VCJD transmission

BSE is not transmissible to humans. However, there appears to be a strong connection between BSE and a variation of Creutzfeldt-Jakob disease, known as variant Creutzfeldt-Jakob disease (vCJD), another disease grouped with other TSEs. Evidence to date indicates that there has never been a case of vCJD transmission from person to person, but rather it is thought to spread from the consumption of cattle products contaminated with BSE. BSE and vCJD share many characteristics, to the point of being nearly indistinguishable from each other. Clinical studies have shown that mice inoculated with BSE showed the same pattern of incubation time, clinical signs, and brain lesions as mice inoculated with tissues from patients with vCJD. This provides evidence that BSE and vCJD are of the same strain . Furthermore, these two diseases were not similar to other TSEs such as sporadic CJD and known scrapies strains. 

Transmissible spongiform encephalopathies (TSEs)—Brain diseases transmitted from one animal to another. Under a microscope, the brain tissue of animals and people with TSEs resembles a sponge. TSEs include variant Creutzfeldt-Jacob disease (vCJD) in humans, scrapie in sheep and goats, and bovine spongiform encephalopathy (BSE) in cows (mad cow disease). These diseases are spread by consumption of brain tissue and are thought to be caused by prions, a kind of protein. 

Because they are derived from cattle, there is a concern that gelatins might be vehicles for the transmission of the prion agent responsible for bovine spongiform encephalopathy (BSE) in cattle and variant Creutzfeldt-Jakob disease (vGD) in humans. There is at present no evidence that these products have contributed to the transmission of BSE or vCJD. However, the incubation period may be up to several years, and due prudence is warranted when such products are used. 

In human populations, exposure to the BSE agent (probably in contaminated bovine-based food products) has been strongly linked to the 1996 appearance of a new transmissible spongiform encephalopathy of humans called variant Creutzfeldt-Jakob Disease (vCJD). 

A new human transmissible spongiform encephalopathy, variant CJD or vCJD, is identified and distinguished from classic CJD. The UK bans the use of mammalian MBM in feed for all farm animals. The UK introduces slaughter scheme to keep cattle older than 30 months out of food and feed chains. Cattle passports are made mandatory for all cattle bom beginning July 1, 1996. The United States FDA imposes a ban on feeding MBM protein to mminants (36). 

However, the emergence of variant Creutzfeldt-Jakob disease (vCJD) in the UK and France has raised concern about a new theoretical risk of infection in patients treated with blood and blood products (198). Animal experiments in which blood from sheep infected with bovine spongiform encephalopathy and natural scrapie-infected sheep into scrapie-free recipient animals have suggested disease transmission by the blood transfusion route in 2 of 24 sheep with bovine spongiform encephalopathy and in 4 of 21 sheep with scrapie (199). Many European countries have incorporated leukodepletion of all blood products, as leukocytes are believed to play a key role in the pathogenesis of variant Creutzfeldt-Jakob disease (198). In some countries, people who have lived in the UK for a period longer than 6 months between 1980 and 1996 are excluded from blood donation (13). Furthermore, it has been shown that various steps used in the manufacture of plasma-derived products also contribute to reduced infectivity by bovine spongiform encephalopathy (198). 

Bishop MT, Hart P, Aitchison L, Baybutt HN, Plinston C, Thomson V, Tuzi NL, Head MW, Ironside JW, Will RG, Manson JC (2006) Predicting susceptibility and incubation time of human-to-human transmission of vCJD. Lancet Neurol 5 393-398... 

However, it appears that not all TSEs are transferable to all species. Scrapie disease in sheep is not known to be infectious in humans, although it seems infectious in cows. Thus, there appear to be species barriers to infectivity. The potential infectivity of CWD disease to humans is not known, although testing of game animals taken by hunters in the CWD-infected areas has been encouraged. The mechanism for the horizontal spread of CWD in deer and elk is without explanation, and it is not clear if transmission to humans is possible, or what the disease would look like if it were transmitted (Xie et al., 2006). It is intriguing to speculate that prion infectivity could leap from one species to another nonpermissible species by way of a permissible intermediate, possibly as evidenced by sheep scrapie to mad cow to human vCJD. 

The central issue in the progression of disease is how PrP = propagates itself from the source of exposure (food) to the brain. In the examples provided by kuru, the recent mad cow epidemic in the United Kingdom, and the resultant transmission of vCJD to humans, the common link is that infectious prions in contaminated food reached the brain by some mechanism, and once in the brain, seeded the conversion of native host PrP to an infective PrP conformation. 

Prion diseases In the 1990s, four patients were infected with variant Creutzfeldt-Jakob disease (vCJD) after transfusions of non-leukodepleted blood. The incubation periods in the recipients were 6.5-8.3 years after transfusion. Leukocytes are now removed from blood used for transfusion [60, 96, 9T]. Plasma products carry a low risk of transmission of transmissible spongiform encephalopathies because of production processes [96 ]. The estimated risk depends on the prion load. Of plasma products, vCJD-implicated batches of clotting factor concentrates were categorized as likely to transmit prion diseases [9T]. In the UK, and before the exclusion of the use of British plasma, the risk of prion transmission by blood was estimated to be 7-14 per 10000 patients with hemophilia, assuming a vCJD population prevalence of 1 in 10000... 

Prions The use of bovine thrombin in fibrin sealants increases the risk of transmission of bovine spongiform encephalitis (BSE) [11 ]. However, the risk of transmitting prion diseases by giving human blood or blood products is theoretical at present. Because of the long incubation time it is challenging to evaluate the risk [2 ]. In 168 UK cases of variant Creutzfeldt-Jakob disease (vCJD), nine patients had received fractionated plasma products on 12 occasions, intramuscular immunoglobulins for travel on four occasions Rh(D) immunoglobulin for rhesus... 

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