Bacteria, Epidemiology, Infectious Diseases, Microbiology, rare infectious disease

Understanding Parrot Fever (Psittacosis): A Rare Infectious Disease

Author Chandana Balasubramanian , 15-Feb-2024

Parrot Fever, also known as psittacosis, is a rare but significant infectious disease that primarily affects birds, notably parrots, but has the potential to spread to humans. The infection is also known as ornithosis or avian chlamydiosis.


Parrot fever is caused by the bacterium chlamydia psittaci (c.psittaci). C. psittaci is a part of the Chlamydiaceae family of bacteria. 


Overall, thanks to safe bird-handling practices and strict regulations on bird import and distribution, the number of ornithosis cases has dropped significantly. Additionally, once antibiotics were discovered, the case fatality rate substantially lowered the mortality rate of parrot fever, reducing its risk level. 


However, it’s important to keep parrot fever on our radar because it is classified as a Category B bioterror agent. 


Understanding its history, how it spreads, affects humans, signs, diagnosis, treatment, and how to prevent it is key to stopping the disease and protecting public health. This guide simplifies psittacosis, offering an easy-to-understand overview of the illness.



The association between human disease and sick parrots was first recognized in Europe in 1879. J.Ritter is credited with being the first to document a case of parrot fever in 1879. 

A team of researchers including J.W Moulder and A. Matsumoto used C.psittaci to understand chlamydia. In fact, the first high-resolution images of a chlamydia bacteria were of C.psittaci in 1970.

In 1978, Wyrick and Brownridge demonstrated how chlamydia infects immune cells.

The word “psittacosis” comes from the Greek term “psittakos,” which means parrot, as the illness was initially discovered in parrots. The term was coined in 1895. 

Over the years, however, it became evident that a wide range of bird species, both wild and domestic, could be reservoirs for the bacterium Chlamydia psittaci

A thorough study of psittacosis was not conducted until the outbreaks of 1929-1930, which occurred across four continents. Even if the number of cases weren’t so high, there was widespread fear about the disease at the time. So much so, that in January 1930, US President Herbert Hoover banned all import of parrots into the United States. 

The fear among the general public was not entirely unfounded. After all, while penicillin was first discovered in 1928, it was approved for clinical use only in 1943. Tetracyclines, the preferred treatment for psittacosis, was only introduced in 1948. Doxycycline, another antibiotic used to treat psittacosis (ornithosis) was introduced in 1967.



Cases of psittacosis have been reported worldwide, with the bacterial infection mainly occurring in regions where large numbers of birds are kept.

Although the disease is called parrot fever, it affects a diverse range of birds, including but not limited to parrots, pigeons, ducks, and chickens. 

Although human cases are comparatively rare, they predominantly occur following direct or indirect interactions with infected birds. For example, in the United States, less than 10 cases are reported each year since the year 2010.

People who are most at risk for psittacosis are people who keep birds as pets, aviary and pet shop owners and employees, poultry workers, and veterinarians.

Notable outbreaks

Notable outbreaks of psittacosis, also known as parrot fever, have been documented throughout history. However, the incidence dropped significantly after the introduction of preventive measures and safe handling practices related to birds.

1879: Germany: A disease resembling psittacosis was reported in Germany when individuals developed pneumonia after exposure to birds. 

1890s: Paris: A psittacosis outbreak with a 33% mortality rate was reported

1917: USA: Psittacosis cases were reported in Pennsylvania, USA. The infections stemmed from birds being stored in a basement.

1929-1930: Worldwide—‘The Great Parrot Fever Pandemic’: One of the most significant psittacosis outbreaks occurred during the 1929-1930 pandemic, affecting around 750 to 800 people worldwide, resulting in over 100 deaths. These outbreaks are collectively referred to as ‘The Great Parrot Fever Pandemic.’ 

During this time, parrots became a popular source of feathers for fashionable hats and garments. As a result, global trade in parrots flourished and along with it, bird-related diseases.

Countries affected included the UK, USA, Argentina, Germany, Switzerland, Sweden, France, Denmark, Holland, Austria, Czechoslovakia, Poland, Spain, Iceland, and Australia.

1978: USA: 160 human psittacosis cases reported by 33 states.

1979: USA: 116 human cases of psittacosis were reported from 25 states. California had the largest number of cases with 23% of the total cases. Most of the psittacosis cases reported during the outbreaks of 1979 were from people who owned caged birds as pets or for commercial use. Other sources of infection were imported birds.

