Heartworms - how did my dog get infected on preventative?

mytdogJanuary 21, 2010

I adopted a stray dog from the animal shelter. Exact age was unclear but the estimated about 6-8 months old. We have had her a little over a year. The shelter did a heartworm test in Dec 08 before we took her home. She was negative. We went to the vet about 1 week later and she was started on Interceptor pills. She has been given one each month (about 4-6 weeks apart, mostly 4 weeks a part, but maybe a few 5 weeks or 6 weeks). We have used 14 doses from dec 6 08 to now 1/22/10 so it was pretty much on time.

I took her back to the vet monday 1/19/10 and she test positive for heartworms! The vet told me that this test said she had an adult heartworm, it could only be 1 heartworm but she had at least one. Now what to do? I've been treating her for 1 year and read in this forum that if I keep treating her (with Interceptor) for like 2 years, the adult(s) would die. Other places say they can live for 7 years. I really don't want to go the whole keep her quiet for a month. I have very active kids and she is an extremely active and happy dog. It would be impossible to keep her quiet. She loves to run and will run in the house if she can't run outside.

So how did this happen? The vet tell me it was impossible and I must have missed a dose. But I didn't. Is it possible that Interceptor isn't working for my dog and I should switch to another brand? Please help.

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Naomi Miller

I can only offer my two cents because I am not a vet but a long time multiple dog owner who has never had a positive HW test. Any vet will tell you that it is impossible for a dog to get infected while on the product and lead you to think you have made a horrible error in the treatment of your dog.... do not fret..... read and study.

On this forum as well as the net in general, you can find many many articles about using Ivomec injectible for cows as preventative and even treatment of heartworms. I have even read articles in the Merck vet manual... the ingredient in the Ivomec is the same used in the pills we give every month; however, if you study the proportions used in those pills with the suggested usage, you will see that ivomectrin has a huge margin for 'error'. In other words, it would take a very large dose to harm your dog and the dosage in those monthly pills in minute in proportion to what you can use.

Please do a search on this site for Ivomec dosage for dogs... then do some internet research as well....you will find that the recommended doses are much larger than those you have been administering so it would make sense that the small doses are not always enough to do the job well. Also, I have read where many people have used the injectible (BTW, you give it orally to dogs) to treat HW infection as well as prevent it.... the thing to be careful of there is to read, read, read, because a typical dosage may kill the heartworms TOO quickly and cause damage to the heart... but it will work.....there are tons of folks who have used it and most under a vet's recommendation or supervision....there is a difference in 'city' docs and 'country' docs.... before I moved to the country, my vet said ABSOLUTELY NEVER use Ivomec for a dog.....here in the country where vets see more hunting dogs and large diversity in livestock etc, ... they will discuss the options.... if your vet will not discuss it, then read.. it sounds like you have a very small case to deal with and should be easy to take care of at this point.....

I am sure someone with more knowledge and better explanations will come along... but my dogs are happy and healthy...so is my cow, llama, pigs.....you get it, lol....

Good luck ....

    Bookmark   January 22, 2010 at 1:55AM
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I don't know - but my dog is now on heartworm (chewable Sentinel) 12 months a year. Due to the so called "warmer winters" in Canada of all places, many dogs were being brought to the vets in January suffering from stomach ailments - generally after a snow melt and the dogs could get their noses into the ground and eat all sorts of horrible stuff (mine being one inclined to do so - as well as eat snow) and the heartworm meds help prevent such stomach ailments (so far so good). So my husband suggested that since he was on heartworm 12 months a year, he would not need the yearly test. I told him that this was unlikely, but when we were at the vet - I asked - on his behalf. We were told that our form of heartworm, the Sentinel must be given once a month - on schedule. Our clinic has had patients also on heartworm 12 months a year who have developed heartworm - and it was later revealed that sometimes a pill was given say a week late - or a dose missed (this of course usually happening during the summer months) - and the dog was vulnerable during the late/missed dose period since heartworm covers the dog for the previous month.

    Bookmark   January 22, 2010 at 3:21PM
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It's so easy to glibly tell an owner it's their fault, and I suspect a vet who does believes it and isn't doing it out of malice.

I don't imagine too many failures are reported to the manufacturing company either, precisely because it's assumed to be an owners failure to comply properly.

Well, I just read an article by a woman who had the same experience. She absolutely knew her dog was medicated appropriately and documented each administration dose. What it all boiled down to was the tablet is designed to be chewed. Not all dogs will chew it properly but swallow it whole. It evidently (in her instance) failed to provide protection because the medication from an intact pill could not be absorbed properly in the dog's gut.

    Bookmark   January 22, 2010 at 7:48PM
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Is there such a thing as a 'false positive' with this medication?

    Bookmark   January 24, 2010 at 10:06AM
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What happened is that when you adopted the dog, she was already infected with IMMATURE worms, which are undetectable by the test. Once the worms matured, she tested positive. Heartworm prevention kills only L3 and L4 immature worms. If your dog had L5 worms when you got her, they were undetected by the test and progressed to adult worms because the prevention doesn't kill that stage.

I see it all the time. I always require owners who adopt stray dogs to test them 6 months after starting HWP because of that very reason. I often see positive tests at the 6 month mark. I test my dogs every year even though I know they get their prevention, just in case of failure of drug to work. No drug is 100% perfect. If you have proof of purchase from a vet (not the internet) and annual heartworm tests, most manufacturers will pay for heartworm treatment if your dog becomes infected while on their product. Since heartworm treatment is easily $500 or more, it behooves people to do an annual test, even though their dog always takes their medicine perfectly.

Most people know their dogs don't like pills or chewable oral HWP. The vet is responsible for making sure owner is not having problems giving the medication. If there is any question of whether the dog is getting the full dose, the vet should recommend other forms of prevention such as injectible (ProHeart) or topical (Advantage Multi, Revolution).

