Animal Health and Welfare Strategy for Great Britain: The Evidence Base
Chapter 5: Disease
5.1.1 The occurrence of disease is a fact of life. We are concerned here with infectious disease and all animal owners have to deal with some infectious disease from time to time whether it be livestock, poultry or companion animals. The rate at which disease occurs will vary and is dependent on a variety of factors such as the degree of exposure to the infectious agent, the density at which the animals are kept, animal movements, susceptibility, nutrition, etc.
5.1.2 The occurrence of disease has the potential to affect welfare, growth, longevity (and hence production) and profitability. It follows that reductions in the prevalence of disease will generally benefit both the animal and its owner.
5.1.3 The occurrence of disease is not only a potential threat to the profitability of a business but it may also be a cost to government. In 2003 it is estimated that almost 190 million was spent in Great Britain on compensation for livestock diseases and the schemes in place to protect public health (e.g. the Over Thirty Months Slaughter scheme that removes adult cattle from the human food chain). About 87 per cent of this spending was on BSE schemes and about 13 per cent was for bovine TB. The following chart illustrates the amount spent in Great Britain on disease and compensation each year since 1990 (including an estimate for 2003). The effects of the introduction of BSE measures from 1996 and the FMD outbreak in 2001 are particularly noticeable.
Figure 5.1.1: Amount spent on livestock disease compensation and OTMS in Great Britain
Source: Defra, National Assembly for Wales, SEERAD
5.1.4 The AHWS sets out a framework to effect improvements in animal health and welfare. To measure any improvements it is necessary to establish current disease status. This section seeks to do this and covers the groups of diseases:
Source: Defra, National Assembly for Wales, SEERAD
Note: Some diseases fall into two categories e.g. Bovine Tuberculosis is an endemic notifiable disease.
5.1.5 Some of the available information is more complete for some diseases than for others. For example, there are complete and accurate data sets for the incidence of BSE back to 1987 and similar data are available for most other notifiable diseases. For other non-notifiable diseases the data are much less complete.
5.1.6 By its very nature, disease is dynamic and subject to changes in prevalence and distribution and any data set relating to a current disease problem is only useful if it is regularly updated. Defra's website www.defra.gov.uk has regularly updated statistics on certain diseases and throughout this section appropriate links have been provided to sites which are regularly updated.
5.2 Notifiable diseases
5.2.1 A notifiable disease is a disease named in section 88 of the Animal Health Act 1981 or an Order made under that Act. Orders made under the Act have enacted more recent European Union Legislation on notifiable diseases. A list of notifiable diseases can be found on the Defra website 2. The following people are responsible for reporting the possible presence of a notifiable disease: a) the person in possession of a diseased or suspect animal or carcase; b) a veterinary surgeon or other person who in the course of his duties examines or inspects a diseased or suspect animal or carcase; c) a person who, following analysis by him of a sample taken from an animal or carcase, reasonably suspects that the animal or carcase is diseaed. The responsible person must give notice of the fact as soon as practicable to the Divisional Veterinary Manager.
5.2.2 Many notifiable diseases have never been found in the UK, as far as the records show. Some were last seen as long ago as the 19th century. Those that are currently present in the UK are bovine spongiform encephalopathy (BSE) in cattle, bovine tuberculosis (bovine TB) in cattle and deer, scrapie in sheep and goats, and paramyxovirus in pigeons. Other notifiable diseases seen from 2000 onwards include anthrax, Brucellosis (Brucella abortus), classical swine fever, contagious equine metritis, and Foot and Mouth disease. The following sections provide some basic information on each of these diseases. Links to more detailed information about each disease and the latest statistics have been provided where possible. More detailed information is also available in the annual report of the Chief Veterinary Officer 3.
5.3.1 Veterinary Officers and Local Veterinary Inspectors investigated 7,595 suspected cases of anthrax during 2003. The most common clinical sign is sudden death and it is incidents such as these which trigger notification and investigation. Suspected cases investigated in 2003 included 7,534 cattle, nine deer, four horses, 23 pigs and 25 sheep. Anthrax was not confirmed in any of these investigations. The last confirmed case was in 2002 in a bovine animal. Statistics on the number of anthrax outbreaks in Great Britain since 1887 are available from: http://www.defra.gov.uk/aminalh/diseases/notifiable/statistics/anthrax.htm
5.4 Bovine Spongiform Encephalopathy (BSE)
5.4.1 BSE is a relatively new disease of cattle. It is a neurological disease involving pronounced changes in mental state, abnormalities of posture and movement and of sensation. The clinical disease usually lasts for several weeks and it is characteristically progressive and fatal. It was first recognised and defined in the United Kingdom in November 1986. Over the next few years the epidemic grew considerably and affected all parts of the country but to different degrees. BSE occurs in adult animals in both sexes, typically in four and five year olds.