1981: Norway: 508 lab-confirmed cases of psittacosis in Norway. During this time, while many cases were related to direct exposure to birds, several cases were from small groups of families or schools where no history of bird exposure was noted. This hinted at the potential for human-to-human transmission of psittacosis.

1986: USA: 186 cases reported in turkey processing plants, a farm, and meat-processing plants in the state of Minnesota.

2018: USA: Another notable instance of psittacosis outbreak was in 2018, where a multi-state outbreak of severe illness was reported among poultry slaughter workers in the states of Virginia and Georgia, United States. There were 13 laboratory-confirmed cases

These outbreaks demonstrate the importance of awareness, prevention measures, and regulatory controls in managing the risks associated with psittacosis, both from pet birds and commercial poultry operations.

Additionally, it’s important to note that parrot fever incidence may be higher but because its symptoms are similar to common respiratory diseases, it may be often misdiagnosed.


How does it spread?

Humans can get parrot fever from birds when they:

  • Handle infected birds
  • Inhale aerosolized particles containing Chlamydia psittaci (from the bird’s droppings, urine, or respiratory secretions
  • Get bitten by an infected bird.

While rare, direct transmission between humans can occur. When an infected person coughs, they can release bacteria-containing aerosolized particles that another person may inhale.


Biology of the disease

Upon entering the human body, Chlamydia psittaci targets the respiratory tract, specifically the lungs, where it infects and multiplies within cells. This process leads to inflammation and the onset of psittacosis symptoms.

The body’s immune response to the infection plays a critical role in the disease’s progression; this influences the severity of symptoms, including pneumonia.

It is often hard to detect c.psittaci infections before they infect humans because the bacteria develops in two stages. In the first stage, the inactive elementary body (EB) is highly resistant to environmental factors and it can survive well without a host. Once the bacteria gets inside a host, the EB cells differentiate into reticulate bodies (RB). These reticular cells are active but do not cause infection, yet. 

The RB cells start using the host’s energy cells (ATP) to multiply, which triggers the host’s immune response. In 2-3 days, the RB cells aggregate and form new EB cells, which continue the cycle. This unique cycle makes it harder to grow psittacosis in labs because the c.psittaci bacteria grow using resources found in the host’s cells. 



Psittacosis symptoms in humans can vary widely, from mild, flu-like manifestations to severe pneumonia. In general, the infection in humans is mild.

According to the CDC, typical psittacosis symptoms include:

  • Fever
  • Chills
  • Headache
  • Muscle pain
  • Dry cough.


More severe cases may lead to difficulty breathing, chest pain, and significant fatigue, underlining the disease’s potential severity. 

The incubation period ranges from 5-14 days after exposure to the pathogen.



Psittacosis is difficult to diagnose because its symptoms are fever, chills, headache, muscle aches, and a dry cough—symptoms that are common to most respiratory illnesses.

A thorough patient history is crucial for diagnosing psittacosis. When exposure to parrots or other birds is mentioned, clinicians may consider psittacosis as part of their differential diagnosis.

Laboratory analyses like antibody blood tests can help. A chest X-ray can indicate the presence of pneumonia in the lungs, if present.



Effective treatment of parrot fever involves antibiotics, with Tetracycline and doxycycline being the main choice. 

For those unable to tolerate doxycycline, like young children, azithromycin serves as an alternative.



Identifying infected birds early is challenging since many may show no symptoms. Symptoms may not emerge for months, allowing the bacteria to spread to other birds or humans.

As a result, it is extremely important to practice safe practices when handling birds. Protective gear, such as masks and gloves, is recommended for individuals working closely with birds to minimize exposure risks.

Preventing psittacosis requires a comprehensive approach, emphasizing hygiene, regular cleaning of bird habitats, and adequate ventilation. 

Education and awareness among bird breeders, pet store employees, and veterinarians are key components of early detection and outbreak prevention.



Parrot Fever, also known as psittacosis, shows how diseases can affect both animals and humans. It reminds us how important it is to prevent and control zoonotic diseases through safe handling of animals and birds.

Although psittacosis is mostly linked with birds, there is a risk of spread between humans. 

Following prevention tips and getting medical help when needed can reduce the impact of psittacosis on public health, making it safer for people and birds to live together.


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Chandana Balasubramanian

Chandana Balasubramanian is an experienced healthcare executive who writes on the intersection of healthcare and technology. She is the President of Global Insight Advisory Network, and has a Masters degree in Biomedical Engineering from the University of Wisconsin-Madison, USA.

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