms are still present. University practices
may offer even more sophisticated testing such as nuclear scintigraphy
using radiolabeled antibodies against heartworms.
If heartworms are still present it would be necessary to continue to
consider which form of therapy to go with. While I don't have any
personal experience with heartworm extraction in cats to go on I do think that I
would consider it as an option. Both the presence and eventually the death
of the heartworm are dangerous for the cat and eliminating the worms
surgically would reduce that risk immediately if successful. Since some
sort of imaging to identify the location and number of worms would be a
good idea prior to surgery the testing mentioned above would probably
be part of the work-up for the procedure.
When clinical symptoms diminish we taper off the medications. So far,
this has worked well for us when the cat has been symptom free for 6 weeks
or more prior to withdrawal of the medications but the number of cases we
have treated in this manner is still only about 3 cats. We chose 6 weeks
If possible I'd base these treatment options on confirming that heartworm
disease is still present. If that is not possible and symptoms have
lessened I think it is reasonable to taper off the medications and see
what happens. If that is not possible and symptoms remain then I think the
choice is harder. Since there are few cases to base a decision on you
may just need to make a choice arbitrarily. Trust your instincts if it
comes to this.
Mike Richards, DVM
Dear Dr. Richards,
This is a very sad note for me to write to you and although it's been a
week since Annie died, I'm still devastated over losing her.
In retrospect, as I reread your attached note, I wish I'd followed up on
the surgical option. She was symptom-free for 4 months and although I
retested her for heartworms in September and it came back positive, she was
handling the disease so well that I was blindly optimistic.
Two weeks ago, she had a short wheezing episode and my vet increased her
dosage of Prednisone to a daily rate for 5 days, then back to every other
day. She responded well and there were no further episodes. A week later,
he climbed to the top of a bookcase and jumped 7 feet to the floor. I
saw her up there and was going to get her down before she jumped, but my
other cat started a cat fight in the backyard and I had to run outside. When I
came back in, Annie was walking around and I was relieved she'd gotten
down without incident. Within several hours, she apparently dropped dead. I
didn't see her until I got up the next morning, concerned that she hadn't
come in to curl up on the pillow against my head.
I suspect what happened is the jump from the bookcase jolted a worm that
may have been decomposing and a worm fragment traveled to the pulmonary
artery and she had a pulmonary embolism. It's all I can do to forgive myself for
not getting her down from the bookcase, or for not pursuing the surgical
removal of the worm(s).
I suppose there's some relief in sending you this note, as I type the
words to someone who offered support and advice. I am so angry with this
disease and the feeling of helplessness that accompanies it. I would have done
ANYTHING for Annie, if I'd only known WHAT to do. She was deserving of a
wonderful long life after the terrible neglect she suffered being a feral
or abandoned cat living in a swamp behind a parking lot with snakes and
mosquitoes. With all the love she gave to me I was committed to keeping
her comfortable and safe. If there is anything you can think of that I can do
to help other cats or owners of cats with heartworms, please tell me. I need
some good to come from this.
I have thought about this for some time. I apologize for the delay in
responding. The only thing that I can think of that might help is to let
people know about this disease and how it affected you. With that in mind
I would like to post your note on our website. Your note has influenced my
view of heartworm in cats and its effect on cat owners. Since heartworm
disease tends to be less common in cats than in dogs and since cats are
not always easy to give medications to, I have tended to take a neutral
approach to this preventative. Your note has made me rethink this position
and I have been more vigorous about discussing heartworm preventative with
cat owners. Perhaps that will make a difference for a cat in our practice
Mike Richards, DVM
Reply: Posting my note on your website is fine. Especially if it could help others
take the prevention of this disease seriously.
Focused research needs to be taking place in the area of how to rid cats of
heartworms once they're infected. It seems that a handful of cardiologists
associated with university vet schools are beginning surgical removals, each
using their own method. No one I spoke to had performed more than 8
procedures and they ranged from pulling worms out through the jugular vein
to going directly into the right side of the heart. When trying to decide
what to do for Annie, I asked cardiologists about the risks.
