E. F. Kaleta
Institute for Avian Medicine,
Justus Liebig University, Giessen,
Germany
Summary
Newcastle disease (ND) and highly pathogenic avian influenza (HPAI) are notifiable diseases of commercial chickens and turkeys on a worldwide basis. Both viruses circulate also in many species of domestic fancy breeds, pet and free-living birds in which they cause various forms of clinical signs, morbidity and mortality. Surveillance of all these birds is strongly recommended in an attempt to detect as early as possible virus carriers and shedders. Also, more epidemiological studies need to be done to detect the causative viruses of notifiable diseases in live and dressed poultry, poultry products and by-products as well as in traded non-commercial birds. Since the required isolation, characterization, and pathotyping of viruses causing notifiable diseases may take several weeks to accomplish, strict biosecurity must be maintained and serological monitoring is recommended. Protection by vaccination against major epidemic diseases including ND and HPAI of flocks at risk is suggested.
Introduction
The European Poultry Conference is an ideal forum to discuss and to evaluate the role of poultry in human nutrition. In addition it's suitable for all questions relating to poultry husbandry conditions and therefore also for the health of poultry. The issue of international disease control has been a special topic for more than hundred years. As a consequence of these considerations the OIE (Office International des Epizooties) was formed. On national level, a large number of countries generated their own reporting systems for a selected number of important transmissible diseases.
The Council directive 1999/74/EC of 19 July 1999 laid down minimum standards for the protection of laying hens. This EU legislation affects directly the types of housing and consequently health and productivity. The advocated housing systems shall improve not only poultry welfare in a sense of animal behaviour but also general health. However, various forms of outdoor housing of laying hens result in more and prolonged exposure of hens to a large variety of transmissible agents and make horizontal infections and subsequent disease developments more likely.
It is attempted in this contribution to describe transmissible epidemic diseases and necessary means for their control. Particular reference is made to notifiable diseases, which affect domestic poultry in particular. Poultry can be either infected and become severely sick or act as reservoir or is an optional vector for these agents.
Some historical aspects
In the past, the occurrence, spread and disappearance of major diseases of men and animals was thought to be due to the unpredictable actions of Miasma and Contagion (Henle, 1840; Spinola, 1858). These two interacting components can be transliterated into contemporary terminology as environmental factors and infectious agents.
The first serious attempt to combat a transmissible disease in historical times was a highly lethal condition in cattle (probably Rinderpest) in central Italy. The heavy losses due to that disease called upon the Pope Clements XII. He discussed the disease and the resulting catastrophic consequences with his medics and a number of cardinals. After long deliberations, this consortium came to the conclusion that such a disease must be eradicated. The technical means suggested were killing (clubbing) and immediate burial on the spot of all diseased and all directly exposed cattle. The recommended method was to solve the Rinderpest problem in all other countries as well.
Since this obviously new approach was blessed with divine benediction, very soon other European countries followed the papal recommendations. Principal arguments to use the club for the destruction of cattle were: (i) inhibition of spread and infection, (ii) elimination of the yet unknown cause of the disease, (iii) the early use of the club will reduce the amount of the contagion, and (iv) protection zones with strictly forbidden access of cattle to pastures and roads and interruption of animal trade will prevent the spread of the contagium animale with live animals, products and people, and will finally eliminate the cause of the disease.
Surprisingly, already in 1788 the physician Wolstein casts doubts on the success of the papal order to kill animals affected with a highly transmissible disease. Wolstein (1778) argued that climatic, environmental and seasonal conditions alter the body and create sickness in conjunction with a contagion, which is formed inside the body, and act as poison if injected into other animals. Instead of large scale clubbing, Wolstein recommends appropriate food, clean water and fresh air, avoidance of pus or use of fire, washing of unclean clothing to eliminate the contagion. Further, farmers make their living on live and healthy animals, thus any restriction of movements will result in searches for hideouts and all kinds of excuses thus undermine the regulations of papal origin.
The present situation of control of epidemics
Quite obviously, after almost 300 years of practised disease eradication by clubbing of farm animals, it appears to be wise to try to re-evaluate the pontifical approach of disease elimination. A number of arguments support this option. These include (i) epidemics are no longer considered as God's punishment or the Devil's ill work. Informed people have confidence in the concept of disease causing microorganisms and additional effects of nutritional, environmental and genetic factors, (ii) the epidemics differ from each other in terms of aetiology, pathogenesis, modes of spread, latency, and other prominent characteristics, (iii) hygienic measures and disinfection help to reduce spread of these agents and (iv) effective drugs and vaccines were developed and used to cure or to prevent epidemics.
Although an abundance of information on epidemics was accumulated during the course of the last 100 years, contemporary measures refer still to the pontifical advice of eradication. However, it is also true that an increasing number of scientists and the public voice oppose the mass use of destruction measures.
The final goal of control of epidemics cannot be the total eradication of all transmissible, highly contagious agents. It is necessary and essential to bring these major diseases to an acceptable level. To reach this target, the presently used regulations and laws need further improvements and also substantial additions.
Notifiable diseases of poultry
For domestic poultry, two potentially devastating diseases are classified on international level as notifiable; these are Newcastle disease (ND, also known as atypical fowl pest) and highly pathogenic avian influenza (HPAI, formerly known as classical fowl plague). These two diseases are the only notifiable diseases of poultry. They are different from each other in terms of aetiology but are similar in terms of epidemiology, clinical signs, gross pathology, and means of virus detection.
