Academic journal article Bulletin of the World Health Organization

Public Health Issues Related to Animal and Human Spongiform Encephalopathies: Memorandum from a WHO Meeting

Academic journal article Bulletin of the World Health Organization

Public Health Issues Related to Animal and Human Spongiform Encephalopathies: Memorandum from a WHO Meeting

Article excerpt

Nature of occurrence

Characteristics of the spongiform encephalopathies

Bovine spongiform encephalopathy (BSE) is a member of the group of transmissible spongiform encephalopathies (TSE), whose prototype is scrapie. All these diseases are associated with a transmissible agent, the nature of which is not fully known. However, it displays many virus-like features such as strain variation and mutation, but differs from conventional viruses in being exceptionally resistant to heat, ultraviolet and ionizing radiations, and many chemical disinfectants.

The TSE agents, following long incubation periods, produce clinical diseases which are characteristically progressive and end fatally. These diseases affect the central nervous system in which characteristic spongiform changes are visible by light microscopy. Detergent-treated extracts of affected brain yield scrapie-associated fibrils (SAF) which are visible by electron microscopy. These fibrils consist mainly of an abnormally modified host-coded protein called PrP or prion protein. Because of this, the TSEs are also referred to as "prion diseases". Infection with these agents does not provoke a detectable immunological reaction in the host, so there is at present no practical means of detecting infection in healthy but possibly infected animals. Bioassay in laboratory species, such as mice, is the only way of detecting and measuring the infectivity of these agents. As with scrapie, the transmission of BSE to mice has been accomplished by intracerebral and intraperitoneal injection, as well as by feeding them with affected brain. BSE has also been transmitted to cattle by the injection of infected brain.

Occurrence of BSE and other animal TSEs

BSE was first recognized in the United Kingdom in November 1986. Initial epidemiological studies indicated an extended common source epidemic. Later studies, taking into account the disease's similarity with scrapie, identified the exposure of cattle to a TSE agent through feed that contained ruminant-derived protein (in the form of meat and bone meal) as the likely source of the disease. A key factor explaining the emergence of the disease was the occurrence of certain changes in rendering practices (i.e., processing of animal and abattoir wastes to produce feed) in the United Kingdom in 1981-82.

The total number of confirmed BSE cases in the United Kingdom between November 1986 and 31 August 1991 was 35 627; at the end of this period the annual incidence was 5 cases per 1000 adult cattle. BSE cases have also been reported in the following countries/territories (the cumulative number of cases as at 31 August 1991 is given in brackets): Ireland [41], France [4], Switzerland [7], Oman [2] and the Falkland Islands [1]. The cases in the last two places were imported from the United Kingdom, which is the only country with a high incidence of BSE.

Other major transmissible animal spongiform encephalopathies are described below.

(1) Scrapie is a disease mainly affecting sheep of 2 to 5 years of age, but it can also affect goats. The disease has been diagnosed in many countries of the world and has been endemic for nearly 300 years in the United Kingdom. Scrapie was introduced into Australia and New Zealand through the importation of sheep from the United Kingdom, but stringent measures eradicated the disease in these countries. Australia and New Zealand are accepted by many countries as being scrapie-free.

Scrapie is important as the only TSE of animals which is known to exist as an endemic infection of its natural host, sheep. Epidemiological evidence shows that the transmission of infection is predominantly from infected mothers. In addition, scrapie infection can pass between unrelated sheep, either directly, or as a consequence of contamination of the environment (e.g., by infected fetal membranes). The oral route of infection is implicated in the natural spread of the disease. …

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