TSE tend to have long incubation periods and infected cattle typically do not show clinical signs until they are older than 30 months of age. In the U.K., at a time when testing of animals was not occurring in the food chain, almost 100 animals with an estimated age younger than 30 months were recognized through the presentation of clinical signs. The youngest was a 20 month old animal found in 1992. Most cases of BSE in the U.K. occurred in dairy cows between 3 and 6 years of age although the incubation period can range from less than 20 months up to the natural lifespan of the animal depending on the level of infectious material to which the animal was exposed. The incubation period for BSE usually ranges from 4 to 8 years.
According to U.K. statistics, no cases of BSE have been found in naturally infected cattle younger than 30 months since a 29 month old animal was found in 1996 (click for info). This range of incubations in naturally infected animals is also supported by experimental research, taking the age at recognition as the onset of definite clinical disease.6 The longest incubation period so far in experimentally infected animals is 110 months.7
It is important to recognize that many cases, and in some countries most cases, are not detected by recognition of clinical signs. Active surveillance for BSE in high risk animals (dead, downer animals, etc) or in apparently healthy cattle at slaughter now detects the majority of cases. This approach has the potential to detect infected cattle before they have clear cut clinical signs. This is supported by experimental evidence suggesting that occasional animals may be positive for BSE before 30 months after oral infection. 8 Other studies have demonstrated that where the dose of BSE infective material is small (e.g., 1 gram), the age at detection by testing of the brain rises.9 This is the situation experienced in most countries other than the U.K. in the past, and explains the relatively older ages at which cases are detected.
With BSE, the higher the level of infectious material a calf is exposed to, the shorter the incubation period.6 Two cattle in Japan, a 21 month old and a 23 month old, suspected of having BSE in 2005 subsequently were not confirmed in transmission studies as BSE cases in 2007. While officially considered BSE cases, they were not transmissible to highly sensitive bovinized mice. The final status of these animals may remain unresolved as test material has been exhausted.10 At the onset of the large active surveillance program in the European Union (EU) in 2001, small numbers of young animals were detected: the youngest cases were found at 28 and 29 months in two emergency slaughter animals. The application of current rapid tests to surveillance in the earlier stages of the U.K. epidemic would have significantly increased the numbers of young animals detected, but they still would have represented only a very small proportion of the total detected.
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