Your dog will NOT get heartworms if you are a week or 2 or even 3 late giving the prevention if they are on an ivermectin or milbemycin product. In theory, you could give HWP every other month and still be effective in preventing infection. But since most people do forget to give it on time, it is much better to give prevention every month rather than risk a late dose after 60 days.

Ivermectin has a HUGE safety margin, which is why a 4# chihuahua can get the same dose heartworm prevention as a 22# cocker spaniel.

I'm sorry your dog is infected. I'm pretty sure it isn't your fault, and that she already had heartworms when you found her. I hope her treatment goes well and she is happy and healthy for many years.

    Bookmark   January 24, 2010 at 11:00AM
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I had the same thing happen to my abused choc lab I got at 18 months old.When I picked him up from the vet he was HW negative.Went back 10 months later because of the way he been treated,I was certain previous owners[if you can call them that]had not had him a HW prevetative.Sure enough he tested positive.Had him treated the following week.Vet said he prob had a fairly mild case of them since I had him on HW prevetative.


    Bookmark   January 24, 2010 at 11:29AM
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Thank you Meghane for your input. I wondered about that in immatures. It's an antibody test, isn't it? I imagine things like this are why annual tests are, or should be, required before filling a script.

I didn't want to post a link on the forum about the owner who had given her doses faithfully and the dog still got Heartworm, but the company covered her cost of treatment, and this person wrote her vet had attended a seminar where heartworm med failures were covered. And.....reasons why.

    Bookmark   January 24, 2010 at 12:34PM
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Heartworm tests for dogs detects adult female antigen only. Most dogs have more than 1 adult worm, and most of the time at least 1 of them is female, so that test works well for dogs. I know some vets will Rx HWP even if owner "can't afford" an annual test, but I don't think it's a good idea because 1) no drug is 100% effective 100% of the time 2) there could have been a break in prevention owner was not aware of (dog threw up, etc) and 3) the dog could have already been infected but not by a mature worm.

It is definitely much better for the dog to be treated with Immiticide rather than continue giving prevention and waiting for the worms to die. It takes 1-3 years to clear an infection by giving HWP, and all during that time there is progressive damage to the heart and pulmonary arteries. If the dog has any signs of heart failure, it is even MORE important to treat the dog ASAP. They are not going to get better with 14 inch long worms living in their hearts.

    Bookmark   January 24, 2010 at 3:35PM
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Thank you for all the information. I am new to this. The vet told me it would cost about $800 to treat (they want to do x-rays, and blood work, etc.). I did call the Interceptor company and they reiterated all about the stages. It is likely fromm talking with them, that she was infected with L5 heartworms before leaving the shelter just like one of the posters mentioned. Since the shelter didn't give her any preventative, they won't pay for the whole cost, but they did say they would offer me $250 towards treatment which sounds very nice since they didn't think it was there fault. I guess I should have had her tested again at 6 months, but didn't know and since I bought a 12 pack of heartworm medicine, didn't go back till the 1 year mark. Our shelter will actually treat dogs that are heartworm positive if they are identified at the shelter but since my dog tested negative there, she doesn't qualify.

Also my dog does appear to swallow the pills whole. If they have to be chewed, then that would definitely would be a problem as well. I may try to crush them up from now on and put it in peanut butter or something.

I hate the thought of heartworms in her so I guess I will go ahead and treat, though she is very active and won't stay quiet for any length of time. The $250 towards the treatment will definitely help.

    Bookmark   January 29, 2010 at 11:30AM
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Glad you're going to get her treated. It will be much better for her health. It is best to check for signs of heart disease, liver disease, and other serious health problems prior to administering heartworm treatment. It was also nice of Interceptor to cover some of the costs.

It doesn't matter if they swallow or chew the heartworm prevention. It gets absorbed either way.

Keeping the pets quiet is always the hardest part. But most do just fine, and even with complications ALL of the patients I have treated have survived and done great. Can't say so much for the ones who tried the prevention only method, although *most* of them have lived.

    Bookmark   January 29, 2010 at 8:37PM
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It may not be as impossible to keep her quiet as it may sound. When my pups were (I think) 10 months old (less than 1 yr for sure), one of the girls broke her leg. Since the break was just above the knee and the bones were well aligned, the vet wanted to try to avoid surgery. We had to keep her quiet, couldn't let her romp with her sister, couldn't walk her, etc. She had to be kenneled when we weren't home. I remember having to stop the romping several times but it was easier than I expected. He leg healed with no surgery (and no cast), just quiet time and frequent x-rays.

Good luck and I hope she recovers well.

    Bookmark   January 30, 2010 at 2:21PM
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We adopted a rescue dog less than a year ago who came to us with a mild heartworm infection. Rescue groups often use the heartworm pill dosing method to save money because heartworm is so expensive to treat. Our regular vet wanted nothing to do with this treatment method but we thought we try the rescue group's method because they swear it's 100% effective and it seemed so much kinder to the dog. We were told to give our pup doxycyclin for two weeks which kills the Wohlbachia bacteria upon which the immature worms feed. Without this bacteria the immature worms cannot develop into adult worms. After the two weeks of doxycyclin, we started giving monthly treatments of ivermectin. It's supposed to eventually kill all the worms.

We moved to a new town and our new vet agreed with our first vet, recommending the injection treatment to get the worms out. So we did the injection treatment and are now treating once a month with Interceptor. In about three months we take him back for a blood test to see if all the worms have been killed.

We did not have trouble keeping our dog quiet. He was pretty knocked out by the shots for a few days and then he mostly stayed quiet until he was out of danger. We crated him.

    Bookmark   January 30, 2010 at 5:49PM
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