Caecal threadworms Trichostrongylus tenuis in red grouse Lagopus lagopus scoticus: effects of weather and host density upon estimated worm burdens

Abstract

Trichostrongylus tenuis eggs were counted in faeces from individually marked wild red grouse for 8 years. Egg counts varied seasonally and annually. In some years, a sudden increase in mid-April was consistent with delayed maturation of larvae which had overwintered in the birds in a hypobiotic state. A more gradual increase in summer was probably due to uninterrupted maturation of larvae ingested then. Despite 30-fold year-to-year variation in mean egg counts, relative differences in egg counts among known individuals within years tended to persist across years. Rainfall in previous summers explained much of the year-to-year variation in egg counts, probably because parasite recruitment was greatest during wet summers. Grouse density was only weakly related to worm egg counts. The data were not consistent with the hypothesis that the cyclic-type population fluctuation in red grouse numbers observed at the time of this study was caused by the parasites.

Experimental Prevention of a Population Cycle in Red Grouse

“…We estimated burdens of the cecal threadworm Trichostrongylus tenuis in late April and May from egg counts in cecal droppings from known birds on C and E. Birds were either trapped or located at night by radio-telemetry and their droppings were collected the next morning (Moss et al 1993b). …”

“…In documenting a cyclic fluctuation at Rickarton moor near Kerloch, our aims were: (1) to test the expectation, based on Rickarton bag records, that a cyclic population fluctuation would occur; (2) to test detailed numerical predictions from a model developed from the earlier Kerloch study; (3) to document the demographic causes of population change during a population cycle; (4) to test the prediction that a cyclic decline could be prevented or lessened by removing territorial cocks from part of the moor in spring so that the population would not reach peak density; (5) to test the prediction that removal of cocks from the manipulated population would result in the loss of a similar number of hens; (6) to compare demographic causes of population change in control (anticipated cycling) and experimental (anticipated broken cycle) populations; (7) to compare other differences, such as comb size (indicating androgen status), in control and experimental populations; and (8) to test other hypotheses involving food ) and parasites (Moss et al 1993b …”

Food Intake, Weight Changes and Egg Production in Captive Red Grouse before and during Laying: Effects of the Parasitic Nematode Trichostrongylus tenuis

“…[501] Moss et al 1993, Delahay 1995. Developing larvae are more pathogenic than adult worms Moss 1990, Delahay et al 1995) and the synchronous resumed development of larvae arrested over winter may explain why outbreaks oftrichostrongylosis in wild grouse occur in spring and early summer (Committee of Inquiry 1911, Macintyre 1918, Jenkins et al 1963, Hudson et al 1992).…”

“…Moss et al (1993) recorded large increases in the number of worm eggs in the caecal faeces of wild birds in north-east Scotland in mid-April in some years and attributed these to the maturation of previously arrested larvae. Wild grouse in north-east Scotland generally begin laying in mid to late April and so larvae previously arrested over winter seem to develop at or shortly before the onset of laying.…”

“…Counts of worm eggs per gram (epg) of grouse caecal faeces reflect numbers of adult T. tenuis present in the hosts(Shaw 1988b, Moss et al 1993, Delahay 1995. There was no significant difference in worm egg counts (mean of six weekly counts for each bird) between treatment groups A and D (t,, = -0.68, P > 0.05; seeFig.…”