Papers & Articles

Papers & Articles Epidemiological study of the decline of BSE in Italy G. Ru, C. Maurella, A. Maroni Ponti, F. Ingravalle, M. Caramelli In this paper, data derived from the national database of the Italian surveillance system for bovine spongiform encephalopathy (BSE) are used to describe the Italian epidemic of BSE. Two data flows were established to collect the results of active and passive surveillance, and 25 regional laboratories were involved. The National Reference Centre (CEA) was in charge of the data analysis. Crude and agestandardised estimates of the prevalence and incidence of BSE were obtained to describe the distribution of the disease in terms of the main risk factors (age, breed and herd size), year of birth, time of diagnosis and geographical location. The increased risk was calculated in terms of the incidence rate ratio. During the five years since January 2001, 128 cases of BSE were identified in domestic cows and four were identified in imported cattle. All but one of the cases were detected through active surveillance. The risk of the disease was highest in dairy stock and in large herds. The northern regions of Italy had an incidence of BSE 2·6 times higher than the southern regions. There was a clear decline in the age-standardised prevalence, from one positive case per 10,000 tests in 2001 to one per 100,000 tests in 2005.

Veterinary Record (2007) 161, 511-514 G. Ru, DVM, PhD, MSc, C. Maurella, DVM, F. Ingravalle, BS, M. Caramelli, DVM, PhD, DipECVPH, Istituto Zooprofilaltico Sperimentale di Piemonte, Liguria e Valle d’Aosta, National Reference Centre for TSE, Via Bologna 148, 10154 Torino, Italy A. Maroni Ponti, DVM, Department for Public Animal Health, Nutrition and Food Safety, Ministry of Health, Piazzale Marconi 25, 00144 Roma, Italy

BOVINE spongiform encephalopathy (BSE) is a fatal neurodegenerative disease characterised by a long incubation period (on average five years) and a gradual accumulation in the central nervous system (CNS) of an anomalous isoform of prion protein (PrP), a normally host-encoded protein (Prusiner 1998). The disease does not stimulate an immune or inflammatory response in the host, and diagnostic serum testing techniques are therefore unable to demonstrate the presence of PrPSc (the anomalous form of PrP) as a marker of infection. Moreover, PrPSc occurs mainly in sites in the CNS, that are hard to access, making it difficult to develop diagnostic techniques for use on live animals (Wells and others 1998). Since January 2001, Italy, like most other European countries, has stepped up its epidemiological surveillance of BSE. The two surveillance systems (active and passive) share the same aim, namely, to identify new cases of BSE, but they take different approaches. In passive surveillance, which was established first, the disease is detected on the basis of the clinical signs, whereas in active surveillance all slaughtered or dead animals more than 24 months old are tested. In Italy, the threshold age for testing normally slaughtered animals has changed over the course of the epidemic. Today, all normally slaughtered animals more than 30 months old and animals

3·0

Crude monthly prevalence Age-standardised prevalence

Cases/10,000 tests

2·5

2·0

1·5

1·0

0·5

0 2001

2002

2003 Year

2004

2005

FIG 1: Crude and age-standardised monthly prevalence of BSE in cattle in Italy between January 2001 and December 2005

The Veterinary Record, October 13, 2007

more than 24 months old in the so-called at-risk categories, that is, fallen stock and emergency slaughtered animals, are tested. The aim of this study was to describe the development of the BSE epidemic in Italy, from the discovery of the index case in January 2001 to December 31, 2005. MATERIALS AND METHODS The data were extracted from the results of the BSE surveillance programmes operating in Italy. Using a standard protocol applied throughout the European Union (EU), and based on the World Organisation for Animal Health (OIE) Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (OIE 2004), the staff of the veterinary services of the local health boards take brainstem samples either from slaughtered animals at the slaughterhouse or from dead animals on farm. In addition, they complete a form that accompanies the samples when they are sent to one of the 25 officially designated regional laboratories, which are certified by an external certifying body. Every month, each laboratory or regional office sends an electronic data file (in either ASCII or Microsoft Access format) to the National Reference Centre for Animal Encephalopathies (CEA) providing data on each animal tested in that region in the previous month. The CEA conducts an ad hoc analysis to verify the completeness and congruency of the data received, and then transforms them into a format compatible with the Stata9 software package (Stata Corporation), which is used for all the statistical analyses. The national data are then collated from these single data files into a single large database. The chief data analysis leads to a breakdown by atrisk categories (normal slaughter, emergency slaughter, animals showing clinical signs at the antemortem inspection, fallen stock and clinically suspicious animals slaughtered because they came from an infected herd), and the animals’ distribution by year of birth and geographical origin. The data flow in passive surveillance is similar to that in active surveillance, but in passive surveillance clinically suspicious animals are sent to both the CEA and to the Ministry of Health, and the data are entered into an ad hoc database. The CEA sends monthly reports to the ministry and to the European Commission to communicate the number of positive results and the number of tests, subdivided by risk group. For this study, these surveillance data were used to calculate estimates of the prevalence and incidence of BSE in Italy. The prevalence was calculated as the number of cases identified

