Transmissible Spongiform Encephalopathies and Bushmeat

What are Transmissible Spongiform Encephalopathies?

History of TSEs

It was thought that there were three ways in which prion diseases manifest; sporadic, familial and via an infectious prion protein PrP^res. But current evidence is pointing to the probability that all prion diseases are caused by a "small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids" (Prusiner 1982).

Prion diseases have been around since the 1700s when they were first identified in sheep, the disease was named scrapie because the sheep developed behavioral changes that caused them to scrape their heads against walls and fences. Cuilli and Chelle in 1936 recognised the infectious nature of the disease and its filterable properties (Chesebro 2003 and Wickner 2004).

Kuru is a human TSE and was endemic in New Guinea where it arose among cannibalistic tribes, for example the Fore. The Fore population carried out a ritual where by when a person died the family ate the body (including the brain) the infectious prion proteins that cause the disease were found to be in abundance in the brain and spinal cord. Consumption of the brain was thought to be the mode of transmission. The kuru epidemic began in 1901 and reached its peak in the 1950s, in the 1950s cannibalism discontinued hence nobody from kuru endemic regions born after 1960 has developed kuru (Collins et al. 2001 and Goldfarb 2002).

TSEs were thrust into the limelight in the 1986 when the first case of BSE was recorded. BSE originated from the feeding of rendered cows to cows in the form of meat and bone meal (MBM).

The number of cases of BSE in the UK.

During the late 1980s and early 1990s there was an epidemic of BSE among the UK's population of cattle. Even after the initial feed ban (in cattle and sheep) in 1987 there proceeded to be a number of cases of BSE, this is due to the long incubation period that all TSEs manifest. Kuru for example can incubate in humans from 2 to 40 years (Chesebro 2003 and Goldfarb 2002). The second feed ban was to stop the feeding of MBM to pigs and other farm animals. The human prion protein disease Creutzfeld-Jacob disease (CJD) was thought to occur sporadically in the elderly, incidence was 1-2 people per million a year. But in 1995 a couple of teenagers (a group where CJD was extremely rare) developed CJD symptoms and more cases were identified in 1996. In 1996 the British government stated that there was a link between BSE and the new vCJD which lead to a worldwide ban on British beef (Smith 2003).

There is a correlation between the increase in cases of BSE in the late 1980s and early 1990s and the later increase in cases of vCJD during the mid 1990s to the early 2000s (Chesebro 2003):

^a Cases outside the UK: 6 in France; 1 in Ireland; and 1 in Italy. There has also been 1 case in the USA and 1 case in Canada of people who had previously lived in the UK in the 1980s and early 1990s.
^b Includes 27 without neuropathological confirmation.

There are fears that vCJD has infected more people but the disease has not manifested because the prion disease is incubating in the body. Scientists at the moment are testing for the presence of vCJD in tonsils and appendixes to gage the prevalence of vCJD in the UK (Smith 2003).

The Prion Protein (PrP)

PrP^c protein is the wild type form of the PrP (cellular PrP) and it is found abundantly in neurons and glial cells of the central nervous system and it is also found in lymphoid and periphery tissues (Harris 1999 and Wiliams and Miller 2002). The PrP^c protein (ORF) is encoded by the Prnp gene which is in a single exon, this exon encodes a protein of 250 amino acids but it does vary in different mammalian species. It is attached to cell surfaces via a glycosylphosphatidylinositol (GPI) (Martins et al. 2001).

Weismann et al. (1992) were the first group of people to generate a PrP^c knockout mouse (PrnP mouse, strain PrP^-/- Zrch). They carried out behavioral, anatomical and immunological tests on these mice and wild type mice to ascertain the function of PrP^c. They did not find any differences but they did find that these PrP^-/- mice were resistant to the scrapie strain of PrP (PrP^sc) (Martins et al. 2001). Incidentally the infectious isoforms of PrP were named after scrapie and is therefore called PrP^sc. A different group of PrP^-/- mice (Ngsk) showed symptoms of ataxia (inability to control involuntary movements) due to the loss of Purkinji cells in the cerebellum, therefore PrPc was found to have a role in the survival of Purkinji cells (Harris 1999, Martins 2001).

PrPc binds to laminin. Laminin is important in neuronal differentiation and survival, PrP^-/- mice were found to have a reduction in axonal extensions. PrP-laminin is also important in the adhesion of neuronal cells. Martins et al. (2001) also generated the possibility of PrP having a function in signal generation due to the presence of PrP^c on the synaptic membrane in neurons. The following table shows the percentages of both alpha helices and beta sheets in PrPc and PrPsc. This data was deduced by infrared spectroscopy and circular dichroism (Harris 1999). PrP^c is converted post-translation into the PrP protease resistant form, PrP^sc by exogenous PrP^sc prions that enter the body (normally via ingestion). The normal tertiary structure of PrP^c consists of a high proportion of alpha helices but infectious PrP^sc prions induce a conformational change of these alpha helices into beta sheets, the proportions of which are shown above in table 10. The mechanism by which this conversion occurs is still a matter of debate.

There is a lot of evidence that supports the idea that prions only consist of protein, the protein hypothesis. For example scientists so far have failed to find a scrapie prion nucleic acid. In 1967 Alper exposed PrP^sc to ultraviolet and ionizing radiation, this process usually obliterates nucleic acids but PrP^sc was almost completely resistant. The following figure shows the hypothetical size of the scrapie genome compared to other viruses in the virino-hypothesis. It also shows the amount of X-ray radiation needed to inactivate scrapie and other viruses by 37% (Chesebro et al 2003). Due to there being a number of different strains of scrapie some scientists believe that nucleic acid is the basis of prion diseases, which lead to the development of alternate hypotheses. One hypothesis suggests the possibility of the nucleic acid being extremely small (2-4 kbp see figure 9) and encapsulated in a protein coat (the coat hypothetically would be extremely tough and hence explain resistant nature of the prion), the virino-hypothesis (Chesebro et al 2003 and Sato and Castillo 2004).

