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The epidemiology of helminth infections of growing sheep in Argentina's western pampas
Internarionol
Pergamon 002CL7519(94)00122-7
Journalfor Parawology, Vol. 25. No. 4, pp. 489494, 1995 Copyright 0 1995 Australian Society for Parasitology Elsevier Science Ltd Pnnted in Great Britarn. All rights reserved 0020-7519195 $9.50 + 0.00
The Epidemiology of Helminth Infections of Growing Sheep in Argentina’s Western Pampas V. H. SUAREZ* INTA,
Estacidn Experimental (Received
and M. R. BUSETTI
Agropecuaria Anguil, CC II, 6326 Anguil, La Pampa, Argentina 9 November
1993; accepted
17 June 19941
AbstractSuarez V. H. and Busetti M. R. 1995. The epidemiology of helminth infections of growing sheep in Argentina’s western pampas. International Journal fir Parusifofogy 25: 489294. Seasonal population trends of helminth parasites of growing lambs were investigated over 4 years. Successive worm-free lambs were grazed together with untreated lambs for 20-30 days and then slaughtered for helminth counts 2 weeks after their removal from pasture. Likewise untreated lambs from the same flock were slaughtered from 2 to 15 months of age for worm counts in the same way as tracer lambs. The predominant parasites were Haemonchus, Nematodirus and Triehostrongyfus. Haemonchus contortus was found to be of major importance and the flock aquired massive worm burdens from summer to mid-autumn. Minimum burdens were seen from winter to early spring and maximum L4 stages were found from mid-autumn to early winter. Nematodirus burdens increased from December with a peak in late summer and then decreased to low values. Maximum larval availability was in autumn to early winter. Trichostrongylus (mainly T. colubriformis) populations increased in autumn and peaked in June-July, while the highest larval availability was in autumn. The minor genera recovered were Ostertagiu, Cooperia, Trichuris, Oesophagostomum and Moniezia. Dictyocaulus, Chabertia and Teludorsagia were noted occasionally and liver flukes were not detected. No important and pathogenic numbers of L4 stages were seen and all predominant species were able to survive over summer or winter in pasture. Key words:
sheep; nematode infection; tracer lambs; Argentina
has been well documented (Armour, 1980; Brunsdon, 1980). However, in the Argentina Pampeana region very few studies (Rosa, Lukovich & Niec, 1971; Suarez, 1986) are available on sheep nematode epidemiology. The use of worm-free lambs as tracer animals has been a good method to determine inhibition patterns and the availability of nematode larvae on pastures (Southcott, Major & Barger, 1976). Correlations of tracer data with worm burdens in animals exposed continually to infestation have been satisfactorily utilized for epidemiological surveys (Muller, 1968; Anderson, 1972). Therefore, the purpose of the current study was to define over 4 years the seasonal changes of prevalent parasitic populations of a growing flock and the
INTRODUCTION
Nematode infection is one of the major causes of economic losses in sheep production (Coop & Angus, 1981). This has been demonstrated (Suarez, unpublished results) in the semi-arid Pampeana region of Argentina by frequent outbreaks of clinical parasitosis, chiefly with Haemonchus contortus and by regional studies which compared treated and control animals (Suarez, 1985; Suarez, Larrea, Busetti, Bedotti, Bulman & Ambrustolo, 1990). The importance of knowledge of the seasonal population trends and availability of infective larvae of sheep nematodes for design control programmes
* To whom all correspondence should be addressed. 489
490
V. H. Suarez
seasonal availability from tracer lambs.
