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Parasite Prevention: Preventing Canine Heartworm Disease


Pet Column for the week of May 29, 2014

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Related site - Primary Care for Pets at the University of Illinois Veterinary Teaching Hospital

[Dog on a beach]
Mosquito season is here. Protect pets against mosquito-borne heartworm disease.

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Sarah Netherton
Information Specialist

Mosquito season is here. The bites of mosquitoes not only cause an itchy nuisance for people and pets, but they carry the potential to transmit serious diseases. Dr. Allan Paul, a veterinary parasitologist at the University of Illinois College of Veterinary Medicine in Urbana, explains how dogs can get heartworms from mosquitoes and how to prevent this deadly disease.

“When a mosquito that is carrying the infective heartworm larvae bites an animal, the infection will be transmitted to the animal. The larvae will migrate throughout the pet’s body and, over a period of several months, these worms will mature into adults,” explains Dr. Paul. “The worms live in the animal’s lungs, heart, and blood vessels.”

Proliferation of the worms occurs when the adult worms mate and the female worms then release their offspring, called “microfilariae,” into the bloodstream. According to Dr. Paul, it can take up to seven months after infection before the microfilariae can be detected in the blood, meaning the dog could test negative for the disease during this “pre-patent” period.

Adult male heartworms are usually 4 to 6 inches long, while adult female worms can be up to a foot long. After about one year, heartworms are fully grown and can live in the dog up to seven years.

Dogs are commonly infected with high numbers of these worms. An infected dog can host anywhere from 1 to 250 heartworms at one time.

“The severity of the disease is related the numbers of worms infecting the dog, how long the infection has been going on, and the activity level of the dog,” says Dr. Paul. “An active dog is more likely to develop severe disease from a smaller number of heartworms compared to its less active counterpart.”

Canine heartworm disease is found throughout the United States and has been reported in all 50 states. According to Dr. Paul, the highest rates of infection in dogs not on a heartworm preventative occur within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River, where the infection rate approaches 45 percent.

Clinical signs that an owner may notice that may indicate a heartworm infection include a chronic cough and exercise intolerance. If heartworm disease is suspected, the animal will need to be seen by a veterinarian.

The veterinarian will perform a physical exam, where the dog may appear normal if the disease is mild, while labored breathing and crackles in the lungs may be observed if the disease is severe. Severely affected dogs may also present with a history of anorexia, cachexia (severe weight loss), jaundice (yellowing of the skin and mucous membranes), and fainting. Clinical signs and disease are dependent upon where in the dog’s body the worms have migrated, typically the heart or lungs.

If a veterinarian suspects heartworms, an x-ray of the heart and lungs is the best way to access the severity of the disease and determine the patient’s prognosis. Inflammation of the lungs and enlargement of the arteries leading from the heart to the lungs are often seen, since these places are where the worms are commonly found.

Treatment for a heartworm positive dog is a months-long process that involves multiple injections into the dog’s back, strict exercise restrictions following the injections, antibiotics, and steroids.

What is the best way to prevent a dog from getting heartworm disease? Dr. Paul recommends a monthly preventative that is available at your local veterinary clinic. “Although there are differences of frequency of infection for various groups of dogs, all dogs in regions where mosquitoes are found should be considered at risk, placed on prevention programs, tested annually for heartworm disease, and examined frequently by a veterinarian,” advises Dr. Paul.

For more information about heartworm disease, speak with your veterinarian.