Top 5 Avian Zoonoses

Approximately 15.4 million poultry and 7.5 million nonpoultry birds are kept as pets in the United States.¹ Although nonpoultry pet bird ownership declined 26% from 2016 to 2018, the number of households with poultry as pets increased 0.4% to 1.1%.¹ Bird owners should be educated on techniques to prevent transmission of common avian zoonoses.

Following are the top 5 avian zoonoses affecting pet birds, in the author's opinion.

1. Salmonellosis

Salmonella spp are gram-negative bacteria that cause 1.35 million annual cases of GI illness in humans in the United States.² In 2021, a salmonellosis outbreak associated with backyard poultry ownership in the United States affected 1,135 humans in 48 states, leading to 273 hospitalizations and 2 deaths.³ Although many cases of human salmonellosis are associated with ingestion of contaminated food, an increasing number are associated with backyard poultry ownership.<sup4,5 sup> 

Salmonella spp can infect any avian species, but zoonotic transfer is most common from chickens and ducks and is often associated with improper handling of eggs (see Suggested Reading for safe handling recommendations). Some backyard poultry owners are more likely to seek advice from the internet than from clinicians.⁶ The increasing popularity of backyard poultry ownership can provide an opportunity for educational outreach.¹ 

S Typhimurium, S Enteriditis, S Newport, and S Javiana account for ≈50% of all culture-confirmed infections in humans in the United States.² Although salmonellosis in humans is usually self-limiting and typically does not require antibiotic treatment, outbreaks associated with backyard poultry have been associated with significant morbidity.³

Salmonella spp can cause illness in birds, but serotypes that typically cause illness in humans are not common causes of avian illness or death.⁷ Prophylactic use of antibiotics to reduce or eliminate Salmonella spp in birds is not recommended, as it can lead to the emergence of drug-resistant organisms and is unlikely to completely clear infection.⁸ Fluoroquinolone use in poultry is prohibited in the United States due to the likelihood of further bacterial resistance development. 

If Salmonella spp infection is suspected in a patient based on clinical signs, culture and susceptibility testing should be performed for appropriate identification and treatment. The Food Animal Residue Avoidance Databank can be consulted for information on prohibited drugs or recommended withdrawal times for egg consumption.⁹ Owners of birds, particularly backyard poultry, should be educated on the importance of biosecurity measures (see Suggested Reading).

2. Campylobacteriosis

Campylobacteriosis is a common bacterial food-borne illness in the United States¹⁰; outbreaks have also been associated with puppies and pet stores. Birds are a less common source of Campylobacter spp than puppies or contaminated food. Campylobacteriosis can be subclinical in avian species and may cause GI illness in humans, similar to salmonellosis.

Transmission of Campylobacter spp to humans typically occurs via ingestion of contaminated food or water, handling of infected puppies or their excrement, or direct contact with poultry or other birds. C jejuni is the most common species found in birds, but one study of wild birds also found C coli and C lari; prevalence of all Campylobacter spp infections was highest in crows (23%) and gulls (25%).¹¹ 

Campylobacter spp are estimated to cause >1 million infections annually in humans in the United States and have a high rate of fluoroquinolone resistance.¹² Ciprofloxacin resistance varies from 50% to 100% in poultry samples.¹² 

Biosecurity measures similar to those for Salmonella spp can prevent zoonotic transmission of Campylobacter spp. Owners of birds, particularly backyard poultry, should be educated about the importance of biosecurity measures and can be referred to the CDC website (see Suggested Reading).

3. Chlamydiosis

Chlamydiosis (ie, ornithosis, psittacosis) is caused by the obligate intracellular bacteria Chlamydia psittaci. These bacteria can be found in many avian species and have been historically associated with the pet bird trade. Although birds can become ill with infection, they can also be subclinical carriers. Approximately 10 cases are reported annually in the United States, but cases may be underreported.¹³ Occasional outbreaks may lead to increases in reported cases in certain years. More detailed information on C psittaci infection in humans and birds can be found in the compendium published by the National Association of Public Health Veterinarians.¹⁴

C psittaci is shed through feces and respiratory secretions of infected birds; stress (eg, from transport) can increase shedding. Humans can become infected by inhaling the bacteria, and infection can range from subclinical cases to severe pneumonia or death. Antibiotics (eg, tetracyclines) are usually indicated to improve prognosis.¹⁴ 

To prevent zoonotic transmission, humans working with birds with suspected C psittaci infection should use personal protective equipment (eg, N95 mask) to prevent inhalation of the organism. Prior to necropsy, carcasses should be wetted to reduce aerosolization. Although C psittaci infection in humans and birds is not reportable in all states, local authorities should be notified of any confirmed cases.

4. Avian Influenza

Although 3 common influenza virus types (ie, A, B, C) can infect a variety of species, only type A infects birds. Influenza viruses are further described by hemagglutinin (H) and neuraminidase (N) antigens on their surfaces. Type A influenzas have historically caused large numbers of human infections and deaths, including the 1918 H1N1 pandemic virus. 

Although more recent zoonotic occurrences of avian influenza have not led to large-scale human infection or death, spillover events may occur.¹⁵ Influenza virus mutations are common, but person-to-person direct transmission has not been associated with zoonotic spillover events in recent decades. 

Anseriformes (eg, ducks, geese, swans) and charadriiformes (eg, shore birds) are common subclinical carriers of influenza viruses and can spread infection via flight migration routes.¹⁶ Influenza virus can manifest in birds in 2 clinical forms: highly pathogenic avian influenza, which can cause up to 100% mortality in a flock and is always either H5 or H7, and low pathogenic avian influenza, which can cause few or mild clinical signs.

Avian influenza is shed by birds via feces and respiratory secretions. Although avian influenza has been reported in a pet parrot,¹⁷ humans are usually infected via inhalation when handling infected poultry (eg, chickens, farmed ducks) or cleaning poultry enclosures.¹⁸ Avian influenza outbreaks have occurred in backyard poultry, but spillover to humans has not been reported in recent outbreaks.¹⁹ In the author’s opinion, outbreaks and zoonotic spillover events may increase with increased backyard poultry ownership.

Some humans may not develop symptoms; others may develop typical influenza symptoms, which can progress to pneumonia and possibly death. Avian influenza has an estimated mortality rate of 33% to 100% in humans.²⁰ 

Prevention of influenza virus inhalation is the same as described for chlamydiosis.

5. Coronaviruses

Although avian coronaviruses are not considered zoonotic, there may be confusion surrounding coronavirus disease because of the severe acute respiratory syndrome coronavirus 2 pandemic. 

Coronaviruses are classified as alpha and beta (ie, infection in mammals) or gamma and delta (ie, infection in birds).²¹ Numerous coronaviruses are species-specific, but spillover events can occur. The most recent spillover events (eg, severe acute respiratory syndrome coronavirus in 2002; Middle East respiratory syndrome coronavirus in 2012; severe acute respiratory syndrome coronavirus 2 in 2019) all involved beta coronaviruses.²¹ It is suspected that these viruses were originally found in bat species and transmitted to humans via a mammalian intermediate host. 

The most prominent avian coronavirus is the infectious bronchitis virus, which has a global distribution and causes significant economic losses in the poultry industry. Infection in poultry usually causes respiratory disease that leads to decreased production. Vaccines are available, but infection has not been eliminated in poultry populations.²¹ This avian virus is not zoonotic; therefore, no specific precautions are needed to prevent human infection.

Conclusion

Infectious organisms are constantly evolving and emerging. Safety measures (eg, hand washing, respiratory protection) can help protect veterinary staff and pet owners from identified infectious agents and those not yet recognized.