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By Dr Simon M. Shane, Durham, NC, USA
Poultry diseases continue to represent a serious loss to poultry producers in all areas of the world. Both industrialised and subsistence production is impacted, involving broilers, layers, turkeys, water fowl, as well as minor species. Infections of flocks are becoming more significant as a restraint to international trade, either justifiably to prevent introduction of diseases, or as a manoeuvre to protect domestic industries.
In evaluating the broad field of poultry diseases caused by bacteria, viruses, fungi, protozoon and metazoon pathogens, interaction among disease-causing agents, nutritional deficiencies and environmental stress should all be considered since individual infections seldom occur in isolation, especially in large intensive production systems. Recent studies have demonstrated the balance that exists between host and pathogen, and the role of a functional immune system in responding to infection and establishing an effective and durable protective antibody response at both the cellular and tissue levels.
Catastrophic diseases
For virtually the entire period of our current century poultry producers have been challenged by highly pathogenic avian influenza (HPAI strain H5N1). The infection has become endemic in many countries in Southeast Asia, Northern and West Africa. Sporadic outbreaks occur following contact between migratory birds and non-confined poultry in Eurasia, Central and Western Europe. From 2003 through 2005 there was considerable concern as to the possibility of a mutation of the avian virus or interchange of genetic components resulting in the avian strain becoming pathogenic to humans. Despite the dire predictions of epidemiologists affiliated to the WHO and other bodies, the great “avian flu pandemic” has not materialised. The investment in diagnostic resources, research and vaccinology stimulated by the threat has advanced preparedness for future events.
The loss associated with HPAI H5N1 is not limited to flock mortality or disruption in supply to markets. Index outbreaks were associated with precipitous declines in consumption, seriously affecting the profitability of commercial operations and devastating subsistence farmers reliant on their flocks for income. Enhanced biosecurity in endemic countries coupled with effective vaccination using inactivated emulsion products has blunted the severity of the disease. Affected nations no longer talk of eradication but have accepted an endemic status with attending control and prevention programmes that impose additional costs.
Sporadic outbreaks of HPAI that have occurred in Europe and Canada are regarded as exotic infections and have been effectively eradicated given the enabling legislation, availability of funds, contingency plans, trained personnel, and resources for quarantines, as well as the detection and disposal of infected flocks. In the words of Dr David Swayne, a distinguished avian pathologist in the US, who has extensive worldwide experience with influenza, “HPAI has become the Newcastle disease of the 2000s”.
Velogenic Newcastle disease, which is endemic in most poultry producing areas of the world, with the exception of North America, is largely controlled by applying a combination of biosecurity measures at the national and regional levels coupled with effective vaccination using a combination of live attenuated and inactivated vaccines.
Highly pathogenic bursal disease, which can be regarded as a catastrophic infection, has also moderated in its impact as producers have learned to apply effective vaccination programmes. Advances in molecular biology have resulted in innovative products such as HVT-vectored vaccines carrying components of the antigenic V1 gene to provide protection when administered to chicks either in ovo or by the subcutaneous route.
Erosive diseases
This group of infections involves primarily the respiratory and digestive tracts, as well as the immune system. The primary viral infections, including lentogenic Newcastle disease, infectious bronchitis and laryngotracheitis, can be suppressed by using appropriate vaccines. Infectious bronchitis and infectious bursal disease, as with other pathogens, is characterised by the emergence of variant strains, which necessitate the introduction of new vaccines that are homologous with field viruses. All of the principal viral respiratory diseases interact with pathogenic strains of E. coli, and the severity of infection is influenced by secondary bacterial infection and concurrent environmental stress.
Coccidiosis remains the most costly condition affecting poultry with the majority of expenditure associated with prevention using synthetic and ionophoric anti-coccidial feed additives. The emergence of “drug-free” broiler production has accelerated the development of more effective anti-coccidial vaccines with traditional oocyst suspensions being supplemented by precocious species, incorporated into vaccines. Coccidiosis is frequently a precursor of clostridial necrotic enteritis since in the absence of growth-promoting antibiotic feed additives, Clostridium spp. can proliferate in the jejunum and ileum resulting in necrotic enteritis and botulism.
The systemic diseases, including salmonellosis and pasteurellosis, require a combined approach to prevention, including biosecurity and vaccination. Inaddition to these measures, mycoplasmosis requires elimination of vertical transmission in the chain of production. Diseases impacting the immune system, including infectious bursal disease, Marek’s disease and chicken anemia, can be controlled with effective vaccination. Although a wide range of products are available to immunise flocks, failure in protection occurs as a result of improper storage, reconstitution and administration of vaccines, especially when provided for flocks in drinking water. Administration of vaccines in hatcheries is inherently more effective than in the field due to a higher level of training and supervision of personnel, as well as the use of sophisticated spraying equipment and in ovo injectors.
Avian pathogenic E. coli (APEC) is responsible for mortality and financial losses in turkey, broiler, breeder and egg producing flocks. During the past two years, the availability of a mutant,gene-deleted, live E. coli vaccine has shown considerable promise in ameliorating losses. This is especially the situation when combined with more effective protection against respiratory viruses and resolving improper production and management practices relating to ventilation.
Emerging diseases
Following the appearance of poult enteritis and mortality syndrome (PEMS) in turkeys in the US, ALV-J virus and spiking mortality in broilers, and a resurgence in malabsorption syndrome, there has been considerable interest in virus infections of the intestinal tract of chicks and poults. The relative importance of parvo viruses, rotaviruses, astro viruses and the avian nephritis virus as individual infections or in combination is under intensive study. The availability of more sophisticated diagnostic procedures based on molecular biology has facilitated the recognition of these infections. It is, however, difficult to reproduce field syndromes under laboratory conditions or to develop models to study specific isolates alone and in combination with viral, bacterial and protozoon pathogens.
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