UNRESOLVED QUESTIONS ABOUT FOOTROT ERADICATION

Footrot was considered to be an eradicable disease when it was shown t hat the essential transmitting agent, Dichelobacter nodosus, did not s urvive away from the sheep. Eradication systems have been based on the elimination of cases of the disease and hence the organism. Closer un derstanding of the behaviour of the disease in flocks in different env ironments has led to development of more rational approaches to eradic ation. Techniques for immunisation and for easier treatment methods ha ve been developed. Much of the emphasis placed on eradication derives from the recognition of the economic impact of footrot - from producti on loss and decreased market value of sheep from affected flocks. In N .S.W. eradication of footrot is the policy of the industry and of the State government. Coincidentally with the development of knowledge on prevention, treatment and eradication of footrot, has been an increase in understanding of the complexity of the disease in the field. Where as previously footrot was considered to be simply a contagious disease which caused lameness and loss in affected flocks, it is now clear th at some forms of footrot cause little lameness and trivial direct prod uction loss. In eradication programmes designed for state-wide applica tion a number of important questions arise. These consider The type of footrot which the eradication programme is targeting. So called benig n infections are now excluded but where is the division between benign and virulent? How is the distinction made? How accurate is the test a pplied to the distinction between benign and virulent infections, i.e. , between targeted and non-targeted disease? What is the eradicability of those expressions of footrot which approach the benign class but w hich technically are classed as virulent? Experience suggests that suc cess of eradication increases with the virulence of the infection. Wha t is the potential for cattle and goats to be reservoirs of infection at the lower end of the virulence scale? Cattle are well established a s hosts of the isolates most benign for sheep. How far up the virulenc e scale does their host capacity extend? Do the forms of footrot low d own in the virulence scale really matter? Their production effects are probably minimal and therefore do not justify expensive and disruptiv e eradication procedures. These questions, and the absence of clear an swers to them, are of major importance to all sheep producers. As the prevalence of fully virulent infections is reduced the proportion of t hose lower on the scale will increase. The New South Wales Strategic P lan for eradication requires that footrot is notifiable in protected a reas. Properties diagnosed as having virulent (i.e., non-benign) infec tions will be quarantined and will have to undertake compulsory eradic ation. If too many properties are classified incorrectly and thereby r equired to eliminate a form of disease which is neither economically i mportant nor eradicable, the plan will not be able to proceed with the support of the industry.