West Nile Virus Detection: The Details are Important

The arrival of WN virus in North America has been followed by reports of WN virus detection in many different mosquito species. The list is extensive. Unfortunately, many of these reports have added to the confusion regarding the identity of the primary North American WN virus vectors. After all, with so many mosquito species identified as infected with WN virus, how can anyone know with assurance without adequate confirmation which species should be targeted for control?

It is vital that those in public health and mosquito control understand precisely the significance of different detection methods for WN virus. Elsewhere we have discussed the importance of differentiating the impact of various detection methods. (Buzzwords Oct/Nov 2002). In particular, it is absolutely essential to understand that polymerase chain reaction (PCR) methods detect nucleic acid. PCR is eminently sensitive and it is essential to understand that it is capable of detecting nucleic acid from degraded material, including dead virions.

The sensitivity of PCR is not a new concept and has been demonstrated many times (see Tabachnick et al. 1996. Susceptibility of Culicoides v. sonorensis to infection by PCR detectable BLU virus in cattle blood. J. Amer. Trop. Med. Hyg. 54: 481-485, or Kramer et al. 2001. Detection of SLE and WEE RNA in mosquitoes without maintenance of a cold chain. J. Amer. Mosq. Control Assoc. 17:213-215).

At the recent American Mosquito Control Association National Meeting in Denver, Colorado, Dr. Mike Turell presented research showing that WN virus nucleic acid can be detected in a mosquito in which there was no detectable live WN virus. We have known all along that even though a mosquito is infected with WN virus, the PCR test does not confirm that there is live virus in the salivary glands, or that the mosquito is capable of transmitting that virus. Indeed C. R. Rutledge et al. at the FMEL showed that several PCR-WN-virus positive mosquitoes collected in the field did not transmit WN virus to a sentinel chicken. These mosquitoes were not infectious at the time of collection. They were not capable of transmitting WN virus at the time they were captured.

It is critically important to pay attention to the details of any detection report. Many of the North American mosquito species reported with WN virus were tested using PCR techniques. Their role in actual viral transmission is entirely questionable on that fact alone. In addition, it is likely that the exquisite sensitivity of PCR will enable detection of a viral "remnant" in a mosquito that blood fed on a viremic host long after the blood meal is no longer detectable. This again calls attention to the difficulties in distinguishing between infected and infectious mosquitoes. Even if a mosquito does contain live virus, it may not be able to transmit the virus. Transmission is the only factor that is important in determining which mosquitoes are true vectors. Does the mosquito blood feed on infected amplification hosts? Can it be infected? Does it live long enough to acquire a salivary gland infection? Finally, does it bite a susceptible host and "transmit" the virus? In short, is the mosquito species in question capable of transmitting the virus? The entire suite of factors associated with vector capacity must be considered.

Pay careful attention to the detection information for mosquitoes, birds, horses and humans. Is it PCR? Is it virus isolation showing that the virus is alive? Is it serology showing a past infection? Is it the new antigen dipstick test that detects only antigen that may not be from a living virus? Don't let the detection reports take your eye off the ball. We know that members of the Culex pipiens complex have been involved in WN virus transmission in northeastern North America and that Cx. quinquefasciatus is a player in Florida. WN virus is a near neighbor of SLE, and researchers at the FMEL have shown that Culex nigripalpus did "transmit" WN virus to a sentinel chicken in Florida in 2001. Cx. nigripalpus is a player in north Florida and will undoubtedly be a player in south Florida. Other information from the Northeast indicates that Culex restuans and Culex salinarius may play a role in WN virus transmission, but their role at this time appears to be minor. Very few of the suspect mosquito species have been shown to be vectors of WN virus. Florida Mosquito Control Personnel know what to look for. Track the species that have been shown to transmit the virus in the field to animal hosts, and do not allow yourself to be diverted to what are likely minor players. We have enough of a challenge without confusing the primary targets.

Walter Tabachnick, Ph.D.
FMEL Director