For many dog owners, warmer months can bring both relief and fear. It’s nice to be able to let pets play outdoors without risk of getting too cold, but rising temperatures can also accelerate the transmission of an ugly and insidious parasite.
Canine heartworm disease is a dangerous and life-threatening illness transmitted by mosquitos to pets all across the United States. And by the time summer arrives, it may already be spreading from southern states northward.
What causes heartworm disease?
As the FDA reports, heartworm disease is caused by a parasitic worm called Dirofilaria immitis. The worms are spread to pets by mosquito bites. They mature into adults, mate, and produce offspring while living inside a dog, eventually making their way to the heart, lungs, and blood vessels of an infected animal.
According to Merck Manual Veterinary Manual, as many as 70 species of mosquitoes can transmit heartworms, which require the insects as an intermediary host before developing within mammals.
“The life cycle begins when a female mosquito bites an infected dog and ingests the microfilariae during a blood meal,” VCA Hospitals reports. “The microfilariae develop further for 10 to 30 days in the mosquito’s gut and then enter its mouthparts. At this stage, they are infective larvae and can complete their maturation when they enter a dog. The infective larvae enter the dog’s body when the mosquito bites the dog.
“These infective larvae migrate into the bloodstream and move to the heart and adjacent blood vessels, maturing to adults, mating, and reproducing microfilariae within 6 to 7 months.”
Where is heartworm disease most prevalent?
In the United States, heartworm disease is most common along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states.
Louisiana and Mississippi continue to be the leading states for heartworm infection, the American Heartworm Society reports. Just 5 states in the south now account for 51% of all pet euthanasia in the country, with Louisiana having the highest rate in the U.S.
What does heartworm disease lead to in pets?
Untreated, heartworm can lead to lung disease, heart failure, or irreparable damage to other internal organs, and ultimately death.
“The severity of a heartworm infection and the signs depend on the number of worms present, the immune response of the infected dog, the duration of infection, and the activity level of the animal,” reads Merck Manual Veterinary Manual, “Most dogs are highly susceptible to heartworm infection, and the majority of infective larvae develop into adult worms. The presence of heartworms in the vessels serving the heart and lungs, and in the heart itself, causes irritation and inflammation of the affected vessels or heart chamber. In addition, the dog’s immune system does further damage to the blood vessels, especially in response to dead worms. With longterm infection (over 1 year in duration), the constant irritation will lead to scarring and reduced flexibility of the blood vessels. Heartworms typically live for 3–5 years in an animal’s body.”
Small dogs typically face worse consequences from heartworm infections or treatments than larger dogs because they have smaller blood vessels and heart chambers, and it takes fewer worms to damage or block those vessels.
The FDA breaks heartworm disease into four classes:
- Class 1: No symptoms or mild symptoms such as an occasional cough
- Class 2: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity
- Class 3: More severe symptoms such as a sickly appearance, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays
- Class 4: Also called caval syndrome. There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is risky, and even with surgery, most dogs with caval syndrome die
Can heartworm disease be treated?
The short answer is yes! But it is not always easy.
One of the previously popular treatments for heartworm disease contained high levels of arsenic and toxic side effects frequently occurred, VCA Hospitals reports. Today, new drugs are available that do not have as many side effects, and allow successful treatment of more than 95% of dogs with heartworms.
The earlier the disease can be diagnosed the better. Many dogs already show signs of advanced heartworm disease at the time they are diagnosed.
“Rarely, cases may be so advanced that it is safer to treat organ damage and keep the dog comfortable than it is to risk negative effects associated with killing the heartworms,” VCA Hospitals writes. “Dogs in this condition are not likely to live more than a few weeks or months. Your veterinarian will advise you on the best treatment approach for dogs diagnosed with advanced heartworm disease.”
How can heartworm disease be prevented?
The American Heartworm Society recommends annual heartworm testing and year-round heartworm prevention for dogs and cats in the U.S., even in regions that experience cold winters.
“Heartworm preventives work retroactively, so an animal that acquires an infection one month must be given heartworm preventives in the months that follow to be protected,” the AHS writes. “And with unpredictable weather patterns and the ability of hardy mosquitoes to survive in protected areas—as well as indoors—it’s difficult to predict when heartworms aren’t in season.”
Fortunately, heartworm prevention is highly effective when given faithfully, and the year-round cost of preventing the disease in dogs is a small fraction of the cost of heartworm treatment.
This may mark the last summer for many dogs if they do not receive the heartworm treatment they need this year. But you can make a difference. Pledge to protect pets by understanding the signs of heartworm and sharing the importance of the Good Flights program.
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