Papers & Articles

2001 2002 2003 2004 2005

Cases/10,000 tests

3·00 2·50 2·00 1·50 1·00

3·00

2·00 1·50 1·00 0·50

0·00

0·00 48-59

60-71 72-83 Age (months)

84-95

>96

1995 1997 1999

2·50

0·50

36-47

1994 1996 1998 2000

3·50

Cases 10,000 tests

3·50

1

2

3 4 Age (years)

5

6

FIG 3: Age-specific prevalence of BSE in Italian cattle by year of birth

FIG 2: Age-specific prevalence of BSE in cattle in Italy by year of diagnosis

through active surveillance per 10,000 animals tested. Excluded from the numerator were imported cases, that is, animals born outside Italy and imported and raised in Italy for at least five years (the average incubation time). The incidence estimates were calculated as the number of new cases diagnosed by passive and active surveillance per million live animals over 24 months of age, either per year or for other specified periods. The estimates of descriptive epidemiology were determined by comparing the distribution of cases of BSE by geographical location, time and the most affected cattle category. To calculate the annual incidence rates in the 25 countries of the EU, data on the cattle populations and the number of BSE cases (where found) for each country were obtained from the EU’s report on the monitoring and testing of ruminants for the presence of transmissible spongiform encephalopathy 2005 (Anon 2005). Relative risk estimates were obtained by the calculation of incidence rate ratios (IRRs) and their 95 per cent confidence intervals (CIs). The IRR of BSE of an Italian cow was compared with the IRRs experienced in the rest of Europe by fitting a Poisson regression model and using Italy as the reference category.

more than 24 months old among those in an at-risk category or normally slaughtered. All but one of the cases were detected through active surveillance. Characteristics of infected animals With rare exceptions, the disease was detected in animals more than four years old and prevalently in dairy stock. The

Trentino Alto Adige Friuli-Venezia-Giulia

Lombardy

Veneto

RESULTS Size of the BSE epidemic in Italy between January 2001 and December 2005 Between January 2001 and December 2005, the national surveillance services identified 128 cases of BSE in domestic cattle and four in cattle imported from other EU countries. During this period, approximately 3·5 million animals were tested. Although the 25 officially designated laboratories can choose from 12 tests authorised by the EU for BSE detection (EC Regulation 260/2005), most of the cattle were tested by the Prionics Check Test (Prionics), up to September 2004, and by the Rapid TeSeE test (Bio-Rad). The total crude prevalence of BSE in Italy during the five years was 0·37 cases per 10,000 tests performed on animals

TABLE 1: Relative risk of BSE in Italian herds of different sizes Herd size 0-19 20-99 100-499 >500

Number of cases

Number of herds

Relative risk

95 per cent CI

11 47 60 9

155,035 57,618 16,606 2294

1 11·5 50·7 55·8

6·0-22·1 26·7-96·5 22·9-132·8

CI Confidence interval

=0 >0-20 >20-40 >40-60 >60-80 >80-100 >100

FIG 4: Incidence of BSE in cattle in the provinces of Italy between January 2001 and December 2005

The Veterinary Record, October 13, 2007

Papers & Articles

TABLE 2: Numbers of cases of BSE in Italy among cattle of different ages, the numbers of cattle tested each year from 2001 to 2005, and the crude prevalences (cases per 10,000 tests) Age (months) 42-53 54-65 66-77 78-89 ≥90 Total Prevalence

