TSEs (transmissible spongiform encephalopathies) are neurodegenerative diseases of various mammalian species, the best known of which include BSE (bovine spongiform encephalopathies) in cattle, CJD (Creutzfeldt–Jakob disease) in humans, scrapie in sheep and CWD (chronic wasting disease) in deer. This review examines the emergence of various TSE strains and their transmission, and discusses disease surveillance and control.
The TSEs (transmissible spongiform encephalopathies) are neurodegenerative diseases which affect a variety of mammalian species. The most studied of these diseases are scrapie in sheep, BSE (bovine spongiform encephalopathy) in cattle, CJD (Creutzfeldt–Jakob disease) in humans and, more recently, CWD (chronic wasting disease) in deer. They are all characterized by long asymptomatic phases post-infection which can last for months or years. The long asymptomatic phase is followed by a short clinical phase displaying a variety of neurological abnormalities, which include ataxia and dementia, finally resulting in death which is thought to be due to neuronal loss. Another hallmark of these diseases is the presence of vacuoles in the brain of an affected individual, from which the term spongiosis is derived. The exact origin of these vacuoles is not clear, although is has been hypothesized that they are the product of apoptotic neurons.
One of the most distinctive features of these diseases, however, is the deposition in the brain of the PrP (prion protein) . In the uninfected individual, PrP is a protease-sensitive sialoglycoprotein anchored to the cell membrane through a GPI (glycosylphosphatidylinositol) anchor (PrPC) . During the course of disease, there is a conformational change in PrPC, resulting in the accumulation of an abnormal protease-resistant isoform (PrPSc). It has been proposed that PrPSc is the infectious agent  and that it is also responsible for the neurotoxicity in these diseases, but the precise role of PrPSc in disease still remains to be established.
Although PrPSc deposition, vacuolar pathology and neuronal loss are often found in the same region of the infected brain, this is not always the case, and, in some instances, PrPSc deposition can be detected early on in the disease process before the appearance of neuronal damage . In other cases, PrPSc deposition is not always detected in a terminally infected brain, despite the presence of vacuolar pathology and neuronal loss [5,6]. The exact relationship between PrPSc deposition, vacuolation and neuronal loss therefore remains to be established.
Disease-associated forms of PrP can also be detected by protease treatment of an infectious tissue homogenate, followed by immunoblot analysis using PrP-specific antibodies. These protease-resistant forms of PrP are detected in most tissues which carry infectivity and are generally referred to as PrP-res to denote their protease resistance. The three bands of PrP-res detected correspond to the three different glycoforms of PrP, produced by differential N-linked glycosylation at amino acids 180 and 196.
Declining and emerging TSEs
Since 1986, when BSE was first recognized, approx. 180000 cattle in the U.K. have developed the disease, and many more animals are predicted to have been infected with BSE, but have been slaughtered before the onset of the clinical phase [7,8]. The origin of BSE is not known, although the epidemic was likely to have been maintained and spread through the practice of recycling infected bovine tissues back to cows through cattle feed . The epidemic peaked in 1992, but since that time because of the control measures which were put in place banning the feeding of ruminant protein to ruminants, the epidemic has consistently declined. However, some cases continued to occur in animals born after the ban was put in place, leading to a more extensive ban on feeding mammalian protein to any farmed animal in 1996. A small number of cases in animals born after the 1996 ban have been attributed to leakage of contaminated feed, although other routes of transmission cannot be ruled out .
BSE has now been identified in most European countries and in a number of non-European countries, including Canada and the U.S.A. Since control methods were enforced later in other European countries, the peaks of these epidemics appeared later than in the U.K., although most European countries are now showing a decline in the annual number of cases . Of considerable concern is the possibility that BSE may have been passed to sheep. There is thus extensive surveillance being carried out for the presence of BSE in sheep, but, to date, sheep BSE has not been detected, although BSE in goats has been confirmed recently .
vCJD (variant CJD)
In 1996, the first case of vCJD was announced  and the linkage of vCJD with BSE was subsequently confirmed through techniques based on strain-typing experiments in mice and the gel-banding patterns of disease-associated PrP [14,15]. vCJD differs from sCJD (sporadic CJD) most notably by its early age of onset . An age-related susceptibility has been demonstrated for some TSE agents in mice although the basis for this susceptibility is not yet understood . Thus factors other than diet may contribute to the unusual early onset of vCJD. The clinical presentation of vCJD differs from that of sCJD, often presenting first with psychiatric symptoms including behavioural changes, anxiety and depression . This is followed within weeks or months by a cerebellar ataxia and myoclonus. Memory disturbances and severe cognitive impairment appear later in the course of disease resulting finally in akinetic mutism . Pathologically, vCJD also differs from sCJD with characteristic amyloid plaques surrounded by vacuoles being detected throughout the cerebrum and cerebellum [13,19,20]. These plaques have been termed ‘florid plaques’, and similar plaques have been detected in experimental passage of vCJD to mice .
To date, there have been 161 clinical cases of vCJD in the U.K. and 30 cases outside the U.K., of which some, but not all, are associated with a period of residency in the U.K. All cases of clinical vCJD cases genotyped have occurred in individuals homozygous for the 129M polymorphism in PrP. Gene-targeted transgenic mice homozygous for either human 129M or 129V PRNP genes infected with vCJD have demonstrated that 129VV and 129MV individuals may also be susceptible to vCJD (Figure 1) . Since many more individuals are likely to have been exposed to BSE than have succumbed to clinical disease, there is also concern that there may be a carrier status, where individuals carry infectivity without showing clinical signs of disease, and that such individuals may be able to pass the disease on to others. The possibility of extensive subclinical disease in humans has also been highlighted following a retrospective tonsil and appendix survey that identified PrP accumulation in the appendixes of two 129VV individuals . Carrier status is an important issue not only for vCJD but also throughout TSEs, particularly with sheep scrapie. The National Sheep Scrapie plan aims to eradicate scrapie by eliminating the most susceptible genotypes of sheep from the national flock, but it is not at present known whether scrapie carriers will remain and have the potential to transmit disease . Whether such carrier states exist and their ability to pass disease on are therefore important questions yet to be answered.