Using the jugular approach the risk was worm breakage and instant death.
Going directly into the heart the risk was that, between performing the
ultrasound and the surgery, the worms could move into other areas of the
heart, most likely the pulmonary artery, and not be accessible to removal.
Using the latter approach, in the majority of cases not all worms were able
to be removed, but the cats' chances of survival were improved by lessening
the worm burden. My vet was concerned that the procedure seemed traumatic.
I chose against surgical removal because the risks seemed just as great, or
less, depending on how you look at it, as her chances to survive without it.
And, if she had died on the table I would have felt just as badly, and
Your influence in the field of veterinary medicine to encourage
cardiologists to stay focused on this issue would be helpful, I'm sure. It
seems that there are quite a few unfortunate kitties at shelters infected
with heartworms who could be surgical candidates. As a cat owner, it's
difficult to offer your pet as part of an experiment.
I am encouraged to read an article in the Kalamazoo Gazette dated Nov. 4,
1998 which states: "Cat lover gives $1 million to MSU (Michigan State
University) vet school". It goes on to say that upon her death, this woman's
donation is intended "to help establish a new center that will research the causes
of life-threatening conditions that affect cats and to develop new treatment and
In addition to the money she is willing to the college, she "also gave
$25,000 in cash to help the fund-raising campaign for a new endowed chair
for feline health." Surprisingly, she only owns one cat, a 19-year old
Siamese, but she is quoted as saying "We tend not to spend a lot of time
looking at the behavioral health of cats." Since MSU is on of the vet
schools I spoke with who is already removing heartworms from cats
surgically, maybe this donation will put them in the forefront and cat
owners dealing with this tragedy of a disease will have somewhere to go with
Heartworm Prevention in cats
Q: Dear Dr. Richards,
I just received my first copy of VetInfo Digest and absolutely love it.
I hope not to take advantage but have two questions.....
1) What are your thoughts on heartworm prevention for indoor cats? I
have 13 cats, all indoor, vaccinated and spayed. Ages range from 2 to 8. Don't see such literature on the subject until lately where studies are showing more
cats have died from it than thought (autopsies were done). I firmly believe
in the best care for all my felines...but have found alot of vets only
want to sell, sell, sell medication. I like your approach to 3 year vaccines.
Unfortunately, here in Fla. the law states you must have a rabies vaccine
A: Dear Karen-
Heartworm disease is less common in cats than it is in dogs. While this is
definitely an estimate, I think that it occurs at about 10% of the rate
that it occurs in dogs. In an area like Southern Florida with a virtually
continual mosquito population the infection rate in dogs approaches 100% in
outside dogs. So an outside cat probably has about a 10% chance of getting
heartworms. Being inside makes it less likely that heartworms will develop
in a cat but does not eliminate the possibility. In a review of a number of
cases of heartworms indoor cats represented about 3% of the cats affected.
So it is possible for an indoor cat to be infected or else the people in
the study weren't being totally truthful about their cat's indoor only
status. The odds of an indoor cat getting infected are pretty low, though.
Heartworm medication appears to be pretty safe. So the question comes down
to economics. If money were no object at all, then I think I'd recommend
putting all cats on heartworm preventative in areas in which heartworms
occurs. Money is usually at least somewhat important in the decision making
process, though. Heartworm preventative for 13 cats is going to be a pretty
big expense. It is very likely that you would be better off saving the
money you might spend on this to attend to other health concerns. It is
inevitable that you will run into problems such as kidney failure,
diabetes, hyperthyroidism and periodontal disease as your cats age and it
would be good to be prepared for the cost of these things. They are more
likely to occur than heartworms in cats kept indoors.
Mike Richards, DVM
Feline Heartworm disease
Q: Dr. Richards,
Let me give you some background on Annie, who was diagnosed in May
during a routine chest x-ray and follow-up Heska test for heartworms. A
recent ultrasound confirmed several heartworms in the right side of her
heart. We were unable to look at her lungs with the ultrasound, as it
was explained to me that nothing distinctive would appear except for a
gray image. The x-ray did indicate significant lung inflammation,
Her symptoms have been sporadic. Several weeks ago she would awake in
the night gasping for breath. Her respiratory rate was racing
intermittently. I dragged her into the vet several times out of concern.
