Heartworms are Spread by Mosquitoes. Heartworm Medications are Spread by Fear.

It’s getting warmer outside — time for sellers of heartworm medications to start scaring you to death.  You are likely aware of Veterinarians recommending ‘Heartworm Prevention’ only during warm summer months, but now urge you to keep your dog on medication year round. The question is: why the change?

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

Heartworm “prevention” is a major health decision for pet parents and multi-billion dollar Big Business for drug companies, veterinarians, testing laboratories and on-line sellers of medication. When health intersects money, there’s 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 meds would die of infestation. But they don’t.)

Every holistic vet I’ve consulted had 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. A vet and homeopath, concluded “that it was not the heartworms that caused disease, but the other factors that damaged the 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, and getting suppressive drug treatment for other symptoms….”

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

The heartworm industry authority,  offers a wealth of information. Their website is a public service but also a marketing tool aimed at buyers and resellers of heartworm meds. Sponsors of this website are a Who’s Who of drug companies. Fort Dodge Animal Health (Wyeth), Merial and Pfizer are “Platinum Sponsors.” Bayer merits Silver. Novartis, Schering-Plough, Virbac and Eli Lilly get Bronze. Most of these companies have sales reps that regularly call on vets and show them how to sell you heartworm meds. With any purchase of any drug, we recommend you ask for information regarding possible adverse effects, the necessity for taking this drug and available alternatives

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 to give your dog a dangerous heartworm infestation:

How Heartworms Infect Dogs: It’s Not Easy!

Step 1: To infect your dog, you need mosquitoes (so you need warm temperatures and standing water). More specifically, you need a hungry female mosquito of an appropriate species. Female mosquitoes act as airborne incubators for premature baby heartworms (called microfilariae). Without the proper mosquito, dogs can’t get heartworms. Period.

That means dogs can’t “catch” heartworms from other dogs or mammals or from dog park lawns. Puppies can’t “catch” heartworms from their mothers and moms can’t pass heartworm immunity to pups.

Step 2: Our hungry mosquito needs access to a dog already infected with sexually mature male and female heartworms that have produced babies.

Step 3: The heartworm babies must be at the L1 stage of development when the mosquito bites the dog and withdraws blood.

Step 4:  Ten to fourteen days later — if the temperature is right –the microfilariae mature inside the mosquito to the infective L3 stage then migrate to the mosquito’s mouth. (Yum!)

Step 5:  Madame mosquito transmits 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.

Step 6:   Only if the dog’s immune system doesn’t rid the dog 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, but the kiddies will die unless a mosquito carrying L3’s intervenes.  Otherwise, the adults will live several years then die.

In summation, a particular species of mosquito must bite a dog infected with circulating L1 heartworm babies, must carry the babies to stage L3 and then must bite your dog . The adult worms and babies will eventually die off in the dog unless your dog is bitten again!  Oh, and one more thing.

Heartworms Development Requires Sustained Day & Night Weather Above 57˚F

In Step 4 above I wrote that heartworm larvae develop “if the temperature is right.”

“Development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27C (80F). Below a threshold temperature of 14C (57F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically.”

“In regions where average daily temperatures remain at or below about 62˚F (17˚ C) from late fall to early spring, insufficient heat accumulates to allow maturation of infective larvae in the intermediate host [the mosquito], precluding transmission of the parasite.”

laboratory studies show that maturation of the worms requires “the equivalent of a steady 24-hour daily temperature in excess of 64°F (18°C) for approximately one month.”  In other words, it has to be warm day AND night or development is retarded even if the average temperature is sufficiently warm. They add, that at 80° F, “10 to 14 days are required for development of microfilariae to the infective stage.”

“If the mean monthly temperature is only a few degrees above 14 degrees centigrade [57 degrees F] it can take so many days for infective larvae to develop that the likelihood of the female mosquito living that long is remote.”

I have never found this temperature-dependent information on a website promoting “preventatives,” but only in more scholarly works not easily accessed by the public. There is, as far as I can find, only one mention of temperature on the Heartworm Society (on the canine heartworm page) and none in the Merck/Merial Veterinary Manual site or Merial’s heartworm video.

