Abnormal prions, proteins that can cause Creutzfeldt-Jakob disease (CJD), can be detected in muscle and may be transmitted during surgery, hints a new study.

Researchers thought that the prions were largely confined to the brain and nervous system. Eight of 32 patients who died from CJD had prions in their muscle, found Adriano Aguzzi and his co-workers at University Hospital Zurich, Switzerland, who used a sensitive test on autopsy tissue. A different, but overlapping third of the patients, carried prions in their spleen

The discovery suggests that prions could cling to surgical equipment used on CJD patients and then infect others - normal sterilisation techniques do not kill the hardy proteins. The risks of transmission are low, but it might occur if hospitals do not discard all CJD-contaminated surgical tools or strip them of prions using chemicals and ultra-high temperature.
Until now, most attention has been focused on treating equipment after neurosurgery, from which five patients have caught CJD. Transmission might also happen from patients incubating the disease who have operations before they begin to show symptoms.
Aguzzi's discovery resurrects the idea that BSE-infected cows might harbour trace levels of prions in their muscle meat.

In a parallel development, Professor Stanley Prusiner of the University of California, San Francisco, has found surprisingly high prion levels in the muscles of mice infected with BSE. Until now, levels of prions in infected cow and sheep meat and muscle tissue were always believed to be low enough not to pose a significant risk of transmission and the emphasis was to remove the brain and spinal cord, which harbour much higher levels.

The aim of the experiment was to determine whether prions are produced or merely accumulate intramuscularly. Transgenic mice producing either mouse or Syrian hamster PrP exclusively in muscle were used. Inoculating these mice intramuscularly with prions resulted in the formation of high titers of nascent prions in muscle. Both high prion titers and the disease-causing isoform of the prion protein (PrP^Sc) were found in the skeletal muscle of wild-type mice inoculated with either the Me7 or Rocky Mountain Laboratory strain of murine prions. Particular muscles accumulated distinct levels of PrP^Sc, with the highest levels observed in muscle from the hind limb.

In contrast, inoculating mice in which PrP expression was targeted to hepatocytes resulted in low prion titers. Hepatocytes are the chief functional cells of the liver and perform an astonishing number of metabolic, endocrine and secretory functions. Roughly 80% of the mass of the liver is contributed by hepatocytes.
The data demonstrate that factors in addition to the amount of PrP produced determine the tropism of prions for certain tissues.
That some muscles are intrinsically capable of accumulating substantial titers of prions is of particular concern. In particular, it was felt that significant dietary exposure to prions might occur through the consumption of meat, even if it is largely free of neural and lymphatic tissue, a comprehensive effort to map the distribution of prions in the muscle of infected livestock is needed.

The experiment seems to tie in perfectly with existing knowledge of PrP. PrpSc has the ability to transform PrP into PrpSc. So, injecting material into muscle engineered to produce PrP, as the brain does, caused accumulations as found in the brain.
The real issue is whether cattle produce PrP in their muscles. This is an unknown area.

Both the Swiss experience and Prusiner's experiment demonstrate that skeletal muscle tissue does have the capability to host infectious material. In the Swiss cases, prions were found in muscles of patients suffering from CJD unrelated to BSE. So the prions must have migrated from the brain. Whether such a migration would occur in beef remains to be seen.

Prusiner's experiment relied upon mice producing normal prions in muscle tissue. something cattle (to the best of anyone's knowledge) do not do. So exact parallels are hard to be drawn. However, migration is a distinct possibility. The conclusion has to be that skeletal muscle tissue is not the prion-free zone it was thought to be. Although the concentrations, and therefore opportunities to comprise the gut barrier, are profoundly less.