of free-living
and M. R. Busetti
larvae as assessed
MATERIALS AND METHODS The study was conducted at Anguil Agricultural Research Station (EEA-Anguil), in the western Pampeana region of Argentina. Data from the following 4 sets of “growing grazing periods” (GGP) of lambs were obtained: 1980/1981, 1981/1982, 1982/1983 and 198611987. Each one had the same length, from 15 October to 30 November of the next year. Animals and experimental pastures. Corriedale sheep and some cross-bred Milchschaf X Corridedale were used in the study. Lambs were born in late winter and weaned in late spring. Pastures were contaminated throughout each GGP by a naturally infected flock, i.e. from October to late December by ewes and their lambs and then from December to late November of the next year by growing weaned lambs from 3 to 14 months of age. Animals were not treated with anthelmintics (except for severely sick lambs). The contaminating flock was grazed on sown perennial pastures (mainly lucerne and weeping lovegrass) or annual crops (oat, common rye, sorghum and aftermath residues) at appropriate stocking rates from 3 to 15 head/ha. Worm counrs. Tracer lambs of 46 months of age, treated with anthelmintics and reared under helminth-free conditions were utilized. One tracer was introduced every month of each GGP to graze alongside the contaminating flock for 20-30 days. After removal, tracers were held in confinement for 2 weeks before slaughter to asess the level of inhibited larval development. Groups of “permanent lambs” (PL) were randomly selected at the start of each GGP from the contaminating flock for sequential monthly slaughter to monitor parasite worm burdens. After removal from pastures the PL were managed in the same way as tracer lambs. Post-mortem procedure. The abomasum and the small and large intestines were examined for worms according to the method of Charles & Baker (1988). After mixing the contents and washings of the abomasum or small intestine,
5% of each sample was examined in 5 aliquots. The abomasal and intestinal mucosa was subjected to immersion in warm normal saline solution according to the procedure of Jackson, Jackson & Smith (1984). Lungs were
examined according to Oakley (1980). Mature worms, early fourth-stage larvae (EL4), and developing fourth-stage larvae (DL4) were identified using the descriptions of Douvres ( 1957). Faecal examination. Faecal samples were collected monthly from 15 sheep of the contaminating flock for worm egg counts (Roberts & O’Sullivan, 1949). Meteorological observations. Regional climate was described by Suarez (1990). Averages for rdinfall and temperature were recorded at the experimental site during the study and the mean data over the last 70 years are presented in Fig. 1. Statistical analysis. The significance of differences in seasonal trends of worm burdens of each predominant parasite species within months and between GGP and their interaction was assessed by analysis of variance (SAS, 1988) after transformation of worm counts (x) as y = log 10 (x + 0. 1). EL4 and DL4 counts were pooled with adult worm counts for subsequent analysis. Monthly EL4 percentages were tested by chi-square test.
Table
l-Prevalence
and mean recovered
Genera
Prevalance Flock (PL) Tracer
Haemonchus Nematodirus Trichostrongylus T. axei Ostertagia Teladorsagia Cooperia Triehuris Oesophagostomum Chabertia Dictvocaulus Moniezia Worm
abundance
int
89.4 87.3 83.4 31.5 27.3 1.9 18.9 68.4 7.3 3.1 4.2 24.2
91.6 95.8 79.1 29.1 33.3 0.4 6.2
0.4
of helminths
Abundance Flock (PL) Tracer 1891 392 416 147 26 0.2 2.7 3.7 0.3 0.06 0.2 0.2
676 534 118 20 44 0.1 0.2
x 1000
P*rma”ents
J
F
A
MY
J”
J
AQ
s
0
N
(PL)
D
Months
Fig. 1. Maximum (EP. M. Max. Temp.) and minimum (Ep. M. Min. Temp.) monthly mean temperature and mean monthly precipitation (Ep. rainfall) during experimental periods (Ep. ) in relation to 30 years averages (Av.) of the same parameters.
D I SummerF
1 IutumnJ
Fig. 2. Geometric
mean
Spring
1
dnter
I &ring
Mo”ths
numbers of Haemonchus from PL.
recovered
Helminth infections of sheep in Argentina Worm
counts 1
-
SPd”Q
S”iAr I *
DJ
Fig. 3. Geometric mean numbers of Huemonchus from tracer lambs.
recovered
RESULTS
Table 1 presents the overall prevalences and abudances (Margolis, Esch, Holmes, Kuris & Schad, 1982) of genera recovered. The most prevalent species were Haemonchus contortus, Nematodirus spp. (N. spathiger 72%, N. oiratianus IWO, N. abnormalis 12%) and intestinal Trichostrongylus spp. (T. colubriformis 95%, T. vitrinus 5%). Interactions of predominant worm burdens between GGP and month were not found. Total Haemonchus burdens acquired from PL and tracer lambs during different GGPs showed the greatest (BO.008) numbers from January to April. Then the burdens dropped sharply during May and June and minimum acquisition was noted during winter and early spring (Figs. 2 and 3). Occurrence of EL4 stage in tracer lambs reached a peak (X2 = 82, P
counts
h
Fig. 5. Geometric mean numbers of Nematodirus from tracer lambs.
December to a peak level in late summer, thereafter falling progressively and remaining lower (PcO.03) from late winter in older lambs (Fig. 4). The total Nematodirus burdens of tracer lambs reached higher (BO.01) levels in autumn and early winter (Fig. 5). Immature stages predomina;ed over the autumn and early winter months, PL (X- = 158, P
--
ok--,
-71---1I-
Months
D I
Fig. 4. Geometric mean numbers of Nematodirus from PL.
recovered
recovered
SumFmer
I LurnnJ
I
Win*ter
I &ring
Fig. 6. Geometric mean numbers of intestinal strongylus
recovered
from
PL and tracer
lambs.
