List A diseases are defined by the World organisation for animal health (Office International des Epizooties, OIE, located in Paris) as transmissible diseases which:
have the potential for very serious and rapid spread, irrespective of national borders;
are of serious socio-economic or public health consequence;
are of major importance in the international trade of animals and animal products.
The List A comprises at present (2000) of 14 viral diseases and one Mycoplasma disease, Contagious bovine pleuropneumonia.
The domestic animals affected by List A diseases belong to species referred to below:
Since the present List A of diseases was established the significance of certain diseases has changed, diagnostic techniques and control procedures have improved and the international movements of animals and animal products have greatly increased. The list has for years helped animal health authorities to set priorities for their research and control programmes. In the light of recently added responsibilities for OIE under the WTO SPS Agreement, however, it appears appropriate to examine the justification of the list and of the diseases included in the list.
The reservoir of List A disease agents in wildlife and in domestic animal populations, the route and the mode of disease transmission play a major role, when evaluating disease threat and deciding on control measures to be applied.
The threat or risk of a List A disease entering a susceptible domestic animal population can arise from several possible sources. Most of the risk factors for a primary outbreak originate from outside the region where the primary outbreak is confirmed. The transmission of the infection can take place via animals incubating disease, diseased animals, infected or contaminated products of animal origin, inadequately cleaned and disinfected means of transport, people and, for certain diseases, air currents or vectors. Primary outbreaks, however, may also be caused by a source present within the affected area such as infected wildlife, laboratories handling List A disease virus and vaccines which have been inadequately inactivated/attenuated or contaminated.
Until recently disease threat or risk were evaluated on rather subjective assessments. Whenever possible this type of assessment should be replaced by a quantitative risk-analysis, which gives a transparent, objective and defensible estimate of the risk posed by a particular action.
In the context of controlling List A diseases the term "control" in this paper shall refer to prevention (measures to exclude infection from an unaffected area), control (measures to reduce the frequency of existing disease) and eradication (measures to eliminate pathogens from a defined area). All countries with a developed animal production have adopted legislation concerning the control of List A diseases. Within the EU this legislation includes rules governing trade and rules to be applied in the event of an outbreak. The later rules require stamping-out of infected holdings, restrictions on movement within and from an infected area and the potential use of vaccination in emergency situations. Difficulties encountered in recent years concerning the control of List A diseases have primarily been related to outbreaks in densely populated livestock areas and in areas where a disease has been endemic in the wildlife population. Information on List A diseases which have occurred during 1990-1999 is shown in table 1.
Table 1 - Reported outbreaks of List A diseases, 1990-1999, in the EU
A precondition for success in the control of a List A disease outbreak includes a comprehensive and well-rehearsed contingency plan. The plan must clearly describe: the legal power held as regards disease notification, stamping-out, payment of compensation, movement controls, vaccination and use of penalties. Other aspects of the plan refer to the chain of command; the establishment and operation of disease control centres; the use of diagnostic laboratories; training of staff and publicity. In the long-term measures of importance for better disease control include: increased disease surveillance; better protection measures at farm level and protection measures relating to movement of animals.