Heartworm Prevention in Dogs

Heartworm Prevention in Dogs

Don’t take it lightly: Conventional preventatives are still the best way to protect  your dog.

Article by Mary Straus, published in the Whole  Dog Journal, March 2006

Photo of Raven

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Photo of Raven, a Scottish Deerhound who was infected by heartworms. Read her story below.


People have learned of the benefits of a natural diet and limited vaccinations,  and have seen the health improvements in their dogs from these changes.  Now, many want to know if they can discontinue administering heartworm  preventatives to their dogs, or whether those can be replaced by natural  options.

Heartworm preventatives can cause serious side effects in some dogs,  including depression, lethargy, vomiting, anorexia, diarrhea, dilation  of the pupil, loss of balance, staggering, convulsions, and hypersalivation.  Some dogs are especially prone to side effects from ivermectin, the main  ingredient in one of the most widely used heartworm preventatives. Also,  some of the preventatives are combined with drugs aimed at killing other  pests such as fleas, mites, roundworms, and hookworms.

On the other hand, heartworm can be a devastating disease. Dogs with  moderate or severe infestations display a chronic cough and can’t engage  in much activity, as worms choke their heart and major blood vessels, reducing  their blood (and thus oxygen) supply. The disease often leaves its victims  incapacitated, incapable of doing much more than a slow walk without gasping  for air, and kills many dogs. Even the treatment for heartworm disease  can be deadly, regardless of which method is used, so it is important to  understand the risks that you take if you choose not to give your dog heartworm  preventative.

In fact, most (certainly not all) holistic veterinarians consider the  use of pharmaceutical preventatives to be less harmful than a heartworm  infection.

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Some argue, but . . .

As the co-moderator of an e-mail list on dog health and nutrition, I frequently  see people allege that as long as you have a healthy dog, feed a raw diet,  and do not over vaccinate, your dog will not get heartworms. If only this  were true! These measures may help to some degree, but they are not foolproof.  The only way to know for sure that your dog is protected is to give heartworm  preventatives.

Christie Keith, who lives in an area of Northern California where heartworm  is relatively uncommon and has raised Scottish Deerhounds naturally for  over 19 years, learned this the worst way.

“I went 16 years not using any form of allopathic preventative on my  dogs. At the end of that 16-year period, on routine testing, I found that  two of my dogs were heartworm-positive,” says Keith. “One of the positive  dogs was Raven, who is a deerhound I bought from another breeder. She came  to me at 17 weeks with bad ear infections and severe allergies, and no  one could argue that Raven was healthy or had a normal immune system.

Photo of Bran, a Scottish Deerhound was became infected with heartworms

”In contrast, my dog Bran (pictured at right) was a third-generation, naturally reared dog  of my own breeding. He was unvaccinated other than minimally for rabies.  He was raw-fed. His mother and her mother were raw-fed and unvaccinated  other than minimally for rabies. He was, by any definition available, extremely  healthy and robust. He had never been sick a day in his life.”

Christie successfully treated both her dogs, though Raven almost died  of a pulmonary embolism during treatment. Bran became heartworm-free after  several months of using the “slow kill” method of heartworm treatment,  with no sign of any adverse effects. Unfortunately, Bran died of acute  renal failure not long after that. Necropsy results were inconclusive,  showing that Bran had glomerulonephritis, but not why.

In her research to try to find the cause of her dog’s death, Christie  discovered that glomerulonephritis is a potential side effect of heartworm  infection. Although she and her vets eventually came to the conclusion  that Bran’s renal failure was caused by Lyme nephritis rather than heartworm  disease, it was disturbing to realize that heartworms can affect more than  the heart and lungs.

“I have no intention of ever living through what I lived through  with Raven and Bran. I can’t keep silent when I see people starting to  believe that healthy animals don’t get heartworm and that we can blithely  forgo using preventatives if we don’t overvaccinate and feed raw. It’s  just not so. And it’s not realistic to rely on the health and natural disease  resistance of our dogs to protect them from a threat that they are exposed  to frequently, as is the case in heartworm-endemic areas.”

“No creature is in a static state of health 24 hours a day, 7 days a  week. If our dogs are frequently exposed to an infectious parasite, eventually  they may well succumb to it, no matter how healthy they are normally.”

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“Alternative” preventatives?

