The only way heartworm disease is transmitted is through the bite of an infectious mosquito. Several species of mosquito are capable of transmitting heartworm.
Heartworm disease (caused by the worm Dirofilaria immitis) primarily affects dogs, but it can also affect other canids, such as wolves, coyotes and foxes, as well as raccoons. In these types of animal reservoir hosts, microfilariae (small worms) are produced by mature heartworms, and the microfilariae are ingested by mosquitoes during a blood meal. While in the mosquito, the microfilariae develop into larvae, and these larvae can be transmitted through subsequent mosquito blood meals. It takes about six months for heartworm larvae to mature into adults once in a new host. The worm larvae migrate from the mosquito bite site into the abdomen and thoracic cavity of the host, eventually entering the blood stream and the heart.
Heartworms in dogs and other canids can result in serious injury to the heart, lungs and arteries, or even death. Heartworm signs in dogs can include cough, tiredness, vomiting and weight loss. Acute symptoms can include convulsions, blindness, difficulty breathing, fainting and sudden death. Adult heartworms are large (the size and shape of cooked spaghetti) and are 4–14 inches long. In general, dogs exhibit symptoms when 50–100 mature heartworms are present.
Symptoms may not occur in dogs for up to a year post-infection; therefore, dogs with advanced infections are typically the ones that show symptoms. At advanced stages of infection, heartworms are more difficult to eliminate safely, hence prevention of infection is best. Humans, cats, ferrets and a variety of other animals can be infected with heartworms, but they can’t serve as reservoirs to infect other mosquitoes because adult heartworms can’t produce microfilariae in these types of hosts. Only 20 percent of infected cats have microfilariae in the blood stream, compared to 80 percent of dogs. Cats and humans also show a much lower infection rate than dogs and other canids.
Most humans experience no symptoms when infected with heartworms. Heartworm in humans may result in lung issues rather than heart complications. Humans can experience inflamed lung tissues due to dying worms that may appear as lesions on lungs. In humans, the worm larvae generally die before they become large adult worms. Lesions caused by worms can be surgically removed to resolve infection.
All U.S. states are affected by heartworms, although the southeastern U.S. shows more cases than other regions. The American Heartworm Society provides updated heartworm distribution maps every three years. These maps are based on data compiled by thousands of veterinary clinics. Risk of heartworm infection differs based on variables such as climate, potential pathogen reservoirs and mosquito vectors.
The best way to prevent heartworm disease is to avoid mosquito bites. Mosquito surveillance and control can suppress or eliminate infected mosquitoes in an affected area. Once infected mosquitoes are eliminated and animals serving as potential reservoirs are treated in an affected area, the transmission cycle can be broken.
If pets are housed indoors and are not exposed to mosquito bites, there is a lower incidence of infection. Pets should be given monthly preventative heartworm medication year-round (provided by their veterinarian) to help prevent heartworm larvae. Dogs can still be infected with heartworms transmitted by mosquitoes, but the worms do not survive in the dogs. Veterinarians recommend that dogs and cats should be tested once a year for heartworm infection.
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