Tropical Biomedicine 22(2): 131137 (2005)
Haemonchus contortus: Parasite problem No. 1 from
Tropics - Polar Circle. Problems and prospects for control
based on epidemiology
Peter J. Wallera and Chandrawathani P.b
a SWEPAR,National Veterinary Instutute, SE-751 89, Uppsala, Sweden
b Veterinary Research Institute, 59 Jalan Sultan Azlan Shah, 31400 Ipoh, Malaysia
Abstract. There is no doubt that on a global basis, Haemonchus contortus is by far the most
important parasite of small ruminants (sheep and goats). This is particularly more so now,
with the development of high levels of resistance to both the broad and narrow spectrum
anthelmintic drugs in H. contortus throughout the world. Epidemiological studies describe
the lower environmental limits for haemonchosis to occur in sheep, as being a mean monthly
temperature of 18C and approximately 50mm rainfall. Thus it has been generally recognised
that H. contortus is a problem parasite restricted to the warm, wet countries where sheep
and goats are raised. However, recent evidence shows that this parasite is apparently common
even in northern Europe. Thus the need for sustainable control strategies for H. contortus is
becoming much more pressing. This report highlights two examples of sustainable and highly
efficient control programmes for H. contortus, that can be implemented in regions at the
extremes of its geographic range (Malaysia and Sweden), where the authors have had direct
involvement.
INTRODUCTION clinical haemonchosis. On a worm-for-
worm basis, H. contortus is considered to
Among the diseases that constrain the be the most pathogenic parasite of small
survival and productivity of sheep and ruminants. It has very high biotic potential
goats, gastrointestinal nematode infection and at times when transmission of this
ranks highest on a global index, with parasite is favoured, losses can occur in all
Haemonchus contortus being of classes of animals. Although it occurs in
overwhelming importance (Perry et al., mixed infections with other nematode
2002). This blood-sucking parasite is parasites, it invariably dominates the
infamous throughout the humid tropics/ faecal worm egg contamination on
subtropics. Annual treatment costs due to pastures. H. contortus is also prominent
this parasite alone have been recently amongst the reports of anthelmintic
estimated to be $26m, $46m and $103m for resistance that has emerged in all
Kenya (Anon., 1999), South Africa (Horak, countries of the world that produce small
pers. comm.) and India (McLeod, 2004), ruminants.
respectively. Although the free-living stages of H.
Haemonchus contortus is probably the contortus are not as tolerant to
only nematode parasite of sheep and goats unfavourable climatic (cold, but
that can be accurately diagnosed without particularly dry) conditions as the other
the aid of laboratory testing. Signs of acute important nematode parasites of sheep
anaemia are obvious. Past history and (Donald, 1968; Waller & Donald, 1970),
discounting other less common conditions the very high biotic potential and
causing anaemia, will strongly suggest pathogenicity of this parasite ensure that
131
it is a major problem in the humid tropics (Chandrawathani et al., 2003; 2004a) see
and subtropics (Anon, 1991; Waller et al., Table 1.
1996; Chandrawathani et al., 1999; Anon, Clearly this situation was
2001; Perry et al., 2002). However, as unsustainable not only in maintaining
Crofton et al. (1965) postulated several viability of the government farms but
decades ago, H. contortus, in common also because it facilitated the distribution
with other nematode parasites of livestock, of highly, multiple resistant parasites
exhibits considerable ecological and together with the animals to small holder
biological plasticity to overcome farmers. Changes in management were
unfavourable conditions either in the implemented, based on previous
external, or host, environment. Obvious epidemiological studies that showed
examples in the latter are the ability of short-term rotational grazing (2-3 days only
parasites to overcome extreme selective on each plot, returning to their original plot
pressures within the host imposed by the after 30 days), provided good level of
use of anthelmintics. H. contortus is control against H. contortus (Banks et
notorious for the development of al., 1990; Barger et al., 1994; Sani &
anthelmintic resistance, which certainly Chandrawathani, 1996). However, control
would have emerged independently in this was further enhanced by daily
parasite species in many countries, if not supplementation of animals with the
regions within countries, of the world nematode destroying fungus, Duddingtonia
(Waller, 1997; Sangster, 1999). flagrans. The level of H. contortus
infection on pasture was consistently
H. contortus in the Tropics lower on the fungal supplemented pasture
Management in Malaysia and this was reflected in significantly
Government-owned small ruminant heavier weight gains of lambs that
breeding farms in Malaysia provide a received the combination of rapid
source of sheep and goats to small holder pasture rotation and fungal supplement
farmers in the country. Despite the (Chandrawathani et al., 2004b) see
intensive use of anthelmintics on these Figures 1 and 2.
farms, annual losses exceeding 25% of the
total flock attributed primarily to H. H. contortus at the Polar Circle
contortus, are commonplace. Recent Management in Sweden
investigations into the drug resistance Although H. contortus is particularly
status on several of these farms, showed adapted to the warm, wet conditions of the
total anthelmintic failure to all drugs tropics / subtropics, there is an apparent
Table 1. Anthelmintic Resistance Status on Malaysian Government Farms
(adapted from Chandrawathani et al., 2003; 2004a)
Percentage Reduction following drug treatment
Farm
Benzimidazole Levamisole Closantel Ivermectin
Ranau 122% (R*) 32% (R) 76% (R)S 17% (R)
Purutan 017% (R)* 22% (R) 03% (R)S 54% (R)
Bongawan 002% (R)* 82% (R) 35% (R)S 41% (R)
Telupid 023% (R)* 88% (R) 53% (R)S 52% (R)
Lahad Datu 040% (R)* 85% (R) 25% (R)S 67% (R)
Gajah Mati 042% (R)* 65% (R) 96% (SR) 73% (R)
R*: resistance based on Faecal Egg Count Reduction Test (FECRT) results.