5.4.2 Since the early 1990s the number of confirmed cases has declined. In 2003, disease was confirmed in 175 animals presented as clinical suspects (passive surveillance). A further 374 cases were confirmed from just under 400 hundred thousand animals tested in the active surveillance programme. This compares with the 2002 figures of 445 clinical cases and 594 cases from testing just over 330 thousand animals in the active surveillance programme. In 2003 an estimated 160 million was spent by government in Great Britain on compensation under the Over Thirty Months Scheme and on disease compensation. A wide range of BSE statistics are available from: http://www.defra.gov.uk/animalh/bse/bse-statistics/level-3-incidence.html
Figure 5.4.1: Passive surveillance: confirmed cases of BSE in animals slaughtered as suspects in GB 1988-2003
5.5.1 Scrapie has been present in this country for over 200 years with no indication of any risk to human health. However, in light of the link between BSE and vCJD, scrapie has assumed a much greater significance. Sheep are likely to have been exposed to the same infected feed that resulted in BSE in cattle. There is therefore a theoretical possibility that BSE is present in sheep. In scrapie, infection can be found in a wide variety of tissues and there is evidence of horizontal transmission. If BSE were present in sheep and if it behaved as scrapie, it would present a potential threat to human health. Much of the current international interest in scrapie is a result of this possibility. Cases of natural scrapie in GB are examined for any evidence of BSE using the latest laboratory techniques. The number of reported cases of scrapie declined from mid-2003, possibly reflecting awareness of impending compulsory flock controls under EU Regulation 999/2001 (as amended).
5.5.2 More scrapie statistics are available from the Defra website at: http://www.defra.gov.uk/animalh/bse/bse-science/scrapie/inciden.html
Table 5.5.1 Summary of scrapie cases in Great Britain 1993-2003
5.6 Bovine Tuberculosis
5.6.1 Tuberculosis is an infectious and contagious disease of humans and animals caused by several species of a family of bacteria called Mycobacteriaceae. Mycobacterium bovis (M. bovis) is the bacterium that causes TB in cattle (also known as bovine TB). Although cattle are the main reservoir and natural host of M. bovis, humans and a wide range of mammals are also susceptible to this bacterium. In addition to cattle, a number of wild animals can act as reservoirs of M. bovis in different regions of the world, posing major impediments to eradicating the disease in cattle. About 1 per cent of bacteriologically confirmed cases of TB in humans in the UK can be attributed to M. bovis.
5.6.2 Bovine TB currently affects only a small proportion of the national herd. About 5 per cent of herds in Great Britain were affected by TB restrictions at some point in 2003, though in hotspot areas such as the south west of England, the percentage was much higher. The TB testing regime was severely disrupted by the Foot and Mouth Disease and when it resumed was initially targeted towards high risk herds and those that had overdue tests. In 2003 provisional figures show that 3.4 per cent to 3.5 per cent of tests on unrestricted herds resulted in a confirmed new herd incident compared with 2.8 per cent in 2000 (pre-FMD). Over 23 thousand cattle were compulsorily slaughtered in 2003 compared with 8.4 thousand cattle in 2000. Government pays the full market value for all animals slaughtered under TB control measures and in the financial year 2002/03 the bill for this was 31.1 million.
5.6.3 Monthly bovine TB figures from 1996 onwards are regularly updated on the web at http://statistics.defra.gov.uk/esg/datasets/tbweb.xls . The following chart shows the percentage of tests on unrestricted herds in Great Britain resulting in a confirmed new herd incident.
Figure 5.6.1: Percentage of tests on unrestricted herds in Great Britain resulting in a confirmed new incident of bovine TB
5.7.1 During 2003 Brucella abortus was isolated in four cattle herds in Scotland and the origin of infection identified as cattle imported from the Republic of Ireland. As a result two entire herds with evidence of active infection were slaughtered, and in two other herds seropositive infected cattle which had not yet calved were identified and slaughtered. These were the first isolates of
B. abortus from cattle in Great Britain since 1993. In March 2004 Brucella abortus was found in a herd in England. Despite these isolated incidents Great Britain has remained an Officially Brucellosis Free Region of the EU since 1991.
