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Transmission dynamics of helminth parasites of pigs on continuous pasture: Oesophagostomum dentatum and Hyostrongylus rubidus
Pergamon
A. ROEPSTORFF*
and K. D. MURRELLt
Danish Centrefor Experimental Parasitology, Royal Veterinary and Agricultural Bdowsoej 13, DK-1870 Frederiksberg C, Copenhagen, Denmark i Receirred 31 May
1996: accepted
13 .Iunuary
Uniwrsitt:.
1997 I
Abstract-Roepstorff A. & Murrell K. D. 1997. Tra osmkion dynamics of Hmintb parasites on conthous psrshw: Oesop&ugostomum dentatum and Hyostrongylus rnbidks. Intern&owl Jotunalfor Pwaait~~ 27: 553462. An increase ia alternative outdoor pig production systems is occurriag ~JI Denmark, aedtlrlsshidy elncidate tbe tranwissi on patterns of Oesophagostkmwm dentutnm and Hyo8hmgyks r&k uousiy on a pasture. A groop of inoculated witb low numbers of both be tbs.Tbescp@were I by serial necropsy and samplkg of faeces, gram pad so& A no of pigs was similarly followed on the same pasture in Year 2 belminth-naHe tracer pigs during all seasons. Tbe pasture mmdting ia a dirt lot by tbe autumn of Year 2. Tbe area was soon coat pasture infectivity and increasing worm burdens in late summer;tbanthefm&ersnfl In May of Year 2, newly exposed pigs became only lightly infected (mostIy 0. datetwm),andIWtr was oherved in Joly-August of Year 2, probably due to an unua%mRy dry summer aml a hck of vegetation. The IWO&S indicate that both 0. dentutum and H. rubidw 8~ v factors, because sigsdlicant transmission occurred only under tbe most favoura with protecting vegetation as in Year 1). Transmission was r of Year 2, wbaa experienced in tbe winter between Years 1 and 2 and during tbe dry lad&g. Co&suo~ grazing actually reduced tr&ssion of 0. dentotum aad H. rt&idw IM?CS&W ofwz reducth ia v&n. Tbii, bowever, is not a de&able alternative farming system, becmme of its adverse emkmmental effects. This environmental impact may be mitigated by employment of a pasture rota&m system in place of continuous grazing. 0 1997 Australian Society for Parasitology. Pwblisbed by Elsevier Science Ltd. Kyv
words:
Oesc~phagostomum
duntutum;
Hwsrrongylus
ruhidus;
pigs: transmission; pasture infectrviiy:
tracer pigs.
are reared indoors, as a result of the marked intensification of modern production systems. Therefore. current parasite control strategies have been developed from research on parasite occurrence and transmission in pigs reared in these confined. indoor systems, and today only Ascaris suurn and &sophugostomum spp.are common in the Danish indoor pig-industry (Roepstorff & Jorsal, 1989). WWstrongylus rubidus was previously common in grazing Danish sows (Jacobs & Andreassen, 1967), and severe clinical outbreaks were encountered (Larsen. 1967).
INTRODUCTION
Gastrointestinal parasitism in domestic pigs is determined largely by their housing facilities and management systems (reviewed by Roepstorff & Nansen, 1994). Currently, almost all pigs in northern Europe
*To whom correspondence should be addressed. Tel: + 45 35 28 21 75: Fax: f45 35 28 2114; E-mail: [email protected]. tPresent ARS-IJSDA,
address: Beltsville Beltsville. MD
Agricultural Research 20705, U.S.A.
Center.
553
A. Roepstorff and K. D. Murrell
554
but with the decline of grazing, this parasite has not been observed in recent years. More recently, in response to animal welfare and agricultural sustainability concerns, an increase in alternative pig production systems is being experienced. These alternative systems usually include production in extensive free-range grazing conditions, especially for sows and piglets. It is clear from published research that access to outdoor facilities increases the risk of infection with helminths (e.g., Pattison et al.. 1980; Alfredsen, 1983; Biehl, 1984; Mercy et a/., 1989). Swine parasitism may increase, therefore, along with the growth in extensive pig production systems. This has raised the question of whether our understanding of the ecology of pig parasites, derived from research on confined pigs, is relevant to outdoor pasture and pig lot environments. Only a few studies have been carried out on outdoor transmission patterns of porcine helminths (e.g., Rose & Small, 1980a, 1983; Burden & Hammet, 1979; Dangolla et al., 1994). Although all of these studies documented high transmission rates on outdoor rearing systems, no serious parasitic infections were found in a recent study in 10 large outdoor sow herds in Denmark (A. Feenstra, personal communication, 1996) and, with the exception of A. suum, this may also be true of some organic farming systems (Roepstorff et al., 1992). This study was designed to elucidate the transmission patterns of 0. dentatum and H. rubidus in pigs allowed to graze continuously on a pasture. Both species are transmitted as free-living infective (L,) larvae and they represent potentially serious parasite threats to pigs raised in outdoor systems. A concurrent study was also conducted on A. suum and T. suis, parasites that are transmitted as infective eggs; the results of that project are reported separately (Roepstorff & Murrell, 1997). MATERIALS
AND
METHODS
This study was designed to evaluate parasite transmission in a continuous grazing system over a 3-year period (Year 1, 1993; Year 2, 1994; Year 3, 1995). Experimental pigs. Danish Landrace/Yorkshire/Duroc crossbred female pigs were obtained from a specific-pathogen-free (SPF) herd. Previous repeated faecal examinations had indicated that the herd was helminth free. No excretion of eggs was observed in the pigs selected for the study. Nevertheless, all pigs were routinely dewormed with 4% Panacur@ (5 mg fenbendazole kg-i body weight mixed in the fodder on both of 2 consecutive days) immediately after turnout on to the study pasture. Feeding. The pigs were fed a standard feed regimen and the ration was adjusted every second week according to body weight. The diet consisted of ground barley. soy meal, minerals and vitamins.