2001

2002

2003

Cases

Tests

Cases

Tests

Cases

Tests

5 20 15 6 2 48 1·3

73,698 69,363 57,947 44,603 110,762 356,373

0 7 10 11 6 34 0·6

106,687 100,496 85,914 66,764 181,640 541,501

0 1 10 9 11 31 0·5

115,181 108,624 91,587 73,212 190,557 579,161

risk of infection in beef cattle was significantly lower than that of other breeds; compared with the Piemontese breed (one of the main beef breeds in Italy), the relative risk (RR) was five times higher in Holsteins (RR 4·7, 95 per cent CI 1·2 to 19), six times higher in brown Swiss cattle (RR 5·7, 95 per cent CI 1·3 to 24) and nine times higher in Simmentals (RR 9·0, 95 per cent CI 2 to 41). Only in crossbred animals and in sparsely represented breeds (Herens and Rendena) was the RR lower than that of the Piemontese breed, although the difference was not statistically significant (RR 0·8, 95 per cent CI 0·2 to 3·8). There was a clear positive association between increasing herd size and the probability of contracting the infection (Table 1). Development of the BSE epidemic The monthly trends of BSE prevalence during the five-year study period is shown in Fig 1. The monthly prevalence decreased steadily and then stabilised in 2005. The ratio of the cases detected to the number of tests performed ranged from 1:10,000 to 1:100,000. Table 2 shows the number of cases and animals tested by age and calendar year. The data refer only to animals more than 42 months old, because no younger animals were BSE positive. A comparison of age-specific prevalence by year of testing (2001 to 2005) (Fig 2) shows that there was a general decline in the size of the epidemic, with progressively lower curves, and that the disease was detected in increasingly older ani-

Portugal UK

Spain Ireland Czech Republic Slovakia

Country

Luxembourg

2004 Cases Tests 0 1 0 2 4 7 0·1

113,763 107,656 90,148 70,940 198,680 581,187

2005 Cases Tests 0 1 2 0 5 9 0·1

104,446 99,455 84,492 65,149 194,963 548,505

Total tests 409,329 386,139 325,596 255,519 681,639 2,606,727

mals. When the same data were reviewed in terms of birth cohorts (Fig 3), the prevalence peaked, together with the risk of infection, among animals born in 1996, followed by a decrease in younger cohorts. Geographical distribution of BSE in Italy Fig 4 shows the geographical distribution of the incidence of BSE. The spread of BSE affected the entire country, partly in association with the distribution of the cattle population; however, BSE appeared to be particularly concentrated in some provinces of the north east, particularly Lombardy, Friuli-Venezia-Giulia, Trentino Alto Adige and Veneto. There was also a higher than average incidence in certain provinces of the central southern regions of the country. However, a comparison of the incidence of BSE in the north versus the central southern regions (Table 3) shows that the risk of infection was 2·6 times higher in the northern stock than in the southern stock. BSE in Italy compared with the rest of Europe Within Europe, Italy ranks among the countries with the lowest risk of contracting BSE in 2005 (Fig 5). DISCUSSION Active surveillance since 2001 has improved the detection of in Italy. It is very likely that earlier cases were missed by the older surveillance system owing to its lack of sensitivity and the reliance on voluntary reporting. This conclusion was reached in a geographical risk assessment published by the European Commission in 2000 (Anon 2000a). The report pointed out that during the 1990s the number of cases of neurological and progressive disease in cattle over 24 months old examined, was very low, and that only in 1999 were the minimum requirements of the OIE guidelines met. Moreover, compensation corresponding to the full market value has been paid to farmers for reporting suspect cases only since 1997. For these reasons, the opportunity to diagnose cases of BSE has been limited. BSE

Poland TABLE 3: Incidence of BSE in cattle in Italy between January 2001 and December 2005, and the relative risk (RR) in the northern and the central southern regions (including Sicily and Sardinia)

Germany Slovenia France

Region North* Central southern

Italy Austria

Susceptible population of cattle 1,825,326 933,940 Cases 107 21 Incidence (cases per million 58·6 22·5 cattle more than 24 months old) RR (95 per cent CI) 2·61 (1·63-4·16) 1 P