Vidyya Medical News Service
*
Volume 6 Issue 139 Published - 14:00 UTC 08:00 EST 18-May-2004 Next Update - 14:00 UTC 08:00 EST 19-May-2004
little clear gif used for spacer
 
 

Bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease

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.



© Vidyya.
 
 

  More Today in Vidyya