Table 2.

Summary of the scenarios tested

Scenario	Number of worm generations or yearsa	sh (%)	sp (%)	p0	x	Model typeb	Mechanism to reduce sheep WECc
Evaluation of the genotype-by-genotype interaction
1	80 gen	20	0	0.001, 0.01,	1.0	reduced	-
				0.1, 0.3
2	80 gen	0 to 30	0 to 30	0.3	0.2, 0.5, 0.8	reduced	-
Evaluation of the host-parasite interaction properties
3	1 yr	-	-	0	0	full	-
4	1 yr	25	10	0.5d	0.5	full	E, M, F
Testing hypotheses 1 and 2
5	20 yr	25	10	0.3	0.0 to 1.0	reduced, full	E, M, F

^a the reduced model is defined by worm generations and the full model is defined by years of simulation; results show that there are 2.2 worm generations per year of simulation; s_h = survival advantage of the B allele in sheep; s_p = survival trade-off on pasture for the B allele; p₀ = initial frequency of the B allele in worms; x = frequency of the low WEC A allele in sheep; ^b see text for details of the full model, which includes complexities of the host-parasite relationship, and the reduced model, which explicitly excludes these factors; ^c postulated mechanisms to reduce WEC in sheep through the ‘resistance’ A allele: E = reduced worm establishment, M = increased worm mortality, and F = reduced female worm fecundity; ^d to evaluate the effect of the low WEC allele for each mechanism of the sheep ‘resistance’ A allele, the worm allele frequency was fixed (p = 0.5) during the transition stages of the worm lifecycle.