Parasites. The EH-strain of 0. dentarm was originally isolated from a Danish indoor sow herd (Roepstorff et al., 1987). and has been maintained by serial passage in helminthnai’ve pigs. The infective larvae were isolated from a faecesvermiculite culture (room temperature), and stored at IO’C until use. The strain of H. rubidus employed was received from Dr D. Barth of MSD, Hannover, Germany, in 1990. Serial passages and larval culture methods were similar to those for 0. dentatum. Experimentalpastures. The experimental pasture (8000 m’) was utilized for continuous grazing and was purposely contaminated with parasites as described below. Two other pastures, maintained in helminth-free conditions (No. I, 4000 m’; No. 2, 3200 m’) were also used for the study. These pastures were used to hold parasite-free pigs prior to their introduction to the experimental pasture. On each pasture, the pigs had access to an insulated house, shade (trees or an open shed) and a wallowing facility. Previously, the pastures had been grazed for many years by cattle. The soil was light, sandy and well drained, with a high humus content. The vegetation was chiefly Stellaria media and the grasses Festuca rubra, Loliumperenne, Poa trivialis and Elytrigia repens. Care was taken to prevent transfer of infective eggs/larvae from the experimental pasture to the clean pastures by using separate tools, boots and requiring all personnel to change protective clothing when moving from the experimentally contaminated pasture to clean pastures. Climate. The precipitation was recorded daily and the temperature continuously at 2-m height at a weather station 500m away (at Department of Agricultural Sciences, Section of Soil and Water and Plant Nutrition, Royal Veterinary and Agricultural University). Experimental protocol. pigs). Twenty-eight pigs
Group
I (continuously
exposed
(20 kg mean body weight) were turned out on the experimental pasture at calendar week 18 (early May) of Year 1. All these pigs were inoculated twice by stomach tube with the following numbers of infective larvae: 500 larvae of 0. dentatum (lgand 19 May, week 20) and 2500 larvae of H. rubidus ( 15 and 16 June. week 24). The pigs were also inoculated with 200 eggs of As&is suum (week 20, mid-May) and 1000 eggs of Trichuris suis (week 28, midJuly), as reported elsewhere (Roepstorff & Murrell, 1997). Subsets of 3 Gr. 1 pigs were removed from the pasture for parasite analysis at weeks 24, 28, 32 and 36. The remaining 16 pigs were necropsied at week 40 (early October). This protocol ensured the presence of a large group of pigs on the experimental pasture during the whole summer of Year 1. Group 2 (tracer pigs). Fifteen pigs (20 kg mean body weight) were turned out on a clean pasture (No. 1) at week 18 (early May), Year 1. Thereafter, subsets of 3 tracer pigs were moved to the experimental pasture every fourth week starting at week 22 (early June) and ending in week 38 (late September). All subsets of tracer pigs were exposed to pasture contamination for 14days, which ensured that the parasites picked up by the tracer pigs would not reach patency before removal of the pigs to helminth-free indoor facilities for an additional 4 weeks prior to necropsy. To avoid fighting, when the tracer pigs were introduced to the continuously exposed pigs, both groups of pigs (Gr. 1 and the introduced tracer pigs) were sprayed with a commercial repellant on their skin for the first 34 days after transfer. Because 1 strongyle egg was found in a tracer pig on clean pasture No. 1 in August 1993. the tracers were immediately dewormed twice with fenbendazole (recommended dose rate), and thereafter all tracer pigs were routinely dewormed every 4th week (not
0. dentatum
and H. rubidus
less than 7 days prior to being moved to the experimental pasture). Group 3 (tracer pigs). Twenty pigs (21 kg mean body weight) were placed on the second clean pasture (No. 2) in weei< 41 (mid-October) of Year 1. Subgroups ofat least 3 pigs were treated according to the same protocol followed for the Gr. 2 tracers, starting in week 43 (late October) and continuing to early April (exposed in weeks 13-15). There were no permanently exposed pigs on the experimental pasture during the winter period between Years 1 and 2. Group 4 (continuously exposed pigs). In Year 2 (1994) 28 pigs (19 kg mean body weight) were allowed to adjust to the outdoor environment for I week on clean pasture No. 2, before being moved to the experimentally contaminated pasture at week 21 (late May). They were kept on this pasture until necropsied as subgroups of 3 pigs at weeks 27, 31, 35 and 39. The remaining 16 pigs were all necropsied at week 43 (late October). Group S (tracer pigs). Twenty-one pigs were placed on clean pasture No. 2 at week 20 (mid-May), Year 2; their mean body weight was 30 kg, somewhat larger than the Gr. 4 pigs. Subsets of at least 3 pigs followed the Gr. 2 tracer pig protocol, starting at week 21. Group 6 (tracer pigs). A single group of 3 tracer pigs (37 kg mean body weight) grazed a IOOO-m2 enclosure within the experimenral pasture during week 27-29 (mid-July) of Year 3. Sampling. Faecal samples were collected rectally from the exposed pigs (Gr. 1 and 4) and all exposed tracer pigs every 2nd week. Samples were made every 4th week from tracer pigs held on the clean pastures. All pigs were weighed every 4th week. Herhage larvae counts. By walking a W-route across the pasture, grass samples (200-300 g) were collected randomly from the experimental pasture every week (summer) or every 2nd week (winter). Similar sampling was carried out on the clean pastures every 4th week. As the vegetation cover gradually diminished, grass samples were supplemented with soil samples (200-300 g) collected in the same manner. After late autumn of Year 1 no vegetation remained on the experimental pasture and it was possible only to sample the soil. Parasitological techniques. Faecal egg counts were carried out by the McMaster concentration technique (sensitivity: 2Oeggsg. ’ (epg)), described by Roepstoti & Nansen (in press). All faecal samples that contained eggs of 0. dentaturn/H. rubidus were cultured for 8 days in vermiculite (room temperature) and 200 L3 were identified to species (Ahcata, 1935). The fractions of 0. dentatum and H. rubidus eggs were multiplied by the total strongyle epg to calculate the faecal egg counts for the 2 species. The agar method of Mwegoha h Jorgensen (1977) was used to recover infective larvae from grass samples. After sieving and sedimentation of the soil samples ( N IOOg), the sediment was mixed with agar and thereafter treated as for grass samples. Necropsy. The pigs were not fed on the day of slaughter. They were euthanized using a captive bolt pistol or by CO, suffocation followed by exsanguination, and the stomach and the large intestine were immediately removed. Aliquots of 209.6 of the contents were washed with a jet stream of water on a 56-pm (stomach) or a 212~pm (large intestine) sieve. The mucosa was scraped off the stomach wall, digested in pepsin.-HCI (1% pepsin, pH w 1.5) in a stomacher apparatus (30 min at 4o’;C) and subsequently sieved (38 pm). The large intestinal wall was incubated in 0.9% NaCl (1 million i.u. benzylpenicillin potassium and 1 g dihydrostreptomycin
transmission
in pigs on pasture
was added per 10 1) at 38°C overnight; the mcubation thud was then sieved (20pm). All samples were fixed with iodine (80 g iodine and 400 g potassium iodinein 800ml dist. water) and decolorized with sodium thiosulphate immediately before counting under a stereomicroscope. Calculations. All calculations were performed SAS’” Release 6.04 software package.
using
the
RESULT Climate
The weekly rainfall and daily mean temperature data are shown in Fig. 1, together with mean ctimate data from Denmark (from Rosensrn & Lindhardt, 1994). The early summer of Year 1 (1993) was warm and dry, while the late summer and autumn were cold and wet. There were prolonged freezing periods in the winter between Years 1 and 2. The spring and early summer of Year 2 were colder than normal. However. the summer of Year 2 was much hotter and drier than normal (20 mm rain in 8 weeks), and an almost similar drought period was experienced in Year 3.
The pastures
The experimental pasture changed radically during the summer of Year 1. At turn out in May, the beight of the vegetation was - IOcm, but as the pigs rooted they turned over the upper 1Ocm of soil. Thus. by August approx. two-thirds of the pasture was denuded of vegetation. By the time Gr. 3 was introduced on the pasture (early October), all the original vegetation had been destroyed, and only a few young seed plants were present. In Year 2, the experimental pasture consisted only of bare soil. Because of the lack of rooting by pigs new vegetation had appeared by July of 1995 (Year 3). when the final tracer pigs were introduced.
The pigs
Despite the abrupt change in the environment of the pigs at turn out, the only adaptation problem observed was sunburning (especially in Gr. 1 and 2), but this did not seem to influence food intake or common well-being. No other clinical symptoms were observed. The use o,f repellants did not completely prevent fighting when tracer pigs were introduced to the continuously exposed pigs on the experimental pasture, and although all tracer pigs used the common feeding area and housing together with the other pigs, they were frequently observed grazing separately.
