Poultry diseases of various kinds permanently occur in all corners of the world. Some have regional impact, while others spread further. Let us take a look at an update on the most relevant diseases, their impact on world trade, and alternatives in treatment and prevention.
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.
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.
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.
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.
The potential zoonotic significance of highly pathogenic avian influenza highlighted the significance of commercial and subsistence poultry in the epidemiology of human infections. Greater concern over food safety is placing pressure on egg, broiler and turkey producers to reduce levels of Salmonella in products. Campylobacteriosis remains a serious food-borne pathogen in poultry meat. The routes of transmission, reservoir hosts and suppression of Campylobacter infection and decontamination are not as clearly understood as with Salmonella spp. Egg-borne Salmonella Enteritidis is controlled by implementation of egg quality assurance programmes, which are based on the elimination of vertical transmission through breeders, vaccination, high levels of biosecurity, and maintaining a cold chain from the time of packing through to point of sale. Education programmes to promote good handling procedures in commercial and domestic kitchens have also proven effective in reducing the incidence rates of salmonellosis in consumers.
The appearance of HPAI, SARS and Nipah virus, of which only HPAI is associated with avian species, has demonstrated the need for cooperation among poultry veterinarians, infectious disease specialists and epidemiologists, wildlife biologists and ornithologists. Cooperation in early detection, characterisation of viruses, determining methods of transmission between and amongst species and the role of travellers and migratory birds is becoming an established principle of disease control.
Since the mid 1980s, the routine use of growth promoting antibiotics has come under extreme scrutiny by public health authorities with the Scandinavian nations and the EU being the most aggressive in replacement with Consumer Acceptable Production Enhancers (CAPES). Elimination of antibiotics can be achieved but at a cost in terms of elevated mortality due to clostridial enterotoxemia, decreased growth rate and depressed feed conversion efficiency.
The use of therapeutic antibiotics varies widely among nations with minimal application in North America to widespread, flagrant and injudicious administration in many developing countries representing a severe health threat. It is evident that antibiotics will be phased out even in countries where use is still allowed, especially if these nations have export markets. Finding alternatives to antibiotics to compensate for decreased growth rate or to prevent specific diseases represents a challenge to scientists affiliated with intensive animal production. Some promise is shown by probiotics, including lactobacilli and other competitive exclusion organisms, and feed additive probiotics such as mannoligo saccharides. Botanicals have been shown to prevent some infections under controlled conditions, but large-scale production of these compounds showing consistent quality and efficacy at a cost that provides a beneficial return has precluded universal adoption by producers. Scientific journals attest to the sophistication of new assay and identification procedures including polymerase chain reaction and its derivative, gene sequencing to establish phylogenetic relationships and to predict pathogenicity. Both automated ELISA for laboratory monitoring of antibody protection and solid state ELISA for rapid diagnostic kits are widely deployed.
New practices ahead
The challenge to production efficiency represented by poultry diseases and parasitism will be countered by developing a new generation of vectored vaccines and educating a new cohort of field veterinarians empowered to improve biosecurity and preventive practice. Unfortunately, the current world economic downturn has reduced funding for research in universities, the public sector and the large bio-pharmaceutical companies. Resources available to combat poultry disease are constrained by declining budgets and a concentration on species other than poultry that offer a relatively greater return on R&D expenditure.