Transmission and Life Cycle

    Little is known about the transmission of dirofilariasis infection to humans.  It is generally understood to occur by accidental infection by the agency of the same insects that infect the parasite's natural hosts, namely various species of mosquito.  Transmission occurs via the blood when the mosquito vector takes its blood meal from an infected animal that has circulating microfilariae.  Microfilariae enter through the insect's proboscis, and then develop into infective juvenile larvae inside the insect's salivary gland.

Figure 1: Microfilariae are ingested by a mosquito during a blood meal.  From
Companion Animal Surgery: http://www.comvet.com/html/heartworm.html.

Figure 2: Early larval stage inside mosquito's salivary gland.  From
Companion Animal Surgery: http://www.comvet.com/html/heartworm.html.

    These infective juveniles are introduced into a new animal host when the mosquito feeds again.  Since the insect vector is generally indiscriminate in its blood meal host,  a mosquito carrying the parasite can potentially transmit the infective juvenile larvae to humans by depositing the infective larvae on the skin as it takes its blood meal from the human.  Humans are a dead-end host for Dirofilaria.  The larvae will either die at the site of inoculation or migrate into the subcutaneous tissues, where they will incompletely mature.  In the case of pulmonary filariasis, incompletely matured worms migrate to the right side of the heart.  However, since they are unable to complete maturation, they die and embolize to the pulmonary vasculature, causing solitary pulmonary nodules to form.  Since the worms are unable to achieve sexual maturation, microfilaremia is not seen in human infections.


Figure 3: Natural life cycle of D. immitis.  From Ohio State University: http://www.biosci.ohio-state.edu/~parasite/dirofilaria.html.
 

Figure 4: The microfilariae stage of the Dirofilaria life cycle is not present in humans.  Image from
Ohio State University: http://www.biosci.ohio-state.edu/~parasite/dirofilaria.html.
 

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