According to the Oxford English Dictionary, the definition of a pathogen is “a microorganism that causes disease,” such as a virus or a bacterium.  However, the latest addition to the pathogen family, an infectious agent that consists solely of proteins, does not quite fit the existing definition.  This infectious protein, discovered in the latter half of the twentieth century, is called a prion.

Unlike viruses or bacteria, prions are simply misfolded proteins and do not contain genetic material.  Yet they are infectious because they induce normal proteins to change their structures and convert to the misfolded form.  Creutzfeldt-Jakob disease (CJD) is the most common prion disease in humans.  Although a certain form of CJD has been reported in 12 countries around the world, a recent study suggests that the disease is caused by only one strain of prion.

Figure 1. (Left) Normal Protein. (Right) Prion protein.

A strain is a genetic variant of a microorganism.  Unlike humans and other organisms that reproduce sexually, microorganisms do not have species.  For example, there are numerous strains of HIV, the virus that causes AIDS.  However, these strains differ genetically and vary in their virulence and resistance to medications.

Strains of microorganisms are easy to identify based on their genomes.  However, prions do not contain genetic material, so it is necessary to research how different prion strains differ on a biochemical level.  This research is important because it could help elucidate how prions are able to jump from one species to another.

The form of CJD examined in this study is called Variant CJD (vCJD).  This form is particularly relevant to prion strain research because it is caused by a between species transmission.  Humans become infected after consuming meat infected with Bovine Spongiform Encephalopathy (BSE), more commonly known as Mad Cow Disease.  Therefore, it was found that the same form of prion causes both vCJD and BSE.

How did scientists prove that a common strain of agent is responsible for all vCJD cases?  They used transmission studies with mice species to define the strain characteristics of distinct cases of vCJD several countries.  If the transmission properties turned out to be very similar, the research would suggest that a common strain of vCJD was responsible for all the cases of the disease.

First, the researchers inoculated mice with brain material from human vCJD patients from four different countries.  Then, they calculated the incubation period, the time between infection and when the mouse began to be symptomatic.  Results were compared to two reference cases from the United Kingdom.

The results were very similar to the reference cases, thus supporting the hypothesis that a common strain of agent is responsible for all vCJD cases, regardless of geographic origin.  The only exception to the results was a disparity between the average incubation times of different mouse species.  Therefore, further research is necessary to determine whether there are any differences in strain properties between the cases of vCJD in the five countries.

The probability of you becoming infected with vCJD is less than one in a million.  So then why was there such a huge frenzy over Mad Cow Disease in the 1990s?  If you are one of the unlucky few infected with vCJD, your prognosis is dismal.  The prions cause your brain tissue to develop holes, resulting in the characteristic sponge-like texture, and eventually kill you.

Figure 2. (Left) Normal human brain tissue. (Right) Human brain infected with vCJD.

The first symptoms include anxiety and depression followed by unusual pain sensations in the face and extremities.  Then, you lose control of your muscles and you are no longer able to move or speak.  Think Alzheimer’s or dementia but on a much shorter timeframe.  Unfortunately, the disease is always fatal, and there is no known cure.

This study increases our understanding of vCJD, which is crucial to controlling the disease in the case of future outbreaks.  Further research is necessary, but these results are promising in their support of the hypothesis that there is just one strain of vCJD responsible for all cases.

Link to paper

Paper Citation: Diack AB, Ritchie D, Bishop M, Pinion V, Brandel J-P, Haik S, et al. Constant transmission properties of variant Creutzfeldt-Jakob disease in 5 countries. Emerging Infectious Diseases [Internet]. October 2012.

Link to figure 1

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