The society also reports, “Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas and presence of animal reservoirs (such as infected dogs or coyotes).”

Does Year Round Medicating Bring Extra Protection?

Applying suncreen at night is useless. So is taking heartworm medication when climate conditions prevent transmission.  Only a small percentage of climes permit year-round transmission. Everyone else is unnecessarily subsidizing drug companies and “preventatives” sellers and, more importantly, exposing their dog to unnecessary risks.

Two exceptions: 1) “Forgetful” and irresponsible pet parents who won’t begin the medication on time or build their dog’s natural immunity might want to medicate year round, although that means they have to remember to give meds every month.  2) If your dog contracts heartworms within a few years of beginning medication … and you can show you gave meds year round … and your dog had the required blood tests (2 or 3),  you may benefit a little financially because drug companies will pay for dog’s treatment. (Read the guarantee terms published by an on-line seller.)

Are Heartworm Preventatives Safe?

You’ve seen those scary photos of worm-strangled hearts, right? Shouldn’t you give meds year round just in case? Isn’t safe better than sorry?

But is that harmless little pill or yummie medical “brownie” really safe?  No drug is completely free of risk and  adverse reactions.  I can find no long-term studies regarding cancer risks and organ damage for dogs receiving heartworm insecticides year round (or even for a few months). Such a study would be difficult to conduct and very expensive.  Who would fund such a study – or publish any negative findings?

One clue to the possibility of adverse reactions should be label warnings: call your doctor immediately if ingested; keep away from children; wash your hands immediately after use…. How can medication be good for dogs but so dangerous foryou?

Another question: is your dog healthy enough for these medications? The “Heartworm Prevention” page states: “Healthy kidneys and normal liver functions are essential in metabolizing most medications.”  Many dogs, including my Jiggy, do not have healthy organ function. I wonder how many unhealthy animals are nevertheless on meds?

Adverse Reactions to Heartworm Medications

With any drug, study FDA and manufacturer information before medicating.

These adverse reactions have been reported to the FDA by manufacturers. (Click the links for more information; write or call manufacturers with any questions). Terms you might not understand include ataxia (gross lack of coordination of muscle movements), pruritus (itchy dermatologic condition), urticaria (hives), mydriasis (excessive pupil dilation), and erythema (skin redness). Other terms should be self-explanatory.

HEARTGARD and TriHeartPlus (ivermectin): Depression/lethargy, vomiting, anorexia, diarrhea, mydriasis, ataxia staggering, convulsions and hypersalivation. INTERCEPTOR (milbemycin oxime) reports the above reactions plus weakness. Sentinel (milbemycin oxime) reports vomiting, depression/lethargy, pruritus, urticaria, diarrhea, anorexia, skin congestion, ataxia, convulsions, hypersalivation and weakness.

REVOLUTION® (selamectin), Topical Parasiticide For Dogs and Cats: pre-approval reactions of vomiting, loose stool or diarrhea with or without blood, anorexia, lethargy, salivation, tachypnea, and muscle tremors. Post-approval experience included the above plus pruritis, urticaria, erythema, ataxia, fever, and rare reports of death and seizures in dogs.

Proheart 6 :  severe allergic reactions (anaphylaxis): facial swelling, itching, difficulty breathing, collapse;  lethargy (sluggishness); not eating or losing interest in food; any change in activity level; seizures; vomiting and/or diarrhea (with and without blood); weight loss; pale gums, increased thirst or urination, weakness, bleeding, bruising; rare instances of death. This product was voluntarily withdrawn from the market in 2004 because of deaths but has been reintroduced. Read my post Heartworm Protection: Do We Need ProHeart 6?

For any other brand, research the product or its active ingredient before even thinking of administering it.

Also, never give any meds without first learning if any vitamins, minerals, herbal products or drugs interact negatively with the medication. Note age restrictions. Most importantly,  learn what symptoms alert you to a reaction. Important note: Collies, Australian Shepards and related breeds have a sensitivity to ivermectin (Heartgard and others).

Beware any website or person professing the absolute safety of any medication.  I’d like adverse reactions for pet medications to be included in all TV ads, as they are for meds for humans — but I don’t expect it.