The Reason I Buy Hw Meds From My Vet

[JJNg]

I'll preface this by saying that this is probably way more detail that most people are interested in. But living and working in a HW endemic area, this is an issue that I pay close attention to.

 

Exerpts from WDJ:

There is evidence in some heartworm populations for genetic variations that are associated with decreased in vitro susceptibility to the macrocyclic lactones.”

Translation: investigators have verified that one strain of heartworms shows resistance to heartworm preventives in the lab.

I couldn't tell if the translation comment above was from the article, or your interpretation, but resistance to HW preventatives has not been proven in any studies.

 

Heartworm preventatives work by killing the L3 larvae that are transmitted to dogs by mosquitoes. It is these L3 larvae that can grow to become adult heartworms. Due to difficulty in obtaining L3 larvae for testing (they would have to be dissected out of mosquitoes), all of the 'resistance' (in vitro susceptibility) studies have been done on microfilaria (essentially pre-L1 stage).

 

Microfilaria are the microscopic 'baby heartworms' that are produced by adult heartworms in a dog. Microfilaria cannot become adult heartworms until they are picked up by a mosquito and develop into L3 larva inside the mosquito. But they are available in large numbers in an infected dog's bloodstream. In general, microfilaria tend to be harder to kill than L3 larva. And while there may be some correlation between microfilaria that are less susceptible to the HW preventative drugs, I'm not sure you can use that information to directly extrapolate to L3 resistance to HW preventatives.

 

The supposedly resistant strain of HW that's been identified has been named MP3. It was found by testing microfilaria from a dog in Georgia that had never been on HW preventative medication. There is no reason to believe that the heartworms in this dog have anything to do with the numerous reports of HW preventative failure in the Mississippi River Valley.

 

There is always natural variation in how susceptible individual organisms are to drugs. So this variation in susceptibility of different heartworm strains is something that's always existed - it is not a new phenomena that has been created by use of preventative medications. Yes, it is true that resistant strains will probably eventually be selected for by the use of preventatives, but it has not been proven that this is why there have been an increasing number of dogs getting heartworms despite being on appropriate doses/frequency of preventatives.

 

In the past, failure of HW preventatives has always been blamed on lack of compliance, missed doses, etc. However, there have been an increasing number of reports (mostly from the Mississippi River Valley) of dogs getting heartworms where the vets are confident that the owners are using HW preventatives correctly. The immediate conclusion that many people have jumped to is resistance, but an alternate theory is that changes in climate and increases in mosquito population has simply bombarded these dogs with huge numbers of heartworm larvae that overwhelm HW preventative control.

 

Most of the studies that show HW preventatives are 100% effective were challenge studies that experimentally infected dogs with numbers in the area of 100 L3 larvae. However, what happens if a dog is naturally infected with 1,000 or 10,000 larvae in a given month? Despite what the studies show, I have always maintained that HW preventatives are not 100% effective. They may be 99.9% effective, which would look like it's 100% when you're testing with studies that use 100 larvae. But if a dog is exposed to 10,000 larvae, 99.9% effective would translate to a dog getting about 10 adult heartworms. Given that a single mosquito bite can transmit up to 10-12 larvae, it's feasible that an outside dog in a severely mosquito-infested area could easily be exposed to that many larvae in a month.

 

Anyway, bottom line is that I believe the jury is still out on whether there is truly heartworm resistance developing at this time. Regardless of whether there is or not, I think this info makes it even more important to HW test regularly (I recommend yearly), even if you do keep your dog on HW preventative year-round.

[seeh2o]

Thanks for your post. I took the quotes directly from WDJ, not my interpretation (however, I did underline).

[racindog]

That was the point, and I don't think any of us missed it (I didn't). The only problem is, that the above claim about the company paying for the treatment is not correct. As Irene illustrated, there are many ways the company will use as reasons not to pay - especially now that incidences of HW positives in dogs on treatment is up and the treatments are more expensive - and there are plenty of others, including that the above dog came from a "rescue/unlnown history" and didnt have two negative tests six months apart. Ten years ago they paid most claims if you did everything right and spent the extra money, but that's no longer the case.

 

EXACTLY. Can't say it any better.

[Guest sja5032]

In response to a previous post- Unfortunately there is actually a shortage of Immiticide, the drug used to treat heartworms. It is completely unavailable right now, but the news it that it should be available in short supply at the end of October. There's currently a waiting list through Merial to order the product for currently infected dogs.

 

 

Why doesn't the vet use ivermectin then? It isn't the best treatment but many shelters will use it because it is much cheaper. It would be better than letting them grow while you wait around.

 

Before anyone freaks out, the whole dogs will die if you give them reg. heartworm meds while positive is not totally true. Yes they can die, but for the same reasons they can die from the treatment. The only difference is, with the treatment they are being monitored and kept inactive.