Tricho-
V. H. Suarez and M. R. Busetti
492
Table 2-Least
square means of each predominant nematode recovered from permanent lambs (PL) and tracers
Genera
Haemonchus Tracer WI
1980181
63" 9oab 209" 907c
1981182 1982183 1986187
55" 474b 504b 611b
Nemaiodirus Tracer (PL) 183" 146" 262" 32b
46" 410b 815b 402a
Trichostrong.vlus PL Tracer 41" 73a 89" 121"
6” 19"b 71b 76b
Different superscripts are significantly different (P < 0.05). summer and early autumn. Insignificant numbers of Chabertia ovina (recovered only from 3 PL) and Dictyocaulus spp. (recovered in 4 PL) were detected during late winter and spring when sheep were grazed permanently on perennial pastures. Moniezia expansa was present thoughout the year, but the highest prevalence occurred between December and April in the youngest lambs. No liver flukes were seen. Table 2 shows the average worm count of each predominant species found in tracer and PL lambs during each GGP. Counts in tracer lambs in the 1980/81 GGP were significantly lower than in other years (P
s 0 N D J F M A M J .I A s 0 NMrJ”fh Spring
1Summw
Autumn
( Winter
1
Fig. 7. Mean egg counts of the contaminating flock (ewes and growing lambs) during each growing grazing period (GGP).
through winter and early spring when Trichostrongylus genus predominated (62%) in faecal cultures. DISCUSSION
Summer-autumn rainfall records of our survey period were higher than expected compared with the 70 year average. Such an increase could have favoured nematode development and could be associated with larger worm counts. The predominant nematodes showed different seasonal patterns. The most important seasonal changes in population structures were detected for Haemonchus contortus. Occurrence of EL4 stages in tracer lambs reached a maximum in autumn to early winter, which was noted previously in adult ewes by Suarez (1986), in sheep studies in Uruguay (Nari, Petraccia, Solari & Cardozo, 1982) and Australia (Southcott et al., 1976). An autumn inhibition of Haemonchus development can be attributed to changes exerted on larvae, induced by the environmental stimulus according to observations in Canada and Great Britain (Blitz & Gibbs, 1972; Connan, 1975). Another factor possibly involved is the degree of host acquired immunity. In our study, the PL group at mid-autumn had an abrupt decrease of egg counts and worm burdens which reflected the development of host resistance to Huemonchus infection. Several studies suggested that the Huemonchus populations were regulated mainly by a larval establishment reduction mechanism (immune exclusion: Miller, Jackson, Newlands & Apppleyard, 1983) and by a loss of established worms, which was related to the current rate of larval intake and to the host’s previous experience of infection (Barger, Le Jambre, Georgi & Davies, 1985). These workers also showed that the proportion of incoming larvae arrested in their development increases as the infection progresses. Tracer lambs results showed pronounced seasonal changes in Huemonchus availability. Larvae were abundant during warmer months, and over the December-March period faecal contamination was
Helminth infections of sheep in Argentina rapidly translated to pastures because Haemonchus, a warm weather worm, seems to parallel temperature changes and rainfall. However, it should be noted that small numbers of Haemonchus larvae were available during winter. Similar patterns for this nematode were shown by Southcott et al. (1976) in a similar rainfall region of Australia, since larval availability increases rapidly in summer and declines slowly in autumn. High percentages of L4 stages of Nematodirus were recovered from PL slaughtered during late autumn and early winter, at a time when their Nematodirus burdens decreased. They can be considered to be inhibited in their development and could be the result of a host immune reaction, at the same time Nematodirus species were picked up by tracer lambs in large numbers (mainly adult stages), in direct contrast to decreasing burdens recovered from PL,. However, whether inhibition was due primarily to immunological or environmental factors cannot be decided by this study. Accumulation of infective larvae on pasture during autumn and early winter could be the result of favourable climatic conditions, the accumulation arising from eggs deposited during the previous summer-autumn. Findings of Gibson (1973) and Southcott et al. (1976) would explain the presence of Nematodirus larvae at these autumn high levels, since these authors showed that considerable time may elapse between egg deposited and the appearance of infective larvae on pasture. Trichostrongylus colubriformis were available for tracers during the 4 seasons, but larvae were more abundant on pasture during autumn when relative humidity was above 60%, which may be associated with the peak of PL infestation seen during early winter. Negligible proportions of larval stages were therefore no arrested recovered, development patterns for T. cofubriformis could be suggested. Relatively few T. axei were recovered from tracers and numbers remained low in PL group, but occurrence was greater when the sheep flock grazed on pastures previously contaminated by cattle, and agreed with previous studies (Suarez et al., 1993). Arundel & Hamilton (1975) showed that in sheep there was an increase in the number of those nematodes normally found in cattle, such as T. axei, when sheep grazed with bovines. Low counts of Ostertagia ostertagi and Cooperia oncophora were always the case. These species are primarily parasites of cattle (Borgsteede, 1981) and disappear when bovines are absent from the GGP systems. Negligible numbers of Teladorsagia circumcincta were recorded in a few lambs in contrast with