The Species Barrier and Susceptibility

A number of experiments have been carried out on animal models for example NHPs, rats, mice and hamsters. Scientists found that intraspecies transmission (within members of the same species) of infectious prion proteins (or transmission to a very closely related species) occurs more easily and has shorter incubation periods compared to interspecies transmission i.e. between different species (Chesebro 2003 and Wickner et al. 2004). This is due to the species barrier. The species barrier is the molecular differences between species, which is represented by differences in the PrnP gene.

If the infectious prion protein is transmitted to a different species the incubation period may be longer than that seen in transmission to a member of the same species but if the prion protein is infectious in the new species then successive infection of members of this species can take place. With each successive inoculation the prion protein becomes more infectious and the incubation times reduce in each successive inoculation. For example BSE can be experimentally transmitted from cattle to mice but due to the species barrier transmission mice are 500 times less likely to become infected compared to cattle (UK Research and Development of TSEs 2004).

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Chronic Wasting Disease

"The forest and woodland is often known as the bush and therefore the wild animal products derived from the bush are known as bushmeat." This is my definition of bushmeat. In the United States and Canada wild deer and elk are hunted for their meat, therefore they are types of bushmeat.

Scientists are unsure as to where and when CWD first arose in mammals, CWD was first identified in Colorado (1967) in a captive mule deer (Williams and Miller 2002). Chronic wasting disease or CWD is a TSE of three species of the Cervidae family; the Rocky Mountain elk (Cervus elephus nelsoni), white-tailed deer and the mule deer (Odocoileus spp) (Belay et al. 2004, Sigurdson et al. 2001 and Williams and Miller 2002). CWD is present in both free-ranging elk and deer populations as well as populations in game farms. In WY, UT and NM CWD is only present in free-ranging deer and elk populations in the other shaded areas CWD is found in farmed deer and elk and the "black" states have both farmed and free-ranging deer and elk (Chesebro 2003). CWD is also found in Canada and has even been found in the Republic of Korea via the importation of infected animals (Belay 2004 and Williams and Miller 2002). Prevalence of CWD in cervid populations has been found to vary, cases of CWD are higher in densely populated herds. Williams and Miller (2002) reported that in one survey 90% of mule deer on a game farm died or had to be put down because of CWD infection. Sigurdson et al. (2001) and Raymond et al. (2000) stated that prevalence of CWD among deer and elk in Wyoming and Colorado is roughly 15%.

Prevalence of CWD in the USA.

Clinical manifestations of CWD include behavioral changes, a decreased appetite and increased salivation, ataxia, polyuria (excretion of large amounts of urine) and polydipsia (excessive thirst), head tremors and weight loss, "wasting" (Belay et al. 2004 and Williams and Miller 2002). Therefore the only other way of ascertaining visibly whether a cervid is infected other than looking for the clinical signs is via a postmortem. CWD is experimentally identified using immunohistochemistry (IHC) on tissue samples or via the discovery of a spongiform appearance of the brain. CWD is characterized by lesions which can be found in various regions of the brain for example the medulla oblongata, thalamus and hypothalamus as well as the spinal cord, eyes, endocrine organs and lymphoid tissues that run along the alimentary canal (Belay et al. 2004, Miller 2000 and Sigurdson et al. 2001).

Cases of CJD and GSS have been investigated because of a possible link to CWD infection (Belay et al. 2004). Due to elk and deer both being types of bushmeat there is a risk of CWD being transmitted to humans especially if CWD infected animals are consumed. There have been reported human fatalities of CWD via the consumption of CWD infected animals (Hoey 2003). But many of these supposed CWD fatalities were disregarded after data and tissue samples were analysed further to show that many of these people had died of natural causes, developed sporadic CJD or had GSS even if they had consumed deer or elk from CWD endemic areas in the past (Belay et al. 2004).

Raymond et al. (2000) carried out important studies into the effect of the molecular species barrier and its effect on the conversion of human PrP^c. They tested (in vitro) the level of conversion of PrP^c (both the Met-Met and Val-Val allelic forms) by PrP^CWD. Results showed that the level of folding was 14 times lower than that seen between inter-cervid conversions and it is 5 times lower compared to the conversion of PrP^c by PrP^sc (from CJD patients). They also carried out statistical tests on the obtained data and concluded that these results were not significant therefore the differences in the PrP proteins are great enough for CWD not to be passed onto humans.

Oral transmission between cervid species is thought to propagate CWD for example contaminated pastures and water supplies. There is a distinct possibility of infectivity being transmitted to sheep and cattle that graze in the same areas as infected cervids (Belay et al 2004). Raymond et al. (2000) carried out PrP conversion studies to ascertain the susceptibility of cattle and sheep to CWD. They found that conversion of cattle PrP^c to PrP^CWD was 5-12 times lower than that seen between inter-cervid conversions. Sheep with the AQ allelic PrP^c form were two-fold less capable of PrP^c conversion by PrP^CWD compared to inter-cervid conversion but the AQ PrP^c could be converted more easily compared to human and cattle CWD conversion.

Although experimental evidence states that there is no significance in the conversion of human PrP^c by PrP^CWD (Raymond et al. 2000) there is still a fear that with the increasing prevalence of CWD in cervid populations and the possibility of CWD infecting sheep and cattle there could be another epidemic of TSEs in humans similar to that of the vCJD outbreak in the UK in the 1980s-1990s.

© MicrobiologyBytes 2007.