In 1907, the 'British Royal Commission' proved that childrens were at risk from bovine TB (1). Today this Zoonotic bTB (caused by M. bovis) is present worldwide. Tuberculosis, phthisis, also known as 'Scrofula', is a threat to public health, domestic livestock, wildlife, and also to trade industry. The status of this disease as a re-emerging zoonosis is today of great concern (4).
The first case of bovine infection from man to cattle was reported by Magnusson in 1937 (2, 3). Bovine TB is now classified by OIE as a "List B" disease.
Countries following the FAO and the OIE recommendation, try to apply appropriate and effective control measures to stop the appearance of new cases of TB (incidence) (5, 23). This can be difficult to achieve because the infectious dose could be a very low amount of bacilli (22).
Low incidence
As 'incidence' we understand the amount of new cases occurring in a defined period of time. The probability of developing a specific disease during a specified period of time is the incidence rate.
Incidence rate = new cases in specified period of time x factor (100,1000…)
Total population at risk during the period
'Low incidence' (LI) occurs when new cases emerge in a lower and slower rate than before within a period of time in the targeted population. Is this low incidence measure real and significant enough to state that bTB is under control and even leading to a possible eradication? The answer is probably, No.
Low incidence might not reflect the real situation on a national level. Low incidence of bTB can suggest that the disease is generally not spreading fast, but has a constant, slow trend within the cattle population. The incidence could be higher in some part of the country and very low in others. We need the 'prevalence', in order to better understand how TB in a population could be quantified.
'Prevalence' is the total number of existing cases occurring at one particular time. :
Prevalence = chronic conditions
Diseases - count the total number of disease individuals
'Prevalence rate' is the amount of the population who has a disease at a given time.
Prevalence rate = existing cases at the specified point of time x factor
Total population at risk during the period
(15)
In livestock the disease is deeply predisposed by husbandry and managerial factors which are of highest importance. Environmental factors such as animal stocking density, movement between groups, quarantine, the new stock, environmental circumstances (humidity, temperature, wind, rain seasons), housing, bedding, watering, ventilation, sanitation and food balance also play vital part .
In diseases outbreaks in animal groups, normally both clinical and sub clinical cases exist in the group (Iceberg Concept). In those endemic diseases, more of the infections in a group have a sub clinical (silent) presentation (see figure). It is fundamental to identify the condition in the group in question or the case classification (negative, exposed, suspect, or reactor).
(24)
There is a group of countries like Australia (30,31), Poland and others, which has achieved free bTB status employing rigorous methods of surveillance monitoring and control (23,41, Apx 1).
Others such as EEUU (27, 28, 29 ) and Spain , show generally the moderate but continuous diminution of the disease (36).
Spain has improved the incidence rate. There has also been a change in the prevalence rate of bTB among the animal population (25). This disease can be endemic.
Prevalence / Incidence , of b.TB during 1986 - 2006 in Spain.
PROGRAMA NACIONAL DE ERRADICACIÓN DE TUBERCULOSIS BOVINA. AÑOS 2008-2010..(25)
Spain - a low incidence country
In Spain the prevalence is higher in beef herds and breeding fighting bulls' herds than in dairy herds, which may be due to production related differences between these types of herds. Beef herds are kept under more extensive conditions, which allow contacts with other herds and wildlife, via communal pasture (4).
BTB presence in the wildlife species had been proven as a source of infection for domestics animals and a menace to endangered species (6,8).
It seems that species sharing the same ecosystems are transmitting the disease. M. bovis has also been detected in carcasses of deer, hare, wild boar, Iberian lynx and fox found in surveys of wildlife killed in national parks and private estates in central, southern and west of the Iberian Peninsula (7, 4).
Figure1. Main bovine tuberculosis risk factors classified into animal, herd and region/country levels.
Worldwide bovine tuberculosis risk factors Vet. Res. (2009) 40:50……………………..(4)
Spain has a control programme in place, at national and regional level. According to their data about 97% of the herds are free of bovine TB (bTB) (18).This was achieved by the application of regime tests and slaughter at the national level. In the Central organization there is the Ministry of environmental, rural and sea. On the regional level there are official veterinary services with provincial and district headquarters (20,21). REPORT OF THE "BOVINE TUBERCULOSIS" SUB-GROUP TASK FORCE, Spain, 14-15 November 2007 (18)
Spain as a member state of the EU Commission has its own Sub-group Task Force, which report back on the progress of its bTB eradication programme (18).
In order to control and prevent outbreaks of bTB various forms of surveillance programmes have been established and information gathered and utilised from findings of these.
What is disease surveillance?
"Epidemiological surveillance is defined as an observational method based on
continuous recording to follow health status or risk factors in a defined population, and particularly to detect the appearance of pathological processes and study their development over time and in space, with a view to adopting appropriate control measures "(19;Toma et al., 1991).
Surveillance is defined as a 'mechanism applied to collect and interpret data on the health of animal population, to accurately describe their health status with respect to specific diseases of concern' (28).
