is the most common way of life; more than 50% of all animal
species are parasites. Parasites occur in all animal species
and they may have a profound effect on the health of people,
domestic animals and wildlife.
the study of parasitism; a multidisciplinary subject covering
many topics including morphology, taxonomy, biology, behaviour,
life-cycles, pathogenesis, epidemiology, ecology, physiology,
biochemistry, genetics and molecular biology, as well as the
diagnosis, immunology and treatment of infections.
Parasites live at
the expense of their hosts whereas other symbiotes may be
mutualists (living in mutual benefit with host) or commensals
(living without benefit or detriment to host). Parasites may
infect the gastrointestinal tracts or circulatory systems
of their hosts, they may invade different tissues and organs
or they may live on the external surfaces of their hosts.
Many infections may be asymptomatic whereas others may cause
acute (transient) or chronic (persistent) clinical diseases
ranging markedly in severity (mild to fatal).
infections may cause mortality (foetal, neonatal, adult death),
morbidity (disease manifest by enteritis, fever, anaemia,
etc.), production losses (reduced meat, milk, fibre production),
and tissue lesions (reduced marketability of product). Despite
many advances in parasite treatment and control, infections
still persist due to many factors, including urbanization
(crowding together); more intensive farming systems, greater
translocation of animals, further land and marine development,
inadequate effluent disposal, emergence of parasite drug resistance,
and spread of vector insecticide resistance.
types of organisms have adopted a parasitic mode of existence;
that is, they require a host for their own survival. Three
major groups of parasites are recognized: protozoa (belonging
to the kingdom Protista), and helminths and arthropods (belonging
to the kingdom Animalia, or Metazoa).
10,000 species of single-celled protozoa have been described
in the gut, blood or tissues of vertebrate and invertebrate
hosts. Parasitic flagellates cause enteric diseases
such as giardiasis, urogenital diseases such as trichomoniasis,
systemic diseases such as sleeping sickness, and tissue
diseases such as Chaga's disease and kala azar. Parasitic
amoebae cause dysentery, meningoencephalitis and corneal
lesions. Spore-forming sporozoa cause many serious diseases:
Apicomplexa cause coccidiosis, malaria and tick fevers;
Microspora parasitize fish and insects; and Ascetospora
cause seasonal mortalities in oysters. Parasitic ciliates
cause diarrhoea or lesions in humans and animals while
commensal species cause serious fouling problems in
50,000 species of multicellular helminths (worms) have
been described from a wide range of hosts. Roundworms
(nematodes) cause much morbidity and mortality in humans
and animals throughout the world. Serious infections
include filiariases, hookworm and threadworm diseases.
Larval and adult tapeworms (cestodes) may be found in
many vertebrate hosts. Some species do not cause clinical
disease whereas others may cause severe weight loss,
diarrhoea, abdominal pain or space-occupying lesions.
Flukes (trematodes) include many important species such
as sheep liver fluke and human schistosomes or blood
Thousands of arthropods are parasitic at some stage
in their life-cycles. Many cause serious diseases and
limit agricultural productivity. Parasitic insects include
biting and sucking lice which may cause skin lesions
or anaemia, fleas which may cause allergic dermatitis,
and various flies which suck blood as adults or produce
larvae which feed on host tissues. Parasitic arachnids
include ticks which feed on blood and may cause anaemia
or paralysis and mites which feed on skin and may cause
mild itching, hair loss or severe mange.