The Truth About Heartworms in Pets

Written by: Dr. Jean Hofve, Holistic Veterinarian, DVM

Natural Heartworm Prevention & Support

What Causes Heartworm in Dogs & Cats?

Mosquitoes transmit heartworms. Tiny heartworm larvae, called microfilaria, circulate in the blood, and are sucked up by the bug when it feeds on an infected host animal. For heartworms, their natural host is the dog. Once inside the mosquito, the larvae must develop through more stages before they can infect another dog. For that to occur, outside temperatures must remain above 57 degrees F, day and night, for a minimum of 8 days. The warmer the temperature, the faster the larvae will mature. If the temperature drops below that critical level, larval development will stop; but the larvae don't die—development will re-start at the same point when the weather warms back up. Larvae reach their infective stage in 8 to 30 days (the latter being the entire lifespan of the average mosquito).

When an infected mosquito bites a dog or cat, the heartworm microfilaria are deposited on the skin, where they crawl into the bite wound and enter the bloodstream. Inside the body, they get ready to "settle down and raise a family." In dogs, the heartworm's natural host, the larvae migrate to the heart and eventually develop into adult worms, reproduce, fill the blood with microfilaria, and pass it on to the next mosquito. The maturation process takes 6-7 months.

"Heartworms have been diagnosed even in cats who spend 100% of their time indoors." - Dr. Jean Hofve

What do heartworms do?

Once in the bloodstream, the microfilaria migrate to the right side of the heart and attach there, where they can grow into adulthood.

According to the American Heartworm Society, "Clinical signs of heartworm disease may not be recognized in the early stages, as the number of heartworms in an animal tends to accumulate gradually over a period of months and sometimes years and after repeated mosquito bites. Recently infected dogs may exhibit no signs of the disease. In contrast, heavily infected dogs may eventually show clinical signs, including a mild, persistent cough, reluctance to move or exercise, fatigue after only moderate exercise, reduced appetite and weight loss."

It takes microfilaria about 6-7 months to mature into adults and start reproducing. Clinical signs are not typically seen before that. Adult worms can live up to 7 years in the dog.

In cats, adult worms can develop, but cannot reproduce; they take about 9 months to mature and tend to live only a year or two. However, adult heartworms are about a foot long, so it only takes 1 or 2 to fill up a cat's tiny heart and cause serious problems.

As it turns out, cats have pretty good defenses of their own. In 80% of cases, the cat's own immune system kills the larvae and clears the infection. Nevertheless, microfilaria can still cause significant inflammation in the lungs, even in cats who never show any signs of infection. Feline heartworms may be commonly misdiagnosed as asthma or bronchitis, when it is actually Heartworm Associated Respiratory Disease (HARD). Also, for the 20% of cats who do become persistently infected, severe respiratory or cardiac disease can occur.

Are there natural approaches for heartworm?

Treatment of a mature heartworm infection can be very dangerous. When the arsenic-based drug is given to an infected dog, the massive die-off of the worms may cause severe inflammation and even respiratory failure. Not all dogs survive this process. Accordingly, prevention is the best option!

Alternatively, many veterinarians advocate simply giving the regular heartworm preventative to kill off any microfilaria already present and keep newly deposited larvae from developing, while waiting for the adult worms to die. This may be a more practical alternative for cats or dogs that do not have a severe infestation.

Seasonal vs. year-round heartworm prevention

Except for the warmest parts of the U.S. (mainly in the southeast), heartworms are completely seasonal.

  • In many areas of the country (northern and mountain states, for instance), such warm temperatures simply don't exist for most of the year, and sustained warm temperatures don't occur until at least June.
  • Only in Florida and south Texas is year-round heartworm transmission possible. Within 150 miles of the Gulf Coast, heartworm risk exists 9 months out of the year.
  • In the rest of the country, heartworm transmission is possible between 3 and 7 months out of the year.
  • Hawaii and Alaska have each had a few cases of canine heartworm, but the incidence in those states is very low.

It should be obvious that during seasons where there are no mosquitoes, there is no risk of heartworm. Evidently that little fact escaped the attention of the veterinarian who prescribed heartworm protection—in December–for a puppy living high in the Colorado mountains. At that altitude, temperatures are never warm enough for heartworms!

