This study demonstrates that male guinea-pigs inoculated intra-urethrally with low doses of Chlamydia psittaci (guinea-pig inclusion conjunctivitis strain, GP-IC) can develop microbiologically inapparent urogenital infection without showing clinical signs or detectable infected cells in urethral scrapings. These inapparently infected animals acted as asymptomatic shedders, transmitting eye infection to normal guinea-pigs housed with them. Treatment with 5-iododeoxyuridine transiently induced appearance of infectious organisms in urethral scrapings from donor animals, confirming organism persistence. In contrast, guinea-pigs that recovered from overt urethral infection developed serum antibodies, remained resistant to urethral reinfection, and failed to transmit eye infection to cagemates. The findings suggest that inapparent chlamydial infection represents a persistent, low-level shedding state distinct from overt infection, potentially relevant to understanding latency mechanisms in chlamydial diseases. The induction of inapparent infection appeared primarily dose-dependent, with subthreshold inocula promoting asymptomatic shedding in animals that would develop overt infection at higher doses.

Key findings

• Low-dose intra-urethral inoculation of GP-IC induced microbiologically inapparent infection in guinea-pigs that transmitted eye disease to cage-mates despite absence of detectable urethral infection
• Drug treatment with 5-iododeoxyuridine transiently activated shedding of infectious organisms from inapparently infected animals, demonstrating organism persistence below detection limits
• Animals recovering from overt urethral infection developed serum antibodies and were resistant to challenge, but inapparently infected animals lacked antibodies and remained susceptible to reinfection
• Inapparent infection appeared to be primarily dose-dependent, with subthreshold inocula causing asymptomatic shedding in majority of non-responding animals