A. Roepstorff
556
and K. D. Murrell
JFMAMJJASONDJFMAMJJASONDJFMAMJJAs
YEAR
1 (1993)
YEAR 2 (1994)
Fig. 1. Climate records measured at Department of Agricultural Science, RVAU, pasture. Curve (left v-axis): daily mean air temperature (“C). Columns (right y-axis): are plotted comparable average climate data measured in Denmark
Transmission of 0. dentatum The Gr. 1 pigs, inoculated with 2 x 500infective larvae at week 20, yielded a mean of 949 adults when worm recoveries were carried out 4 weeks later (Table 1). Thereafter, intestinal worm counts increased to 26 500 by early September, then dropped to 10800 worms in early October. The largest numbers of immature worms were found at weeks 32 and 36 (3200 and 9700 larvae per pig, respectively, corresponding to 67% and 37% of the total worm burdens); this coincided with the greatest pasture contamination (see below) and preceded the large increase in total worm counts in September. The mean faecal egg count (Fig. 2) of the Gr. 1 pigs increased rapidly to > 3000 epg by week 24; it then fluctuated between 3000 and 5500 epg until all the pigs were slaughtered. The epg counts did not correspond to the actual worm burdens. The Gr. 2 tracer pigs became infected when first exposed to the experimental pasture at weeks 2628, i.e. approximately 4weeks after the initial contamination of the pasture by the Gr. 1 pigs and 6-8 weeks after the initial larval inoculation of the Gr. 1 pigs. The highest level of transmission apparently occurred in early August; tracers from that period harboured the highest worm burden (- 21000 worms). In comparison, the Gr. 2
YEAR 3 (1995) approx. 500m from weekly precipitation from 1961 to 1990.
the experimental (mm). Overlying
tracers, grazing in early September, harboured only -800 0. dentatum, indicating that transmission had decreased drastically by late summer. The winter tracers (Gr. 3) acquired very low numbers of larvae; only CL30 worms resulted from exposure in the MarchApril period. The new group of continuously exposed pigs in Year 2 (Gr. 4), turned out on the experimental pasture in week 21 (late May), quickly acquired low numbers of 0. dentatum; the first positive egg counts were observed by 4weeks after exposure to the pasture. The relatively low worm burdens peaked in July-August (only -50 worms), indicating that no transmission had occurred after the Year 1 overwintering larvae had disappeared from the pasture. Similarly, the Gr. 5 tracer pigs acquired only 5-35 0. dentatum in late May and none in June and July. In the August-October period, O-60 larvae were picked up by the tracers. Just 1 single worm was recovered in the 3 tracers exposed in the following summer of Year 3 (1995). The results of the grass and soil samples are shown in Fig. 3. In Year 1, the number of infective larvae kg-’ dry grass increased to l&30 000 by week 29 (late July), with a peak of nearly 60 000 kgg ’ in week 32 (midAugust). Because of pig rooting activity it became
0. dentatum
and H. rubidus
transmission
in pigs on pasture
c
t4
0
0
09
w
00
e
(x10,000)
L3 per kg wet soil (x100)
P
h,
L3 per kg dry grass
=;
WI
G
o\
0. dentatum
and H. rubidus transmission in pigs on pa:,ture
impossible thereafter to collect grass, and sampling was stopped by early October. Sampling of soil showed that the number of infective 0. dentatum larvae decreased from 130-l 200 per kg in the autumn (September-October) to O-50 per kg in the winter period. Low numbers of 0. dentatum larvae (O68 kg ’ soil) were recovered in the early summer (weeks 21-28) of Year 2, i.e. during the first 7 weeks after turnout of the Gr. 4 pigs. No larvae were found in mid July-early September, i.e. during the drought of that year (Fig. 1). The soil again yielded low numbers of infective larvae in late September and October. Transmission qf‘ H. rubidus The pigs of Gr. 1, inoculated with 2 x 2500 infective larvae of H. ruhidus at week 24 (Year 1) yielded a mean of 275 stomach worms, when worm recoveries were carried out 4weeks later (Table 2). The mean worm burden of Gr. I increased thereafter to 2560 by early September (week 36), and subsequently dropped to 1200 in early October (week 44). The faecal egg counts increased to > 800 epg 4 weeks after the initial infection. and then fluctuated between 150 and 1000 epg. The egg excretion rates did not appear to be related to the adult worm burdens. None of the Gr. 2 tracer pigs became infected with H. rubidus until early August, when a mean of 1262 worms was recovered (Table I!). Thereafter. transmission decreased drastically; only 25-125 and O-30 larvae, respectively, were acquired by the tracer pigs 4 and 8 weeks later. This reduction in transmission continued into the winter period. Tracers of Gr. 3 yielded a total of only 1 worm from the last 3 sets of tracers exposed in JanuaryApril (Year 2). Very few H. rubidus were found in the continuously exposed Gr. 4 pigs in Year 2, and none was found in the tracer pigs of Gr. 5 and 6, which were exposed on the pasture from July-November (Year 2) and August (Year 3). The grass samples (Fig. 4) yielded fluctuating numbers of infective H. rubidus kgg dry grass from August to early October 1993 (Year 1). with 1 sample (mid-August) containing 4600 L3 per kg. H. rubidus larvae were recovered from soil only in May of Year 2. DISCUSSION
The present study showed that both species of nematodes were able to be transmitted effectively to grazing pigs, and that their individual transmission dynamics and the impact of environmental factors on their transmission patterns were quite similar. Because tracer pigs acquired infections when grazing the experimental pasture approximately 4 weeks after the first egg contamination of the pasture (68weeks after
inoculation of the Gr I pigs), a short larval gene] .ttion time on pasture occurs during summer. The direct estimations of larvae on grass and soil, as well as the data on transmission of infective larvae to tracer pigs. support this. These data also reveal that the numbers of infective larvae of both species on the pasture. after reaching high levels in August, experienced a sudden decline in September. The decline in larval availabiiitt at the end of the summer is also seen in the drop (> 50%) in worm burdens in the continuously grazed pigs (Gr. 1) after early September. Because a ?tricL relationship between pasture contamination and acquired worm burdens is often lacking (Kendall & Small, 1974~ Rose & Small, 1980a), the present reductions in worm burdens may also reflecr the importance of dose-dependent regulatory tnechanisms, as has been proposed previously for 0. cii~orum by Christensen et al. (1995) and Roepstortf cjt tri. (1996), and for H. rubidus by Kendall & Small ( I974b ). The natural regulation of the host’s worm populations is likely the result of multiple factors, including both host-mediated regulation and the regulation o!‘ the free-living larval population by biotic and abiotic: factors. The free-living stages of 0. denrutum and tf. ruhi&s have nearly identical temperature and r&live humidity requirements, as has been demonstrated in controlled laboratory experiments (Fossing (‘I trl.. 1995; Rose & Small, 1980b, 1982). These studies also documented the great similarities between the species with regard to developmental rates and survival profiles on pastures. Rose & Small (1983) reported identical pasture contamination patterns for both parasites in a commercial herd. in which the grass contamination reached maximum in late summer. a pattern also observed in the present study. Our results demonstrate that although both species experrence considerable mortalny. coinciding with falling temperatures and the disappearance of the vegetation. a few larvae of both species can overwinter, as previously observed in tt-ie mild winters in England (Rose & Small. 1980b, 1982) and in the cold winters rtr Germany for 0. dwltatum, (Haupt, 1969). M. N. Larsen (M.Sc. thesis, University of Copenhagen, 1996) showed that a Iew (< 1%) 0. dentotum and If. ruhidus L3 could develop in faeces buried in bare sot I, but almost none survived when the faeces were &aced in short grass and exposed to the sun and the wind. The number of 0. cietztatum larvae in grass samples taken in the late summer of 1994 (Year 2) was roughlv 10 x higher than previous reports of herbage recoveries for this species (e.g., Kendall & Small, 1974~ Rose &Small, 198Oa, 1983; Nansen et al., 1996). An expianation may lie in the fact that the samples of sparse and low vegetation from the well-grazed pasture III
(6 July) (3 Aug.) (3 1 Aug.) (28 Sept.) (26 Oct.)
27 31 35 39 43
3 3 3 3 16
3 3 3 3 16
No. of pigs
Hyostrongylus
worm
0 0 0 0 0
0
(-)
4
0 214 161 2345 1138
Adult
(the pigs were necropsied
C-1 C-1 ;I;
Group
Group 1 C-1 (Cl) (81-258) (136255) (O-278)
pigs
4 weeks
Year
3
Year 2
2
after exposure).
Year
1
l-Year
Year (-) (200-316) (88-217) (8243720) (11&3130)
H. rubidus
counts (mean and min.-max.) for all groups of continuously in week 24 (total of 1000 L,) to initiate parasitic contamination
Continuously exposed Immature H. rubidus
rubidus
“Week of necropsy. hWeeks of exposure to the contaminated pasture ‘Nearly all worms in the tracer pigs were adults.
