Chlamydia trachomatis is a unique obligate intracellular bacterium that remains the leading cause of sexually transmitted bacterial diseases and preventable blindness worldwide. Chronic ocular infections are referred to as trachoma, and predominate in developing countries. Since 2001, the World Health Organization has promoted control strategies including antibiotics, improved hygiene, and environmental measures with limited success. Consequently, a vaccine is urgently needed. Integral to vaccine design is an understanding of the interactions of the pathogen and host immune response. Various animal models of trachoma show that urogenital C. trachomatis strains and other species of the family Chlamydiaceae produce severe conjunctival inflammation and scarring similar to that of the ocular C. trachomatis strains. However, we do not know the extent of organisms that may be involved in human trachoma. Furthermore, C. trachomatis heat shock protein 60 (Hsp60) has been implicated in inflammation and conjunctival scarring but the role of other Chlamydiaceae Hsp60 in disease pathogenesis has not been examined. In this study, we set out to identify whether other Chlamydiaceae species are present in trachoma, and determine their association with severity of clinical disease and with mucosal and systemic immune responses to Chlamydiaceae species-specific Hsp60 to further investigate the immunopathogenesis of this blinding disease.
Methods and Findings
We randomly selected nine of 49 households in a trachoma-endemic region of Nepal. Trachoma was graded, and real-time, quantitative (k)PCR was used to detect genomic DNA and cDNA (from RNA) for Chlamydiaceae ompA and 16S rRNA genes, respectively, from conjunctival swabs. IgG antibody responses to recombinant (r) Chlamydiaceae species-specific Hsp60 were determined for tears and sera. Surprisingly, all three species—C. trachomatis, Chlamydophila psittaci, and Chlamydophila pneumoniae—were detected in eight (89%) study households; one household had no members infected with C. pneumoniae. Of 80 (63%; n = 127) infected individuals, 28 (35%) had infection with C. psittaci, or C. pneumoniae, or both; single and dual infections with C. psittaci and C. pneumoniae were significantly associated with severe conjunctival inflammation (OR 4.25 [95% confidence interval (CI), 2.9–11.3], p = 0.009] as were single infections with C. trachomatis (OR 5.7 [95% CI, 3.8–10.1], p = 0.002). Of the 80 infected individuals, 75 (93.8%) were also positive for 16S rRNA by kPCR for the same organism identified by ompA. Individuals with tear IgG immunoreactivity to Chlamydiaceae rHsp60 were eight times more likely than individuals without tear immunoreactivity to be infected (95% CI 6.4–15.1; p = 0.003), 6.2 times more likely to have severe inflammation (95% CI 4.4–12.6; p = 0.001), and 5.7 times more likely to have scarring (95% CI 3.9–11.1; p = 0.019) while individuals with serum IgG immunoreactivity were 4.1 times more likely to be infected (95% CI 3.1–10.1; p = 0.014).
We provide substantial evidence for the involvement of C. psittaci and C. pneumoniae, in addition to C. trachomatis, in trachoma. The distribution of Chlamydiaceae species by household and age suggests that these infections are widespread and not just sporadic occurrences. Infection with multiple species may explain the failure to detect chlamydiae among active trachoma cases, when only C. trachomatis is assayed for, and the failure of clinically active cases to resolve their disease following what would be considered effective C. trachomatis treatment. The evidence for viable (RNA-positive) organisms of all three species in single and coinfections, the significant association of these infections with severe inflammation, and the significant association of tear and serum IgG responses to Chlamydiaceae Hsp60 with inflammation and scarring, support the role of all three species in disease pathogenesis. Thus, while our findings should be confirmed in other trachoma-endemic countries, our data suggest that a reevaluation of treatment regimens and vaccine design may be required. Understanding the full impact of Chlamydiaceae species on the epidemiology, immunopathology, and disease outcome of trachoma presents a new challenge for Chlamydiaceae research.
In a study of trachoma cases within households in Nepal, Deborah Dean and colleagues find involvement of the Chlamydia species C. psittaci and C. pneumoniae in addition to C. trachomatis.
