Ct bacterial loads are highest in those with TI [19]. The presence of TF and/or TI defines active trachoma. Ct can often be isolated from cases of active trachoma but, because follicles can persist for months or years after the infection has resolved, even the most sensitive nucleic

acid detection systems often fail to identify infection in subjects with active trachoma. Some, but not all cases of active trachoma develop conjunctival scarring, but this process usually takes several years. Ct cannot usually be isolated from subjects with scarring trachoma. In human volunteer studies, and in experimental infections in non-human primates, selleck chemicals scarring sequelae were not seen following a single infection [20], [21], [22], [23] and [24]. In trachoma endemic communities, the prevalence of scarring increases with age. It is more common in women, who are more frequently in contact with young children (the main reservoir of infection). People with intense inflammatory trachoma and persistent or recurrent Ct infection are more likely to develop scarring [25] and [26]. SP600125 As the scarring progresses and the scars contract, the lashes may turn inward and rub against the cornea

(trachomatous trichiasis, or TT), which is painful and causes corneal damage that may result in blindness. Experimental studies in humans and NHPs showed that re-challenge with the same strain of Ct results in an attenuated clinical response compared to primary infection, with a lower bacterial load [17], [20] and [21]. In trachoma not endemic communities the prevalence of ocular Ct infection decreases with age, and the highest bacterial loads are found in young children, suggesting that a degree of protective immunity develops following natural infection. A study in a trachoma endemic community in The Gambia, in which members

of affected households were examined and tested for ocular Ct infection every two weeks over a 6-month period in the absence of treatment, showed that the duration of episodes of disease and of infection was age dependent. The duration of untreated infection was estimated to be approximately 15 weeks in children aged 0–4 years, and 8 weeks in older children and adults [27] and [28]. The estimated incidence of infection was also lower in older individuals. The conclusion from this study is that protective immunity develops following natural infection, and is associated with both a reduced incidence and a reduced duration of infection. Experiments in baboons and in the Taiwanese monkey (Macaca cyclops) in the 1960s evaluated the protective efficacy of whole organism chlamydial vaccines, delivered parenterally, against ocular infection [21] and [29]. In both species it was shown that vaccines can provide short term, strain-specific protection against ocular Ct infection, which is of relatively short duration (less than 2 years).