5.8 Classical Swine Fever (CSF)
5.8.1 CSF was first confirmed in this country in 1864 and was initially allowed to spread unchecked until 1878, when legislation for its control was introduced. The disease persisted for many years until it was finally eradicated from Great Britain in 1966 (see below for details of outbreaks). Since then there have been sporadic outbreaks in 1971 and 1986.
5.8.2 A more serious outbreak in East Anglia in 2000 affected 16 farms. A total of 74,793 pigs including those on contact farms were slaughtered to eradicate the disease. The case of this most recent outbreak was not finally established but was most likely the result of pigs eating a contaminated imported pork product. Statistics on the number of CSF outbreaks in Great Britain since 1879 are available from: ( http://www.defra.gov.uk/animalh/diseases/notifiable/disease/classicalsf.htm)
5.9 Contagious Equine Metritis
5.9.1 In 2002 a contagious equine metritis organism (CEMO) outbreak resulted in restrictions being placed on a 24 'at risk' stallions and mares under the Infectious Diseases of Horses Order 1987, as a result of tracings from the original infected stallion. Swab samples taken to establish the CEMO status of these restricted horses revealed the presence of the causal organism Taylorella equigenitalis in one mare in late 2002, and a further mare in February 2003. Both mares had been served by the original infected stallion. No further cases have since been detected in Great Britain as a result of either outbreak tracings or routine industry pre-breeding testing.
5.10 Foot and Mouth Disease (FMD)
5.10.1 Foot-and-mouth disease (FMD) is an acute infectious disease, which causes fever, followed by the development of vesicles (blisters) - chiefly in the mouth and on the feet. The disease is caused by a virus of which there are seven 'types', each producing the same symptoms, and distinguishable only in the laboratory. Until 2001 Great Britain had been FMD free for many years. In 2001 there were 2,026 cases in Great Britain involving the slaughter of over 4 million livestock. Statistics on the number of FMD outbreaks in Great Britain since 1877 are available from: ( http://www.defra.gov.uk/animalh/diseases/notifiable/statistics/fmd.htm)
5.11.1 Paramyxovirus (PMV) of pigeons is a virus belonging to the Newcastle Disease Groups, group 1 paramyxoviruses. It was first reported in the Sudan and subsequently moved towards Europe, reaching first Italy, Spain and Portugal then Holland, Belgium, Germany and France. In June 1983, racing pigeons were found to be infected in Cornwall, the first case in Great Britain. Since then hundreds of cases have been identified. Statistics are available from the Defra website: http://www.defra.gov.uk/animalh/diseases/notifiable/statistics/pigeon_stats.htm
5.12.1 Zoonoses are diseases and infections which can spread naturally between vertebrate animals and people. People may become infected by a variety of routes including contaminated food and water (foodborne and waterborne), direct contact with the animal or its contaminated environment, and through insect vectors. Successful management of the risks to public and animal health posed by zoonoses requires close collaboration between all those involved in managing animal health, producing food, safeguarding public health and the environment.
5.12.2 The 'Zoonoses Report UK 2002', gives an overview of Zoonoses in the United Kingdom. It was published early in 2004 and is avaialble on the Defra web site 4 along with earlier reports. These reports aim to be useful both to the professionals who deal with zoonotic diseases and to the non-specialists who wish to have an insight into zoonoses, their prevalence and importance. The introduction discusses some of the drawbacks in using the figures as an absolute measure of the incidence of zoonotic diseases.
5.12.3 At the beginning of 2003 a survey of cattle, sheep and pigs arriving at GB slaughterhouses was started. The survey is to determine the national prevalence in these animals of a number of zoonotic organisms including salmonella, campylobacter, Verocytotoxigenic E. coli O157 (VTEC O157), yersinia and cryptosporidia, along with the susceptibility to antimicrobials of isolated organisms. The study will be completed early in 2004 with the aim of analysing and reporting the results in autumn 2004, which should give a comparison with the situation at the last similar survey in 1999-2000.
5.13.1 All laboratories are required to report details of the isolation of salmonella from samples taken from animals, their environment and feed, as defined in the Zoonoses Order 1989. These data are collated, analysed and published annually in 'Salmonella in Livestock Production GB 5. A similar analysis of the data collected in 2003 will be published in summer 2004. Additionally, 144 advisory visits were made in 2003 to farms to provide advice on the control of salmonella in livestock.
