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Worms vs. coccidia

Nematodes (worms) and coccidia (Eimeria spp.) are quite different in their morphology and in the way they interact with their hosts. Worms are extracellular parasites and are considered to be controlled by Th2 immune responses while coccidia infect the host at an intracellular level and generally elicit the development of a Th1 immune response.

Flickr image used with permission of Margaret Sweeney

Scottish researchers conducted a study to estimate the genetic parameters of parasite infection indicator traits in Scottish Blackface sheep with natural worm and coccidia infection. Data on fecal egg counts (FEC) from different species of strongyle parasites and fecal oocyst counts (FOC) from coccidian parasites were collected on 3-month-old lambs. Other data recorded included live weight and fecal soiling score (dagginess; dg score). Data were obtained from 3,731 lambs sampled between 2011 and 2017.

Coccidia were the most prevalent parasite (99.5%), while strongyles and Nematodirus had a prevalence of 95.4% and 72.7%, respectively. Heritability estimates were 0.16, 0.17, 0.09 , 0.09, 0.03, and 0.33 for FEC (strongyles), FEC (Nematodirus), FOC, dag score, and live weight, respectively. Significant positive genetic correlations between FEC (strongyles) FEC (Nematodirus), and FOC at 3 months of age showed that co-selection of sheep for resistance to these different parasites is feasible.

Furthermore, no significant antagonistic genetic correlations were found between live weight and FEC, so selection for increased resistance to parasite infection is not expected to adversely affect weight gain. However, there were significant antagonistic phenotypic and genetic relationships between dag score and live weight, indicating that clinical signs of disease may be a more meaningful indicator of the impact of parasite burden on productivity.

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