February 2004 The Need For a Tiered Arbovirus Alert System Note: Dear Readers, the following article was written by Jonathan F. Day, Professor and Walter J. Tabachnick, Director and Professor, University of Florida - IFAS who works at the Florida Medical Entomology Laboratory in Vero Beach, Florida. The article was recently published in the newsletter of the Florida Mosquito Control Association called Buzz Words.
Three years of West Nile surveillance in Florida clearly indicates 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 activities. 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 tier 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 though-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 Florida Department of Health (FL DOH)/County Health Department-declared Medical Advisory. 2) Increase the efforts associated with well thought-out and designed local surveillance programs. 3) Florida Department of Consumer Services (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 Florida DOH’s 5-D/1-S message which is; Dusk and Dawn - avoid going outside during these times of the day, this is when mosquitoes are most active. Dress - wear long sleeved shirts, pants, and socks to reduce the amount of exposed skin. Deet - apply mosquito repellents containing deet to areas of exposed skin. Drainage - empty and remove from the weather, containers around the home or business that collect water, make sure rain gutters function properly and are cleaned, and drain any areas on your property that retain water for more than a couple of days. Screen all doors and windows and repair any screened areas that need repair.
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. 6) Coordinate the dissemination of public information through the EOC. 7) Cancel outdoor nighttime events. 8) 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 West Nile virus transmission reported in Colorado during the summer of 2003, there would be 10,000 human cases reported throughout the state. 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 indicates a substantial increase in human risk. |