The UK foot and mouth disease outbreak — The AftermathDaniel 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 aftermathDaniel 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 2001In 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 the epidemic lasted 212 days and caused outbreaks on 2,364 different premises.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'..."
In 2001 the epidemic lasted 214 days and resulted in the identification of infection and culling of herds on 2,026 premises
"a comparison with a well-managed (our emphasis) traditional policy is yet to be published"
political value of the 2001 policyThe authors point out arguments in favour of a CP cull :
".. a clearly defined policy with simple goals can be of both logistical and political value."
The effect of the 2001 policies on the progress of the diseaseThey 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,
"IP culling alone might have been sufficient to bring the disease under control …Although requiring more rigorous investigation, this is likely to have been true in other affected regions in the UK where infection was widespread."
SVS staffThe 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.
"This response was inadequate - the success of traditional methods depends on having the necessary resources to identify and cull IPs and DCs as early as possible."The article also notes that a consequence of the 2001 outbreak has been the
"extension of the legal authority to cull, which now includes "any animal the Secretary of State thinks should be slaughtered with a view to preventing the spread of foot-and-mouth disease", as outlined in the Animal Health Act of 2002"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 policyA 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:
"overestimation of the early infectiousness of IPs would lead to models exaggerating the importance of CP culling. The sensitivity of the models to this assumption remains unclear."
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 IPThis 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
"an exaggerated estimate of the value of CP culling"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
detailed analysis of the proportion of IPs that were confirmed as infected by laboratory tests is yet to be published.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.
"Subsequent analysis has revealed that the role of animal movement in the early spread of infection might have been underestimated, leading to a potential overestimation of the number of premises that were infected after the movement ban was imposed. Both sources of error could cause the models to overestimate the control effort that was required" and "although it is well known that after movement restrictions were imposed in late February the transmission kernel changed to reflect much higher levels of local transmission, a constant transmission kernel was assumed in all models thereafter"
Recommendations in the articleOne 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
"The aims of a successful control policy need to be defined more precisely"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"
ConclusionsWhile traditional methods have worked in the past, the authors urge that
"we need to learn why and when these measures can fail…… as implemented in 2001, they did not work to an acceptable standard"
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
"a marriage of the value of the expert advice so staunchly defended by the veterinary practice, with the benefits of modern surveillance, diagnostic and data management technologies and the analytical capabilities of theoretical modelling at the strategic level. This will require drive, focus and coordinated cross-disciplinary communication, and patience, good listeners, and open minds.... ."