March 26, 2013

Facts About Heartworm Prevention

Perhaps one of the hardest decisions a dog or cat owner will make to help our pets thrive is to stop seeking the reassurance of "preventative" medicine, namely heart-worm pills and topical flea and tick treatments.

With the season soon upon us, let's look at the facts about heart-worm medicine.

Dog owners have been scared into believing that one bite from an infected mosquito and your dog will keel over dead with six-inch worms in its heart the next day. Okay. That's an exaggeration. So is everything you've ever seen or heard about the disease itself.

Fact #1: Heartworm infection is not the scourge many would have us believe. Transmission occurs only during mosquito season.

Heartworms go through several life stages before they become adults infecting the pulmonary artery of the host animal. The worms require the mosquito as an intermediate stage to complete their life cycles. The rate of development in the mosquito is temperature-dependent, requiring about two weeks of temperature at or above 27°C (80°F). Below a threshold temperature of 14°C (57°F), development cannot occur, and the cycle will be halted.1 As a result, transmission is limited to warm months, and duration of the transmission season varies geographically.

Fact #2: Heartworm is far from common in healthy animals.

According to Terrierman's Daily Dose: Data on heartworm incidence rates at the local level reinforces how rare heart worm really is. For example, California is coded red-hot in a map of high-transmission areas, with 500 cases. And yet, when a total of 4,350 dogs in 103 Los Angeles County cities coming from 21 participating animal hospitals were tested, only 18 heartworm-positive tests turned up.

Fact #3: There is no such thing as heartworm prevention; there is only heartworm poison.

Ivermectin (Heartgard, Heartgard Plus, Iverhart, Merial and Verbac) or Milbemycin (Interceptor, Safeheart, Sentinal and Norvartis) are nematode poisons designed to kill heartworm microfilaria (microscopic worms.) A single dose will kill all microfilarial infection that occurred up to 90 days earlier (i.e. all Stage 3, 4 and young Stage 5 heartworm infections).

Fact #4: Heartworm infection is not rapid and will not kill your dog overnight. It takes about three months for microfilaria to mature to a larval stage in your dog or cat. It takes even longer for adult heartworms to develop - about three months. One dose of an ivermectin treatment at any time before this period between infection and the development of adult worms is all it takes to stop heartworm dead.

Fact #5: The same heart-worm medicine you give your dog as prevention is also the cure in the unlikely event that your dog is infected. Have a look at this video featuring the Old Country Vet, Dr. Busby, who speaks out quite firmly on how this works.

Access this video on heart-worm treatment and other videos by Dr. Busby, the Old Country Vet, at his website.

The fact is that a healthy immune system is the best prevention against common parasites.

Dr. Jean Dodds recently addressed the use (and dangers) of heart-worm medicine for dogs.

The recent dramatic increase in immunological diseases has been attributed to the effects of environmental influences on these genetically susceptible individuals. An increasing number of breeds are at relatively high risk for these problems. The genetic influences are compounded by the fact that immunological recognition and reactivity is continually challenged throughout life by an array of environmental agents that serve to promote failure of the body’s self-tolerance. This produces or triggers a variety of autoimmune diseases affecting the thyroid, blood, eyes, skin, muscles, joints and specific organs. Environmental agents known to be involved include the effects of drugs, toxins, chemicals, viruses and other infectious agents, vaccines, hormonal and nutritional influences, and stress. Perhaps our biggest challenge in preventing and controlling these serious and increasing problems is to identify and remove/reduce the environmental factors involved.
Some individual animals affected with autoimmune diseases and their immediate relatives have been shown to react adversely to commercial, monthly heartworm preventives. When an individual’s immune system is compromised, any regular exposure to particular kinds of drugs, chemicals or toxins can produce significant adverse effects, whereas these exposures are well-tolerated by animals by animals with healthy immune systems that do not carry the genetic susceptibility to these disorders. It is important to emphasize that the licensed drug or chemical is safe unless used in a genetically or physiologically susceptible companion animal. These adverse reactions usually occur within the first 10-14 days after the monthly product has been administered and typically begin after an animal has had 2-5 doses. Occasionally, animals that have been taking monthly preventives for a relatively long time will develop subsequent product intolerance. This usually indicates that some underlying disease process has emerged to explain the problem. Based on cumulative data, it is my recommendation that dogs affected with autoimmune diseases and their immediate relatives receive only plain daily heartworm preventive (Dimmitrol = diethylcarmbazine). If heartworm disease is not prevalent where the animals live, routine use of heartworm preventives is not recommended. This is especially important for dogs suffering from chronic diseases of the skin, hair and coat, or those with bone marrow, thyroid or liver disease.

Jan Rasmusen, author of Scared Poopless, the Straight Scoop on Dog Care has completed some excellent work on heartworm medication safety as well as prevention options.
Remember, you kill heartworm babies after the fact. You can only “prevent” them by avoiding mosquitoes.  (You can also kill them with a healthy immune system.) This means starting meds 30-45 after the weather warms and mosquitoes appear. Also, Washington State University warns, “If your pet travels to heartworm areas, prevention needs to be administered within 30 days of exposure to infected mosquitoes. Adult dogs (older than 6 mos.) need to be tested before starting preventative.”
Dr. Margo Roman, an integrative vet from in Massachusetts, documentary film maker and Founder of the first-ever Integrative Health Pet Expo in Massachusetts this fall [2009], tells me she begins medication six weeks after sees mosquitoes. This allows 2 weeks for the microfilariae (baby heartworms) to mature  inside a mosquito to the infective stage and be transferred to a dog, plus 30 days additional days covered by the medication working backwards to kill those babies.
When should you stop heartworm medicine? 
Dr. Roman recommends stopping meds after the first frost for people living in an area with cold winters.  In other areas, vets recommend stopping 30-45 days after weather consistently falls below 57 F degrees and you see no mosquitoes.
Once I was convinced that this parasite was easy to identify in plenty of time to reverse its course, I stopped using all heartworm and flea treatments.

Instead, I had my dog tested for infection four times a year. The test is a simple blood draw that costs about $25. It was far easier on my dog and my budget - and much safer - than filling his blood stream with pesticide. Bottom line, if a heart-worm test comes back positive, eighteen months of the same ivermectin prescribed as a preventative safely rids your dog of the parasites. Eventually, we reduced testing to twice yearly and never gave heart-worms another thought.

Tomorrow, I'll talk about how to stop letting fleas bug you. 

  1. ^ a b Knight, David (1998-05-01). "Heartworm". Seasonality of Heartworm Infection and Implications for Chemoprophylaxis. Topics in Companion Animal Medicine.

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