Antibody responses of sheep to challenge with Trichostrongylus colubriformis and the effect of dexamethasone treatment.

Eight-month-old random bred Romney wether lambs were reared nematode-free in pens and assigned to 4 groups each of 5 lambs. Lambs in 3 groups were infected orally, twice a week, with 5000 Trichostrongylus colubriformis infective larvae (L3), a control group remained unifected throughout. Two infected groups were treated with dexamethasone (0.5 mg kg-1 bodyweight), one between days -7 and 77, the other between days 77-154. Nematode challenge infection was withheld from the third group from day 133 after anthelmintic treatment. Nematode eggs in sheep faeces (FEC) were monitored at weekly intervals. T. colubriformis-specific antibody levels were determined twice a week and specific immunoglobulin isotypes (IgA, IgG1, IgG2 and IgM) determined weekly in serum samples using ELISA. Resistance, as measured by FEC, was expressed by 35 days after L3 infection began but sheep dosed with dexamethasone did not develop resistance. Extant resistance was abrogated in sheep dosed with dexamethasone. Nematode challenge resulted in elevated serum levels of antibodies to T. colubriformis L3 excretory/secretory antigens, these consisted predominantly of IgG1 and IgM. The IgG1 response was more persistent than the IgM response. Specific serum IgA and IgG2 responses were low, but significant, in nematode-challenge sheep. Dexamethasone treatment prevented the antibody responses and resulted in a rapid reduction of extant antibody levels in resistant sheep. Weight gain was reduced by nematode challenge with or without dexamethasone treatment compared with control sheep.

Serum mast cell proteinase responses of sheep to challenge with Trichostrongylus colubriformis and the effect of dexamethasone treatment.

Eight-month-old random bred Romney wethered lambs were reared nematode-free in pens and assigned to 4 groups of 5 lambs. Lambs in 2 groups were dosed orally, twice a week, with 5000 Trichostrongylus colubriformis infective larvae (L3) for the duration of the experiment. These 2 groups were treated weekly with dexamethasone (0.5 mg kg(-1) body-weight), one between days -7 and 70, the other between days 77 and 147. A third group was dosed with L3 until anthelmintic treatment on day 133. A fourth group remained uninfected throughout and served as a control group. Nematode eggs in sheep faeces (FEC) were monitored at weekly intervals. Serum samples were taken twice a week and assayed for sheep mast cell proteinase (SMCP). Serum levels of SMCP in uninfected control sheep were 459 +/- 190 pg ml(-1). Twenty-eight days after nematode dosing commenced, SMCP levels were significantly above control sheep levels and after 49 days reached a plateau level of 1154 +/- 364 pg ml(-1). The SMCP response persisted even after cessation of dosing, and SMCP levels remained significantly above control levels to the end of the experiment (day 213). Dexamethasone treatment prevented elevation of SMCP and resulted in a rapid reduction of extent SMCP levels in resistant sheep. Overall, serum levels of SMCP were significantly correlated (P<0.001) with specific anti-T. colubriformis L3 antibody in serum (r = 0.601, d.f. = 78), blood eosinophils (r = 0.609, d.f. = 78) and log(FEC+15) (r = -0.521, d.f. = 78). These results show that serum levels of SMCP correlate with other indicators of parasitism and may have potential use as a non-invasive indicator of gastrointestinal mast cell responses to nematode infection.

Association of blood eosinophilia with the expression of resistance in Romney lambs to nematodes.

The dynamics of blood eosinophilia was studied in Romney lambs dosed twice weekly with 5000 infective larvae of Trichostrongylus colubriformis. A marked rise in blood eosinophil counts of the dosed lambs coincided with the start of the decline of faecal egg counts (FEC). Eosinophilia was also studied in grazing Romney lambs from a breeding programme, based on selection for high or low FEC. Comparison of the sire groups suggested that the magnitude of the eosinophil response was under genetic control. The significant negative correlations between blood eosinophil counts and FECs confirmed the association between eosinophilia and the expression of resistance in Romney lambs to nematodes. However, comparison of the mean blood eosinophil counts and FECs for both the ram and ewe lambs of the resistance sire groups suggested that the association of eosinophilia and resistance may be greater for some sire groups than others.