aureus infections. The conjunctiva is particularly susceptible to infection.
The epithelial surface enclosed by the eyelids is a warm moist enclosed
environment in which microorganisms can rapidly become established.
However, microorganisms must avoid being rinsed away by tears and some
pathogens, such as Chlamydia trachomatis (Figure 3.3), attach specifically
to conjunctival cells. An estimated 500 million people are infected with
different serotypes ofChlamydia trachomatis, making trachoma the most
significant eye infection worldwide. The disease blinds approximately
1% of infected individuals while many others suffer visual impairment.
Chlamydia trachomatis is transmitted by contact with contaminated flies,
fingers and towels although trachoma itself results from chronic repeated
infections. This is much more likely to occur in regions where restricted
access to water prevents regular washing of the hands and face. Chlamydia
is also a sexually transmitted disease (Section 3.6) and there is evidence that
untreated chlamydial infections can lead to premature delivery and babies
born to infected mothers can be infected in their eyes and respiratory tracts.
Chlamydia is a leading cause of early infant pneumonia and conjunctivitis
(pink eye) in the newborn.
A laboratory diagnosis of trachoma can be carried out using samples of
conjunctival fluid or scrapings. The usual treatment is with oral or topical
antibiotics, such as tetracycline or doxycycline (Section 3.11).Chlamydia
infections account for only a fifth of cases of conjunctivitis; others are caused
by bacteria such as Streptococcus pneumoniae and Leptospira spp.
Serious infections of the inner eye with Pseudomonas aeruginosa may
follow trauma or after invasion by the protozoan Toxoplasma gondii
causing chorioretinitis and possible blindness. This widespread protozoan
is not a serious threat unless acquired in utero or when an individual is
immunosuppressed, perhaps as a result of taking drugs to prevent transplant
rejection (Chapter 6). Infection occurs by swallowing oocysts released by
infected cats or by eating meat containing tissue cysts.
The eyes may also be infected by parasitic worms, for example, larval forms
of the tapeworm Echinococcus granulosus that is transmitted by eggs passed
from infected dogs. Infection by the larvae of the nematode Toxocara canis,
which occurs naturally in the intestine of dogs, is, however, more common. An
infection can lead to the detachment of the retina. River blindness in Africa and
Central America is caused by the helminth Onchocerca volvulus. Simulium flies
carrying larvae obtained from the skin of infected hosts transmit the infection.
These flies develop in rivers, hence the name of the disease. River blindness is
a serious infection with over 300 000 people infected worldwide. The rates of
blindness, which is irreversible, may reach 40%. The usual treatment is with
anthelminthic drugs.
Infections of the outer ear may cause pain or irritation and are resistant to
some antibiotics. The middle ear can be colonized by viruses and bacteria
from the upper respiratory tract causing acute otitis media with swelling
and blockage of the Eustachian tube and may lead to deafness, though this
is generally temporary. Microorganisms that cause middle ear infections
include the virus that causes mumps. This may be followed by secondary
infections with Streptococcus pneumoniae and Haemophilus influenzae.
This is very common in children. Indeed, otitis media is the most frequent
illness diagnosed in young children. The general symptoms, apart from a
devastating earache, are fever, vomiting and diarrhea. The vesicles of the
tympanum become dilated, with bulging of the drum itself occurring in the
later stages of infection. If treatment is inadequate, then acute attacks may
eventually perforate the eardrum, produce chronic discharges and defective
hearing. The usual treatment is with oral antibiotics (Section 3.11), such as
ampicillin, amoxycillin, erythromycin and cefixime.
INFECTIONS OF THE EYES, EARS AND CENTRAL NERVOUS SYSTEMCZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY )(
Figure 3.3 Light micrograph of Chlamydia
trachomatis, growing in cultured eukaryotic
cells, stained with iodine. Courtesy of School of
Biochemistry and Microbiology, University of Leeds,
UK.