Heart worms are about six inches long. They live mostly in the heart and the large blood vessel that bring oxygen-rich blood from the lungs to the heart. Adult male and female worms living in the heart produce thousands of microscopic baby worms which circulate throughout the body. These baby heartworms do not grow to adulthood in the dog where they were born. (If they did, the dog would quickly die and that would be bad for both the dog and the heartworms.) Before baby heartworms can develop further, they must find their way to a mosquito.
So then, a mosquito comes along and bites the infected dog, sucking up baby heartworms. This probably isn't too good for the mosquito, but it is what the worms have been waiting for. During the next month, the heartworm babies develop into heartworm teenagers, a stage partway between baby and adult.
Now, the mosquito bites another dog, infecting the new dog with teenage heartworms, ready to develop into adulthood. After six or seven more months the life cycle is complete: new adult male and female heartworms are producing thousands of baby heartworms.
The tree hole mosquito, which breeds in oak trees at our elevation, is very good at spreading heartworms. It lives longer than most mosquitoes, takes repeated blood meals, and even seems to remember where it found food the last time. Since the mosquito remembers where it got a good meal last time, it is likely to deliver teenager heartworms back to the to the same dog they originally came from as babies or miss that particular dog and give them to the dogs that live next door. As a result we often see clusters of very severe heartworm infestation.
In 1972, when the heartworm problem in this part of the United States was first being recognized, 25% of all heartworm infections which were reported in the state of California that year were reported by our hospital. Entomologists from U. C. Berkley came to Placerville to catch heartworm-infected mosquitoes for their studies.
We have a lot of heartworms here. Unprotected dogs living outdoors will almost certainly catch heartworms. Indoor dogs going outside in the morning and evening will probably catch heartworms. Statistics show that long hair offers no protection whatever.
There are no symptoms at all until the disease is very advanced. Then, the symptoms are those of congestive heart failure: dull coat, lack of energy, coughing, difficulty breathing, perhaps fainting spells and an enlarged abdomen. Waiting for symptoms to develop is not a realistic alternative to prevention.
There are several different drugs used for heartworm prevention, all of them highly effective and easy to use. You have your choice between a monthly good-tasting pill, a monthly application to the skin or a once-ever-six-months injection. All are highly effective and cost about the same, although there is a new semi-generic product that's a bit cheaper. In cold climates, prevention medication is unnecessary during the winter. In Placerville, we have mosquitoes year 'round, and although we could theoretically skip the medication when it is sufficiently cold, the weather is different every year. Our recommendation is once a month, year 'round, permanently
No medication is perfect, and none of us have perfect memories. Dogs should be periodically retested. In situations where pet owners feel confident that the medication is being given regularly, testing every two years is adequate.
Although heartworms can be fatal, and treatment for the disease involves risk, the condition is nearly always curable. Treatment requires careful medical care and complete rest at home afterwards. See heartworm treatment in dogs.
Because cats are not a natural host for the parasite, heartworm disease in cats is a very different problem. Cats are much less likely to get heartworms than dogs are. When they do, symptoms are unpredictable and seemingly illogical.
Usually there are no symptoms. When noted, symptoms resemble those of other more common problems.
We do not have a satisfactory drug for heartworm treatment in cats. If we did, using it would be risky because cats do not deal well with dead heartworms. They sometimes absorb the dead worms successfully; they sometimes die from arterial obstruction or from allergic reaction to the dead worms. So, if we could kill feline heartworms, it is not clear that we should. As it stands now, the best treatment for feline heartworm disease is to start on prevention (so they don't get more) and try to control the symptoms, if there are any.
Primarily because of the low heartworm numbers found in cats and the fact that heartworms do not reproduce in cats, testing is much less reliable than in dogs. Although the American Heartworm Society considers it "good medical practice" to test cats before starting on prevention and it is hard to argue with this, we feel that effort and expense required to test the entire cat population would be better devoted to doing something more useful. After all, even if we know that a cat has heartworms, our treatment is exactly the same as for a cat that doesn't have heartworms: give medication to keep form getting more heartworms and watch for symptoms.
If your cat has unexplained vomiting problems, or especially if there is difficult or noisy breathing, heartworm testing is an important part of the diagnostic workup. If you "just want to know", that is also a good reason to test for heartworms. However, we do not ordinarily test cats that look good and act normal.
We urge everyone in our area to protect their cats from heartworm disease. If your cat is entirely indoors and you don't get mosquitoes in the house, she may not need heartworm protection, although a surprisingly high proportion of feline heartworm cases occur in "indoor" cats.
Heartguard for Cats is a once-a-month, good tasting kitty treat medication. Most cats enjoy it or accept it readily mixed with their food. If necessary, Heartguard can be given like a pill. More often, we recommend Revolution, which controls heartworms, intestinal worms, fleas and ear mites and is also easier to use.