Submitted by CCLRC member Autumn Davidson, DVM

Heartworm prevention is advised now in all 50 states in the USA for both dogs and cats. The incidence of heartworm varies by geography, and can be seen on a great website about animal parasites, www.capcvet.org produced and updated by the Companion Animal Parasite Council.

Tulare County is listed as having a low incidence of heartworm disease, but it is present, especially in the foothills. We have the vector (mosquito) and we have reservoirs (coyotes, infected dogs). The mosquito crop is particularly robust this year because of the rain! Mosquitos ingest heartworm microfilaria when they get a blood meal from a dog, then infect the next dog with immature heartworms.

Many prescription heartworm preventatives are on the market, manufactured specifically for dogs and FDA approved. The products I like best are given orally once a month, and are flavored such that dogs readily gobble them. In fact, the packages need to be kept out of their reach! The best products combine ingredients that prevent both heartworms and common, insidious intestinal parasites like roundworms (which are dangerous to people). Some prevent fleas as well. These drugs should be given with the guidance of the prescribing veterinarian, because they are drugs and are not always innocuous. Some breeds of dogs are best treated with certain drugs.

I give my dogs preventative year around. I don’t want to worry about exposure to infected mosquitos at any time, and I’ve seen mosquitos in Three Rivers almost year around. If I travel, I don’t want to worry about getting the dogs restarted on preventative in time. It is easier to remember it on the first of every month. Even indoor cats have been reported to have contracted heartworms. Roundworms can be contracted year-round.

The CAPC advises annual heartworm testing even for dogs on preventative. Why is this? Two main reasons: 1. No drug is 100% effective. 2. If heartworm infection is diagnosed early, it can be treated before serious consequences (lung and heart disease) occur. Untreated heartworm infection can be lethal. Additionally, the dog is exposing other dogs via mosquito bites.

The older drugs used for heartworm prevention could cause serious problems if given to dogs already infected; this is no longer the case, but dogs should still be tested to assure they are clear before starting preventative (if they are 6 month of age or older), and ideally 6 months later (in case they were infected with larval heartworm when the test was first performed). (The most accurate heartworm test used today looks for adult heartworms, not larvae.) If your veterinarian follows the guidance of CAPC they will advise annual testing. Many veterinarians will not REQUIRE it to refill the preventative prescription, but they feel obligated to educated clients that it is advised. You maybe asked to sign a waiver if you choose not to test, that way the veterinarian cannot be accused of malpractice.

In order to get a prescription for heartworm prevention, your dog must be a current patient of a veterinarian. The California Veterinary Medical Board requires veterinarians to have examined a pet and recorded the findings of that exam within a year, to establish the “veterinary-client- patient” relationship. If a veterinarian prescribes drugs for a dog that is not a current patient, they have violated their license laws. Veterinarians should then be willing to dispense the preventative, or provide a written prescription that can be filled on line or at a pharmacy.