Regulation of Teladorsagia circumcincta and Trichostrongylus colubriformis worm populations by grazing sheep with differing resistance status

Abstract

-In an experiment lasting 4 years, changes in the Teladorsagia circumcincta andTrichostrongylus colubriformis populations were compared in lambs and adult sheep with differing resistance statuses. Two flocks of 30 rams (resistant R and susceptible S) grazed separate pastures and 8 rams were slaughtered in the middle and at the end of each grazing season. Five groups of tracer lambs were added each year to estimate the pasture infectivity and were killed for worm counts. The availability of animals with differing resistance statuses (rams and tracer lambs) and differing levels of infection made it possible to investigate the number, size and fecundity of worms of these two species. The inflammatory response was measured in the rams by counting the globule leukocytes, mast cells and eosinophils in the fundic, pyloric and intestinal mucosa. In the tracer lambs, the daily egg production by the female worms of both species was negatively correlated with the worm burden. Worm length accounted for 60 and 70% of the variation in the number of eggs in utero for T. circumcincta and T. colubriformis respectively. Worm length was closely associated with the resistance status of the host; there were greater differences between lambs, and S and R rams for T. colubriformis. T. circumcincta worm lengths were not affected by the worm number. Globule leukocyte counts were related to the worm burdens, and mast cell counts to worm length in the R and S rams. The number, size and fecundity of the worms may well be regulated by similar mechanisms in both species, but T. colubriformis seemed to be more intensively regulated than T. circumcincta. This finding could be useful in devising more effective methods of parasite control.sheep-nematoda / Teladorsagia circumcincta / Trichostrongylus colubriformis / population dynamics / resistance

Comparative morphological and molecular identification of Haemonchus species in sheep

“…There is a significant association between increased IgA activity against larvae and reduced Hc female length (Strain & Stear, 2001;Saddiqi et al, 2011). Moreover, host breed (Aumont et al, 2003), host gender (Gruner et al, 2004), diet (Hoste et al, 2008) and concurrent parasitic infection (Reinecke et al, 1982;Terefe et al, 2005) can affect nematode length. Morphological features that detect the length of a specific character used in species identification may be affected as well.…”

Markers/parameters for the evaluation of natural resistance status of small ruminants against gastrointestinal nematodes

“…circumcincta developed in two stages: first, in association with the development of a specific local IgA response that resulted in the reduction of average egg production per worm; and second, in association with the production of globule leucocytes in the abomasal mucosa that resulted in the control of number of worms. In several studies, the number of globule leucocytes was negatively correlated with the number of worms, suggesting that this cell type has a specific role in the resistance of the host (Douch et al, 1986;Stear et al, 1995;Balic et al, 2000b;Gruner et al, 2004). Significant differences in the globule leucocyte counts were also reported in St. Croix (resistant) and Dorset (susceptible) sheep (Gamble and Zajac, 1992) and Crioula Lanada sheep (resistant) and in Corriedale (susceptible) lambs (Bricarello et al, 2004).…”

“…It has been reported that mast cell counts in both the abomasal and the intestinal mucosa have an inverse relation to the length of female worms in both resistant and susceptible rams (Gruner et al, 2004) with Tel. circumcincta, and a positive correlation between mast cells and worm fecundity has also been reported.…”

The immunology and genetics of resistance of sheep to Teladorsagia circumcincta

“…Evaluation of estimated breeding values for FAMACHA scores indicate its heritability (Riley and Van Wyk, 2009) and has now been used as a phenotypic marker for selection in Brazil (Molento et al, 2009). However, FEC has been the most widely-used parameter in identifying sheep nematode resistance (Smith et al, 1984;Gill, 1991;Gruner et al, 2004;Davies et al, 2005). Heritability values vary from 0.30 -0.48 in T. circumcincta (Stear et al, 2009), T. colubriformis (Sreter et al, 1994;Douch et al, 1996) and H. contortus (Gruner et al, 2004) making it a viable indicator trait for selection.…”

“…However, FEC has been the most widely-used parameter in identifying sheep nematode resistance (Smith et al, 1984;Gill, 1991;Gruner et al, 2004;Davies et al, 2005). Heritability values vary from 0.30 -0.48 in T. circumcincta (Stear et al, 2009), T. colubriformis (Sreter et al, 1994;Douch et al, 1996) and H. contortus (Gruner et al, 2004) making it a viable indicator trait for selection. The genetic component of FEC is further evidenced by its association with three different genotypes at the diallelic adenosine deaminase locus (Gulland et al, 1993) and major histocompatibility complex (MHC)-linked microsatellites in a wild population of Soay sheep (Beraldi et al, 2007).…”