(15 June) (13 July) (10 Aug.) (7 Sept.) (5 Oct.)
(date?
Week
24 28 32 36 40
Z-The
Table
(date)b
27-29
Apr.)
July)
June) July) Aug.) 1 Aug.) Sept.) Oct.)
Mar.)
May-8 June-6 July-3 Aug.-3 Sept.-28 Oct.-26
(5 July-19
21-23 (25 25-27 (22 29-3 1 (20 33-35 (17 37-39(14 4143 (12
13-15 (29 Mar.-l2
Group 3
3 3 3 3 5 4
Group
5
3
Group 3 3
7-9 (15 Feb.-l
Nov.) Dec.)
Group 3 3 3 3 3
6
5
3
2
0
0 0 0 0 0 0
0
2
17 0
12 0
0 0 1262 62 15
Total
infected
(-
(-) (--) (-) (-) (-) (-)
C-)
(O-5)
(040) (-1
(O-25) (-)
)
(-1 (-) (9241833) (25-l 15) (O-30)
H. rubidus’
1 pigs were experimentally
Tracer pigs No. of pigs
3 3
(26 Oct.-9 (23 Nov.-7
(1 June-l 5 June) (29 June-l 3 July) (27 July-10 Aug.) (24 Aug.-7 Sept.) (21 Sept.-5 Oct.)
of exposure
51-l (21 Dec.4 Jan.) 3-5 (18 Jan.-l Feb.)
4345 4749
22-24 26-28 30-32 34-36 38-40
Weeks
exposed pigs and tracer pigs. The Cr. of the experimental pasture
5 t: E
b
E
? ia F1 P 2 ;i 6
0. denfatum
and H. ruhidus
transmission
in pigs on pasture
GRASS
0
20
24
28
32
36
40
44
48
52
Week Fig. 4.
RIIIIII~~~~~1rl--4
8
number
12
16
20
24
28
32
36
40
Y’EAR 2
Numbers of infective larvae of H. rubidus in grass ( x , per kg dry weight. left v-axis) and in soil (0. per kg wet weight, right y-axis) of the experimental pasture. Note that the right and left !I-axes have different scales.
this study had relatively more stems close to the ground than did samples collected on a pasture with high vegetation, as was typical of the previous studies. Close proximity to the soil surface may facilitate translation of infective larvae from soil to vegetation. The tracer animal technique for assessing the risk of infective helminth transmission has proved to be extremely valuable in ruminant livestock studies (e.g., Armour & Ogbourne, 1982). The use of tracer pigs in the present study also allowed the determination of transmission patterns of parasites in a continuously grazed pasture system. Because the tracer pig worm acquisition data related well to the results from grass/soil sampling, it may be concluded that the tracer principle is also valid for pig studies, although the hierarchical structure of groups of pigs, leading to different grazing behaviour of introduced tracer animals, might have some influence on the results. The overall results of the present study showed that 2 of the potentially most important helminths for alternative outdoor swine rearing systems have similar epidemiological characteristics, and they provide an opportunity to design a control strategy suitable for both of these species, which are transmitted as infective larvae. 0. dentatum and H. rubidus are most actively transmitted when temperatureand moisture in the parasite’smicroclimate are high. and most
adverselyaffected by dry summerson bare soil and during the winter period. A managementschemethat can take advantageof theseenvironmentafinfluences will yield pig rearing systemsthat can minimtze the useof anthelminthics.assuggestedfor ruminants by Nansen(1987);suchmanagementwill beof particular importancefor organicpig rearing.However, the pasture systememployedin this study (continuousgrazing) is not a desirablealternative, despite the fact that both 0. dentatum and H. rubidus were almost eradicated.Under heavy rooting pressure,the soil is completely denudedof vegetation, rendering it vulnerable to erosion; surfacerun-off of animal waste, and the risk of ground water contaminationmay also be increased.Furthermore. the dirt lot did not totally prevent helminth transmission,becauseboth A. suuyll and T. suiswere transmitted very effectively during the dry summerof Year 2 (RoepstorlT& Murrell. 1997). For these reasons,consideration should be given to an outdoor rearingsystemthat includespasture rotation schemes (Roepstortf& Nansen.1994).
Acknowledgements-Technicians Niels Mtdtgaard and Jsrgen Nielsen are kindly acknowledged for taking good cam of the pigs on the pasture. Marlene Sorensen, Niels Peter K. Hansen and Birgitte Ssnderby are thanked for their tcchnrcal
562
A. Roepstorff
assistance in the laboratory. The Department of Agricultural Sciences. Section of Soil and Water and Plant Nutrition, The Royal Veterinary and Agricultural University. is acknowledged for the climate data registrations. The Danish Veterinary and Agricultural Research Council (grant 13-483 I1) and the Danish National Research Foundation provided financial support.
REFERENCES Alfredsen S. A. 1983. Miljlaets innflytelse pa gastrointestinale parasitter hos purke. Norsk Veterinrertidsskrift 95: 48l487. Alicata J. E. 1935. Early developmental stages of nematodes occurring in swine. U.S. Department of Agriculture Technicai Bulletin 489: l-96. Armour J. & Ogbourne C. P. 1982. Bovine ostertagiasis: a review and annotated bibliography. Commonwealth Institute of Parasitology, Miscellaneous Publication 7: 1-21. Biehl L. G. 1984. Internal parasitism of feeder pigs in Southern Illinois. Agri-Practice 5: 20-26. Burden D. J. & Hammet N. C. 1979. The development and survival of Trichuris suis ova on pasture plots in the south of England. Research in Veterinary Science 26: 6670. Christensen C. M., Barnes E. H., Nansen P., Roepstorff A. & Slotved H.-C. 1995. Exuerimental Oesoohaaosromum dentatum infection in the pig: worm popula;onH resulting from single infections with three doses of larvae. International Journalfor Parasitology 25: 1491-1498. Dangolla A., Bjsm H. & Nansen P. 1994. A field experiment on the epidemiology of Oesophagostomum dentatum and H.yostrongylus rubidus infections in a flock of outdoor reared pigs in Denmark. Acta Veterinaria Scandinavica 35: 307-314. Fossing E. C., Knudsen T. S. B., Bj0rn H. & Nansen P. 1995. DeveIopment of the free-living stages of Hyostrongyfus rubidus and Oesophagostomum spp. at different temperatures and relative humidities. Journal of Helminthology 69: 7-11. Haupt W. 1969. Ein Beitrag zur Uberwinterungsfahigkeit der dritten invasionsfahigen Oesophagostomum-larven des Schweines auf der Weide. Archiv fiir Experimentelle Veterindrmedizin 23: 1211-1215. Jacobs D. E. & Andreassen J. 1967. Gastro-intestinal helminthiasis and the adult pig in Denmark. II. The geographical distribution of Hyostrongylus rubidus and Oesophagostomum spp. Nordisk Veterincrmedicin 19: 462465. Kendall S. B., Small A. J. 1974a. Hyostrongylus rubidus in sows at pasture. Veterinary Record 9s: 388-390. Kendall S. B. & Small A. J. 1974b. Biology of Hyostrongylus rubidus. VII. Some factors affecting populations of Hyostrongylus rubidus in the pig. Journal of Comparative Patholology 84~ 169-179. Larsen S. 1967. Gastritis in sows due to Hyostrongylus rubidus-in one case associated with Ballantidium eoli. Acia Veterinaria Scandinauica 8: 347-359. Mercy A. R., Chanet G. & de Emms Y. 1989. Survey of internal parasites in Western Australian pig herds. 2.
