Evidence for Bovine Spongiform Encephalopathy as the Source of New Variant Creutzfeldt-Jakob Disease in Humans
Writer’s comment: The review paper was my favorite assignment in English 102. It allowed me to become an “expert” on a current topic from my own field and then to summarize what I had learned in a way that both stretched and solidified my understanding of the crucial questions and how they are being answered. As a pre-vet student who had just spent the summer as a dairy intern, I found bovine spongiform encephalopathy—mad cow disease—to be the perfect topic to study. My fascination with the unique and mysterious nature of the disease led me through both the reading of complicated research papers and the process of organizing a review of the relevant articles.
- Harmony McPherson
Instructor’s comment: Harmony McPherson prepared this paper for my advanced composition course, English 102: Writing in the Biological Sciences. Examining the flurry of research that followed the epidemic of “mad cow disease” that disrupted British life in the late 1980s and early 1990s, Harmony concentrates on questions about the transmissibility of the infectious agent from cattle to humans. I was particularly impressed by her ability to process and organize information drawn from a healthy number of difficult articles and to write about this area of research in vigorous, “advanced” prose.
- Sondra Reid, English Department
Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases found in mammals and associated with a particular brain glycoprotein called prion protein (PrP). In the diseased state, PrP assumes a new conformation (designated PrPres, for proteinase-resistant, or PrPSc), which is laid down in deposits in various areas of the brain, accompanied by spongiform change of the surrounding tissue. TSEs, which include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans, are unique among infectious diseases in that they seem to be transmitted by the prion protein alone. When introduced to normal neural tissue, the PrPres form catalyzes the conformational change of PrP to PrPres, producing pathological changes over time. Furthermore, different strains of prion disease seem to be encoded by the protein’s conformation rather than its sequence (Collinge, Sidle, Meads, Ironside, & Hill, 1996).
The British BSE epidemic of the 1980s and 1990s, deemed to be the result of feeding beef protein products to cattle, raised concerns about the safety of beef consumption for humans. In 1996, these fears appeared to be confirmed by the emergence in Britain of a new form of CJD (Will et al., 1996). Since then, research on the possible link between BSE and new variant CJD (vCJD) has followed two lines of inquiry—identifying the infectious agent in both diseases and determining the potential for BSE transmission from cattle to humans.
The initial hypothesis that vCJD had a bovine source was based primarily on its temporal and geographical coincidence with the BSE epidemic. It was unlikely that a new TSE would arise spontaneously, and this was clearly a new form of disease in humans (Collinge et al., 1996). R.G. Will and his colleagues (1996) at the National CJD Surveillance Unit in Edinburgh studied the initial ten cases of vCJD, which were distinct from sporadic CJD (spCJD) and iatrogenic CJD (iCJD) in the relatively young age of the victims (18–41 years, vs. an average age of onset for spCJD of 65 years) and the atypical clinical symptoms seen. Post-mortem examination and cerebral biopsy revealed a consistent neuropathological profile of spongiform change and PrP deposits or plaques in specific areas of the brain. An unusual type of plaque with an eosinophilic center and surrounding spongiform change was seen in all vCJD cases, but these florid plaques had never been found in spCJD.
To determine whether this new form of CJD was pathologically similar to BSE, M.E. Bruce and a group of researchers in Edinburgh (1997) injected several strains of inbred mice intracerebrally with brain homogenate from spCJD, vCJD, and BSE cases. They found that in incubation times and neuropathological profiles, the mouse cases of vCJD and BSE were clearly distinct from spCJD but highly consistent with each other. The pathological signature of vCJD, based on semiquantitative scoring of disease effects in various areas of the brain, was, in fact, almost identical to that of BSE.
J. Collinge’s research team in London also compared the prion strains molecularly using Western blots (gel electrophoresis and immunostaining). Western blots of prion strains normally show three bands, representing the di-, mono-, and unglycosylated forms of the protein. When brain homogenates from sporadic, iatrogenic, and new variant cases of CJD were run on Western blots after proteolytic treatment, the vCJD prions showed a significantly higher proportion of the diglycosylated form than did the spCJD and iCJD cases. The researchers then compared the vCJD pattern to that found in mice and a macaque experimentally infected with BSE, and in a cat with natural BSE. In each case, the BSE prion displayed the same high proportion of diglycosylated form that vCJD had shown. These results suggested that BSE and vCJD prions are the same strain, perhaps possessing a special affinity for glycosylation due to their conformation (Collinge et al., 1996; Hill et al., 1997).