As a result, her Prednisone dosage was increased from 5mg every other
day, to 5mg twice a day. A bronchodilator, Aminophylline, was added at
25mg twice a day. And, in response to increased lung inflammation shown
in a second x-ray (30 days after the first), a diuretic Furosemide, was
added at 6mg once per day.
Soon after increasing her meds, I gave her the monthly Heartgard tablet.
That night she had breathing difficulties and severe vomiting and
diarrhea for the next 48 hours. We stopped all food and meds until she
stabilized. In response to being unable to keep her meds down, she was
given an injection of 6mg methylprednisolone acetate. This, on July 9.
Since then (1 week has lapsed), she has been symptom free. Her appetite
is good and she is social. Her current dosages have been adjusted a bit
since the symptoms have lessened. The diuretic every other day, instead
of daily. The Prednisone is back on the smaller dosage of 5mg every
other day, because of the recent injection that my vet says could help
her for up to 6 weeks. And the bronchodilator remains at twice per day.
Now that you have been introduced to Annie, who is a beautiful
Siamese-mix once homeless cat who was living in a mosquito infested
Florida swamp, and whom I am totally committed to, I have some questions
1) What are your thoughts regarding her symptoms, condition and dosages?
2) Is it possible that the Heartgard caused the severe vomiting and
diarrhea, when prior to this there was none?
3) Is there any benefit to giving her the Heartgard if she is no longer
exposed to mosquitoes?
4) There are 6 other cats in the household. I assume her condition does
not expose them to any risks, however, considering her lowered immune
system caused by the Prednisone, is it best to keep her away from the
5) I understand that adulticide is an option for killing the heartworms,
although risky. Dr. Dillon at Auburn University discusses
thiacetarsamide at his website (www.vetmed.auburn.edu/fhd). Dr. Sung
Shin, S. Korea, says the most recent and relatively safest is immiticide
(melarsomine hydrochloride), although he is not experienced with cats.
What are your thoughts?
6) The University of California - Davis is surgically removing
heartworms from cats through the jugular vein. It involves
echocardiography to guide the instruments. In dogs, fluoroscopy is used,
but the opinion is that echocardiography has more advantages in the cat.
Do you know anything about this?
7) From all my research on feline heartworms, it is my understanding
that the greatest risk to the cat is when a heartworm dies. At this
point it travels to the lungs and can cause pulmonary embolism. Are we
helpless to this event? Do the meds help prevent the risk of embolism?
Is there anything else that I can do?
8) Aspirin was recommended by the specialist performing the ultrasound
to lower the risk of clots, however, Dr. Dillon's website discusses
increased inflammation that can result from aspirin. Do you have any
9) Have you experienced cats outliving their heartworms and the
resulting damage to internal organs and becoming normal, healthy cats
again? Do you have any ideas on the chances of survival?
Thank you for providing further information to help me support Annie
during her disease.
A: Dear Nancy-
Annie is lucky to have found you.
1) I think that you may have just seen what happens when a heartworm dies
in a cat. The symptoms of the disease tend to get very much worse, very
rapidly. If it is possible to nurse the cat through the acute crisis things
will usually settle down until the next heartworm dies. This is conjecture,
as you are aware, and it is possible that the existing heartworms are just
causing a lot of reaction in Annie.
2)It is not as common for cats to have microfilaria (circulating baby
heartworms) as it is for dogs to have them. Sometimes, when microfilaria
are present, there can be an adverse reaction to Heartgard (Rx) or
Interceptor (Rx). So I'd think it was possible that the reaction may have
been due to the heartworm preventative but am not at all sure it is the
most likely cause. I haven't seen any reports of adverse reactions like
this to Heartgard in cats so it is important to realize that I am
extrapolating from clinical experience with dogs.