493
other findings in the humid Pampeana region of Argentia (Rosa et al., 1971), which suggests that dry late winter conditions may not favour the development of this nematode. The lowest level of total worm numbers recovered from tracers were found during 1981/82 GGP, when the annual pastures (negligible initial contamination) predominated over the perennial pastures in the sheep diet. PL brudens reached the maximum numbers during 1986/87 GGP, when during the whole period sheep grazed on perennial pastures that were contaminated continuously. The post-parturient rise in the faecal egg counts observed in the ewes is a well-documented phenomenon (Armour, 1980) and provides a source of primary infection to susceptible lambs. Previous experiments (Suarez, 1986) showed that this peak results more from increased fecundity of resident worms, or activation of arrested stages. than from increased acquisition of parasites. Results of this study showed that flock acquired massive worm burdens during the warner months of the year from summer to mid-autumn. The major parasite was Haemonchus contortus and to a lesser degree Nematodirus species and Trichostrongylus colubriformis. Ackno~rledgemenfs-The authors offer their sincere thanks to Mr Carlos Garcia, Jorge Gavella, Nelson Zentt and Hector Jordanes for their assistance. We acknowledge the financial help of Merck Sharp & Dohme (Argentina). REFERENCES Anderson N. 1972. Trichostrongylid infections of sheep in a winter rainfall region. I. Epizootiological studies in the Western District of Victoria, 1966-67. Australian Journal of Argriculrural Research 23: 1113-l 129. Armour J. 1980. The epidemiology of helminth disease in farm animals. Veterinary Parasitology 6: 746. Arundel J. H. & Hamilton D. 1975. The effect of mixed grazing of sheep and cattle on worm burdens in lambs. Australian Veterinary Journal 51: 43&439. Barger I. A., Le Jambre L. F., Georgi J. R. & Davies H. I. 1985. Regulation of Haemonchus contortus populations in sheep exposed to continuous infection. International Journal for Parasitology 15: 529-533. Blitz N. M. & Gibbs H. C. 1972. Studies on the arrested development of Haemonchus contortus in sheep. I. The introduction of arrested development. International Journal for Parasitology 2: 5-12. Borgsteede F. H. M. 1981. Experimental cross-infections with gastrointestinal nematodes of sheep and cattle. Zeitschrift fir Parasitenkunde 65: l-10. Brunsdon R. V. 1980. Principles of helminth control. Veterinary Parasitology 6: 185-215. Charles T. P. & Baker N. F. 1988. Seasonal prevalence of gastro-intestinal nematodes of beef calves grazed on
494
V. H. Suarez
irrigated pastures in the lower Sacramento Valley of California. American Journal for Veterinary Research 49: 566-571. Connan R. M. 1975. Inhibited development in Haemonthus contortus. Parasitology II: 239-246. Coop R. L. & Angus K. W. 1981. How helminths affect sheep. In Practice 3: 411. Douvres F. W. 1957. Keys of the identification and differentiation of the immature parasitic stages of gastrointestinal nematodes of cattle. American Journal of Veterinary Research 18: 81-85. Gibson T. E. 1973. Ovine parasitic gastroenteritis. Nematodiriasis In: Helminth Diseases of Cattle, Sheep and Horses in Europe (Edited by Urquhart G. & Armour J.), pp. 49-50. Proceedings of a workshop, University of Glasgow Veterinary School, 9914 April 1973. Jackson E., Jackson F. & Smith W. D. 1984. Comparison of saline incubation and pepsin digestion as methods for recovering Ostertugiu circumcincta larvae from the abomasum of sheep. Research in Veterinary Science 36: 380-381. Margolis L., Esch G. W., Holmes J. C., Kuris A. M. & Schad G. A. 1982. The use of ecological terms in parasitology (Report of an ad hoc Committee of the American Society of Parasitologists). Journal of Parasitology 68: 131-133. Miller H. R. P., Jackson F.. Newlands G. & Appleyard W. T. 1983. Immune exclusion, a mechanism of protection against the ovine nematode Haemonchus contortus. Research in Veterinary Science 35: 357-363. Muller G. L. 1968. The epizootiology of helminth infestation in sheep in the South-Western Districts of the Cape. Onderstepoort Journal of Veterinary Research 35: 159-194. Nari A., Petraccia C., Solari M. A. & Cardozo H. 1982. La inhibition de1 desarrollo larvario en nematodes gastrointestinales de ovinos con especial referencia a Haemonchus contortus. Veterinaria (Uruguay) 18: 78-88. Oakley G. A. 1980. The recovery of Dictyocaulus viviparus from bovine lungs by lung perfusion: a modification of Inderbitzen’s method. Research in Veterinary Science 29: 395-396. Roberts F. H. S. & O’Sullivan P. J. 1949. Methods for
