Foot and mouth disease study questions need for mass culling

foot and mouth disease, culled cattle - Credit: Murdo MacLeodCattle infected with foot and mouth disease (FMD) are only infectious for a brief window of time, around half what was previously thought, according to a study published today in the journal Science.

Outbreaks of the devastating virus, which affects cattle, sheep, pigs, deer and other animals, led to mass culling of livestock in the UK in 2001 and 2007 in order to contain the spread of the disease.

The new research questions the need for such heavy-handed control measures, instead emphasising that efforts should be directed at early detection of infection and rapid segregation of infected individuals from healthy herds.

An incursion of the disease into New Zealand would have widespread economic impacts from the banning of agricultural exports — a fact that has led to intensive biosecurity controls, as well as national planning and preparedness efforts.

The Science Media Centre asked New Zealand experts in foot and mouth disease for their responses to the new study’s findings and its claims.

Dr Robert Sanson, Epidemiologist, AsureQuality Limited, comments:

“This paper gives scientific proof to what has been suspected for some time, that the infectiousness of the virus is not related to the quantity of virus released per se. Rather, it is the timing of the released virus relative to the onset of clinical signs that determines its ability to cause infection. This period of peak infectivity is shown to be shorter than the virus emission period.

“In terms of what this information will mean to FMD control policy, one needs to consider the overall risk of a farm. A herd will contain individuals at many different stages of infectiousness and as a unit will remain infectious for a long period. As long as there is active infection on a farm, the entire infected group of animals must be considered as high risk, as in the absence of suitable tests that can detect animals in the latent stage of disease, once an animal becomes clinical, it will be starting to release highly infectious viral particles. Even if the clinical animals are slaughtered promptly, any released virus will retain its infectivity, until rendered harmless by environmental processes. This means there is still a risk of ongoing contamination from this virus, possibly for some days, depending on what has been contaminated, temperature and whether it is exposed to UV light or not etc.

“This study should be repeated in other species, particularly sheep and pigs.”

Prof Roger Morris, Animal Health and Epidemiology, EpiCentre, Massey University comments:

“The paper adds some valuable new information about the timing of the infectious period and its duration, using an experimental design which has not been attempted previously (it is extremely expensive). The most important finding is the relatively short period of peak infectiousness, which is centred around the time of onset of clinical signs. However the conclusions reached go beyond the evidence provided. The transmission of FMD virus within and between herds is not a fixed process determined by averages, but shows considerable variation, and the variation is critical to determining the pattern of spread. The authors only obtained data for eight transmissions involving seven source animals, which is far too small to draw firm conclusions about such important issues as whether some animals become infectious before they become clinical.

“The new data, seen in the context of all the other information available about control of foot and mouth disease, is helpful in refining strategies, but will not allow major changes to be made in control plans for the disease, despite the claims in the paper.”