EPIDEMIOLOGICAL ANALYSIS AND INTERPRETATION OF SERIOUS POULTRY DISEASES A. James Veterinary Epidemiology and Economics Research Unit, University of Reading, Department of Agriculture P.O. Box 236, Reading RG6 6AT, United Kingdom Original: English Summary: At present, the available information on the occurrence and impact of most poultry diseases is very limited. Epidemiological analysis and interpretation of serious poultry diseases is important at producer, national and international levels. For producers, it allows monitoring of production and early warning of disease and other problems. Analysis of the monitoring data can help to identify the causes of health problems, and allow the development of more effective preventive strategies. It also assists rational economic decision-making on health management policy options. At the national level, epidemiological analysis and interpretation permits better decision-making on national disease control policies and priorities. The efficiency of national poultry disease surveillance and monitoring programmes can be improved by the use of epidemiological principles in their design. With the present trend toward liberalisation of world trade, epidemiological analysis is increasingly important in demonstrating the true disease status of potential trading partners, and in assessing the risk associated with trade in different commodities. Increased trade in poultry products should result in improved economic welfare for both producers and consumers, and in general improvements in the world poultry health status. 1. POULTRY INDUSTRIES Poultry industries in most countries are characterised by enormous variation in scale of production and level of management. Poultry systems include individuals owning a few free-range birds, as well as industrial units containing many thousands of birds under intensive management and tightly controlled conditions. This diversity of production systems has important implications for disease control and epidemiological analysis. In economic and social terms, poultry production is probably the most important form of livestock production. For many countries intensive poultry production offers the best prospect of improving the quantity and quality of protein in human diets. Improvements in production technology have led to long-term reductions in the real price of poultry products. At the other end of the scale, village poultry make small but vital contributions to the income and nutrition of rural communities. Intensive poultry production is a major economic activity, and accounts for a significant fraction of gross national product for many countries. It is more comparable to an industrial process than other agricultural enterprises. There are high levels of capital investment and recurrent costs, and output is correspondingly high. Very high standards of management and control are required to maintain productivity at economic levels. Village poultry, on the other hand, require minimal investment and recurrent input. The birds utilise scavenged feed resources that would otherwise be wasted. Levels of production are frequently very low, but the offtake that is obtained has cost very little to produce. In many countries, traditionally-reared birds are preferred by consumers and command a much higher market price than intensively-reared birds. There is a trend towards vertical integration in intensive poultry production, with large companies seeking to encompass the entire production and marketing process by acquiring interests in breeding, feed production, rearing and marketing. By this means they hope to achieve better control of the quality of inputs as well as assuring market outlets. Such enterprises have the potential to manage disease constraints throughout the production chain. Some of these companies are multinational, and international trade in poultry inputs and products is of increasing importance. Countries with comparative advantage in the production and processing of poultry are developing important export industries. To maintain and develop this trade will require internationally-recognised assurances on the health status of exported products. - 159 -
Village poultry are subject to a wide range of production constraints, including predation, disease and poor nutrition. To improve levels of production requires that all of these constraints be addressed. This can usually only be achieved by containing the birds and adopting a small-scale intensive production system. However, these systems cannot easily utilise scavenged feed resources, and so there remains the opportunity to produce something from nothing with freerange poultry. For this reason, free-range village poultry are likely to continue to be present in many countries, and this has important implications for poultry disease control programmes. 2. SERIOUS POULTRY DISEASES Only two poultry diseases, highly pathogenic avian influenza (fowl plague) and Newcastle disease, are included in the OIE List A diseases (FAO 1 /OIE 2 /WHO 3, 1997). However, thirteen more are included in List B and five in List C. While these diseases are important, serious economic loss also results from other diseases that are not subject to international reporting. These include non-infectious diseases, external and internal parasites, and diseases of more local significance such as hydropericardium syndrome. The available information on occurrence of the OIE List A and List B poultry diseases is summarised in Table 1. It is clear that important diseases are widespread, but also that the information is incomplete. For most of the diseases, there is very little information on actual or potential economic impact. Fowl plague Newcastle disease Table 1: Occurrence of poultry diseases Avian infectious bronchitis Avian infectious laryngotracheitis Avian tuberculosis Duck virus hepatitis Duck virus enteritis Fowl cholera Fowl pox Fowl typhoid (S.gallinarum) Infectious bursal disease Marek's disease Mycoplasmosis (M.gallisepticum) Psittacosis and ornithosis Pullorum disease (S.pullorum) Country Afghanistan Australia 0 0 ++ + + 0 0 + ++ 0 ++ ++ ++ + + Bangladesh Bhutan 0 ++ 0? 