an, 3 K. D. Murrell Relationship to anthelmintic usage and parasite control practices. Australian Veterinary Journal 66: 69. Mwegoha W. M. &Jorgensen R. J. 1977. Recovery of infective 3rd stage larvae of Haemonchus contortus and Ostertagia ostertagi by migration in agar gel. Acfa Veterinaria Scandinavica 18: 293-299. Nansen P. 1987. Production losses and control of helminths in ruminants of temperate regions. International Journal for Parasitology 17: 425433. Nansen P.. Larsen M., Roepstorff A., Grranvold J., Wolstrup J. & Henriksen S. Aa. 1996. Control of Oesophagostomum dentatum and Hyostrongylus rubidus in outdoor-reared pigs by daily feeding with the microfungus Duddingtoniu flagrat&. Parasitogical Research 82: 580-584. Pattison H. D.. Thomas R. J. & Smith W. C. 1980. A survev of gastrointestinal parasitism in pigs. Veterinary Record 1W: 415418. Roepstorff A. & Jorsal S. E. 1989. Prevalence of helminth infections in swine in Denmark. Veterinary Parasitol0g.v 33: 231-239. Roepstorff A. & Murrell K. D. 1997. Transmission dynamics of helminth parasites on continuous pasture: Ascaris suum and Triehuris suis. International Journal for Parasitology 21: 563-572. Roepstorff A. & Nansen P. 1994. Epidemiology and control of helminth infections in pigs under intensive and nonintensive production systems. Veterinary Parasitology 54: 69985. Roepstorff A. 8~ Nansen P. The Epidemiology, Diagnosis and Control of Helminth Parasites of Swine. A FAO Handbook. FAO. Rome. In press. Roepstorff A. & Bjrarn H. & Nansen P. 1987. Resistance of Oesophagostomum spp. in pigs to pyrantel citrate. Veterinary Parasitology 24: 2299239. Roepstorff A., Bjnrrn H., Nansen P., Barnes E. H. & Christensen C. M. 1996. Experimental Oesophagostomum dentatum infections in the pig: Worm populations resulting from trickle infections with three dose levels of larvae. International Journalfor Parasitology 26: 399408. Roepstorff A., Jorgensen R. J., Nansen P., Henriksen S. Aa., Skovgaard Pedersen J. & Andreasen M. 1992. Parasitter hos okologiske svin, pp. 1-36. Landsudvalget for svin, Danske Slagterier, Copenhagen. Rose J. H. & Small A. J. 1980a. Transmission of Oesophagostomum spp. among sows at pasture. Veterinary Record 107: 223-225. Rose J. H. & Small A. J. 1980b. Observations on the development and survival of the free-living stages of Oesophagostomum dentatum both in their natural environments out-of-doors and under controlled conditions in the laboratory. Parasitology 81: 507-517. Rose J. H. & Small A. J. 1982. Observations on the development and survival of the free-living stages of Hyostrong.ylus rubidus both in their natural environments outof-doors and under controlled conditions in the laboratory. Parasitology 85: 33343. Rose J. H. & Small A. J. 1983. Observations on the effect of anthelmintic treatment on the transmission of Hyostrongylus rubidus and Oesophagostomum spp. among sows at pasture. Journal of Helminthology 57: l-8. Rosenorn S. & Lindhardt K. 1994. Dansk vejr i 100 dr, pp. 1-213. Lademann a/s, Copenhagen.
A. ROEPSTORFF*
and K. D. MURRELLt
Danish Centrefor Experimental Parasitology, Royal Veterinary and Agricultural Bdowsoej 13, DK-1870 Frederiksberg C, Copenhagen, Denmark i Receirred 31 May
1996: accepted
13 .Iunuary
Uniwrsitt:.
1997 I
Abstract-Roepstorff A. & Murrell K. D. 1997. Tra osmkion dynamics of Hmintb parasites on conthous psrshw: Oesop&ugostomum dentatum and Hyostrongylus rnbidks. Intern&owl Jotunalfor Pwaait~~ 27: 553462. An increase ia alternative outdoor pig production systems is occurriag ~JI Denmark, aedtlrlsshidy elncidate tbe tranwissi on patterns of Oesophagostkmwm dentutnm and Hyo8hmgyks r&k uousiy on a pasture. A groop of inoculated witb low numbers of both be tbs.Tbescp@were I by serial necropsy and samplkg of faeces, gram pad so& A no of pigs was similarly followed on the same pasture in Year 2 belminth-naHe tracer pigs during all seasons. Tbe pasture mmdting ia a dirt lot by tbe autumn of Year 2. Tbe area was soon coat pasture infectivity and increasing worm burdens in late summer;tbanthefm&ersnfl In May of Year 2, newly exposed pigs became only lightly infected (mostIy 0. datetwm),andIWtr was oherved in Joly-August of Year 2, probably due to an unua%mRy dry summer aml a hck of vegetation. The IWO&S indicate that both 0. dentutum and H. rubidw 8~ v factors, because sigsdlicant transmission occurred only under tbe most favoura with protecting vegetation as in Year 1). Transmission was r of Year 2, wbaa experienced in tbe winter between Years 1 and 2 and during tbe dry lad&g. Co&suo~ grazing actually reduced tr&ssion of 0. dentotum aad H. rt&idw IM?CS&W ofwz reducth ia v&n. Tbii, bowever, is not a de&able alternative farming system, becmme of its adverse emkmmental effects. This environmental impact may be mitigated by employment of a pasture rota&m system in place of continuous grazing. 0 1997 Australian Society for Parasitology. Pwblisbed by Elsevier Science Ltd. Kyv
words:
Oesc~phagostomum
duntutum;
Hwsrrongylus
ruhidus;
pigs: transmission; pasture infectrviiy:
tracer pigs.
are reared indoors, as a result of the marked intensification of modern production systems. Therefore. current parasite control strategies have been developed from research on parasite occurrence and transmission in pigs reared in these confined. indoor systems, and today only Ascaris suurn and &sophugostomum spp.are common in the Danish indoor pig-industry (Roepstorff & Jorsal, 1989). WWstrongylus rubidus was previously common in grazing Danish sows (Jacobs & Andreassen, 1967), and severe clinical outbreaks were encountered (Larsen. 1967).
INTRODUCTION
Gastrointestinal parasitism in domestic pigs is determined largely by their housing facilities and management systems (reviewed by Roepstorff & Nansen, 1994). Currently, almost all pigs in northern Europe
*To whom correspondence should be addressed. Tel: + 45 35 28 21 75: Fax: f45 35 28 2114; E-mail: [email protected]. tPresent ARS-IJSDA,
address: Beltsville Beltsville. MD
Agricultural Research 20705, U.S.A.
Center.
553
A. Roepstorff and K. D. Murrell
554
but with the decline of grazing, this parasite has not been observed in recent years. More recently, in response to animal welfare and agricultural sustainability concerns, an increase in alternative pig production systems is being experienced. These alternative systems usually include production in extensive free-range grazing conditions, especially for sows and piglets. It is clear from published research that access to outdoor facilities increases the risk of infection with helminths (e.g., Pattison et al.. 1980; Alfredsen, 1983; Biehl, 1984; Mercy et a/., 1989). Swine parasitism may increase, therefore, along with the growth in extensive pig production systems. This has raised the question of whether our understanding of the ecology of pig parasites, derived from research on confined pigs, is relevant to outdoor pasture and pig lot environments. Only a few studies have been carried out on outdoor transmission patterns of porcine helminths (e.g., Rose & Small, 1980a, 1983; Burden & Hammet, 1979; Dangolla et al., 1994). Although all of these studies documented high transmission rates on outdoor rearing systems, no serious parasitic infections were found in a recent study in 10 large outdoor sow herds in Denmark (A. Feenstra, personal communication, 1996) and, with the exception of A. suum, this may also be true of some organic farming systems (Roepstorff et al., 1992). This study was designed to elucidate the transmission patterns of 0. dentatum and H. rubidus in pigs allowed to graze continuously on a pasture. Both species are transmitted as free-living infective (L,) larvae and they represent potentially serious parasite threats to pigs raised in outdoor systems. A concurrent study was also conducted on A. suum and T. suis, parasites that are transmitted as infective eggs; the results of that project are reported separately (Roepstorff & Murrell, 1997). MATERIALS
AND
METHODS
This study was designed to evaluate parasite transmission in a continuous grazing system over a 3-year period (Year 1, 1993; Year 2, 1994; Year 3, 1995). Experimental pigs. Danish Landrace/Yorkshire/Duroc crossbred female pigs were obtained from a specific-pathogen-free (SPF) herd. Previous repeated faecal examinations had indicated that the herd was helminth free. No excretion of eggs was observed in the pigs selected for the study. Nevertheless, all pigs were routinely dewormed with 4% Panacur@ (5 mg fenbendazole kg-i body weight mixed in the fodder on both of 2 consecutive days) immediately after turnout on to the study pasture. Feeding. The pigs were fed a standard feed regimen and the ration was adjusted every second week according to body weight. The diet consisted of ground barley. soy meal, minerals and vitamins.