From the epidemiological point of view, basically, the two notifiable diseases may affect three major groups of susceptible birds. These are (i) birds living in enclosed environments with high levels of biosecurity: commercial poultry and breeding stock, (ii) birds in backyard housing systems of variable forms of biosecurity: fancy birds attending exhibitions, trade shows and other similar activities, and (iii) wild birds living free, migrating within countries / regions or twice annually between countries or continents. All three groups of birds are (almost) equally susceptible to infection and develop variable forms of disease. However, access and monitoring (sampling of blood, faeces and eggs) of these birds poses different problems.
In confirmed cases of ND or HPAI, primary outbreaks in a region require according to internationally accepted regulations mass destruction of infected and exposed poultry. National reference laboratories were established to diagnose the responsible virus and task forces are ready for immediate action. Recent years witnessed a number of major outbreaks in commercial and back yard poultry which resulted in high levels of mortality, severe economic losses and disturbance / interruption of local or international trade in animals and animal products.
In many parts of the world, mass destruction of poultry is considered as a huge waste of valuable resources to feed mankind. This view is strongly supported by an ever-increasing size of a hungry human population especially in developing countries. This point receives further momentum by the fact that we witness a slowly decreasing area of farmable land and an increasing public concern in developed regions of the world.
Basically, the development of both epidemics could be successfully prevented by prophylactic vaccination. Raised arguments against the widespread prophylactic use of vaccines are - among others - that vaccinated poultry may be super infected and will be subsequently for a short period of time virus carrier and virus shedder which may cause unrecognized virus transmission to susceptible unvaccinated poultry.
Arguments in favour of vaccination are - among others - related to maintenance of the economic valuable poultry, prevention or at least significant reduction of virus multiplication in immune birds, absence of pain and suffering due to acquired disease and last but not least the beneficial effects on welfare of poultry. Cost-benefit-studies are in favour of vaccination (Kaleta, 1992). As a consequence, prophylactic vaccination against Newcastle disease is mandatory in some and completely illegal in other countries. Successful vaccination against all forms of HPAI is feasible - as it is in humans, horses, pigs and others. Yet, so far vaccination of poultry against HPAI has not been authorized in any of the major poultry producing and exporting countries.
The epidemiology of both viruses, NDV and HPAIV, is well known (Alexander, 1997). Both viruses are transported with live and dressed poultry, products and manure of poultry, and circulate in wild bird populations. An example is given in Table 1 which illustrates that antibodies against avian influenza A virus of the haemagglutinin subtypes H5 and H7 are present in some Canada geese that are free-living in the north-western part of Germany. Using and definitely applying the well established, firm knowledge on the epidemiology of ND and HPAI viruses does suggest that lateral spread to domestic poultry farms and the subsequent virus dispersion within poultry houses can be detected and countermeasure explored and implemented.
Measures for international disease control
A major disadvantage in the contemporary control of both notifiable diseases of poultry is the fact that all practised measures focus on poultry itself and not (yet) on their agents and their natural hosts outside the poultry world. In addition, trade in live poultry, poultry products and transport of manure needs to be included in monitoring and control measures. In view of the devastating damage to poultry and the negative effects on international trade/movements of birds, I am suggesting the following:
- Further improvements and worldwide availability of methods for sampling, assay, and evaluation procedures for viruses which cause devastating diseases;
- Enforcement of surveillance of domestic and imported farmed, pet and free-living birds using reliable parameters for sampling, virus assays, and interpretation;
- Use of these parameters also for traded poultry and all poultry products including fresh manure on port of entry;
- Out-door housing and free runs of large poultry flocks should be either abandoned or regularly monitored and vaccinated against major epidemic diseases, especially against ND and HPAI;
- Investments of farmers in improved biosecurity should be more honoured that the level of compensation payments for destroyed poultry;
- Establishment or further improvement of international communication systems that are based on an abundance of mutual trust.
References
Alexander, D. J. (1997). Newcastle disease and other avian paramyxoviridae infections. In Diseases of poultry 10th ed. (pp. 541-569). B. W. Calnek (Ed.) Ames: Iowa State University Press.
Henle, J. (1840). Pathologische Untersuchungen. August Hirschfeld Verlag, Berlin.
Kaleta, E. F. (1992). Paramyxovirusinfektionen. In Krankheiten des Wirtschaftsgeflügels. (Vol. II, pp. 587-661). G. Heider und G. Monreal (Hrsg.). Gustav Fischer Verlag, Jena und Stuttgart.
Kaleta, E. F. (1997). Epidemiology of avian diseases. Acta Veterinaria Hungarica, 45, (3), 267-280.
Office International des Epizooties. World Organisation for Animal Health. Manual of standards for diagnostic tests and vaccines. 3rd ed. Copyright OIE, Paris, France.
Spinola, W. T. J. (1858). Handbuch der speziellen Pathologie und Therapie für Thierärzte. Verlag August Hirschfeld, Berlin.
Wolstein, J. G. (1788). Anmerkungen über die Viehseuchen in Oesterreich. Nebst einer Abhandlung gegen das Umbringen der Thiere in Seuchen. Joseph Camesina Verlag, Wien.
From Proceedings of 11th European Poultry Conference, Bremen, Germany.
