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Questions and Answers Regarding Bovine
Spongiform Encephalopathy (BSE) and Variant Creutzfeldt-Jakob Disease
(vCJD)
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Cattle at a trough.
(Image courtesy
Ermias Belay) |
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On this page:
What is bovine spongiform
encephalopathy?
Bovine spongiform encephalopathy (BSE) is a progressive
neurological disorder of cattle that results from
infection by an unconventional transmissible agent.
More than 184,000
cases of BSE have been confirmed in the United Kingdom
alone in more than 35,000 herds. Regularly updated
numbers of reported BSE cases, by country, are available
on the website of the Office International Des Epizooties
at: http://www.oie.int/eng/info/en_esb.htm.
The BSE epidemic in the United Kingdom peaked in
January 1993 at almost 1,000 new cases per week.
The outbreak may have resulted from the feeding of
scrapie-containing sheep meat-and-bone meal to cattle.
There is strong evidence and general agreement that
the outbreak was amplified by feeding rendered bovine
meat-and-bone meal to young calves.
The nature of the transmissible agent is unknown.
Currently, the most accepted theory is that the agent
is a modified form of a normal cell surface component
known as prion protein. The pathogenic form of the
protein is both less soluble and more resistant to
enzyme degradation than the normal form.
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Is BSE occurring
in the United States?
Yes, the first known case of BSE in the United States was identified in December 2003. On December 23, 2003, the U.S. Department of Agriculture (USDA) announced a presumptive diagnosis of BSE in an adult Holstein cow from Washington State. This diagnosis was confirmed by an international reference laboratory in Weybridge, England, on December 25. Preliminary trace-back based on an ear-tag identification number suggested that the BSE-infected cow was imported into the United States from Canada in August 2001. The preliminary trace-back identification of the animal was later confirmed by genetic testing.
On June 24, 2005, the U.S. Department of Agriculture announced receipt of final results from The Veterinary Laboratories Agency in Weybridge, England, confirming BSE in a cow that had conflicting test results in 2004. This cow was from Texas and represented the first endemic case of BSE in the United States.
On March 13, 2006, the U.S. Department of Agriculture (USDA) announced the confirmation of bovine spongiform encephalopathy (BSE) in a cow in Alabama. The newly confirmed case was identified in a non-ambulatory (downer) cow on a farm in Alabama. The animal was euthanized by a local veterinarian and buried on the farm. The age of the cow was estimated by examination of the dentition as 10-years-old. It had no ear tags or distinctive marks; the herd of origin could not be identified despite an intense investigation (see Alabama BSE Investigation, Final Epidemiology Report, May 2006).
For more information about BSE in the United States, see the Animal and Plant Health Inspection Service, USDA BSE site.
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Is BSE a foodborne hazard in the United
States?
Strong evidence indicates that BSE has been transmitted to humans primarily
in the United Kingdom, causing a variant form of Creutzfeldt-Jakob disease
(vCJD). In the United Kingdom, where over 1 million cattle may have been
infected with BSE, a substantial species barrier appears to protect humans
from widespread illness. Since variant CJD was first reported in 1996, a total of 195 patients with this disease from 11 countries have been identified. As of August 11, 2006, variant CJD cases have been reported from the following countries: 162 from the United Kingdom, 20 from France, 4 from Ireland, 2 from the United States (including the current case), and one each from Canada, Italy, Japan, Netherlands, Portugal, Saudi Arabia, and Spain. Similar to the two U.S. cases, two of the four cases from Ireland and the single cases from Canada and Japan were likely exposed to the BSE agent while residing in the United Kingdom. One of the 20 French cases may also have been infected in the United Kingdom.The
risk to human health from BSE in the United States is extremely low.
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What is the
variant form of CJD that the experts in the United
Kingdom believe might be related to the BSE outbreak
in cattle?
In contrast to the classic form of CJD, the variant
form in the United Kingdom predominantly affects
younger persons (median age at death around 29 years)
and has atypical clinical features. These atypical
features include prominent psychiatric or sensory
symptoms at the time of clinical presentation or
early in the course of the illness, delayed onset
of neurologic abnormalities, duration of illness
of at least 6 months, and a diffusely abnormal non-diagnostic
electroencephalogram.
The characteristic neuropathologic profile of variant CJD includes, in both
the cerebellum and cerebrum, numerous kuru-type amyloid plaques surrounded
by vacuoles and prion protein (PrP) accumulation at high concentration indicated
by immunohistochemical analysis.
Recently published data indicate that the epidemic of variant CJD in the United
Kingdom may have already reached a peak. A listing of monthly updated numbers
of CJD and variant CJD cases in the United Kingdom is available at: http://www.dh.gov.uk/PolicyAndGuidance/HealthAndSocialCareTopics/CJD/fs/en
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Is
there evidence directly linking this newly recognized
variant of CJD to BSE exposure?
There is strong epidemiologic and laboratory evidence
for a causal association between variant CJD and
BSE. The absence of confirmed cases of variant CJD
in other geographic areas free of BSE supports a
causal association.
In addition, the interval between the most likely
period for the initial extended exposure of the population
to potentially BSE-contaminated food (1984-1986)
and onset of initial variant CJD cases (1994-1996)
is consistent with known incubation periods for CJD.
An experimental study reported in June 1996 showed
that three cynomologus macaque monkeys inoculated
with brain tissue obtained from cattle with BSE had
clinical and neuropathological features strikingly
similar to those of variant CJD (Nature 1996;381:743-4).
