Poultry Diseases

Viral respiratory diseases of significance are caused by only a handful of viruses represented by Newcastle disease virus (NDV), infectious bronchitis virus (IBV), infectious laryngotracheitis virus (ILTV), avian metapneumovirus (MPV), and avian influenza virus (AIV). Despite the relatively small number of types of viruses circulating in the field, the diseases and syndromes associated with them still represent an enormous economic and epizootiological challenge to the poultry industry and the veterinary profession. Some of such viruses are of relatively low virulence but can easily become enzootic in some areas. Others may be extremely lethal and may cause short-lived outbreaks. Strategies for control and prevention of respiratory disease are diverse and depend on many factors. Some of such strategies are only partially effective and others have shown very positive results in many ways, and include (not exclusively) a combination of biosecurity, effective vaccination, rapid and accurate diagnostics, and a comprehensive modern approach to disease control. A sound understanding of the interactions between poultry genetics, nutrition, husbandry and infectious disease is needed in order to succeed in poultry disease prevention and control, including the control of respiratory diseases. Some items important for disease control are discussed herein in the context of respiratory disease caused by viruses.

Interaction between genetics, nutrition, husbandry and infectious disease. 
 
Livability of broilers and layers has improved continuously over the past few decades. Part of this improvement should be credited to population genetics methodologies applied towards selection of individuals capable of withstanding the pressures of intensive poultry production while improving the performance for meat and egg production, a task not easy to accomplish. Improvements in poultry nutrition and feeding methodologies has also contributed to continuous progress in economic performance without compromising poultry health. Poultry husbandry, defined as the combination of methodologies for keeping domestic poultry in the most favorable conditions for health, welfare and economic performance, has also accomplished substantial advances without which economic performance could not happen. Finally, infectious diseases can be expressed regardless of any of the factors aforementioned, but without a proper balance of all of them infectious disease can easily become overwhelmingly difficult to control.

The role of biosecurity in the prevention and control of respiratory viruses

Biosecurity is a loose term that may signify different things for many people. Regardless of the definition of biosecurity each person might have, it is clear to most people that biosecurity is a requirement for disease control. However, the poultry industry still has many risks derived from every day practices (Nespeca et al. 1997; te Winkel 1997; East 2007; Dorea et al. 2010; Halvorson 2011; Van Steenwinkel et al. 2011) and many of such practices are not usually thought of carefully. Generally and in practical terms, biosecurity intends to prevent the introduction of infectious disease into a naïve population and/or the prevention of exporting infectious disease to naïve populations of poultry. Proper farm design while making a conscious effort to prevent disease has been successful in averting many costly conditions (Halvorson 2011). Some respiratory diseases of viral origin are practically impossible to prevent under ordinary circumstances. Infectious bronchitis is one example of a disease that can easily occur despite proper biosecurity, especially in areas with a high poultry density (Halvorson 2011). However, IBV being an infectious agent that is species specific should not be impossible to contain to a certain extent. Other diseases such as infectious laryngotracheitis are much more dependent on biosecurity because the virus can easily be transported mechanically between houses, farms or even geographical areas (Dufour-Zavala 2008). Newcastle disease and avian influenza are examples of viral respiratory diseases that have a more complex ecology since their etiologies can be shared by poultry and other avian species, and thus prevention often has to rely on vaccination (Anonymous 1970; Anonymous 1971; Anonymous 1971; Alexander 1995; Alhaji and Odetokun 2011). All too frequently one  can witness situations in which despite multiple live and killed vaccinations against NDV or after several applications of recombinant vectored and killed vaccines against avian influenza flocks of commercial poultry still suffer from severe outbreaks of either Newcastle disease or avian influenza. Countries where such diseases are enzootic often violate basic principles of biosecurity by allowing farm workers and visitors to enter poultry premises without showering in and changing clothes and footwear. It is not uncommon to observe outbreaks of avian influenza or Newcastle disease in poultry houses that are not bird proof. In addition, the rules for handling and transportation of contaminated poultry litter may not be followed, placing at risk many populations of susceptible or partially protected poultry.

