Serological cross-reactivity among chlamydial strains in a family outbreak of psittacosis.

Three members of a family of nine persons contracted psittacosis with severe pneumonia, respiratory failure, delirium, hepatitis and renal involvement. A newly purchased cockatiel was probably the primary source of infection but person-to-person transmission is likely to have taken place between twin brothers who shared a bedroom, one of whom had no direct contact with birds. Type-specific chlamydial serological tests identified the infecting agent as Chlamydia psittaci. The highest titres in the initial samples of serum from the patients, however, were to C. psittaci TWAR (Taiwan Acute Respiratory) and serological cross-reactivity among chlamydial strains was demonstrated. This study of a clearly defined outbreak of psittacosis provides useful information for those undertaking larger population surveys of chlamydial disease and emphasises the need for detailed serological investigation of cases.