Incubation Period
    Upon infection in dogs with D. immitis, the infective larvae migrate to the subcutaneous tissues where they mature for several months.  Once the nematode is fully mature, it migrates to the right ventricle of the heart.  After six to seven months, the fully mature worm is able to produce microfilariae, which enter the host dog's bloodstream (Shah).     Upon infection in humans, worms are unable to reach maturity.  In the case of subcutaneous Dirofilaria infection, if the infectious larvae is not immediately destroyed by the human immune response to the the infection, it grows gradually and reaches adulthood in about five months.  The worm eventually becomes constrained within an inflammatory nodule, where it survives for several months or years and eventually dies and is destroyed (Pampiglione, et al).  Since subcutaneous dirofilariasis infections generally result in painless nodules that are asymptomatic, the infection may persist for long incubation periods before an individual notices it and seeks medical assistance.
    In the case of human pulmonary dirofilariasis, the infectious larvae migrate to the veinous circulation, die in the right ventricle, embolize into the pulmonary artery, and organize into a nectrotic and fibrotic nodule.  The shortest documented time between a negative chest x-ray and the appearance of a pulmonary nodule is 5 months (Shah).
Ocularal dirofilariasis infections cause early intense irritation, which prompts early surgical removal of the worm and thereby keeps the incubation period short.

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