Íslenskar landbúnaðarrannsóknir - 01.09.1974, Síða 11

SHEEP PARASITES IN ICELAND 9
period 20th of June — mid September, we
might imagine a drop in the number of
eggs, starting in August and continuing for
several months due to the mortality of the
nematodes. Figures of „other strongyles" (Fig.
4, 5 and 12) do show that the number of
eggs does fall in late September until Nov-
ember. (This decrease can also be explained
by other factors, see below).
In late September the sheep remrn to the
farm, where the rate of infection is probably
even greater than in the spring. This should
result in an increase in the number of eggs
in faeces, beginning in early November and
continuing for several months. Fig. 4, 5 and
12 show that the number of eggs does in-
crease but only slightly, in November, rea-
ching a maximum of several hundred eggs
in late November or early December at
which time the number begins to decline.
Why is the rise in number of eggs not
higher in November—December? Investiga-
tions have shown (f. ex. Ayalew and Gibbs
1973, Field et al. 1960) that most of the
nematode larvae which the sheep pick up in
the fall do not mature at once but lie dor-
mant in the intestine until the spring. More-
over, it has been found (f. ex. Connan
1968, Sullivan and Donald 1970) that
the egg production of those nematodes which
are already mamre decreases in the fall and
early winter.
It gets colder in October-November and
during the night the temperamre frequently
drops below freezing point. Therefore the
risk of infection diminishes rapidly. About
mid-November the sheep are placed in houses
with slatted floors and fed hay until May.
During this period the sheep rarely leave the
houses and infection can be expected to be
minimal. The combined factors of reduced
infection, decreased egg laying of the mature
worms and inhibited mamration of the larvae
in the sheep lead to a fall in egg number
in December which reaches a minimum in
January or February (Fig. 5).
In March-April the number of eggs be-
gins to increase again and reaches a maxi-
mum in April-May with a mean approaching
two thousand eggs pr g faeces (F'ig. 5). This
great increase in number of eggs of „other
strongyles", the so-called „spring rise" is well
known in other countries. The following
have been put forward as the main factors
involved:
1. New infection.
2. The larvae which did not mamre in the
fall or winter become mature and start
laying eggs.
3. The mature worms which decreased their
egg-laying in the fall increase it again.
(For reviews see Ayalew and Gibbs
1973, Connan 1968a, Dunsmore 1965,
Gibbs 1967).
It is interesting to compare the difference
in the number of eggs in the sheep that
received thiabendazole in December with
those that did not receive treatment (Fig. 12
and fig. 5). Assuming thiabendazole killed
all the larvae and adult worms, then only
factor 1. could be responsible for spring rise
in those sheep that received thiabendazole.
By comparing the figures it can be roughly
estimated that factor 1. is responsible for
about 40% of the number of eggs in the
spring in those sheep that did not receíve
thiabendazole, factors 2 and 3 about 50%
and the remaining 10% is the egg laying
during the winter.
If we assume that thiabendazole did not
kill all larvae and adult worms, or if we
assume that larvae and adult worms already
present in the sheep diminish further infesta-
tion then factor 1 becomes of relatively