Chlamydia psittaci infection of horses with respiratory disease.

The study discovered that horses with acute respiratory disease were infected by two strains of Chlamydia psittaci, a bacterium type. These strains were proven non-toxic but caused latent infection and activated an antibody response in test animals.

Identification of Chlamydia psittaci in Horses

• The researchers isolated two strains of Chlamydia psittaci (termed as NS 121 and NS 172) from the nasal tracts of horses suffering from acute respiratory disease.
• The characteristics which identified these agents as Chlamydia included their specific shape, stain-absorbing properties, ability to grow inside of chicken embryos, failure to cultivate on bacterial media, and the presence of a common chlamydial complement fixing group antigen.

Confirmation of Chlamydia psittaci

• To be certain of the strains’ identities as C. psittaci, the investigators tested for resistance to sodium sulphadiazine, a common method for distinguishing this bacterium.

Pathogenic and Antibody Responses

• However, unlike typical C. psittaci strains, the ones identified here did not cause disease when introduced to mice and guinea pigs, making them non-pathogenic in those animals.
• The researchers found that these novel strains resulted in a latent infection, meaning they remained in a dormant state within the host’s body, potentially able to cause illness at a later time.
• Notably, these strains still prompted an immune response, which suggests that the host’s immune systems recognized and responded to their presence.

Serological Response in Horses

• The scientists were able to observe an increase in the levels of specific antibodies against the infection in one of the horses from which acute and convalescent (recovering) sera could be collected.

Exclusion of Chlamydia psittaci in Other Cases

• Despite the discovery in two cases, the researchers were unable to isolate the chlamydia bacteria from material taken from 14 aborted foals, four synovial fluids from horses with acute polyarthritis, and nasal, tracheal, and lung material from another 276 horses.