Computer models of parasite populations and anthelmintic ... - CSIRO

Table 3b Trichostrongylus
DRUG USED Low-Efficacy Oral Short-Acting Oral
RESISTANCE LEVEL low mod. high low mod. high
Trichostrongylus NSW 2143 3051 - 1299 2519 3063
Years 1-13 14-20 - 1-9 10-13 14-20
R-allele% 0-44 44-74 - 0-46 46-80 80-96
Trichostrongylus SA 1993 - - 2054 1881 -
Years 1-20 - - 1-13 14-20 -
R-allele% 0-39 - - 0-45 45-71 -
Ostertagia shows the greater increase in worm burdens as R-allele frequency increases. Trichostrongylus
burdens in the summer rainfall zone were most compromised by use of a low-efficacy drench when R-
allele frequency was low. The 65% increase in average worm burden would probably lead to some subclinical
production loss. Trichostrongylus and Ostertagia populations were controlled by the low-efficacy
drug in both environments because preparation of safe pastures for young sheep was assumed (i.e.
drenching was not the sole worm control measure). The more potent short-acting oral drench did control
worms at a lower level than the low-efficacy drug, but selected for resistance at a slightly faster rate. The
results indicate that both drugs can be used to control these species despite having different strengths and
weaknesses.
Table 3c Haemonchus
DRUG USED Low-Efficacy Oral Short-Acting Oral
RESISTANCE LEVEL low mod. high low mod. high
Haemonchus 99%* 130 344 - 102 250 348
Years 1-10 11-20 - 1-6 7-14 15-20
R-allele% 0-49 49-74 - 0-31 31-80 80-86
Haemonchus 80% # 160 464 - 225 153 410
Years 1-10 11-20 - 1-5 6-10 11-20
R-allele% 0-40 40-80 - 0-48 48-82 82-92
* indicates 99% CLS efficacy when no resistance to CLS is assumed.
# CLS efficacy of 80% when CLS-resistance is present.
Haemonchus populations were mainly controlled by CLS even when its efficacy against resident worms
and incoming larvae was reduced by about 20%. Consequently little impact on worm populations of drug
resistance to the broad-spectrum (BS) drenches was observed in these simulations, particularly as BS
treatments are usually accompanied by a CLS treatment. It needs to be emphasised that CLS resistance
remained static in these simulations so that the comparison between low-efficacy and potent BS drenches
could be made. The results (Table 3c) show that Haemonchus can be controlled when CLS is 80%
effective, but in reality this level of resistance would not last. Thus CLS could be relied on to control
Haemonchus when it is 80-90% effective but only as a short-term measure.
3.4. Acknowledgements - We are grateful for financial support from Australian woolgrowers through the
Australian Wool Research and Promotion Organisation in development of these models. The case study
simulation was published in the Proceedings of the Australian Sheep Veterinary Society 1999 AVA
Conference, Hobart 1999 (B. Besier editor) and we wish to thank the ASVS, a Special Interest Group of
the Australian Veterinary Association, for permitting us to reproduce this material.
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