The Relationship Between Bovine Spongiform Encephalopathy,
Creutzfeldt-Jakob Disease & Prions
Bovine Spongiform Encephalopathy (BSE) or Mad Cow Disease is a
chronic, progressive degenerative, neurological disease of
cattle that has been linked to a form of Creutzfeldt-Jakob
disease (CJD), a fatal degenerative brain disease in humans. BSE
belongs to a family of diseases known as the transmissible
spongiform encephalopathies (TSEs). TSE animal diseases found in
the United States include scrapie in sheep and goats, chronic
wasting disease in deer and elk, transmissible spongiform
encephalopathy in mink, feline spongiform encephalopathy in
cats, and in humans: kuru, both classic and variant
Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker
syndrome, and fatal familial insomnia.
In November 1986, the first reported cases of BSE occurred in
Britain. During the period 1989 to early 2001, more than 1
million cases of BSE were identified. Other countries around the
world at this same time also reported similar cases of BSE.
These countries included Denmark, France, Germany, Italy,
Ireland, Belgium, The Netherlands, Spain, Portugal, Spain and
Switzerland, Canada, the Falkland Islands and Oman.
Symptoms of BSE in cattle include loss of muscular coordination,
inability to stand up, listlessness, decreased milk production,
and loss of body weight. Affected animals also show signs of
behavioral change, for example, nervousness, aggression and a
lack of interest in surroundings. The period from infection to
onset of the disease, known as the incubation period, is from
two to eight or more years. Once symptoms develop, animals
progressively deteriorate and die within several months.
Where is the BSE agent found in cattle? Autopsies of diseased
cattle brains revealed anatomical and morphological changes
consistent with that of spongy like texture, similar to that of
Swiss cheese. Current scientific research confirms that BSE
infectivity occurs in the brain, trigeminal ganglia, tonsils,
spinal cord, dorsal root ganglion, and distal ileum of the small
intestine of cattle experimentally infected with the BSE agent.
Research also confirms that BSE infectivity is in the brain,
spinal cord, and retina of the eyes of cattle infected with the
agent under field conditions. Although bone marrow has
demonstrated infectivity in experimentally infected cattle,
these findings are not conclusive.
What causes BSE? The general consensus among scientists and
researchers is that the causative agent believed to cause BSE is
an abnormal form protein called prion. This abnormal protein is
found in brain, spinal cord, tonsils and small intestines of
cattle. The exact origin of the causative agent, prion, is
somewhat controversial. Some researchers believe that prions
come from cattle bones, hide and other inedible tissue after
slaughter. While others believe that the origin of prions came
from sheep. Yet other sources recently indicated that prions may
be of human origin. Irregardless of the origin of prions, cattle
and other ruminants can become infected only after consuming
contaminated feed containing recycled animal tissues and prions.
Prions are extremely resistant to heat, ionizing radiation,
normal sterilization process and common disinfectants that
normally inactivate viruses and bacteria. This may explain why
rendered recycled animal tissue may still be infective after
processing.
Can BSE be transmitted from one cow to another cow or ruminant?
BSE is not a contagious disease. There is no evidence that the
disease is transmitted through direct contact or
animal-to-animal spread. The primary means by which animals
become infected is through consumption of feed contaminated with
the infectious BSE agent or prions.
Is BSE transferable to humans? Since the initial report of the
disease, there has been much speculation that it might be
transferable to humans through beef products. The appearance of
CJD in several dairy farmers in Britain in the early 1990s
heightened the alarm. In March 1996 the British Ministry of
Health announced the discovery of ten cases of a unique type of
CJD, called new variant CJD or vCJD. This type of CJD differs
from the classical form in that victims are all under the age of
42 (the classical form of the disease typically develops around
age 65), the victims display unusual psychiatric problems,
distinct brain tissue changes are identified during an autopsy,
and the victims have no family history of the disease. The
British government found that the victims may have contracted
the disease through contact with BSE-infected cattle before the
eradication of suspected animals had taken effect.
What causes vCJD? Since 1996, a number of studies have confirmed
that BSE in cattle can be transmitted to humans and cause vCJD.
Studies have linked the time and location of the BSE epidemic in
cattle to more than 94 human cases of vCJD found in Britain,
France, and Ireland. In studies in which scientists injected
monkeys and mice with brain tissue from BSE-infected cows or
brain tissue from vCJD-infected humans, the animals develop the
same type of brain degeneration. This degeneration is
distinguishable from the degeneration following injection with
brain tissue from cases of the classical form of CJD. The
British government in 1996 announced that BSE may be transmitted
to humans.
Creutzfeldt-Jakob Disease (CJD) form of human spongiform
encephalopathy caused by an infection of the brain, probably by
a particle called a prion. The disease causes fatal degradation
of brain tissue and produces a dementia that affects men and
women, often between the ages of 50 and 65. Some 90 percent of
cases progress to death within one year, sometimes within one
month. Symptoms include loss of speech, difficult swallowing,
rigid limbs, and contraction of the facial muscles, with death
often resulting from a complication following these symptoms.
The remaining 10 percent of cases develop dementia and may
slowly decline over several years. There is no record of anyone
recovering from the disease and there is no known treatment.
Creutzfeldt-Jakob disease is found worldwide but is relatively
rare, affecting about 0.9 people per million in the United
States. The mean age of death is 67 years. Deaths from
Creutzfeldt-Jakob disease are uncommon in people under age 50.
In people age 70 to 74 years, the disease causes 5.7 deaths per
million people.
Although the hypothesis of the cattle-to-human transmission
remains unproven, it is supported by the results of a number of
scientific studies, including two released in 1997. Laboratory
mice injected with brain tissue from BSE-infected cows and
another group injected with brain tissue from vCJD-infected
humans both developed the same symptoms of brain degeneration
and the infection was ultimately fatal in both groups. In
addition, researchers found the same prion strain in both groups
of mice. In one of the studies, researchers injected a third
group of mice with tissue from humans who had died of classical
CJD; these mice developed no symptoms and survived the trial.
Is there a BSE test for meat? At present there exists no test
for BSE. The only confirmatory test is done at post-mortem and
involves microscopic examination of brain tissue using
immunocytochemistry to confirm suspected cases of the disease.
How long can BSE be in an animal before it shows signs of the
disease? The incubation period is from 30 months to eight years.
Following the manifestation of clinical signs the animal's
condition deteriorates very rapidly from two weeks to six
months, resulting in death.
What measures are necessary to prevent BSE? (i) Introduce
compulsory destruction of suspect and disease animals; (ii) Stop
feeding of rendered animal tissue to cows and other ruminants;
(iii) Ban imports of diseases cattle or other ruminant carcasses
from countries known to have BSE; (iv) Ban imports of live cows
and other ruminants from countries known to have BSE; (v) Sick
or disease animals should not enter the human food supply and
slaughtered animals used for human food should have ante-mortem
and post-mortem health certificates; (vi) Introduce mandatory
testing and surveillance to detect early cases of BSE; (vii)
Although scientific evidence shows that cooking does not kill
the BSE infective agent, all meat products should be thoroughly
cooked for at least 70