Clearly more information is required. The distribution of cases of BSE in the national herd. What was expected with the 'nugget' hypothesis was that one infected batch of feed might go to one herd but the next infected batch was just as likely to go to that herd again as to any other another herd. What seems to have happened however, is that once a herd has had one case, it is more likely to get another than a herd that has not already had a case. One explanation of this is that once a farmer had seen one cow with BSE , he was more likely to recognise it again.
Farmers talk about 'infected herds'.
Model 2 will explain these cases but only in small herds would this be statistically valid. Epidemiology seen fits precisely in what would be expected with vertical transmission. An infective batch would affect the mothers and hence the offspring in later years would be more likely to develop disease. My own attempt at doing a sibling study was poor. D. Steady age distribution of BSE in cattle born in different years.