Canine heartworm disease continues to be a major, life-threatening issue for dogs all over the world. It has been found in all 50 of the United States, and the American Heartworm Society recently released a new study showing an increase in the incidence of heartworm disease throughout the country.

I am often amazed at the lackadaisical attitude that some pet owners have toward heartworm disease. It is easily preventable by giving a medication monthly. Why would anyone choose not to do this for their pet?

The heartworm blood parasite, Dirofilaria immitis, is spread by mosquitoes that feed on an infected dog by taking blood. The mosquito transmits the microfilaria — heartworm offspring — to other animals when it bites them. The microfilaria mature and migrate to the heart and pulmonary vessels, where they produce thousands of infective larvae. Symptoms of heartworm disease include difficult breathing, coughing, exercise intolerance, weight loss and heart failure. Sudden death with no obvious symptoms might also occur.

Heartworm preventative should be given every month, year-round in Missouri. It is easy to give and comes in a variety of forms. Chewable tablets, flavortabs and topicals are all available. For those with difficulty remembering to give medications, you can sign up through several different companies to receive monthly e-mails or post card reminders. Stickers to put on the calendar to help you remember also come in every box of heartworm preventative.

Dogs should have a yearly blood test for heartworms. This is recommended even for those on the monthly preventative. Missed doses, dogs that threw up the medication, confusion over who was giving the medication and tricky dogs who spit out medications are all reasons why yearly tests are recommended.

Testing is one aspect of heartworms that causes a lot of confusion. Heartworms have a very long life cycle, so it takes six to eight months from the time of infection before a dog will test positive. This explains why a dog that tests negative and is given preventative every month for a year might test positive the next year. The test kits used in most veterinary hospitals detect the presence of the feline heartworm uterine antigen. This means that a dog with only male heartworms will not test positive.

There are also some misconceptions about heartworms. One is that inside pets do not need to be given monthly preventative. Even if your dog never leaves the house to go for a walk, play or take care of bathroom duties, it should still be on preventative. Mosquitoes can and do get in the house. It takes the bite of only one infected mosquito to transmit heartworms.

Owners of dogs with long or thick fur coats sometimes question the necessity of giving heartworm preventative. Do not underestimate the determination of mosquitoes. They will find a way to get a blood meal through all that fur.

My favorite excuse that people sometimes give for not using heartworm preventative is that they have had dogs for years and they never got heartworms. You could probably cross the street without looking both ways many times without getting hit by a car, but is that really a good idea?

Heartworms can infect cats and many other species. Heartworm disease of cats poses many challenges and will be discussed in my next column. For more information regarding prevention of heartworms in dogs, contact your veterinarian or go to the American Heartworm Society Web site at www.heartwormsociety.org.