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Update
2002: Bovine Spongiform Encephalopathy and Variant Creutzfeldt-Jakob
Disease |
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Description
Since 1996,
strong evidence has accumulated for a causal relationship between ongoing
outbreaks in Europe of disease in cattle called bovine spongiform encephalopathy
(BSE, or "mad cow disease") and a disease in humans originally
called new variant Creutzfeldt-Jakob disease or more recently simply variant
CJD (vCJD). Both disorders are invariably fatal brain diseases with unusually
long incubation periods measured in years and are caused by an unconventional
transmissible agent. Although there is very strong evidence that the agent
responsible for the human disease is the same agent responsible for the
BSE outbreaks in cattle, the specific foods that might be associated with
the transmission of this agent from cattle to humans are unknown. However,
bioassays have identified the presence of the BSE agent in the brain,
spinal cord, retina, dorsal root ganglia (nervous tissue located near
the backbone), distal ileum, and the bone marrow of cattle experimentally
infected with this agent by the oral route.
In addition
to cattle, sheep are susceptible to experimental infection with the BSE
agent by the oral route. Thus, in countries where flocks of sheep and
goats may have been exposed to the BSE agent through contaminated feed,
a theoretical risk exists that these animals might have developed infections
caused by the BSE agent and that these infections are being maintained
in the flocks, even in the absence of continued exposure to contaminated
feed (for example, through maternal transmission). Regardless, as of July
2002, cattle remain the only known food animal species with disease caused
by the BSE agent.
Occurrence
From 1995
through June 2002, a total of 124 human cases of vCJD were reported in
the United Kingdom, 6 cases in France, and 1 case each in Ireland, Italy,
and the United States. The case-patients from Ireland and the United States
had each lived in the United Kingdom for more than 5 years during the
UK BSE epidemic. Based on analyses published in April 2002 using exponential
growth models, the best estimate of the annual increase in the number
of vCJD cases in the United Kingdom since the outbreak began was 18% per
year, which is equivalent to a doubling every 4.2 years (95% confidence
interval, 2.7 to 9.1 years). This estimate of the doubling time is longer,
albeit not statistically significantly different, than a similarly calculated
estimate a year earlier, which found a doubling of vCJD cases in the United
Kingdom every 3 years. Continued surveillance will be necessary to determine
whether this change in the best estimates of the doubling time portends
an epidemic in humans that is reaching its peak or reflects chance variability
in a possibly exponential increase of new cases.
From 1986
through 2001, >98% of cases of BSE worldwide were reported from the
United Kingdom, where the disease was first described. During this same
period, the number of countries reporting at least one indigenous BSE
case increased. Almost all of these were in Europe. There were 4 such
European countries through 1993, 8 through 1998, 11 through 2000, and
18 through 2001. The 19th European country, Poland, reported its initial
BSE case in 2002. In addition, as of July 2002, two countries outside
of Europe –- Japan and Israel –- reported their first indigenous
BSE cases in 2001 and 2002, respectively. The spread of the BSE agent
from the United Kingdom or potentially from other countries with BSE was
most likely through the importation of infected live cattle or BSE-contaminated
animal feed. The proportion of the annual total number of BSE cases worldwide
reported outside of the United Kingdom rose to >25% in 2000 and >45%
in 2001. This increase reflected the declining large (over 182,000 total
cases) epidemic of BSE in the United Kingdom and the increasing number
of other countries with improved surveillance and higher rates of BSE.
In 2001,
only two countries, the United Kingdom and Portugal, reported a BSE incidence
rate of >100 indigenous cases per million cattle aged over 24 months.
Although reported BSE rates must be interpreted with considerable caution
because the sensitivity of surveillance may vary over time and by country,
for 2001, the reported BSE rates for other countries (in decreasing order)
were as follows: Republic of Ireland (62 BSE cases per million), Switzerland
(49 cases per million), Belgium (28), Spain (24), Germany (20), France
(20), Slovakia (18), Italy (14), and the Netherlands (10). The reported
rates for Denmark, Slovenia, Greece, the Czech Republic, Finland, Japan,
and Austria were between 1 and 7 BSE cases per million. The numbers of
reported BSE cases and incidence rates, by country, are available on the
Internet website of the Office International Des Epizooties, at http://www.oie.int/eng/info/en_esb.htm.
