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Parasite Control (Management Causes of Anthemintic resistance (Blanket…
Parasite Control
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Refugia
Traditional practice = treating all lambs and moving to new clean pasture meaning all nematodes on the field are resistant. Refugia is the practice ensuring susceptible worm populations are diluted with the refugia population
long acting formulations can cause issues as milk fat will contain the drug meaning its ingested by lambs - decreased susceptible to resistant worm ratio
Refugia = Nematodes that are not exposed to anthelmintics, these an be on the field or in an untreated animal.
Targeted Treatment
Targeted treatment is when the whole flock is treated based on the knowledge of risk and parameters that quantify severity of infection but some animals must be left untreated
Example 1: prediction - assessment of fields to predict risk e.g when lambs and ewes where last on the field. Prediction can also take place using models - Nematodirus battus infection can be predicted based on climate models as a cold period followed by milk temperatures is needed for eggs to hatch (impact of climate change)
Example 2: FEC - Counting can indicate infection levels but will only show numbers of adult worms not larval stages, FEC also does not differentiate between species and doesn't account for animal resilience.
Targeted selective treatment is when individual animals are treated on infection severity and performance indicators - it can however be hard to identify individual animals
It is predicted that 80% of worms are in 20% of animals, TST is used to identify and treat animals with high burdens
Example 1: Lamb DLWG- models can be used to predicted DLWG, if animals are not meeting these predictions PGE can be suspected and treated. This method has been shown to reduce use and the development of resistance while having no impact on lamb performance
Disadvantages: It is a reactive not proactive system, Resilient animals aren't treated so will continuously contaminate pasture. The model might be effected by other factors that aren't nematodes such as mineral deficiencies. Unpractical on many farms
Example 1: DISCO- Diarrhoea score, practice is widely used by farmers but research is limited. Evidence has shown a correlation between DISCO and FEC however other factors can cause diarrhoea
Example 3: BCS - Used in peri-parturient ewes, stress of lambing can reduce immune function and awake worms from hypobosis. Cornelius et al., 2014 showed that merino ewes of a BCS greater than 2.7 did not benefit from anthelmintic treatment. Its thought that a higher BCS means more energy to keep up immune function
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