Bovine Spongiform Encephalopathy Health Article

BOVINE SPONGIFORM ENCEPHALOPATHY

Few more challenging food safety issues exist today than that of bovine spongiform encephalopathy (BSE) and the human form of the disease, variant Creutzfeldt-Jakob disease (vCJD). While vCJD remains rare at this time, the lack of a screening diagnostic test, uncertainty about the extent of exposure to the agent among humans and animals, the long incubation period, and the resulting devastating and inevitably fatal disease combine to create a situation of extreme difficulty for those responsible for public-policy development. The true extent of the unfolding epidemic is unknown, and since the largest exposures to BSE occurred before the relationship to a human disease was recognized, the full impact on human health may not be known for many years to come. This makes it difficult to determine appropriate actions to take to protect the consumer.

THE EPIDEMIC OF BSE

BSE appeared as a completely novel disease of cattle in the United Kingdom (UK), the first known case being diagnosed retrospectively in 1985. By 1986, BSE was recognized as a transmissible spongiform encephalopathy (TSE) of cattle and considered to be analogous to scrapie, a disease of sheep that is not known to cause human disease. Regardless, investigations were conducted to determine how it was being transmitted among cattle. These studies determined that protein supplement feeds made with meat and bone meal (MBM) were the most likely source of the disease. MBM is extracted from cattle and sheep carcasses through a long cooking process called rendering. Rendering is an established practice, used since the turn of the twentieth century. Through rendering, any tissue remaining on animal carcasses after removal of principle tissues is converted into a cake-like material that is used in multiple industries, including the production of protein supplements for animal feeding. Recycling of the agent through rendering led to the rapid and diffuse spreading of BSE throughout most of the United Kingdom (see Figure 1).

Cattle feed sold in the European market spread the disease further. The first cases outside of the UK appeared in 1989 in the Republic of Ireland. By 1990, two other countries (Portugal and Switzerland) were affected, and France reported its first case in 1992. As of December 2000, eleven European countries outside of the UK had reports of BSE in their national herds (Republic of Ireland, Switzerland, Portugal, France, Belgium, Luxembourg, Netherlands, Liechtenstein; and in 2000, Denmark, Spain, and Germany). Three other countries (Canada, Oman, and Maldives Islands) have reported cases, but only in imported cattle. The UK reported approximately 180,000 cases (as of December 2000), with just over 1,300 BSE cases reported elsewhere.

BSE IN HUMANS

In April 1996, Dr. Robert Will of the United Kingdom Edinburgh CJD Surveillance Unit announced that ten persons had been identified with a novel form of Creutzfeldt-Jakob disease (CJD), and that these cases were sufficiently alike and sufficiently distinguishable from sporadic CJD that they could be considered a new variant of CJD

Figure 1

(vCJD). Surveillance for vCJD is conducted throughout the European Union (EU), and in some non-EU countries including the United States, Canada, and Australia. As of April 4, 2001, the number of vCJD cases reported in the UK was ninety-seven, with three cases reported in France and once in the Republic of Ireland.

Clinically, the illness begins with behavioral changes, but its progression is inevitable and unrelenting. The course is relatively prolonged (two years or more) with both mental and physical deterioration. The patient is eventually left in a vegetative state, unable to speak or move, and death is inevitable, as there is no treatment apart from supportive nursing. The average age of onset is 29 years (cases have ranged from 14 to 74 years of age). Diagnosis is made by magnetic resonance imaging, tonsillar biopsy, and cerebro-spinal fluid testing, and must be confirmed by neuropathology. EEG, used for the diagnosis of sporadic CJD, is not helpful. Brain biopsy is not recommended unless a treatable differential diagnosis is sought.

It is now widely considered that the same agent causes both BSE and vCJD. A number of causes have been proposed—including organophosphates, vaccines, and other novel agents— however, scientific support for these theories has been weak. The route of exposure for humans in unclear, but it seems reasonable to suspect cattle-based food.

Discussion of the potential health impact for humans began early, and by 2001 a wide variety of measures had been implemented, including the removal of animals known to have BSE from the human food chain and the removal during slaughter of known high-risk tissues from animal carcasses regardless of their BSE status (certain tissues of sheep and goats are also removed because of fears raised that sheep and goats may be harboring BSE without developing disease). These tissues included initially the brain, eyes, spinal cord and terminal ileum, but the list was later expanded to include the whole head (excluding tongue), spinal column, thymus, intestine, and dorsal root ganglion. In addition, reflecting current knowledge about the spread of infectivity, animals over certain ages (from 6 to 30 months of age depending on the level of safety required) are not consumed. Also to be considered is the possible contamination of skeletal muscle meat due to certain slaughter techniques that embolize brain tissue throughout the body, or which cause surface contamination of edible parts of the carcass.