to a virus. Studies have shown it can specifically modulate the immune response during early virus infection, and mosquito feeding can exacerbate WNV infection, leading to higher viremia and more severe forms of disease. 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. 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. 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. For neuroinvasive disease the additional risk factors include older age (>50+), male sex, hypertension, and diabetes mellitus. 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 people with West Nile disease, while the incidence of the gene in the general population is only 1.0%. The most at risk occupations in the U.S. are outdoor workers, for example farmers, loggers, landscapers/groundskeepers, construction workers, painters, summer camp workers and pavers. Two reports of accidental exposure by laboratory personnel working with infected fluids or tissues have been received. While this appears to be a rare occurrence, it highlights the need for proper handling of infected materials. The World Health Organization states that there are no known cases of health care workers acquiring the virus from infected patients when the appropriate infection control precautions are observed. Diagnosis Preliminary diagnosis is often based on the patient's clinical symptoms, places and dates of travel (if patient is from a nonendemic country or area), activities, and epidemiologic history of the location where infection occurred. A recent history of mosquito bites and an acute febrile illness associated with neurologic signs and symptoms should cause clinical suspicion of WNV. Diagnosis of West Nile virus infections is generally accomplished by serologic testing of blood serum or cerebrospinal fluid (CSF), which is obtained via a lumbar puncture. Initial screening could be done using the