Recently, an animal-rights advocate tried to persuade me to become a vegetarian. Among the health benefits he claimed was reduced risk from food-born diseases.
He claimed that if a plant protein turned into a disease-causing prion, your immune system would destroy it. He claimed that prions from animal protein could sneak in under the radar and cause diseases such as Mad Cow Disease.
He also claimed that bacteria and viruses were significantly more likely to cross from one species to another if the species were closely related (i.e. vertibrate to vertibrate) than if the species were distantly related (i.e. plant to vertibrate).
He could not cite any literature to support this thesis, but he claimed that scientific studies had been published to support it.
Can any of the physicians or scientists among the Dopers either confirm or refute this?
I’m not a scientist or a physician, but I think that’s bovine excrement. Many plants contain alkaloids, which are toxic. It’s a defense mechanism against predators. Many of those plants are quite toxic to us. Toadstools, for example, are very poisonous. Some cause minor ailments, but others can cause serious lung infections.
Many microorganisms, BTW, are fungi, which are plants: histoplasmosis, sporotricosis, madura foot, etc.
I’m not sure about prions, but here’s my two cents. As I understand it, Mad Cow Disease is just one of many afflictions that mammals develop when they are fed things their bodies are not designed to digest. Cows are herbivores. In the interest of saving money, they are fed a blended mix of just about anything laying around. This may include hay or normal food, but may also include newspaper, shredded plastic, excrement, dead cows and other animals (that for one reason or another aren’t slaughtered for human consumption).
Once the disease gets in to a cow (or chicken, or what have you) it can afflict any animal that eats the meat of the diseased animal. I’m not sure of the specifics about how this happens, but I’m sure someone can answer that one seperately.
As far as vegetarianism vs. omnivorism goes, it’s a case of how high on the food pyramid you choose to eat. If you eat a cow, you eat it, and (ideally) the hay it’s eaten. (Although you also get the pesticides that went on the hay, and the Miracle Grow that the farmer sprayed on it while it was growing, concentrated in the cow’s system.) If you eat a chicken you eat it and (ideally) the grain it’s eaten. If you eat a wolf or bear you add another level on the pyramid (but then, wolves and bears aren’t usually farmed for food, so let’s forget them). Eating a vegetarian diet cuts out the middle “man”. Since cows and chickens and fish are fed all sorts of antibiotics, steroids, “vitamins”, and other products along with their barely digestable diet, when you eat them, you eat all of that, as well.
When you eat vegetables and soy products, you have a more direct say as to what you eat. If you eat organic, or grow your own, you can even be sure that you’re not getting pesticides and the rest of the things that many farmers put on their crops.
I’m a vegetarian, but not a doctor (I feel like I should draw a Venn Diagram)… But this is my understanding (do correct me if I’m wrong):
Prions do exist, and are the cause of some infectious diseases. Two human diseases caused by prions are Crutzfeld-Jacob Disease (CJD) and Kuru. The idea of prions are they modify normal proteins in the body to be like the prion, eventually killing the host. The disease is then transmitted when the body of the host is consumed by other animals. In general, they are transmitted only by eating animals (humans included), not from plants. Unlike other infectious agents, prions contain no DNA/RNA. Like viruses and bacteria, prions are just another category of infectious agent.
In the case of mad cow disease (CJD in humans, Bovine Spongiform Encephalopathy (BSE) in cows), it is believed to be transmitted by eating cows which have BSE. The worry is that lots of cows have BSE since other animals are routinely used in the feed of cows.
To my knowledge, the human body does not have an immunological response to prions, and they are quite difficult to destroy.
On the other hand, I rarely talk about prions in trying to promote animal rights. It’s much easier to talk about heart disease, cancer, obesity and related disorders, to say nothing of the antibiotic resistant bacteria. These are far and away bigger killers than prions. Oh, and cows are just so damn cute :).
Short, you are absolutely correct. You might have added that prions are proteins and that, apparently, they exist in everyone’s brains, but something goes wrong and they become modified and then modify all the other prions, degrading them as well.
Vegetables are lower in the eating chain, but that doesn’t mean that they are disease causing free.
I’ve never heard of a plant disease caused by prions.
Basically true. You won’t get tobacco mosaic by consuming infected tobacco. Nor, AFAIK, can you get any other plant disease.
