Abstract

Avian infectious diseases continue to be an important concern for human and avian health throughout the world. Many factors have and continue to make avian infectious disease a prevalent and ongoing concern, including:

1.  Birds harbor many pathogens, which can affect not only humans but other mammals and multiple species of birds.¹⁴ These pathogens may not cause identifiable clinical signs.

2.  Migration and flight facilitate infectious agent spread over large distances and from wildlife to captive populations.⁵

3.  Infectious diseases have caused extinction in avian species, especially island dwellers.¹⁴

4.  Habitat, ecosystem, and biodiversity degradation can increase human and likely other species’ exposure to infectious disease.^6,9

5.  There is a lack of knowledge regarding prevalence and incidence of many avian diseases, and a lack of funding for continuing research of effective prevention strategies.⁸

6.  Avian management often is based on open systems.

7.  Infectious disease testing is expensive, poorly comparable between labs, and may not be appropriate for use in a nontarget species.⁷

Common infectious disease concerns among captive avian species, based on practical experience and from a consultation and management perspective, continue to be: chlamydiosis, avian influenza, West Nile virus (WNV), avian bornavirus, avian polyoma virus, and psittacine circovirus. This presentation will give a broad overview and then focus on diseases for which consults are commonly sought. Agents causing true zoonotic infections of birds and those proven to move from an infected bird to a human and cause disease are few (Table 1). Generally, they cause mild morbidity (not mortality) and are responsive to treatment. You should wear gloves, mask, and a lab coat and preferably work in a hooded space for all avian necropsies.

Table 1. Bacterial, fungal, viral, and parasitic etiologies of zoonotic concern of birds

^a Bacterial
^b Fungal
^c Viral
^d Parasitic

Potential zoonoses have been encountered in birds and occur in humans, but the infectious routes have not been proven. Of particular importance are avian-associated diseases, a source of confusion for many veterinarians and animal managers. Aspergillus spp. can cause acute fulminant disease in avian species, but infected birds are not a source of disease for people. People are only subject to rare middle ear disease or bronchopulmonary allergy based on environmental exposure to this ubiquitous organism. Histoplasma spp. and Cryptococcus spp. grow in avian feces, not within the bird (one documented exception in a very sick, likely immunocompromised psittacine chick).¹² Wear gloves and a mask and keep avian enclosures from becoming caked with feces and urates, but do not fear infection from bird contact based on this fungal contaminant. Secretion of rabies virus has been documented to occur from a single great horned owl after experimental injection of rabies virus and steroids.

A number of encephalitis viruses that birds may carry usually require an insect vector (usually a mosquito) in order to be passed from the bird to a person. While WNV remains the hot topic, eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Saint Louis encephalitis (STLE) viruses are all primarily carried by birds. The cycles for these viruses are between mosquitoes and passerine birds. Birds often have no or minimal clinical signs. Humans, horses, and small companion mammals are examples of dead-end hosts, which suffer disease and do not spread or amplify virus. Notable exceptions include alligators and crows and geese, which may spread WNV laterally without a vector. If sentinel for EEE or WEE was needed, an emu would be valuable, as EEE and WEE may cause acute death, severe neurologic signs, or hemorrhagic diathesis; but most people vaccinate emus for these diseases. These diseases are discoverable via antibody serology, PCR, or other viral assays; however, most diagnoses are determined postmortem. Raptors are generally recommended to be vaccinated and then have titers checked for WNV (National Veterinary Services Laboratory).¹⁶ Ratites are vaccinated for EEE and WEE.

While psittacosis remains the most commonly reported zoonotic disease of avian species, the authors receive few consultation calls regarding this bacterium. This is likely due to procedures that remain in place based on previous importation procedures for psittacines. Chlamydiosis is not only a disease of psittacine birds, but it can affect all species and may have a prevalence approaching ∼30% in free-living pigeons and doves. Recent public health reports from Sweden document increased cases of psittacosis based on bird feeder exposure of humans and free-living passerines.¹³ Chlamydiosis of raptor species is also now thought to be more common.^3,19 Human-to-human transmission of psittacosis also is possible.^4,20 A complete blood count serves as the initial screening test for psittacosis in the authors’ practice, as infected birds often have leukocyte counts >30,000 with a prominent lymphocytosis and/or monocytosis (>2000) in chronic stages of the disease. For case classification, further diagnosis, and treatment, practitioners should be familiar with current recommendations from the compendium for chlamydiosis prevention and control.¹⁵

Influenza (type A) remains a concern worldwide based on human health, mammal health, and human food supplies (chickens, turkeys). As an RNA virus, avian influenza is one of the most rapidly evolving life forms on earth. Aquatic birds are the primary natural reservoir for most subtypes of influenza A viruses.^1,2,11 Based on the vast silent reservoir in aquatic birds, influenza viruses cannot be eradicated. Most cause asymptomatic or mild infection in birds, where the range of symptoms depends on the viral properties. Viruses that cause severe disease in birds and result in high death rates (of chickens!) are called highly pathogenic avian influenza (HPAI). Recent examples include a January 15, 2016 HPAI H7N8 outbreak in commercial turkeys in Indiana and a March 5, 2017 HPAI H7N9 outbreak in commercial poultry in Tennessee, both of North American wild bird origin.

Viruses that cause outbreaks in poultry but are not generally associated with severe disease are called low pathogenic avian influenza (LPAI). No vaccine is currently approved for use in all avian species in the United States for influenza. Of the six licensed vaccines for poultry, label claims are limited to chickens, and none provide adequate protection.¹⁷ The five steps for responding to an outbreak continue to be: quarantine, eradicate, monitor region, disinfect, and test. As of 2016, based on requests for proposals to the private sector, multiple AI vaccines are either currently licensed or under development. For those under development, U.S. Department of Agriculture (USDA) is working closely with the manufacturers to expedite the review and approval of products to ensure quick availability. The USDA then intends to stockpile vaccine as protection during an outbreak. The vaccination would be a suppressive emergency approach, where commercial poultry in a defined geographic area with rapidly spreading disease would be vaccinated.

Influenza viruses that humans can contract include avian influenza virus subtypes A (H5N1), A (H7N9), and A (H9N2) and swine influenza virus subtypes A (H1N1) and (H3N2).¹⁸ Most human cases of A (H5N1) and A (H7N9) infection have been associated with direct or indirect contact with infected live or dead poultry. For swine influenza, close proximity to infected pigs or visiting locations where pigs are exhibited has been reported for most human cases; some limited human-to-human transmission has occurred. Most human influenza infections are from direct contact or contaminated environments. Subsequent transmission to other people is inefficient. To minimize public health risk, surveillance of animal and human populations is essential.

Avian bornaviruses are widespread and may affect multiple species (Table 2). In many cases, infection may have minimal clinical effects. Much basic information regarding pathogenesis, infective route, disinfection, and treatment remains to be elucidated. No bird should be euthanized based on a single positive PCR test for avian bornavirus.¹⁰ No avian bornavirus has caused documented infection or disease in humans; however, recent bornaviral infections in tree squirrels have caused encephalitis and death in humans.

Table 2. Recent bornaviruses reclassification

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