UK researchers find infectious period of the disease shorter than thought, lessening need for pre-emptive culls.
A remarkable experimental study has yielded new information on the transmission of one of the world's most damaging animal diseases.
Researchers from the United Kingdom studied the direct transmission of foot-and-mouth disease from one cow to another in a unique experimental set-up that might also find applications in the study of other pathogens.
Foot-and-mouth is a highly infectious disease that can have a huge impact on farmers' livelihoods. Endemic in many parts of Africa and South America, the virus that causes the disease has been eradicated in much of Europe and North America. However, sporadic outbreaks can cause massive economic losses, and control measures have proved controversial.
In 2001, a foot-and-mouth outbreak in UK cattle triggered a mass cull and restricted access to many parts of the countryside. A second outbreak in 2007 — eventually traced back to a poorly maintained laboratory site — saw lighter control measures and less damage.
Now a paper published in Science1 suggests that the aggressive approach taken in 2001 to control infection may not be necessary. It shows that the window in which infected cattle can transmit the disease to other animals is actually shorter than previously believed and, crucially, that the infectious period occurs after the appearance of disease symptoms.
Bryan Charleston, a foot-and-mouth expert at the UK Institute for Animal Health in Pirbright, Surrey — which is on the site where the virus leaked in 2007 — and his team exposed eight cows to one form of the virus. They then attempted to transmit the virus from these 'source' cows to other bovines in a biosecure compound on the Pirbright site, while monitoring a complex set of data such as blood samples, temperature and lesions on the animals.
"It is, as far as we can tell, the first study in a target host of an actual viral disease where we've looked at transmission parameters by carrying out one-to-one infections rather than looking at proxies," Charleston says.
In 28 attempts there were only 8 successful transmissions of foot-and-mouth. The team found that the cattle in their study were not infectious until around 0.5 days after clinical signs appeared, and that the infectious period lasted 1.7 days on average.
Previous estimates — based on isolation of the virus from infected animals — have come up with significantly longer periods of infectivity. Charleston and his co-authors suggest that the fact that cattle are less likely to be infectious before showing clinical signs means that, during an epidemic, culling on farms that are at risk of infection could be unnecessary. Careful monitoring for signs of infection could be used instead.
In the 2001 outbreak, some 700,000 cattle were culled to fight the disease, says co-author Mark Woolhouse, an infectious-disease epidemiologist at the University of Edinburgh, UK. Only a "small fraction" of these actually had the virus, he says.
From science to slaughter?
Neil Ferguson, a mathematical biologist at Imperial College London, says that there has been some debate in the foot-and-mouth community about whether the disease was transmitted pre-clinically or not.
He adds: "The paper is fantastic in terms of being one of the few studies that quantify how infectious animals are as a function of how long they've been infected and what their symptoms are. It puts greater emphasis on really trying to speed up diagnoses of infection on farms."
However, improving the speed of diagnosis could prove logistically difficult in practice. Although those controlling the 2007 outbreak eschewed the mass culling used in 2001, the later incident was geographically more contained, so less-aggressive measures were used to stop the disease from spreading.
Matthew Keeling, an expert on disease modelling at the University of Warwick, near Coventry, UK, also points out that much of the modelling used to predict disease spread and best responses to outbreaks actually works on the level of the farm, rather than of the individual animal.
He agrees that the study does highlight the benefits of early detection. "It tells us if you're very observant, you're able to detect the signs very early," he notes.
Charleston hopes that despite the huge effort it took to gather this information, the techniques used could be valuable if applied to other infectious diseases such as influenza. The difference in foot-and-mouth disease infectiousness predicted previously and that found through the experimental study shows a need for better evidence when forming policy on the control of acute diseases, he says.
The pioneering experiment has been welcomed by many researchers in the field, but Keeling points out that these one-to-one infection studies are not an easy undertaking: "The amount of work that went into this for just eight animals being infected was enormous."
Charleston, B. et al. Science 332, 726-729 (2011).