10.1 Introduction

Respiratory diseases of gamebirds are similar to respiratory diseases of other avian species, more specifically chickens and turkeys. As a general rule, pheasants, quail, and partridges respond similarly to chickens. Peafowl respond similarly to turkeys. Due to how they are raised, in contrast to commercial poultry, gamebirds are at higher risk of respiratory disease from migratory waterfowl, wildlife, and other pests. Causes of respiratory disease include viruses, bacteria, fungi, and parasites.

10.2 Viral Respiratory Diseases

Viral respiratory diseases include avian influenza (AI), Newcastle disease, infectious laryngotracheitis (ILT), coronavirus infection in pheasants, and quail bronchitis viruses (QBVs).

Avian influenza is caused by an enveloped RNA virus in the orthomyxovirus family. The AI virus is considered to be a type A influenza which also infects human beings. Humans may also be infected with type B and C influenza viruses. Type A influenza viruses may infect other animals such as swine, horses, dogs, cats, mink, seals, and whales and is named for the respective species that it infects, such as swine influenza when it infects swine and AI when it infects birds. Wild waterfowl, namely ducks, geese, and swans, appear to be the reservoir for all type A influenza viruses where they cycle as low pathogenic strains (LPAI) and mutate to highly pathogenic avian influenza (HPAI). LPAI causes little disease and death while HPAI causes dramatic disease and death loss. LPAI H5 and H7 viruses circulate for a time in gallinaceous birds and develop into HPAI viruses [1].

Influenza viruses are further classified by the surface antigens on their envelope which are known as H for the hemagglutinin antigen and N for the neuraminidase antigen. Hemagglutinin is responsible for helping the virus attach to the cell membrane. Since viral envelopes consist of lipid, derived from the host cell, soap and water, cooking temperatures, ultraviolet light, and almost any disinfectant can disrupt the membrane, preventing the virus from attaching to the host cell and thus preventing infection. Neuraminidase antigen, also on the virus envelope, allows the newly assembled viruses to escape from host cells. In humans, drugs developed to combat flu viruses are antineuraminidase compounds. There are currently 18 H types and 11 N types recognized [2]. Therefore, AI viruses can be any combination of H type and N type with a total of 198 different possibilities. In the avian species, only H5 and H7 types are of major concern since they have, in the past, been responsible for catastrophic influenza outbreaks, also known as HPAI [1].

Although thought of as primarily a respiratory infection, common AI viruses may also produce gastrointestinal infections. Yet under the right conditions, the H antigen may have the correct amino acid sequence (multiple basic amino acids at the cleavage site) which allows those viruses to be cleaved by enzymes found throughout the body, thus producing general systemic disease. It is through this mechanism that low LPAI viruses become HPAI [3, 4]. In the USA, there have been multiple outbreaks of HPAI [5]. They have cycled between domestic flocks as LPAI and yet seem to change overnight to HPAI. Diagnosis requires laboratory testing, and no treatment or vaccination is used in the USA.

Newcastle disease is caused by an enveloped RNA Avulavirus in the paramyxovirus family and is commonly referred to as avian paramyxovirus 1. It too is considered to be a respiratory disease yet is often mixed with neurologic disease as well. The type and severity of disease caused by Newcastle disease virus (NDV) vary with the type of infecting virus. There are NDV viruses that cause little or no disease, called lentogenic strains, those that cause moderate disease, called mesogenic strains, and those that cause catastrophic disease, called velogenic strains. The milder strains are thought to be endemic in poultry while velogenic strains thought to be endemic in feral pigeons, some domestic poultry, coastal seabirds, and cormorants [1]. Velogenic strains can cause lesions within the body organs and are referred to as viscerotropic velogenic (vvNDV) or neurologic disease alone and as neurotropic velogenic (nvNDV). Although vvNDV is found throughout the world, it rarely occurs in the USA. It is therefore often referred to in the USA as exotic NDV. Over the years, California has had repeated outbreaks of vvNDV due to smuggling of fighting chickens [6, 7]. Diagnosis requires laboratory tests and no treatment is available. Vaccination, with primarily lentogenic strains and some mesogenic strains, is commonly used in commercial poultry flocks in the USA.

Unfortunately, in either HPAI or vvNDV outbreak scenarios, it is very hard to determine which virus is causing the problem without laboratory tests. Both viruses cause large death losses, with or without clinical illness, within flocks and similar, if not equivalent, clinical signs and gross lesions, making it difficult to tell which virus is the source of the outbreak. Gross lesions in both HPAI and vvNDV consist of hemorrhage in multiple internal organs including proventriculus and intestines, hemorrhage and ulceration within the esophagus and hemorrhage in the trachea. Other lesions of vasculitis may include swelling of the face and cyanosis of skin, shanks, and feet. The reproductive system may also be affected.

Diagnosis in either AI and NDV is by polymerase chain reaction (PCR), virus isolation, and other virus identifying techniques. PCR tests are available to first identify the causative virus as either AI or NDV virus and then to differentiate AI as H5 or H7 and NDV as velogenic.

For information concerning HPAI and vNDV, please access the web pages of the United States Department of Agriculture at www.aphis.usda.gov/aphis.

Infectious laryngotracheitis is a herpesvirus that normally infects chickens. In gamebirds, the disease has been reported in pheasants and in a peafowl [8]. The classic lesion of ILT is hemorrhage with or without fibrin in the larynx and proximal trachea (Figure 10.1). Other organs are not generally affected but in some cases, the virus infection may extend to the lungs. ILT may be suspected upon necropsy and confirmed with histopathology (Figure 10.2) and other laboratory tests such as PCR and/or virus isolation. There is no treatment available for ILT yet reportedly increasing the house temperature a few degrees may shorten and lessen the disease effects. Vaccination is used in commercial chickens. Extreme care should be used when deciding to vaccinate for ILT since the chicken embryo‐origin and tissue culture‐derived ILT vaccine can cause outbreaks in nonvaccinated birds when it spreads from bird to bird [9]. Only genetically modified ILT vaccines should be used since they do not contain the entire virus and thus will not spread and cause disease.

Coronavirus is closely related to infectious bronchitis virus, a respiratory disease in chickens, with some strains causing kidney damage and gout. Coronaviruses, which are enveloped RNA viruses, have been recovered from pheasants with respiratory disease and kidney damage [10]. Mortality has been recorded as 15%, and possibly up to 45%. Although not standard, diagnosis would be a combination of lesions (renal disease), virus isolation with identification. Of course, there is no treatment and no known vaccine is available.

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