Page 145 - The Veterinary Laboratory and Field Manual 3rd Edition

114 Susan C. Cork and Mani Lejeune

There is also a group of helminths of lesser vet- use snails as intermediate hosts; whereas, most
erinary importance, the acanthocephala (thorny cestodes exist by cleverly exploiting the preda-
headed worms). tor–prey relationship between the intermediate
Nematodes are the most abundant of the and definitive hosts (for example, Taenia spp.).
worms but not all are parasitic, there are many Adult trematodes invariably reside in the organs
free-living forms and some parasitize plants. of the gastrointestinal system (for example,
There are a wide range of parasitic nematodes, Paramphistomum sp., Fasciola sp. and so on) with
the adults of which have a predilection for a one exception (Schistosoma sp. in blood vessels).
particular organ in the host. While the majority Similarly, adult cestodes generally reside in
reside in the gastrointestinal tract (for example, the gastrointestinal tract of the definitive host
Haemonchus sp., Trichostrongylus sp., Ascaris sp., (usually a predator), however, the larval forms
Trichuris sp.), some live in the lungs (for example, tend to have a predilection for various organ
Dictyocaulus sp., Protostrongylus sp.), peritoneal systems in the intermediate hosts (usually prey
cavity (Setaria sp.), kidney (Stephanurus sp.), eye species).
(Thelazia sp.), connective tissue (Onchocerca sp.) Some ‘parasitic’ worms appear to live ‘in bal-
and meninges (Paraelaphostrongylus tenuis). The ance’ with the host causing little apparent harm
life cycle of nematodes can be simple; with the (for example, Cooperia sp. of sheep). However,
eggs hatching in the environment and develop- some are highly pathogenic and cause poor pro-
ing into infective larvae that are ingested by duction and ill health in livestock (for example,
the host and reach the gastrointestinal tract to Haemonchus contortus of sheep). The disease
develop into adults, for example, Haemonchus caused by helminths may be subclinical or clini-
contortus. Others have complex life cycles with cal. Clinical signs depend on (1) the number and
larval stages migrating through organs such species of parasite present, (2) the location of
as the liver or lungs before reaching gastroin- the parasite(s) in the body and (3) the general
testinal tract to reside as adults, for example, immune status of the host animals (which can
Ascaris suum. The life cycle can also be direct be determined by age, previous exposure, con-
or indirect. The latter requires development of current disease, nutrition and so on). Clinical
early parasitic stages in a suitable intermedi- disease is also exhibited if the host animal
ate host (for example, houseflies for Habronema becomes physiologically stressed as a result of
muscae). With the majority of helminths having lack of feed, harsh climate, concurrent disease,
an environmental component to their life cycle pregnancy or during seasonal migration. Not all
the diseases that they cause can be seasonal and animals in a herd or flock will be equally infected
are effected by climatic conditions. In addition, with a particular parasite species because some
some parasitic helminths have specific host and animals may be genetically susceptible, whereas
habitat preferences limiting their distribution to others may have a degree of resistance. Parasites
specific parts of the world. residing in resistant animals will often shed
Trematodes and cestodes are considered fewer eggs than those in more susceptible ani-
to be more primitive than nematodes. They mals. This is exemplified by the fact that in a
are largely hermaphrodites (that is, the sexes horse herd with strongyle infection only 20–30%
are not separate) with the notable exception are heavy egg shedders, the remaining animals
being the schistisomes. Most trematodes and are often either low or moderate shedders. For
cestodes that are parasitic to animals have an this reason, eggs counts are often used to deter-
indirect life cycle requiring an intermediate mine which animals in a herd need to be given
host. For example, the majority of trematodes treatment (anthelmintic).

Vet Lab.indb 114 26/03/2019 10:25