A debate about when to give heartworm preventatives was published in an April 2009 journal article ("Ask the Expert: Year-Round Heartworm Prevention: Two Viewpoints," by By Dwight Bowman and James Lok, published in NAVC Clinician's Brief, the official publication of the North American Veterinary Conference, 2009/04/01). Both authors are university professors in parasitology.

The argument presented by Dr. Bowman in favor of year-round heartworm medication focused on just two points:

  • The speculation that "scenarios can arise where transmission may occur in cooler climates in the 'off season.'
  • The completely undrelated notion of prevention of extra drugs added to the heartworm preventative.

Arguing on the other side, Dr. Lok lays out the case for appropriate seasonal control. He concludes, "Besides incurring unnecessary costs for the client, indiscriminate application of broad-spectrum medications can engender further confusion about the primary imperative for these medications—heartworm prevention—and when they are most crucial—during the season of heartworm transmission."

If the weather is unseasonably warm for long enough, exceptions to those recommendations should be made.

Having looked at both sides of the debate, I have to agree with those who suggest that giving year-round heartworm medication to animals in states where year-round transmission does not occur is doing an injustice to both the animals being given drugs they don't need, as well as the pocketbooks of their guardians.

Natural Heartworm Prevention

The most common preventative drugs for heartworm are

  • Ivermectin (Heartgard®)
  • Milbemycin (Interceptor®)
  • Selamectin (Revolution®)

While these drugs are generally safe and effective, there are always exceptions:

  • Toxicity associated with ivermectin can include depression, ataxia (trouble balancing or unsteady walking), and blindness, but these are uncommon at the low doses used in heartworm preventatives.
  • Ivermectin should be used with caution in collies and related breeds such as Old English Sheepdogs and Australian Shepherds, who may be more sensitive to the drug's neurological effects.
  • Milbemycin, the most common alternative drug for collie breeds, can cause depression/lethargy, vomiting, ataxia, anorexia, diarrhea, convulsions, weakness and hypersalivation.
  • Selamectin is also used to treat ear mites and some worms; adverse reactions may include hair loss at the site of application, diarrhea, vomiting, muscle tremors, anorexia, lethargy, salivation, rapid breathing, and contact allergy.

Another serious and growing trend is the resistance of heartworms to these drugs. This means that we are selecting for superworms that may be able to survive and grow even in animals on heartworm preventatives. As with all cases of resistance, the correct response is to lessen the use of the drug and reserve it only for when it is absolutely necessary.

Only Natural Pet HW Protect Herbal Formula is a natural product intended for use as a preventative to be used during mosquito season as part of a comprehensive heartworm control program.

An herbal approach to heartworm prevention is unlike a traditional preventative, which chemically kills all heartworm larvae. Still, it may be an effective and more natural method to help keep animals in peak conditions. Consistent dosing is essential for proper protection, along with heartworm testing at least every 6 months.

Shop Only Natural Pet Heartworm Products >


  1. The temperature needs to stay above 57 degrees for 8 to 30 days.
  2. A mosquito has to bite a dog with microfilaria in its bloodstream.
  3. That mosquito has to then bite your dog or cat 8-30 days later.
  4. You must give the heartworm preventative medication within 6 weeks of the mosquito bite to kill microfilaria in the blood and prevent the larvae from growing to adulthood.

FAQ & Common Heartworm Myths

Are Heartworms Contagious?

No, heartworm disease is only spread through the bite of a mosquito. For example, a dog would not pass the disease to another dog just by being near them.

Can Humans Get Heartworm?

Yes, humans can get heartworms through a mosquito bite just like dogs and cats can. This would be the only way for heartworms to enter a human’s bloodstream.

What to Feed a Dog with Heartworms?

Some dogs and cats may begin to lose their appetite and weight. We recommend feeding them easy-to-digest foods and putting away the fetch toys to help keep them calm.


Knight DH, Lok JB. Seasonality of heartworm infections and implications for chemoprophylaxis. Clin Tech Sm An Pract. 1998 May;13(2):77-82.

Atkins C. Feline heartworm disease. NAVC Clinician's Brief. Http:// Accessed 5/20/2009.

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