Infectious Disease and Microbiology – Chlamydophila Species

Overview

Chlamydophila species are obligate intracellular Gram-negative bacteria that primarily cause respiratory infections. The two clinically important organisms are Chlamydophila pneumoniae and Chlamydophila psittaci.

C. pneumoniae is an established cause of community-acquired atypical pneumonia, while C. psittaci causes psittacosis, a zoonotic infection typically acquired from birds.

Important Species

Chlamydophila pneumoniae

C. pneumoniae, historically referred to as TWAR, is transmitted from person to person and causes a proportion of community-acquired respiratory infections.

Chlamydophila psittaci

C. psittaci is associated mainly with birds and causes psittacosis in humans.

Exposure is particularly important among:

• Bird owners

• Poultry workers

• Veterinarians

• Pet-shop employees

• Animal handlers

Microbiologic Characteristics

Chlamydophila species are:

• Gram-negative bacteria

• Obligate intracellular organisms

• Unable to replicate independently outside host cells

Like other chlamydial organisms, they undergo a specialized intracellular developmental cycle.

Epidemiology

C. pneumoniae has a worldwide distribution and spreads primarily through respiratory secretions from person to person.

C. psittaci is maintained in birds and some domestic animals. Humans usually acquire infection by inhaling aerosolized material contaminated with infected bird secretions, respiratory droplets, or dried feces.

Chlamydophila pneumoniae Infection

Respiratory Disease

C. pneumoniae is an important cause of atypical respiratory infection.

Clinical syndromes include:

• Pharyngitis

• Sinusitis

• Bronchitis

• Community-acquired pneumonia

The illness is often gradual in onset.

Patients may develop:

• Low-grade fever

• Headache

• Malaise

• Sore throat

• Hoarseness

• Persistent dry cough

The cough may persist for several weeks even after other symptoms improve.

Pneumonia

Pneumonia caused by C. pneumoniae is usually relatively mild but may occasionally be more significant in older adults or patients with chronic underlying disease.

The presentation may resemble infections caused by:

• Mycoplasma pneumoniae

• Respiratory viruses

• Legionella species

Chlamydophila psittaci Infection

Psittacosis

C. psittaci causes psittacosis, also called ornithosis.

The classic epidemiologic clue is:

Bird exposure + atypical pneumonia

Symptoms may include:

• High fever

• Severe headache

• Myalgias

• Malaise

• Dry cough

• Dyspnea

• Pneumonia

Some patients develop systemic manifestations involving the liver, nervous system, or other organs.

Associated Animals

Birds are the most important reservoirs, particularly:

• Parrots

• Parakeets

• Cockatiels

• Pigeons

• Poultry

Infected birds may appear healthy while shedding the organism.

Possible Association With Atherosclerosis

C. pneumoniae has been investigated as a possible contributor to the development of atherosclerosis.

Although organisms and immune responses have been detected in atherosclerotic tissue, a direct causal role has not been established sufficiently to justify antimicrobial therapy for prevention or treatment of atherosclerosis.

Diagnosis

Diagnosis may be made using:

• PCR

• Serology

• Antigen detection

• Cell culture

Because these organisms are intracellular and difficult to cultivate, molecular and serologic methods are generally more practical than routine culture.

PCR

PCR can detect organism-specific nucleic acid in respiratory specimens.

This provides direct evidence of infection and is especially useful when available.

Serology

Traditional methods include:

• Complement fixation

• Microimmunofluorescence

Microimmunofluorescence has historically been used to distinguish antibody responses to different chlamydial species.

For psittacosis, older diagnostic criteria considered a complement-fixing antibody titer greater than 1:16 supportive in an appropriate clinical setting, although modern diagnostic interpretation generally relies on paired serology or molecular testing.

Antigen Detection

Antigen-detection assays may also identify chlamydial antigens in clinical specimens, although PCR has largely become more useful where available.

Cell Culture

Culture is possible but difficult because the organism requires living cells for replication.

It is mainly performed in specialized or reference laboratories.

Differential Diagnosis

For C. pneumoniae respiratory infection, consider:

• Mycoplasma pneumoniae

• Legionella

• Influenza

• Other respiratory viruses

• Typical bacterial pneumonia

For psittacosis, consider:

• Q fever

• Legionnaires’ disease

• Influenza

• Tularemia

• Other causes of atypical pneumonia

A strong bird-exposure history is particularly helpful for distinguishing psittacosis.

Treatment

Doxycycline

For pneumonia caused by C. pneumoniae, the source regimen is:

Doxycycline 100 mg orally every 12 hours for 10–14 days

Doxycycline is also a major treatment option for psittacosis.

Macrolides

Additional effective agents include:

• Azithromycin

• Clarithromycin

These may be especially useful when tetracyclines are unsuitable.

Tetracycline

Tetracycline-class antibiotics are active against these intracellular organisms and have historically been used extensively for both C. pneumoniae and C. psittaci infections.

Fluoroquinolones

Respiratory fluoroquinolones such as:

• Levofloxacin

may also have activity and can be considered in selected patients.

Prevention

For C. pneumoniae, prevention relies on general respiratory hygiene because infection spreads person to person.

For C. psittaci, prevention includes:

• Proper handling of birds

• Avoiding inhalation of dried bird droppings

• Cleaning cages carefully

• Using protective equipment during high-risk occupational exposure

• Veterinary evaluation of infected birds

High-Yield Clinical Patterns

Chlamydophila pneumoniae

Gradual respiratory illness

• ●

Persistent dry cough

• ●

Community-acquired atypical pneumonia

→ Think C. pneumoniae

Chlamydophila psittaci

Bird exposure

• ●

High fever

• ●

Severe headache

• ●

Atypical pneumonia

→ Think psittacosis

Exam Essentials

Genus: Chlamydophila

Important species:

→ C. pneumoniae

→ C. psittaci

Microbiology:

→ Gram-negative obligate intracellular bacteria

C. pneumoniae transmission:

→ Person to person

C. pneumoniae disease:

→ Community-acquired atypical pneumonia

C. psittaci reservoir:

→ Birds

C. psittaci disease:

→ Psittacosis

Classic psittacosis clue:

→ Bird exposure + atypical pneumonia

Diagnosis:

→ PCR, serology, antigen detection, specialized culture

Treatment:

→ Doxycycline

Other options:

→ Azithromycin, clarithromycin, tetracycline, levofloxacin

Key distinction:

C. pneumoniae → person-to-person respiratory infection

C. psittaci → zoonotic bird-associated pneumonia