In February 2001, foot-and-mouth disease (FMD) was confirmed in Great Britain. A major epidemic developed, which peaked around 50 cases a day in late March, declining to under 10 a day by May. By mid-July, 1849 cases had been detected. The main control measures employed were livestock movement restrictions and the rapid slaughter of infected and exposed livestock. The first detected case was in south-east England; infection was traced to a farm in north-east England to which all other cases were linked. The epidemic was large as a result of a combination of events, including a delay in the diagnosis of the index case, the movement of infected sheep to market before FMD was first diagnosed, and the time of year. Virus was introduced at a time when there were many sheep movements around the country and weather conditions supported survival of the virus. The consequence was multiple, effectively primary, introductions of FMD virus into major sheep-keeping areas. Subsequent local spread from these introductions accounted for the majority of cases. The largest local epidemics were in areas with dense sheep populations and livestock dealers who were active during the key period. Most affected farms kept both sheep and cattle. At the time of writing the epidemic was still ongoing; however, this paper provides a basis for scientific discussion of the first five months.

Estimating detailed transmission trees that reflect the relationships between infected individuals or populations during a disease outbreak often provides valuable insights into both the nature of disease transmission and the overall dynamics of the underlying epidemiological process. These trees may be based on epidemiological data that relate to the timing of infection and infectiousness, or genetic data that show the genetic relatedness of pathogens isolated from infected individuals. Genetic data are becoming increasingly important in the estimation of transmission trees of viral pathogens due to their inherently high mutation rate. Here, we propose a maximum-likelihood approach that allows epidemiological and genetic data to be combined within the same analysis to infer probable transmission trees. We apply this approach to data from 20 farms infected during the 2001 UK foot-and-mouth disease outbreak, using complete viral genome sequences from each infected farm and information on when farms were first estimated to have developed clinical disease and when livestock on these farms were culled. Incorporating known infection links due to animal movement prior to imposition of the national movement ban results in the reduction of the number of trees from 41 472 that are consistent with the genetic data to 1728, of which just 4 represent more than 95% of the total likelihood calculated using a model that accounts for the epidemiological data. These trees differ in several ways from those constructed prior to the availability of genetic data.

Clinical and laboratory investigations of five outbreaks of foot-and-mouth disease (FMD) were made during the early stages of the 2001 epidemic in the UK. The first outbreak, confirmed on February 20, was at an abattoir in Essex which specialised in the processing of culled sows and boars. On February 23, the disease was confirmed at a pig farm in Northumberland which held cull sows and boars fed on waste food; the findings indicated that it was the first of the five premises to be infected. The disease had probably been present since early February, and it was the most likely origin of the epidemic. The other premises investigated were a waste food-fed cull sow/boar pig unit in Essex, approximately 30 km from the abattoir, which was probably infected at the same time or before the abattoir, a sheep and cattle farm approximately 6 km from the Northumberland pig farm, which was probably infected by airborne virus from it in the period immediately before February 13, and a sheep and cattle farm in Devon which had clinical disease from February 20 and was probably infected by sheep transported from Northumberland on February 13 which arrived on February 15.

The results of epidemiological investigations suggest that livestock on up to 79 premises, spread widely throughout the British Isles, may have been exposed to infection by foot-and-mouth disease (FMD) virus by the movement of infected sheep before the first case of the disease was confirmed at an abattoir in Essex on February 20, 2001. A further 36 premises may have been infected by this route before the national livestock movement ban was imposed on February 23.

Between January 6 and April 23, 1997, 11 outbreaks of Newcastle disease were confirmed in Great Britain, four in broiler chickens and seven in turkeys. Although the viruses isolated gave intracerebral pathogenicity indices in day-old chicks between 1.65 and 1.95, the clinical signs of disease in field infections were variable and not always associated with high mortality, especially in turkeys. Epidemiological investigations indicated that the majority of the outbreaks occurred as a result of secondary spread by human agency from two or more primary infected flocks. The presence of similar outbreaks in Scandinavian countries in 1996 and the unusual patterns of movement of migratory birds at the end of 1996 and beginning of 1997 suggest they may have been responsible for the primary introduction of the causative virus into Great Britain.