132
Figure 1. Mean tracer worm burdens from Control Group (broken line) and Fungus Treatment
Group (solid line) in the field trial at Infoternak Farm, Malaysia. (from Chandrawathani et al.
2004b).
* Significant difference between Control and Fungus at P<0.05
Figure 2. Mean weight gain of lambs from the Control Group (broken line) and Fungus Treatment
Group (solid line) in the field trial at Infoternak Farm, Malaysia. (from Chandrawathani et al.
2004b).
133
increasing importance of this parasite in Table 2. Monthly Worm Burdens of Tracer Lambs
the temperate countries of Europe (U.K in Sweden (adapted from Waller et al. 2004)
Jackson & Coop (2000); France: Hoste et
al., 2002; Netherlands: M. Eysker, pers. Test No. (date) H. contortus T. circumcincta
comm.; Denmark: Thamsborg, pers.
comm.). This could well be as a 1 (13/5 2/6) 00000 000100
consequence of the adaptation of this 2 (2/6 23/6) 00000 000725 (22%)*
parasite not only to anthelmintic selection 3 (23/6 8/7) 00163 (60%)* 000713 (54%)
and the increasing development of
4 (19/8 4/9) 00850 (100%) 004300 (73%)
resistance, but also to adaptation to
unfavourable, non-chemical conditions 5 (4/9 23/9) 03213 (97%) 005563 (46%)
experienced either by the free-living, or the 6 (23/9 9/10) 28800 (100%) 037838 (88%)
parasitic stages. The parasites could have
7 (9/10 28/10) 55450 (100%) 0 78938 (91%)
either become more cold tolerant for the
development and survival of the free-living
stages, and /or developed special survival Year 2
mechanisms of the parasitic stages within 09 (13/5 10/6) 00625 (100%) 157873 (64%)
the host, to ensure between-year survival.
10 (6/7 31/7) 00013 (100%) 007263 (38%)
Although anthelmintic resistance to
the benzimidazole anthelmintics has been
* Numbers in parenthesis represent the percentage of the
reported in H. contortus in Sweden population arrested in development at the early fourth larval
(Nilsson et al., 1993), it appears that this stage in the mucosa of the abomasums.
is the extent of the resistance problem.
Thus the increased prevalence of this
parasite is likely to be due to some other pastures. These results suggested that
adaptive mechanism. A recent study in this eradication of H. contortus, on a farm-by-
country has shown that H. contortus has farm basis in the first instance, is a
undergone some unique epidemiological practical and realistic possibility for sheep
adaptations (Waller et al., 2004). Virtually (goat) flocks in Sweden. All ruminants on
complete inhibition of development occurs the farm (sheep, goats and cattle) would
once infective larvae are acquired by sheep need to be properly treated with a highly
early in the grazing season. Almost no effective anthelmintic during the winter
survival occurs over-winter on pasture. housing period. This is because H.
The peri-parturient ewes hold the key to contortus is capable of infecting cattle,
between year transmission, as they often particularly young animals, and thus they
show high H. contortus faecal egg counts could potentially act as a reservoir
at the time of turn-out in early spring. This infection if not treated (Southcott &
leads to early contamination of pastures Barger, 1975). A 2-year pilot study on 2
and continuation of infection for a further farms was implemented, where such a
year, with only one parasite generation/ programme was followed. Monitoring by
year (see Table 2). faecal egg counts and infective larval
Winter housing of all small ruminants differentials of ewes and lambs for the
(sheep and goats) is virtually universally subsequent 2 grazing seasons, together
practised throughout Sweden, and with total abomasal worm counts of 10
generally for a period of 4 - 5 months lambs from each farm at the end of the
(December April/May). Thus animals are first grazing year, showed that this
not exposed to infection, and importantly objective was achieved (Waller et al.,
there is no faecal contamination, on 2005) see Table 3.
134
Table 3. Faecal egg counts of ewes and lambs and worm burdens of lambs on 2 farms in Sweden
where eradication of Haemonchus contortus was achieved (from Waller et al. 2005)
Farm 1
Sampling Date Mean Infective Larval Differentiation
Egg count
(epg) H. contortus T.circ. Trich. spp. Ch. ovina Oesoph.
2003
Ewes 30 April 670 36% 32% 8% 24% 0
21 Aug. 25 0 0 0 0 0
25 Sept. 0 0 0 0 0 0
Lambs 21 Aug. 60 0 0 0 0 0
25 Sept. 240 0 33% 24% 17% 26%
2004
Ewes 15 April 0 0 0 0 0 0
1 June 20 0 23% 73% 4% 0
Lambs 8 July 260 0 63% 26% 11% 0
31 Aug. 380 0 7% 64% 16% 8%
2005
Ewes 19 April 200 0 25% 23% 50% 2%
Lambs 9 Aug 580 0 13% 8% 79% 0
Mean Worm Burdens (10) Lambs - October 2004
Nil 1855 75 Nil Nil
Farm 2
H. contortus T.circ. Trich. spp. Ch. ovina Oesoph.
2003
Ewes 9 May 1650 44% 16% 32% 8% 0
19 Aug. 165 + + + - -
29 Sept. 10 - + + + -
Lambs 19 Aug. 80 20% 25% 20% 35% 0
29 Sept. 390 20% 29% 33% 18% -
2004
Ewes 15 April 0 0 0 0 0 0
18 May 10 0 0 0 0 0
Lambs 13 July 425 0 28% 23% 42% 9%
16 Aug. 440 0 25% 45% 30% 0%
2005
Ewes 12 April 215 0 15% 50% 15% 20%
Lambs 29 June 620 0 45% 55% 0 0
Mean Worm Burdens (10) Lambs October 2004
Nil 6183 600 Nil Nil
135
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