Effect of gastro-intestinal nematode infection on sheep performance: a systematic review and meta-analysis

Abstract

Background: Gastrointestinal nematode (GIN) infections are common in domestic sheep and impact directly and indirectly on the health of infected animals as well as on the associated economic production. In this study, we aim at summarizing the current knowledge on the influence of GIN infections on sheep production by conducting a systematic review. A subsequent meta-analysis of relevant studies was performed to provide an estimate of the effect of GIN infections on weight gain, wool production and milk yield.

Methods: A literature search was performed on the CAB, Pubmed and Web of Science database for the period 1960-2012. Inclusion criteria were: 1) Measurement of at least one production parameter. 2) Comparison between groups of sheep with different nematode burdens. 3) Same conditions regarding all aspects except parasite burden between groups. 4) Quantitative measurements of one or more production traits.

Results: Altogether, 88 studies describing 218 trials were included in this review. The majority of studies (86%) reported that GIN infections had a negative effect on production but this was reported to be statistically significant in only 43% of the studies. Meta-analysis indicated that performances of sheep infected with nematodes was 85, 90 and 78% of the performance in uninfected individuals for weight gain, wool production and milk yield respectively. Our results suggest a possible reporting bias or small study effect for the estimation of the impact of GIN infections on weight gain. Finally, a general linear model provided an estimate for the decrease in weight gain in relation to the increase in faecal egg count of nematodes.

Conclusion: This study underlines the importance of GIN infections for sheep production and highlights the need to improve parasite management in sheep, in particular in face of challenges such as anthelmintic resistance.

Fig. 1

Forest plots of 94 trials included in the meta-analysis of impact of gastro-intestinal nematode infection on weight gain (a, n = 70), wool production (b, n = 19) and milk yield (c, n = 5) in sheep. Black dots represent the log-transformed ratio of performance of the infected over the control group in each trial. Dot sizes are proportional to the sample sizes in the trial and horizontal bars give the standard error of the estimate. Vertical dotted lines indicate the zero (no effect of nematode infection on production) and vertical continuous lines show the overall estimate for all the trials in each performance trait

Fig. 2

Funnel plots with 95 % pseudo-confidence limits of 94 trials included in the meta-analysis of impact of nematodes on weight gain (a, n = 70), wool production (b, n = 19) and milk yield (c, n = 5) in sheep. Treatment effect (log-transformed ratio of performance of infected over control animals) is given on the X-axis and standard error of the estimate is represented on the Y-axis

Fig. 3

Decrease in weight gain of sheep by increasing infection level with mixed species of gastrointestinal nematodes. Mean difference in faecal egg counts between low parasite burden animals (LPAR) and high parasite burden animals (HPAR) is used as an indicator of level of infection and shown on the X-axis. Y-axis shows the ratio of weight gain of HPAR over LPAR. The continuous line shows the estimated effect of nematode infection with a 95 % confidence interval (dotted lines) computed with a Generalized Linear Model using the results of 73 trials (black dots with sizes proportional to sample size of the trials)