The clinical condition known as psittacosis has many names, but ultimately, it is the presence of the bacterium Chlamydia psittaci in a patient’s body. The name “psittacosis” can be somewhat misleading given it is not limited to the psittacine family and all bird lineages can pass on this zoonotic disease. However, avian-to-human transmission is more commonly associated with the psittacine species, likely due to their more frequent popularity as companion species. Other common names for infection with Chlamydia psittaci depend on global location but include avian chlamydiosis, parrot fever and ornithosis.
Chlamydia psittaci infection, transmission and spread
The Gram-negative bacterium invades and, as an obligate intracellular pathogen, multiplies in the cells of an infected avian patient.
Chlamydia psittaci is comparatively infectious and has been documented worldwide in 30 orders of birds, covering over 467 species (Chu et al., 2023). There are 10 known genotypes of C. psittaci and each one is associated with a specific animal host – all are classified as zoonotic (Table 1).
Zoonosis to humans is considered uncommon, with only 58 cases reported in six years in the United States; only two were confirmed by culture (Chu et al., 2023). However, other sources state that an average of 250 cases are reported annually in the United States alone (Forbes, 2008). Further still, Ravichandran et al. (2021) state that psittacosis is often overlooked in importance and that the global implications cannot be undervalued.
| Genotype (subgroups) | Endemic hosts | Other hosts |
|---|---|---|
| A (A‐VS1, A‐6BC, A‐8455) | Psittacine birds (Psittacidae) | Turkeys, ducks, pigeons and Passeriformes |
| B | Pigeons (Columbiformes) | Chickens, turkeys, ducks, Psittacidae and Passeriformes |
| C | Waterfowl (Anseriformes), such as ducks and geese | Chickens, ducks and pigeons |
| D (D‐NJ1, D‐9 N) | Turkeys | Pigeons, chickens |
| E, CAL‐10, MP, OR MN | Humans | Turkeys, pigeons, ducks, ostriches and rheas |
| F | Psittacine isolates VS225, prk Daruma, 84/2334 (110) and 10,433‐MA |
Belgian turkey farm |
| E/B (EB‐E30, EB‐859, EB‐ KKCP) | Ducks | Parrots, turkeys and pigeons |
| M56 | Outbreaks in muskrats and hares | |
| WC | Outbreaks of enteritis in cattle |
As a zoonosis, psittacosis can cause numerous clinical signs, such as pneumonia, that result in serious illness or even death. In the global poultry industry, C. psittaci is deemed a significant cause of loss and, therefore, is not considered a rare disease. In fact, it is often described as underestimated in poultry workers, not just in recorded avian cases (Smith et al., 2005).
A note on mortality in avian and human patients
The number of psittacosis infections is highly variable. So, while mortality reports of 40 to 50 percent are often reported in avian patients (Spickler, 2017), it is dependent on multiple factors – not least how quickly treatment is provided to the patient. In turn, the risk of death from this disease in human cases can be 20 percent when left untreated; however, this is very dependent on the locale (Dembek et al., 2023). It is, therefore, clear that appropriate biosecurity cannot be underestimated.
Pathophysiology and clinical signs
An incubation period of 2 to 14 days is commonly accepted for psittacosis infection, with the mode of transmission most commonly faecal contamination, discharge and feathers/feather dust.
Latent infection is not uncommon, and a bird can have ‘peaks’ and ‘troughs’ of the disease over an extended period
It is worth noting that latent infection is not uncommon, and a bird can have “peaks” and “troughs” of the disease over an extended period. The “peak” is often seen during times of higher stress, such as environmental and seasonal changes (Fudge, 1996). These cases are termed chronically infected, but acute cases are also seen. The difference in definition between acute and chronic illness is largely dependent on the species, patient’s age, stress factors, treatment history, level of infection and strain type (Forbes, 2008).
Symptoms are largely nondescript but can include (Schlossberg, 2000):
- Anorexia
- Lethargy
- Discharge from the nasal passages
- Diarrhoea
- Off-colour urates
- Poor feather condition
- Onset of death
Confirmation of infection is normally confirmed following a polymerase chain reaction (PCR) test. Samples for these tests are often collected via the cloacal route or by faecal collection over three days.
Biosecurity for nursing the psittacosis patient
If you suspect or have confirmed that your patient has Chlamydia psittaci infection, the infected individual should be considered a red-level biosecurity threat (Gregory, 2023). The reasons for this are varied but largely centre on the risk to other patients and the staff in your veterinary practice.
Appropriate personal protective equipment (PPE) should be worn, with a disposable apron, mask and gloves as a minimum. Using appropriate footbaths for entering and leaving the area where the bird is housed is similarly essential, as it minimises the risk of transference to other non-infected areas. While the bacterium may not survive long in the environment, it does, however, remain infectious for several months on organic materials. As such, ensuring the correct level of cleanliness is imperative to the biosecurity of the practice to prevent the risk of infection spread and the risk of reinfecting the patient.
Treating and managing the psittacosis patient
Once isolated in an appropriate setting, the patient should be appropriately medicated – likely with a course of doxycycline (often provided orally).
As with all species, minimising stress is imperative for infected avian patients, so practitioners should be highly efficient when handling them to give medication and administer treatment, etc. Stress can worsen clinical signs and potentially lead to a negative outcome by destabilising a relatively stable patient.
It is important to note that the treatment for Chlamydia psittaci is not always straightforward and can be required for an extended period, with around 21 to 45 days the accepted duration, but this is ultimately species-dependent (Sanchez–Migallon, 2010).
Treatment for Chlamydia psittaci is not always straightforward and can be required for an extended period, with around 21 to 45 days the accepted duration,
If the patient is unstable, it is likely that hospitalisation will occur so supportive treatment can be put in place. This supportive care can include oxygen therapy, gavage feeding, fluid provision and, of course, medication.
Once stabilised, it is imperative that the owner continues the full course of treatment. Prior to the patient returning home, owners should conduct a full and thorough disinfection of all items to prevent reinfection. A dietary discussion with a veterinary professional must also occur before releasing the pet to ensure a reduction in calcium, which can impede the absorption of tetracyclines, is adhered to.
Conclusion
Globally, a Chlamydia psittaci occurrence of 1 percent has been recorded in wild bird populations, with a high of 30 percent seen in select species, such as budgerigars (Spickler, 2017). As such, infection with this bacterium does occur frequently, making it likely that you will see it in your veterinary practice. Should an infected individual present to your clinic, you cannot neglect the risks to the infected individual and other patients. Though the risk to veterinary staff may be lower, a duty of care to your colleagues and patients should ensure that appropriate measures are in place to prevent the spread of infection.
