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.