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

of free-living stages and host regulation of Haemonchus and Ostertagia populations, were used to simulate
drug selection in these species.
3.1. Model Assumptions: Table 1 gives the assumptions for the activity of a low-efficacy and short-acting
oral anthelmintic against each worm and larval genotype. Note that the only difference between the 2
drugs is the efficacy against resident worms on the day of treatment; exclusion of larvae following
treatment is presumed to be the same for both drugs. The frequency of the gene for drug resistance was
initially set at 0.01% for all simulations; this yields an approximate efficacy of 80% and 99% for the lowefficacy
and short-acting oral anthelmintics, respectively. SS, RS and RR represent susceptible
homozygous, heterozygous and resistant homozygous genotypes, respectively and assumes a single gene
determines resistance [5,6]. For the short-acting oral drench drug resistance was assumed to be a dominant
trait [3,6,14] in Haemonchus and Ostertagia simulations and a co-dominant trait for Trichostrongylus
simulations. For all species, resistance to the low-efficacy oral drug in parasitic stage worms was assumed
to be dominant. Selection for drug resistance was simulated and evaluated by determining increases in this
gene frequency in larvae on pasture and in grazing sheep.
Table 1. Assumed anthelmintic efficacy against resident worms and incoming larvae.
Drug efficacy (%) against resident worms at the time of treatment.
Worm Species: Trichostrongylus Ostertagia-Haemonchus
Worm genotype: SS RS RR SS RS RR
Treatment type
Low-efficacy oral 80 0 0 80 0 0
Short-acting oral 99 50 0 99 4 2
Drug efficacy (%) against incoming larvae (L3) for 3 days post treatment.
Worm Species: Trichostrongylus Ostertagia-Haemonchus
L3 genotype: SS RS RR SS RS RR
Treatment type
Low-efficacy oral 99 50 0 99 0 0
Short-acting oral 99 50 0 99 0 0
3.2. Management Simulated: Frequency and timing of broad-spectrum treatment used in the two worm
control programs were Wormkill [4] for the summer rainfall environment (Armidale, NSW, Australia) and
“summer drenching” for a winter rainfall environment (Kybybolite, South Australia) which experiences a
hot dry summer. In both environments lambs are treated at weaning and moved to a safe pasture.
Trichostrongylus and Ostertagia populations were simulated in both environments. Haemonchus was not
simulated at Kybybolite as it is not a problem species in this area. At Kybybolite a weaning drench
(12/10) and move to safe pastures combined with a single summer treatment (17/11) was sufficient for
good parasite control, ewes also received a treatment on 17/11. Under Wormkill lambs were drenched on
22/12, 22/2 and 1/5 while ewes only received one broad-spectrum drench on the 22/12.
3.2.1. Haemonchus control: For Haemonchus the simulations included treatments with Closantel (CLS) as
recommended by the Wormkill program (2/11, 22/12 and 22/2 for both ewes and lambs). Two levels were
chosen for CLS efficacy as resistance to CLS occurs in this region. CLS efficacy was set at 80% or 99%
for Haemonchus simulations, however, the evolution of CLS resistance was not modelled in these
simulations, that is, CLS efficacy remained at these levels throughout the simulations. CLS was assumed
to have persistent activity against incoming larvae for 30 days. In the absence of CLS resistance, CLS
treatment excluded 99%, 90% and 60% of L3 for days 1-10, 11-20 and 21-30 respectively after treatment.
If CLS resistance was assumed, CLS treatment excluded 80%, 70% and 50% of L3 for days 1-10, 11-20
and 21-30, respectively, after treatment.
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