Post-parturient Rise Research Articles

Peri-parturient rise of Cryptosporidium oocysts in cows: New insights provided by duplex quantitative real-time PCR

In order to clarify if a peri-parturient rise of Cryptosporidium parvum oocysts occurs in cows, faecal samples from 42 cows on two farms were collected. These samples were taken during the pre-parturient, the peri-parturient and the post-parturient periods. Two methods were used to detect the oocysts, a nested-PCR coupled with sequencing and a duplex real-time PCR (qPCR) that quantified Cryptosporidium spp. DNA concentration. The qPCR results were adjusted using a hierarchical Bayesian model taking into account within and between run variation. Generalised Estimating Equation models (GEE) were used to determine if peri-parturient cows were at greater risk of being infected than pre- or post-parturient cows. Fourteen dairy cows exhibited a peri-parturient and post-parturient rise in the excretion of Cryptosporidium spp. oocysts, other than the zoonotic C. parvum. The cows in the suckler beef farm were the only ones infected with the zoonotic species C. parvum at calving. Due to the low concentration of oocysts excreted mainly from species other than C. parvum, it would appear unlikely that cows act as a source of infection for their calves or contribute significantly to environmental contamination.

Quantifying shape of lactation curves, and benchmark curves for common dairy breeds and parities

The MilkBot®(DairySight LLC,Argyle, NY; http://milkbot.com) lactation model provides a means of quantifying both shape and magnitude oflactation curves as a set of parameter values, each of which is associated with a single aspect of lactation curve shape. Lactation data may be fitted to the model to summarize a lactation as a set of parameter values which summarize the lactation as a whole. The scale parameter controls magnitude without changing the shape of the curve, the ramp parameter controls steepness of the post-parturient rise in milk production, the decay parameter controls the rate of late lactation decline, and the offset parameter defines a theoretical offset between the start of milk production and calving. The decay parameter is easily re-expressed mathematically as persistence to quantify the rate of decline in production after peak milk. Time and quantity of peak milk, or production for any day or period in the lactation may be calculated directly from parameter values.
 Aggregate normal lactation curves for mean and median milk production of Holstein, Jersey, Crossbred, Guernsey, Ayrshire, and Brown Swiss dairy cattle, stratified by parity, are calculated from a DHIA data set collected from 2005 through mid-2008 and covering over six million lactations and 51 million milk weights, mainly from farms in the eastern United States. These constitute benchmark curves that may be used as standards for normal milk production, or to quantify changes in normal productivity over time or with respect to other variables, or in econometric studies.

Use of Lotus corniculatus containing condensed tannins to increase lamb and wool production under commercial dryland farming conditions without the use of anthelmintics

Two grazing experiments were conducted for 12 and 13 weeks, respectively, over the spring periods of 2000 and 2001 at Massey University's Riverside farm in the Wairarapa (New Zealand) to compare effects of grazing Lotus corniculatus (birdsfoot trefoil; cv. Grasslands Goldie) or perennial ryegrass ( Lolium perenne)/white clover ( Trifolium repens) dominant pasture during lactation on ewe and lamb live weight (LW), wool production, faecal nematode egg counts (FEC) and dag score. Ewes and their lambs (mainly twins) were rotationally grazed on lotus or pasture ( n = 50) without any anthelminitc treatment at a herbage allowance of 6.5 and 8.0 kg green DM/ewe/days for Experiments 1 and 2, respectively. Total condensed tannins (CT) concentration in the diet selected was 24 to 27 g CT/kg DM for lotus and 1.4 to 1.5 g CT/kg DM for pasture. In vitro organic matter digestibility and metabolisable energy (ME) concentration were higher for lotus than for pasture in both experiments, whilst the concentrations of neutral detergent fibre (NDF) was lower in lotus than in pasture. The LW gain, weaning LW and wool production were consistently higher ( P < 0.001) for lambs grazing lotus, in both Experiment 1 (258 versus 189 g/days; 36.1 versus 30.1 and 1.17 versus 0.98 kg) and in Experiment 2 (247 versus 162 g/days; 31.8 versus 24.1 and 1.17 versus 0.81 kg), respectively. Ewe and lamb dag score were strongly and positively correlated with dag weight ( P < 0.001), and generally increased with time in sheep grazing pasture; while grazing on lotus consistently reduced dag score. FEC in ewes grazing pasture showed a post-parturient rise (PPR) following lambing, whilst ewes grazing lotus had a reduced PPR in FEC. Up to day 70, FEC in lambs grazing lotus was lower than that for lambs grazing pasture, but between day 70 and the end of both experiments (approximately day 90), FEC in lambs grazing lotus increased to similar values as for pasture-fed lambs. FEC was not correlated with dag score or dag weight in ewes or lambs grazing pasture, but these indices were weakly and positively correlated in ewes and lambs grazing lotus, suggesting that lowering FEC on lotus also reduced dag formation. Under dryland farming conditions, the use of Lotus corniculatus (cv. Grasslands Goldie) during the spring/early summer lactation period can be used to increase lamb growth and wool production, whilst eliminating the need for pre-lambing anthelmintic drenching and, probably, reducing the amount of insecticide needed to control flystrike. These effects are probably due to the CT in lotus reducing rumen protein degradability and controlling internal parasites, and to the higher digestibility and voluntary feed intake (VFI) of lotus compared to perennial ryegrass ( Lolium perenne)/white clover ( Trifolium repens) pasture. The absence of endophyte in lotus may also have contributed to these effects.

