MadSci Network: Virology

Subject: Re: Food irradiation and viral diseases

Posted by Tom Wilson
Grade level: M.D. PhD, Pathology, Div. of Molecular Oncology, Washington University Medical School
City: State/Province:
Area of science: Virology

Interesting questions.

QUESTION 1:  In a broad sense, yes, there are numerous viral diseases spread

through food, most typically by what we call the "fecal-oral" route.  These

come about about when someone doesn't wash their hands before preparing food,

thus contaminating the food and anyone who eats it.  Hepatitis A virus is

well known for this route of spread, and frequently causes outbreaks, 

often from a single worker in a  fast food restaurant.  But as you can see,

this contamination is human-to-human spread at the point of food preparation,

and not infection of humans from viruses inherent in the food stuff itself.  

To understand the answer to this more focused question appropriate to the 

role of irradiation, we need to understand that viruses are "organisms" 

that grow by infecting cells and that are completely incapable of growth 

independently.   Contrast this with the situation with bacteria and parasites 

which grow independently, and are simply growing ON the foodstuff, sometimes 

as a natural result of the normal life cycle of that food item, sometimes as 

part of the process of "rotting".  So to rephrase your question, what you are 

asking is "are there any viruses that *infect* food organisms that can also 

infect humans?".  The answer to this question is "not very many".  This is  

because viruses have to bind to a specific kind of cell and be able to grow 

within it in order to propagate, which is to say they exhibit "cell tropism".  

As a result, most viruses are species specific, and most viruses that infect 

cows, for example, do not effectively infect humans, and when they do, the 

disease is often very attenuated.  There are definitely exceptions - e.g. the 

rabies virus can productively infect most mammalian species, and in fact you can 

get rabies from eating meat from a rabid animal (but I certainly hope that not 

very many rabid animals make it to slaughterhouse!).  But in the end, the 

contraction of a viral disease from a foodstuff itself would be a very rare 

occurence, and certainly viral diseases figure in very little in the potential 

benefits of food irradiation.

QUESTION 2:  To understand the answer to this question, we first need to

know the mechanism by which irradiation sterilizes things.  While many types

of  molecules are potentially affected by ionizing radiation (and in fact

some specific modifications in lipids have been used as a means of detecting

whether or not food has been irradiated) the only one that really matters is

DNA, or more generally nucleic acids, because these are irreversible.

Gamma rays cause mutations in nucleic acids, mostly strand breaks, any one

of which could be repaired by the cell, ANY cell (be it yours, the muscle

cell in the meat, or the bacteria).  But when there are too many breaks the

cell cannot recover its genetic information  and dies.  Implicit in this

last sentence is that there is a definite dose-response relationship, with

some organisms requiring more irradiation to be effectively killed.  The

second thing we need to know is that viruses are like cells in that they

all have a nucleic acid "genome", sometimes DNA, but sometimes RNA.  So they

are subject to the same kinds of damage and loss of "genetic information" as

any cell, and as a result are "killed" by radiation.  (I put "killed" in

quotes because it is debatable whether viruses are truly "alive" to begin

with, since they can't replicate indepently).

QUSTION 3:  Based on the above, I think you will see that no virus will be

resistant to the effect of irradiation.  Although I must admit I don't know

where viruses  fit on the dose-reponse curve (presumably they would be

killed easily since they have no independent means of nucleic acid repair).

QUESTION 4:  Now you're getting even trickier, even if you don't know it!

Mad cow disease is one of a class of diseases called "spongiform

encephalopathies".  Humans get  very similar diseases called

Creutzfeldt-Jacob and (much more rarely) Kuru.  These are "infectious"

diseases in that you can catch them from someone else, essentially by being

exposed to their brain.  Almost everyone now accepts (myself included),

however, that these diseases are transmitted by an extremely unusual

mechanism.  There is no infectious "organism" in the classical sense of the term,

but rather it is an "infectious protein" called a "prion"  that is

completely devoid of nucleic acid, i.e. there is NO genetic information.

This is way too complicated to detail here, but what appears to happen is

that a specific protein normally found in your brain (called PrP) can become

changed to an altered structure.  This stucture has the remarkable

ability to cause other normally structured PrP protein to adopt the altered

structural form, thus providing a means of "propogating" the change (and by

some unkown mechanism causing the disease).  If you happen to get exposed to

the altered PrP protein, that change can now propogate in you.  So, to

answer your question, irradiation would NOT be effective in eliminating the

risk of catching a prion based disease such as mad cow disease, since the

agent responsible is a protein and has no nucleic acid component to be

irreversibly damaged.  Highly unusual!!  (a further note - there is actually

no good evidence that people can get the PrP based disease from eating beef

from an "infected" cow, and this is actually UNlikely for many reasons.

Nonetheless, prudence would dictate that people not eat a cow that has mad

cow disease!!)

Allow me one final editorial comment.  It has been shown repeatedly that

food irradiaton is effective in sterilizing food and essentially eliminating

the risk of food-borne infectious disease, and safe in that nutritional

value is preserved and no "oogie-boogies" will come crawling out of the

food.  However, fear of those oogie-boogies has severely limited the use of

this procedure.  In the  majority of the USA, food is clean enough  that

sterilization is simply not necessary, but in certain rural areas and third

world situations, food sterilization by irradiation could be of real value, with no oogie-boogies.

I have enjoyed answering your questions, and I hope you find my answers

enlightening at some level.  I am always amazed by the insightful and intriguing questions that come up on this service.  Thanks!

Tom Wilson MD PhD