Mad Cow Disease and Variant Creutzfeldt-Jakob Dis (cont.)

Mad Cow Disease Causes

Prion diseases are unique and can be transmitted in a variety of ways:

• Some forms can be inherited such as familial CJD, Gerstmann-Sträussler-Scheinker disease (GSS), and fatal familial insomnia (FFI). The disease is caused by a gene mutation in the prion gene. Other inherited prion diseases are more rare.
  
  
  • Prion disease may develop sporadically, for no apparent reason and with no pattern such as sporadic CJD. Cases can occur in men and women of all ages, but the average age is 62 years. The prevalence of sporadic CJD is about 1 case per million people each year throughout the world, even among vegetarians. Sporadic CJD is the most common type of human prion disease.
    
    
  • Sporadic prion disease may be introduced into a human through infected surgical instruments or transplant tissues.

• Infectious prion disease such as new variant CJD (nvCJD) is likely caused by eating BSE-infected meat from cattle. No cases have been reported in the US (one Florida case occurred in a woman who had lived in England).
  
  
• A few seemingly sporadic cases in young men in Michigan may point to transmission to hunters from deer or elk with chronic wasting disease (CWD), a form of animal prion disease. CWD is becoming common in this animal population in the US. Because hunters eat the meat of these animals, there is a potential for the prion disease to cross from hunted to hunter.
  
  

One additional form of human prion disease is called kuru. It was found among native peoples of New Guinea who practiced ritualistic cannibalism (eating of humans, and their brains, specifically). Probably the illness began when a person with sporadic CJD was eaten. Death from kuru takes about 1 year following the onset of symptoms; however, the average incubation period was about 12 years and can be as long as 40 years. With the end of such rituals, this disease has nearly disappeared in New Guinea.  

The disease has been experimentally transmitted among cattle and from cattle to monkeys who eat infected tissue (particularly brain tissue) in lab tests.  

But the question remains: How do cattle develop BSE? Feed is the major route for transmission among cattle, according to veterinary medicine experts at Iowa State University. When ranchers and farmers feed cattle with products made from other cattle or sheep, such as ruminant feed, they are recycling diseased animal protein in feed containing meat and bone meal, thus causing the disease in cattle.

Prions may be transmitted among laboratory animals also through broken skin, suggesting the possibility of similar transmission to humans who contact infected tissue or products and who have broken skin.