Infectivity of BSE containing material.

If the infectivity of BSE is similar to that of scrapie, it is expected that the dose of BSE infected brain needed to transmit to mice orally would be 125,000 to 200,000 times greater than the dose required to transmit by intracerebral injection.

Infection is dose dependent but the required dose for BSE transmission to cattle is small. An oral dose of 500 mg to 1 g (0.02 to 0.04 oz.) of infected brain appears sufficient to infect.

The BSE agent has been found only in brain tissue, spinal cord and retina of cattle naturally affected with BSE. It has not been found in meat or milk. Similarly, it is not transmissible through meat or milk (see table below). Traces of the BSE agent are detectable in the small intestines of calves that had been fed large doses of meat and bone meal (MBM) from BSE-infected animals.

Transmission via the oral route (from feed) appears to vary by species. It is a major route of natural transmission of BSE to cattle, which consume little MBM relative to number of infections, but probably a minor one in domestic cats, which consume large amount of MBM relative to number of infections (~100 cases of feline TSE in Britain).

The mechanism by which the infectious agent moves from the digestive tract to the central nervous system has not been determined. While it has been speculated that spleen-derived white cells may carry the infectious agent through the lymph nodes, no BSE infectivity has been shown in the lymph nodes of BSE-infected cattle

Taylor et al have shown that scrapie can be transmitted to mice through cuts and abrasions on the skin which would indicate that blood or peripheral nerves transmit the infective agent to the CNS. However neither blood nor muscle have been shown to be infective. It should be noted that scrapie brain extracts were applied to wounds to achieve this infection. This experiment was meant to determine the safety of handling scrapie brain tissue during autopsy and does not imply that sheep transmit infection via abrasions.

 

Infectivity of bovine tissues and body fluids
Highest infectivity	brain spinal cord eye
Medium infectivity	spleen tonsil lymph node ileum proximal and distal colon pituitary gland adrenal gland cerebrospinal fluid placenta pineal gland
Low infectivity	peripheral nerve liver lung pancreas bone marrow thymus nasal mucosa
No infectivity	skeletal muscle (meat) heart mammary gland - milk, colostrum blood - blood clot, serum or plasma kidney thyroid gland salivary gland ovary uterus testis seminal vesicle fetal tissues hair skin bone cartilaginous tissue connective tissue bile saliva urine feces