Metabolic Profile Test in Iran: Variations of Metabolites Around Parturition at Dairy Cattle

profile or profile was defined as a set or combination of blood constituents analyzed together in one test. Metabolic Profile test (MPT) was used as a diagnostic approach in Iran in recent years. Reference values of metabolites were determined by Seifi (1996). Calving, with the demands for initiating milk production, places high-producing cows in a critical nutrient balance and may affect profiles for 60 or more days after calving. Production level is one aspect of the periparturient flux that may reflect in the profile. Glucose and PCV are negatively correlated and total proteins; Mg and cholesterol are positively correlated with the post parturient rise in milk production. The aim of this study was to evaluate the variations of serum metabolites around parturition. The investigation was conducted in two dairy farms in Mashhad city in north east of Iran. One hundred cows were bled on four occasions: 20-10 days prepartum, 3-7 days, 30 days and 56-60 days postpartum. Blood samples were drawn from Jugular vein and analyzed by computerized analyzer (RA1000 Technicon). The following metabolites and enzymes were measured on serum samples: glucose, urea nitrogen, cholesterol, albumin, total protein, calcium, inorganic phosphorus and Aspartate Aminotransferase (AST). Data were analyzed for statistical significance by analysis of variance using SPSS program for Windows. A p value less than 0.05 were considered as significant. In farm 1, prepartum variations in urea nitrogen, cholesterol, albumin, calcium, AST, PCV and total protein and in farm 2 prepartum variations in glucose, cholesterol, calcium, phosphorus, AST, PCV, and total protein were significant (p< 0.05). Serum glucose level decreased markedly 3-7 days postpartum and gradually increased at 30 and 5660 days postpartum. At least 92% of dairy cows suffer negative energy balance at early lactation. At this stage, the level of milk production reaches to peak earlier than dry matter intake and the result is low blood glucose. The peak of urea nitrogen concentration was found preparturition and it decreased postpartum. The concentration of serum cholesterol was lower at 10-20 days prepartum and after calving gradually increased. The peak of serum calcium levels was found 10-20 days before calving and it was markedly decreased after parturition. Serum total protein and PCV values were lowest at preparturition sampling. AST activity levels were lower at first sampling and reached highest after parturition. AST relates to metabolism, and it seems increased activity of the enzyme in high producing dairy cows is due to increased of body metabolic and hepatic activities . Reference

Resistance of Sabi and Dorper ewes to gastro-intestinal nematode infections in an African semi-arid environment

Sabi and Dorper ewes were evaluated for resistance to gastro-intestinal nematode parasites (predominantly Haemonchus contortus) at the Matopos Research Station in Zimbabwe. Live weights (LWT), blood packed red cell volume (PCV) and faecal egg counts (FEC) were recorded at mating, 1 month before lambing and 1–3-month post-lambing for five separate lambings between 1996 and 2000. Dorper ewes were heavier ( P<0.01) than Sabi ewes at all sampling times. Sabi ewes were more resistant to internal parasites than Dorper ewes as shown by their significantly lower FEC and significantly higher PCV at all sampling times except at 2-month post-lambing for FEC and at weaning (3-month post-lambing) for PCV. At all sampling times except at weaning individual ewes were treated with an anthelmintic when their FEC was greater than 1500 eggs/g (epg) and/or their PCV was less than 20%. The proportion of Dorper ewes that was treated with anthelmintic was significantly higher than that of Sabi ewes at all sampling dates. This effect was particularly marked 1- and 2-month post-lambing. There was a post-parturient rise in FEC in the 2-month period after lambing for both breeds.