Vaccines and vaccination for the prevention of viral respiratory diseases

There is no question that many vaccines and vaccination equipment have improved over the years. Some of the most relevant improvements of recent years consist of improved killed vaccine emulsions; development of new variant virus vaccines; development of viral vector recombinant vaccines; balanced titer in multivalent vaccines in order to minimize interference; development of hyperconcentrated inactivated vaccines against Newcastle disease and avian influenza for application at hatch or in very young chickens; attenuation of various subtypes of metapneumovirus of chicken and turkey origin; design and development of live vaccines for in ovo application; etc. Progress has also been made in adequacy of vaccination equipment for layers in production; in ovo vaccination; dual application of separate inactivated vaccines in one shot; etc. However, many of these improvements in vaccines and vaccination equipment have often motivated the poultry industry to continue to cut cost by fractionating vaccine doses, using off-label vaccination routes, combining vaccines that may or may not interfere with each other, or in some cases, use vaccines or vaccine strains that are not authorized in a particular geographical area. Despite the many advances in vaccines and vaccination equipment the poultry industry still lacks availability of vaccines that will fully protect chickens against diseases such as infectious laryngotracheitis without paying a heavy price for some of the disadvantages that commercial vaccines may have(Dufour- Zavala 2008; Johnson et al. 2010). For example, the most protective vaccines against infectious laryngotracheitis are by far the chicken embryo origin (CEO) vaccines (Johnson et al. 2010). At the same time, CEO vaccines can be extremely reactive, may induce a loss of several points in feed conversion, may establish latency and thus eventual recrudescence with backpassaging of vaccine virus in naïve chickens to regain virulence, etc. (Dufour-Zavala 2008). The live attenuated virus alternatives such as tissue culture origin (TCO) vaccines are unfortunately limiting in that they must be given by eye and thus become impractical and costly in terms of application. Newer and safer alternatives such as recombinant vaccines are only partially protective and in many cases insufficient to face a heavy challenge in the field. In addition, onset of protection by recombinant vaccines appears to be much slower than that of live attenuated vaccines. Finally, in some areas the commercial cost of live attenuated vaccines may be only one fourth or one fifth of the cost of some recombinant vaccines. Thus, much progress is still due in the development of safe, effective vaccines against infectious laryngotracheitis. 

Relevance of rapid and accurate diagnostic systems

Accurate, economical,practical and expedient laboratorydiagnostic tools are absolutely critical for effective control of disease outbreaks (Dufour-Zavala 2008). Considering the high poultry density and the very active dynamics of bird, litter, vehicle and personnel movement in today´s poultry industry it would be difficult to conceive effective disease prevention and control without adequate laboratory systems designed to monitor and detect infectious diseases. The advent and increasing use of molecular detection methods, automated systems for antibody detection and development of in situ antigen capture tools for virus protein detection in the field have all made a very significant contribution in the field of disease prevention and control. Outbreaks of low pathogenic or highly pathogenic avian influenza (HPAI) can easily be detected in areas that routinely monitor flocks for antibody detection. In addition, local laboratories must be well equipped to support the poultry industry with molecular detection capabilities. Antigen capture kits for field and laboratory use have been extremely successful in establishing presumptive diagnoses with a relatively high confidence level. The use of molecular detection methodologies has allowed for very rapid and accurate detection of disease agents of extreme significance such as HPAI or exotic Newcastle disease (END). In addition, molecular methodologies permit rapid characterization of disease agents through methods such as the use of melting curves on real time PCR and/or through partial genome sequencing. Antibody detection using enzyme-linked immunosorbent assays (ELISA) is not a very recent method, but today ELISA reactions can be run very rapidly by the thousands with the aid of robotic machines, which also represents a very significant improvement in disease monitoring and diagnostics.

