June 2003

The Invisible Killer of Man’s Best Friend

            Dog heartworm is a common, mosquito-borne, parasitic disease in canines worldwide occurring in tropical, subtropical and temperate regions. In the United States, it was once limited to the south and southeast regions but is now found throughout the continental United States and Canada. Heartworm is a serious and potentially fatal disease in dogs. It is caused by the parasitic nematode (a roundworm) called Dirofilaria immitis. The adult, reproductive worms reside in the heart where they cause disease by clogging the heart and major vessels leading from the heart. They interfere with the valve action in the heart and by clogging the major vessels, restrict blood supply to other organs, particularly the lungs, liver and kidneys which causes damage that leads to their malfunction. The worms are 6 inches (males) to 14 inches (females) long and about 1/8 inch diameter, about the thickness of cooked spaghetti. They live and mate in an animal’s heart for up to five years. During their lifetime, the females produce millions of young called microfilaria. The microfilaria live mainly in the small blood vessels, entering the bloodstream during early morning and early evening hours, the time when mosquitoes are most active. This worm cannot complete its entire life cycle in a dog. The heartworm must go through several life stages inside a mosquito before it can become infective to another canine. The mosquito picks up microfilaria when it feeds on an infected animal. The microfilaria develop for 10-30 days inside the mosquito then migrate from the digestive tract to the mosquito’s mouthparts. Susceptible animals are infected when the infected mosquito feeds. As the mosquito feeds on its host, the infective heartworm larvae crawl out from the mouthparts on to the animal’s skin, they are not injected by the mosquito into the animal. These tiny infective worms proceed to burrow into the host animal using the puncture wound the mosquito made to get its bloodmeal. Worms that enter animals other than canines generally die within several days. However, once inside a canine host, the worms lodge themselves into tissue where they remain for up to 7 months completing their larval stages. After completing their larval stage they leave the tissue and enter the bloodstream, move to the heart and associated major vessels, grow to maturity in 2 to 3 months and start reproducing, producing microfilaria, thus completing their life cycle. In this parasitic cycle, canines act as the host and reservoir while the mosquito is the intermediate host and vector (transmitting agent). The natural reservoir for heartworm (before dogs were domesticated) are wolves, foxes, racoons, skunks, opossums, and other canines. Worldwide, there are over 70 mosquito species capable of supporting the development of and then vectoring dog heartworm. In Florida there are 20 species and in our area of the Florida panhandle there are 7 common species of mosquitoes capable of transmitting the heartworm parasite.

            Visible signs of heartworm may not appear until a full year after the animal was infected. In fact, the disease may be well advanced before the dog shows any symptoms. Consequently, the disease is diagnosed mostly in dogs 4 to 8 years old. The symptoms are a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. These signs are most noticeable following exercise, some dogs may even faint. Severely diseased dogs can even die suddenly during exercise or excitement.

            Diagnosis of heartworm infection can be confirmed by taking a blood sample and finding microfilaria under a microscope. This test is 80% effective. A more reliable method is to take x-rays. X-rays will show an enlargement of the heart and bulging of the large artery leaving the heart to the lungs. It is very important to diagnose a heartworm infection early. Much of the damage caused by heartworm occurs before there are any outward signs.

            Treatment of dogs with heartworm has some risk involved, the younger the animal, the better able they are to tolerate the treatment. Killing the adult worms is accomplished by the use of drugs containing arsenic. These medications are designed to kill and slowly disintegrate the worms over a period of time, surgery, to physically remove the worms, is another option. Once the adult worms are killed, then the microfilaria need to be eliminated, again by the administration of a drug. Survival from this treatment regime is about 95%. Once the animal has been rid of the adult worms and microfilaria, treatment is complete, however, you cannot sit back and relax, the animal can become reinfected.

            A heartworm prevention program needs to be established immediately and continued for the remainder of the animal’s life. Dogs can be treated with a daily chewable or a monthly chewable tablet. All of the medications used to prevent infection are safe and very effective and cost about the same. Other actions that can reduce the possibility of infection is to keep your dog out of mosquito infested areas, screening the animal’s living quarters or keeping your pet indoors. Indoor dogs have a much lower incidence of infection. Mosquito control in residential areas helps to minimize the incidence of heartworm infection by breaking the transmission cycle of the parasite. In addition to these preventive measures, a blood sample inspection for microfilaria, once a year, by your veterinarian, will diagnose an infection early or provide you assurance that your dog is not infected. After all, young or old, mongrel or purebred, isn’t Fido’s well-being worth it?