A. Vlassoff

Genetic and phenotypic relationships among anti-Trichostrongylus colubriformis antibody level, faecal egg count and body weight traits in grazing Romney sheep

Serum levels of an antibody (Ab) and immunoglobulin G, (IgG,) to the larval (L3) stage of the internal parasite, Trichostrongylus colubrifomis, were determined in 3 years in Romney lambs. Animals from two pairs of selection flocks, bred for increased or reduced resistance to internal parasites, were compared for Ab or IgG, levels, and a progeny test flock was recorded to provide data for genetic parameters (n = 64 sires). Lambs were exposed to a natural parasite challenge on pasture, following an anthelmintic drench treatment at weaning. Blood and faecal samples from 4- to 6-month-old lambs (January samples) were taken when the mean FEZ of a monitor group reached 800 to 1500 eggs/g. Lambs were again drenched with anthelmintic and a further cycle of the protocol was carried out (March samples). Selection flock differences in March (High flock minus Low flock means) averaged - 0.73, - 0.93 and 1.27 phenotypic standard deviations for log,Ab, log, IgG, and log, (FEC + 100) at one site, and -0.89, - 0.94 and 1.89 phenotypic standard deviations at the other. Repeatabilities and he&abilities of log,Ab, log, IgG,, and log, (FEC + 100) were estimated. For data collected in March, the genetic correlations between log,Ab or log IgG, and log, (FEC + 100) were - 0.56 f 0.18 and - 0.35 f. 0.19 respectively, whilst they were -0.48 f 0.17 and -0.47 + 0.15 between log,Ab or log, IgG, and the average of log, (FEC + 100) in January, March and May. The results suggest that ram selection based on serum antibody levels in March would result in 51-67s of the genetic gain in reducing FEC that could be achieved by using FEC directly.

Responses of Romney sheep to selection for resistance or susceptibility to nematode infection

Divergent breeding lines of Romney sheep, selected as lambs for consistently low or high faecal worm egg count (FEC) following natural multispecific challenge by nematode parasites, have been maintained at Wallaceville Animal Research Centre since 1979. From the start of the trial until 1992, 821 and 736 experimental lambs in lines selected for increased resistance (R) and increased susceptibility (S), respectively, were generated, with the use of 44 different sires. In order to assess genetic responses to selection, FEC and productivity data were analysed using restricted maximum likelihood procedures. By 1988 the two lines had diverged in average loge (FEC+100) by 0·67 log units, and by 1992 divergence had increased to 1·48 log units, representing 2·90 genetic standard deviations of divergence. No significant correlated responses were observed in live weights. However, by 1992 the mean score for breech soiling (dags) in the R line was proportionally 0·41 greater than in the S line (F v. 0·92, respectively). It is concluded that selective breeding can change the degree of resistance or susceptibility to nematode infection in Romney sheep, and selection for productivity needs to be continued at the same time in any practical breeding programme.

Epidemiology of nematodosis in Romney lambs selectively bred for resistance or susceptibility to nematode infection

