Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
202
Strains of cholera
There are many different strains of V. cholerae. Until
the 1990s, only the strain known as O1 caused cholera.
Between 1817 and 1923, there were six pandemics of
cholera. Each originated in what is now Bangladesh,
and was caused by the ‘classical’ strain of cholera, O1.
A seventh pandemic began in 1961 when a variety of O1,
named ‘El Tor’, originated in Indonesia. El Tor soon spread
to India, then to Italy in 1973, reaching South America in
January 1991, where it caused an epidemic in Peru. The
discharge of a ship’s sewage into the sea may have been
responsible. Within days of the start of the epidemic, the
disease had spread 2000 km along the coast, and within
four weeks had moved inland. In February and March of
1991, an average of 2550 cases a day were being reported.
People in neighbouring countries were soon infected. In
Peru, many sewers discharge straight onto shellfish beds.
Organisms eaten as seafood, especially filter-feeders such
as oysters and mussels, become contaminated because they
concentrate cholera bacteria when sewage is pumped into
the sea. Fish and shellfish are often eaten raw. Because the
epidemic developed so rapidly in Peru, it is thought that the
disease probably spread through contaminated seafood.
A new strain, known as V. cholerae O139, originated
in Chennai (then called Madras) in October 1992 and has
spread to other parts of India and Bangladesh. This strain
threatens to be responsible for an eighth pandemic. It took
El Tor two years to displace the ‘classical’ strain in India;
O139 replaced El Tor in less than two months, suggesting
that it may be more virulent. Many adult cases have been
reported, suggesting that previous exposure to El Tor has
not given immunity to O139 (Chapter 11).
QUESTIONS
10.2 List the ways in which cholera is transmitted from
person to person.
10.3 One person can excrete 10 13 cholera bacteria a day.
An infective dose is 10 6 . How many people could one
person infect in one day?
10.4 Explain why there is such a high risk of cholera
following natural disasters such as earthquakes,
hurricanes, typhoons and floods.
10.5 Describe the precautions that a visitor to a country
where cholera is endemic can take to avoid catching
the disease.
Malaria
Transmission of malaria
The features of this disease are summarised in Table 10.3.
Most cases of malaria are caused by one of four species of
the protoctist Plasmodium, whose life cycle is shown in
Figure 10.4. Genetic analysis of infections shows that some
species of Plasmodium that cause malaria in monkeys also
affect humans.
Female Anopheles mosquitoes feed on human blood to
obtain the protein they need to develop their eggs. If the
person they bite is infected with Plasmodium, they will
take up some of the pathogen’s gametes with the blood
meal. Male and female gametes fuse in the mosquito’s
gut and develop to form infective stages, which move to
the mosquito’s salivary glands. When the mosquito feeds
again, she injects an anticoagulant from her salivary
glands that prevents the blood meal from clotting, so that it
flows out of the host into the mosquito. The infective stages
pass from the mosquito’s salivary glands into the human’s
blood together with the anticoagulant in the saliva. The
parasites enter the red blood cells, where they multiply
(Figures 10.5 and 10.6).
The female Anopheles mosquito is therefore a vector of
malaria and she transmits the disease when she passes the
infective stages into an uninfected person. Malaria may
also be transmitted during blood transfusion and when
unsterile needles are re-used. Plasmodium can also pass
across the placenta from mother to fetus.
Pathogen
Method of
transmission
Global distribution
Incubation period
Site of action of
pathogen
Clinical features
Method of
diagnosis
Annual incidence
worldwide
Annual mortality
worldwide
Table 10.3 The features of malaria.
Plasmodium falciparum, P. vivax,
P. ovale, P. malariae
insect vector: female Anopheles
mosquito
throughout the tropics and subtropics
(endemic in 106 countries)
from a week to a year
liver, red blood cells, brain
fever, anaemia, nausea, headaches,
muscle pain, shivering, sweating,
enlarged spleen
microscopical examination of
blood (Figure 10.5); dip stick test for
malaria antigens in blood
about 207 million cases of malaria in
2012 (about 80% are in Africa)
about 630 000 deaths in 2012 (about
90% are in Africa)