A study published in 1996 indicated that a Western blot analysis of infecting
prions obtained from 10 variant CJD patients and BSE-infected animals had similar
molecular characteristics that were distinct from prions obtained from patients
with other types of CJD (Nature 1996;383:685-90).
An experimental study involving inoculation of a panel of inbred mice with
the agents causing BSE and variant CJD substantially increased the strength
of the scientific evidence for a causal association between variant CJD and
BSE (Nature 1997;389:498-501). In this study, groups of inbred mice and a group
of cross-bred mice inoculated with brain homogenates from variant CJD cases
were reported to have had latency periods and lesion profiles consistent with
the BSE pattern.
The latency period, neuropathology, and disease-causing PrP isoforms in transgenic
mice expressing bovine PrP that were inoculated with variant CJD, BSE, and
scrapie brain extracts provided additional evidence supporting the link between
BSE and variant CJD (Proc Natl Acad Sci 1999;96:15137-42).
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Has
CDC initiated increased surveillance efforts to determine
whether the newly recognized variant of CJD (vCJD) occurs
in the United States?
Yes. The possibility that BSE can spread to humans has focused increased attention on the desirability of enhancing national surveillance for Creutzfeldt-Jakob disease (CJD) in the United States.
The Centers for Disease Control and Prevention (CDC) monitors the trends and current incidence of CJD in the United States using several surveillance mechanisms. On a routine basis, CDC reviews the national multiple cause-of-death data taken from death certificates and compiled by the National Center for Health Statistics, CDC. In addition, with the support of the Council of State and Territorial Epidemiologists, CDC conducts follow-up review of clinical and neuropathology records of CJD decedents aged <55 years who are identified through the national mortality data analysis or reported by health care workers. This is the age group in which almost all of the vCJD cases worldwide have occurred to date.
In 1996-97, CDC established, in collaboration with the American Association of Neuropathologists, the National Prion Disease Pathology Surveillance Center at Case Western Reserve University, which performs special state-of-the-art diagnostic tests for prion diseases, including post-mortem tests for vCJD. These tests are provided free of charge to all U.S. physicians. For more information about the center, visit its website at: http://www.cjdsurveillance.com.
Currently, CDC works with selected state health departments on various enhanced CJD surveillance projects and education programs regarding the importance of autopsy to both the surveillance and diagnosis of CJD. In addition, CDC collects, reviews and when indicated, actively investigates specific reports by health care personnel or institutions in all states of possible iatrogenic CJD and variant CJD cases.
These surveillance methods for CJD enhance the ability to identify cases of variant CJD when such cases occur in the United States.
For more information about surveillance and diagnosis of CJD, see the following article: Creutzfeldt-Jakob Disease surveillance and diagnosis. Belay ED, Holman RC, Schonberger LB. CID September 15 2005;41:834-836.
A summary of the analysis of multiple cause-of-death data was published in the Journal of the American Medical Association on November 8, 2000 (Volume 284, No. 18, pp. 2322-3) and in Clinics of Laboratory Medicine in December 2002 (Volume 22, pp. 849-62).
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Have any cases of variant CJD (vCJD) been reported in the United States?
Yes, three cases of vCJD have been reported from the United States. By convention, variant CJD cases are ascribed to the country of initial symptom onset, regardless of where the exposure occurred. There is strong evidence that suggests that two of the three cases were exposed to the BSE agent in the United Kingdom and that the third was exposed while living in Saudi Arabia.
The first patient was born in the United Kingdom in the late 1970's and lived there until a move to Florida in 1992. The patient had onset of symptoms in November 2001 and died in June of 2004. The patient never donated or received blood, plasma, or organs, never received human growth hormone, nor did the patient ever have major surgery other than having wisdom teeth extracted in 2001. Additionally, there was no family history of CJD.
The second patient resided in Texas during 2001-2005. Symptoms began in early 2005 while the patient was in Texas. He then returned to the United Kingdom, where his illness progressed, and a diagnosis of variant CJD was made. The diagnosis was confirmed neuropathologically in early 2006 at the time of the patient's death. While living in the United States, the patient had no history of hospitalization, of having invasive medical procedures, or of donation or receipt of blood and blood products. The patient almost certainly acquired the disease in the United Kingdom. He was born in the United Kingdom and lived there throughout the defined period of risk (1980-1996) for human exposure to the agent of bovine spongiform encephalopathy (BSE, commonly known as "mad cow" disease). His stay in the United States was too brief relative to what is known about the incubation period for variant CJD.
The third patient was born and raised in Saudi Arabia and has lived in the United States since late 2005. The patient occasionally stayed in the United States for up to 3 months at a time since 2001 and there was a shorter visit in 1989. The patient's onset of symptoms occurred in Spring 2006. In late November 2006, the Clinical Prion Research Team at the University of California San Francisco Memory and Aging Center confirmed the vCJD clinical diagnosis by pathologic study of adenoid and brain biopsy tissues. The patient has no history of receipt of blood, a past neurosurgical procedure, or residing in or visiting countries of Europe. Based on the patient's history, the occurrence of a previously reported Saudi case of vCJD attributed to likely consumption of BSE-contaminated cattle products in Saudi Arabia, and the expected greater than 7 year incubation period for food-related vCJD, this U.S. case-patient was most likely infected from contaminated cattle products consumed as a child when living in Saudi Arabia (1). The patient has no history of donating blood and the public health investigation has identified no known risk of transmission to U.S. residents from this patient.
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Is
BSE a foodborne hazard for travelers to Europe?
The current risk for infection with the BSE agent
among travelers to Europe is extremely small, if
it exists at all. Information describing this risk
is available in the document titled "Risk
for Travelers" available on this site.
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