Susceptibility to BSE or vCJD comparison of bovine transgenic mice (BOV) and human transgenic mice with different amino acids at codon 129 (HUMM; HUMV; HUVV)
More recently, two cases of possible human-to-human transmission of vCJD by blood transfusion have been reported [24,25]. While blood transmission of vCJD had been predicted from experimental studies in sheep, which had demonstrated that blood is an efficient route of transmission of both BSE and sheep scrapie [26,27], it was unexpected that the second of these human cases appeared in an individual carrying 129MV PrP. From previous studies of iatrogenic CJD cases, 129MV heterozygotes might have been expected to be more resistant or have considerably longer incubation times of disease than either of the homozygous 129MM or 129VV individuals. However, clinical disease was not present in the 129MV case, suggesting that longer incubation times may indeed occur in these individuals, raising further concerns over the extent of subclinical disease in the population and the ability of such individuals to transmit disease.
CWD was first identified as a fatal wasting disease of captive mule deer in the late 1960s in Colorado and was classified as a TSE in 1978 . More recently, CWD has become a serious problem in both game farms and wild deer and elk populations in North America, being detected at >10% incidence in certain wild deer populations in Colorado and Wyoming. The mechanism of transmission is not known; however, evidence supports lateral transmission through direct animal-to-animal contact or by indirect exposure to infectivity through contaminated feed or water sources . New foci are thought to have arisen through commerce in live farmed deer and elk. Wild ruminants with CWD are found on the same ranges as cattle and sheep, raising concerns for transmission of CWD to these species. Moreover, there is also the concern that CWD-like BSE could cross the human species barrier. In vitro studies have indicated an inefficient conversion of human PrP into a protease-resistant form by CWD-infected brain homogenate, raising the possibility of low level transmission to humans . Epidemiological studies to date have not indicated any link between CJD and CWD; however, such studies are limited. Studies involving inoculation of CWD in mice have shown transmission in transgenic mice expressing cervid PrP, but not in wild-type mice or transgenic mice expressing human PrP (129MM), suggesting that a species barrier may prevent the spread of this agent [31,32]. However, more studies are required to establish possible CWD transmissions to other species such as cows or sheep.
One of the major problems still surrounding TSEs is the absence of a pre-mortem diagnostic test. While clinical signs of disease can give strong evidence of not only the presence of TSE, but also the strain of TSE in the case of human disease, final confirmation of disease is only obtained following post-mortem examination of brain tissue. Although macroscopic examination can be used as a major diagnostic tool, the definitive confirmation of TSE disease is reliant on the direct measurement of infectivity by bioassay of tissue in mice [14,33,34]. However, these assays take many months to perform, and are of limited use for large-scale screening programmes. Considerable effort has therefore gone into developing more rapid tests for detecting infectivity. The need for surveillance for BSE and scrapie in Europe has lead to the development of a number of rapid diagnostic tests which are based on either immunoblot or ELISA systems . Understanding the exact relationship between disease-associated forms of PrP and infectivity is one of the major problems in assessing the reliability of PrP-res-based diagnostic assay systems. This issue has been compounded further by the identification of TSE disease in animals which do not appear to have a protease-resistant form of PrP [5,6]. Indeed, a model of disease has been shown to have high levels of infectivity in brain with undetectable levels of PrP-res by either immunoblot or ELISA-based systems, suggesting that PrP-res is not always a reliable diagnostic marker of disease (R.M. Barron, unpublished results).
New strains of TSEs
The extensive surveillance for BSE and scrapie in Europe and elsewhere has revealed the existence of a number of previously unrecognized TSE strains. Based on molecular strain typing and pathological examination of the brain, a new form of BSE has been identified in two older cattle in an Italian surveillance system . It has not yet been established whether this represents a newly diversified strain of TSE or whether this strain has existed for some time and been identified through the increased surveillance. The ability of this strain to transmit to humans is also under investigation in transgenic mouse studies. New strains of cattle TSE have also been reported by groups in France and Japan [37,38].
During the surveillance of sheep at slaughter a number of atypical cases of scrapie have been identified in Germany, France  and Norway . These cases demonstrate a molecular profile and distribution of PrPSc not recognized previously in sheep scrapie. Atypical cases have also appeared in sheep carrying a PrP genotype which was previously thought to have been resistant to scrapie. BSE has also been experimentally transmitted to sheep of this resistant genotype , raising concerns that no sheep are truly resistant to disease and that new scrapie strains may propagate in sheep previously thought to be resistant to disease.
While considerable progress has been made in the study of TSEs since the emergence of BSE, there are still major gaps in our knowledge of the basic mechanisms of these diseases. The species barrier and mechanisms of host resistance and susceptibility are not understood. Moreover, we have yet to establish exactly what defines a TSE strain and what determines how that strain targets specific areas of the brain. Importantly, the exact nature of the infectious agent remains to be determined. Our ability to diagnose, control, eradicate or treat these diseases is still dependent on providing answers to these fundamental questions.
Preparing for the Pandemic: Universities and Public Health: Education session at BioScience2006, held at SECC Glasgow, U.K., 23–27 July 2006. Edited by K. Gartland (Glasgow Caledonian, U.K.).