A Florida Mosquito Control Arbovirus Response Plan

Florida mosquito control has a responsibility to mitigate the impact of mosquito-borne disease on human health and well-being through the efficient, effective and environmentally proper use of mosquito control methods. The arrival of West Nile in Florida has brought pressures on mosquito control to develop appropriate responses commensurate with the threat from West Nile transmission. It would be helpful to have guidelines for mosquito control organizations to assist them in interpreting mosquito-borne disease information that may be available to their local jurisdictions. The Florida Medical Entomology Laboratory does report assessments on a statewide level on its Encephalitis Information System (EIS) ( Guidelines, however, are needed that provide a framework for mosquito control agencies to use locally available arthropod-borne pathogen and disease information to apply mosquito control efforts commensurate with the extent of arthropod-borne disease and/or the risk of disease to their human clientele.

A Florida Mosquito Control Arbovirus Response Plan (FMCARP) must take into account the great diversity in mosquito control organizations in Florida and the diversity of the issues each faces due to the variety of ecologies in different regions, and the variety of available resources for mosquito control in the state. An FMCARP should integrate its guidelines for mosquito control agencies in Florida with the companion Florida Department of Health Mosquito Illness Response Plan. Florida mosquito control agencies require a FMCARP containing specific guidelines for mosquito control efforts commensurate with public health risks from mosquitoes. The Department of Health Illness Response Plan is not meant to provide such guidelines. The FMCARP should consider several factors to assess the status of mosquito-borne disease and the impact on a mosquito control program's response.

A. Population Size. The absolute size of the human population in a jurisdiction is a critical factor in determining the problem from an arthropod-borne disease. It must be understood that precisely the same risk of mosquito-borne human disease in districts or counties with a larger numbers of humans will have a larger number of human cases compared to smaller counties. For example, Indian River County and Miami Dade County have the same disease incidence for West Nile, i.e., an incidence of 10 cases per 100,000 people in each county. There is no difference in risk in the two counties. However, there are 12.5 cases in Indian River, population size 120,000, but 230 in Miami Dade, population size 2,300,000. This is an important consideration.

B. Time of the Year. Guidelines consider the time of the year that the information is collected. The same information collected in the early Florida transmission season (May-August) may demand a more aggressive response than the same information collected in the later transmission season (September-December) in Florida.

C. Risk of Disease vs. Actual Occurrence of Disease. Guidance should be provided for the "risk" for human disease when the numbers of human cases are not known, or have not occurred yet, but is projected on the basis of other information. In addition, guidance is needed based on the actual "occurrence" of human cases. Other information to determine "risk" may be any, some, or all of the following: surveillance information (mosquitoes, wild birds, sentinel chickens, equines) in the local jurisdiction or in the absence of surveillance information, information obtained from a geographically associated county that has such surveillance information. Once human cases are occurring, then responses are needed that are commensurate with both incidence and absolute numbers of cases.

D. Reporting Interval. Guidelines should account for specific reporting periods. For example, surveillance information is appropriate for the specific time period in which the information is collected. Surveillance information used should be based on the shortest surveillance time period being implemented, i.e., usually a weekly reporting period.

E. Surveillance Information. There is wide diversity in the available surveillance information throughout Florida. Some localities have well developed surveillance information that can be used prior to and during the occurrence of human West Nile cases to assess risk and apply appropriate mosquito and disease control strategies. Each of the different surveillance tools may provide different information which will need to be assessed and evaluated by knowledgeable mosquito and mosquito-borne disease epidemiologists relative to the tool being used, location of the information, and time of year. Certainly the timeliness of having surveillance information is critical.

F. Surveillance, Human Population Size and Estimating Risk. It is possible to obtain crude estimates of the risk of human West Nile cases using sentinel chicken seroconversions rates to estimate the frequency of mosquito transmission in a specific area. This can be used to gauge the magnitude of overall risk. Such risk estimates are more accurate if the estimate is confined to the smallest local human population that is near the sentinel chicken flocks. Also, information on the mosquito attack rates will greatly improve the estimates. Finally, information on the mosquito attack rates on humans will also improve the estimate.

There is still much to be done to improve mosquito control's ability to respond and mitigate a Florida West Nile virus epidemic. Guidelines to assess human risk and guidelines for appropriate mosquito control responses commensurate with human risk are important steps. Future development of an FMCARP should be based on discussions and input from Florida Mosquito Control Districts and the members of the Florida Mosquito Control Association.

Walter J. Tabachnick
Florida Medical Entomology Laboratory
University of Florida/IFAS