On Wednesday, September 10, 2003 Jean Kline, County Health Department Administrator for the Indian River County Health Department (IRCHD), requested from the Florida Department of Health (FL DOH) that Indian River County be placed on Medical Alert for West Nile (WN) virus. This request was made despite the absence of WN-positive human cases in Indian River County, but in the presence of surveillance data that indicated an elevated risk of virus transmission.

On Tuesday, September 9 Dr. Donald Shroyer from the Indian River Mosquito Control District (IRMCD) had provided the IRCHD with his most recent arboviral surveillance data which showed: 1. Seven (of 8) IRMCD sentinel chicken sites with at least one WN-positive sentinel chicken. 2. Twenty-one confirmed WN-positive sentinel chickens in IRC. 3. Documentation of a substantial increase in blood feeding by Culex nigripalpus following heavy rainfall on September 2-4.

In his 9-9-03 correspondence to the IRCHD Shroyer stated, "Our surveillance system has given us all the "early warning" we can ever expect to see, and should not be ignored. A Medical Alert now would be both prudent, and readily justified. For this county in particular, it would be professional irresponsibility to wait for the appearance of a human case before acting".

Indian River County's request for a Medical Alert was denied because the FL DOH policy at that time was to reserve the issuance of a Medical Alert for counties with at least one confirmed WN case. Jean Kline then requested that FL DOH issue a Medical Advisory for IRC. This request was granted, making IRC the only Florida County in 2003 to be placed under a Medical Advisory.

The events in IRC during September 2003 served to highlight the problems encountered by having only a single alert level for arthropod-borne virus transmission risk to humans. In the months following September 2003, it became clear that IRC should not have been on Medical Alert. However, surveillance data leading up to early September clearly indicated that WN virus was present and was being actively transmitted within the County. To wait for a human case before issuing a warning was, in Shroyer's words, "professionally irresponsible". Issuing a Medical Advisory was a perfect compromise. It allowed the IRCHD to send out a blast fax which alerted all county physician groups (ER, walk-in clinics, neurology, and primary care physicians) to the possibility of human WN cases. The Medical Advisory also resulted in local press coverage about mosquitoes, WN virus, and disease transmission. Finally, by issuing a Medical Advisory, the IRCHD increased public awareness of WN virus transmission while leaving a Medical Alert intact for future use in the event of substantial WN virus transmission to the citizens of IRC.

A single alert status forces Florida into several undesirable positions as the Florida arboviral season progresses from the Maintenance Phase (January through March) to the Amplification Phase (April through June) and on to the Early Transmission Phase (July through September) when the bulk of human WN cases appear. Because of the sporadic nature of WN virus transmission during most years, single human cases are expected to occur over wide geographic areas from June through November. These sporadic cases are extremely difficult to predict in time and space and single human cases within a county often do not represent a significant increase of WN virus transmission to residents of that county. Therefore, elevating a county to Medical Alert status based on a single human case may be proper early in the year when that case represents the front end of a building epidemic. However, when transmission of WN virus is truly sporadic, the single human case may not represent a true increase in human risk. The risk of WN virus transmission in counties on Medical Alert with a single human case may be no different than the risk in adjacent counties without a case that are not on Medical Alert. Florida must develop a tiered arboviral response policy for 2004 that has built-in appropriate responses for Florida's public health professionals.

Three years of WN surveillance in Florida clearly indicate that a multi-tiered arboviral alert system is desperately needed. The risk of arboviral transmission in Florida is never zero. The risk is greatly reduced during the Florida dry season from mid-November through mid-May but Florida residents should always be advised to avoid mosquito bites. From mid-May through mid-November the risk of mosquito-borne virus transmission increases substantially. During some years environmental conditions will limit virus amplification and the risk of substantial numbers of human cases will remain low. During other years environmental conditions will favor viral amplification during the critical Amplification Phase (April-June) of the Florida arboviral cycle. During these years, the risk of a large number of human cases dramatically increases, necessitating the implementation of a multi-tiered alert system.

Collectively, Florida has an internationally recognized arboviral surveillance program that is based on excellent local surveillance programs. During the arboviral transmission season (mid-May through mid-November) risk must be evaluated and alerts issued on the local level. Many factors must be tracked to evaluate the risk of a significant level of arboviral transmission to humans within a given county. Some of the more important risk factors include:

1. Environmental conditions - especially rainfall and drought cycles throughout the year.
2. Vector populations - especially abundance, parity, blood feeding status, and oviposition behavior.
3. Avian amplification host populations - especially reproductive success during the major breeding season (April-July) and immunological status.
4. Arboviral transmission as measured by thoughtfully maintained sentinel chicken surveillance programs.