and M. R. Busetti egg counts and larval cultures for strongyles infecting the gastrointestinal tract of cattle. Australian Journal of Agricultural Research 1: 99-103. Rosa W. A. J., Lukovich R. & Niec R. 1971. Parasitism0 gastrointestinal de1 10s ovinos y bovinos en la zona sur de la provincia de Buenos Aires (Tres Arroyos, Cnel. Pringles y Cnel. Dorrego). Revista de Znvestigaciones Agropecuarias, Repdblica Argentina S. 4, Vol. VIII, 3: 71-83. SAS, 1988. Statistical Analysis Institute, language guide for personal computers, 6th Ed. SAS Institute Inc., Cary, NC, 1028 pp. Southcott W. H., Major G. W. & Barger I. A. 1976. Seasonal pasture contamination and availability of nematodes for grazing sheep. Australian Journal of Agricultural Research 27: 277-286. Suarez V. H. 1985 Parasitosis gastrointestinal en ovinos en Region Semiarida Pampeana. I. Corriedale Resultados de1 period0 1981/82. Revista Argentina de Produccidn Animal 5: 3-4: 243-255. Suarez, V. H. 1986. Epizootiologia de 10s parasites gastrointestinales en ovejas en la Region Semiarida Pampeana. Revista de Medicina Veterinaria (Buenos Aires) 67, 4: 190-202. Suarez V. H. 1990. Inhibition patterns and seasonal availability of nematode for beef cattle grazing on Argentina’s Western Pampas. International Journal for Parasitology 20: 1031-1036. Suarez V. H., Larrea S., Busetti M. R., Bedotti D. O., Bulman G. M. & Ambrustolo R. R. 1990. Nematodes gastrointestinales ovinos: Su control y efecto sobre 10s parametros epizootiologicos, hematologicos y productivos en la Region Semiarida Pampeana (Argentina). Therios 73: 156173. Suarez V. H. & Cabaret J. 1992. Interbreeding in the subfamily Ostertagiinae (Nematoda: Trichostrongylidae) of ruminants. Journal of Parasitology 78: 402405. Suarez V. H., Busetti M. R.. Bedotti D. 0. & Fort M. C. 1993. Parasitosis intemas de 10s ovinos en la provincia de La Pampa. Revista de la Facultad de Agronomia (Universidad National de La Pampa, Argentina). 7, 2, in press.
Pergamon 002CL7519(94)00122-7
Journalfor Parawology, Vol. 25. No. 4, pp. 489494, 1995 Copyright 0 1995 Australian Society for Parasitology Elsevier Science Ltd Pnnted in Great Britarn. All rights reserved 0020-7519195 $9.50 + 0.00
The Epidemiology of Helminth Infections of Growing Sheep in Argentina’s Western Pampas V. H. SUAREZ* INTA,
Estacidn Experimental (Received
and M. R. BUSETTI
Agropecuaria Anguil, CC II, 6326 Anguil, La Pampa, Argentina 9 November
1993; accepted
17 June 19941
AbstractSuarez V. H. and Busetti M. R. 1995. The epidemiology of helminth infections of growing sheep in Argentina’s western pampas. International Journal fir Parusifofogy 25: 489294. Seasonal population trends of helminth parasites of growing lambs were investigated over 4 years. Successive worm-free lambs were grazed together with untreated lambs for 20-30 days and then slaughtered for helminth counts 2 weeks after their removal from pasture. Likewise untreated lambs from the same flock were slaughtered from 2 to 15 months of age for worm counts in the same way as tracer lambs. The predominant parasites were Haemonchus, Nematodirus and Triehostrongyfus. Haemonchus contortus was found to be of major importance and the flock aquired massive worm burdens from summer to mid-autumn. Minimum burdens were seen from winter to early spring and maximum L4 stages were found from mid-autumn to early winter. Nematodirus burdens increased from December with a peak in late summer and then decreased to low values. Maximum larval availability was in autumn to early winter. Trichostrongylus (mainly T. colubriformis) populations increased in autumn and peaked in June-July, while the highest larval availability was in autumn. The minor genera recovered were Ostertagiu, Cooperia, Trichuris, Oesophagostomum and Moniezia. Dictyocaulus, Chabertia and Teludorsagia were noted occasionally and liver flukes were not detected. No important and pathogenic numbers of L4 stages were seen and all predominant species were able to survive over summer or winter in pasture. Key words:
sheep; nematode infection; tracer lambs; Argentina
has been well documented (Armour, 1980; Brunsdon, 1980). However, in the Argentina Pampeana region very few studies (Rosa, Lukovich & Niec, 1971; Suarez, 1986) are available on sheep nematode epidemiology. The use of worm-free lambs as tracer animals has been a good method to determine inhibition patterns and the availability of nematode larvae on pastures (Southcott, Major & Barger, 1976). Correlations of tracer data with worm burdens in animals exposed continually to infestation have been satisfactorily utilized for epidemiological surveys (Muller, 1968; Anderson, 1972). Therefore, the purpose of the current study was to define over 4 years the seasonal changes of prevalent parasitic populations of a growing flock and the
INTRODUCTION
Nematode infection is one of the major causes of economic losses in sheep production (Coop & Angus, 1981). This has been demonstrated (Suarez, unpublished results) in the semi-arid Pampeana region of Argentina by frequent outbreaks of clinical parasitosis, chiefly with Haemonchus contortus and by regional studies which compared treated and control animals (Suarez, 1985; Suarez, Larrea, Busetti, Bedotti, Bulman & Ambrustolo, 1990). The importance of knowledge of the seasonal population trends and availability of infective larvae of sheep nematodes for design control programmes
* To whom all correspondence should be addressed. 489
490
V. H. Suarez
seasonal availability from tracer lambs.
