This is a part of the Sick Saturdays feature, in which we have a sick discussion on the symptoms and biology behind infectious illnesses.

Diseases transmitted between animals and humans are known as zoonotic infections. Zika virus is just one of many zoonoses spread by mosquitoes. But did you know that you can get pneumonia from birds? Psittacosis is a rare bird-borne bacterial illness that can cause a variety of symptoms or none at all in humans. 

Bacteriology, pathophysiology, and transmission

Psittacosis (the P is silent) comes from the Latin word psittacus, meaning parrot. Chlamydophila psittaci is the cause of the disease – yes, that bacterium is related to Chlamydia trachomatis, a sexually transmitted species of Chlamydiaceae that causes the disease often simply called chlamydia. C. trachomatis also causes the eye disease trachoma, which is the leading cause of blindness in the world.

All bacteria in the genus Chlamydia are obligate intracellular parasites, meaning they must live inside another cell to be viable (similar to what a virus has to do). While Chlamydia bacteria can survive outside of cells in an elementary body that is resistant to environmental stressors, they must reside in a cell to be biologically active. The elementary form of the bacterium is what is transmitted between hosts. Once the elementary body is endocytosed into an animal cell, it turns into an active reticulate body.

In birds, psittacosis is also known as ornithosis, avian chlamydiosis, or parrot fever. The bacterium C. psittaci is most commonly found in species in the orders Psittaciformes (parrots, cockatoos, lories) and Galliformes (chickens, pheasants, turkeys), though it can infect any bird. 

Psittacosis is spread from birds to humans usually through the respiratory route. Birds shed C. psittaci in their feces and when exhaling. When a human inhales aerosols containing the bacteria, an infection can occur. Additionally, a person can get sick if they eat food contaminated with C. psittaci. Rarer forms of transmission include contaminated objects (fomites) and insect bites. Human-to-human transmission is rare. The bacteria also infects other non-avian domestic animals, but they are not known to transmit psittacosis. 

People at the highest risk of contracting psittacosis are those who come into contact with birds regularly, such as veterinarians, pet shop employees, bird owners, and zookeepers. Poultry workers who handle bird carcasses are also at risk, as bird blood and tissues may still be infectious. 

Once infection sets in, C. psittaci infects epithelial cells in the lungs and begins replicating, which triggers an immune response. Lots of neutrophils come to the site of infection due to the release of chemokines – signaling proteins that attract white blood cells. The acute chemokine-mediated response then becomes an inflammatory cascade, and other immune cells are recruited to the area. Resulting tissue damage allows C. psittaci to spread through the bloodstream. The infection process is relatively similar between humans and birds. 

Symptoms, diagnosis, and treatment

Some people with chlamydial infections, including sexually transmitted chlamydia and psittacosis, have no symptoms. With an incubation period of 5 to 14 days, a C. psittaci infection typically initially causes flu-like symptoms, such as fever, chills, headaches, and muscle aches. An intense headache is the most frequent and prominent symptom. 

Respiratory issues including a dry cough, difficulty breathing, sore throat, pneumonia (which can be severe), or rarely, chest pain, are also present in psittacosis. Other possible symptoms are repeated nosebleeds and an enlarged liver or spleen. Further into the infection, as it becomes more systemic, a person might have a slow heart rate, inflamed heart lining (pericarditis), jaundice, face rash, or sensitivity to light. Contracting psittacosis has been associated with schizophrenia, a correlation which has also been seen with a number of other bacterial and viral infections.

Note that while psittacosis may sound scary, it is quite rare in the United States, with about 100 cases or fewer being reported annually. However, the disease is probably underdiagnosed since its minor symptoms are generalized.

Culturing is the preferred method for diagnosing many bacterial infections. But because Chlamydia bacteria are obligately intracellular, they cannot be easily cultured. This makes diagnosing psittacosis a bit complex. 

Sometimes, C. psittaci are cultured, but they are also detected through serological tests and molecular analyses. Serological tests look at whether a patient’s blood contains antibodies to the bacterium. Molecular methods such as polymerase chain reaction (PCR) give the most accurate results in a short amount of time, but for a high monetary cost, as they require specialized equipment to perform.

Doxycycline has proven especially useful against psittacosis; 100 mg is given orally or intravenously twice a day for 10 to 14 days. In individuals that cannot take tetracycline antibiotics, macrolide antibiotics such as azithromycin or erythromycin are recommended instead. 

Symptoms of ornithosis include discharge from the nose or eye, difficulty breathing, loss of appetite, and diarrhea. However, birds with ornithosis may also be asymptomatic. Methods for diagnosing ornithosis in birds are similar to those used for humans, since the same bacterium is being detected. A 45-day regimen of doxycycline is usually prescribed to treat ornithosis. 

The textbook I used in my medical bacteriology class, Clinical Bacteriology by Struthers and Westran, has a photo of a parrot coughing on a human to depict psittacosis. I think it is quite cute. I hope you enjoyed this lesson on psittacosis and I encourage you to share your knowledge with others!


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