Some holistic practitioners recommend various herbal or homeopathic preparations  for heartworm prevention, and anecdotal evidence from some dog owners can  be found on many discussion lists devoted to natural dog care. However,  consumers should be aware that none of these alternatives have  been  studied for safety or efficacy, nor are there any studies indicating that  they are effective at protecting against heartworm infection. In addition,  some herbal dewormers, such as wormwood and black walnut, are potentially  toxic when used at dosage levels needed to control intestinal parasites.  Holistic veterinarian Dr. Susan Wynn, author of Emerging  Therapies: Using Herbs and Nutraceuticals in Small Animals (AAHA  Press, 1999) and other books, in reference to herbal dewormers for intestinal  parasites, said in a chat on,  “Use a conventional dewormer. They are safer – MUCH – and more effective,  than herbal dewormers.”

Some holistic practitioners advocate the use of homeopathic nosodes  for heartworm prevention. Again, there are no studies indicating that they  are effective. In his book, Homeopathic  Care for Cats and Dogs, Don Hamilton, DVM, says “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.”

What is known, is that conventional heartworm preventatives are the  best form of protection currently available. Fortunately for those of us  who worry about the side effects of using the conventional drug preventatives,  there are numerous ways you can minimize their use and still protect your  dog. I’ll discuss these methods after introducing the most common preventatives.

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Conventional preventatives

The two most common (and generally considered safe) heartworm preventative  ingredients used today are ivermectin (used in Heartgard by Merial, and  other products) and milbemycin oxime (used in Interceptor by Novartis).

There is also an older, daily heartworm preventative available, diethylcarbamazine  or DEC. For many years, this drug was available from Pfizer as “Filaribits.”  Though Filaribits has been discontinued, you can still find generic versions  of DEC.

DEC is very safe in terms of side effects, but can be life-threatening  if given to a heartworm-infected dog with circulating microfilariae, due  to the risk of a rapid die-off of the microfilariae and resulting anaphylactic  reaction. Also, missing just one or two days of medication can allow your  dog to become infected. If you use DEC, it is essential that you test for  heartworms before starting this drug, and every six months while using  it. (Avoid Filaribits Plus, which has oxybendazole added to control intestinal  parasites  and has been known to cause liver damage.)

There are other heartworm products that include drugs for other purposes. Heartgard  Plus adds pyrantel to control intestinal parasites, including roundworms  and hookworms. Adult dogs rarely have problems with roundworms, but if  your yard has been infested with hookworms, this product might be good  to use until the hookworms have been eliminated.

Sentinel is a combination of the products Interceptor and Program  (lufenuron). Lufenuron is a medication that acts to prevent fleas from  reproducing; it’s not a pesticide and does not kill fleas or keep them  from biting your dog. This may be helpful  for a short time if you  have a flea infestation, and employ several nontoxic methods to get the  flea problem under control, such as diatomaceous earth to treat the house  and nematodes to treat the yard.

I’m less enthusiastic about selamectin (found in Revolution by  Pfizer), a more recent entry to the market. Selamectin is a topical product  that is also indicated for fleas, one kind of tick, ear mites, and the  mites that cause sarcoptic mange. While this may well be great if your  dog had mange, fleas, ticks, and ear mites, I strongly prefer drugs  with a minimal and targeted action over ones with broad-spectrum activity.

The injectable product moxidectin (ProHeart 6 by Fort Dodge)  has been withdrawn from the US market due to numerous reports of adverse  effects, including death. I do not recommend the use of injectable heartworm  preventatives at all, as there is no way to remove them from your dog’s  system if there is a bad reaction, and the time release drug will continue  to affect your dog for months.

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Temperature and timing



Based on new information regarding possible reduced heartworm preventative efficacy, I now feel it’s best to give heartworm preventives year-round, not only for better protection against infection, but also to ensure that the manufacturer will pay for treatment should your dog become infected with heartworms.See New Information Regarding Heartworm Resistance for details.

So, how can you reduce your dog’s exposure to conventional heartworm preventative  drugs, without decreasing his protection from the nasty parasite?

First, it is not necessary to give heartworm preventatives year-round  in most parts of the country. Heartworm development in the mosquito is  dependent upon environmental temperatures. Heartworm larvae cannot develop  to the stage needed to infect dogs until temperatures have been over 57  degrees Fahrenheit (14 degrees Centigrade), day and night, for at least  one to two weeks. The amount of time it takes will vary depending on how  warm it is: the warmer the temperatures, the faster the heartworm larvae  develop.

If temperatures drop below that point at any time during the cycle,  development may be prevented, but I wouldn’t rely on this. Temperatures  can vary according to where the mosquito lives, and may be warmer under  the eaves of houses or in other protected areas than the general ambient  temperature.

Heartworm preventatives work by killing heartworm larvae that have already  infected the dog, but before they can mature into adult worms that cause  damage. When you give your dog heartworm preventative, you are killing  any larvae that have infected your dog within the last one to two months.  Any larvae that have been in your dog longer than 60 days are more likely  to survive the treatment and go on to mature into adult worms.