Parasites. The EH-strain of 0. dentarm was originally isolated from a Danish indoor sow herd (Roepstorff et al., 1987). and has been maintained by serial passage in helminthnai’ve pigs. The infective larvae were isolated from a faecesvermiculite culture (room temperature), and stored at IO’C until use. The strain of H. rubidus employed was received from Dr D. Barth of MSD, Hannover, Germany, in 1990. Serial passages and larval culture methods were similar to those for 0. dentatum. Experimentalpastures. The experimental pasture (8000 m’) was utilized for continuous grazing and was purposely contaminated with parasites as described below. Two other pastures, maintained in helminth-free conditions (No. I, 4000 m’; No. 2, 3200 m’) were also used for the study. These pastures were used to hold parasite-free pigs prior to their introduction to the experimental pasture. On each pasture, the pigs had access to an insulated house, shade (trees or an open shed) and a wallowing facility. Previously, the pastures had been grazed for many years by cattle. The soil was light, sandy and well drained, with a high humus content. The vegetation was chiefly Stellaria media and the grasses Festuca rubra, Loliumperenne, Poa trivialis and Elytrigia repens. Care was taken to prevent transfer of infective eggs/larvae from the experimental pasture to the clean pastures by using separate tools, boots and requiring all personnel to change protective clothing when moving from the experimentally contaminated pasture to clean pastures. Climate. The precipitation was recorded daily and the temperature continuously at 2-m height at a weather station 500m away (at Department of Agricultural Sciences, Section of Soil and Water and Plant Nutrition, Royal Veterinary and Agricultural University). Experimental protocol. pigs). Twenty-eight pigs
Group
I (continuously
exposed
(20 kg mean body weight) were turned out on the experimental pasture at calendar week 18 (early May) of Year 1. All these pigs were inoculated twice by stomach tube with the following numbers of infective larvae: 500 larvae of 0. dentatum (lgand 19 May, week 20) and 2500 larvae of H. rubidus ( 15 and 16 June. week 24). The pigs were also inoculated with 200 eggs of As&is suum (week 20, mid-May) and 1000 eggs of Trichuris suis (week 28, midJuly), as reported elsewhere (Roepstorff & Murrell, 1997). Subsets of 3 Gr. 1 pigs were removed from the pasture for parasite analysis at weeks 24, 28, 32 and 36. The remaining 16 pigs were necropsied at week 40 (early October). This protocol ensured the presence of a large group of pigs on the experimental pasture during the whole summer of Year 1. Group 2 (tracer pigs). Fifteen pigs (20 kg mean body weight) were turned out on a clean pasture (No. 1) at week 18 (early May), Year 1. Thereafter, subsets of 3 tracer pigs were moved to the experimental pasture every fourth week starting at week 22 (early June) and ending in week 38 (late September). All subsets of tracer pigs were exposed to pasture contamination for 14days, which ensured that the parasites picked up by the tracer pigs would not reach patency before removal of the pigs to helminth-free indoor facilities for an additional 4 weeks prior to necropsy. To avoid fighting, when the tracer pigs were introduced to the continuously exposed pigs, both groups of pigs (Gr. 1 and the introduced tracer pigs) were sprayed with a commercial repellant on their skin for the first 34 days after transfer. Because 1 strongyle egg was found in a tracer pig on clean pasture No. 1 in August 1993. the tracers were immediately dewormed twice with fenbendazole (recommended dose rate), and thereafter all tracer pigs were routinely dewormed every 4th week (not
0. dentatum
and H. rubidus
less than 7 days prior to being moved to the experimental pasture). Group 3 (tracer pigs). Twenty pigs (21 kg mean body weight) were placed on the second clean pasture (No. 2) in weei< 41 (mid-October) of Year 1. Subgroups ofat least 3 pigs were treated according to the same protocol followed for the Gr. 2 tracers, starting in week 43 (late October) and continuing to early April (exposed in weeks 13-15). There were no permanently exposed pigs on the experimental pasture during the winter period between Years 1 and 2. Group 4 (continuously exposed pigs). In Year 2 (1994) 28 pigs (19 kg mean body weight) were allowed to adjust to the outdoor environment for I week on clean pasture No. 2, before being moved to the experimentally contaminated pasture at week 21 (late May). They were kept on this pasture until necropsied as subgroups of 3 pigs at weeks 27, 31, 35 and 39. The remaining 16 pigs were all necropsied at week 43 (late October). Group S (tracer pigs). Twenty-one pigs were placed on clean pasture No. 2 at week 20 (mid-May), Year 2; their mean body weight was 30 kg, somewhat larger than the Gr. 4 pigs. Subsets of at least 3 pigs followed the Gr. 2 tracer pig protocol, starting at week 21. Group 6 (tracer pigs). A single group of 3 tracer pigs (37 kg mean body weight) grazed a IOOO-m2 enclosure within the experimenral pasture during week 27-29 (mid-July) of Year 3. Sampling. Faecal samples were collected rectally from the exposed pigs (Gr. 1 and 4) and all exposed tracer pigs every 2nd week. Samples were made every 4th week from tracer pigs held on the clean pastures. All pigs were weighed every 4th week. Herhage larvae counts. By walking a W-route across the pasture, grass samples (200-300 g) were collected randomly from the experimental pasture every week (summer) or every 2nd week (winter). Similar sampling was carried out on the clean pastures every 4th week. As the vegetation cover gradually diminished, grass samples were supplemented with soil samples (200-300 g) collected in the same manner. After late autumn of Year 1 no vegetation remained on the experimental pasture and it was possible only to sample the soil. Parasitological techniques. Faecal egg counts were carried out by the McMaster concentration technique (sensitivity: 2Oeggsg. ’ (epg)), described by Roepstoti & Nansen (in press). All faecal samples that contained eggs of 0. dentaturn/H. rubidus were cultured for 8 days in vermiculite (room temperature) and 200 L3 were identified to species (Ahcata, 1935). The fractions of 0. dentatum and H. rubidus eggs were multiplied by the total strongyle epg to calculate the faecal egg counts for the 2 species. The agar method of Mwegoha h Jorgensen (1977) was used to recover infective larvae from grass samples. After sieving and sedimentation of the soil samples ( N IOOg), the sediment was mixed with agar and thereafter treated as for grass samples. Necropsy. The pigs were not fed on the day of slaughter. They were euthanized using a captive bolt pistol or by CO, suffocation followed by exsanguination, and the stomach and the large intestine were immediately removed. Aliquots of 209.6 of the contents were washed with a jet stream of water on a 56-pm (stomach) or a 212~pm (large intestine) sieve. The mucosa was scraped off the stomach wall, digested in pepsin.-HCI (1% pepsin, pH w 1.5) in a stomacher apparatus (30 min at 4o’;C) and subsequently sieved (38 pm). The large intestinal wall was incubated in 0.9% NaCl (1 million i.u. benzylpenicillin potassium and 1 g dihydrostreptomycin
transmission
in pigs on pasture
was added per 10 1) at 38°C overnight; the mcubation thud was then sieved (20pm). All samples were fixed with iodine (80 g iodine and 400 g potassium iodinein 800ml dist. water) and decolorized with sodium thiosulphate immediately before counting under a stereomicroscope. Calculations. All calculations were performed SAS’” Release 6.04 software package.
using
the
RESULT Climate
The weekly rainfall and daily mean temperature data are shown in Fig. 1, together with mean ctimate data from Denmark (from Rosensrn & Lindhardt, 1994). The early summer of Year 1 (1993) was warm and dry, while the late summer and autumn were cold and wet. There were prolonged freezing periods in the winter between Years 1 and 2. The spring and early summer of Year 2 were colder than normal. However. the summer of Year 2 was much hotter and drier than normal (20 mm rain in 8 weeks), and an almost similar drought period was experienced in Year 3.
