infection after malaria. This disease is caused by flatworms
mainly Schistosoma haematobium, S. japonicum, and
S. mansoni and is contracted in water infested with larvae
that develop in freshwater snails. Schistosomiasis is endemic
in 76 countries mainly located in Africa, South America,
Caribbean islands, eastern Mediterranean, and Southeast
Asia. More than 600 million people are at risk of infection
and 200 million are infected with schistosomiasis, with 20
million cases of severe illness.
Malaria also deserves to be cited as an example since it is
the first global pandemic parasitosis. In fact, 107 countries
are at risk of malaria, representing three billion people (46 %
of the estimated world population of 6.5 billion people). An
estimated 350–500 million clinical malaria episodes annually cause the death of 1.5–2.5 million people, including
many children (Murray et al. 2012). Plasmodium, the causative agent of malaria, is transmitted by mosquitoes of the
genus Anopheles that suffer only marginally from the parasitic infection. A negative effect of the parasite on fertility of
the host mosquito has been identified (Ahmed and Hurd
2006). In humans, Plasmodium infects red blood and liver
cells during its development cycle, which is one of the most
complex cycles in the parasitic world (Box 11.4, Fig. 11.14).
Emergence and Reemergence of Diseases
In the context of epidemics, many new diseases are termed
emerging diseases. An emerging disease means a disease
whose incidence increases significantly in a given population in a given region, compared to the usual situation of this
disease. An exemplary emerging disease is the toxic shock
syndrome caused by diffusion into the body of toxins produced by the bacterium Staphylococcus aureus, more commonly known as golden staph. Toxic shock syndrome is
increasing especially among women as a result of using
tampons (during the menstrual cycle), which increases the
risk of infection. Avian influenza, as indicated above with
highly pathogenic H5N1 virus, decimates sensitive wild
birds and poultry and is emerging in different regions of
the world. Under certain conditions, the H5N1 virus can
cross the species barrier and cause fatal infections in
humans, especially in Southeast Asia where the proximity
between human population and livestock is the major risk of
contamination.
Reemerging diseases are those that reappear after a silent
period, short or long, and often in a different form, sometimes more severe. The AIDS pandemic causes multiple
microbial coinfections involved in the reemergence of
diseases such as tuberculosis. Epidemics of chikungunya or
dengue hemorrhagic fevers, human diseases due to viral
agents transmitted by blood-sucking mosquitoes, reemerge
after long silent periods (7–30 years) in the regions of Africa
and Southeast Asia. In 2005, a new strain of highly pathogenic Chikungunya virus carrying a mutation at position 221
(alanine substitution by valine) of the envelope protein E1
was involved in severe and debilitating symptoms as well as
in human deaths in the islands of the Indian Ocean
(Schuffenecker et al. 2006).
Factors involved in disease emergence and reemergence
can vary considerably and are generally poorly known, but
there are many risk factors worsening epidemiological
changes. These factors include general environmental
perturbations (global warming) that will interfere with the
circulation pattern of infectious agents and their vectors,
urbanization and increased human populations that lead to
human impact on the environment, aging of human
populations that become weakly immunocompetent, and
new land uses and practices. Manufacture of animal meal
from the carcass of dead animals, potentially contaminated
with prion, and its use in animal feed are one of the causes of
the emergence and reemergence of mad cow disease. As
well as viruses, prion is an entity that is not independent of
the host cell for its multiplication. It should be noted that the
prion protein (PRoteinaceous Infectious particle ONly) is
present naturally in a nonpathogenic form in mammals
(including humans), where it is involved in the development
of the nervous system of the embryo. It would also play a
protective role against oxidative stress and programmed cell
death. It is the mutated form of the prion protein that
becomes pathogenic. With the ability to multiply exponentially and self aggregate, the mutated prion destroys neuronal cells and causes deposits in the brain.
International trade can lead to the spread of pathogens, as
suggested for H5N1 avian influenza outbreaks which reflect
the poultry trade on the Trans-Siberian route (Gautier-Clerc
et al. 2007). Other causes are related to the intrinsic adaptive
capacity of infectious agents and their ability to acquire
genetic information by horizontal transfer (cf. Chap. 12).
These genetic events may lead to the selection of resistance
to biocides (antibiotics and other antimicrobials), which
poses serious problems in human and veterinary medicine
(Box 11.5).
The Conflictual Nature of Animal–Microbe Interactions
Any pathology arises from uncontrolled conflictual
interactions between potentially infectious agents and their
hosts. The example of commensal bacteria in animals
including humans is striking. These bacteria are called normal flora, confined to specific areas and live in balance with
the immune system of healthy individuals. Upon environmental (rain, cold, heat) or immune (injury, poisoning) disturbance, the balance of the interaction can be broken, and
the resulting imbalance can lead to the expression of a more
or less severe disease in the healthy individual carrier. The
infectious agent escapes the control of the host, colonizes
unusual tissues and reaches excessive numbers, produces
toxins, and can eventually kill its host.