0 0 0 + + + + 0 + Cambodia +++ +++ 0 0? 0 0 +++ ++ ++ 0 0 ++ 0 + China (P.R.) 0 ++ ++ ++ + + 0 ++ + 0 ++ ++ + + ++ India 0 ++ + + + + + ++ ++ + ++ + + 0 + Indonesia 0 ++ 0 0 + + 0 + 0 0 0 + Iran 0 ++ ++ + 0 0 0 + + 0 ++ ++ +++ 0 0 Iraq Japan 0 + + 0 0 0 0 + 0 + + + + 0 Korea 0 ++ + + 0 0 0 0 + + + + + Laos 0 +++ ++ + + 0 +++ +++ ++ + + + + 0 + Malaysia 0 + + + 0 + + + + 0 + + 0 0 + Mongolia 0? 0 0 + 0 0 + 0 0 0 0 0 0 + Myanmar ++ ++ + + + ++ ++ + +++ + +++ +++ New Caledonia 0 0 ++ + 0 0 0 + ++ 0 + ++ + + 0 New Zealand 0 0 ++ ++ + 0 + ++ 0 + ++ + + 0 Pakistan +++ + + 0 0 0 0 + + ++ ++ + ++ 0 ++ Philippines 0 + + + 0 0 0 + + + + + + + + Russia 0 + 0 + 0 0 0 + + 0 + + + 0 ++ Singapore 0 0 0 0 0 + 0 0 0 0 0 0 0 + Sri Lanka 0 ++ 0 0 0 0 0 ++ +++ 0 + ++ 0 0 ++ Taipei China Thailand USA 0 0 + + + + + + + 0 + + + + + Vanuatu 0 0 + 0 0 0 0 + 0 + 0 + Vietnam 0 + 0 0 0 0 0 + + + + + + 0 + Source: FAO/OIE/WHO Animal Health Yearbook for 1995 0: reported absent +: low sporadic occurrence +++: high occurence?: suspected but not confirmed ++: enzootic 1 Food and Agriculture Organization of the United Nations 2 Office International des Epizooties 3 World Health Organization - 160 -
Eleven of the 28 Member Countries (39%) provided reports on the epidemiological analysis and interpretation of serious poultry diseases. The country reports on occurrence of disease show similar patterns to these data. The important diseases reported as most commonly seen include: Newcastle disease, infectious bursal disease (Gumboro), salmonellosis, Marek s disease and mycoplasmosis. The diseases identified as having caused serious economic loss in the last decade are the same, with the following additions: coccidiosis, avian influenza and hydropericardium syndrome. There are wide variations in the list of notifiable poultry diseases in different countries. In some countries no poultry diseases are notifiable, while others include many of the OIE List A and List B diseases. Most countries reported that they either had diagnostic facilities for the poultry diseases considered important, or had arrangements to send samples to other countries. The reports are summarised in Table 2. Disease Table 2: Summary of information in Member Country reports. *Commonly seen *Notifiable *Serious loss in last 10 yrs *OIE action recommended Newcastle disease 4 9 5 3 IBD (Gumboro) 5 3 4 4 Salmonellosis 5 4 1 3 Marek s disease 2 2 1 1 Mycoplasmosis 2 1 Coccidiosis 1 1 Avian influenza 6 1 3 Inclusion body hepatitis 1 Chronic respiratory disease 1 Leukosis 1 Hydropericardium syndrome 1 *No. countries (of 11 responding). 3. EPIDEMIOLOGICAL ANALYSIS Epidemiology can be defined simply as the study of disease in populations. This is in contrast to the diagnosis or treatment of individual animals. The word population could either mean a single herd or flock, a village population, or the animals in a region or country. Epidemiology is especially relevant to the control of poultry diseases. It would hardly ever be economic for a veterinarian to diagnose and treat a disease problem in a single bird. To justify the cost of an intervention, it is necessary that it should benefit a large number of birds. Poultry are usually kept in large numbers in close contact with each other. Under these conditions, infectious diseases can spread very rapidly through the population. Poultry populations tend to be homogenous with respect to their genetics, environment and feeding, so non-infectious diseases are likely to affect the whole population at the same time. All of these factors indicate that health management in poultry should be implemented at a population level, using epidemiological approaches to the prevention, diagnosis and treatment of disease. The methods used in epidemiology all depend on information. In diagnosing a disease in an individual animal, all of the necessary information can be obtained by examination and testing of that single animal. However, in diagnosing a disease problem at population level, much more information is required. This might come from herd records, questioning the owner or collecting information from samples of animals in the population to estimate the status of the whole population. Economic factors are of critical importance, both in understanding the causes of disease in poultry populations, and in making decisions on disease prevention and control. In poultry more than any other type of livestock, environmental and management factors are inextricably linked with disease as determinants of productivity. Intensive poultry production is a highly competitive business with slim profit margins. A minor failure of health control, or excessive expenditure on drugs or vaccines, can easily turn the profit on a poultry enterprise into a loss. Therefore epidemiological analysis is almost certain to include economic analysis, recognising economic factors as both determinants of the epidemiology and as results of disease control policy. - 161 -
