Foot-and-mouth disease control measures could be reduced

FMDV and when they show clinical signs of the infection,” said Woolhouse. “Normally, we only know if a person or animal is infected with disease when their clinical signs appear. But, what we didn’t know before this is how those signs relate to infectiousness. In the case of FMDV, the clinical signs and infectiousness seem to occur around the same time.”

In twenty-eight attempts to infect healthy cows with FMDV (by placing them in close proximity to an infected cow for eight hours), the researchers only observed eight successful transmissions of the virus. In light of their results, Charleston and his colleagues suggest that cows with FMDV only become infectious for a brief period of time — approximately 0.5 days after clinical signs of the disease appear.

“We now know that there is a window where, if affected cattle are detected and removed from the herd promptly, there may be no need for pre-emptive culling in the immediate area of an infected farm,” said Woolhouse. “We have an opportunity now to develop new test systems which can detect infected animals earlier and reduce the spread of the disease.”

Their findings are consistent with a rarely tested theory that disease symptoms may be functionally linked to infectiousness.

“If you do things like measure virus in the blood, you’re taking no account of the clinical state of the animal,” said Woolhouse. “People might imagine that the clinical signs of a virus—the symptoms, such as sneezing — have something to do with its transmission. But, while there has been a lot of thoughtful speculation on the topic, there haven’t been many actual studies.”

Charleston and his team are now calling for practical tools that could diagnose foot-and-mouth disease in the field before clinical signs appear. According to the researchers, if FMDV could be detected in livestock just twenty-four hours before clinical signs appear, then farmers might have time to remove the infected animals before they transmit the virus.

“If the benefits of this research are going to be realized in the field, we are going to have to implement pre-clinical diagnostics,” said Woolhouse. “It’s technically and logistically challenging, but our work shows that the potential benefits would be much greater than we’ve previously realized. So, at the very least, we should take a look at the possibilities for detecting FMDV early on.”

The researchers also propose that similar studies could reveal much more about other animal (and human) pathogens in the future.

“We urgently need to evaluate other infections,” said Woolhouse. “Until we do that, we can’t evaluate how effective control measures like quarantining individuals, prophylaxis, anti-virals or the pre-emptive culling of livestock are going to be.”

The funding for this research was born out of a special initiative, launched by the U.K. Biotechnology and Biological Sciences Research Council after the horrific 2001 outbreak of foot-and-mouth disease in that country. The researchers involved say that such direct experiments are vital to our understanding of public health.

“If you’re going to make informed decisions about controlling infectious diseases, you need the right kinds of scientific evidence—and this study provides that, even if it wasn’t easy or cheap to come by,” concluded Woolhouse. “People have used short-cuts before and we can end up with misleading information. This new research tells me that we can’t afford to take those short-cuts. This is the kind of work we need to be doing to learn how to manage infectious diseases in the future.”

— Read more in Bryan Charleston et al., “Relationship Between Clinical Signs and Transmission of an Infectious Disease and the Implications for Control,” Science 332 no. 6030 (6 May 2011): 726-29 (DOI: 10.1126/science.1199884)