Field trials were undertaken to compare nematode population dynamics, lamb productivity and levels of breech soiling in experimental flocks of Romney lambs selectively bred for increased resistance or susceptibility to nematode infection. In each year of the 2 year study, spring-born ewe lambs derived from Wallaceville Animal Research Centres divergent nematode-resistant and nematode-susceptible breeding lines were grazed as separate flocks on matched farmlets from weaning (at 3 months old) until they were approximately 10-11 months old. Allocation of farmlets was reversed between Years 1 and 2 of the study to account for any possible paddock-related effects. Within each year both flocks were subjected to identical management conditions, including anthelmintic treatment (which was administered only when the overall mean faecal worm egg count measured across both genotypes reached 1500 eggs g-1). In both years, by mid-autumn (April) nematode larval infestation levels on pasture were approximately 5-6-fold greater on the farmlet grazed by susceptible (S) genotype lambs than on that grazed by their resistant (R) counterparts (Year 1: 2506 cf. 544 larvae kg-1 herbage; Year 2: 431 cf. 74 larvae kg-1 herbage). This led to 51-fold and 56-fold differences in faecal egg count between R and S lambs by late autumn (May) and winter (July) in Years 1 and 2, respectively. Although mean growth rates were similar in the R and S lambs over summer (while pasture infestation levels on the farmlets were still in the process of diverging), significantly higher growth rates occurred in the R than in the S lambs over autumn-winter in both years of the study (P < 0.01). In contrast, no significant differences in growth rate occurred in either year between male lambs derived from the nematode-resistant and nematode-susceptible breeding lines which were grazed together on another area of the Wallaceville farm from weaning until late autumn. Despite the substantially lower pasture infestation levels encountered by the R ewe lambs, they nevertheless temporarily suffered more breech soiling (dags) than their S counterparts (P < 0.01) in both years. Yearling fleece-weights of the R and S genotypes did not differ significantly in either year. Although the results of our study confirmed that there are potentially significant epidemiological benefits to be derived from breeding sheep for resistance to nematode infection, these benefits did not appear to be associated with large advantages in animal performance. Further work is needed to establish how these results should be interpreted with respect to anthelmintic drench requirements of genetically resistant animals.

Genetic and phenotypic relationships among Trichostrongylus colubriformis-specific immunoglobulin E, anti-Trichostrongylus colubriformis antibody, immunoglobulin G1, faecal egg count and body weight traits in grazing Romney lambs

Variation in total and Trichostrongylus colubriformis-specific immunoglobulin E (IgE) was investigated in four- and six-month old Romney lambs reared on pasture, where they were exposed to natural challenge with nematode parasites. The lambs, which were from an experimental progeny test flock (n=64 sires), were born in spring over three years and weaned at an average age of three months (December). Data from the flock were analysed to obtain heritability estimates for IgE and genetic correlations between IgE and other immunological, parasitological and production traits. In addition, correlated responses of IgE to long-term genetic selection for high, control or low faecal nematode egg count (FEC) were investigated in a set of experimental breeding lines. Repeatabilities and heritabilities of IgE traits in the progeny-test flock were similar to those calculated previously for anti-T. colubriformis antibody (Ab), immunoglobulin G1 (IgG1) and FEC (generally >0.30). Genetic correlations of the loge transformed IgE traits (January and March samples) with loge (FEC+100) on three sampling occasions were all negative (−0.22 to −0.37). However, positive correlations (0.17 and 0.43) were found between loge IgE and dag score, indicating a tendency for there to be more severe breech soiling in lambs with elevated serum IgE. In the 1995 and 1996 selection-line lamb crops (after 17 and 18 years of selective breeding for high or low FEC), both total and T. colubriformis-specific IgE levels were higher (by between 59 and 103%) in the low than in the high line (P<0.001). IgE levels in the controls were intermediate between those of the high and low lines but closer to those of the low line. The results are in line with other evidence which suggests that greater genetic resistance to gastrointestinal nematode infection in Romney lambs is associated with an increase in activity of the TH2 arm of the immune system which mediates inflammatory responses against multicellular parasites.

