2. Objetivos
6.3. Fase 2: Mediciones de ruido
These are control mechanisms or strategies other than the use of chemoprophylaxis (Athanasiadouet al., 2000) aimed at combating helminth problems. The development of resistance against nearly all anthelmintics (Leathwick et al., 2009; Waller, 2006; 2006b; Waller & Thamsborg, 2004), coupled with increasing demand for organic agriculture which strictly prohibits the use of synthetic products (Hordegen et al., 2003; Waller, 2006b) combined with rising consumer concerns about chemical use on farms, has encouraged research into alternative strategies for control of internal parasites particularly in sheep in NZ (Niezen et al., 1993). These include the amalgamation of chemotherapy with grazing management, especially alternate grazing with different stock classes; pasture spelling and renovation; making hay or silage; use of tannin-
containing forages in a grazing rotation; immunomodulants; use of nematophagous fungi; vaccines and targeted inactivation of genes regulating nematode development (Leathwick et al., 2009; Molan et al., 2002; Pomroy, 2006; Sangster, 1999; Sykes & Coop, 2001; Vlassoff et al., 2001; Waller, 1998, 2006; Williams, 1997).
One way to reduce the parasite burden on pastures is through grazing management. This involves a planned rotation programme aimed at limiting exposure to infective larvae on pasture. Knowledge of the life cycle of parasites becomes paramount here. In wet tropical climates, studies have shown that peak larval concentration of H. contortus and Trichostrongylus spp. occurred on pasture about 7 days post contamination but fell to hardly detectable levels within 28 – 42 days (Barger et al., 1994; Waller, 2006). In investigating this, Barger et al. (1994) set up a grazing trial consisting of 10 paddocks in which each paddock was grazed in a sequence for less than 7 days (3.5 days) to avoid auto-infestation and then spelled for 31.5 days (parasite larvae die within this period). The result showed that the egg counts of goats which grazed in the rotation system were less than half those that grazed outside this rotation system. In the temperate regions, larvae do not necessarily die this quickly. An alternative is interchange grazing between sheep and cattle which has proved to be considerably beneficial in worm control. In this system, grazing management systems exploit host specificity in which case species that are pathogenic to one host will not affect the alternative host or will be less pathogenic or prolific (Waller, 2006). Specifically, this involves alternation of the separate host species at intervals of 60 – 180 days with use of anthelmintic (although not always) at times of alternation (Waller, 2006). On the other hand, there is a possibility in the interchange grazing system of cattle nematodes showing clinical disease in sheep or vice-versa and so relying heavily on this system may prove disastrous.
Biological control of nematodes is another alternative method gaining ground in some parts of the world (Waller, 2006). Several types of biological control mechanisms have been recognized, but the interest here has been the nematophagous fungi (Duddingtonia flagrans; (Larsen et al., 1994; Waller & Faedo, 1996; Waller & Larsen, 1993). The fungus exploits the free-living stages of parasites as a source of food (Waller, 2006). It has very important attributes in that chlamydospores of this fungus are able to survive gut passage of livestock and are able to grow rapidly in freshly deposited dung and have nematophagous abilities (Waller, 2006). Control is achieved by the fungus capturing
and killing the infective larval stages before they migrate from dung to pasture where they could be ingested by grazing animals to complete their life cycle (Ketzis et al., 2006). In one experiment, Chandrawanthani et al. (2002) found that dose rates of approximately 1x106D. Flagrans spores/animal/day reduced the percentage of infective larvae development in faecal cultures by more than 90%. This could become a very useful tool where anthelmintics resistance is pronounced.
Plants have recently been studied in connection with antiparasitic effects against nematodes and other beneficial effects on livestock. It has been shown through studies that some plant species that contain CT have the potential to reduce the degree of parasite infestation, allowing the sheep to withstand helminth infection and improve growth rates in sheep (Waghorn, 2008). It was shown that drenched lambs grazing CT- containing legumes (Sulla spp.) grew at a comparable rate to lambs grazing non-CT- containing Lucerne (226 g/day vs 243 g/day) but faster than lambs grazing perennial ryegrass/white clover pasture (166 g/day; Niezen et al., 1998b). On the contrary, undrenched lambs grazing CT-containing legumes had higher daily gains (Sulla; 175 g/day) than when grazed either lucerne (121 g/day) or pasture (88 g/day). Furthermore, Niezen et al. (1998a) reported that parasite burdens at slaughter were similar for undrenched lambs that grazed Lotus spp. and pasture but were nevertheless always lower for growing animals grazing sulla. This clearly shows that forages containing CT when grazed by ruminants are able to sustain growth even in a parasitised state.
Work done by Diaz Lira et al. (2008) showed that grazing of weaned lambs, that had not been drenched, in willow fodder blocks showed some marked reduction in the burden of some of the most important internal parasites (Nematodirus spathiger, Trichostrongylus vitrinus and Trichostrongyluscolubriformis), compared with undrenched lambs grazing on conventional grass-based pastures, although grazing fodder blocks was not as effective as anthelmintic drenching. It was also shown in the same experiment that non- drenched lambs grazing in willow fodder blocks grew at comparable rates (154 g/day vs. 155 g/day) to drenched lambs grazing perennial ryegrass/white clover pasture but faster (111 g/day) than non-drenched lambs grazing perennial ryegrass/ white clover pasture. More recently, Musonda et al. (2009) concluded that grazing willow blocks played a beneficial role in sustainable farming systems in which the use of anthelmintics