Once considered confined to the southern US where winters are mild, heartworms now infest dogs and some outdoor cats in all parts of the country. Many veterinary clinics display a preserved heart infected with heartworm in a jar. However, seeing is not necessarily believing; although clients can view the infested heart loaded with long, spaghetti like worms every time they visit, many gamble that their pets will never be bitten by an infected mosquito.

Heartworm prevention costs money and requires commitment: blood must be drawn to examine for signs of infestation before the preventive drug can be given, and the drug must be administered regularly whether the daily dose or the once-a-month type is chosen. But the effort and the money are well-spent to keep a loved pet from the discomfort and debilitation of the infliction.

Like most parasites, the heartworm goes through several life stages before emergence as an adult and needs at least two hosts to complete the cycle. The mosquito serves as the host for the larval stage of the worm, called microfilariae. The mosquito ingests the larva when it bites an infected dog and deposits its cargo in an uninfected dog when seeking another blood meal. The microfilariae burrow into the dog, and for three or four months, undergo several changes to reach adult form. They then travel to the right side of the heart through a vein and await the opportunity to reproduce. Adult heartworms can reach 12 inches in length and can remain in the dog's heart for several years.

Female heartworms bear live young -- thousands of them in a day. These young -- the microfilariae -- circulate in the bloodstream for as long as three years, waiting to hitch a ride in a bloodsucking mosquito. They undergo changes in the mosquito that prepares them to infect the dog, and they transfer back to the original host species the next time the mosquito bites. The process of change in the mosquito takes about 10 days in warm climates, but can take six weeks in colder temperatures.

The worms grow and multiply, infesting the chambers on the right side of the heart and the arteries in the lungs. They can also lodge in the veins of the liver and the veins entering the heart. The first sign of heartworm infestation may not manifest for a year after infection, and even then the soft cough that increases with exercise may be dismissed as unimportant by the owner. But the cough worsens and the dog may actually faint from exertion; he tires easily , is weak and listless, loses weight and condition, and may cough up blood. Breathing becomes more difficult as the disease progresses. The progression is traumatic: the dog's quality of life diminishes drastically and he can no longer retrieve a Frisbee or take a long walk in the park without respiratory distress. Congestive heart failure ensues, and the once-active, outgoing pet is in grave danger.

If a blood test or the onset of symptoms alert owner and veterinarian to the presence of this devastating parasite, treatment is possible and successful if the disease has not progressed too far. The first step is to evaluate the pet and treat any secondary problems of heart failure or liver or kidney insufficiency so that he can withstand the treatment. The next step is to kill the adult worms -- with an arsenic compound. The drug is administered in two doses each day for two days, followed by several weeks of inactivity to give the pet's system a chance to absorb the dead worms. Exertion can cause the dead worms to dislodge, travel to the lungs, and cause death.

Six weeks after the administration of the drug to kill the adult worms, further treatment to kill the microfilariae is needed. The pet is dosed daily for a week , then the blood test is repeated. If microfilariae are still present, the dose can be increased. Follow-up studies should be done in a year.

Even if the pet, the owner, and the owner's pocketbook survive the treatment, dog or cats can be reinfected the next time a mosquito bites. So, before mosquito season opens again, owners should have their pets tested for heartworm and placed on preventive. Most veterinarians use two tests before declaring a dog free of heartworm; the first test checks for microfilariae in the blood and the second, done with the same sample, checks for adult worms in the heart that can be present even without the larvae in the blood.

Preventive doses can be given daily or monthly and are only available from a veterinarian. Some of the drugs also rid the pets of other parasitic worms. Many veterinarians recommend and many owners use a year-round heartworm prevention program. If your pet has already had his yearly check-up, call your veterinarian to schedule a heartworm check. If he's due for yearly vaccination, be sure to include a heartworm check in the visit!

Three of the most commonly recommended preventative medications are Heartgard, Interceptor, and Filaribits. Ask your veterinarian for the best recommendation for your pet.