West Nile Epidemic? These are the Questions We Will Ask

Florida mosquito control and public health professionals have made many advances in their capabilities to deal with West Nile virus since its arrival in Florida in 2001. We have improved our understanding of West Nile issues through numerous meetings and workshops, like the FMEL-sponsored West Nile workshop that resulted in the 2003 FMCA Technical Bulletin, Volume 4, Florida Mosquito Control Response to the Challenge of West Nile Virus. Florida State agencies, DOH and DACS, continue to revise and improve their response plans using new information. There is still a lot to do.

The FMEL generates a web-based Encephalitis Information System ( to periodically communicate the current arboviral risk assessment in Florida. The target audience of the FMEL EIS includes Florida mosquito control and public health professionals as well as Florida citizens. The EIS assessments are based on arboviral, meteorological, climatological, and biological surveillance available at each reporting period for the entire state. The goal of the FMEL EIS is to assist the community in determining the extent of the risk to human health and well-being from pathogens like West Nile, St. Louis encephalitis, and eastern equine encephalitis viruses.

One of the lessons that West Nile has provided is the need to continually assess information in order to effective gauge the risk of human cases. This is one of our greatest challenges. The Florida Coordinating Council on Mosquito Control is working with DOH and with DACS to provide guidance about how different triggers and observations can be used to assess danger. This assessment will carry with it recommendations for the appropriate mosquito control actions to reduce and mitigate the predicted danger.

In mid-July, 2004 a cluster of human West Nile case was reported in Miami-Dade County. As per the DOH Arbovirus Disease Response Plan, the state and county DOH placed Miami-Dade on medical alert. The difficult questions for mosquito control and public health is "What does a geographically isolated grouping of human cases represent? Is this the beginning of a West Nile epidemic in Miami-Dade with 100s of human cases expected? What are the appropriate mosquito control and public health responses that need to be taken?" Perhaps we will have some answers by the time this column is delivered to Buzz Words readers. We can only hope that the answers and resulting actions come from surveillance information rather than from a large number of human cases.

The FMEL communicates with many mosquito control districts in Florida. We receive calls and e-mails where we discuss information and provide assistance in interpreting surveillance information. We are glad to do this and we encourage all Florida mosquito control agencies to contact us for this service.

How can we answer the critical question, "Is this the beginning of an epidemic?" This column is meant to provide Florida mosquito control with a template of questions that we will ask as we try to make a risk assessment for a county where high levels of arboviral transmission are suspected to be ongoing. Some counties might have answers for all of the questions. Other counties might have very little information. Of course, the more information available, the more accurate will be the assessment.

Questions regarding suspected foci of arboviral transmission:

  1. What indicators does the county have for West Nile transmission?
    • a. Wild avian serology and virus solation?
    • b. Dead Birds?
    • c. Horses?
    • d. Humans?
  2. What is the temporal and geographical distribution of the indicators?
    • a. Where were the positives infected?
    • b. When were the positives infected?
  3. Does the county have a sentinel chicken surveillance program?
    • a. How many flocks? How many birds per flock? Are the flocks run year round?
    • b. How often is each bird bled?
    • c. What are the weekly seroconversion rates?
    • d. What mosquito species and what relative number of each species are found at each flock? How do these numbers change over time, daily, weekly, monthly, seasonally?
  4. Does the county monitor mosquito populations?
    • a. What species of mosquitoes are present? What vector species are present and where? What are the spatio-temporal
    • b. distributions of the major vectors, Culex nigripalpus and Culex quinquefasciatus? What about the spatio-temporal distributions of minor vectors such as Culex restuans and Cx. salinarius.
    • c. Are the above species focal in their distribution? Do any overlap with large avian populations? Are they present where there are humans?
  5. What have been the seasonal water levels?
    • a. What has been the drought levels preceding the suspected transmission event?
    • b. What was the rainfall pattern following any extended periods of drought?
    • c. How have mosquito populations responded to the drought/wetting cycles?

These are some of the questions we will ask. Counties with no surveillance will unfortunately only be able to provide information on the human cases. In these counties, humans will act as sentinels. This is the best that can be done for counties that lack arboviral surveillance programs. It is impossible to mount a proactive response against arboviral amplification and transmission when humans are used as sentinels. In these cases, it is not optimal to provide a response that is appropriate to the risk.

The first human cases of West Nile virus in Miami-Dade provide a wakeup call indicating the need to further improve our capabilities. Miami-Dade mosquito control is to be congratulated on getting control operations moving quickly. Unfortunately, the first date of onset was weeks before actual notification. Then there were also the dead bird reports. Although the presence of dead birds pose problems in interpreting transmission risk clearly, the reports of over 100 birds (3-4 times reports in previous months) could have triggered notice to the public in the form of an advisory, with communications to mosquito control in June. All well before the first reported human case. Certainly the dead birds could also have triggered more surveillance information from sentinels and mosquito pools in the region. Such information is essential for risk assessment. However, when the primary trigger is unfortunately human sentinels, it is absolutely essential for the human case reports to be communicated more quickly.

These are the questions. The FMEL can provide assistance in using whatever information is available to answer "Is this the beginning of an epidemic"?

Walter J. Tabachnick, Director,
Jonathan Day, Professor
Florida Medical Entomology Laboratory
University of Florida/IFAS