It's that time of year again and questions about HEARTWORMS are starting to appear on many of the lists.

I've been searching the net for information on heartworm. There are many sites out there with "information" the problem with much of it is that it is either wrong or incomplete. One site says simply that a mosquito bites an infected dog then bites another dog and infects it. While that, to some extent, in the most general of ways, is true it is very misleading because it just doesn't happen that way. The entire process is much more complicated than that. 

This is a compilation of information I've been collecting for a couple of years.

The heartworm has 5 separate larval stages referred to simply as L1, L2, L3, and so on. 
It also has two separate cycles which combined makeup the lifecycle of the heartworm. 
One cycle takes place in a mosquito and the other inside a dog or cat.

The mosquito is infested when it bites a dog which is harboring L1 (MICROFILARIAE). This can only happen if the dog is also harboring the L5, or adult, male and female heartworm as the Microfilariae are their offspring. These Microfilariae can live for up two years in the dogs blood but must be taken up by a mosquito in order to develop any further. If they are not they will simply die of old age.

Once the mosquito is infested, the larva must go through to stages of development or molts L2, and L3, before they can infect another dog. This, mosquito, stage takes anywhere from two weeks to several months depending on the weather. The warmer the weather the faster the development.

The importance of temperature:
While the larva are developing in the mosquito, the temperature MUST remain above 
57 degrees Fahrenheit at all times, day and night. If at any time during the mosquito stage the temperature drops below 57 F, the development is halted and must start over. 
It is only the L3 larva which are capable of infesting your dog.

Now lets say that a mosquito has bitten an infested dog and the temperature has remained above 57for a minimum of 14 days since that bite and the mosquito bites your dog. Still your dog is not infested because the L3 larva are deposited in a tiny droplet of mosquito saliva adjacent to the bite not injected into your dog as many would have us believe. Providing the humidity and temperature are such that the droplet does not evaporate, the L3 larva must swim through the saliva and into the mosquito bite, thereby entering your dogs system.
Once inside your dog the L3 larva must spend the next two weeks or so developing into the L4 larva. During this period of time the larva is living in the skin, not the blood of the newly infested dog. The L4 will continue to live and develop in the skin for the next three or four months where it develops into the L5 stage.

Once it makes this development into the F5 it then leaves the skin and enters the circulatory system. The L5 or young adult then migrates to the heart and pulmonary arteries. Once there it will mate approximately 5 to 7 months after entering the dogs body. That is of course assuming that the dog has been infested with both male and female larva. This mating produces the Microfilariae.

A word on "preventatives"

The most popular heartworm "preventives" Heart guard and Interceptor are not really preventives at all, rather they act by killing the microfilariae, L3, and/or L4 larva in an infested dog. Interceptor kills the L3s, and L4s while Heartguard will kill the L4s and some of the youngest L5s. In other words they're poisons. Neither of them kills the fully adult heartworms. 

There are basically two standard tests for heartworm. One is called the antigen or occult test which tests for the antigens produced by the adult female heartworm. This test does not show the presence of microfilaria. The other is the microfilaria test. This test, of course, tests for microfilariae. Both Heartguard and Interceptor kill microfilariae. Therefore if ones dogs have been on either of these products they will test negative for heartworm when given the microfilaria test even though they may be infested with adult heartworms. It is not common, but it does happen. 

There have been many reports of dogs having very bad reactions to both Heartguard and Interceptor. Giving ones dog doses of poison month after month to kill something which probably isn't there anyway doesn't make an awful lot of sense to me. 

In his book "Homeopathic Care For Cats and Dogs", under the heading Heartworm, Dr. Don Hamilton says: 
"This is a serious disease that primarily affects dogs... It can be treated homeopathically but this should be under the care of an experienced veterinarian. 

Heartworm preventives are generally very effective at protecting dogs against the disease....In dogs the "monthly" preventives are effective if given at six week intervals, and possibly even at seven- or eight week intervals.... The daily preventives are almost a thing of the past, but these are usually effective if given every other day.