Comprehensive approach to disease control 

Poultry veterinarians and technical personnel must understand that infectious diseases appear and disappear for a reason, and often times our busy lives are not conducive to truly analyzing every significant disease situation so we can learn from it in order to prevent it or manage it better the next time it occurs. Catastrophic respiratory diseases such as END or HPAI are not easy to overlook. High mortality, s in egg production, misshapen eggs, increased downgrades at the egg packing plants, egg shell discoloration, egg deformities and clinical signs consistent with HPAI and/or END are difficult to ignore. What really becomes quite challenging is to confront respiratory disease complexes where respiratory viruses act in concert with the environment, management practices, opportunistic and primary pathogens, nutrition, etc. Any or several of such factors can be off-balance and thus may make disease control more challenging than if the disease in question was caused by a single entity. It is absolutely critical to approach disease control from as many angles as possible. For example, ILT cannot easily be controlled by simply implementing proper biosecurity measures, or by just establishing an intensive vaccination program. It is important to not overlook the role of weather, the environment, the infrastructure of the premises in the affected area, interactions with other diseases, management practices, poultry litter handling and moving, live haul practices, etc. (Dufour-Zavala 2008). All such items and many more should be part of a comprehensive approach to disease control. It is not possible to control avian influenza effectively without regulating effectively poultry commercialization, litter management and transportation, proper biosecurity and in some cases, vaccination. Lastly, the use of geographic information systems (GIS) for localization of poultry premises and related premises is absolutely critical for any attempts of infectious disease control. GIS allows for easy and rapid localization of outbreak sites, determination of truck routes for poultry, poultry products and contaminated manure, etc. (Dufour-Zavala 2008).

Regardless of any and all new methodologies used for disease control, and despite an ever growing body of knowledge about poultry diseases, vaccine and vaccination equipment development, better infrastructure compatible with disease control, effective Government regulations, improved genetics and nutrition, etc., viral respiratory diseases are still a continuous challenge to the poultry industry, the allied industry and the professionals that work in the field. Great progress has been made but it is still clearly insufficient. Infectious laryngotracheitis is widely distributed in the poultry industry of many countries of the Americas and in other regions of the world. Clearly, there are no suitable vaccine or vaccination alternatives that can be considered safe, effective, and without the risk of establishing latency, recrudescence and reversion to virulence (Dufour-Zavala 2008). Recombinant vaccines against ILT are an improvement but not the entire solution to the problem (Johnson et al. 2010). Infectious bronchitis continues to be one of the most important respiratory pathogens. It never goes away and it continuously generates new antigenic types and occasionally new pathotypes that cannot be easily prevented by way of the use of currently available vaccines. Low pathogenic avian influenza has unfortunately become enzootic in some areas and the industries affected have actually become complacent about it and unwilling or incapable of combating the disease. As long as there is at least one vaccine available, industry will not look into the possibility of eradication. Exotic, velogenic, neurotropic or very virulent Newcastle disease continues to be one of the most economically devastating diseases of poultry, particularly in geographical areas where it tends to be enzootic despite intensive vaccination against it (Anonymous 1970; Anonymous 1971; Anonymous 1971; van den Wijngaard 1993; Leslie 2000; Cattoli et al. 2010; Msoffe et al. 2010). Avian metapneumovirus infection appears to be of lesser importance. However, either lack of understanding, overemphasizing of its importance and role in respiratory disease, or true virulence of field isolates in some areas has encouraged the poultry industry to vaccinate sometimes indiscriminately against MPV with live vaccines, even in flocks that happen to be infected with Mycoplasma gallisepticum and/or M. synoviae, often times with disastrous consequences. Often times the motivation to vaccinate against MPV stems from simple serological evidence of exposure while ignoring all possible complicating or opportunistic agents and predisposing factors.

Clearly, there is still progress to be made in the control of respiratory diseases of viral origin. A comprehensive approach is needed for disease control requiring consideration of the interactions between poultry genetics, nutrition, husbandry and infectious disease. Biosecurity, proper vaccine use and vaccination practices are certainly critical items in the fight against respiratory diseases.

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