FAECAL NEMATODE EGG COUNTS IN LACTATING EWES FROM ROMNEY FLOCKS SELECTIVELY BRED FOR DIVERGENCE IN LAMB FAECAL EGG COUNT

Faecal nematode egg counts (FECs) were examined in lactating ewes from divergent flocks of Romney sheep which had been selectively bred from 1979 to 1996 for or against FEC in 4 to 7 month old lambs. Faecal samples were obtained from the ewes while under normal grazing management, 1 to 2 months after lambing in spring for each of 6 years between 1987 and 1996 (no. = 785 records; 298 animals). Analyses were carried out on log e (FEC + 100)–transformed data using animal-model maximum likelihood procedures, accounting for repeated records on ewes, within and between lactations. An examination of non-genetic effects indicated that there was no significant effect of age class of ewe on FEC but ewes which gave birth to single lambs had significantly lower post-parturient FECs than those bearing twins (back-transformed means of 184 v. 276 egg per g, respectively; P P e (FEC + 100) in their lambs in the same years. Heritability and repeatability estimates for log e (FEC + 100) in ewes were 0·37 (s.e. 0·06) and 0·46 (s.e. 0·03) respectively. Genetic correlation estimates between a ewes post-parturient log e (FEC + 200) and her log e (FEC + 100) as a lamb, based on analysis of (co)variance or realized responses, were 0·70 or 0·58 respectively. The phenotypic correlation between a ewes log e (FEC + 100) and that of her lamb(s) in the same lactation was 0·29 (s.e. 0·06) ( P

Distinguishing morphological features of the third larval stage of ovine Trichostrongylus spp.

In this paper we describe a procedure that enables the identification of species of infective third stage (L(3)) Trichostrongylus larvae. Lambs were infected with putatively monospecific infections of three species of Trichostrongylus commonly found in New Zealand (T. axei, T. colubriformis and T. vitrinus) and Teladorsagia (Ostertagia) circumcincta. After recovering L(3) from faecal cultures, the lambs were slaughtered and adult male worms recovered and examined for spicule morphology to verify identification. L(3) were examined for morphological features and measurement of their length. Further L(3) were exsheathed and examined under high power optics to observe posterior morphological features (tubercles). The posterior of T. colubriformis has a three-tubercle structure whereas T. vitrinus has a single tubercle and T. axei none. However, the tails of T. circumcincta also lack tubercles and thus T. axei cannot be readily distinguished from them on this feature. The range of lengths of L(3) of Trichostrongylus spp. (600-858 microm) and T. circumcincta (700-914 microm) were found to overlap considerably. The shape of the anterior end of these two species differs and this in combination with length provides an indication of the proportion of T. axei and T. circumcincta in a culture. A combination of tubercle number, with overall length and anterior morphology of L(3), can be used to differentiate nematode populations of T. axei, T. colubriformis, T. vitrinus and T. circumcincta.

Genetic parameters for Nematodirus spp. egg counts in Romney lambs in New Zealand

Genetic analyses were carried out on Nematodirus spp. egg counts (NEM) of lambs from a set of Romney breeding lines. The lines had been under selection for 22 seasons (1979 to 2000) for divergence in resistance to infection by strongyle nematodes other than NEM, using faecal egg count (FEC) as the selection criterion. Heritabilities and genetic correlations for NEM were estimated using residual maximum likelihood procedures. Correlated responses in NEM were also determined. Heritability estimates for log e (NEM + 100) in lambs at 4 months of age (NEM1) or 6 months of age (NEM2) were 0·15 (s.e. 0·03) and 0·26 (s.e. 0·04) respectively (c.f. 0·28 (s.e. 0·02) and 0·35 (s.e. 0·02) for log e (FEC + 100)). The genetic correlation between log e (NEM1 + 100) and log e (NEM2 + 100) was 0·85 (s.e. 0·08), while the genetic correlations between measurements of log e (NEM + 100) and log e (FEC + 100) on both sampling occasions had a weighted average of 0·43, with estimates ranging from 0·30 (s.e. 0·08) to 0·52 (s.e. 0·07). Divergence in log e (NEM + 100) between the high and low FEC lines, estimated over both sampling times combined, was 1·07 phenotypic standard deviations, compared with 3·6 phenotypic standard deviations for log e (FEC + 100). Expressed in terms of back-transformed eggs per g, the high and low FEC lines differed by factors of 7·6 and 32·2 for NEM and FEC, respectively. The results support earlier parasitological data indicating that the genetic mechanisms in sheep which are responsible for resistance to other strongyle nematodes probably also influence resistance to Nematodirus infection.