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Heartworm medication

"Giving your dog heartworm medication when climatic conditions prevent transmission is not only pointless, but dangerous for your dog. Why would anyone want to give their beloved pet toxic chemicals when there is absolutely no valid reason to do so?"


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Home  |  The question is: why the change?
So, is the change to year round medications all about money? Or is there more to this story?  |  How heartworms infect dogs: It's not easy!  |  Conclusion

Part 1
The terrible truth about Heartworm medication - truths, omissions
and profits

Heartworms are spread by mosquitoes
but Heartworm medications are spread by fear

It’s getting warmer outside — time for sellers of heartworm medications to start scaring you to death. Television and print ads, which used to push medications only during warm summer months, now urge you to keep your dog on medication year round.

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The question is: why the change?

Drs. David Knight and James Lok of the University of Pennsylvania School of Veterinary Medicine, addressing recommendations for year round medications warn, that the practice of some vets to continuously prescribe monthly chemo-prophylaxis are responsible for:

  • Exaggerating the risk of heartworm transmission
  • Increasing the cost of heartworm protection to their clients unnecessarily
  • Unnecessarily exposing pets to "harmful disease causing chemicals" they don't need

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So, is the change to year round medications all about money? Or is there more to this story?

Heartworm prevention is a major health decision for pet parents, and a multi-billion dollar business for drug companies, vets, testing labs and on-line sellers of medication. When health intersects money, there is a lot of room for conflict of interest.

Only by understanding the business aspects and the truth about heartworm transmission can you make an informed decision about if, how, and when to protect your dog with commercial products.

While everyone agrees that heartworm infestations can be life-threatening, infestation is far from inevitable nor is it the immutable death sentence advertisers would have you believe. Otherwise, all dogs (and cats) not on medications would die of infestation. But of course they don't!

Every holistic vet I've researched has concerns about the long-term safety of heartworm medications. Well known vet, author and columnist Martin Goldstein wrote in his wonderful book "The Nature of Animal Healing", that he sees heartworms as less epidemic, than the disease-causing toxicity of heartworm medicine.

Dr. Jeff Levy, vet and homeopath, concluded that, it was not the heartworms that caused disease, but rather the other factors which damaged dogs' health, to the point that they could no longer compensate for an otherwise tolerable parasite load. Those factors include:

  • Being vaccinated yearly
  • Eating commercial dog food
  • Being prescribed suppressive drug treatment for other symptoms

Heartworm medications do not, by the way, prevent heartworms. They are toxic chemicals (poisons) that kill heartworm larvae (called microfilariae) contracted during the previous 30-45 days, and maybe longer, due to what is called the reach back effect

The reach back effect is a technique common to Heartguard (a heartworm drug manufacturer), meaning if a dog has contracted heartworm in the previous four months, their medication will effectively clear the infection if medication is continued every month for at least 12 months afterwards. This effectively locks unsuspecting pet parents into purchasing one year's supply of Heartguard medication for their pooch.

The heartworm industry authority, The American Heartworm Society offers a font of information. However, do bear in mind their website is not only a public service, but also a marketing tool aimed at buyers and resellers of heartworm medications.

Sponsors of this website are a who's who of drug companies, e.g. Fort Dodge Animal Health (Wyeth), Merial, Pfizer, Bayer Novartis, Schering-Plough, Virbac and Eli Lilly. Most of these companies have sales representatives who regularly call on vets, and show them how to sell heartworm medications to pet parents.

With any purchase of any drug concerning your furry friend, we recommend you ask the following information:r

  • The reason for giving your dog the drug
  • If there are any side effects or adverse symptoms, e.g. what are they? and what should you do if any should arise?
  • Whether there are any safer alternatives available?

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How heartworms infect dogs: It's not easy!

Well, now that we've looked behind the scenes of the heartworm industry, let's take a look at how the heartworms themselves (called Dirofilaria immitis) do business.

Seven steps must be completed in the mosquitoes life cycle to give your dog a dangerous heartworm infestation:

Step 1

In order for your dog to become infected with heartworm three conditions have to exist simultaneously:

  • A hungry female mosquito of an appropriate specie.
  • Sustained warm temperatures of 80° Fahrenheit or above (day and night) for a minimum of two weeks or more.
  • Standing water

Female mosquitoes act as airborne incubators for premature baby heartworms (called microfilariae). It is very important to understand that without the appropriate species of mosquitoes, dogs can't get heartworms. Period.