Also, your dog may become infected the day after you give heartworm  preventative, the drugs do not provide any future protection at  all.

If your goal is to provide full protection for your dog with minimal  drug administration, you’ll have to monitor the temperatures in your area.  Mosquitoes may be capable of transmitting heartworm larvae to your dog  around two weeks after your local temperature has stayed above 57 degrees  Fahrenheit day and night.

Give the season’s first dose of preventative four to six weeks after  that to destroy any larvae that infected your dog during that time. Thus,  the first dose should be given six to eight weeks after daytime and nighttime  temperatures first exceed 57°F. Continue to give the preventative every  four to six weeks, with the last dose given after temperatures drop below  that level on a regular basis.

For some parts of the country, this can mean giving preventatives only  between July and October, while in others, where temperatures remain mild  all year, they may have to be given year-round.

If you do not give your dog heartworm preventatives (because the area  you live in is very low risk or because the temperatures are not right  for heartworms to develop), and then take your dog to a an area where heartworm  is a problem, you must treat him with heartworm preventative upon your  return to protect him.

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Dosage amounts



Based on new information regarding possible reduced heartworm preventative efficacy, I strongly advise against giving lower than recommended dosages of heartworm preventatives.See New Information Regarding Heartworm Resistance for details.

With at least one drug, you can give your dog less than the recommended  dosage of preventative, without compromising safety.

Milbemycin oxime, the active ingredient in Interceptor, has been approved  by the FDA at one-fifth the regular dosage to kill heartworms only, without  controlling intestinal parasites, including roundworms, whipworms and hookworms.  Novartis received FDA approval for a product, “Safeheart”, with this lowered dosage of milbemycin,  but it appears that they don’t intend to market it. (You can read the FDA approvals showing that  milbemycin oxime will control heartworm at one-fifth the dosage found in  Interceptor on the FDA’s web site; see “Resources” below).

The actual recommended dosage of milbemycin oxime for heartworm prevention  only is 0.05 mg per pound of body weight (0.1 mg per kg). Contrast this  with the recommended dosage of Interceptor for control of heartworm and  intestinal parasites: 0.23 mg milbemycin oxime per pound (0.5 mg/kg) of  body weight. Heartworm can be prevented at a much lower dose than that  needed to control intestinal parasites.

Safeheart contains 2.3 mg of milbemycin oxime for dogs from 2 to 50  pounds, and 5.75 mg for dogs 50 to 125 pounds. Interceptor contains 2.3  mg for dogs up to 10 pounds, and 5.75 mg for dogs 11 to 25 pounds. So if  your dog weighs more than 50 pounds, you can give the Interceptor for dogs  11-25 pounds, otherwise you can use the one for dogs up to 10 pounds.

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Frequency of preventatives



Based on new information regarding possible reduced heartworm preventative efficacy, I strongly advise against giving heartworm preventives less often than monthly.See New Information Regarding Heartworm Resistance for details.

It may not be necessary to give heartworm preventatives every month. The  monthly dosage schedule was devised to make it easy for people to remember  when to administer the drugs, and to ensure that dogs would still be protected  if a dose were somehow not swallowed or later vomited before being absorbed.

The FDA approvals cite studies showing that Heartgard, Interceptor and  Revolution provide protection beyond 30 days. If you are very good about  remembering to give medications, and you can watch your dog after administering  the pill to be sure that it is not spit out or later vomited, it may be  safe to use heartworm preventatives less frequently than every 30 days.  Dosing your dog every 45 days is a conservative way to stretch your dog’s  dosage schedule.

The original FDA approval for Heartgard states, “The target dose of  6 mcg per kilogram of bodyweight was selected from titration study 10855  as the lowest dose providing 100% protection when the dosing interval was  extended to 60 days to simulate a missed-dose circumstance.”

The drug manufacturers’ pre-approval tests indicate that even longer  dosing schedules may convey protection from heartworm – but I wouldn’t  stake my dogs’ well-being on dosage schedules extending beyond a somewhat  arbitrary 45 days.

The original FDA approval for Heartgard states, “The target dose of  6 mcg per kilogram of bodyweight was selected from titration study 10855  as the lowest dose providing 100 percent protection when the dosing interval  was extended to 60 days to simulate a missed-dose circumstance.”

The original FDA approval for Interceptor states, “Complete (100%) protection  was achieved in dogs treated at 30 days post infection, with 95% protection  at 60 and 90 days.” This does not apply to Safeheart, which was tested  only at a 30 day dosing interval.

