The epidemiology and pathological effects of Trichostrongylus tenuis (Nematoda) in red grouse (Lagopus lagopus Scoticus)
The intensity and prevalence of infection with T. tenuis adults and larval stages is described for eight populations of red grouse in northern England and Scotland. During winter most larvae were present as arrested exsheathed third stage larvae. Arrested larval burdens were positively correlated with adult worm counts but the predictive value of this relationship was poor. Numbers of arrested larvae varied widely amongst locations and were weakly correlated with grouse density and maximum autumn temperatures. Exposure of infective larvae to low temperatures for 4-12 weeks before infection of captive grouse failed to induce substantial numbers to arrest. The highest numbers of inhibited larvae were observed in grouse given chilled L3 from a worm population known to exhibit high levels of arrest in wild grouse. Larval stages not usually associated with arrested development also occurred well after the usual prepatent period. Developing larvae and adult worms present at the onset of laying in hen grouse did not significantly reduce egg production. However, developing worms caused hen grouse to lose significantly more body weight during laying. Hen grouse may have retained the ability to lay despite infection, at the cost of investment in reserves for incubation. Developing larvae caused a severe energy imbalance in captive grouse at 12-16 dpi as late 4th-stage larvae developed into adult worms. At 12 dpi infected birds had significantly elevated resting metabolic rates and at 16 dpi voluntary food intake was significantly depressed relative to controls. After this, effects on energy balance diminished but there followed a gradual reduction in grouse body weight and condition. The resumed development of arrested larvae in wild grouse in spring presumably has similar effects and could reduce their fecundity and survival.