Commentary on Research Findings: Nonviremic Transmission of West Nile: Implications for Understanding West Nile Epidemiology
Reference: Higgs et al. 2005. Nonviremic transmission of West Nile virus. PNAS 102(25): 71-74
This interesting article demonstrates that uninfected Culex pipiens quinquefasciatus mosquitoes can be infected with West Nile virus (WNV) at low levels by co-feeding on the same host within proximity of biting infected C. p. quinquefasciatus. The authors allowed uninfected C. p. quinquefasciatus to feed on mice concurrently with WNV infected C. p. quinquefasciatus. As many as ca. 5% of the uninfected mosquitoes were shown to become infected with WNV.
This type of transmission is called non-viremic or non-systemic transmission. It is different from the more general transmission mechanism that occurs when an arthropod feeds on a viremic host where the virus is present systemically throughout the blood system. Non-systemic transmission has been demonstrated to occur in ticks for several pathogens, and also in blackflies and sand flies for vesicular stomatitis virus (VSV).
The epidemiological implications of non-systemic transmission were described by Lord and Tabachnick (2002. The influence of non-systemic transmission on the epidemiology of insect borne arboviruses: a case study of vesicular stomatitis epidemiology in the western U. S. J. Med. Entomol., 39: 417-426). A key aspect of nonsystemic transmission is the distribution of vectors in space and time on an individual host. We currently do not have a good understanding of this distribution for mosquitoes, but it will be critical in determining the significance of nonsystemic transmission of arboviruses.
What are the lessons for mosquito control for understanding and controlling West Nile? Some mosquitoes may become infected with WNV by feeding on hosts that are not viremic. This could be any host; human, horse, chicken, reptile, any animal that attracts more than one mosquito. The oft used term "dead end host" should be invoked carefully. Non-systemic transmission increases the potential for WN to spread without the lag period required for hosts to develop viremias. This could increase the speed with which the virus spreads after introduction to a new area. On the other hand, the major feature in WNV amplification and the potential for epidemics is still the role of viremic birds. The potential numbers of infected mosquitoes produced by viremic birds dwarfs the potential for non-viremic infections on other animal hosts. The numbers of mosquitoes that will become infected by the low probability of feeding in proximity to a transmitting mosquito will always be very small in comparison. Our experience with WNV in Florida confirms this.
Elsewhere in previous Buzz Words columns the reasons for Florida having escaped a West Nile "big event" were attributed to the timing of the bird breeding season, and wetting events. The timing of these events is critical to producing needed WNV amplification in naïve bird populations. Though it is likely that some proportion of mosquitoes have been infected in Florida by co-feeding, without bird amplification we have yet to see substantial transmission. The role of co-feeding in the transmission of WNV in Florida has not been apparent, although it may have played a role in the rapid appearance of the virus throughout the state in 2001. Further interpretation of the role of co-feeding will require more data and analysis. For example, it would be naïve to assume that Florida's sentinel chickens have been important to West Nile epidemiology. The evidence for the past four years fails to support this. Florida has yet to experience the West Nile big event, despite maintaining the largest sentinel chicken surveillance program in the U. S.
We still have a lot to learn about the role of nonsystemic transmission of WNV and other arboviruses. See Lord and Tabachnick for further discussion and Randolph et al. (1996. Co-feeding ticks: epidemiological significance for tick-borne pathogen transmission. Parasitology Today 118:177-186.).
Walter J. Tabachnick - Retired
Cynthia C. Lord, Associate Professor
C. Roxanne Connelly-Connelly
Florida Medical Entomology Laboratory
University of Florida/IFAS
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