Componentes del sistema de agua

To prevent the development of anthelmintic resistance in GINs it is necessary to identify the risk factors and apply the strategies, mentioned previously, to prevent anthelmintic resistance. Additionaly it is necessary to have a thorough knowledge about the differences between goats and sheep as most information about goats has been accumulated from sheep data (Hoste et al., 2010; Jackson et al., 2012). Besides the differences in behavioural, immunological and physiological characteristics of these two hosts, it is advisable to understand differences in the pharmacology of these species. It has been reported that goats metabolize anthelmintics more rapidly than other livestock (Kaplan 2010). For example, some anthelmintic families such as benzimidazole, imidazothiazole/tetrahydropyrimidine, macrocyclic lactone and amino-acetonitrile are more rapidly cleared from the blood of goats, in comparison to sheep, if they are orally administered (Jackson et al., 2012). Additionally, it has been reported that goats require higher dose rates than other livestock to achieve proper efficacy (Kaplan 2010). Therefore, goats should be given a dose 1.5 (levamisole) to 2 times (for other drugs) higher than for sheep and cattle (Kaplan 2010).

Currently a range of techniques and practices have been proposed to control GIN infection, and these are summarized in Table 2.1.

23

Table 2.1. Anthelmintic strategies applied to control GINs in small ruminants Practices or

intervention

Effects and precautions

Strategic treatment

The principle of strategic treatment is to control infections at the beginning of the grazing season in order to minimize the paddock infectivity when the grazing season progresses (Torres-Acosta and Hoste 2008). Alternatively this practice is applied when most of parasites are inside the host and not on the pastures (Navarre and Pugh 2002). Strategic treatment practice has been reported to be suitable for farms with sheep and goats that have one main seasonal period of reproduction (Torres-Acosta and Hoste 2008) and during the winter when GINs are in a hypobiotic state (Navarre and Pugh 2002). To apply this practice, it is necessary to have good epidemiological information to define when animals will need treatments (Torres-Acosta and Hoste 2008) and to use anthelmintics that can kill encysted larvae (Navarre and Pugh 2002).

In more temperate to subtropical conditions, this practice become less effective because larvae survive in the enviroment for longer periods (Navarre and Pugh 2002). In such conditions, it has been recommended to use anthelmintics at the beginning of the wet season or once in the dry season, summer or autumn (Besier 2008), but these practices directly compromise refugia and select heavily for anthelmintic resistance (Besier 2008; Torres-Acosta and Hoste 2008).

Move and dose A variation of strategic treatment practice is the “move and dose” approach that has been widely applied in temperate and tropical conditions (Torres- Acosta and Hoste 2008). The background of this approach is to treat animals before moving them to a “clean” pasture (Torres-Acosta and Hoste 2008). However, similar to the practices mentioned above, this approach can select heavily for anthelmintic resistance (Torres-Acosta and Hoste 2008; Falzon et al., 2014). This practice should be discouraged, and alternative solutions should be investigated (Falzon et al., 2014).

Treat all and stay

This approach was proposed by Mobini (2010). The rationale is that if all animals are to be treated, they should remain in the pasture or paddock (for at least 2 to 3 weeks after treatment to pick up unselected larvae for propagation of the susceptible worms in the new paddock). This could prevent them from contaminating a new pasture with only resistant parasites which survived from previous treatments (Mobini 2010).

24 Suppressive

treatment

Suppressive treatment is applied before the end of the pre-patent period of the parasites, aiming to eliminate nearly all worms from the environment (Torres-Acosta and Hoste 2008) and requires use of anthelmintics at regular intervals, normally every 2 to 3 weeks with short acting anthelmintics or every 5 - 7 weeks with long acting anthelmintics (Torres-Acosta and Hoste 2008). However, this practice has been reported to be labour-intensive, fails to identify animals with superior immunity to parasites (Navarre and Pugh 2002) and, most importantly, results in anthelmintic resistance (Navarre and Pugh 2002; Torres-Acosta and Hoste 2008). To prevent or slow down the development of anthelmintic resistance, it is necessary to have adequate dosages and rotational use of effective anthelmintics (Navarre and Pugh 2002).

