Prevalence and intensity of gastrointestinal nematodes in slaughter lambs from central Alberta

Abstract

Two trichostrongyles, Teladorsagia ostertagi and Nematodirus helvetianus, accounted for > 99% of nematodes recovered from gastrointestinal tracts of 47 lambs pastured in central Alberta during the summer of 2000. Their prevalence and mean intensity increased from < 10% and < 50 worms/host, in late June, to > 80% and approximately 1000 worms/host, by mid-July, respectively.

In sheep, as in other ruminants, the epidemiology of nematode gastrointestinal parasitism is heavily influenced by climatic conditions (1). A limited amount of recent information is available on the epidemiology of these nematodes in sheep reared in the cold, dry climates of northwestern North America (1). The current study was carried out to examine the changes in prevalence and intensity of gastrointestinal nematodes in lambs reared in central Alberta during the early part of the pasture season.

Forty-seven gastrointestinal tracts of lambs were collected and frozen immediately, approximately biweekly from May 15 to July 31, 2000, from an abattoir near Innisfail in central Alberta. The lambs were raised on 3 ranches within 60 km of the abattoir; they had not been treated with anthelmintics prior to slaughter.

Standard protocols were used to evaluate nematode prevalence (proportion of infected hosts in a sampling) and intensity (numbers of worms per infected host) in samples of lambs (2). Nematode identification was based primarily upon the structure of male spicules (3). Each tract was separated into “stomach” (rumen, reticulum, omasum, abomasum), small intestine, cecum, and large intestine. Total counts were made for stomach and cecal nematodes. To quantify nematodes present in the small intestine, each tract was divided into 10 sections so that each section represented 10% of the total length. Washings from each section were placed into a 1000 mL graduated cylinder. Nematode counts were estimated as the average number of worms present in 2, 100 mL aliquots multiplied by the dilution factor.

Fecal egg counts were determined by using a standard centrifugation-floatation technique. Three, 1 g samples of feces were removed from the large intestine of each host. Each sample was mixed into a cup containing 15 mL of water. The solution was strained through a tea strainer and placed into a 15 mL centrifuge tube, which was then centrifuged at 1500 rpm for 3 min. The supernatant was discarded and the pellet was resuspended in a saturated sugar solution (specific gravity = 1.28). Additional sucrose solution was added to achieve a positive meniscus on each tube, whereupon a 22 mm² glass cover slip was placed on top of the tube, which was centrifuged at 1500 rpm for 3 min. The coverslip was then removed and placed on a slide for quantification of eggs.

Nonparametric Kruskal-Wallis (for n > 2 samples) and Mann-Whitney (for n = 2 samples) tests were used to compare median parasite intensities among sampling periods. Spearman's correlations were used to test for associations between parasite intensity and egg counts.

Teladorsagia ostertagi and Nematodirus helvetianus had the highest overall prevalences, and their peak mean intensities were 2 to 3 orders of magnitude greater than those for Trichuris ovis (Table 1). The prevalences and mean intensities for these 2 species were low within May and June samples. In 8 of the 9 each species in each month combinations, prevalence never exceeded 20% and intensities were < 2 worms host. Thereafter, prevalence and intensity increased markedly. For T. ostertagi and N. helvetianus, there was a significant increase in median intensity between the end of June and the end July (H = 9.0, P = 0.011; H = 6.6, P = 0.036, respectively). Median T. ostertagi intensity was significantly correlated with median intensity of N. helvetianus (n = 19, Spearman's Rho = 0.70, P = 0.0028) indicating that hosts heavily infected with one species tended to be heavily infected with the other.

Table 1.

Eggs were found in 16 out of 19 lambs in the 2 samples collected during July. The frequency distribution of egg production was bimodal, with samples from 11 hosts containing less than 100 eggs/gram (epg) and 5 containing between 200 and 400 epg. In general, lambs containing large numbers of worms tended to contain large numbers of eggs in their feces (n = 16, Spearman's Rho = 0.81, P = 0.0002). This correlation remained significant even when the 5 hosts from the right-hand side of the frequency distribution of egg counts were excluded from analysis.

Prevalence and intensity of nematodes, as well as fecal egg counts, increased as the summer progressed. These data indicated that lambs reared on pasture in central Alberta were acquiring nematode infections in late June, with the eggs being detectable in feces approximately 2 wk later. This suggested that pasture contamination by infected ewes is the source of infection for lambs. Alternatively, the possibility that over-wintered larvae were the source for these infections could not be discounted. Tracer lamb studies or herbage sampling throughout the pasture season could be used to distinguish these alternatives.

Lambs collected from pastures in Nebraska harbored similar species of nematodes with intensities that were generally higher in July than in January (4). In addition, lambs and adult sheep from pasture in Saskatchewan were found to have nematode species similar to those of the current study (5). The samples from Saskatchewan tended to have a higher number of species, but it is unclear from the data whether the greater diversity was in adult sheep or lambs.

More recent epidemiological studies on nematode parasitism of sheep and lambs from similar climatic regions show similar patterns of infection to those found in this study. Lambs from dry regions of Greece had fecal egg counts that increased rapidly from early June and peaked in early August (6). Similarly, in India, fecal egg counts rose in July and peaked in September (7). In the latter study, infected ewes were considered the source of infection for lambs.

The significant correlation between nematode intensity and fecal egg count indicates that heavily infected lambs tend also to release large numbers of eggs in their feces. This result is surprising given the high variation in fecal egg counts and our low sample sizes. Numerous studies have shown a poor correlation between fecal egg counts and nematode intensity, particularly in cattle (8). However, our results provide preliminary evidence that estimation of fecal egg counts within similar-aged lambs sampled in mid- to late-summer can be used to identify heavily infected lambs for targeted therapy.