The lifecycle of the disease starts with an arthropod (mosquito, Simuliidae (blackfly), Tabanid(horsefly), Stomoxys calcitrans(stablefly)) bites an infected animal and acquires the L1 stage filaria and then later bites an uninfected animal or human at which point the L3 stage filaria escape the arthropod, drop to the surface and crawl into the open incision left by the arthropod. The L3 filaria will then transform into the L4 adult stage within the new host.
In most mammals the adult will produce microfilaria (L1) which will circulate in the blood where they are able to be picked up by the next insect bite. In humans however the microfilaria have not been observed to circulate or at least not in sufficient quantities for microscopic examination to help diagnose the disease.
HDRI Theory: There have however been many observations in specific case studies where the aspirated fluids from skin/tissue nodules where microfilaria have been observed. It is believed that these microfilaria were present due to the nature of the skin/tissue nodules protecting the inner tissues from the human immune system which would have otherwise killed off the microfilaria. HDRI hopes to determine if this is actually the case because of the immunological consequences of such an immune response could drive morbidity in several significant ways.
Because there are no circulating microfilaria in humans the specific parasites have entered a host which is an evolutionary dead end as any microfilaria will not have the chance to find a new host. This of course does not mean that the host is going to fare any better than any other host. There are likely significant stresses on the human immune system as Dirofilaria has been titled the “Masers of Immune Regulation” for a reason.
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 Foil, L. D. (1989). Tabanids as vectors of disease agents. Parasitology Today, 5(3), 88–96. https://doi.org/10.1016/0169-4758(89)90009-4
 Baleba, S. B. S. (2021). Water immersion tolerance by larval instars of stable fly, Stomoxys calcitrans, L1758 (Diptera: Muscidae) impairs the fitness performance of their subsequent stages. BMC Ecology and Evolution, 21(1), 1–10. https://doi.org/10.1186/s12862-021-01810-z