1. Non-transmissible TSE may acquire infectivity by passage through another species. In the above example, a form of Scrapie does not infect mice, but this form of Scrapie is transmissible to mice if 'passaged' through hampsters.
2. Again, a non-transmissible TSE may acquire infectivity by passage through another species. In the above example, Kuru does not infect ferrets or goats, but Kuru is transmissible to ferrets or goats if 'passaged' through either primates or cats.
3. Sheep Scrapie does not infect humans. However, one scenario for the appearrance of BSE suggests that cattle were fed meat and bone meal (MBM) containing sheep offal. This feeding practice may have infected cattle with Scrapie causing the disease now called 'Mad Cow Disease" or BSE. Under these conditions, Scrapie may have been 'passaged' through cattle and can now be transmitted to humans causing vCJD.
4. Another scenario for the appearrance of BSE suggests that a spontaneous
case of BSE occurred in a cattle. Again, due to the process of feeding MBM containing,
in this case, cattle offal, the disease was spread. BSE is directly transmissible
to humans causing vCJD. A new concern
from this scenario is the potential for BSE to be passaged through sheep (or
other animal species) to humans. Under experimental conditions BSE can be transmitted
to sheep, mice, pigs, primates (macaques and marmosets), mink and other cattle.
Similar to cattle and especially in the UK, sheep have been fed MBM containing
cattle offal.
The ultimate question - are there sheep which have BSE that can be transmitted to humans?
This latter possibility is responsible for the 1998 quarantine of three sheep herds in Vermont. The sheep were imported; either Belgian or Dutch sheep that may have eaten MBM containing BSE cattle offal. Four of the sheep had a BSE-like infection. At this time (March 2001) no one knows whether the sheep have a form of Scrapie (non-human transmissible) or BSE (possibly transmissible to humans). Under the US laws set up to prevent BSE from entering the US, the three herds of sheep are to be destroyed, the brains are to be examined and the transmissibility and clinical presentation of the infection tested in animals. The type of TSE lesion in the test animals should delineate between Scrapie and BSE, i.e. this could identify the source of the infection.
Either scenario, Scrapie as the source of BSE or the potential for BSE to infect sheep,