Fróðskaparrit - 31.12.2000, Page 187

SNIKAR A SEYðl IFØROYUM
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treatment the previous autumn. The rise in
the spring is expected to be due to a phe-
nomenon described in several studies from
other countries, the periparturient egg rise.
This is seen in ewes in the period two
weeks before lambing until six weeks after
lambing. The faecal egg discharge is in-
tense in this period and it builds up a mas-
sive population of infective larvae on the
pasture, to be ingested by the susceptible
lamb generation.
Nematodirus spp
Nematodirus battus: There was a signifi-
cant difference between epg in lambs and
ewes with a mean epg in ewes never higher
than 4, and with a mean of 102 epg in lambs
that were about five months old. This indi-
cates development of a considerable degree
of resistance in sheep older then one year.
Faecal egg count in winter drops to zero for
all lambs and ewes, most likely due to ac-
quired resistance and anthelmintic therapy
in the autumn and winter, and rises to a
mean count of 70 epg for lambs in the
spring (Fig. 4).
The typical pattern of transmission is
considered to be that only one parasitic
generation of N. battus occurs per season.
Eggs develop to infective third-stage lar-
vae, but do not hatch until the larvae have
been exposed to freezing (Gibson and
Everett, 1981) followed by periods with
moist conditions and mean night/day tem-
peratures above 10°C (Thomas and
Stevens, 1960). These conditions lead to a
synchronous hatching in the spring, fol-
lowed by transmission to the lambs con-
centrated in a few weeks. Autumn trans-
mission has been reported in England,
Scotland, and Norway, but the spring trans-
mission is considered typical (Rickard et
al., 1989).
In this study, lambs younger than five
months have not been examined. It is,
therefore, not possible to comment on
transmission to younger lambs, but the re-
sults indicate that there is a spring trans-
mission. In addition, both faecal egg
counts and post mortem examinations indi-
cate that autumn transmission is common.
Studies in Oregon show that transmission
seems to be year round with major peaks in
the autumn and through the winter. It is
suggested that these results might be corre-
lated to high precipitation rather than to
temperature (Rickard et al., 1989). If these
suggestions are reliable, precipitation
might be a major factor in N. battus devel-
opment and transmission in the Faroe Is-
lands, as rain is abundant and precipitation
totals are highest in autumn and winter, and
mean night/day temperature only just ex-
ceeds 10°C (Fig. 28). The relative humidi-
ty is high, normally 88% annually in Tór-
shavn, and is highest around August (Cap-
pelen and Laursen, 1998).
The egg counts for other Nematodirus
spp. showed that the distribution was more
even between seasons and between lambs
and ewes than found for N. battus. It is
likely that these Nematodirus spp. found in
the faecal examinations were Nematodirus
filicollis, as these were the only Nema-
todirus spp. found in the post mortem ex-
aminations described above.