Massimo Tranquillo , Giorgio Zanardi, Dominga Avisani
Osservatorio Epidemiologico Veterinario della Regione Lombardia, Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia-Romagna, Brescia
BSE is a degenerative, neurological disease to which humans are susceptible. It is transmitted by prion protein PrP(Sc). BSE made its first appearance in the United Kingdom in 1985, where it appears to have been spread when muscle tissue of cattle was recycled and used in animal feed. Currently, the member countries of the European Community conduct active surveillance for BSE in order to identify and remove PrP(Sc)-positive animals from the food chain.
In Italy active surveillance began on the 1st of January in 2001. In the Italian program, rapid testing using the Prionics test was implemented on spinal cord samples from the following classes of cattle: 1) Animals >30 months of age who died in the stall or during transport; 2) Animals undergoing slaughter at >30 months and destined for human consumption; 3) Animals >30 months undergoing emergency slaughter and those that were ill before death. Testing began for categories 1) and 3) on July 1st 2001 and on September 12th 2001 for the animals belonging to category 2).
Presented here are the results of the active and passive surveillance conducted in 2001-02, and a description of the 30 cases of BSE that were found in Lombardy, a region that accounts 40% of the cattle of the country and 70% of the samples examined for BSE in Italy.
Data on the characteristics of cattle that were examined with the rapid test and of the number with positive tests were obtained at the BSE Archive in Veterinary Epidemiologic Observatory of the Lombardy region (OEVR). Detailed information on the 30 animals that were determined to have BSE was also obtained from the same Archive.
The period prevalence 2001-02 was calculated for the whole population and for the population of animals considered to have a higher probability of being positive (“high risk”): animals that died in stall, those slaughtered on an urgent basis, and those whose productivity was compromised.
Following the decision to lower the age limit for animals to be screened, the cattle slaughtered in 2002 were younger than those in 2001 (1). For this reason, rates were standardized by date of birth and age cohort.
In the two-year period of 2001-02, a total of 352,466 cattle raised in Lombardy were examined. Of these, 295,876 underwent routine slaughter (category 2), while 56,590 belonged to the groups “at risk“. There were 30 positive cases, all confirmed by the Transmissible Spongiform Encephalopathy Center for Animal Encephalopathies in Turin, yielding a period prevalence of 8.5/100 000 (95% confidence interval 5.7-12.1).
Overall, the rate of positivity was higher in the cattle “at risk” compared to those undergoing routine slaughter (12.4/100,000 vs 7.8/100,000), although this difference was not statistically significant.
Of the 30 positive cases, 19 were diagnosed in 2001, and 11 in 2002. Twelve (40%) were from in Brescia, 8 (27%) from Cremona and 5 (17%) from Mantova; the remaining 5 came from Bergamo, Lecco, Pavia, Lodi and Milano. All were all born before May ’97. Most (23 animals, 77%) were routinely slaughtered. Two died in the stall, and five were animals that were ill before death; none were animals that had been slaughtered on an emergency basis Of the 23 routinely slaughtered animals, 18 (78%) had clinical findings, three of which had neurological problems.
The Table summarizes the results for each of the catetories, standardized by birth cohort and age. Although there is a lower prevalence in 2002 compared with 2001, rates of positivity were low and the confidence intervals for the categories overlapped.
In Lombardy in 2002, in the face of inconsistent reporting of suspect cases, a special plan was activated that was designed to enhance the reporting of suspect clinical cases of BSE. During 2002, the Veterinary Services of the Local Health Authorities of the Region identified 75 suspect cases, of which none were confirmed as positive by the diagnostic center in Turin.
Passive surveillance systems for BSE are subject to a number of influences, including the sensitivity and specificity of the clinical diagnosis, the awareness and willingness of veterinarians and cattle raisers to identify and report animals with symptoms suggestive of BSE. Passive surveillance system alone therefore does not represent an ideal means of guaranteeing the health of the public. For this reason, an active surveillance system in which routinely slaughtered animals and those at risk are systematically examined, remains essential.
As can be seen, the most likely estimation of number of cases of BSE among cattle in Lombardy is 8-9 cases per 100,000 animals. Applying this rate to the total population of cattle in Lombardy (736,000 head as of January 1st 2001), the estimated total would be 60-70 cases. Thus, according to the data, we predict that there will be approximately 30-40 cases in the next two years. Each year, the additional surveillance data obtained from the active surveillance system will permit further refinements in these predictions.
Comment
Umberto Agrimi
Laboratorio di Medicina Veterinaria, ISS
To date, there have been 102 confirmed cases of BSE in cattle in Italy. Although the crisis of 2001 is over, surveillance and data analysis activities remain intense. The active surveillance by means of the rapid test has done its job; it has revealed the disease where it had not earlier been found, it has confirmed the geographical risk-categories established by the EU, and has clarified the incidence levels in European countries. Today it also beginning to provide information useful for predicting future trends in the epidemic. The evaluations conducted by the scientific board of EU indicated that the period highest exposure of Italian cattle to contaminated feed was in1995-96. If this is correct, we may hope that we are on the downslope of the epidemic curve. The changes in Italy parallel the findings of other European countries demonstrating a decline in cases over time. The coming years will clarify if we are really close to a resolution of the problem or not.
Speaking of the rapid test, it is useful at this point to raise some of the misunderstandings concerning its use. Unlike most other surveillance systems, the entire population of adult cattle undergoing slaughter are tested. The need to calm consumers has turned the rapid test from being an instrument for epidemiologic research to being a tool for protecting publich health. In reality, the rapid test reveals the presence of the condition only a short time before the symptoms begin to appear; this means that there is a period of years where the disease is undetectable. Also, because detection requires levels of approximately 103-104DL50/g; animals with negative tests may still carry significant levels of the infective agent. This is why the tissues considered at risk from all animals are banned even from negative animals. The security of the consumers is in fact relying on the elimination of considered “specified risk materials”.
To this picture can be added that the transmissability of BSE to man-- due to factors that are only in part identified, e.g. the so-called “barrier of species”as well the unknown pathogenic traits of the agent strain – seems to appear lower than was initially suspected. A total of 147 cases of Creutzfeldt-Jakob, the human version of BSE, have been identified to date (including 1 probable case in Italy); 137 of these were reported the UK. However, the trend of the epidemic curve seems to be downward. The models of prediction are becoming more reliable with time and are, luckily, providing us with a less pessimistic view of the future.
Bibliography
1. Tranquillo M, Zanardi G, Avisani D. BSE in Lombardia: risultati dopo due anni di sorveglianza.
