MadSci Network: 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