3) Recently I read a review of feline heartworm disease (I think that a
brief abstract of the paper is on our subscriber site. I can not remember the exact details
but something like 100 cats with respiratory disease were tested for
heartworms in Texas and Louisiana. I think 16 had heartworms and 3 of those
were cats thought to be totally indoor residents. No one really knows what
the incidence of heartworms in cats is. This makes it hard to give really
accurate risk/benefit advice but the risk of heartworm infection in a
house cat is low. How low is hard to say.
4) Heartworms are only transmitted through mosquito bites and only from
animals with circulating microfilaria. This would seem to make the risk to
your other cats negligible. There may be more risk to Annie from the other
cats but it is probably not highly significant, either. I am assuming that
Annie doesn't have something else, like feline leukemia virus infection or
feline immunodeficiency virus.
5)I have talked to the Merial technical vets on a couple of occasions about
adult heartworm treatment and they strongly advise NOT attempting to kill
adult heartworms in a cat. It is my understanding that over 50% of cats die
when attempts are made to treat adult heartworms and that the natural death
rate is thought to be about 50% as well. So there does not appear to be any
benefit to attempting to treat heartworms in cats at the present time.
Heartworms do not live as long in cats as they do in dogs and we have had
several cats in our practice live through infections and become heartworm
negative. We have also had a number of deaths associated with this disease.
I really think our clinical experience supports a survival rate of 50% or
6)I have surgically removed heartworms from dogs, without the aid of
sophisticated imaging instrumentation but in the manner described, and some
of those dogs did very well. I have not attempted to do this in cats and
was not aware that it was being done at UCD but it may be a viable option.
The best thing to do to find out is call and ask their success rate with
this treatment technique.
7)I think that it helps if the cat is on prednisone at the time the
heartworm dies. This is only a clinical impression, not supported by any
reported studies that I know of. That is the only medication that I have
strong feelings about. Aspirin usage is/was/will be controversial,
especially in cats. There are studies to support its use, studies saying it
doesn't make any difference and reports of detrimental effects in a few
dogs. I haven't seen anything about cats with heartworm disease and aspirin
use in the literature but it is always possible (or even likely) that there
may be something published I don't know about.
8) I don't like to use aspirin and prednisone at the same time and I like
prednisone better so I use it. Some vets do use these medications in
9) I think I answered this one earlier. We have several patients who has
been heartworm antigen positive and lived through the heartworm infection
to become heartworm antigen negative. These cats had clinical signs
suggestive of the disease in addition to positive tests. Only one of these
cats also had ultrasonagraphy and I don't think it was followed up after
the symptoms stopped and the cat went off meds successfully but the cat has
been doing fine for a couple of years now on no medications. The damage to
cats seems to be more severe and the recovery period post infection longer
than in dogs but we do have patients who appear to have made complete
10) I think that very close observation and aggressive treatment of any
suspected crisis is very important. You and your vet seem to have managed
this very well in the last episode but keep vigilant. Also, I find that
oxygen therapy is very helpful if it is possible to implement during a crisis.
Hope this helps.
Mike Richards, DVM
Advanced heartworm disease
Q: My 17 year old cat, Muffin, was diagnosed with advanced heartworm disease in February of this year. This was determined through both in office and a mail in test. (In fact, Heska called my vet back to make sure that the sample was really from a cat. They said that they had never seen such a high positive reading.) However, Muffin also is diabetic. My vet does not want to treat her with prednisone, feeling that it will seriously affect the blood sugar levels. She is on glipizide for her diabetes. This is not keeping her at the desired levels, but does provide some reduction. (My husband said that he did not want to start insulin injection, mostly due to our unsteady working hours) We do have her on theo-dur tablets for breathing.
My questions are: Is there anything else that we might want to consider doing to help with the heartworm disease? What are the things that I should look out for in conjunction with the heartworm disease? How severe a factor will her age be in "pulling through" and getting rid of the heartworms? How will I know that she has outlived the heartworms? Any other advise that you might have on treatment for either the heartworms or the diabetes would be greatly appreciated. BTW the website is great. I found out more from the questions and answers here, than I have been able to find on many other sites combined. Thanks for all the effort and time that is put into maintaining this forum. Dawna
A: Dawna- I hate it when a cat or dog has a problem with interferes with treatment of another problem but that is the case with Muffin.