Comparison of the post-parturient rise in faecal egg counts of indigenous and cross-bred ewes.

The breed differences in post-parturient rise (PPR) in faecal egg counts in lambing ewes of different breeds naturally infected with Haemonchus contortus were compared. The ewes of Nali, 50% Nali x 50% Russian Merino/Corriedale and 37.5% Nali x 62.5% Russian Merino/Corriedale were treated with fenbendazole while ewes of the above three breeds as well as 25% Nali x 75% Russian Merino/Corriedale, were kept untreated. Observations from 3 weeks before lambing to 12 weeks post-lambing at weekly intervals revealed that in treated ewes, egg counts of Nali did not differ significantly with 50% Nali whereas 37.5% Nali had significantly higher egg counts than those of Nali and 50% Nali. In untreated ewes egg counts of 50% Nali were significantly higher than that of Nali on some occasions, while 25% Nali had significantly higher egg counts than the other three breeds. None of the ewes from the Nali breed showed signs of haemonchosis, whereas few lambing ewes of the three Nali crosses (50%, 37.5% and 25% Nali) showed signs of haemonchosis and one ewe of the latter breed died.

Effect of genetic type, lactation and management on helminth infection of ewes in an intensive grazing system on irrigated pasture

A survey of helminth infection was conducted in a flock of 290 ewes distributed into Romanov (R), Merinos d'Arles (M) and Romanov × Merinos (R × M) genetic types, grazing irrigated pasture in the south of France. Faecal egg and larval counts were done seven times per year from 1981 to 1984 on homogeneous groups of ewes and then individually once to four times every autumn from 1985 to 1988. Helminth fauna was diverse and more abundant during autumn. High levels of strongyle infection occurred in the ewes that remained on the same irrigated pastures during summer. Moving to Alpian pastures during the summer lowered autumnal infection. Significant differences between genotypes in intensity of infection were observed in the order R > R × M . M for strongyles ( Teladorsagia circumcincta and/or Trichoslrongylus vitrinus, Chahertia ovina and/ or Oesophagostomum venulosum, Nematodirus spp.), Momezia spp. and Dictyocauius filaria. The hierarchy was reversed for Fasciola hepatica infection and not consistent from one year to another for protostrongylid infections. The effect of lactation intensity on the postparturient rise was studied by equilibrating number of ewes according to reproductive status (zero, one or two lambs in lactation). Merino ewes with two lambs in lactation, as well as primiparous Romanov ewes, had significantly higher strongyle infections than the others. The repeatabilities of the larval and egg counts between the four trial years were 0.24, 0.23 and 0.16, respectively, for protostrongyles, Nematodirus and strongyles, with higher intra-annual values for protostrongyles and inconsistently significant results for strongyles due to the presence of several species.

Effect of grazing management on minimising effects of parasitic nematodes on lamb production

An integrated management system involving cattle, ewes and lambs developed to minimise levels of parasitic nematode contamination of pasture was evaluated in a 2-year (1989-1991) study. Four experimental farmlets were managed as self-containcd units over a May-May year. Over the two years, stocking rate averaged 15 su/ha and cattle:sheep ratio 35:65. Three treatments (lambs regularly drenched, ewes drenched post-lambing/ lambs undrenchedand undrenchedewes andlambs) were similarly managed (set-stocked, lambing to weaning, 7-day shifts post-weaning, a 77-day interval between consecutive lamb grazings and a strict rotation of lambs followed by ewes with cattle midway between the ewe and subsequent lamb grazing). A fourth treatment differed in each year: in year 1, lambs were set stocked over summer and in year 2 cattle followed lambs, and ewes were positioned mid-way in the rotation between lambs and cattle, In both years, the management system developed to minimise nematode parasitism in lambs mamtained low levels of infective larvae on pasture (~80 L3 larvae/kg pasture), and a faecal egg count in lambs of less than 1500 epg; clinical parasitism was not observed. However, regular anthelmintic drenching of lambs significantly (P~0.01) increased liveweight gain of lambs from weaning in November to May by an average 35% over the 2 years. In year two, grazing ewes ahead of lambs significantly increased (PcO.01) pasture larvae levels and lamb faecal egg counts and decreased liveweight gain compared with grazing cattle ahead. Drenching ewes to minimise effects of the post-parturient rise in faecal egg count had no influence on nematode levels in lambs or on lamb performance. Keywords parasitic nematodes, grazing management, lamb growth