of free-living
and M. R. Busetti
larvae as assessed
MATERIALS AND METHODS The study was conducted at Anguil Agricultural Research Station (EEA-Anguil), in the western Pampeana region of Argentina. Data from the following 4 sets of “growing grazing periods” (GGP) of lambs were obtained: 1980/1981, 1981/1982, 1982/1983 and 198611987. Each one had the same length, from 15 October to 30 November of the next year. Animals and experimental pastures. Corriedale sheep and some cross-bred Milchschaf X Corridedale were used in the study. Lambs were born in late winter and weaned in late spring. Pastures were contaminated throughout each GGP by a naturally infected flock, i.e. from October to late December by ewes and their lambs and then from December to late November of the next year by growing weaned lambs from 3 to 14 months of age. Animals were not treated with anthelmintics (except for severely sick lambs). The contaminating flock was grazed on sown perennial pastures (mainly lucerne and weeping lovegrass) or annual crops (oat, common rye, sorghum and aftermath residues) at appropriate stocking rates from 3 to 15 head/ha. Worm counrs. Tracer lambs of 46 months of age, treated with anthelmintics and reared under helminth-free conditions were utilized. One tracer was introduced every month of each GGP to graze alongside the contaminating flock for 20-30 days. After removal, tracers were held in confinement for 2 weeks before slaughter to asess the level of inhibited larval development. Groups of “permanent lambs” (PL) were randomly selected at the start of each GGP from the contaminating flock for sequential monthly slaughter to monitor parasite worm burdens. After removal from pastures the PL were managed in the same way as tracer lambs. Post-mortem procedure. The abomasum and the small and large intestines were examined for worms according to the method of Charles & Baker (1988). After mixing the contents and washings of the abomasum or small intestine,
5% of each sample was examined in 5 aliquots. The abomasal and intestinal mucosa was subjected to immersion in warm normal saline solution according to the procedure of Jackson, Jackson & Smith (1984). Lungs were
examined according to Oakley (1980). Mature worms, early fourth-stage larvae (EL4), and developing fourth-stage larvae (DL4) were identified using the descriptions of Douvres ( 1957). Faecal examination. Faecal samples were collected monthly from 15 sheep of the contaminating flock for worm egg counts (Roberts & O’Sullivan, 1949). Meteorological observations. Regional climate was described by Suarez (1990). Averages for rdinfall and temperature were recorded at the experimental site during the study and the mean data over the last 70 years are presented in Fig. 1. Statistical analysis. The significance of differences in seasonal trends of worm burdens of each predominant parasite species within months and between GGP and their interaction was assessed by analysis of variance (SAS, 1988) after transformation of worm counts (x) as y = log 10 (x + 0. 1). EL4 and DL4 counts were pooled with adult worm counts for subsequent analysis. Monthly EL4 percentages were tested by chi-square test.
Table
l-Prevalence
and mean recovered
Genera
Prevalance Flock (PL) Tracer
Haemonchus Nematodirus Trichostrongylus T. axei Ostertagia Teladorsagia Cooperia Triehuris Oesophagostomum Chabertia Dictvocaulus Moniezia Worm
abundance
int
89.4 87.3 83.4 31.5 27.3 1.9 18.9 68.4 7.3 3.1 4.2 24.2
91.6 95.8 79.1 29.1 33.3 0.4 6.2
0.4
of helminths
Abundance Flock (PL) Tracer 1891 392 416 147 26 0.2 2.7 3.7 0.3 0.06 0.2 0.2
676 534 118 20 44 0.1 0.2
x 1000
P*rma”ents
J
F
A
MY
J”
J
AQ
s
0
N
(PL)
D
Months
Fig. 1. Maximum (EP. M. Max. Temp.) and minimum (Ep. M. Min. Temp.) monthly mean temperature and mean monthly precipitation (Ep. rainfall) during experimental periods (Ep. ) in relation to 30 years averages (Av.) of the same parameters.