I would be very hesitant to use prednisone in a cat only marginally controlled with glipizide. I do not know of any other recommended treatment in cats, either. It is possible that anti-histamines may help some, that other immunosuppresive medications may have some effect (I sort of doubt this one) or that very judicious use of aspirin may be beneficial but those are the only things I can think of.
Heartworms tend to live 1 to 2 years in cats. We try to remember to discuss whether or not people are seeing coughing or other symptoms (vomiting, weight loss) on yearly exam visits. If not, we test the cats again if they were positive on an antigen test or retake X-rays to see if the changes from the heartworms are resolving. It is still sort of a guess as to what is going on, though. The Heska test is an antibody test and is likely to remain positive even after the heartworms are gone but I am not sure of that.
Muffin may very well live through the heartworm infection even without treatment. I hope so.
Mike Richards, DVM
Heartworm prevention in cats
Q: I have read all the heart worm stuff here...why won't anyone say whether or not to use a heartworm preventive?
A: I suspect most vets don't yet have much feel for using heartworm preventatives in cats. The incidence of heartworms is much lower in cats than it is in dogs, making it a little harder to justify the cost of preventative if money is tight. Cats don't take medications as easily as dogs do there is more possibility that the medication won't be usable. I give it to my cats, though. I think anyone with outside cats or inside/outside cats in a heartworm endemic area should consider it.
Mike Richards, DVM
Heartworm Preventive for Cats:
Merck (tm) recently released a version of their Heartgard (Rx) heartworm preventive, containing ivermectin, for use in cats. The product is a chewable pill formulation which is available in two dosage sizes, one for cats weighing 5 lbs. or less and one for cats 5 to 15 lbs. of body weight. The medication is administered once a month, on the same day each month, for heartworm prevention. It may be used in cats already infected with heartworms to prevent introduction of new worms into the cat's cardiovascular system.
The manufacturer recommends testing cats prior to placing them on Heartgard (Rx), which follows the recommendations of the American Heartworm Society. While testing of cats for heartworm disease is not as accurate as testing dogs for heartworm disease, there are tests available from Idexx and Heska for cats.
This is a prescription medication. It will also control infection with hookworms in cats. The package insert indicates it is safe for use in kittens over six weeks of age and that it is safe to use during pregnancy. There are no known conflicts with other medications at this time. It is packaged in cartons of 6 tablets and should be used from the beginning of heartworm (mosquito) season until one month after mosquitoes are gone. For some areas of the country it will be necessary to use this medication on an all-year basis.
Mike Richards, DVM
Heartworm medication and cats
Q: I have started noticing ads suggesting we should start giving heartworm medicine to cats. I live in Kansas City, my two spayed girl felines are strictly indoor cats and I've always been reluctant to administer anything of a medicinal nature unless it's absolutely necessary to fix an illness. Is heartworm medicine really necessary? I understand the need to give it to dogs, a friend of mine recently had to have his Am Staff treated for a severe case of 'em and it was really hard on the dog. But cats? Let me know, whenever. I'll be checking the internet. Thanks.
A: p- Cats are less commonly affected by heartworm disease than dogs. The exact difference in the rate of infestation probably varies from place to place but it appears to run about 10% of the infection rate of dogs in the same area. So where I live, I expect an infection rate in unprotected outdoor cats of about 4 to 5%. Since heartworm disease is carried by mosquitoes, exposure to them has to be considered. There are very few dogs who really live totally indoor lives. There are many cats who do this. I think that the heartworm preventive is pretty safe. All of these variables have to be taken into account in the decision to use heartworm preventative.
I tell my clients with outdoor cats that they definitely should consider the use of the preventives. I tell my clients with indoor cats that if it makes them feel safer and they don't mind spending the money it's fine with me if they want to use preventative but I don't feel the need to insist that they consider it for these cats.
Hope that helps clarify the situation for you.