The pastures
The experimental pasture changed radically during the summer of Year 1. At turn out in May, the beight of the vegetation was - IOcm, but as the pigs rooted they turned over the upper 1Ocm of soil. Thus. by August approx. two-thirds of the pasture was denuded of vegetation. By the time Gr. 3 was introduced on the pasture (early October), all the original vegetation had been destroyed, and only a few young seed plants were present. In Year 2, the experimental pasture consisted only of bare soil. Because of the lack of rooting by pigs new vegetation had appeared by July of 1995 (Year 3). when the final tracer pigs were introduced.
The pigs
Despite the abrupt change in the environment of the pigs at turn out, the only adaptation problem observed was sunburning (especially in Gr. 1 and 2), but this did not seem to influence food intake or common well-being. No other clinical symptoms were observed. The use o,f repellants did not completely prevent fighting when tracer pigs were introduced to the continuously exposed pigs on the experimental pasture, and although all tracer pigs used the common feeding area and housing together with the other pigs, they were frequently observed grazing separately.
A. Roepstorff
556
and K. D. Murrell
JFMAMJJASONDJFMAMJJASONDJFMAMJJAs
YEAR
1 (1993)
YEAR 2 (1994)
Fig. 1. Climate records measured at Department of Agricultural Science, RVAU, pasture. Curve (left v-axis): daily mean air temperature (“C). Columns (right y-axis): are plotted comparable average climate data measured in Denmark
Transmission of 0. dentatum The Gr. 1 pigs, inoculated with 2 x 500infective larvae at week 20, yielded a mean of 949 adults when worm recoveries were carried out 4 weeks later (Table 1). Thereafter, intestinal worm counts increased to 26 500 by early September, then dropped to 10800 worms in early October. The largest numbers of immature worms were found at weeks 32 and 36 (3200 and 9700 larvae per pig, respectively, corresponding to 67% and 37% of the total worm burdens); this coincided with the greatest pasture contamination (see below) and preceded the large increase in total worm counts in September. The mean faecal egg count (Fig. 2) of the Gr. 1 pigs increased rapidly to > 3000 epg by week 24; it then fluctuated between 3000 and 5500 epg until all the pigs were slaughtered. The epg counts did not correspond to the actual worm burdens. The Gr. 2 tracer pigs became infected when first exposed to the experimental pasture at weeks 2628, i.e. approximately 4weeks after the initial contamination of the pasture by the Gr. 1 pigs and 6-8 weeks after the initial larval inoculation of the Gr. 1 pigs. The highest level of transmission apparently occurred in early August; tracers from that period harboured the highest worm burden (- 21000 worms). In comparison, the Gr. 2
YEAR 3 (1995) approx. 500m from weekly precipitation from 1961 to 1990.
the experimental (mm). Overlying
tracers, grazing in early September, harboured only -800 0. dentatum, indicating that transmission had decreased drastically by late summer. The winter tracers (Gr. 3) acquired very low numbers of larvae; only CL30 worms resulted from exposure in the MarchApril period. The new group of continuously exposed pigs in Year 2 (Gr. 4), turned out on the experimental pasture in week 21 (late May), quickly acquired low numbers of 0. dentatum; the first positive egg counts were observed by 4weeks after exposure to the pasture. The relatively low worm burdens peaked in July-August (only -50 worms), indicating that no transmission had occurred after the Year 1 overwintering larvae had disappeared from the pasture. Similarly, the Gr. 5 tracer pigs acquired only 5-35 0. dentatum in late May and none in June and July. In the August-October period, O-60 larvae were picked up by the tracers. Just 1 single worm was recovered in the 3 tracers exposed in the following summer of Year 3 (1995). The results of the grass and soil samples are shown in Fig. 3. In Year 1, the number of infective larvae kg-’ dry grass increased to l&30 000 by week 29 (late July), with a peak of nearly 60 000 kgg ’ in week 32 (midAugust). Because of pig rooting activity it became
0. dentatum
and H. rubidus
transmission
in pigs on pasture
c
t4
0
0
09
w
00
e
(x10,000)
L3 per kg wet soil (x100)
P
h,
L3 per kg dry grass
=;
WI
G
o\
0. dentatum
and H. rubidus transmission in pigs on pa:,ture
impossible thereafter to collect grass, and sampling was stopped by early October. Sampling of soil showed that the number of infective 0. dentatum larvae decreased from 130-l 200 per kg in the autumn (September-October) to O-50 per kg in the winter period. Low numbers of 0. dentatum larvae (O68 kg ’ soil) were recovered in the early summer (weeks 21-28) of Year 2, i.e. during the first 7 weeks after turnout of the Gr. 4 pigs. No larvae were found in mid July-early September, i.e. during the drought of that year (Fig. 1). The soil again yielded low numbers of infective larvae in late September and October. Transmission qf‘ H. rubidus The pigs of Gr. 1, inoculated with 2 x 2500 infective larvae of H. ruhidus at week 24 (Year 1) yielded a mean of 275 stomach worms, when worm recoveries were carried out 4weeks later (Table 2). The mean worm burden of Gr. I increased thereafter to 2560 by early September (week 36), and subsequently dropped to 1200 in early October (week 44). The faecal egg counts increased to > 800 epg 4 weeks after the initial infection. and then fluctuated between 150 and 1000 epg. The egg excretion rates did not appear to be related to the adult worm burdens. None of the Gr. 2 tracer pigs became infected with H. rubidus until early August, when a mean of 1262 worms was recovered (Table I!). Thereafter. transmission decreased drastically; only 25-125 and O-30 larvae, respectively, were acquired by the tracer pigs 4 and 8 weeks later. This reduction in transmission continued into the winter period. Tracers of Gr. 3 yielded a total of only 1 worm from the last 3 sets of tracers exposed in JanuaryApril (Year 2). Very few H. rubidus were found in the continuously exposed Gr. 4 pigs in Year 2, and none was found in the tracer pigs of Gr. 5 and 6, which were exposed on the pasture from July-November (Year 2) and August (Year 3). The grass samples (Fig. 4) yielded fluctuating numbers of infective H. rubidus kgg dry grass from August to early October 1993 (Year 1). with 1 sample (mid-August) containing 4600 L3 per kg. H. rubidus larvae were recovered from soil only in May of Year 2. DISCUSSION
The present study showed that both species of nematodes were able to be transmitted effectively to grazing pigs, and that their individual transmission dynamics and the impact of environmental factors on their transmission patterns were quite similar. Because tracer pigs acquired infections when grazing the experimental pasture approximately 4 weeks after the first egg contamination of the pasture (68weeks after
inoculation of the Gr I pigs), a short larval gene] .ttion time on pasture occurs during summer. The direct estimations of larvae on grass and soil, as well as the data on transmission of infective larvae to tracer pigs. support this. These data also reveal that the numbers of infective larvae of both species on the pasture. after reaching high levels in August, experienced a sudden decline in September. The decline in larval availabiiitt at the end of the summer is also seen in the drop (> 50%) in worm burdens in the continuously grazed pigs (Gr. 1) after early September. Because a ?tricL relationship between pasture contamination and acquired worm burdens is often lacking (Kendall & Small, 1974~ Rose & Small, 1980a), the present reductions in worm burdens may also reflecr the importance of dose-dependent regulatory tnechanisms, as has been proposed previously for 0. cii~orum by Christensen et al. (1995) and Roepstortf cjt tri. (1996), and for H. rubidus by Kendall & Small ( I974b ). The natural regulation of the host’s worm populations is likely the result of multiple factors, including both host-mediated regulation and the regulation o!‘ the free-living larval population by biotic and abiotic: factors. The free-living stages of 0. denrutum and tf. ruhi&s have nearly identical temperature and r&live humidity requirements, as has been demonstrated in controlled laboratory experiments (Fossing (‘I trl.. 1995; Rose & Small, 1980b, 1982). These studies also documented the great similarities between the species with regard to developmental rates and survival profiles on pastures. Rose & Small (1983) reported identical pasture contamination patterns for both parasites in a commercial herd. in which the grass contamination reached maximum in late summer. a pattern also observed in the present study. Our results demonstrate that although both species experrence considerable mortalny. coinciding with falling temperatures and the disappearance of the vegetation. a few larvae of both species can overwinter, as previously observed in tt-ie mild winters in England (Rose & Small. 1980b, 1982) and in the cold winters rtr Germany for 0. dwltatum, (Haupt, 1969). M. N. Larsen (M.Sc. thesis, University of Copenhagen, 1996) showed that a Iew (< 1%) 0. dentotum and If. ruhidus L3 could develop in faeces buried in bare sot I, but almost none survived when the faeces were &aced in short grass and exposed to the sun and the wind. The number of 0. cietztatum larvae in grass samples taken in the late summer of 1994 (Year 2) was roughlv 10 x higher than previous reports of herbage recoveries for this species (e.g., Kendall & Small, 1974~ Rose &Small, 198Oa, 1983; Nansen et al., 1996). An expianation may lie in the fact that the samples of sparse and low vegetation from the well-grazed pasture III
(6 July) (3 Aug.) (3 1 Aug.) (28 Sept.) (26 Oct.)
