The approach of this study was to determine the potential causative factors that were common to all affected animals. Already in the early months of the investigation the most likely factor was thought to be infection by a slow virus or unconventional prion protein agent, although toxic causes could not be excluded and so had to be investigated. Data were collected on the use of vaccines, hormones, organophosphorous (OP) pesticides, synthetic pyrethroid (insecticide) sprays and anthelmintics (used to treat parasitic worms). Weed killers and herbicides had not been used on 22.9 per cent of the farms and pesticides had not been used on 68.9 per cent of the farms. Vaccines, hormones, OPs, anthelmintics and other treatments were also excluded as common factors.

Genetic factors were also considered in 1987, given what was known about other TSEs and the possibility of transmission to offspring. It was known, for example, that autosomal dominant 15 forms of CJD existed in which the disease was transmitted to 50 per cent of offspring on average, and also that genetic susceptibility and resistance factors to sheep scrapie existed. Analysis of parental data on 501 BSE cases revealed the involvement of 239 sires. Search of the pedigree database for bulls revealed that the number of bulls connected to at least ten BSE cases was not indicative of a single gene mutation common to all affected cattle. The data also excluded autosomal recessive 16 inheritance of BSE, although genetic factors influencing susceptibility to infection were still a possibility.
As scrapie in sheep and goats was the only TSE known to affect farm animals in the UK, it was the main candidate for the source of the epidemic in cattle. However, the epidemiology study revealed that sheep were not held on 20 per cent of the farms with BSE-affected cattle. Thus direct or indirect contact with sheep on affected farms was also excluded.

The one factor common to all affected farms was the use of commercial cattle feed, either as finished rations such as pelleted calf feed and dairy cow cake, or as protein supplements used in home-mix rations. These had been fed at some time to all of the cases for which accurate records were available. For many years animal protein in the form of MBM had been incorporated into cattle feed to provide a rich source of protein.
MBM was produced by rendering offal and other waste material produced during the preparation of carcasses of cattle, sheep, pigs and poultry for human consumption. It was thus possible that MBM provided a vector for the transmission of scrapie from sheep to cattle.Further anecdotal evidence that MBM was the vector for the agent responsible for the BSE epidemic was provided by the occurrence of a similar TSE in a nyala and a gemsbok from awildlife park. Disease was diagnosed in these animals in 1986 and 1987 respectively, and it was established that both had been fed cattle feed containing MBM. Although the incubation times for both the nyala and the gemsbok were surprisingly short (the animals had only been exposed to MBM 3 months and between 3 and 15 months respectively before the onset of illness), pathological results from the experimental transmission of tissues from these animals to mice were similar to those found in mice inoculated with material from BSE-affected cattle. It was also noted that the early cases of BSE had occurred almost exclusively among dairy herds, an observation that correlated with the heavy feeding of cattle concentrates to dairy cows and their calves.