That means dogs can't "catch" heartworms from:

  • Other dogs or from dog park lawns
  • Other family pets, e.g. cats or horses
  • Wildlife in areas around where they live, e.g. foxes or coyotes
  • Puppies can't “catch” heartworms from their mothers
  • Mama dogs can't pass heartworm immunity to pups

Step 2

Our hungry female mosquito needs access to a dog already infected with sexually mature male and female heartworms and heartworm babies.

Step 3

The heartworm babies in the infected dog must be at the L1 stage of development when the female mosquito bites the dog and withdraws blood.

Step 4

Ten to fourteen days later, if the temperature is right, the heartworm babies mature inside the mosquito to the infective L3 stage and then migrate to the mosquito's mouth. (Yum!)

Step 5

Madame mosquito continues the cycle by transmitting the L3's to your dog's skin with a bite. Then, if all conditions are right, the L3's develop in the skin for three to four months (to the L5 stage), before making their way into your dog's blood. But, your dog still isn't doomed yet!

Step 6

Only if the dog's immune system doesn't naturally get rid of these worms do the heartworms develop to adulthood.

Step 7

It takes approximately six months for the surviving larvae to achieve maturity. At this point, the adult heartworms may produce babies if there are both males and females present, but the babies will die, unless a mosquito carrying L3's intervenes and bites the dog to carry on the heartworm cycle to the next dog it bites. Otherwise, the adults will live several years and then die off.

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Published studies about heartworms in summary

The University of Pennsylvania Veterinarian School

The University of Pennsylvania Veterinarian School in a study funded by Merial found:

  • Development in the mosquito is temperature dependent, requiring approximately two weeks of temperatures at or above 27 Centigrade (80 Fahrenheit)
  • Below a threshold temperature of 14 Centigrade (57 Fahrenheit), development cannot occur, and the heartworm life cycle stops dead in its tracks
  • Transmission is limited to warm months, and the duration of the transmission season varies geographically.

Washington State University Veterinarian School

The Washington State University Veterinarian School reports that laboratory studies indicate that maturation of heartworms requires "the equivalent of a steady 24-hour daily temperature in excess of 64°Fahrenheit (18°Centigrade) for approximately one month".

In other words, it has to be warm day and night, otherwise development is retarded even if the average temperature is sufficiently warm. They add, that at 80° Fahrenheit, "10 to 14 days are required for development of heartworm babies to progress to the infective stage".

The American Heartworm Society

The American Heartworm Society also reports, factors affecting the level of risk of heartworm infection to include:
  • The climate, temperature and humidity
  • The species of mosquitoes in the area
  • The presence of mosquito breeding areas
  • The presence of animal reservoirs, such as infected dogs or coyotes.

Bearing mind the content of the above report, it is interesting to note, that a recent news release from the American Heartworm Society states:“By giving heartworm prevention every month, forgetful pet owners will have their pets protected when they need it most". But doesn't that also mean they get it when they need it least? Or need it not at all? Are you a “forgetful” owner?

Jerold Theis, DVM, PhD

Jerold Theis says, "If the average monthly temperature is only a few degrees above 14 degrees Centigrade (57 degrees Fahrenheit) it can take so many days for infective larvae to develop, that the likelihood of the female mosquito living that long is extremely remote".

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In all my research on heart worm disease I have never found this temperature-dependent information on a website promoting "preventatives", it is only published in more scholarly works not easily accessed by the general public.

There is, as far as I can find, only one mention of temperature on website of the American Heartworm Society (on the canine heartworm page).

There is no mention of the temperature factor importance in the Merck and Merial Veterinary Manual site or Merial’s heartworm video, even though Merial funded the University of Pennsylvania study!

Please proceed to Part 2, Heartworm medication, Options to fear-based recommendations

Dogs Naturally Magazine
The American Heartworm Society

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This article and information forms part of the Carole's Doggie World Library and is presented for informational purposes only and not intended as an endorsement of any product. The information is not intended to be a substitute for visits to your local veterinarian. Instead, the content offers the reader information researched and written by Carole Curtis for

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Your dogs suffer in silence because they cant tell you about . . .
their painful teeth    |     their flea problems    |     or their allergies

Follow these links and soak up the free information to gain a happier, healthier dog who thinks you are the best person on the planet!

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