Many of the reported increases in 2001 presumably reflect European Union
legislation passed in June 2000 that required improved BSE monitoring
by Member States after January 1, 2001. The legislation called for rapid
post-mortem tests on a targeted sample of animals at highest risk for
BSE.
In addition
to the countries with confirmed BSE in 2001, 14 other countries had been
classified by the European Union’s Scientific Steering Committee
by June 2002 as likely having, or confirmed as having, BSE at a lower
level. These countries included Albania, Bulgaria, Croatia, Cyprus Republic,
Estonia, Hungary, Latvia, Lithuania, Luxembourg, Poland, Romania, San
Marino Republic, Slovic Republic, and Turkey. Finally, in January 2002,
the US Food and Drug Administration (FDA) published its new guidance to
reduce the theoretical risk of transmission of the agents of CJD and vCJD
by blood and blood products. This document included a US Department of
Agriculture (USDA) list of European countries with BSE or possibly at
increased risk of BSE that FDA indicated was applicable for determining
blood donor deferrals; one deferral criterion, for example, included living
cumulatively for 5 years or more in Europe from 1980 until the present.
Among the European countries on the USDA list that were not previously
mentioned in this chapter include Bosnia-Herzegovina, Liechtenstein, Macedonia,
Norway, Sweden, and the Federal Republic of Yugoslavia. Information is
being generated rapidly on BSE issues. Updated sources should be consulted.
Risk to Travelers
The current
risk of acquiring vCJD from eating beef (muscle meat) and beef products
produced from cattle in countries with or at possibly increased risk of
BSE cannot be precisely determined. Among many uncertainties affecting
this determination, for example, are the incubation period between exposure
to the infective agent and onset of illness, the ultimate number and age
distribution of vCJD cases in the United Kingdom that will result from
earlier BSE exposures, the sensitivities of each country’s surveillance
for BSE and vCJD, the compliance with and effectiveness of public health
measures instituted in each country to prevent BSE contamination of human
food, and details about cattle products from one country that were distributed
and consumed in others. Nevertheless, in the United Kingdom, the current
risk of acquiring vCJD from eating beef and beef products appears to be
extremely small, perhaps about one case per 10 billion servings.
In other countries of the world, this current risk, if it exists at all,
would not likely be any higher than that in the United Kingdom, particularly
if BSE-related, public health control measures are being well implemented.
Such measures would be especially important in a country such as Portugal,
which like the United Kingdom has a relatively high incidence rate of
reported BSE. Although the current risk of acquiring vCJD through beef
and beef products in Europe is presumably very low, it should be noted
that the aforementioned published guidance on US blood donor deferral
criteria focuses on the time (cumulatively 3 months or more) that a person
lived in the United Kingdom from 1980 through 1996, whereas for the rest
of Europe it focuses on the time (cumulatively 5 years or more) a person
lived in these countries from 1980 through to the present. The lack of
a time limit on the latter criterion reflects existing uncertainties related
to human food chain protection in some countries outside of the United
Kingdom. There are also concerns about overly complicating the questions
posed to prospective US blood donors during the blood-collection screening
process.
Preventive Measures
Public health
control measures, such as enhanced BSE surveillance, the culling of sick
animals, and bans of specified risk materials (SRM), have been instituted
in countries of Europe to prevent potentially BSE-infected tissues from
entering the human food chain. The most stringent of these control measures,
including an “Over Thirty Months Scheme” that excludes all
animals older than 30 months from the human food and animal feed chains,
have been applied in the United Kingdom and appear to be highly effective.
In June 2000, the European Union Commission on Food Safety and Animal
Welfare strengthened the European Union’s set of control measures
in relation to BSE by adopting a decision requiring all member states
to remove SRMs from the animal feed and human food chains as of October
1, 2000; such bans had already been instituted in most member states.
Also noteworthy among European Union’s set of control measures are
the banning of the use of mechanically recovered meat from the vertebral
column of cattle, sheep, and goats for human food, and the BSE testing
of all cattle aged over 30 months destined for human consumption.
To reduce
the possible current risk of acquiring vCJD from food, travelers to Europe
or other areas with indigenous cases of BSE or at possibly increased risk
of BSE may wish to consider either:
(1) avoiding
beef and beef products altogether or
(2) selecting
beef or beef products, such as solid pieces of muscle meat (versus calf
brains or beef products such as burgers and sausages), which might have
a reduced opportunity for contamination with tissues that may harbor
the BSE agent.
Milk and
milk products from cows are not believed to pose any risk for transmitting
the BSE agent.
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