Monday, February 14, 2011

Dirofilaria immitis: Eat your heart out

Dirofilaria immitis, although it is just another parasitic organism, grabs our attention more than any other parasite, especially those of you who have pets. We see signs in pet stores and watch commercials on T.V. that tell us the dangers of dog heartworm. Transmitted by mosquito bites, heartworm, Dirofilaria immitis, is a common filarial parasitic nematode that infects primarily dogs and other canine species. It usually resides in the arteries, heart, lungs, as well as other blood vessels in dogs. Heartworms cause serious vascular damages and can be fatal, especially in working dogs with high levels of physical activity. Heartworm can be found all over, prevalent in areas with mosquitoes[1].

Heartworms belong to class of nematoda called secernentea. Dirofilaria immitis is characterized by its cylindrical threadlike body with a tough exterior. A sharp spear at the top of their heads is used to puncture cells. Both male and female heartworms have similar characteristics but differ in size. The female worm is 10 to 12 inches long while males are half the size of the female. They receive their nutrients from the red blood cells surrounding the heart and lungs of their hosts. An adult female worm can release up to 5,000 larvae per day into the blood stream[2]. One interesting fact about this parasite is that it carries endosymbiotic Wolbachia bacteria, whose metabolites affect the host's immune response[3].

Dirofilaria immitis is capable of infecting a wide variety of mammals. While dogs are considered the definitive host of heartworms, they can infect more than 30 species including cats and humans (very rare). Most infected dogs do not show any signs of disease for as long as two years. Unfortunately, by that time the disease is well advanced. The signs of the disease depend on the number of adults present and the degree of damage done to the heart and other organs. Few of the most obvious signs are: chronic cough, shortness in breath, loss of stamina and weakness. All of these signs are most noticeable following exercise, when some dogs even faint from lack of air supply to their lungs. The disease is diagnosed mostly in four to eight year old dogs. Moreover, various mosquito species are intermediate hosts of Dirofilaria immitis larvae. They are capable of supporting larval development to the infected larval stage as well as transmitting the disease[4].

Knowing the life cycle and how Dirofiliaria immitis reproduces is a key factor in understanding why this parasite is such a problem. The life cycle begins when a female black mosquito takes a blood meal from an infected host. The blood contains microfilariae, long thin embryos of Dirofilaria immitis, that look like tiny worms. Microfilariae lives about two weeks in the mosquito salivary gland before becoming inactive. It develops to the infective larval stage inside the mosquito and migrate within the insect's body to its mouth parts[1]. Next time an infected mosquito bites a dog, cat or any canine, heartworm larvae drops onto the host's skin and migrates in through the wound. In the dermal layer of the skin, and the muscle tissue, the larvae continues to mature for eight to ten weeks, attaining a length of about 25mm as they migrate through tissue heading to the heart and lung vessels[5]. Within 70 to 90 days, the larvae finally reaches the right side of the lungs and the pulmonary artery, where they impede blood flow and cause damage to the heart and lung vessels. Proving both male and female worms are present, Dirofiliaria immitis adult worms mate and females produce eggs, which develop into the long thin microfilariae that are released into the bloodstream. The offspring are produced 7 to 9 months after the mosquito bite. Is it these microfilariae that mosquitos take in when they draw blood from the infected host and the cycle starts all over again. Cats rarely have this microfilariae circulating for the female mosquitoes to pick it up[1].

Heartworms are primarily found in the right ventricle of its host's heart. On a geographical level, Dirofilaria immitis occurs all over the world, including 50 states in America, where there are a large number of mosquitoes. Mosquitoes thrive in warm and wet conditions, therefore tropical countries and regions that are closer to the equator have a greater presence of Dirofilaria immitis. The maturation of larvae ceases when the temperature drops below 57oF but the transmission never reaches zero in winter. Moreover, from an ecological standpoint most of the third world and other developing countries are not capable of medicating host's affected with this parasite because they are not able to afford the medication[1].

Dirofilaria immitis is an example of a type C parasite that uses vectors or biting organisms to inject themselves through the host integument. Vectors are considered to be invertebrate animals, usually arthropods and in this case mosquitoes are the intermediate species. The pathogen multiplies within the arthropod vector and transmitted when the arthropod takes a blood meal. Furthermore, what is remarkable in these Dirofilaria immits is the usage of double strategy of manipulating both space and time to enhance fitness. Only the microfilariae in the peripheral blood vessels is transmitted when the mosquito takes a blood meal. Also, mosquitoes are nocturnal, only bite in the evening and at night. Therefore, because microfilariae can only be ingested during nighttime, the number of microfilariae present in the peripheral vessels rises at nightfall[6].


[6] Combes, C.(2005). The Art Of Being A parasite. University of Chicago


  1. One of the main host being mosquito, is there a way to prevent the parasite from being transmitted from the mosquito to the next host or will ridding the mosquito rid of the life cycle of the parasite? Can the parasite find other ways to move to the next host if there is no mosquitos present?

  2. You mentioned this parasite was a host to the Wolbachia bacteria; how does this bacteria affect it's immune system? Also, what would be the treatment for both the animals and for the unlucky human who becomes a host?