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
Also see these related articles:
- Heartworm Treatment
- Time to Step It Up (Update on heartworm resistance WDJ July 2011)
- Are Heartworms Developing Resistance to Preventatives? (WDJl March, 2011)
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.
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.
”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.”
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 doghobbyist.com, “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.
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.
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.
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.
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.
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.
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.
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.”
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.
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 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.
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.
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: www.vetmed.wsu.edu/depts-VCPL/
Dosages 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 products such as Comfortis or Trifexis (also called Vethical AcuGuard and ComboGuard) that contain spinosad, a newer flea-control product that increases the risk of neurological side effects from ivermectin and milbemycin oxime. 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.
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).
SYMPTOMS OF INFECTION
- 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
See NADA 138-412 for Heartgard, NADA 140-915 for Safeheart and Interceptor, and NADA 141-152 for Revolution.
Canine Heartworm Disease
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.
You can contact me if you have any comments, but I regret to say that I can no longer respond to questions about individual dogs. See my Contact page for more information. My name is Mary Straus and you can email me at either or