Autumn-born lambs — Responses to use of anthelmintics and selenium

Abstract Extract Madam:—Out of season (autumn/winter) lambing is expanding in northern areas. These lambs need to grow at moderate rates through the late winter feed trough so they can fully utilise the spring flush to reach heavy weights before summer. Autumn-born lambs are likely to face high parasite roundworm challenge pre- and post-weaning. Strongyle faecal egg counts of autumn and spring-born lambs at weaning averaged 2000 and 400 eggs/g respectively (n=30) under an all-sheep, splitseason lambing policy near Kaikohe. (7) McSporran, K.D. and Andrewes, W.G.K. 1987. Field study of the post-parturient rise in faecal egg counts of lactating ewes. The effects of season, genotype of ewe and number of lambs born per ewe. Inter. J. Parasit., (pers. comm) [Google Scholar] Roundworm infestation can also reduce selenium concentration in the liver. (2) Andrews, E.D., Hogan, K.G. and Brunsdon, R.V. 1971. Some effects of anthelmintic dosing on the mineral status of hoggets. Proc. N.Z. Soc. Anim. Prod., 31: 65–65. [Google Scholar] Although Se level in pasture tends to be higher in autumn/winter than in spring/summer (l) Andrews, E.D., Grant, A.B. and Stephenson, B.J. 1964. Weight responses of sheep to cobalt and selenium in relation to vitamin B12 and selenium concentrations in liver and kidney. N.Z. J. agric. Res., 13: 17–27. [Taylor & Francis Online] , [Google Scholar] this could be offset by lower pasture availability. Better winter growth rates have been recorded in weaned calves from a combination anthelmintic/selenium drench than from either of these alone. (5) McLeod, C.C., Wolff, J.E. and Schwarz, G. 1975. Effect of anthelmintic and selenium drenching on liveweight gain in young beef cattle. N.Z. J. exp. Agric., 4: 219–225. [Google Scholar]

Observations on the post-parturient rise of nematode egg-output in cattle

The nematode egg-output from 20 young cows with a grazing history of two seasons and all calving for the first time, was followed from 3 weeks before calving until 14 weeks afterwards. The egg-output was measured as the number of larvae cultured per gram of faeces (L.P.G.) accompanied by larval differentiation. In the first 2 weeks after parturition there was a marked rise in parasite egg-output mainly caused by Ostertagia spp., Trichstronglus spp., and Cooperia oncophora. A second peak of C. oncophora occured 4 weeks later. Peaks of Oesophagostomum spp., Haemonchus spp. and Bunostomum spp. were also later. Epidemiolgical consequences with respect to pasture contamination due to the post-parturient rise are possible, particularly when calving takes place in spring or early summer and the cows are kept indoors for only a few days.

The Epidemiology and Control of some Nematode Infections in Grazing Animals

This chapter discusses the epidemiology and control of some nematode infections in grazing animals. The epidemiological effects of differences in the reaction of different hosts to a common parasite are given. The connection between arrested development and the increase in worm egg output seen in many host–nematode systems at or following parturition, and which has come to be called the postparturient, periparturient, or lactation rise, is largely incidental. The chapter discusses the parasitic gastro-enteritis in sheep. The central issue appeared to be whether disease-producing infections were built up by exponential population growth through several generations. As the grazing season progresses, the relative numbers of worms of different species present in lambs change. The worm eggs passed by ewes in the course of the postparturient rise were the source of nearly all the worms that were the cause of disease in their lambs. Strategies for the control of parasitic gastro-enteritis in lambs are presented. Cattle exposed to regular infection gradually acquire a resistance to the establishment of Ostertagia ostertagi (O.ostertagi). The chapter discusses the methods for controling the parasitic gastro-enteritis in cattle and the parasitic bronchitis in cattle and the methods for controling it.

An experimental investigation of the postparturient rise of faecal egg count of Ostertagia circumcincta as a source of infection for lambs

A group of three ewes and six lambs running on worm free pasture exposed to infection arising from a stimulated postparturient rise of faecal egg count was compared with a similar group not exposed to a postparturient rise. The second group remained almost worm free and gained weight steadily whilst the first acquired a high worm burden and ceased to gain weight from the time they began to acquire worms. Methods of controlling the worm infection and preventing weight loss are discussed.