The assessment of arboviral epidemic risk begins in January by tracking all of the risk factors listed above. The proximity of substantial numbers of arboviral-positive sentinel chickens to the Amplification Phase (April-June) of the Florida arboviral cycle will be one of the best indicators of a building epidemic. As the transmission season progresses and risk factors start to drop into place we may wish to consider the following tiered alert levels. The significance of the suggested tiered level is that it does not depend entirely on the appearance of human cases to establish a response policy. The following levels will be reached based on surveillance information that we would hope would precede the appearance of substantial numbers of human cases in Florida.

Level 1 (Background arboviral transmission): Detection of arbovirus nucleic acid or antibody in sentinel flocks, wild or domestic birds, mammals, or mosquitoes by a local surveillance program. The earlier in the year this occurs, the greater the concern. Responses to Level 1 risk would include: 1) maintain a well thought-out and designed local surveillance program. 2) Maintain vector control activities already in place. 3) Consider a local advisory about personal protection against biting arthropods. 4) Communicate with all local physicians (ER, walk-in clinics, neurology, and primary care physicians) about the possibility of human mosquito-borne arboviral cases.

Level 2 (Medical Advisory): Widespread regional detection of arbovirus nucleic acid or antibody in sentinel flocks, wild or domestic birds, mammals or mosquitoes. Arboviral detection may have a wide geographical distribution but will be focused in time. The earlier in the year this occurs, the greater the concern. A 10% increase in arboviral detection above background, especially early in the year may initiate a Level 2 response. Responses to Level 2 risk include: 1) Consideration of a FL DOH/County Health Department-declared Medical Advisory. 2) Increase the efforts associated with well thought-out and designed local surveillance programs. 3) Florida DACS may consider the need to issue a mosquito declaration in counties that need added mosquito control measures including a request for external resources to use in aerial adulticiding and larviciding activities. 4) Communicate with local physicians and media stressing the FL DOH's 5-D/1-S message.

Level 3 (Medical Alert): Detection of five or more Florida-acquired human arboviral cases per week and/or a 40% increase in arboviral detection above background, especially early in the year. Responses to Level 3 risk include: 1) Consideration of a FL DOH/County Health Department-declared Medical Alert. 2) Maintain increased well thought-out and designed local surveillance programs. 3) Florida DACS and local vector control programs may consider increased aerial adulticiding and larviciding based on the likely impact of these activities on focused high risk vector populations and larval habitats. 4) Communicate increased concern to local physicians and media stressing the importance of mosquito avoidance.

Level 4 (Medical Emergency): Detection of a geographically widespread but temporarily focused distribution of human arboviral cases at a level of 20 cases per week and/or a 50% increase in arboviral detection above background, especially during June and July. There may be as many as 50 new cases reported each day. Responses to Level 4 risk include: 1) Consideration of a FL DOH/County Health Department-declared Medical Emergency. 2) Maintain well thought-out and designed local surveillance programs, although under a Level 4 scenario the value of increased surveillance is greatly reduced except in areas where substantial numbers on human cases have not been reported. 3) Establish an Emergency Operation Center (EOC). 4) Coordinate requests for vector control assistance through the EOC. 5) Maximize adult mosquito control efforts in high-risk areas including retirement centers, hospitals, and adult communities. 5) Coordinate the dissemination of public information through the EOC. 6) Cancel outdoor nighttime events. 7) Close local, state and federal parks and recreation areas to nighttime events and overnight camping.

In all likelihood most of our Florida summers will be spent at Response Level 1. We know, however, that there is great potential for the occurrence of widespread arboviral epidemics throughout Florida. If Florida ever experiences the level of WN virus transmission reported in Colorado during the summer of 2003 there will be 10,000 human cases reported throughout the state. Duval County will report 1,400 cases; Orange County 1,700; Hillsborough County 2,000; Pinellas County 1,700 and Palm Beach County 2,100. Every summer in Florida we should plan for a Level 1 arboviral risk response, but we should be capable of implementing a Level 4 response when local surveillance data indicate a substantial increase in human risk.

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

• Back to Buzz Words Archive