27 31 35 39 43
3 3 3 3 16
3 3 3 3 16
No. of pigs
Hyostrongylus
worm
0 0 0 0 0
0
(-)
4
0 214 161 2345 1138
Adult
(the pigs were necropsied
C-1 C-1 ;I;
Group
Group 1 C-1 (Cl) (81-258) (136255) (O-278)
pigs
4 weeks
Year
3
Year 2
2
after exposure).
Year
1
l-Year
Year (-) (200-316) (88-217) (8243720) (11&3130)
H. rubidus
counts (mean and min.-max.) for all groups of continuously in week 24 (total of 1000 L,) to initiate parasitic contamination
Continuously exposed Immature H. rubidus
rubidus
“Week of necropsy. hWeeks of exposure to the contaminated pasture ‘Nearly all worms in the tracer pigs were adults.
(15 June) (13 July) (10 Aug.) (7 Sept.) (5 Oct.)
(date?
Week
24 28 32 36 40
Z-The
Table
(date)b
27-29
Apr.)
July)
June) July) Aug.) 1 Aug.) Sept.) Oct.)
Mar.)
May-8 June-6 July-3 Aug.-3 Sept.-28 Oct.-26
(5 July-19
21-23 (25 25-27 (22 29-3 1 (20 33-35 (17 37-39(14 4143 (12
13-15 (29 Mar.-l2
Group 3
3 3 3 3 5 4
Group
5
3
Group 3 3
7-9 (15 Feb.-l
Nov.) Dec.)
Group 3 3 3 3 3
6
5
3
2
0
0 0 0 0 0 0
0
2
17 0
12 0
0 0 1262 62 15
Total
infected
(-
(-) (--) (-) (-) (-) (-)
C-)
(O-5)
(040) (-1
(O-25) (-)
)
(-1 (-) (9241833) (25-l 15) (O-30)
H. rubidus’
1 pigs were experimentally
Tracer pigs No. of pigs
3 3
(26 Oct.-9 (23 Nov.-7
(1 June-l 5 June) (29 June-l 3 July) (27 July-10 Aug.) (24 Aug.-7 Sept.) (21 Sept.-5 Oct.)
of exposure
51-l (21 Dec.4 Jan.) 3-5 (18 Jan.-l Feb.)
4345 4749
22-24 26-28 30-32 34-36 38-40
Weeks
exposed pigs and tracer pigs. The Cr. of the experimental pasture
5 t: E
b
E
? ia F1 P 2 ;i 6
0. denfatum
and H. ruhidus
transmission
in pigs on pasture
GRASS
0
20
24
28
32
36
40
44
48
52
Week Fig. 4.
RIIIIII~~~~~1rl--4
8
number
12
16
20
24
28
32
36
40
Y’EAR 2
Numbers of infective larvae of H. rubidus in grass ( x , per kg dry weight. left v-axis) and in soil (0. per kg wet weight, right y-axis) of the experimental pasture. Note that the right and left !I-axes have different scales.
this study had relatively more stems close to the ground than did samples collected on a pasture with high vegetation, as was typical of the previous studies. Close proximity to the soil surface may facilitate translation of infective larvae from soil to vegetation. The tracer animal technique for assessing the risk of infective helminth transmission has proved to be extremely valuable in ruminant livestock studies (e.g., Armour & Ogbourne, 1982). The use of tracer pigs in the present study also allowed the determination of transmission patterns of parasites in a continuously grazed pasture system. Because the tracer pig worm acquisition data related well to the results from grass/soil sampling, it may be concluded that the tracer principle is also valid for pig studies, although the hierarchical structure of groups of pigs, leading to different grazing behaviour of introduced tracer animals, might have some influence on the results. The overall results of the present study showed that 2 of the potentially most important helminths for alternative outdoor swine rearing systems have similar epidemiological characteristics, and they provide an opportunity to design a control strategy suitable for both of these species, which are transmitted as infective larvae. 0. dentatum and H. rubidus are most actively transmitted when temperatureand moisture in the parasite’smicroclimate are high. and most
adverselyaffected by dry summerson bare soil and during the winter period. A managementschemethat can take advantageof theseenvironmentafinfluences will yield pig rearing systemsthat can minimtze the useof anthelminthics.assuggestedfor ruminants by Nansen(1987);suchmanagementwill beof particular importancefor organicpig rearing.However, the pasture systememployedin this study (continuousgrazing) is not a desirablealternative, despite the fact that both 0. dentatum and H. rubidus were almost eradicated.Under heavy rooting pressure,the soil is completely denudedof vegetation, rendering it vulnerable to erosion; surfacerun-off of animal waste, and the risk of ground water contaminationmay also be increased.Furthermore. the dirt lot did not totally prevent helminth transmission,becauseboth A. suuyll and T. suiswere transmitted very effectively during the dry summerof Year 2 (RoepstorlT& Murrell. 1997). For these reasons,consideration should be given to an outdoor rearingsystemthat includespasture rotation schemes (Roepstortf& Nansen.1994).
Acknowledgements-Technicians Niels Mtdtgaard and Jsrgen Nielsen are kindly acknowledged for taking good cam of the pigs on the pasture. Marlene Sorensen, Niels Peter K. Hansen and Birgitte Ssnderby are thanked for their tcchnrcal
562
A. Roepstorff
assistance in the laboratory. The Department of Agricultural Sciences. Section of Soil and Water and Plant Nutrition, The Royal Veterinary and Agricultural University. is acknowledged for the climate data registrations. The Danish Veterinary and Agricultural Research Council (grant 13-483 I1) and the Danish National Research Foundation provided financial support.
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