Six million people—most of whom live in crowded, unhygienic conditions in developing countries—are blind because of an infectious disease called trachoma. It is generally accepted that trachoma is caused by Chlamydia trachomatis, bacteria that pass easily between people on hands and clothing. Infection usually occurs first during childhood, but people do not become blind until adulthood. Successive infections cause progressive scarring of the inside of the eyelid. Eventually, the eyelashes turn inward and rub painfully over the front of the eye (the cornea). This causes corneal scarring, loss of corneal transparency and, finally, irreversible blindness. C. trachomatis infections can be prevented by improving personal hygiene (in particular, facial cleanliness in children) and by reducing fly breeding sites, and they can be treated with antibiotics. However, C. trachomatis and other organisms appear to be developing drug resistance to antibiotics commonly used to treat these infections. In addition, early scarring and in-turned eyelashes can be treated surgically, although recurrence of the in-turned eyelashes frequently occurs months to years after surgery.
Why Was This Study Done?
The World Health Organization has been promoting these “SAFE” interventions (surgery, antibiotics, facial cleanliness, and environmental improvement) since 2001 with the aim of eliminating trachoma by 2020. However, these control measures have had limited success so far and it looks like a vaccine may also be needed. To develop an effective vaccine, scientists need to know whether all cases of human trachoma are caused by so-called ocular strains of C. trachomatis. Might C. trachomatis strains that are usually associated with sexually transmitted disease (urogenital strains) or different species in the family Chlamydiaceae also cause human trachoma as work in animals has suggested? In this study, the researchers have investigated which Chlamydiaceae species are associated with trachoma in a region of Nepal where the disease is endemic (always present).
What Did the Researchers Do and Find?
The researchers examined all the members for trachoma in nine randomly selected households in a Nepali village. They then used sensitive molecular biology methods to identify the species in the family Chlamydiaceae and strains present in the eyes of the infected individuals. One third of them were infected with only C. trachomatis (mainly ocular strains but also some urogenital strains), one in five were infected with only Chlamydophila psittaci, and one in ten with only Chlamydophila pneumoniae. The other infected individuals had mixed infections. Infection with C. psittaci and/or C. pneumoniae was strongly associated with severe eye inflammation as was infection with C. trachomatis alone. The researchers also asked whether there were any antibodies (proteins made by the immune system that recognize infectious organisms) in the tears or blood of the infected individuals that recognized the Hsp60 protein of each Chlamydiaceae species; an immune response to C. trachomatis Hsp60 is thought to be involved in the inflammation and scarring seen in trachoma. Individuals with antibodies in their tears to Chlamydiaceae Hsp60, the researchers report, were eight times as likely to be actively infected with these bacteria and six times as likely to have severe eye inflammation as individuals without the antibodies.
What Do These Findings Mean?
These findings provide evidence for the widespread involvement of C. psittaci, C. pneumoniae, and urogenital strains of C. trachomatis as well as ocular strains of C. trachomatis in trachoma and might explain why some people with active trachoma do not have C. trachomatis in their eye secretions and why antibiotics that kill C. trachomatis effectively do not cure all cases of trachoma. However, because live bacteria were not isolated from patients and shown to cause disease in a model system, these findings do not prove that Chlamydiaceae other than C. trachomatis cause trachoma. Some or all of the bacterial strains and species detected in this study may be innocent bystanders although the strong association between their presence and severe inflammation and the association between antibody responses to Chlamydiaceae Hsp60 and inflammation argues against this possibility. If the involvement of multiple Chlamydiaceae strains and species is confirmed and extended in other trachoma-endemic regions, then future antimicrobial therapies and vaccines will need to deal with all these bacteria and not just C. trachomatis.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050014.
The MedlinePlus encyclopedia contains a page on trachoma (in English and Spanish)
The World Health Organization provides information on trachoma (mainly in English but some information is available in French, Russian, and Spanish)
The US Centers for Disease Control and Prevention provides a technical fact sheet on trachoma
The charity Sightsavers International also provides information on trachoma and global efforts to eliminate the disease
The Carter Center provides an overview of trachoma control and a description of its trachoma control program