A new analysis of the spread of Britain's Foot and Mouth disease epidemic shows that extended culling programs were essential for bringing the epidemic under control.
The study by researchers from Imperial College, London, which was fast-tracked for publication in the scientific journal Nature today, also shows that the number of cases could have been reduced by 16 per cent (saving 30 per cent of animals culled) if the infected premises and contiguous premises cull policy had been fully implemented from 1 April.
Professor Neil Ferguson, Dr. Christl Donnelly and Professor Roy Anderson are all members of the Department of Infectious Disease Epidemiology, part of the Faculty of Medicine at Imperial College, London. Since February 2001, they have been employing mathematical and statistical techniques to track the foot and mouth epidemic and evaluate options for control of the disease.
Neil Ferguson and Roy Anderson are mathematical epidemiologists; Christl Donnelly is an epidemiological statistician. As a team, they combine these techniques to develop models which are fitted to data as they become available. The department was founded in November 2000 and is headed by Professor Anderson.
The staff of the department also carry out research on a wide range of infectious diseases including, HIV, influenza viruses, pneumococcal and meningococcal bacteria, BSE, vCJD, and a variety of parasitic infections.
The authors of the report call for continued vigilance in order to speed the elimination of the disease from Great Britain.
Professor Neil Ferguson, first author of the report, said:
"This is the most thorough analysis yet of the epidemic. It has enabled us to evaluate the different policies in turn and it clearly shows the need for a rapid and complete contiguous culling policy for disease control and eventual elimination."
The study also helps to explain why some regions were harder hit than others.
The researchers used geographical data representing the number of animals, the species of animal on the farms, and the fragmentation of the farms around the country to make highly predictive Foot and Mouth "risk maps" of Great Britain.
The farm risk factors the maps identified include:
* Large farms with many animals are significantly more infectious and susceptible than smaller ones;
* Cattle farms are most susceptible, sheep farms less so, and pig farms least susceptible;
* Fragmented farms -- made up of scattered fields -- are at much higher risk of transmission.
Fragmentation, say the authors, is one reason why Cumbrian farms were so hard hit compared to those in Devon or Wales. Farms in Cumbria are significantly more fragmented and this led to increased transmission in the area, probably as a result of greater movement of people and vehicles between land parcels.
The analysis also leads to calculation of a new median distance of transmission of 4km, which, the authors say, "suggests that most transmission probably occurred through the movement of animals, personnel or vehicles, rather than through animal contact or windborne spread."
The UK risk maps also highlight areas of predicted high transmission risk such as the Derbyshire Dales and southwest Wales, which have not yet been affected.
"These areas in particular warrant heightened surveillance and continued vigilance in maintaining movement controls and biosecurity measures," said Professor Ferguson.
The Imperial team does not predict an end date for Britain to be clear of the epidemic. However, they say that as colder conditions take hold, farmers need to be alert to the risks of relaxing controls in the coming months.
Dr. Christl Donnelly, an author of the research, said:
"We would be very concerned if in the cooler months ahead there were a relaxation of movement restrictions and biosecurity controls."
"We will not have learnt the lesson of 1967 if restrictions are relaxed. As the weather gets cooler and the virus is able to survive longer, we are in danger of seeing significant outbreaks of the epidemic again," she said. - By Tom Miller
Imperial College of Science, Technology and Medicine
[Contact: Professor Neil Ferguson, Tom Miller ]