On the other hand, non-meat food can be a source of infection just as meat can. You can get salmonella, cyclosporidiosis, hepatitis A, and other stuff from lettuce, sprouts, and other plants you put on your salad. For instance, if lettuce is fertilized with manure from a cow that has E. coli O157:H7 in its gut, then that lettuce could have the disease-causing bacteria on them.
IANAB, but I believe fungi have been put in a different kingdom than the plants. I don’t know which one it is though.
I have also yet to hear of any plant prion diseases. However, if any were to arise, they would be just as dangerous as animal prion diseases. There is no reason for the body to selectively destroy plant proteins over animal proteins. I would say that the development of plant prion diseases would be pretty unlikely, but they would not be safer than animal prion diseases.
As for viruses, there are known viruses that infect both plants and animals, but they’re rare and poorly understood. Generally speaking, it is true that closely related species are more likely to share viruses than distantly related species. However, there are a lot of other factors involved as well.
And finally, fungi are indeed their own kingdom, neither plants nor animals. There are a lot of very significant differences.
First of all I have personally cared for 3 patients who were infected with CJD. It was truly a gut-wrentching experience for all involved. I live in a fairly small town and as far as I know, none of these patients knew or had any common contacts with each other. But anyway…
You forgot to elaborate on Kuru.
Kuru is a disease nearly identical to CJD, possibly may even be the same thing as that and Bovine encephalopathy, at any rate it is a disease that was found only in certain canabalistic tribes. It seems these prions live in the central nervous system, of which the brain is a part.
Kuru is passed on by tribe members eating the brains of othere tribe members who were infected with the disease.
Is there any difference at all between CJD and BSE? Or are they the same thing in two differnet loci (humans & cows)?
That was a plot point in the novel Shadows on the Aegean by Suzanne Frank. (Not a very good novel, BTW.) She attributed the downfall of Minoan civilization to – well, the Thera eruption certainly screwed things up big time, of course, but there was also – ritual human cannibalism of brains. She describes slicing a thin section of someone’s brain and holding it up to the light: tiny pinpricks of light show through. Then they munch it. Most of them come down with CJD.
IIRC, proteins normally change their physical configuration in response to chemical signals in order to fulfill various functions. Pathogenic prions change in such a way that they are no longer able to unfold. That leaves the pathogen almost indistinguishable from a normal protein–it’s chemical composition and structure is almost identical to something that’s supposed to be present. (This also makes blood tests a problem.) The problem is that it doesn’t go away, and it causes other molecules to become similarly “stuck”. I think the folded configuration also stabilizes it somewhat against heat, so cooking is less likely to break it down.
While prion diseases are pretty horrifying, they’re not very impressive as an argument for vegetarianism. As dtilque pointed out, you can certainly become ill, even fatally so, from eating plants (even non-toxic ones). You can also fall prey to various dietary deficiencies, particularly B12, if you don’t plan your diet very carefully and take proper supplements. I wouldn’t entirely rule out the possibility of plant prion diseases, though I submit that they’re unlikely on any large scale–how would they spread through plant populations?
The thought is that BSE is just the cow version of CJD. The prions are still the same.
I’ve never heard of plant prions either. There is some evidence that prion-like protein behavior is seen in a specific yeast protein, but this protein is not PrP, the protein responsible for CJD and BSE. So don’t worry about catching it from yeast.
Has this been proved? I’ll admit the evidence is compelling, but I didn’t know that the existence of rogue “infectious proteins” that warp DNA had actually been proved.
Mjollnir, there is no question that prions exist–we have even identified the genes that code for them. Likewise, it seems generally accepted that they are involved in various forms of spongiform encephalopathy in several species. The debate among researchers (which may or may not have been settled by now) is over whether or not malformed prions are infectious on their own. They certainly don’t affect DNA; the prion theory of infection holds that warped prions cause similar malformations to arise in other prion proteins. This has been shown to occur in an experiment in which normal human prions are exposed to BSE prions, resulting in a change to a form similar to the pathological PrP[sup]SC[/sup]. The results were referenced in a February 1998 article in Chemical and Engineering News, although I have not seen the full results. The same article also describes the viral theory of transmission, which holds that the changes in the protein are the result of exposure to viral nucleic acids. No doubt a bit more searching on Google would turn up more detailed and recent references.
I think it should be pointed out that there are many types of prions, and that some are as far as is known harmless. Some prions are needed, for instance one type of prion is found in yeast, and is necessary for the yeast to reproduce. So, prions are pretty ubiquitous in life. Each prion affects one kind of protean, and molds it into another kind, this may or may not be harmless.
