Page 25 - parasitology for medical and clinical laboratoryprofessionals

Background of Parasitology 5

Those individuals who are in good health and well over half the bacterial and parasitic infections suffered
practice good hygiene and food preparation safety are by humans and the causative organisms number into the
less likely to become infected that those who are not hundreds of species. Many livestock pathogens and those
careful in their lifestyles and sanitation practices. It has that infect domestic carnivores such as dogs and cats may
been documented that parasitic infections on a wide- affect multiple host species, including humans (Haydon,
spread basis occur more frequently in poor and impover- Cleaveland, Taylor and Laurenson, 2002).
ished regions with a low level of sanitation. Parasites also
stimulate an immune response when the body produces Accidental Host
antibodies against an invasive organism in the same way
as with bacterial and viral infections. In many cases, an Another term sometimes heard when a parasitic infection
indirect identification of an organism may be possible by is experienced is that of accidental host. This refers to the
testing for specific antibodies to known organisms. infection of an animal or human with a parasite or other
Three important terms are used in the subject of host organism not normally found in the host. Vectors are also
descriptions. The main host is called the definitive host, involved in the transmission of many parasites, and may be
and is the organism in which the adult form of the para- either biological, such as a mosquito or tick, or mechani-
site is found. This is a sexually mature form of the parasitic cal, where the parasites are transmitted by food products
organism. Some parasites require more than one level of or by flies that walk on infected wastes and then pass the
host in order to complete a life cycle, which includes mat- parasites from one area to another. Humans may become
uration and reproduction. An intermediate host involves “accidental” hosts when human infection is not required
a life cycle where a species of host that differs from that of for propagation and continued survival of the infectious
the definitive host is necessary for competion of the devel- agent found in nature. Most emerging or “new” infectious
opmental stages of a parasite. An example of this relation- diseases are zoonotic (animal origin). These zoonotic dis-
ship is a widespread type of parasite such as the malarial ease agents may be viruses, bacteria, multicellular parasites
organism, Plasmodium vivax. The intermediate host is or prions, the agent associated with “mad cow disease.”
the human, where asexual or larval forms of the parasite
are found, and the definitive host is a species of mosquito
where the parasites undergo sexual reproduction. TRANSMISSION OF PARASITIC
INFECTIONS
Reservoir Host
The parasitic life cycle is vital in the transmission of
A reservoir host is an organism in which the parasite is parasitic infections. Some parasites spend their entire life
harbored until it is transmitted to the main or primary cycles in one host, whereas others require more than one
host. The reservoir host may not be harmed extensively host at different developmental stages. Some are para-
or at all while harboring the parasites and is linked to sitic only during periods of development by becoming
connected populations or environments where a given free-living at certain stages. Many parasites also require
organism can permanently reside until it is transmitted to certain environmental conditions for themselves and
a defined target population as a pathogen. Multiple res- their hosts, so many parasites are found in the warmer
ervoir hosts might exist, and confirmation of the destruc- climates of the world where there is a greater diversity
tion of a reservoir occurs when the target population is of animal life. Knowledge of the life cycle for suspect or-
free of disease and the parasite has been eliminated. If ganisms is necessary for interrupting the life cycle and
diseases in the target population are controlled, the pres- thereby minimizing the number of infections, as well as
ence of a reservoir for the causative organism may never knowing the correct specimen type required for identify-
be determined. Practical approaches in identifying res- ing the particular parasite.
ervoirs require field work for identification and effective The ways humans are infected (modes of transmis-
control of the reservoir hosts. sion) are numerous and often complex. Humans may
Reservoir hosts may harbor infectious agents that even infect animals with their own diseases, such as tu-
can infect more than one host species (target population). berculosis. Animals in which an infectious agent can
Zoonoses are infections that are common in animals and reproduce with no ill effect on the animal are known as
may also infect humans. These diseases may comprise reservoirs. Humans come into contact, either directly or