Biology Reference
In-Depth Information
Prevalence of G. lamblia is around 20-30% in the developing world, 45
with up to 100% of children acquiring the infection before the age of 3. 42 In
the developed world, where water treatment is better and more widespread,
the prevalence is lower but outbreaks do occur. In 1985 there were par-
ticularly serious cases in both the UK 46 and United States. 47 In the United
States, G. lamblia was the most common intestinal protozoan infection in
the early 2000s 41 and it has been estimated that there are 2 million cases
annually, although many individuals are asymptomatic. 40 More recently, in
late 2004, over 1000 cases were reported in Norway, resulting from leaking
sewage and ineffective water treatment. 48 Giardia lamblia is zoonotic, with
common animal reservoirs being beavers, cattle, cats and dogs. 41
Giardia lamblia cysts are around 8 × 12 µm in size, containing two tro-
phozoites, and the cyst structure lacks the presence of a noticeable suture
line or operculum, which demarcates the site of excystation in other intes-
tinal parasite ova stages. 40 The cyst wall is robust and protects the internal
trophozoites. 49 A review of the details of the cell biology of Giardia is given
by Gillin and Reiner, 50 and studies of the make-up of the cyst wall are still
ongoing. 40
2.3.5. Toxoplasma
Toxoplasma gondii is capable of infecting over 350 vertebrate species. How-
ever, only cats serve as final hosts, where sexual reproduction in the intes-
tine produces oocysts, which are secreted into the environment. Toxoplasma
oocysts are approximately 10×12µm in size. The initially noninfectious
oocysts undergo sporulation upon exposure to air, moisture and adequate
temperatures, thus becoming infectious. Ingestion of sporulated oocysts can
lead to infection in a wide variety of hosts. The life cycle of this parasite is
complex, involving many stages, and we refer the reader to Figure 2.10 and
a recent review. 51 While many people are asymptomatic or experience mild
flu-like symptoms, toxoplasmosis is a serious disease that can cause blind-
ness, encephalitis and death in immunocompromised individuals and in a
developing fetus.
Major outbreaks include the one linked to the municipal water sup-
ply inVancouver in 1995 when more than 7000 people became infected,
with 100 cases of acute toxoplasmosis recorded. 52 More recently, in Bra-
zil, 176 people contracted toxoplasmosis after a drinking water reservoir
was contaminated with cat feces. Four outbreaks were reported between
2001 and 2010, with the largest infecting around 250 people in India
in 2005.
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