Sunday, 23 June 2013

Agricultural Shows; a virus' dream?

This week saw the annual event when farmers descend from all parts of Scotland to compare their animals: The Royal Highland Show. I went along and, after a while wandering around the tractors, had a look around the livestock. Looking at the plethora of pampered sheep and cattle, a few things occurred to me. 
First was a half-hearted feeling of 'missed opportunity'. Sheep in particular, but also cattle, had come from across Scotland (and beyond). Imagine if a blood sample of every animal had been taken. Schmallenberg virus (SBV) is known to have reached, and circulated, in Scotland, but nobody knows to what extent. Perhaps testing these animals may have given a cross-section of SBV distribution across Scotland. Maybe even an idea of variation in breed susceptibility; although there's nothing really suggesting that such differences exist.

Show time: a Jacob sheep gets a final trim in the sheep yard prior to showing at the Royal Highland Show.
Second was considering the potential for transmission and dissemination of pathogens across wide geographical areas. In 2001 in the UK the dispersal of Foot and Mouth Disease Virus to various regions was due, at least in part, to the mixing and distribution of sheep from Hexham Market. Agricultural shows represent a similar type of mixing event, and last for days rather than hours. In the case of Foot and Mouth, the clinical signs are mild in sheep, thus allowing infected animals to go unnoticed.

Bringing animals to the show with clinical signs of disease is unlikely to happen as the idea is obviously to show the best animals. That restricts the possibility of infectious animals to those in a pre-clinical incubation stage. However, shows lasting for several days gives sufficient time in which to develop a viraemia and allow transmission, either via direct contact, fomites or vectors. Unsurprisingly there are already studies on the role of agricultural shows. One regarding UK shows (Webb, 2006) revealed competitors at shows to form a large network, with some competitors travelling to shows up to 600 km apart. Clearly there is potential, but thus far nothing seems to have been nailed down showing an outbreak resulting from mixing at agricultural shows.

Fig. 4
The impact of time-between shows: the number of days between shows affects the network, no time limit results in a mass network (a) whereas time limits of 14 days (b), 10 days (c) or 7 days (d) reduce the number of nodes, effectively separating shows and breaking links. From Webb 2006.

Third was the zoonotic potential.
It is well established that  cross-species transmission of zoonotic pathogens occurs at the interface between the animal source and humans. These shows represent perfect scenarios for such interactions. Largely absent from the Royal Highland Show were pigs, which have previously been shown to be a source of influenza A infection. Pigs may therefore represent more of a risk due to their ability to sustain various zoonotic agents. Likewise shows where chickens and ducks are present, offering the possibility for the transmission of avian influenza.  

Odd choice: A sheep decides a coat is preferable to hay.

The idea of catching bugs from farm animals at these kind of events, as well as at petting farms for kids etc., is not new. And it is true, infections do happen. The knee-jerk response of a modern nanny state society, at least in the UK, would therefore dictate the banning of such scenarios, including all agricultural shows without question. But this is another case of putting risk into perspective; of all the millions of animals, and the millions of contacts with humans that have happened at these shows over the years, how many infections have occurred, or at least been serious enough to cause alarm? The simple answer: not many. For now though, the relative importance that agricultural shows play in the transmission, evolution and species transfer of viruses remains largely unknown.

Webb, C.R. (2006) Investigating the potential spread of infectious diseases of sheep via agricultural shows in Great Britain. Epidemiol Infect. 134(1)31-40. doi: 10.1017/S095026880500467X

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