Related Prion Diseases I

BSE belongs to a group of related diseases known as the transmissible spongiform encephalopathies (TSEs). The causative agents of these diseases are thought to have very similar characteristics. All cause fatal spongiform changes in the brain.

The group includes:

• scrapie, which affects sheep and goats (USDA-APHIS additional information on scrapie)
• transmissible mink encephalopathy
• chronic wasting disease of mule deer and rocky mountain elk
• TSEs have also been reported in Great Britain in kudus, an eland, a nyala, and a gemsbok (4 types of antelope)
• feline spongiform encephalopathy
• four rare diseases in humans

The incubation period of these diseases tends to be quite long. Mice inoculated with BSE brain homogenates develop neurological signs of the disease 300 to 450 days later, indicting prolonged incubation period prior to onset. While onset of BSE is observed in mature cattle, the prolonged incubation period indicates that exposure occurs in calfhood. Calves have been estimated to be at much higher risk of exposure (up to 30 times) than adults. The incubation period in cattle is commonly 3-5 years (but the range can be considerably wider). Scrapie has an incubation period of up to 60 months and has been endemic in Great Britain for 250 years.

Scrapie

Scrapie in sheep is endemic in all countries except Australia and New Zealand. One third of all British flocks have scrapie. It is estimated that about 1% of U.S. sheep have scrapie: scrapie is not a reportable disease in the US. Some 90-95% of cases occur in the Suffolk or Hampshire breeds. Typically no more than 5% of the animals in a herd are infected. Scrapie may be naturally transmitted between sheep through the placenta or by contact with the placenta, however, vertical transmission (from dam to offspring) has been disputed (Ridley and Baker, 1995).

Scrapie is a prion disease. Like BSE, it is caused by the transmission of protease-resistant prion proteins.

The scrapie agent can be recovered from:

• lymphatic tissue
• tonsils
• spleen
• thymus
• placenta
• intestinal tract
• as well as nervous tissue of affected sheep

The scrapie infective agent was not found in:

• skeletal muscle (red meat)
• heart
• mammary gland
• milk
• blood
• skin
• feces

Scrapie has an incubation period of 2 to 5 years. Incubation length may depend on the 'strain' of Scrapie.

Scrapie is an acquired infection. Infection can be from pasture where placental tissue carrying the agent is ingested. Scrapie can be selectively "bred out" indicating a genetic component.

The presence of multiple strains of scrapie indicate different initial causes, presumably sporadic or inherited. New cases of sporadic scrapie may appear periodically as may DNA mutations that result in scrapie; however determining the cause once the founder animal has been lost is difficult due to the means of tranmission.

It appears unlikely or at least extremely unusual for scrapie to be transmitted to man either occupationally or by eating sheep meat. While scrapie is common in sheep, CJD is very rare in man.

The clinical signs that sheep express with scrapie include:

• impaired social behavior - unusual restlessness, nervousness
• possible change in fleece color
• ataxia - loss of motor coordination, gait abnormalities (high-stepping, hopping, swaying)
• pruritus - itching with scratching and rubbing, biting of feet and limbs, lip smacking; resulting in skin damage and wool loss
• wasting, emaciation
• hyperaesthesia - extreme agitation, oversensitivity to stress (trembling or convulsions with stress)
• loss of ability to eat as a result of ataxia (not loss of appetite per se)
• eventual death