D I SummerF
1 IutumnJ
Fig. 2. Geometric
mean
Spring
1
dnter
I &ring
Mo”ths
numbers of Haemonchus from PL.
recovered
Helminth infections of sheep in Argentina Worm
counts 1
-
SPd”Q
S”iAr I *
DJ
Fig. 3. Geometric mean numbers of Huemonchus from tracer lambs.
recovered
RESULTS
Table 1 presents the overall prevalences and abudances (Margolis, Esch, Holmes, Kuris & Schad, 1982) of genera recovered. The most prevalent species were Haemonchus contortus, Nematodirus spp. (N. spathiger 72%, N. oiratianus IWO, N. abnormalis 12%) and intestinal Trichostrongylus spp. (T. colubriformis 95%, T. vitrinus 5%). Interactions of predominant worm burdens between GGP and month were not found. Total Haemonchus burdens acquired from PL and tracer lambs during different GGPs showed the greatest (BO.008) numbers from January to April. Then the burdens dropped sharply during May and June and minimum acquisition was noted during winter and early spring (Figs. 2 and 3). Occurrence of EL4 stage in tracer lambs reached a peak (X2 = 82, P
counts
h
Fig. 5. Geometric mean numbers of Nematodirus from tracer lambs.
December to a peak level in late summer, thereafter falling progressively and remaining lower (PcO.03) from late winter in older lambs (Fig. 4). The total Nematodirus burdens of tracer lambs reached higher (BO.01) levels in autumn and early winter (Fig. 5). Immature stages predomina;ed over the autumn and early winter months, PL (X- = 158, P
--
ok--,
-71---1I-
Months
D I
Fig. 4. Geometric mean numbers of Nematodirus from PL.
recovered
recovered
SumFmer
I LurnnJ
I
Win*ter
I &ring
Fig. 6. Geometric mean numbers of intestinal strongylus
recovered
from
PL and tracer
lambs.
Tricho-
V. H. Suarez and M. R. Busetti
492
Table 2-Least
square means of each predominant nematode recovered from permanent lambs (PL) and tracers
Genera
Haemonchus Tracer WI
1980181
63" 9oab 209" 907c
1981182 1982183 1986187
55" 474b 504b 611b
Nemaiodirus Tracer (PL) 183" 146" 262" 32b
46" 410b 815b 402a
Trichostrong.vlus PL Tracer 41" 73a 89" 121"
6” 19"b 71b 76b
Different superscripts are significantly different (P < 0.05). summer and early autumn. Insignificant numbers of Chabertia ovina (recovered only from 3 PL) and Dictyocaulus spp. (recovered in 4 PL) were detected during late winter and spring when sheep were grazed permanently on perennial pastures. Moniezia expansa was present thoughout the year, but the highest prevalence occurred between December and April in the youngest lambs. No liver flukes were seen. Table 2 shows the average worm count of each predominant species found in tracer and PL lambs during each GGP. Counts in tracer lambs in the 1980/81 GGP were significantly lower than in other years (P
s 0 N D J F M A M J .I A s 0 NMrJ”fh Spring
1Summw
Autumn
( Winter
1
Fig. 7. Mean egg counts of the contaminating flock (ewes and growing lambs) during each growing grazing period (GGP).
through winter and early spring when Trichostrongylus genus predominated (62%) in faecal cultures. DISCUSSION
Summer-autumn rainfall records of our survey period were higher than expected compared with the 70 year average. Such an increase could have favoured nematode development and could be associated with larger worm counts. The predominant nematodes showed different seasonal patterns. The most important seasonal changes in population structures were detected for Haemonchus contortus. Occurrence of EL4 stages in tracer lambs reached a maximum in autumn to early winter, which was noted previously in adult ewes by Suarez (1986), in sheep studies in Uruguay (Nari, Petraccia, Solari & Cardozo, 1982) and Australia (Southcott et al., 1976). An autumn inhibition of Haemonchus development can be attributed to changes exerted on larvae, induced by the environmental stimulus according to observations in Canada and Great Britain (Blitz & Gibbs, 1972; Connan, 1975). Another factor possibly involved is the degree of host acquired immunity. In our study, the PL group at mid-autumn had an abrupt decrease of egg counts and worm burdens which reflected the development of host resistance to Huemonchus infection. Several studies suggested that the Huemonchus populations were regulated mainly by a larval establishment reduction mechanism (immune exclusion: Miller, Jackson, Newlands & Apppleyard, 1983) and by a loss of established worms, which was related to the current rate of larval intake and to the host’s previous experience of infection (Barger, Le Jambre, Georgi & Davies, 1985). These workers also showed that the proportion of incoming larvae arrested in their development increases as the infection progresses. Tracer lambs results showed pronounced seasonal changes in Huemonchus availability. Larvae were abundant during warmer months, and over the December-March period faecal contamination was