Mike Richards, DVM
Q: I have a cat that has been diagnosed with heartworm-before preventive came out. Want to help him survive the dying off of the worms. Have him on 5 mg of pred every other day. Should I also have him on preventative med? What else can I do to maintain this cat?? He was diagnosed with in house blood test--no Xrays to this point. Do you recommend xray? Should I restrict his activity? Thank you for your advice.
A: I usually take X-rays of these cats, just to be sure that something else isn't going on. The X-rays also help to make sure the test is not a false positive (which is pretty rare in cats, I think). Prednisone is the mainstay of treatment since the mortality rate of treatment with adulticide medications in cats is supposed to be about 50%. No one seems to want to take that big a risk and I don't blame them.
I am not sure if there will be reactions to the preventives in the presence of heartworms. This appears to be very rare in dogs and I assume it will be in cats, too, but I am not sure. I want to wait and see what is reported as people start to use the preventive medications for cats. Right now, I do not feel comfortable recommending the use of heartworm preventatives in cats positive for heartworms.
If you want to recheck the values, Heska has just come out with a heartworm test specifically for cats. The blood has to be sent to them but it is a way of checking the accuracy of the previous test.
Mike Richards, DVM
Q: I've heard that Heartgard has currently produced a new product for Felines. I was wondering if you have heard any statistics about this, or if their are any test to diagnose it (i.e. Occult, or Direct test)? Thanks for any info!
A: I have seen one study on the prevalence of heartworms in outdoor cats in an area in which heartworm disease is endemic. This study found that 4% of cats checked at shelters in an endemic area were positive for heartworms, if I remember correctly. I have seen estimates from around 1% to up to 10% based mostly on the "best guess" technique.
It is possible to diagnose heartworm disease in cats. One of the "standard" test kits, made by Idexx, is licensed for use in testing cats but probably can only detect around 40% of the cases of heartworms in cats. The test kit requires a minimum "antigen load" to be positive and cats often have a smaller number of heartworms than dogs, making the test less accurate. There is a new test on the market for cats, marketed by Heska. It is a mail-in test and is reported to be very accurate approaching 100% accuracy. Very few cats have sufficient numbers of microfilaria to test positive on direct or filtration/concentration testing methods. X-rays have been useful to confirm heartworm disease in cats when the testing was questionable.
We have at least 4 or 5 cats currently under our care who have tested positive for heartworms and have clinical signs of the disease. Over the years, we have diagnosed a significant number of cases of this on post-mortem exams -- perhaps as many as 20 or more cases. We practice in an area in which heartworm disease is very common because of the close proximity of the Chesapeake Bay and tidal water and relatively warm climate. We will offer the new heartworm preventives for cat. I have no idea at this time how much interest there will be among cat owners in using prevenatives, though.
Mike Richards, DVM
Control of HW symptoms
Q: Dr.Richards, Thank you for your prompt reply for info on feline heartworm disease. What can I expect when this cat goes into crises when a worm dies off. How can I best help him @ this point? Thank you again.
A: We have seen several cases of sudden death associated with the die off of heartworms in affected cats. Fortunately, not all of the cats have this effect. Some cats do not show any signs of problems at all, that the owners are able to detect, other than slight increases in the signs already present, such as coughing or retching. These tend to come up as part of the history on physical exams after the problem is over and we are presuming that heartworm die off explained the increase in symptoms seen for a while. We have had one cat make it to us in a crisis state from pulmonary embolism associated with the death of a heartworm - at least we think that was the problem since the cat did live. We just used high doses of injectable corticosteroids (dexamethasone in this case), strict cage rest and oxygen therapy. It worked but I don't know if it is the best treatment method or if were were just lucky. I haven't seen a lot written about care of cats with this complication. I think that cats do better if they are kept on prednisone at a level that controls the symptoms of heartworm disease. Other than that, not encouraging excessive exercise seems wise. Heartworms are not supposed to live as long in cats as they do in dogs and they should die off within 2 to 3 years. I hope that you only realize problems occurred in retrospect, too.