4. APPLICATIONS OF EPIDEMIOLOGICAL ANALYSIS AT PRODUCER LEVEL Faced with a wide range of infectious and non-infectious diseases, poultry producers can use epidemiological techniques to identify and diagnose health problems in their flocks, and to assist in decision-making on disease prevention and control. At the heart of these applications is information. Simple monitoring of such factors as output, water and feed input, vaccination regimes and mortality can provide early warning of problems in health, management or both. Laboratory analysis may be needed to complete the diagnosis, but the monitoring data alert the manager to the existence of the problem. The same data may also be of value in identifying the causes of the health problem; for example, by linking the disease with birds from a particular source, or with particular batches of feed or vaccine. Monitoring data can also be used in the economic assessment of the enterprise, and to assist in decision-making on health and management issues. More advanced epidemiological surveillance of commercial poultry flocks may include regular sampling of birds to monitor levels of endemic diseases and parasites, or to confirm the effectiveness of vaccination programmes. Clearly it would be uneconomic to test entire flocks in this way. Random samples of birds are selected, and the results from the sample then provide an estimate of the status of the whole flock. The reliability of the estimate depends on the sample size. Epidemiological formulae and tables allow confidence limits for the status of the whole flock to be calculated. Though these methods are applicable to large-scale enterprises, small-scale producers generally have neither the knowledge nor the resources to use them. In consequence, it is often the case that practically nothing is known about the health status of small poultry flocks, unless there are major outbreaks of notifiable diseases. Even then, it is all too common to find that the causes of widespread mortality among village poultry are unknown. The main constraint to improving the health of small-scale poultry flocks is the high cost of delivering services to small numbers of birds. The problem is even more acute in extensive free-range systems, where other constraints such as predation and poor nutrition can mean that the overall benefit of disease control is marginal. There is no benefit in protecting a bird from Newcastle disease if it will eventually fall victim to some other disease or a predator. 5. APPLICATIONS OF EPIDEMIOLOGICAL ANALYSIS AT NATIONAL LEVEL At the national level, epidemiological analysis is used to support disease control policy. This can take the form of epidemiological surveillance to determine whether a disease is present in the country. Epidemiological monitoring is used to estimate the prevalence and incidence of diseases that are present. Epidemiological analysis may help to identify patterns of disease in the national flock, and suggest more effective control policies. Economic analysis is used in conjunction with epidemiological analysis to evaluate alternative disease control policies. At this level, economic analysis must include much wider issues than are relevant to the individual producer. For example, human health must be taken into account in the case of zoonotic diseases, and the eradication of a disease might open export markets with price consequences for both producers and domestic consumers. Epidemiological surveillance and monitoring are both used to determine the disease status of a population. Monitoring is intended to estimate the prevalence of a disease or a disease sign, while surveillance is intended to confirm that the prevalence is zero. In both cases, the sample design should recognise that a national poultry population is not one homogenous unit, but is divided into flocks. Moreover, there are different types of flocks; for example, layer flocks, broiler flocks, parent flocks and village flocks. Given this heterogeneity of population, it would be meaningless to produce one figure as a national prevalence estimate. It would also be extremely difficult to devise a statistically-valid sample design and to calculate the reliability (standard error) of the estimate. Instead, the population should first be divided into strata, defined as different types of flocks and production systems. Each stratum, for example commercial layer flocks, is then treated as an independent sub-population for sampling purposes. Within the stratum at least two prevalence estimates are required: the proportion of flocks affected, and the prevalence within affected flocks. The sampling process is therefore to select a random sample of flocks, and then to select a random sample of birds from each selected flock. The sample sizes depend upon the required accuracy of the prevalence estimates and the expected prevalence in the population. Accurate estimates of high prevalence require larger sample sizes than equivalent estimates of lower prevalence. Tables of required sample sizes in both surveillance and monitoring are given in most of the standard epidemiological reference books. The numbers of animals to be tested can often appear to be very large. However, careful review of the objectives of the investigation can often reduce the burden. For example, in serological monitoring it is often the case that accurate estimates of the status of individual flocks are not required. The objective is to obtain a population overview. In this case, only a relatively small number of birds from each flock need to be tested. - 162 -
