Dirofilaria immitis
Dirofilaria immitis, commonly known as heartworm, is a parasitic worm primarily affecting dogs but also found in other animals like foxes and wolves. Transmitted through the bite of infected mosquitoes, these long, thin worms invade the heart and associated blood vessels, leading to heartworm disease, which can cause severe health issues such as heart damage, heart failure, lung disease, and pulmonary embolisms. While heartworm disease is common among dogs, it is rare in humans, with infections typically resulting in non-reproductive adult worms that can settle in the lungs or other tissues. Diagnosis in dogs often involves blood tests and imaging, while human cases are identified through chest X-rays. Treatment for infected dogs includes a series of medications to eliminate the worms, but it carries risks of complications like pulmonary embolisms. Prevention is best achieved through monthly heartworm preventive medications and minimizing exposure to mosquitoes. Regular veterinary check-ups are also essential for monitoring heartworm presence.
Dirofilaria immitis
Dirofilaria immitis (D. immitis), more commonly known as heartworm, is a class of worms very similar to roundworms. Heartworms are very long and thin parasitic worms that are passed through the bite of a mosquito typically to animals such as dogs, foxes, and wolves. In animals, these parasites invade the right side of the heart and blood vessels that connect the heart to the lungs. They cause heartworm disease, which leads to a variety of serious problems to the infected animal such as heart damage, heart failure, lung disease, and pulmonary embolisms (blockages in the blood vessels in the lungs). Heartworm disease is more common in dogs but very rare in humans.
Heartworm Disease
Many species make up the Dirofilaria genus of worms. Only three cause dirofilariasis, or disease caused by the infestation of filarial worms, in humans: D. immitis, D. repens, and D. tenuis. The typical hosts for these diseases are dogs, foxes, wolves, and raccoons. D. immitis is found in dogs, foxes, and wolves and is the most common form found in the United States. D. repens is not found in the United States but is common in Europe. D. tenuis typically affects only racoons in North America.
D. immitis, which causes heartworm disease in the United States, can only be passed by the bite of an infected mosquito. More than seventy different species of mosquitoes can transmit heartworms. Heartworm disease cannot be passed by contact from dogs to other animals or humans. Heartworms were first found in the southeastern United States in 1847. They have since migrated to the rest of the fifty states due to the movement of infected hosts. The first human case of heartworm disease was identified in 1941.
When an infected mosquito bites an animal, it transmits the heartworm larvae to the animal. The larvae make their way to the heart and mature into adults. They grow to an average of fourteen inches long. The adult heartworms then mate and produce microfilariae. This process takes about six to seven months. Heartworms can live in dogs for five to seven years, while microfilariae can live for about two years. Dogs that have heartworm disease can have several hundred heartworms living in their hearts, blood vessels, and lungs.
Humans also can become infected by the bite of a mosquito. Larvae can mature into adults, but they cannot mate or produce microfilariae. The adults can settle in the lungs and chest cavity or other places such as under the skin or in the eyes, brain, or testicles.
Signs and Symptoms
Many dogs with heartworm disease do not show any signs of being ill. Others may show some symptoms. Some dogs may be lethargic, eat less, and lose weight. The most common first sign of the disease is a cough. Other signs are fatigue during physical exercise and an extended abdomen due to fluid accumulation.
Like dogs, humans usually show no signs of having heartworm disease. Areas of inflammation, cough, chest pain, and fever are common symptoms. The areas of inflammation can be caused by dying adult worms in pulmonary arteries. These show up as lesions on X-rays of the lungs. The worms also can be found as nodules (lumps) under the skin.
Diagnosis
Blood tests that look for antigens and antibodies are performed to identify heartworm disease in dogs. In addition, X-rays and ultrasounds can be used to examine the heart and lungs to determine the presence of heartworms.
In humans, blood tests are not used to identify heartworm disease. Heartworm disease can be diagnosed by taking chest X-rays to look for lesions in the lungs. In addition, the tissue from inflamed areas or nodules may be tested for the presence of heartworms.
Treatment
If heartworm disease is not treated, the infected animal can die. Treatment depends on how severe the infection is in an animal. In most cases, dogs are given two months of a heartworm preventive medication such as Heartgard to kill heartworm larvae. After this, the dog receives an injection of an adulticide (drug to kill the adult heartworms) called melarsomine, which goes by the brand name Immiticide. Melarsomine is the only drug approved by the US Food and Drug Administration (FDA) to treat heartworm disease in dogs; however, it contains arsenic (a poisonous chemical), which may make some dogs ill and cause tissue damage.
A month later, the dog receives two more melarsomine injections. At six months, the dog is given a blood test to test for antigens. If the tests show heartworms remain in the blood, another melarsomine injection is given. In addition, the dog will remain on the heartworm preventive medication during treatment and for rest of its life.
Melarsomine kills adult heartworms, but the dead heartworms can cause pulmonary embolisms. Signs of pulmonary embolisms include cough, coughing up blood, fever, and heart failure. During heartworm treatment, dogs should be kept quiet and calm and not allowed to exercise vigorously. In severe cases, adult heartworms will need to be removed surgically from the heart.
For humans, treatment involves the surgical removal of lesions in the lungs or nodules in the skin. Medications typically are not needed to treat the disease in humans.
Prevention
The best way to prevent a dog from getting heartworm disease is by giving it a monthly heartworm preventive medication. These medications do not kill adult heartworms, but they do kill heartworm larvae. Some medications also control other parasites such as fleas and ticks. These medications should be used year-round. Preventive medications can be obtained from a veterinarian. In addition, veterinarians should test animals periodically for heartworm disease.
Avoiding exposure to mosquitoes is the best form of heartworm prevention for both animals and humans. Some ways to prevent exposure to mosquitoes is by avoiding the outdoors at times when mosquitoes are present, wearing insect repellant, and covering any exposed skin.
Bibliography
Eckstein, Sandy. "Heartworms in Dogs: Facts and Myths." WebMD. WebMD, LLC. 16 Dec. 2011. Web. 3 Feb. 2016. http://pets.webmd.com/dogs/guide/heartworms-in-dogs-facts-and-myths
"Heartworm Basics." American Heartworm Society. American Heartworm Society. Web. 3 Feb. 2016. https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics#my-dog-was-treated-for-heartworm-4-months-ago-and-his-heartworm-test-is-still-positive-what-does-this-mean
"Heartworm (Dirofilaria immitis) Infection & Prevention in Dogs." Doctors Foster and Smith PetEducation.com. Petco Wellness, LLC. Web. 3 Feb. 2016. http://www.peteducation.com/article.cfm?c=2+2096&aid=743
"Parasites – Dirofilariasis." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention. Web. 3 Feb. 2016. http://www.cdc.gov/parasites/dirofilariasis/index.html