The term surveillance is used for the recognition of new or exotic diseases, and monitoring is aimed at detecting increases in established infection levels that may signal the recurrence of a disease outbreak. Surveillance programmes are often used to incorporate both surveillance and monitoring activities (MOSS) (28).There are very clear definitions in the report of the ISVEE conference, held in Durban.(2009)
Epidemiological surveillance forms part of descriptive epidemiology because it aims to provide a reliable picture of the epidemiological situation regarding one or more diseases (13).
The concept of disease surveillance is shown in Figure 1.(26).
Surveillance methods
Obligatory surveillance: Bovine TB is a nationwide programme disease which requires the declaration of all suspected bTB cases by farmers, veterinarians, abattoirs inspectors, and everybody in contact with livestock.
Routine surveillance: Call passive surveillance, also known as 'scanning' surveillance, is an on-going observation of the prevalent disease profile of a susceptible population. With the information from this data we can detect any abnormal changes or emerging diseases and obtain a general picture of the disease situation.
Active surveillance: Also called tailored programmes, targets a specific disease or condition within a defined population. The presence of the disease can be measured or its absence verified. At first a clear case definition must be developed. Surveillance is then carried out in structured population-based inspections (methodical testing at slaughter, random surveys, examinations for infection in non-symptoms animals including wildlife) or in structured target surveillance actions (disease reporting, targeting testing, ante-mortem inspections, laboratory investigations, sentinel elements, field records, livestock's herds, wildlife disease statistics).
"Effectives controls requires an understanding of the epidemiology of a diseases, including its infections dynamics within domestics as well as wildlife populations"(32 chapter 8, p. 363).
Data elements
Data are obtained from different sources: Findings in abattoirs, in the field, in research laboratories, zoos, feral animal keepers, private veterinary practices and from state veterinary surveillance. There are good definitions in the web of USDA. This site provides broad information on animal health/disease areas (40)
We can define the data by the event under surveillance. The event and the population at risk (numerator/denominator), have to be measurable. Then we need to identify the sources and the data providers, based in the appropriated census to obtain a real status of the disease.
Data collection is a team project, collectors and providers and everybody else involved participate to achieve the objectives. It is based on risk assessment. To be beneficial procedures must be focused, systematic, efficient, reliable and economical. The goal is to access hidden pockets of an animal population, to avoid uncontrolled situations in the future. The main goal here is the eradication the bTB, and to gain the status of freedom of the disease.
In disease surveillance, compulsory and voluntary notifications are common sources for data collection. In remote areas with poor infrastructure participatory appraisals can be a valuable source of information.
Laboratories play an important part for the provision of valuable and reliable data. An active and full functional diagnostic laboratory service is therefore fundamental for any surveillance activity. Analysis of data is also performed by the laboratories. Here we find two concepts to consider, first sensibility, second feasibility. The threshold of sensibility must be the same in the entire test to have consistency in the analytical process; the targeted surveillance must be realistic and possible to execute.
Once the data is validated it is fed into a centralised database for the access and distribution to all stakeholders, normally by via an internet interface. The outcome must be clearly accessible for all parts involved.
The use of epidemiological data can provide the evaluation of the disease and its consequences. The internet has become an important tool for the decentralisation of data entry. Appropriate analysis of data provides a good global representation of the situation (11,14). Geographic information systems GIS are used to support this process (16).
Objectives of national surveillance
TB is a notifiable disease worldwide. The OIE provides recommendations in its 'Terrestrial Animal Health Code' and all aspects of surveillance are addressed and recommendations given to follow (17).
The aim is to apply a national surveillance, monitoring and control system which will aid the eradication of the disease.
The main objectives for a national surveillance should be:
To estimate the size of the problem within the domestic and wildlife animal population.
To detect outbreaks in animals both farmed and in wildlife population, monitor the course of such outbreaks and evaluate the impact.
To identify bovine tuberculosis in the case that it is brought to the country.
To have preventive controls and eradication measures in place on defined animal populations to measure advancement and efficacy in the eradication program.
To have the capacity to verify disease freedom or low threshold of risk for trading objectives.
Conclusion
BTB is a life threatening disease that affects animals and humans alike. Since its discovery efforts have been made to track it, to treat the disease and to control its spread with the aim to eventually eradicate it. Despite these efforts bTB is re- appearing in an alarming way amongst animals and humans. Surveillance has become a key element to avoid, monitor and stop the spread of the disease.
Some countries have been successful in reducing and maintaining its spread, or even eradicating it through effective surveillance of their domestics and wildlife animal population.
The problem is complex, wide reaching and time consuming. It involves institutions, the farmers, the veterinarian services, the regional and governmental organisations; even environmental factors play an important part. There is a need to coordinate, on a regional level as much as on a national one. Exchange of information and communication is necessarily combined to ensure that data is processed and evaluated correctly, timely and used appropriately (37).
Surveillance and effective control programmes for bTB, adapted to the needs of each country and region, should be implemented and supported by regional, governmental and international bodies, in a coordinated manner.
Developing countries often lack of the necessary facilities, and therefore need additional support from the developed world to aid their eradication of this disease.
(37) Epidemiological Surveillance Systems
