Ministry of Agriculture (MAFF) epidemiologists believe that the breeding herd initially infected ("index" case) probably became infected in early June. The original source of the outbreaks is believed by MAFF to be an infected pork product probably coming from a "discarded sandwich" (a public footpath runs close to the sow paddocks of the outdoor breeding herd where the epidemic is believed to have originated).
The "discarded sandwich" theory is a possibility, but a factor weighing against it is that the sandwich would have had to contain uncooked meat. Also, the pigmeat in UK sandwiches almost always comes from the UK or mainland Europe and there was very little CSF in Europe during the previous nine months. Furthermore, the UK virus specific genotype is previously unknown in Europe. The "discarded sandwich" theory is attractive in an industry which would like reduce imports, but at present it is based entirely on circumstantial evidence. Theoretically, some alternative possibilities might be:
- Undetected infected pigs in the vicinity*
- Malicious activity
- Illegal importation of pig products or semen
- People or equipment travelling from Asia (see below)
* there have been feral pigs in Norfolk in the past
Typing of the UK virus
The Veterinary Laboratories Agency's early analysis of the UK virus' genome indicated that the genotype was 2.1. Viruses of the 2.1 subgroup were originally reported in South East Asia and have now been reported from a number of East Asian countries. Viruses of subgroup 2.1 have appeared infrequently in pigs in Western Europe during the last 10 years and have never been isolated from European wild boar. The UK virus strain is related to, but not identical with, the viruses isolated from limited CSF outbreaks in Austria and Switzerland in 1993, and in Italy in 1992 and 1995.
The first European isolation of 2.1 virus was in Vienna in 1993 when a shipment of wild boar meat was confiscated after illegal importation from China. Later in 1993, a 2.1 virus strain was isolated from an outbreak of CSF in domestic pigs in Switzerland, also originating in imported wild boar meat from China.
In 1997 a CSF virus belonging to subgroup 2.1, but unrelated to the current UK strain, caused the huge epidemic in Belgium, Germany, Italy, Netherlands & Spain.
The current UK virus has been typed by the EU Reference Laboratory (Hannover) for swine fever, using RNA sequencing ("fingerprinting"). The Laboratory confirmed the 2.1 sub-grouping. However, the sequence data (specific genetic strain) of the recent UK East Anglian isolate differs from previous 2.1 CSF viruses found in Europe, but similar strains have been reported in Asia.
Typing of CSF viruses
The EU Reference Laboratory is at the Institute of Virology, School of Veterinary Medicine, Hannover, Germany. The Laboratory maintains a collection of over 600 CSF virus isolates derived from outbreaks all over the world. It works in close co-operation with National Swine Fever Reference Laboratories of EU Member States. Nucleic acid segments from swine fever viruses are sequenced classified into sub-groups. This allows identification of specific clusters of infection and understanding of the origin of a CSF virus strain.
Recent CSF outbreaks reported elsewhere
CSF outbreaks this year have been reported by Bulgaria, Germany and Thailand. Most of us assume that CSF viruses are enzootic (endemic) in China. Last year CSF outbreaks were reported in Argentina, Croatia, Germany, Italy and Luxembourg.
MAFF report to OIE on source and spread of UK epidemic
Local & International spread of CSF virus
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