Tactical treatment

Tactical treatment is applied to remove parasites from their host before they enter their reproductive phase (Navarre and Pugh 2002), aiming to minimise the pathogenic and economic effects of pasture infectivity (Torres-Acosta and Hoste 2008). It involves drenching some or all animals in the flock when pastures are already contaminated with worm larvae e.g. during spring and summer in mild temperate climates, or during the wet season in tropical areas (Torres-Acosta and Hoste 2008). Tactical treatment practice can reduce the risk of anthelmintic resistance because treatments are performed when GIN larvae are already present (van Wyk (2001), and resistant worms are easily diluted with new infection from pasture (Torres- Acosta and Hoste 2008). The success of this practice depends on the detection of early signs of GIN infection (Torres-Acosta and Hoste 2008). However, this is not easy because: (1) clinical signs of a natural GIN infection may be easily confused with other health problems causing anaemia, undernourishment, diarrhoea or even chronic wasting (Torres-Acosta and Hoste 2008); (2) when animals have confirmed clinical signs of nematodiasis, then infection has often already caused considerable losses in productivity (Torres-Acosta and Hoste 2008); and (3) another common instance of mismanagement occurs when farmers treat their herds with a broad spectrum anthelmintic when they detect animals eliminating proglottids of Moniezia spp. in faeces (Torres-Acosta and Hoste 2008).

Selective treatment

Within goat herds, GINs are not equally distributed amongst individuals (Vlassof et al., 1999; Hoste et al., 2001). This means that only a small proportion of animals in the flock carry a large worm burden and the rest will naturally have low worm burdens (Hoste et al., 2002). Therefore, selective treatment practice or targeted selective treatment practice has been proposed (van Wyk et al., 2006), aiming to drench only animals that have large GIN infections and leave the others untreated (Torres-Acosta and Hoste 2008). Selective treatment have been used primarily to maintain refugia in GINs of small ruminants (Höglund et al., 2009). Selective treatment can reduce the number of treatments, the cost of anthelmintic treatment and labour, selection of superior animals for breeding, and most importantly, the reduction of selection pressure for anthelmintic resistance (Fleming et al., 2006; van Wyk et al., 2006; Torres-Acosta and Hoste 2008; Leathwick and Besier 2014). However, the greatest obstacle for selective treatment

25

practice is the identification of animals that need to be treated (Fleming et al., 2006; van Wyk et al., 2006; Torres-Acosta and Hoste 2008) and the practicality of implementation for the owner (Besier and Love 2012).

Generally, the identification of animals that need to be treated is primarily based on different phenotypic markers, either related to clinical signs or production losses (Torres-Acosta and Hoste 2008). According to Jackson et al., (2012), based on the epidemiology and pathogenicity of GINs in small ruminants, selective treatment can be divided into two approaches, namely selective treatment e.g. the FAMACHA© system, developed in South Africa primarily for use with infections of the haematophagous species H. contortus, and a selective approach for non-haematophagous species that was first developed in France for use with lactating goats.

FEC are a criteria to assess individuals that should be treated in a flock, but this approach is quite time consuming and relatively costly (Kenyon et al., 2009; Woodgate and Besier 2010). With haemonchosis, the FAMACHA© system seems more practical than FEC (Woodgate and Besier 2010) and appears to be the option of choice (Fleming et al., 2006; van Wyk et al., 2006; Besier 2008; Kaplan 2010). With other GINs such as Teladorsagia,

Trichostrongylus, Ostertagia, and Nematodirus, host milk yields, body condition scores, and body weight changes are important indices when selective

treatment practices are applied (Hoste et al., 2002; van Wyk et al., 2006; Besier 2008). Combined

selective treatment (C- TST)

Proposed by Torres-Acosta et al., (2014) C-TST combines FAMACHA©, body condition scores and FEC to identify goats at risk of severe GIN infection. In sheep production, it has been reported that the use of more complex objective indices, based on weight-gains in relation to individual animal performance, has the potential to discriminate between animals in their likely response to drenching (Busin et al., 2013; Kenyon et al., 2013; Leathwick and Besier 2014). For example, field trials involving large flocks in Australia demonstrated that using indices for individual animal drenching decisions, based on body condition scores, could enable a large proportion of animals to remain untreated with no significant production penalty (Leathwick and Besier 2014).

26

In conclusion anthelmintics still have an important role in controlling GINs in goats although worldwide there is anthelmintic resistance in GIN populations to all classes of anthelmintics. To ensure the effectiveness of anthelmintic treatment, it is necessary to have a thourough knowledge of risk factors to propose effective strategies to prevent the development of anthelmintic resistance of GINs. Selective treatment seems to be the best treatment to maintain anthelmintic efficacy and reduce the development of anthelmintic resistance. However, further investigation is needed to develop and validate indicators to select animals for treatment. A better understanding of the proportion of untreated animals necessary to maintain effective refugia under conditions of differing parasite species, environments and animal management regimes is still required.