Background: West Nile virus (WNV) was first reported in the U.S. in 1997. It is spread by mosquitoes that bite an infected animal (usually a bird), and then bite another animal, transmitting the virus. In addition to birds, the virus can be spread to humans and other mammals including dogs, cats, and horses. It can also be transmitted via blood transfusion or organ transplant from an infected person. Many humans experience no symptoms, but about 20% will contract “West Nile fever”, with fever, headache, body aches, nausea, and rash that can last for weeks. In a few cases (less than1%), the illness will become more serious, leading to permanent neurological effects such as muscle weakness, vision loss, and coma. There is no specific treatment for WNV illness. Diagnosis of WNV infection is accomplished by detecting virus proteins (by ELISA) or genetic material (by PCR) in blood or cerebrospinal fluid samples. (Since the virus genetic material is RNA, reverse transcription (RT) is used to copy the RNA into cDNA for the PCR test). Antibodies to WNV can also be detected in the patient’s blood by ELISA, but these may not be detectable until later in the infection process.
Note: An ELISA test is considered positive if the color production (absorbance) for a sample is at least twice the negative control value. PCR results are recorded as viral load values, i.e. how many copies of the virus were detected.
Case A. The annual blood drive in Mitchell, SD is usually held during September each year. However, this year there was an outbreak of West Nile infections in mid-August. No one died, but at least 30 people were diagnosed with West Nile fever, and five cases were severe enough to require hospitalization. It is likely that many more people were infected with WNV but did not have symptoms, so all of the donated blood will be screened for antibodies to WNV. Any samples testing positive for antibodies will be tested for WNV genetic material by PCR. The presence of WNV cDNA would indicate an active infection, otherwise the individual has probably recovered from the infection.
Procedure: To analyze this case, run an ELISA on the set of 10 donated blood samples provided, testing them for antibodies to WNV proteins. If any of the samples test positive for antibodies, test those samples for WNV genetic material by PCR.
- How many blood samples, if any, tested positive for WNV antibodies?
- Did any of the samples test positive for WNV genetic material?
- What is the significance of finding WNV antibodies or genetic material in blood?
- Is the number of positives what you would expect, based on the number of cases of West Nile fever?
- Should the individuals who donated the blood that tested positive be notified?
- What can be done to reduce the number of WNV infections next season?
Case B. Rachel has lived in Anaheim, CA, all of her life, and had never noticed anything like this. For the past several weeks she had been finding dead birds, mostly crows and a few sparrows, in her yard. She has also seen them in the neighbor’s yards when she walked her dog. She estimates that she has seen at least 35 dead birds. Rachel finally contacts the California Department of Health, which collects some of the birds for West Nile virus testing. Lab technicians take samples from brain tissue and test them for the presence of WNV proteins by ELISA. Although no humans have reported symptoms of West Nile disease, health officials decide to test individuals for the presence of antibodies to WNV in their blood, to determine the extent of human infections. Rachel, her husband, and two children are tested.
To analyze this case, run an ELISA on the six samples of brain tissue from dead birds (four crows and two sparrows) for the presence of WNV protein. Also, test the blood samples from Rachel’s family members for antibodies to WNV proteins.
- Did the birds die from WNV infection?
- Were any members of Rachel’s family infected with WNV?
- Are there any consequences if someone is infected with WNV but exhibits no symptoms?
- Should more people in the area be tested?
- What can the residents do to protect themselves from WNV infection?
- What impact will that many dead birds have on the local crow population?
- What is the potential environmental impact reducing crow numbers?