11 Microorganisms and Biotic Interactions
425
mainly Schistosoma haematobium, S. japonicum, and
S. mansoni and is contracted in water infested with larvae
that develop in freshwater snails. Schistosomiasis is endemic
in 76 countries mainly located in Africa, South America,
Caribbean islands, eastern Mediterranean, and Southeast
Asia. More than 600 million people are at risk of infection
and 200 million are infected with schistosomiasis, with 20
million cases of severe illness.
Malaria also deserves to be cited as an example since it is
the first global pandemic parasitosis. In fact, 107 countries
are at risk of malaria, representing three billion people (46 %
of the estimated world population of 6.5 billion people). An
estimated 350–500 million clinical malaria episodes annually cause the death of 1.5–2.5 million people, including
many children (Murray et al. 2012). Plasmodium, the causative agent of malaria, is transmitted by mosquitoes of the
genus Anopheles that suffer only marginally from the parasitic infection. A negative effect of the parasite on fertility of
the host mosquito has been identified (Ahmed and Hurd
2006). In humans, Plasmodium infects red blood and liver
cells during its development cycle, which is one of the most
complex cycles in the parasitic world (Box 11.4, Fig. 11.14).
Emergence and Reemergence of Diseases
In the context of epidemics, many new diseases are termed
emerging diseases. An emerging disease means a disease
whose incidence increases significantly in a given population in a given region, compared to the usual situation of this
disease. An exemplary emerging disease is the toxic shock
syndrome caused by diffusion into the body of toxins produced by the bacterium Staphylococcus aureus, more commonly known as golden staph. Toxic shock syndrome is
increasing especially among women as a result of using
tampons (during the menstrual cycle), which increases the
risk of infection. Avian influenza, as indicated above with
highly pathogenic H5N1 virus, decimates sensitive wild
birds and poultry and is emerging in different regions of
the world. Under certain conditions, the H5N1 virus can
cross the species barrier and cause fatal infections in
humans, especially in Southeast Asia where the proximity
between human population and livestock is the major risk of
contamination.
Reemerging diseases are those that reappear after a silent
period, short or long, and often in a different form, sometimes more severe. The AIDS pandemic causes multiple
microbial coinfections involved in the reemergence of
diseases such as tuberculosis. Epidemics of chikungunya or
dengue hemorrhagic fevers, human diseases due to viral
agents transmitted by blood-sucking mosquitoes, reemerge
after long silent periods (7–30 years) in the regions of Africa
and Southeast Asia. In 2005, a new strain of highly pathogenic Chikungunya virus carrying a mutation at position 221
(alanine substitution by valine) of the envelope protein E1
was involved in severe and debilitating symptoms as well as
in human deaths in the islands of the Indian Ocean
(Schuffenecker et al. 2006).
Factors involved in disease emergence and reemergence
can vary considerably and are generally poorly known, but
there are many risk factors worsening epidemiological
changes. These factors include general environmental
perturbations (global warming) that will interfere with the
circulation pattern of infectious agents and their vectors,
urbanization and increased human populations that lead to
human impact on the environment, aging of human
populations that become weakly immunocompetent, and
new land uses and practices. Manufacture of animal meal
from the carcass of dead animals, potentially contaminated
with prion, and its use in animal feed are one of the causes of
the emergence and reemergence of mad cow disease. As
well as viruses, prion is an entity that is not independent of
the host cell for its multiplication. It should be noted that the
prion protein (PRoteinaceous Infectious particle ONly) is
present naturally in a nonpathogenic form in mammals
(including humans), where it is involved in the development
of the nervous system of the embryo. It would also play a
protective role against oxidative stress and programmed cell
death. It is the mutated form of the prion protein that
becomes pathogenic. With the ability to multiply exponentially and self aggregate, the mutated prion destroys neuronal cells and causes deposits in the brain.
International trade can lead to the spread of pathogens, as
suggested for H5N1 avian influenza outbreaks which reflect
the poultry trade on the Trans-Siberian route (Gautier-Clerc
et al. 2007). Other causes are related to the intrinsic adaptive
capacity of infectious agents and their ability to acquire
genetic information by horizontal transfer (cf. Chap. 12).
These genetic events may lead to the selection of resistance
to biocides (antibiotics and other antimicrobials), which
poses serious problems in human and veterinary medicine
(Box 11.5).
The Conflictual Nature of Animal–Microbe Interactions
Any pathology arises from uncontrolled conflictual
interactions between potentially infectious agents and their
hosts. The example of commensal bacteria in animals
including humans is striking. These bacteria are called normal flora, confined to specific areas and live in balance with
the immune system of healthy individuals. Upon environmental (rain, cold, heat) or immune (injury, poisoning) disturbance, the balance of the interaction can be broken, and
the resulting imbalance can lead to the expression of a more
or less severe disease in the healthy individual carrier. The
infectious agent escapes the control of the host, colonizes
unusual tissues and reaches excessive numbers, produces
toxins, and can eventually kill its host.
11 Microorganisms and Biotic Interactions
425