The original FDA approval for Revolution states, “Selamectin applied  topically as a single dose of 3 or 6 mg/kg was 100% effective in preventing  the maturation of heartworms in dogs following inoculation with infective D.  immitis larvae 30 or 45 days prior to treatment, and 6 mg/kg [the recommended  dosage amount] was 100% effective in preventing maturation of heartworms  following inoculation of infective larvae 60 days prior to treatment.”

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Splitting pills

The issue of splitting heartworm pills comes up frequently. I have spoken  to representatives from Merial (maker of Heartgard) and Novartis (maker  of Interceptor). Both said that the active ingredients are mixed into their  products before the pills are formed, and therefore should be evenly distributed  (though they cannot guarantee this). However, both manufacturers advise  against pill splitting.

Splitting pills is inexact and may result in the dog getting less or  more of the medication. If you do decide to split the pills, use a pill  splitter (available at any drug store) and do not try to give the minimum  dosage, as you cannot be certain that your dog will get enough of the medication.

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No guarantees



Based on new information regarding possible reduced heartworm preventative efficacy, I strongly advise against giving heartworm preventives less often than monthly, or giving lower than recommended dosages.It’s best to give heartworm preventives year-round as well, not only for better protection against infection, but also to ensure that the manufacturer will pay for treatment should your dog become infected with heartworms. I also recommend annual testing for dogs, especially for those who live in heartworm-endemic areas, even if you give preventives all year round.

Manufacturers will only honor preventative guarantees when products are purchased from a veterinarian and given year-round according to label instructions. At least two online sites offer their own guarantees against product failures: 1-800-PetMeds, PetCareRx, and Drs. Foster & Smith.

See New Information Regarding Heartworm Resistance for details.

It is important to realize that, if you do decide to modify the way these  medications are given — by splitting pills, giving pills less often than  monthly, or using reduced dosages — the guarantees provided by the manufacturers  will be invalidated. Under normal usage, if your dog develops a heartworm  infection while on one of these heartworm preventatives, the company will  pay for treatment, but this is not true if you are using the drugs other  than as directed on the label.

It is important to understand the risk that heartworm infection poses  to your dog. Rather than relying on unproven alternative methods of heartworm  prevention, or the unreliable method of depending on your dog’s health  to keep him from getting infected, all of the methods discussed above will  offer you ways of safely reducing your usage of conventional heartworm  preventatives, while still giving your dog complete protection from heartworm  infection.

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Heartworm Testing


  When heartworm preventatives were first introduced, the only product available  used diethylcarbamazine (DEC) and was given on a daily basis. DEC is extremely  dangerous if given to a dog infected with adult heartworms, as it can cause  a rapid die-off of any microfilariae, resulting in an anaphylactic. For  this reason,  it is  important to never give DEC without first  testing to be sure that your dog is free of heartworms before beginning  treatment and every six months to a year thereafter.The newer heartworm drugs are less dangerous to dogs who are infected  with adult heartworms. They can even be used to kill the microfilariae  produced by the adult worms in the body, and have some effect against the  adult worms.Heartworm testing is still recommended before administration of these  drugs. It’s best to know ahead of time whether there are microfilariae  present, so you can be ready to treat the dog for an anaphylactic reaction  caused by the microfilariae’s rapid die-off, and to choose the safest preventative  to use if the dog is infected. Ivermectin (Heartgard) is safer in this  regard than milbemycin oxime (Interceptor), which has a much stronger effect  against the microfilariae. Next month’s article, on treatment for heartworm  infection, will have more information on this topic.

The most common current method of heartworm testing is called antigen  testing. This type of test can identify only adult female heartworms, and  therefore will not show a positive result until approximately five to seven  months after the dog has been infected, the time needed for the larvae  to develop into adult worms in the body. For this reason, it is no use  doing a heartworm test on any dog younger than five months. Heartworm tests  are very sensitive, but they are not 100 percent reliable. They are highly  specific, with very few false positives, but they are not always able to  detect very low heartworm burdens, or infections with only male heartworms.

It is generally recommended to do a heartworm test on any dog over the  age of six months before initially starting  preventatives. If you  give preventatives only part of the year, you may want to do a heartworm  test before restarting the medication in the spring or summer, especially  if there is any question about the timing of starting and stopping the  drugs the previous year.

If you give preventatives year round, it is still recommended to test  for heartworm infection every two to three years, just for added security,  particularly if you use minimal dosage amounts or increased time between  doses. Note that your dog needs to have a yearly veterinary exam in order  to get a prescription for preventatives, even if your dog does not need  to be tested for heartworm.

