Estimating Downwind Contamination

Once an outbreak has been detected, responders can use dispersion models to further characterize the outbreak. A key aspect of characterization is identification of exposed populations who are not yet symptomatic so that prophylactic treatment can be directed toward them. Using estimates of release location and time—perhaps those obtained from the use of dispersion models in the analysis of biosurveillance data— responders can "simulate'' the outbreak to inform decisions about prophylactic treatment of populations.

An example of how authorities have used dispersion models in the past to respond to known outbreaks is a FMD outbreak that started in northern France. On March 4,1981, French veterinary authorities and the World Organization for Animal Health notified the Ministry of Agriculture in the United Kingdom (UK) of outbreaks of FMD in northern France (Donaldson et al., 1982). Some previous outbreaks of FMD in southern Great Britain were the result of windborne spread of FMD virus across the English Channel from France. Thus, the Ministry of Agriculture on March 6 and March 12 asked the Meteorological Office of the UK about the likelihood of spread of FMD across the English Channel to the UK.

The Meteorological Office of the UK advised the Ministry of Agriculture that conditions favorable for windborne spread had occurred. Given recent wind directions and other weather conditions, the Meteorological Office predicted that airborne concentrations of virus at two islands in the English Channel— Jersey and the Isle of Wight—were high enough to cause infection in animals. The risk was low for southern England.

Subsequent to the receipt of this advice, the Ministry of Agriculture received notification of FMD on Jersey and the Isle of Wight. No outbreaks developed in southern England.

Additional research into the outbreaks confirmed that the strain of FMD virus on the two islands and the strain in northern France were identical. This research also ruled out other modes of transmission of FMD virus such as movement of animals or equipment.

The study makes no mention of any heightened surveillance or preventive action (such as vaccination) based on the information about a threat to the islands. However on the same day that the first farmer reported symptoms in his cattle (on the Isle of Wight), veterinarians diagnosed the outbreak and imposed restrictions on the movement of animals. Prior to these outbreaks of FMD, the United Kingdom had a plan in place to deploy to the site of any FMD outbreak a team of experts. These experts included individuals from the Animal Virus Research Institute, the Ministry of Agriculture's state veterinary service, and other appropriate experts such as meteorologists and statisticians. Such a team deployed to the Isle of Wight after the outbreak was detected and collected the epidemiological data reported in the study.

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