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The UK foot and mouth disease outbreak — The Aftermath

Daniel T. Haydon, Rowland R. Kao & R. Paul Kitching

Nature Reviews - Microbiology August 2004 Vol 2 No 8 PERSPECTIVES - Opinion (registration and subscription required) http://www.nature.com/cgi-taf/DynaPage.taf?file=/nrmicro/journal/v2/n8/index.html

The article can be read on payment of a subscription to the magazine. What follows is a brief commentary on the article in our own words, with our own emphasis and our own sub headings. We cannot do justice to the article - but feel some of its key points at least should reach the warmwell readership - most of whom cannot afford to subscribe to scientific journals.

The UK foot-and-mouth disease outbreak - the aftermath

Daniel T. Haydon, Rowland R. Kao and R. Paul Kitching

Commentary from warmwell.com

"…unprecedented numbers of animals were slaughtered in a new and untested control procedure, largely formulated and justified with the use of necessarily hastily developed computer models…..uncertainties in the data and the reliance of these models on assumptions that are necessarily crude and also difficult to verify, it is difficult to make the argument that mathematical models showed that implementation of widespread and intensive culling was the only tenable option."

Comparing 1967 and 2001

In comparing the 1967 outbreak with 2001, the authors note that in 1967 slaughter was mostly confined to infected premises alone, whereas in 2001 animals were culled on a further 8,131 premises associated with, IPs "in some way"

Before reading their article we had not realised the huge contrast between 1967 and 2001 in numbers of animals killed. The two outbreaks were similar in many ways:

In 1967 approximately 442,000 animals were slaughtered to control the epidemic, whereas in 2001, say the authors, "... at least 4 million animals were slaughtered for the purposes of disease control, with at least a further 2.5 million animals destroyed in 'welfare culls'..."

"a comparison with a well-managed (our emphasis) traditional policy is yet to be published"

political value of the 2001 policy

The authors point out arguments in favour of a CP cull :

The effect of the 2001 policies on the progress of the disease

They demonstrate that reconstructions show the epidemic peaked between "19 March and 21 March and the number of reported cases peaked on 26 March before the new policy measures were implemented"

Evidently then it cannot be claimed that the contiguous cull policies were in themselves responsible for the reduction.

Graphs in the article show that in Devon,

SVS staff

The article notes that the SVS were stretched beyond their capacity because of the drastic reductions in their numbers (field staff reduced from about 600 in 1967, to 220 in 2001), because so few had any practical experience of dealing with FMD, and because the scale of the outbreak before the movement ban made impossible a successful imposition of traditional control methods.

According to the article fewer than 10% of IPs were culled in the 24 hours after disease was reported, and only 35% were subject to culling within 48 hours.

The article also notes that a consequence of the 2001 outbreak has been the The authors suggest that had a movement ban on all livestock been in position immediately after confirmation of the first case it could have halved the size of the epidemic .

Models leading policy

A new departure for 2001 was the mathematical modelling. While the authors conceded that models allow an objective exploration of data they suggest that there has been too little published examination of the consequences of the assumptions made by the models:

The authors suggest that in the future, models ought to be developed and used only in circumstances in which both their strengths and inevitable short-comings are recognized and widely understood. This, it is implied, was certainly not the case in 2001.

Relationship between the probability of transmission and the distance to an IP

This is known as the 'transmission kernel'. Another element lacking in published literature about the outbreak, according to the article, is an exploration of the underlying biases in estimates of 'who infected whom' An overestimation of the importance of local spread could have led to Interestingly, the authors note that "a large pig farm left infectious for at least two weeks during meteorological conditions favourable to transmission is thought only to have infected 1 - 10 neighbouring farms before it was culled"

We also learn (and it is now August 2004) that

They note that there was little aerosol spread during the 2001 outbreak and that this should have made the outbreak easier, and not more difficult, to contain.

Recommendations in the article

One most clear piece of advice contained in the article is that "we must learn how information from quantitative models should be incorporated into policy formulation in a balanced way, mindful of its persuasive but often illusory level of numerical precision."

The authors suggest that modelling needs to have integrated into it the implications of limited logistical and human resources. This the 2001 models certainly did not have - and yet the influence of the models on the choice of policy is well known.

When the authors say that

they are possibly making reference to another interesting sentence within the article: "the main arguments in favour of a CP cull are simpler decision-making and ease of management, together with the benefit that, in a time of great chaos and uncertainty, a clearly defined policy with simple goals can be of both logistical and political value."

While this is no doubt so, an alternative aim for a successful control policy might be to minimise animal suffering and loss and to keep as limited as possible the social and psychological impact of the disease on the human population.

They echo the best Inquiry conclusions that coordinating policy centrally should not compromise the vital need to allow for local control tactics.

They say that we need to know much more about "the precise mechanisms that allow the local spread of disease. We need to develop rigorous protocols for exploring phenotypic variability which might characterize different viral strains, and not simply track, but react to, the locations of strains worldwide"

Conclusions

While traditional methods have worked in the past, the authors urge that

As for the latest DEFRA FMD contingency plan, the authors say that the timing of the decisions regarding control options is crucial. A list of criteria are required so that the seriousness of outbreaks can be measured early and "an appropriately measured response is selected."

They make a plea for