Page 173 - The Veterinary Laboratory and Field Manual 3rd Edition

142 Susan C. Cork and Mani Lejeune

number of parasite eggs. In a good rotational not be grazed on pasture contaminated by
grazing system, the stock person usually last year’s young stock.
ensures that pasture is spelled after young • Wet or moist pastures: wet places like the
livestock have been grazing because young edges of pools of water are suitable for the
animals often pass large numbers of para- development and maintenance of nematode
site eggs in their faeces until they gain some larvae or trematode metacercariae, and if
resistance. possible should be fenced off.
• Stabled animals: raised hay-racks ensure that
controL MEaSurES feed is not contaminated by bedding or faeces
There are a wide range of effective drugs avail- so that infective larvae do not migrate into it
able for the treatment and prevention of parasitic from the floor.
disease but it cannot be overemphasized that the
use of anthelmintics is only effective if concur- A note on anthelmintic resistance
rent husbandry practices are also implemented
to reduce the burden of infective larvae on In recent years, anthelmintic resistance has been
pasture. spreading at an alarming rate. It is widespread
in nematode parasites of ruminants, pigs and
• Periodic use of anthelmintics (chemo- horses. Resistance can develop wherever anthel-
prophylaxis): regular treatment of animals in mintics are frequently used and this has been
order to keep them free of the most patho- seen with commonly used formulations of benz-
genic (harmful) worms. There are a wide imidazole, levamisole/morantel and ivermectin.
range of products available for treatment and/ In South America, 100% resistance to almost all
or prevention of parasitic disease and advice known anthelmintics on the market has been
should be sought from the regional veterinary reported in small ruminant populations. The
officer or animal health advisor with regard to risk of resistance developing is greatest where
the best choice and use of available anthel- animals have not been given a sufficient dose of
mintics. Many of the pour-on and injectable the anthelmintic and in cases where the same
preparations used to control ectoparasites product is used repeatedly over a period of time.
may also be effective against some, but not The issue also appears to have emerged due
all, helminths. to the unnecessary usage of anthelminthic for
• Avoid overcrowding: overcrowding (high ‘blanket treatment’ of all animals in a herd.
stocking rate) tends to increase the concen- Improvements in our understanding of the
tration of worm eggs on pasture and should biochemistry and molecular genetics of parasite-
therefore be avoided. anthelmintic interactions has helped us to better
• Rotation of pasture: pasture rotation allows understand what leads to resistance, for exam-
the animals to graze on ‘clean pastures’ at ple, levamisole/morantel resistance appears to
intervals while the infected paddocks can be be associated with alterations in cholinergic
left to become ‘clean’. It is, however, important receptors in resistant nematodes whereas benz-
that pasture should be left without grazing imidazole resistance appears to be associated
livestock for sufficient time (this depends on with an alteration in beta-tubulin genes which
climatic factors and land use) so that part of reduces or abolishes the high affinity binding
the population of infective larvae die. of benzimidazoles for tubulin in these para-
• Young stock: calves, lambs and other young sites. Further discussion on the mode of action
stock are the most susceptible so they should of anthelmintics and anthelmintic resistance is

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