Chlamydia Species - General Characteristics

Introduction

Chlamydia species include host adapted and non-host adapted species varying in virulence for different hosts. They can casue various diseases, including respiratory, enteric, pleural and reproductive diseases in animals and humans; specific conditions include conjunctivitis, arthritis, abortion, urethritis, enteritis, pneumonia, encephalomyelitis. Disease manifestation varies from subclinical to severe systemic infections - intestinal infections are often subclinical and persistent.Human infections are commonly acquired from infected birds with psittacosis or ornithosis, causing respiratory infections in the handler.

Bacterial Characteristics

Chlamydia species are obligate intracellular Gram negative bacteria with ribosomes, DNA and RNA and an outer membrane containing lipopolysaccharides (LPS). The peptidoglycan cell wall is resistant to lysozymes.

These bacteria only grow in the presence of living eukaryotic cells as they are unable to synthesise ATP therefore requiring intermediates from host cells. There are two morphological forms:

Elementary body, the infective extracellular form, which is small, metabolically inert and osmotically stable; surrounded by cytoplasmic membrane, outer membrane with LPS, but no peptidoglycan
Reticulate body: larger, metabolically active, osmotically fragile

The elementary body survives in the environment for several days.

Pathogenesis and Pathogenicity

The elementary body enters the host epithelial cells by receptor-mediated endocytosis. It is subsequently transformation into a larger reticulate body within endosome known as an inclusion, which divides by binary fission to form many new chlamydia cells. The reticulate bodies mature and condense to form elementary bodies which are released from the dying host cells approximately 72 hours after infection to infect other cells. Persistent infections can occur if replication is delayed by environmental conditions such as the presence of interferon gamma.

Many infections are subclinical due to the intracellular existence of chlamydia which avoids the body's inflammatory response. Chronic infections may fail to induce any immune response, or may repeatedly stimulate the immune system, causing a delayed hypersensitivity reaction and tissue damage. Prolonged faecal shedding of organisms is a common sequelae. Clinical signs of infection depend on the route of infection and degree of exposure.

Clinical infections can occur in non-natural host species.

Diagnosis of Infection

Direct microscopy of smears and tissues e.g. organs from aborted foetuses, liver/spleen from avian cases
Kosters (modified Ziehl-Neelsen) stain of placental smears shows small red rods
Blue inclusions in cytoplasm of Giemsa-stained cells
Methylene blue stain with darkfield microscopy
Fluorescent antibody stain
Antigen detection kits for diagnosis from swabs
ELISA to detect Chlamydophila LPS
Isolation in embryonated eggs and McCoy cells as well as animal tissues
PCR to detect chlamydial DNA
Serological tests: complement fixation, ELISA, indirect immunofluorescence

Species Specific Infections

Sheep

Sporadic or outbreaks of lamb polyarthritis
High morbidity, low mortality
Commonly occurs together with conjunctivitis
Most recover but may remain chronically lame

Cattle

Severe disease in young calves
High mortality
Can be seen in smears of synovial fluid from swollen joints
Oedematous and hyperaemic surrounding tissue
Possibly due to intrauterine infection

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