Case Study Footandmouth Disease In The United Kingdom 2001

In February 2001, the first case of foot-and-mouth disease (FMD) in 20 years was detected in a pig in a slaughterhouse in the United Kingdom. By that time the epidemic had already begun. Eleven months later, at least 6 million animals had been destroyed, including 4.9 million sheep, 700,000 cattle, 400,000 pigs, 2,000 goats, and 1,000 deer. Direct and indirect losses were calculated to exceed £12 billion (www.defra.gov.uk).51-53

Losses involved not only the agricultural sector of the economy but also sectors integrated with agricultural and tourism-related industries. The social costs of the outbreak and the long-term effects on farming and rural communities are incalculable.54-58

What was the source of the virus causing this catastrophic outbreak? And was its introduction into the United Kingdom accidental or deliberate? Today, neither question can be answered with any confidence. Why is this?

Paradoxically, more than a century ago, FMD virus (FMDV) was the first animal virus discovered and only the second viral agent described.59 Years later it was shown to belong to the same virus family, the Picornaviruses,60,61 as such important human pathogens as poliovirus, Hepatitis A, rhinovirus, and others. Like them, FMDV contains a positive-sense, single-stranded RNA genome of approximately 8,000 nucleotides that is directly translated into a single polyprotein upon entry into the host cell cytoplasm. The genome contains a single open reading frame (ORF) coding for a polyprotein flanked by two untranslated regions (UTRs), both predicted to display complex secondary structures.60,61 Despite the large body of molecular knowledge concerning biochemical and genetic properties of FMDV that has accumulated over the years, fundamental questions remain unanswered about the virus and the disease.

Current estimates of FMDV genome mutation rates suggest that, on average, about one base misincorporation is likely to occur each time a single FMDV genome replicates. This should result in the introduction of every possible one-step mutation from the progenitor genotype within a single infected animal many times a day.62 Since genomic RNA is made up of four nucleotides, a genome of 10,000 nucleotides could occupy a total of 410,000 points in sequence space: however, only a tiny fraction of this immense number is actually realized by RNA viral genomes.50,63 The significance of this, for forensic purposes, is that a single replicative cycle is theoretically sufficient to incorporate genetic changes in the progeny. Such a high rate of mutation makes any single-nucleotide change too common to be informative. Multiple nucleotide differences must be used for elaboration of distance matrices and phylogenetic trees.64

Over the last 15 years an impressive amount of sequence information from the carboxy-terminal region of the FMDV capsid protein VP1 gene has accumulated, driven in part because this region carries neutralizing antibody epitopes important for selection of suitable vaccines.65-69 It is precisely the immunological pressure applied from the host in this area, and the resultant genetic hypervariability in the virus, that make it theoretically appropriate for analysis of phylogenetic relationships between isolates. Rates of nucleotide substitution for VP1 range from 0.5-1.5 X 10-2 nucleotide substitutions/per site/per year (nt/s/y).62

Currently, approximately 2,000 partial or complete VP1 sequences of the seven FMDV serotypes are available: over 1,000 are serotype O.70 In general, these sequences show that viral populations that are circulating and causing disease outbreaks around the world are genetically heterogeneous but related, being distributed by regions in genetically distinct virus populations known as "topotypes" that can be differentiated based on nucleotide differences of up to 15%.65,70 These molecular methods enable scientists to unambiguously identify the strain of virus responsible for an outbreak, but do not allow precise identification of strain origin in terms of geography. In addition, the observations of unrelated and nonsystematic VP1 sequence changes provide additional confusion regarding actual phylogenetic relationships65,67,71-74 that impairs use of VP1 phylogenetics as a forensic tool. The FMDV studies of Moya et al.75 strongly suggest that phylogenetic reconstruction methods can infer erroneous phylogenies due to nucleotide convergences between isolates belonging to different experimental lineages that join by accident in time or space of sampling. They also point out that diverse evolutionary mechanisms acting under differing experimental dynamics generate alterations and change the frequencies of genetic variants, which can lead to the misinterpretation of the real evolutionary history.

Over the last 21 years, a strain of FMDV serotype Pan Asia O has spread from India throughout Southern Asia and the Middle East.76 During 2000, this strain caused disease outbreaks in the Republic of Korea, Japan, Russia, Mongolia, and South Africa. In February 2001, the Pan Asia O strain spread to the U.K. Studies of sequences of the VP1-coding region from approximately 30 Pan Asia O isolates demonstrated that the U.K. virus was closely related to all of them, and nearly identical to the South African isolate O/SAR/1/2000.77 Notably, nucleotide sequences of the VP1 coding regions of 30 Pan Asia O viruses isolated over an 11-year period differed by no more than 5%, making meaningful phylogenetic and forensic analysis extremely difficult.78 Complete genome sequence analysis of eight Asian, African, and European isolates of Pan Asia O strains confirmed the close relationship between the South Africa and U.K. outbreaks, but failed to identify, or even imply, the mechanism of introduction or the source attribution for the latter outbreak. The results were consistent with either a common source for both the 2000 South Africa and the 2001 U.K. outbreaks or that O/SAR/1/2000 is the source of the strain that caused the U.K. outbreak.79

This close genetic relationship among Pan Asia O strains circulating in the world today and lack of sufficient knowledge about mutation frequency, mutation rates, mutation and recombination processes, tolerance and constraints of genes and proteins, and evolution and memory of FMD viruses in nature prevent meaningful phylogenetic and forensic analysis. Currently, results obtained lack sufficient statistical support.

This is the reason why neither of the questions posed above can be answered.

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