Prevent Heartworm


Reprinted from Pyr News & Notes, 6/97

by Angie Meroshnekoff

Heartworm disease is a parasitic infection that is transmitted by mosquitoes. When the mosquito bites an infected dog, it draws up microscopic larvae, called microfilaria, along with its meal of blood. During the next two to six weeks, the microfilaria undergo additional changes within the mosquito's body and become able to infect another dog the next time the mosquito bites.

This larva will remain in the dog's tissue surrounding the bite wound for about two months while it grows into an immature adult worm. From there, these immature worms begin to enter the dog's bloodstream and within three to four months, they reach the heart and surrounding arteries. The adult heartworms reach maturity about six to seven months after the initial bite, and can grow 6-14 inches in length. They are now mature enough to begin their own reproductive life cycle within the dog. A single adult heartworm can produce thousands of live microfilaria in a day - each capable of being carried by a mosquito to spread the infestation to another dog. There is no evidence that humans are affected by this parasite.

At first, there are no obvious signs that a dog has become infested, but after a period of time, as the worms block the arteries of the heart, lungs and liver, the infested dog begins to display these symptoms of chronic infestation:


Every dog, whether it lives in the city or the country, indoors or out, should be tested annually for heartworms. The vet draws a blood sample, filters and stains a portion of it, then checks it under a microscope to see circulating microfilaria. To be absolutely accurate, a second portion of that blood sample should also be tested for the presence of antigens to the adult heartworms. Only if both tests are negative, should the dog then be put on regular heartworm preventative.

Even if a dog is believed to be regularly on a heartworm preventative, annual testing is good insurance that all the pills were taken as scheduled and no hidden heartworms are developing. Heartworm has been identified in every State in the union, and in every county in California. Since an infected mosquito can travel up to 15 miles from the original source of infection, any contact with a mosquito must be considered potentially dangerous to your dog.


There are two treatments that kill adult heartworms - the problem with both is that they also place the dog in great danger. The older treatment, still used by most vets, is to inject an arsenic compound, thiacetarsemide (Caparsolate), into the dog. The drug is administered in two doses each day for two days, followed by several weeks of inactivity to give the dog's system a chance to absorb the dead worms. Exertion can cause the dead worms to dislodge, travel to the lungs, and cause death.

Recently, a new drug, Melarsomine dihydrochloride, (Immiticide), was approved that greatly lessens the adverse reactions that many dogs had to the older medication. A series of two injections is given 24 hours apart. During hospitalization, the dog is carefully monitored for any adverse reactions. It still must be kept quiet for six weeks following treatment (that means staying in a crate and being walked on leash when it needs to eliminate), but the danger is from clots caused by the dead worms, not from the medication itself.


Once your dog is tested and found completely free of Heartworms, there is a choice of preventative medication. In California, where mild winters favor the year-round production of mosquitoes, the preventative should be given year round.

There are currently three types of heartworm prevention available. Ask your vet which one is best for your dog.


This product offers the most comprehensive protection available. The manufacturer even has a 100% guarantee that will cover any treatment costs you may incur. The convenient once-a-month tablet protects against heartworms, hookworms, roundworms, and whipworms. It comes in a chewable form so your dog will eat it right out of your hand. Just keep in mind that this little pill is preventing your dog from contracting heartworms, so make sure your dog doesn't spit it out!

Heartgard Plus - (Ivermectin)

This preventative medication is given once-a-month and is effective against heartworms, as well as hookworms and roundworms. It is available in tablets or an easy to give chewable beef cube type treat. In field tests, 98% of the dogs readily ate the chewable form, making it a favorite for owners who have difficulty giving their pets pills. Ivermectin is not usually given to Collies and is contraindicated for dogs that have shown an allergy to it.

Filaribits Plus

This is a once-a-day chewable tablet and is used for dogs who are sensitive to Ivermectin. It protects against heartworms as well as intestinal hookworms and roundworms. Many dogs look forward to this daily "treat," and have been known to remind their owners if they forget to give it!

Whichever preventative you choose for your dog, remember that heartworms are life-threatening and a low-cost annual test, and a daily or monthly preventative are cost-effective ways of assuring your Pyr that s/he can live a normal, active lifespan.

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