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Anthelmintic Resistance

1. What is resistance?

Resistance is the heritable ability of a parasite to tolerate a normally effective dose of an anthelmintic. The parasite is considered resistant if it survives exposure to the standard recommended dose of the anthelmintic and the ability to survive is passed on to its offspring. Resistance can be viewed as drug tolerance, since ‘resistant’ parasites can often be removed by exposure to higher dose rates of anthelmintic up to the maximum dose tolerated by the host. Anthelmintic resistance in cattle can be measured in several ways. These include field tests, such as a simple Wormer Test (WT) as an indication of treatment efficacy, or the more often used Faecal Egg Count Reduction Test (FECRT). (Section 8. has details of these techniques). A fully effective anthelmintic is expected to reduce the FEC to zero after administration. If the reduction is 95% or less, then this is interpreted as the presence of resistance genotypes. (Point B in Fig. 5.1)

Under field conditions however, anthelmintics may continue to give clinical responses in parasitised cattle when the reduction in faecal egg count (FEC) is < 95%. Consequently, farmers remain unaware that resistance to an anthelmintic is present until the reduction reaches approximately 80% or less (point C in Fig. 5.1). Beyond this point there may be production losses from poor worm control and the severity of the resistance will increase rapidly if the anthelmintic remains in use.

This distinction between the detection of resistance using FECRTs, at the 95% level and farmers seeing apparent failure at approximately the 80% level is vital to the slowing of the development of anthelmintic resistance. By detecting resistance at an early stage, cattle farmers can employ the COWS recommendations to prolong the time taken for the worm population on their farm to move from point B to point C (Fig. 5.1). This means the activity of the wormer group(s) concerned can be maintained for longer.

Figure 5.1 The rate at which AR appears in a herd

Next section - Worldwide AR situation