[Guest BrianRke]

Maybe, but maybe not. The usual culprit where HW meds are administered is either human error or the dog having had HW, but not long enough to test positive. I always test every six months with a new adoptee. After two negatives in a row I got to once annually. No matter where you buy the meds, the two reasons above are how the manufacturers are arguing their way out of reimbursing customers whose dog's test positive.

 

That said, Ivermectin injectable given orally is cheaper than anything bought online and perfectly effective.

I agree

[Brooker914]

Ivermectin won't kill the adult worms, only Immiticide will. Ivermectin can be given to heartworm positive dogs to kill the microfilaria (immature stages) in the meantime, but then you'd have to wait for the adults to die, which can take between 2-7 years. In that time frame the dog can die from the heartworms. During the shortage right now, the Heartworm Association recommends treating with Heartgard and Doxycycline, then as soon as we can get Immiticide to kill the adults and cure the dog.

[JJNg]

Why doesn't the vet use ivermectin then?

This is essentially what most vets are doing, and what the American Heartworm Society is recommending if Immiticide is not available. Although most vets are using Heartgard, which is safer to dose than liquid ivermectin. This is pretty standard protocol for cases where the owner can't afford adulticide treatment, or where the dog is not a good candidate.

 

However, as Brooker914 mentioned, ivermectin will not kill the adult worms. It does shorten their lifespan and prevents the accumulation of more adult worms. Without ivermectin, heartworms will live around 4-7 years. But if the dog is consistently given even the tiny, monthly heartworm preventative dose of ivermectin found in Heartgard, the lifespan of most of the adult worms will be shortened to 18-24 months. With the more recent addition of doxycycline to the protocol, that lifespan may be even shorter.

 

Before anyone freaks out, the whole dogs will die if you give them reg. heartworm meds while positive is not totally true. Yes they can die, but for the same reasons they can die from the treatment. The only difference is, with the treatment they are being monitored and kept inactive.

Actually, the reason some HW positive dogs can die if given HW preventative is due to anaphylaxis if the dog has a large number of microfilaria, and the preventative is one that kills the microfilaria quickly. This was a big risk with the old, daily HW preventative Filaribits (does anyone even remember this product?). Of the current monthly products, milbemycin is the more risky ingredient (Interceptor, Sentinel, or Trifexis). The low dose of ivermectin in HW preventatives kills microfilaria very slowly, so Heartgard (and any of the other generic equivalents) are quite safe.

[Guest Swifthounds]

I remember Filarabits.

[Hubcitypam]

I remember Filarabits.

Me too -- in the 80's I think.

[Guest Swifthounds]

I remember Filarabits.

Me too -- in the 80's I think.

 

Vets around here were carrying them into 2001 and until they stopped making them - mostly for dogs of breeds that grew rapidly from one weight category to the next on the monthy meds.

[Guest KsFrets]

...That said, Ivermectin injectable given orally is cheaper than anything bought online and perfectly effective...Why doesn't the vet use ivermectin then?... This essentially what most vets are doing, and what the American Heartworm Society is recommending if Immiticide is not available...

 

So, in theory, a HW negative dog, started and maintained on Ivermectin, should be fine for life. I know that our vet supports it, and educates on it's use. Our group also does the same (that's how we learned about it). But since we have some very knowledgable people here, my question is Ivermectin shelf life. We do keep it in the fridge. I ask because our last bottle ($34 from Tractor supply) lasted our three dogs+fosters for over 3 years! I just bought a new bottle, and so I have another three year's supply. I can't help but wonder if it keeps it's effectiveness over time. Anyone have any insight? Thanks!

[Guest KennelMom]

...That said, Ivermectin injectable given orally is cheaper than anything bought online and perfectly effective...Why doesn't the vet use ivermectin then?... This essentially what most vets are doing, and what the American Heartworm Society is recommending if Immiticide is not available...

 

So, in theory, a HW negative dog, started and maintained on Ivermectin, should be fine for life. I know that our vet supports it, and educates on it's use. Our group also does the same (that's how we learned about it). But since we have some very knowledgable people here, my question is Ivermectin shelf life. We do keep it in the fridge. I ask because our last bottle ($34 from Tractor supply) lasted our three dogs+fosters for over 3 years! I just bought a new bottle, and so I have another three year's supply. I can't help but wonder if it keeps it's effectiveness over time. Anyone have any insight? Thanks!

 

 

From: http://www.drugs.com/vet/ivomec-injection-for-cattle-sheep-and-swine-can.html

Stability

 

IVOMEC Injection has been shown to be stable for five years when stored under normal conditions.

 

But I replace it after a year.

[Guest KsFrets]

Thank you very much for the link! That makes me feel much better, and that's some very interesting information. It works on more parasites than I realized (in swine).

[Guest sheila]

I'm glad I started this discussion. My friend is not a GTer, but I've directed her to this topic so she could begin to research this on her own and I'm sure she will. She is a very dedicated pet owner and will do anything she can to keep her dogs healthy. Heck, in my next life I want to come back as one of her hounds!