Gibb et al. 2005 ). In sheep, within the European flock, worm infestations involving
the abomasal nematode speciesHaemonchus contortusandTeladorsagia circum-
cincta, and the small intestinal parasitesTrichostrongylus colubriformisandNema-
todirus battusare common. They may cause slow growth and hence extended time to
slaughter. Weight loss, emaciation and even death can result from acute haemoncho-
sis and nematodirosis. Welfare concerns arise from the deterioration in body condi-
tion of emaciated sheep. Furthermore, faecal staining of the perineum, resulting from
PGE-induced diarrhoea, is a major risk for cutaneous myiasis (Scott et al. 2007 ). This
condition is the invasion of living tissues by larvae of dipteran flies in the summer.
It arises as a result of female blowflies laying eggs in areas of soiled fleece and the
ensuing larvae lacerating the infested skin. Timed administration of anthelmintics to
ewes during the periparturient period and subsequently to grazing lambs mitigates
production losses, prevents clinical disease and reduces larval pasture contamination.
Details of anthelmintic agents available for use in livestock are presented under
Sect. 2.3 below and in Table 1. Although all chemoprophylactic protocols provide
satisfactory control, drugs of the macrocyclic lactone (ML) group deserve special
mention because they have transformed parasite control strategies in livestock over
the past 28 years. These molecules are characterised by slow clearance, large
volumes of distribution and long terminal half-lives (McKellar and Benchaoui
1996 ) (Table 2 ). Their prolonged persistence in the body prevents the establishment
of incoming larvae for several weeks, thus allowing effective and long-lasting
control of gastro-intestinal nematode infections with low frequency of drug admin-
istration. Members of this class are available in injectable form (mostly oil-based
formulations for subcutaneous injection), oral drenches (aqueous solution or suspen-
sion for oral delivery), sustained release devices (e.g. osmotic bolus delivering
12.7 mg of ivermectin per day over a period of 135 days into the reticulo-rumen
(forestomach) of cattle) and pour-ons (solutions for application along the midline of
the back). In addition to being anthelmintic agents, MLs also have potent ectopar-
asiticide properties. All these attributes have made these molecules the “gold stan-
dard” of parasite control in ruminants. In food producing animals, however, drugs
with long-lasting efficacy often have the drawback of persistent residues in edible
tissues. Most MLs fall into this category, and edible products derived from treated
animals can only enter the food chain after a relatively long withdrawal (waiting)
period. The exception is eprinomectin; this molecule was specifically developed for
its low partitioning into the milk (Alvinerie et al. 1999 ), and therefore lactating dairy
cows can be dosed without the inconvenience of post-treatment milk discard.
2.2 Helminth Parasites of Dogs and Cats
2.2.1 Gastro-Intestinal Nematodes
Clinical disease caused by gastro-intestinal nematodes can range from ill-thrift and
slow growth to severe haemorrhagic enteritis and anaemia, particularly in young
pups and kittens (Magne 2006 ). From the perspective of prevalence and pathogenic
116 H. Benchaoui