Could Prions Cause “Mad Dog” Disease?
From Today’s Scientific American 60-Second Science Podcast an interesting — and rather chilling – report on prion diseases.
Prions are abnormally shaped proteins that spread disease by causing normal proteins to mis-fold into their abnormal shape. Genetic material like DNA or RNA is not involved in the creation or spread of prions. Prion diseases affect the brains and central nervous systems of many mammals — including humans. And though we humans can catch mad cow disease from eating infected beef; prions were believed jump the species barrier only rarely.
As reported in SciAm:
A study published in the September 4th issue of the journal Cell investigates that issue. Scientists from Texas, Spain and Chile took normal hamster proteins and mixed them with misfolded mouse ones. And the mouse prions were able to change the hamster proteins into a new kind of prion that infected both healthy hamsters and mice. The test tube is obviously an unnatural situation, but it shows that prions can leap the species barrier without the aid of any other infectious agent. Scientists hope to learn more about how this process works so they can keep prions in their place-away from humans.
Back in the 90’s, thousands of cattle in Great Britain collapsed from a mysterious malady now known as mad cow disease. And at that time experts assured us that people couldn’t get the disease from eating beef. The species barrier – “a kind of biological Hadrian’s wall” — would protect human carnivores from harm.
As reported in the New York Times back in June, 2003:
So much for scientific hubris. After young people began dying in 1996 from a new variant of Creutzfeldt-Jakob disease, a rare malady thought to occur only in older people, the experts were forced to admit they were wrong.
The people had died because they ate meat from infected cows. A species barrier had crumbled.
Prion diseases, (e.g. BSE, FSE, scrapie and other spongioform encephalopathies) are poorly understood though they occur in most mammals. Scientists have known for some time that prion diseases can be spread when animals cannibalize each other (i.e. eat the remains of others of their own species). Prion species are problematic for several reasons:
- They often incubate for long periods of time before they cause detectable symptoms.
- Symptoms of prion diseases can be highly variable, both between individual animals and between species.
- Some prion diseases can be transmitted from one species to another – but others appear not to cross the species barrier.
- Some animals may act as unaffected carriers of these diseases, transmitting them without being affected themselves.
- Because of the long incubation time common in prion diseases, some species may pass them on before being affected themselves.
- In some cases an intermediate species may act as a host to a subclinical form of a prion infection and then act to transport it between two other species.
- Prions are astonishingly resistant to disinfectants. Bleach, ultraviolet radiation, heat and other common sterilizing agents have little or no effect on them.
- Some evidence indicates that prions can be inherited – passed from parent to offspring before birth.
Research on species barriers and prion diseases is fraught with enigmas, dead-ends, contradictions and confusion.
One of the main jobs our cells carry out is folding up proteins, the workhorse molecules of our bodies. Each protein molecule can be folded in several different ways before it finds its final shape and function. Mistakes can occur during folding but mis-folded proteins are usually found and disposed of by enzymes.
Mis-folded prion proteins are resistant to discovery and/or disposal by enzymes (or stomach acid). The prions remain in the body and may cause damage – like the characteristic spongelike holes in brain cells found in BSE and CJD.
Humans can suffer from prion diseases. Can dogs?
The answer so far has been “no”, but it seems more accurate now to say “we don’t know.”
No evidence of a canine prion disease has been discovered yet (though a feline version, FSE, exists) – but there is currently no way to test for these diseases in live animals. Brain and spinal cord biopsies are the only diagnostic tool currently available so it is possible that a canine prion disease may exist, but have detectable symptoms that are remarkably different from those seen in cows and humans.
If this is the case, such a disease could be misdiagnosed for some period of time before it is discovered.