Where diseases are believed not to exist in the country, epidemiological surveillance programmes can provide a quantified degree of assurance that the disease is not present. For example, a surveillance programme could be designed to produce a 95% probability of detecting disease if it were present at a prevalence of 5% or more in 1% of the flocks in a stratum. It should be noted that sampling can never provide certainty that a disease is not present. To obtain certainty, it would be necessary to test every animal in the population. However, repeated surveillance programmes can produce very high probabilities of detection. A surveillance programme producing a 95% (0.95) probability of detection would have a 0.05 probability of failing to detect the disease. However, if the programme were repeated in the following year, the joint probability of failing to detect in both years would be 0.05 2 or one quarter of one percent. After three years, the probability of failing to detect the disease would be vanishingly small (0.05 3 ). It is also possible to reduce the total number of birds to be tested in disease surveillance programmes by balancing the number of flocks sampled and the number of birds tested from each flock. It is possible to compensate for reduced sample sizes within flocks by increasing the number of flocks tested. Judicious design of such two-stage samples can reduce the number of birds tested by as much as 80%, while maintaining the same probability of disease detection. If the results of sample-based epidemiological surveillance or monitoring are to be representative of the population, it is essential that random sampling methods be used. Purposive sampling methods lack the objectivity of random samples, and risk simply confirming the prejudices of the sample designer. Certainly for international recognition of disease status, random samples are required. Practical methods for selecting random samples in epidemiological research are described in the standard epidemiological reference books. The logistical and economic problems of delivering poultry health services to small-scale producers have already been referred to. However, in some national poultry disease control programmes it may be the case that it would be justified to control disease in small flocks in order to protect the rest of the industry. In such circumstances, it would usually be necessary for government or the industry to subsidise the operations in small flocks. However, commercial producers cannot always be relied upon to support disease eradication programmes, even where eradication would appear to reduce production costs compared to a vaccination policy. In an eradication or disease-free situation, commercial producers are always at risk from re-introduction of disease. If that results in quarantine, the costs to commercial producers can be ruinous. They are usually contractually bound to supply products on schedule to purchasers. Commercial producers seek to manage the production process as reliably as possible. They are willing to accept a scheduled vaccination cost, especially if their competitors all face the requirement. In that case the cost of the vaccination would be passed on to consumers through higher prices. 6. APPLICATIONS OF EPIDEMIOLOGICAL ANALYSIS IN INTERNATIONAL TRADE The use of animal health restrictions as non-tariff barriers to international trade in livestock products is coming under increased scrutiny from the World Trade Organization. In order to restrict trade on animal health grounds, countries have to show that there is a real difference in animal health status between the two countries, and that there is a significant risk of introducing exotic disease through the product being traded. As a guide to the animal health status of countries, the FAO/OIE/WHO Animal Health Yearbook suffers from the serious problem that it mostly relies on passive disease reporting systems. If a disease is not reported, it may be assumed to be absent. This does not provide sufficient assurance of disease-free status to allow importing countries to be confident of the safety of international trade. There is therefore a trend toward the prescription of active disease surveillance standards, as well as effective disease reporting systems, to demonstrate freedom from disease. So far as the evaluation of the disease risk associated with a particular traded commodity is concerned, epidemiologists are increasingly using risk assessment methods. These can be used to quantify the risk of disease introduction through a particular trade pattern. They can also be used to compare different control, quarantine and inspection procedures in terms of their effect in reducing risk. If these applications of epidemiological methods do succeed in liberalising world trade in poultry products, the economic welfare of both producers and consumers should, according to economic theory, be improved. It is also likely that a general improvement in animal health status will result. If facing competition from more efficient producers with better health status in other countries, commercial poultry producers will have to improve their own standards in order to continue in business. This will undermine the present preference of some commercial producers to tolerate any disease control cost, so long as it can be built into the process and the price passed on to the consumer. Producers in countries with disadvantages in poultry production will face real problems from international competition. These include countries with: - 163 -
- harsh climates, where housing costs are higher; - higher poultry feed costs; - higher labour costs; - large populations of village poultry acting as a reservoir for diseases; - stricter laws on animal welfare, resulting in higher housing and management costs. However, few countries have all of the advantages or disadvantages, and so it is unlikely that poultry production will become uneconomic in any countries, especially when the high transport cost of some products, such as eggs, is taken into account. REFERENCE FAO/OIE/WHO (1997). Animal health yearbook 1995. Food and Agriculture Organization of the United Nations. Rome. - 164 -