Although the preventive drugs are generally safe, they can initiate an autoimmune disease in susceptible animals...The homeopathic nosode that is made from heartworm larva is employed commonly as a preventive to avoid the drug side effects. Many question its effectiveness, though I have several clients who use the nosode (apparently successfully) with animals in heartworm endemic areas. Most animals have no trouble with heartworms. I do know of some cases where the nosode did not protect, however. I believe it does offer some protection, though it may be incomplete... If you decide to try the nosode, you must understand that its effectiveness is currently unknown."

To me this quote says pretty clearly that Dr. Hamilton has more faith in the drugs like Heartguard than he does in any homeopathic approach to prevention.

On the other hand, Dr. Martin Goldstein, in his book, "The Nature Of Animal Healing" page 221 say's:
"Granted, heartworm is a serious condition... 
A few caveats are in order, however. Only a small percentage of dogs who get heartworm die of it, especially if they're routinely tested twice yearly for early detection. Even in untreated dogs, after a period of uncomfortable symptoms, the adult worms die... The chances of a microfilaria infected mosquito biting your dog the first time are slim. Of it happening to the same dog twice? Very slim...Early in my career, I saw and treated hundreds of cases of heartworm disease, most with routine medication, yet witnessed only three deaths (the last was in 1979). By comparison, we're seeing, cancer kill dogs on a daily basis. To my mind, the likelihood that toxicity from heartworm pills is contributing to the tremendous amount of immune suppression now occurring, especially in cases of liver disease and cancer, is far greater and more immediate than the threat of the disease they're meant to prevent...

As a precaution, I recommend that all dogs be tested twice a year for heartworm. For clients who insist on a more active form of prevention, I suggest doses of black walnut given two to three times a week, as I've actually reversed clinical heartworm with it..."

Obviously two very differing opinions of the safety of the common heartworm preventive drugs and the efficacy of alternative treatments from two very respected authors and healers.

Other alternative preventives I've run across are:

Soaking an old towel in equal parts of the essential oils of pennyroyal and citronella; tearing it into pieces and hanging it in various places around ones dog runs or property is said to make an excellent repellent.

Citronella, eucalyptus, pennyroyal, rosemary, rue and wormwood are the strongest herbal repellents available, not only for mosquitoes, but for fleas, and ticks as well. 
Rubbing fresh leaves of pennyroyal, rosemary eucalyptus, lavender and/or wormwood on ones dogs' coat and/or ones own skin is also said to be a good temporary repellent.

We keep a small spray bottle with a mixture of the above essential oils plus black walnut on hand for use on ourselves and our dogs. Mixing in equal parts of distilled water makes it considerably less costly and still seems to do a good job. Spraying it on things like lawn or deck furniture can also be very effective.

For a quite different view of the subject as well as some interesting additional information go to

I hope this provides some helpful information.

Langsley T Russell
Bulloved Bulldogs

======== Interceptor & Heartgard Studies ======

Here again is the way to get to the FDA studies for heartworm preventatives:

Go to: 

Type "Nada 138-412" (without the quotes in the search window) and Select for the ivermectin (Heartgard) study.

Type "Nada 140-915" and Select for the milbemyecin(Interceptor) study.

One study states that the effective dose of milbemyecin (the active ingredient that prevents heartworm) is actually .1mg/kg but in order to also control hookworm, the effective dosage is raised to .5mg/kg which is
the amount in Interceptor.

The other study states that the 6mcg/kg dosage of ivermectin is actually good for 60 days and, therefore, the monthly dosage has a wide safety margin.

Type "Nada 141-051" moxidectin or commercial name ProHeart (1997)


Heartguard can be used, according to the manufacturer, on dogs with heartworm disease. It will not kill the adult heartworms but it will kill any "babies" they produce. This is often recommended in dogs who, for other medical reasons, can not be treated for heartworms. It keeps the infestation "under control".

Heartguard also seems to be the safest preventative to give in case your dog is heartworm positive and you don't know this. The other preventatives (Ivermectin and Filib.) will cause a bad reaction in your dog (possibly shock) if heartworm positive. Heartgard tends to even kill young adult worms ... Called the "Reach back Effect" whereas the other Heartworm preventatives don't do this. (I don't know about ProHeart.)