Figure 5.13.1: Salmonella in domestic fowl in Great Britain 1983-2003
5.14 Multiple antimicrobial resistant salmonella serotypes
5.14.1 Antimicrobial resistance is a problem that makes disease control more difficult. As resistance to treatments increases new treatments need to be used or developed to be able to continue to treat a disease effectively. Towards the end of the year multiple antimicrobial resistant strain of Salmonella Java were isolated on a cattle premises in England. These strains, which were resistant to ampicillin, chloramphenicol, streptomycin, sulphonamides, tetracyclines, trimethoprim, cefoperazone (ACSSuTTmCfp), had not been seen in GB before. Further investigations were put in place to better understand these strains and assess the risk they pose to the animal and human population. Similar but not identical strains have been found in human infections.
5.14.2 In 2002, following reports of multiple antimicrobial resistant strains of S. Newport causing concern in the United States, surveillance was enhanced. The USA strains were resistant to ampicillin, chloramphenicol, streptomycin, sulphonamides and tetracyclines, and in addition often had intermediate or full resistance to third generation cephalosporins (ceftriaxone). No similar strains were reported in livestock in Britain in 2003.
5.15 Endemic disease
5.15.1 The term 'endemic' is used to describe those diseases which are constantly present in a population. Some endemic diseases are also notifiable diseases that must be reported to the DVM (e.g. bovine TB). Many other endemic diseases do not need to be reported. Examples include Bovine Virus Diarrhoea and Orf. Where non-notifiable livestock disease is suspected samples can be submitted for laboratory investigation to discover what disease (or diseases) may be present, but such testing is not obligatory.
5.15.2 The Veterinary Investigation Diagnosis Analysis database (VIDA) contains a record of every submission made to Veterinary Laboratories Agency (VLA) Regional Laboratories and Scottish Agricultural College (SAC) Disease Surveillance Centres in Great Britain and has been operating since 1975. In England and Wales there are fifteen such centres which form part of the Surveillance Division of the Veterinary Laboratories Agency. In Scotland there are eight centres, administered by the Scottish Agricultural College.
5.15.3 In 2002 over 123 thousand submissions were received. The total number of submissions recorded by VIDA represent only the material submitted for investigation. Trends in the data can therefore be affected by why a sample was (or was not submitted) from a particular animal. This potential bias is influenced by many factors including the particular clinical presentation of a suspected disease, the level of awareness of a disease and its perceived importance, the value of the animal or animals affected, and the general economic climate. Particular diagnoses may also be affected by improved scientific methods, and knowledge of this may also affect rates of submission; these factors will usually vary differentially with time. This bias should be considered when interpreting both individual figures, and apparent trends, from VIDA data.
5.15.4 The latest VIDA annual report can be found at: http://www.defra.gov.uk/corporate/vla/science/science-vida01.htm
5.15.5 Examples of VIDA endemic disease data where trends are reasonably robus include diagnoses of causes of abortion and mastitis in cattle.
5.15.6 Abortion in cattle: The pattern of causes of abortion in cattle has shown little change over the last four years except for Leptospirosis, which has progressively reduced from 24 per cent to 3 per cent of total abortion diagnoses. Fetopathy due to Infectious Bovine Rhinotracheitis (IBR) has fallen by 85 per cent over the last eight years. The reduction of both Leptospirosis and IBR feopathy may, in part, be due to the introduction of more stringent diagnostic criteria.
Figure 5.15.1: Diagnosis of bovine fetopathy in 2002 as a percentage of all bovine fetopathy diagnoses (n = 1078) (excluding cases where a diagnosis was not reached)
Source: VIDA annual report 2002
5.15.7 Mastitis in cattle: In 2002 there was a total of 6,517 diagnoses of mastitis in cattle. In 19 per cent of diagnoses no pathogen was identified. Of the remaining diagnoses 4,390 were cases of clinical mastitis and 896 were sub-clinical cases of mastitis. Staphylococcus aureus accounted for 34 per cent of the sub-clinical mastitis diagnoses in 2002 (compared with 19 per cent in 1999 and 25 per cent in 2000). This apparent high prevalence possibly combined with increased recognition of the need to carry out bacteriological examination of high cell count cows. The apparent increase of Staphylococcus aureus, for which established control measures exist, is interesting in light of the observation that the labour resource on some dairy farms may have dropped below a critical threshold, as a consequence of low milk prices, possibly resulting in less attention to detail in cow management. The relative proportions of major mastitis pathogens diagnosed in all mastitis submissions during 2002 are shown in figure 5.15.2.
Figure 5.15.2: All incidents of mastitis in cattle in Great Britain as a percentage of total mastitis diagnoses in 2002 (n = 6517)
Source: VIDA annual report 2002