Mike Richards, DVM
Heartworm disease in cat
Q: Dr. Richards, Thank you again for your prompt reply-- good news and bad news huh!? I had my cat Shadow x-rayed yesterday. He already has some damage from the worms. He has had a couple of mini crises--the worst was the other night. His RR went up pretty high. They gave him IV corticosteriods and brought it back down fairly quickly. I was able to bring him back home with me. He is stable now. Do you think 5mg of pred is high enough dosage --every other day. We are considering increasing the dosage to 10mg every day. Could you offer an opinion on this to help me make a decision best for him. I continue to want to believe that I can see hiom thru this ordeal. We have placed him on preventative. Do you have any thoughts about this practice? Thank you for your guidance.
A: I can't see any problem with using the heartworm preventive in a cat with heartworms. It seems logical to at least limit the introduction of new heartworms into the picture, since that is now possible. The early information I have indicates that this should not cause reactions or any further problems.
We have almost always been able to use 5mg of prednisone every other day successfully in controlling the symptoms of heartworm disease in cats. We do have one patient on 10mg every other day but he also has polymyositis (a muscle disorder) that requires the higher dosage. I have to admit that his heartworm disease signs are well controlled on this dosage, almost to the point of being imperceptible, but I still think I'd rather avoid that high a dose of prednisone if I could.
It is definitely worthwhile to hang in there. I do believe that many cats outlive the heartworms.
Mike Richards, DVM
Feline Asthma may be early stage HW disease
Q: Dear Dr. Richards,
Due to the fact that I have been trying to find accurate, up-to-date into on Feline Asthma on the internet & have found it to be a real chore, I was thrilled to find your site with its accurate info. & possibilities for new treatment modalities i.e. Iain's
suggestions-see Asthma in Cats. Though I feel I have a good, caring vet,
asthma isn't a high priority for her as there aren't many cases. Since I'm
a nurse, I feel anything I can do to "source" new info. for her can be
beneficial for both of us. If you have any good sites on current studies
or recent info. on your vet. continuing education concerning asthma
(feline), I'd be most appreciative in receiving those. Keep up the high
quality of your site; many of us value it! Sincerely, L. S. G.
A: L. S. G.
I am not sure if this is on our site, or not. Dr. Phillip Padrid wrote an
article in a Capsule Report a couple of years ago advocating the use of
cyproheptadine (Periactin Rx) for asthma. He was using 2mg per cat given
twice a day. I have not seen other literature references to this but it is
probably pretty safe since Periactin is used a lot as an appetite stimulant
At a continuing education seminar that I attended the speaker said that a
lot of cases of suspected asthma may be heartworm disease. Cats apparently
react very strongly to the early stages of heartworm disease and can arrest
or suppress the growth of the heartworms but develop severe respiratory
disease in the effort, in some cases. This might be worth considering. But
again, there are no other references that I am aware of supporting this
We have had a couple of cases where we suspected asthma where we were able
to find lungworm eggs in stool samples (they are not always present when
this disease is affecting a cat). Both cats responded well to the
administration of ivermectin. There have been some case reports of
ivermectin reactions in cats being treated for ear mites, including some
reported deaths, so it is important to be reasonably sure that lungworms
are a potential problem before using this treatment. I am not sure if there
are other treatments. Our two patients did fine and the coughing cleared up
quickly after treatment.
We see enough cases where we do not find any other discernible cause that I
feel that asthma by itself is not especially uncommon in cats. We have had
the best luck treating with corticosteroids like prednisone, so far.
Veterinary Medicine magazine had a review article on feline asthma in
September of 1990 that had some good information on diagnosis of this
condition. If your vet subscribes to this journal, and saves it, he or she
might still have this issue.
Recently, the use of antibiotics like azithromycin (Zithromax, Rx), which
seem to work well against chronic upper respiratory bacterial infections,
has been helpful in our practice in clearing up a couple of chronic
sneezing/coughing kitties. It has definitely not been the answer for all
cats but it would be worth considering if the response to prednisone isn't
Hope this helps some.
Mike Richards, DVM
Michael Richards, D.V.M. co-owns a small animal general veterinary practice in rural tidewater Virginia. Dr. Richards graduated from Iowa State University's College of Veterinary Medicine in 1979, and has been in private practice ever since. Dr. Richards has been the director of the PetCare Forum...