See the article on Heartworm Treatment for what you should do if a heartworm test comes back positive.

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Ivermectin Toxicity


Ivermectin has a bad reputation among some dog owners, but not all dog  owners need to worry unduly about the drug’s toxicity. Ivermectin toxicity  is genetic, and there is now a test available to determine whether a dog  is sensitive to ivermectin and other drugs. Dogs with ivermectin toxicity  may also be sensitive to loperamide [Imodium], cyclosporine [Atopica], acepromazine,  digoxin, butorphanol [Torbutrol/Torbugesic], and several chemotherapy drugs.  Breeds known to be affected include Collies, Australian Shepherds, Shelties,  Border Collies, Old English Sheepdogs, English Shepherds, McNabs, Long  Haired Whippets, and Silken Windhounds. To learn more about this test,  see the following Web site on multidrug sensitivities: as high as 50 times the amount used to prevent heartworms are used to treat mites on dogs (demodectic mange). In addition to dogs with ivermectin sensitivity, these very high doses of ivermectin are also problematic if combined with Comfortis (spinosad), a newer flea-control product that increases the risk of neurological side effects from ivermectin. Dogs infected with heartworms may suffer an anaphylactic reaction from the death of too many microfilariae at once when given very high doses of ivermectin as well.

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Facts About Heartworm Disease


  Heartworm disease is caused by an infestation of a parasite, Dirofilaria  immitis, commonly called heartworm, with an elaborate life cycle. It starts  in an infected animal; more than 30 species, including dogs and wild animals  such as coyotes, foxes, and ferrets act as “reservoir” species. Adult worms,  residing in the host animal’s heart, lungs, and associated blood vessels,  mate and the females release their young (called microfilariae). These  circulate in the host animal’s blood for up to two years. They develop  into their next stage of life, L1 (for first larval stage) only if ingested  by a mosquito during a mosquito’s blood meal.It takes the L1 larvae 8 to 28 days, depending on environmental temperatures,  to develop into their third stage (L3), when they migrate from the mosquito’s  stomach to its mouth. The L3 larvae enter their next host through the mosquito’s  next bite.As many as 10 to 12 L3 larvae can be transmitted to a dog in a single  mosquito bite. The L3 larvae molt and migrate through the dog’s tissues  in search of major veins, which they infiltrate and use as a path to reach  the heart. It takes them about 90 to 100 days to develop into L5, the form  that breaches the circulatory system. Only ivermectin affects them (and  not all of them) once they have reached the L5 form or beyond. However,  all the drugs affect the L3 and L4 forms, which is why it’s important to  administer a preventative drug at least every 45 days during heartworm  “season.” (Note: DEC must be given every day during heartworm season.)

If no preventatives are used, the larvae continue to develop to sexual  maturity. If both sexes are present, they can mate and produce microfilariae  about six to seven months after the infective mosquito bite that put them  in the dog. Adult heartworms can live three to five years, with males attaining  a length of 17 cm (about 6¾ inches) and females a whopping 27 cm  (more than 10½ inches).


  • Mild disease: Cough
  • Moderate disease: Cough, exercise intolerance, abnormal lung sounds
  • Severe disease: Cough, exercise intolerance, difficulty breathing, abnormal  lung sounds, enlargement of the liver, temporary loss of consciousness  due to poor blood flow to the brain, fluid accumulation in the abdominal  cavity, abnormal heart sounds, death

Image of heartworm life cycle  

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FDA Center for Veterinary Medicine FOI Summaries:   See NADA 138-412 for Heartgard,  NADA 140-915 for Safeheart and Interceptor, and  NADA 141-152 for Revolution.Timing Heartworm Preventatives (US)   (California only)Heartworm Prevention

Canine Heartworm Disease

Adverse Reactions

Are  Heartworms Getting Worse? A seminar in 6 parts

Also see Heartworm Disease in Dogs: Prevention and Treatment for updated  information.

Where to find Dimmitrol (formerly Filaribits), daily heartworm preventative     The company is from Australia, but it appears they will ship it anywhere  in the world, including the US. The shipping fee is reasonable (it says  $4.95 for one bottle).     American store with Australian connections, it appears. This company  lists Dimmitrol, and allowed me to place it in my shopping cart, but it  is a prescription product, and on the Prescription Products web page, it  says “Please note: Prescription Products are not sold by or dispensed by  VetShopOnline. They are sold and dispensed by an Independent Pharmacy.  Prescription Products cannot be returned or replaced.”     Despite the company name, the product is shipped from Australia.     Australian store, has a phone number for American clients.     Another Australian store that ships to the US.