Transmissibility of BSE to Humans
Experimental indications that BSE and vCJD were caused by the same prion led to investigation of how humans could have been infected by cattle. The ability of a prion disease to transmit between species depends primarily on the degree of homology between the infective and host prions (Collinge et al., 1996). The slowed conversion of PrP to PrPres due to differences in prion amino acid sequence is called the species barrier.
American and British researchers led by G.J. Raymond (1997) carried out in vitro studies to estimate the effectiveness of the species barrier protecting humans from BSE. They found a correlation between the efficiency of in vitro conversion of PrP by PrPres strains and the previously determined transmissibilities of those strains. For example, no conversion is seen for hamster prion and a particular genotype of sheep prion, which are known to be resistant to BSE in vivo. BSE PrPres had low efficiency of conversion of human PrP; it changed no more than 2.5% to the disease-causing form, a rate that represents 10% of BSE’s intraspecies efficiency. This rate was similar to the conversion efficiency of scrapie PrPres on human PrP, and on this basis BSE was suggested to be no more of a threat to humans than is scrapie, which has never been linked to human disease (Raymond et al., 1997).
However, evidence from in vivo BSE transmission studies challenged this theory. Collinge’s group used transgenic mice expressing human PrP; these mice developed disease after intracerebral inoculation with either BSE or vCJD, although incubation periods were prolonged and efficiency of transmission was reduced relative to typical CJD (Hill et al., 1997).
Other researchers tried to estimate the danger of BSE to humans by studying primates. French scientists successfully produced vCJD-like disease in three macaques by intracerebral inoculation with BSE (Lasmezas et al., 1996). These results may be particularly valuable because Old-World primates are believed to be the most closely related to humans, and macaque PrP is 96.4% homologous to the human variety. In Britain, BSE was transmitted to four marmosets by intracerebral inoculation, though the florid plaques typical of vCJD were not observed (Baker, Ridley, & Wells, 1993). The marmosets in this study also showed prolonged incubation times (49 months vs. 17 months for passage between cattle), suggesting that a sizable species barrier may exist between ruminants and primates, including humans.
Although these studies indicate that BSE is a threat to primates when injected into the brain, human exposure to the disease has likely been primarily oral, a route of infection that may be 105 to 109 less efficient than the intracerebral pathway (Baker, Ridley, & Wells, 1993). Yet there is evidence that oral exposure can still be lethal. N. Bons et al. (1999), for example, studied naturally occurring spongiform encephalopathy in 20 French lemurs that had been fed beef protein from British cattle during the BSE epidemic. They found neuropathology similar to that in two other lemurs that were experimentally infected with BSE by being fed contaminated bovine brain, indicating that the animals with naturally occurring disease were likely infected with BSE by eating beef protein and that the oral pathway indeed holds danger for humans as well. However, counter-evidence has been presented by Baker, Ridley, & Windle (1996), who note that none of more than 100 marmosets in their colony developed spongiform encephalopathy (except those experimental animals injected intracerebrally), despite each being fed daily meat meal supplements from British cattle for five to ten years during the epidemic.
In spite of the apparent link between vCJD and BSE, the bovine epidemic seems to have had little impact so far on human health. P. Aylin and his colleagues studied English and Welsh death registrations for those in occupations considered to have high risk of contracting BSE—i.e., butchers and slaughterhouse workers, farm workers, and veterinarians. Between 1979 and 1996, there was no increase in deaths from CJD (all types), and no significant trends were observed in deaths from dementia in general. No deaths from CJD of any type were recorded during this period for veterinarians or butchers (Aylin, Bunting, De Stavola, & Coleman, 1999).
C.M. van Duijn et al. (1998) also attempted to establish risk factors for CJD by surveying 405 CJD patients (all types) from six European countries. Although consumption of raw meat slightly increased the risk of contracting the disease, no other significant associations were found that related to cattle, whether through occupation, animal exposure, or diet.
The molecular and neuropathological evidence relating vCJD and BSE prion strains leaves little doubt that the human disease arose from a bovine source. But the method of transmission is less clear. Oral transmission is the most likely method of entry into the human population of BSE, but more evidence for its likelihood and a better understanding of the species barrier need to be gained before the potential danger of the disease to people can be predicted.
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