An ounce of prevention beats a pound of cure
Once considered a parasite of southern climates, the heartworm (Dirofilaria immitis) is now recognized as a major, global pest affecting dogs, wolves, coyotes, and foxes. From its discovery in dogs more than a century ago and the documentation in cats in the 1920s, researchers have devised diagnostic tests, preventives and treatments, but the disease still spreads.
To jog the concern of clients, veterinary clinics display a preserved heart infected with heartworm in a jar and hang posters about heartworm life cycles in examining rooms, but 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 dogs will never be bitten by an infected mosquito.
Heartworm prevention is simple. It involves a blood draw to determine whether the parasite is present and regular dosing with preventive medication. Heartworm infestation is dangerous; untreated dogs die and treated dogs go through weeks of discomfort while the worms are killed and expelled from their bodies.
Parasites go through several life stages before emergence as adults and often need at least two hosts to complete the cycle. In heartworms, a mosquito serves as the intermediate host for the larval stage of the worm, also known as the 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 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.
The time lag between the initial infestation of microfilariae and reproduction by adult worms living in the heart is six-seven months in dogs.
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 prepare them to infect a 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.