SEAC : Statement - 29th July 1996

Statement

- 29th July 1996

Maternal transmission of BSE

The Spongiform Encephalopathy Advisory Committee (SEAC) considered at its meeting on the 19 July 1996 an interim report on a study conducted by the Epidemiology Department, Central Veterinary Laboratory, Weybridge to investigate the possibility of cow to calf transmission of BSE. The study involved assembling two groups of animals with over 300 cattle in each. One group consisted of offspring of confirmed cases of BSE whilst the other group comprised animals born in the same herd and in the same calving season whose dam had reached at least 6 years of age without developing clinical signs of BSE. The animals in the two groups were kept until the age of 7 or until BSE or another disease intervened. BSE occurred in both groups as the cattle were born around the time of the ruminant feed ban in 1988 and the cattle, or at least some of them, in both groups would have been exposed to infected feed.

As at the 14 July 1996, 273 animals in each group had reached the age of 7 and had been slaughtered or had developed disease. 55 animals are still alive and histological results are pending for 8 animals. Of the 273 animals born to dams with BSE, 42 have developed histologically confirmed BSE. In the 273 animals born to mothers who had not developed BSE 13 were histologically confirmed as having BSE. This provides evidence that the risk of maternal transmission is approximately 10% for the BSE infected cows whose calves were studied. The statistical confidence limits for that figure are 5-15%, this range being a reflection of the numbers of animals in the study and of the numbers developing BSE. It is highly unlikely that the results from the histological examination of the brains of the 63 remaining animals will materially alter these findings.

An analysis has been made of the interval between the birth of the animals in the study and the onset of clinical BSE in their mothers. All of the calves in the study were born within 13 months of the clinical onset of BSE in their dams, and the great majority were born within 5 months of clinical onset. Thus the study does not provide a good estimate of the risk to animals born more than 6 months before the onset of BSE in the dam. However, the findings provide some, albeit limited, evidence that there is an enhanced risk of maternal transmission in the last 6 months of the BSE incubation period. It is plausible that the risk of maternal transmission reduces markedly as the interval between the birth of the calf and the onset of BSE in the dam increases. Therefore, the risk of maternal transmission observed under the study conditions is likely to be greater than would be expected for the entire population of cows. Under field conditions, only a fraction of the BSE-infected cows giving birth would be within 6 months of demonstrating clinical signs of BSE because of the long incubation period of the disease. The average incubation period is 60 months and, if the rate of cow to calf transmission over the last 6 months of the incubation is 10% and it is insignificant before that time, then the average transmission from cow to calf over the 60 months duration of infection in an animal prior to developing clinical disease will be 1%. This would be the rate of maternal transmission that would be observed under field conditions.

Maternal transmission at the rates observed in husbandry conditions of the UK dairy herd will not lead to the permanent establishment of BSE even at a low incidence in the UK herd. It will die out, as it already clearly is doing, as a consequence of the restrictions on the primary mode of transmission through infected feed. The study itself provides no new evidence in relation to horizontal transmission. Since the meeting of SEAC on 19 July the Epidemiology Department at CVL has examined the data from this study and that pertaining to the herds from which study animals were taken and has found no evidence of horizontal transmission.

The study tells us nothing about the route of maternal transmission, which could be in utero, at birth or soon after birth. In sheep scrapie where there is also evidence of maternal transmission, infectivity can be detected in the placenta. Furthermore in sheep scrapie there is evidence of in utero transmission from an experiment where the embryo from a scrapie infected sheep was transplanted into a healthy ewe and when that ewe gave birth the lamb eventually went on to develop scrapie. Similar embryo transfer experiments are underway in cattle but results are not yet available. Infectivity has however not been detected in the bovine placenta or in milk, or in blood (see Note 1).

The Committee considered whether evidence of maternal transmission calls into question the existing recommendations to protect public health. These were drawn up on the assumption that BSE could be a risk to man, which is still not proven, and on the assumption that maternal transmission could occur. The Committee have concluded that there is no case for changing its recommendations (see Note 2) in relation to milk, meat, blood or any other product which is currently permitted.

There is no evidence from any of the transmissible spongiform encephalopathies that infectivity can be transmitted through milk. In commercial dairy herds where the bulk of BSE cases arise calves do not receive their mothers milk except for the first few days of life when they receive the special milk produced at that time called colostrum. Colostrum is different in nature from ordinary milk and is not sold for human consumption. In the beef suckler herds it is common practice for calves to be suckled by their mothers for up to six months. Existing data do not provide evidence to suggest that the rate of maternal transmission in beef suckler calves who have prolonged exposure to their mother's milk is any different to that in dairy herds where the only exposure would be to colostrum in the first few days of life. The Committee was pleased to note that the Epidemiology Department at CVL is undertaking further detailed studies on this point, and that the results of these studies will be available very soon.

The Committee recognise the role of the ACDP and the HSE in relation to the occupational risks. The Committee draws the attention of those bodies to these new findings but does not make any recommendations for further action other than that the two bodies should consider the evidence and any implications for occupational health.

The Committee considered the position in relation to the measures to eradicate BSE, particularly in relation to any selective cull implemented by Government. It is clear from the new information that maternal transmission will not perpetuate the disease and that BSE will therefore die out even in the absence of any form of selective cull. The Committee were made aware that preliminary analyses of the effect of these new results on a culling policy had been undertaken but that these were as yet incomplete. Nevertheless the Committee recommends that the results of the completed analyses be taken into account before final decisions are made about the policy for a selective cull.

Finally, the Committee considered what further research might be of high priority in the light of the results of this study and the matter will be the subject of a separate report.

Note 1. Data from standard transmission experiments following parenteral inoculation into mice. Details are given in Table 7 of the latest MAFF Progress Report on BSE, published in June 1996.

Note 2. In its statement of 20 March about the new variant of CJD the SEAC said:

"The Committee does not consider that these findings lead it to revise its advice on the safety of milk."
"If the recommendations set out above [i.e., for the proper enforcement of SBO controls on the deboning of cattle over 30 months] are carried out the Committee concluded that the risk from eating beef is now likely to be extremely small."