Helminth infections of sheep in Argentina rapidly translated to pastures because Haemonchus, a warm weather worm, seems to parallel temperature changes and rainfall. However, it should be noted that small numbers of Haemonchus larvae were available during winter. Similar patterns for this nematode were shown by Southcott et al. (1976) in a similar rainfall region of Australia, since larval availability increases rapidly in summer and declines slowly in autumn. High percentages of L4 stages of Nematodirus were recovered from PL slaughtered during late autumn and early winter, at a time when their Nematodirus burdens decreased. They can be considered to be inhibited in their development and could be the result of a host immune reaction, at the same time Nematodirus species were picked up by tracer lambs in large numbers (mainly adult stages), in direct contrast to decreasing burdens recovered from PL,. However, whether inhibition was due primarily to immunological or environmental factors cannot be decided by this study. Accumulation of infective larvae on pasture during autumn and early winter could be the result of favourable climatic conditions, the accumulation arising from eggs deposited during the previous summer-autumn. Findings of Gibson (1973) and Southcott et al. (1976) would explain the presence of Nematodirus larvae at these autumn high levels, since these authors showed that considerable time may elapse between egg deposited and the appearance of infective larvae on pasture. Trichostrongylus colubriformis were available for tracers during the 4 seasons, but larvae were more abundant on pasture during autumn when relative humidity was above 60%, which may be associated with the peak of PL infestation seen during early winter. Negligible proportions of larval stages were therefore no arrested recovered, development patterns for T. cofubriformis could be suggested. Relatively few T. axei were recovered from tracers and numbers remained low in PL group, but occurrence was greater when the sheep flock grazed on pastures previously contaminated by cattle, and agreed with previous studies (Suarez et al., 1993). Arundel & Hamilton (1975) showed that in sheep there was an increase in the number of those nematodes normally found in cattle, such as T. axei, when sheep grazed with bovines. Low counts of Ostertagia ostertagi and Cooperia oncophora were always the case. These species are primarily parasites of cattle (Borgsteede, 1981) and disappear when bovines are absent from the GGP systems. Negligible numbers of Teladorsagia circumcincta were recorded in a few lambs in contrast with
493
other findings in the humid Pampeana region of Argentia (Rosa et al., 1971), which suggests that dry late winter conditions may not favour the development of this nematode. The lowest level of total worm numbers recovered from tracers were found during 1981/82 GGP, when the annual pastures (negligible initial contamination) predominated over the perennial pastures in the sheep diet. PL brudens reached the maximum numbers during 1986/87 GGP, when during the whole period sheep grazed on perennial pastures that were contaminated continuously. The post-parturient rise in the faecal egg counts observed in the ewes is a well-documented phenomenon (Armour, 1980) and provides a source of primary infection to susceptible lambs. Previous experiments (Suarez, 1986) showed that this peak results more from increased fecundity of resident worms, or activation of arrested stages. than from increased acquisition of parasites. Results of this study showed that flock acquired massive worm burdens during the warner months of the year from summer to mid-autumn. The major parasite was Haemonchus contortus and to a lesser degree Nematodirus species and Trichostrongylus colubriformis. Ackno~rledgemenfs-The authors offer their sincere thanks to Mr Carlos Garcia, Jorge Gavella, Nelson Zentt and Hector Jordanes for their assistance. We acknowledge the financial help of Merck Sharp & Dohme (Argentina). REFERENCES Anderson N. 1972. Trichostrongylid infections of sheep in a winter rainfall region. I. Epizootiological studies in the Western District of Victoria, 1966-67. Australian Journal of Argriculrural Research 23: 1113-l 129. Armour J. 1980. The epidemiology of helminth disease in farm animals. Veterinary Parasitology 6: 746. Arundel J. H. & Hamilton D. 1975. The effect of mixed grazing of sheep and cattle on worm burdens in lambs. Australian Veterinary Journal 51: 43&439. Barger I. A., Le Jambre L. F., Georgi J. R. & Davies H. I. 1985. Regulation of Haemonchus contortus populations in sheep exposed to continuous infection. International Journal for Parasitology 15: 529-533. Blitz N. M. & Gibbs H. C. 1972. Studies on the arrested development of Haemonchus contortus in sheep. I. The introduction of arrested development. International Journal for Parasitology 2: 5-12. Borgsteede F. H. M. 1981. Experimental cross-infections with gastrointestinal nematodes of sheep and cattle. Zeitschrift fir Parasitenkunde 65: l-10. Brunsdon R. V. 1980. Principles of helminth control. Veterinary Parasitology 6: 185-215. Charles T. P. & Baker N. F. 1988. Seasonal prevalence of gastro-intestinal nematodes of beef calves grazed on
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