========== Heartworm Preventative website ===========

An informative website on all four types of heartworm preventatives can be found at: 

It says that ProHeart was only released in 1998 so that's why we haven't heard of it. It's a derivative of ivermectin.

I would like to know why ProHeart is considered safer than Heartgard???

======= 2004 ProHeart UPDATE =======
4000 reports of bad to severe reactions & 400 deaths

Mar 1, 2004 534 pm US/Central
CHICAGO (CBS 2) The Food and Drug Administration approved the drug in 2001 as safe and effective for the prevention of heartworm disease.

Now FDA officials say they're concerned about the unexpected number of adverse reactions that have been reported by veterinarians and pet owners whose dogs were given the drug, and an investigation is underway.
Now some of those dog owners are telling their story for your pet's sake.
Cletius, Vito and Murphy. Three families' beloved pets that suddenly got very sick.
"I was horrified. I had no idea what was going on at the time," dog owner Georgene Paulauski said.
She is talking about what she thinks is Cletius's reaction to ProHeart 6. It is the first and only injectible form of medicine that prevents heartworm disease for six months.
"I feel so guilty about this because I did it for convenience sake of not having to give a monthly pill," Paulauski said.

She's one of more than 4000 who've made reports to the FDA about adverse reactions they believe their dogs suffered from ProHeart 6.  More than 400 of those dogs died or were put to sleep by their owners.

Dr. Bob Rogers, an outspoken critic from Texas, is one of several veterinarians we talked to who say they won't give the shot."I think that's a very alarming number," Veterinarian Dr. Bob Rogers said.
He prefers monthly oral medications that have been on the market for more than a decade.

The reactions to ProHeart 6 reported to the FDA include severe allergic reactions, vomiting, diarrhea, convulsions, weight loss and an auto immune anemia.

Paulauski of Crown Point Indiana filed an adverse reaction report after 5-year-old Cletius got sick a few weeks after his injection.
Cletius was diagnosed with an autoimmune disease in which the body destroys its own red blood sells.
"He's been poked and prodded and to surgery and back and multiple units of blood multiple transfusions," Paulauski said.
The steroids to help Cletius recover caused additional complications.
"They found a massive bleed, a huge huge huge bleed in his belly," Paulauski said.

Another report was filed by this Kankakee couple after their 9-year-old Yorkie became incontinent.
"She wouldn't drink, she wouldn't eat, she started losing her physical strength," said Jim Rentas, the dog's owner.
"And you know what the consequences are of that and your desperation at trying to get this in her is overwhelming," Laurie Rentas said.

Overwhelming for an Oak Forest couple who own 10-month-old Vito. They say that an hour after his shot. Vito couldn't get up. That night he had a seizure.
"He was foaming at the mouth, he had lost control of his bladder and he was seizing so bad I thought he broke he back," Vito's owner, Kim Russ said. "Within two days you know he was a, he was like a vegetable," Tim Russ said. After two anguishing months of treatments the Russ' put Vito to sleep.

The FDA has been checking a number of possibilities from whether there are any impurities in the manufacturing process, to whether the proper dosages are being given. So far nothing abnormal has turned up, but the reviews are continuing.

Fort Dodge Animal Health, the manufacture of ProHeart 6, says its review of the cases show that "no definitive links could be made to the drug." The company says millions of dogs like cooper have been protected without any problems. In a written response to our questions, the company said the adverse reactions reported amount to less than one percent of the 15 million doses sold. The company said most cases that require medical treatment respond quickly to medical intervention.

But since ProHeart 6 is a time-released drug that stays in a dog's system for six months, vets say it can take that long for a dog to recover.

"He's not out of the woods yet. They still aren't promising us anything. If he'll live," Paulauski said.

In the last two years the FDA has required changes to the label for ProHeart6 warning about the possibility of severe allergic reactions and in rare cases death. FDA officials say there is no pattern to the reported problems when it comes to the size, breed or age of the dogs.

Heartworm Cures | Home | Back