Predicting the "Big Event"
If you watched the NASCAR Aaron's stockcar races from Talladega, Alabama on April 20 and 21, 2002 you witnessed "The Big One," not once, but twice in the same weekend. The "Big One" is defined as a crash involving the majority of the cars in a race. In short, a crash that can instantaneously remove half of the population from a racing event, a truly catastrophic event. The first wreck occurred on April 20 during lap 16 of the Busch series race when 29 cars in the 43-car field mixed it up as they came off turn 2 and entered the famous long back straightaway at the Talladega Superspeedway. The second crash occurred in exactly the same spot the next day during lap 164, 6 laps from the end of the race, of the Winston Cup race. This time the crash involved 24 cars from the field of 43.
NASCAR has long known that "The Big One" was a statistical inevitability and even predicted where it would occur, either at Daytona or Talladega Superspeedways. How were they able to make this prediction? Two factors predispose these speedways for a big crash. First, both tracks are 2.5 mile (2.66 in the case of Talladega) tri-ovals. Second, both tracks require carburetor restrictor plates for all of the stockcar participants. How did these factors predispose Talladega for two massive crashes in less than 24 hours? First, the tri-oval track configuration makes the negotiation of turns, especially turn 2, more difficult. The wall is to the right as cars enter the turn. Because of the tri-oval track construction, negotiation of turn 2 requires that the driver make constant steering corrections throughout the turn. Second, NASCAR requires the use of horsepowersapping carburetor restrictor plates to keep cars under 200 mph at both Talladega and Daytona. These restrictor plates bring cars into speed parity and, instead of the cars stringing out around the track, they run together in large groups. A group of cars traveling more than 160 mph, and 3 and 4 abreast, as they enter the technically difficult turn 2 was the recipe for disaster at both of the Aaron's races. NASCAR knows that restrictor plate accidents are inescapable and actually view them as "a show within a show". As long as no one is injured (and no one was hurt in either of the Talladega accidents), and the only result is approximately $2 million worth of junk sheet metal, these "Big Ones" are actually good for business.
Predicting arboviral epidemics, or what we refer to as "The Big Event", in Florida is similar to predicting the big NASCAR wreck. We know where the epidemics are likely to occur because of our rich history of arboviral surveillance in Florida. We also know what factors predispose regions of the state for an arboviral epidemic (see Day, J.F., 2001. Predicting St. Louis encephalitis virus epidemics: Lessons from recent, and not so recent, outbreaks. Ann. Rev. Entomol. 46:111-38 for a complete review). Three mosquito-borne viruses, St. Louis encephalitis (SLE), eastern equine encephalitis (EEE) and West Nile (WN) viruses have the potential of causing serious human disease in Florida.
There are 4 annual transmission patterns observed for mosquito-borne viruses in Florida. 1) During most years no human cases are reported. 2) Focal outbreaks, usually involving fewer than 10 clinical cases in a small geographical area, make up the second transmission pattern. 3) Sporadic transmission frequently occurs in Florida and involves single human cases spread over a wide geographical area. 4) A full-blown epidemic or "The Big Event" is the most dangerous, in terms of economics and public health, arboviral transmission pattern (see Day and Stark, 2000, Frequency of St. Louis encephalitis virus in humans from Florida, USA, J. Med. Entomol. 37:626-33).
A "Big Event" is defined as 25 or more clinical cases within a population of 100,000. The EEE virus has never caused a human epidemic in Florida. In general, clusters of human EEE cases are extremely rare. This is probably because the EEE transmission cycle is more complex than that of any of the other mosquito-borne viruses. The SLE virus, on the other hand, is capable of causing large human epidemics in Florida and throughout North America. Five SLE epidemics have been reported in Florida, one each in 1959, 1961, 1962, 1977, and 1990. We know that WN virus caused large human epidemics, similar to those reported for SLE in North America, in South Africa, Israel, Eastern Europe and Western Europe. It remains to be seen how WN virus will behave in Florida and the rest of North America. My guess is that there will be a Big Event caused by WN virus in Florida, or somewhere else in North America where SLE virus has had a historical presence. I believe that we have not yet seen the full disruptive potential of WN virus. The 62 West Nile cases reported from New York City in 1999 were certainly a wake up call, but I expect that a Big Event in a heavily populated area like Queens will result in many more clinical cases. Likewise, the 10 human cases from Staten Island in 2000 hardly qualify as a big event.
The WN virus transmission pattern observed in Florida during 2001 is best described as sporadic. We saw similar SLE transmission events in the state during 1979, 1980, and 1997. In 2001, 12 clinical WN human cases were reported throughout Florida in a population of about 14 million people. In north Florida a single human case was reported in Washington, Leon, Jefferson, Duval, and Putnam Counties, and 2 cases were reported from Madison County. In north central Florida, a single human case was reported in Marion County. No human cases were reported from peninsular Florida. But then there were the 4 human cases reported in the Florida Keys. WN virus transmission in Florida during 2001 was similar to the traditional sporadic EEE and SLE transmission patterns observed here since the early 1950s. This was especially true in north Florida where EEE and WN viruses were cycling together during the summer of 2001. Both viruses were likely sharing avian amplification hosts, mosquito vectors, and dead-end hosts including horses and humans. Imagine the potential for human disease when the same mosquito transmission rate that resulted in 1 case in 15,000 Jefferson County residents is observed among the 2-3 million residents in the population centers of south Florida.
Sporadic human cases of arboviral infection are impossible to predict. The health care, mosquito control, and research workers in Florida cannot predict sporadic arboviral cases, nor should they feel that they have failed when the inevitable sporadic case appears. Sporadic cases of EEE, SLE and/or WN may be diagnosed every summer. Where and when these isolated cases appear is impossible to predict, in much the same way that the time and place of an individual wreck in a NASCAR race is impossible to foresee. Wrecks are inevitable every Saturday and Sunday of the racing season. Who will be involved and on what lap cannot be predicted. A large crash that wipes out half the field is a rare event, but the track on which this type of wreck will occur is more predictable, because the underlying causes of the accident are understood. Health care workers, mosquito control personnel, and researchers in Florida have spent a great deal of time and money tracking arboviral transmission throughout the state.
We understand where and when large transmission events are likely to occur because we understand the underlying causes of arboviral transmission and have made the investments in time and money to continually track the biological and physical parameters responsible for significant transmission events. The Florida arboviral surveillance program is the best in the World. We should be proud and we should stay the course.
Jonathan F. Day, PhD
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