2.3 Anthelmintic Agents
Six modern classes of broad-spectrum anthelmintic agents are currently available,
or are likely to become available in the near future:
l Benzimidazoles
l Imidazothiazoles/tetrahydropyrimidines
l MLs
l Paraherquamides
l Cyclic depsipeptides
l Amino-acetonitrile derivatives (AADs)
Narrow-spectrum anthelmintic classes with examples, developed for animal health
prophylaxis and therapy, mainly to combat liver fluke or tapeworm infections,
include the following:
l The benzimidazole compound triclabendazole
l Salicylanilides (closantel)
l Pyrazinoisoquinolines (praziquantel)
l Benzenesulphonamides (clorsulon)
l Substituted phenols (nitroxynil eglumine)
The spectrum of activity and mechanisms of action of these agents are summarised
in Table 1.
2.4 Anthelmintic Resistance
Resistance of gastro-intestinal nematodes to anthelmintic agents constitutes a major
threat to the worldwide small ruminant industry. Resistance to benzimidazoles,
levamisole and the MLs is now widespread in all continents including Europe
(Table 3 ). Moreover, multi-drug resistance to all three anthelmintic groups is no
longer uncommon (Kaplan 2004 ). Whilst the most significant impact of anthel-
mintic resistance has been largely confined to sheep and goats, evidence of resistant
Cooperiain cattle is emerging (Coles et al. 2001 ;www.parasol-project.org). In
recent years, guidelines have been published to standardise the in vivo and in vitro
methods available for the detection of nematode resistance (Coles et al. 2006 ).
Resistance in liver fluke has not yet reached the scale experienced with nematodes,
but it has been reported for the salicylanilides, rafoxanide and closantel, with cross
resistance to the halogenated phenol, nitroxynil. A concerning trend observed since
the mid-1990s is the development of resistance to triclabendazole, the principal
drug used to treat liver fluke infections, because of its high activity against imma-
ture migrating stages (Brennan et al. 2007 ). In horses, cyathostomins resistant to the
benzimidazoles and pyrantel are now widespread (Kaplan 2004 ; Traversa et al.
2007b); there is also evidence ofParascaris equorum resistance to the MLs
122 H. Benchaoui