Vertical transmission

Vertical transmission, the transmission of a viral or bacterial disease from the female of the species to her offspring, has been observed in various West Nile virus studies, amongst different species of mosquitoes in both the laboratory and in nature.^[48] Mosquito progeny infected vertically in autumn, may potentially serve as a mechanism for WNV to overwinter and initiate enzootic horizontal transmission the following spring, although it likely plays little role in transmission in the summer and fall.^[49]

Risk factors

Risk factors independently associated with developing a clinical infection with WNV include a suppressed immune system and a patient history of organ transplantation.^[50] For neuroinvasive disease the additional risk factors include older age (>50+), male sex, hypertension, and diabetes mellitus.^[51][52]
A genetic factor also appears to increase susceptibility to West Nile disease. A mutation of the gene CCR5 gives some protection against HIV but leads to more serious complications of WNV infection. Carriers of two mutated copies of CCR5 made up 4.0 to 4.5% of a sample of West Nile disease sufferers, while the incidence of the gene in the general population is only 1.0%.^[53][54]