Different serotypes of Chlamydia trachomatis cause inclusion conjunctivitis and trachoma

To establish infection on the conjunctiva, microorganisms must avoid being rinsed and wiped away in tears. The best way of achieving this is to have a specific mechanism of attachment to conjunctival cells. Chlamydia, for example, has surface molecules that bind specifically to receptors on host cells. This is one of the reasons that, of all the organisms infecting the conjunctiva (Table 25.1), they are among the most successful. There are eight different serotypes of Chlamydia trachomatis responsible for inclusion conjunctivitis (D–K) (Fig. 25.1) and another four serotypes responsible for trachoma (A, B, Ba and C), which, globally, is the most important eye infection in the world.

Figure 25.1 Chlamydial conjunctivitis is the most common form of neonatal conjunctivitis.

(Courtesy of G. Ridgway.)

Eight million people worldwide are visually impaired because of trachoma

Approximately 84 million people worldwide are affected by trachoma. Of these, about 8 million have some degree of visual impairment and the disease accounts for more than 3% of the world’s blindness. Trachoma is endemic in resource-poor countries where prevalence rates in pre-school children can reach 60–90%. Trachoma was known in ancient Egypt 4000    years ago, and tweezers to remove in-turned eyelashes (Fig. 25.2) have been found in royal tombs. Transmission of C. trachomatis is by contact, for example, by contaminated flies, fingers and towels.

Figure 25.2 Chlamydia trachomatis and blindness. The pathogenesis is outlined in (A). Scarring of the cornea (B) results from long-standing ocular trachoma. (Courtesy of R.C. Barnes.) Giemsa stain of an ocular scraping from trachoma (C) shows C. trachomatis as an intracellular inclusion.

(Courtesy of G. Ridgway.)

Trachoma itself is the result of chronic repeated infections (Fig. 25.2), which are especially prevalent when there is poor access to water, preventing regular washing of the hands and face. Under these circumstances, chlamydial infection is frequently spread from one conjunctiva to another and this can be referred to as ‘ocular promiscuity’, comparable with the spread of genital secretions in non-specific urethritis (see Ch. 21). Some chlamydial serotypes can infect the urogenital tract (see Ch. 21) as well as the conjunctiva, and the conjunctiva or lungs of a newborn infant may become infected after passage down an infected birth canal (see Ch. 23). In this situation, systemic treatment with erythromycin is generally needed.

Chlamydial infections are treated with antibiotic and prevented by face washing

Laboratory diagnosis of chlamydial infections (see Chs. 21 and 32) can be carried out using conjunctival fluid or scrapings. Point-of-care tests for rapid diagnosis in the field are available but there are issues regarding sensitivity. Treatment is with topical or oral antibiotics (e.g. azithromycin, doxycycline, etc.). Because infection and reinfection are facilitated by overcrowding, shortage of water and abundant fly populations, the disease can be prevented by improvements in standards of hygiene. In many areas with high rates of endemic trachoma, disease leading to blindness has been sharply reduced or eliminated by socioeconomic development and specific intervention steps (e.g. face washing). This has led the World Health Organization to establish an international alliance for the global elimination of blinding trachoma by the year 2020.

In spite of many decades of research, there are still no vaccines for chlamydial infections. This is partly because immunopathology itself makes a major contribution to the disease, and vaccine-induced immune responses could be harmful.