PHENOTYPIC MARKERS FOR SELECTION OF NEMATODE-RESISTANT SHEEP

Citation
Pgc. Douch et al., PHENOTYPIC MARKERS FOR SELECTION OF NEMATODE-RESISTANT SHEEP, International journal for parasitology, 26(8-9), 1996, pp. 899-911
Citations number
44
Categorie Soggetti
Parasitiology
ISSN journal
00207519
Volume
26
Issue
8-9
Year of publication
1996
Pages
899 - 911
Database
ISI
SICI code
0020-7519(1996)26:8-9<899:PMFSON>2.0.ZU;2-O
Abstract
Nematode-resistance of an animal can be defined as an enhanced natural ability, relative to its peers, to both prevent establishment of larv al nematodes and evict any that do establish. These parameters are not measurable in a practical sense and consequently nematode-resistance has usually been defined in terms of low faecal nematode egg counts (F EC). Studies in New Zealand and Australia have demonstrated that nemat ode-resistance, as measured by FEC, has a heritability of about 0.3 in Romney and Merino sheep. However as a selection trait FEC has practic al Limitations and its use may incur production penalties through with holding drench treatment for prolonged periods or from a need for arti ficial challenge. FEC is influenced by the level and composition of a natural nematode challenge and especially the expression of the immune response. Thus immunological parameters which reflect the underlying genetic resistance could potentially be used as phenotypic markers. Id eally, a useful phenotypic marker would be easy to sample and its assa y would be inexpensive and able to be automated, in addition to being strongly correlated with nematode-resistance. Results from several New Zealand trials have indicated that antibody levels (particularly IgG, ) to excretory/secretory antigens of L(3) nematodes such as Trichostro ngylus colubriformis may meet these criteria. Levels of antibody again st L(3) antigens are also independent of on-farm drenching strategies. Blood eosinophil count has also been considered for use as a selectio n parameter but a high degree of sample variability reduces its potent ial. Other immunological parameters associated with nematode-resistanc e which have potential as phenotypic markers include serum nematode-sp ecific IgE and products of mucosal mast cells such as proteinases. It is likely that as the critical immune responses of sheep to nematodes become more clearly defined, new immunological parameters with potenti al for use as phenotypic markers will be found. The definition of thes e immune responses will also assist in the identification and characte rization of genetic markers. Copyright (C) 1996 Australian Society for Parasitology. Published by Elsevier Science Ltd.