Respiratory diseases are one of the most important health challenges faced by poultry producers. Numerous infectious pathogens – viral, bacterial, and combinations of both – threaten the poultry industry. Severity of illness and mortality rates vary but most of these diseases adversely affect productivity and performance of broiler, layer, and breeder flocks. Moreover, by weakening birds’ respiratory systems and overall health, these diseases open the door to secondary infections such as E. coli. Collectively, respiratory pathogens have tremendous economic impact on the poultry industry.
The Impact of Diseases on Respiratory Health
Respiratory pathogens tend to be highly contagious. Because many respiratory disorders present with similar clinical signs – snicking, watery eyes, rales, conjunctivitis, labored breathing, lethargy, etc. – accurate diagnosis is crucial for determining an appropriate treatment, especially when multiple pathogens or different strains may be present. Knowing the existing pathogens in your complex is crucial to developing a good disease prevention program.
Newcastle Disease Virus*
Newcastle disease virus (NDV) is a highly contagious respiratory disease that primarily infects chickens, but also turkeys and other avian species such as wild and cage birds. All age groups are vulnerable. There are numerous serotypes and strains ranging from low to high virulence, causing a variety of respiratory symptoms as well as neurologic symptoms, poor egg production, and varying degrees of mortality.
Virulent (velogenic) Newcastle disease causes high mortality, severe respiratory disease, and systemic disease throughout the body. These strains can be neurotropic (nvND), causing neurologic signs due to infection of the central nervous system, or viscerotropic (vvND), which results in hemorrhages in the visceral organs and skin, similar to those seen with highly pathogenic avian influenza.
Low virulent (lentogenic or mesogenic) Newcastle disease strains cause mild, non-specific respiratory signs with low mortality. Vaccine strains are derived from this group of viruses.
NDV has been found in most regions of the world, but many areas have eradicated it. The disease can be controlled by vaccination, but strain diversity, local regulations, and other vaccination complexities require strategic selection of appropriate vaccines for a particular region, production site, and disease challenge.
Infectious Bronchitis Virus*
One of the most common avian respiratory pathogens, infectious bronchitis virus (IBV) is a highly contagious avian coronavirus that causes mild to moderate respiratory disease in chickens. In young chickens, most strains of IBV cause low mortality but nearly 100% morbidity. Symptoms include gasping, sneezing, conjunctivitis, and nasal discharge. The most damaging effect of IBV is the predisposition to secondary infections, which can cause high levels of airsacculitis in the flock, leading to production losses and additional labor at processing. This is because IBV damages the bird’s innate immune defenses, opening it to other infectious agents. Some strains of IBV may also cause damage to the kidneys, leading to wet droppings, increased water consumption, and lethargy.
In layers and breeders, IBV infection causes severe drops in egg production (up to 50%), egg shell abnormalities, and decreased hatchability. Depending on severity of infection, these losses in production may be brief or may persist for several weeks, resulting in significant economic impact.
IBV prevention is a constantly moving target. IBV field strains show a high degree of genetic variability, and are always changing, so proper vaccine strain selection is crucial to controlling the problematic strains in specific regions. Strain-specific diagnosis is an essential tool for determining the predominant viruses causing issues within a flock.
Infectious Laryngotracheitis (ILT)
ILT is a common respiratory disease caused by the DNA virus Gallid herpesvirus type 1. It is found across the world in areas of dense poultry production and primarily affects chickens older than 20 days of age, but can also affect younger birds. The disease can cause mortality and decreased egg production among layers.
Clinical signs of ILT range from mild to severe, depending on the strain of the virus. Low pathogenic strains cause clinical signs similar to other respiratory diseases, such as lacrimation, swollen eyelids, oral and nasal discharge, rales and coughing. In more severe infections, difficulty breathing with bloody oral discharge is often observed. At one to two weeks, the incubation period from exposure to the emergence of clinical signs is relatively long.
Transmission of the virus occurs by way of infected bird contact, as well as contaminated equipment and litter. Diagnosis of the disease can be accomplished through observance of clinical signs and necropsy findings, but clinical signs in milder cases may not be sufficient. The most common diagnostic test for ILT is histopathology through microscopic examination of the trachea.
Because there is no reliable treatment for ILT, prevention through vaccination is common among poultry producers.
The term avian influenza (AI) actually encompasses a large number of viruses that infect domestic poultry as well as other species. AI viruses can be broadly divided into two categories based on pathogenicity. The high-pathogenic (HP) forms, the H5 and H7 subtype strains, spread rapidly and cause severe disease and high mortality, leading to devastating financial losses to poultry businesses and, often, trade restrictions.
The low-pathogenic forms get much less media attention but still have significant economic impact. Although mortality rates are much lower and symptoms milder than HPAI, it still adversely affects bird productivity, especially when found in combination with other respiratory diseases.