The Effect of the Postparturient Faecal Egg Count on the Worm Burden of Lambs and on their Weight Gain

A comparison was made between the effects of a simulated postparturient rise of faecal egg count following lambing in March and one following lambing in April on the pasture larval count and on the worm burden of the lambs. The rise following March lambing produced only a low pasture larval count but a high faecal egg count in the lambs. The rise following lambing in April produced a higher pasture larval count but a comparable faecal egg count to that seen in the March group. The worm burdens of the two groups were similar. Both groups experiencing infection derived from the postparturient rise gained less weight than similar groups grazing herbage not exposed to such contamination. Une comparaison a été faite entre les effets d’un accroissement simulé au nombre d’oeufs fécaux après l’agnelage de Mars et l’agnelage suivant d’Avril, sur le nombre larvaire du pâturage et sur la quantité de vers des agneaux. L’accroissement après l’agnelage de Mars produisit seulement un nombre larvaire peu élevé du pâturage, mais un nombre fécal d’oeufs élevé chez les agneaux. L’accroissement après l’agnelage d’Avril produisit un nombre larvaire plus élevé du pâturage que celui observé dans le groups de Mars. Les quantités de vers chez les deux groupes étaient semblables. Les deux groupes qui ont éprouvé une infection dérivée de l’accroissement après l’agnelage gagnèrent moins de poids que des groupes semblables pâturant de l’herbe non exposée à cette contamination. Die Wirkung eines Anstiegs der in den Faeces nach der Geburt gezählten Wurmeier im März und nach dem Wurf im April auf eine Zählung der Larven auf der Weide und die Wurminfestation der Lämmer. Der Anstieg nach dem Lammen im März verursachte eine nur geringe Zahl von fäkalen Wurmeiern bei den Lämmern. Der Anstieg nach dem Aprillammen ergab eine grössere Zahl von Larven auf der Weide. Die Wurmbevölkerung war bei beiden Gruppen einander ähnlich. Beide Gruppen, die durch den Anstieg nach dem Wurf infiziert waren, hatten eine geringere Gewichtzunahme als ähnliche Gruppen, die mit nicht infiziertem Futter ernährt worden waren. Se efectuó una comparación entre los efectos de una numeración de huevos fecales tras el parto, de elevación simulada, tras el parto en Marzo y tras el parto en Abril, sobre la numeración de larvas en los pastos y sobre el grado de infestación de lombrices en los corderos. Le elevación que siguió a los partos de Marzo produjo solamente una numeración baja de larvas en los pastos pero una elevada numeración de huevos fecales en los corderos. La elevación que siguió a los partos de Abril produjo una numeración más elevada de las larvas en los pastos que la observada en el grupo de Marzo. Los grados de infestación de los dos grupos fueron similares. Ambos grupos que experimentaron infecciones derivadas de la elevación tras el parto ganaron menos peso que grupos similares que pacieron en hierba no expuesta a tal contaminación.

The Acquisition of Trichostrongylid Infection by Grazing Lambs

Summary A grazing experiment was carried out during 1966–1968 in an attempt to identify the source of the larvae which initiate infection in grazing lambs. The post-parturient rise in faecal egg count was abolished in the ewes but was replaced by eggs of T. colubriformis passed by artificaily infected 5-month-old Dorset Horn lambs. The acquisition of infection in 2 years appeared to result principally from the larvae derived from the simulated post-parturient rise but in the third year residual pasture infection was important. Paddocks continuously grazed by lambs over a number of years became dangerously infected with Nematodirus spp.

Experiments on the control of gastro-intestinal nematodes in lambs

Grazing experiments carried out in 1967 and 1968 compared the acquisition of the worm burden in 4 groups of lambs, one dosed with anthelmintic and moved to clean pasture at weaning, a second moved to clean pasture at weaning, a third dosed at weaning but left on the pasture they had previously grazed and a fourth weaned and left on the pasture they were grazing. In Semptember the worm burden of the first group was the lowest and worm burden increased in order up to the fourth group which had the highest. The experiments provided evidence that residual herbage larval infection is an important factor in initiating infection in lambs as well as infection derived from the post-parturient rise in faecal egg output of the ewes.

The post-parturient rise in the faecal nematode egg count of ewes: some host-parasite relationships.

Abstract Extract Both the aetiology and practical importance of the post-parturient rise in strongyle egg counts of breeding ewes have been adequately reviewed and discussed (Dunsmore, 1965 Arundel, J. H. and Ford, G. E. 1969. The use of a single anthelmintic treatment to control the post-parturient rise in faecal worm egg count of sheep. Aust. vet. J., 45: 89–93. [Crossref], [PubMed] , [Google Scholar]; Connan, 1968a Brunsdon, R. V. 1964. The seasonal variations in the nematode egg counts of sheep: a comparison of the spring rise phenomenon in breeding and unmated ewes. N.Z. vet. J., 12: 75–80. [Taylor & Francis Online] , [Google Scholar], b Brunsdon, R. V. 1967. The spring-rise phenomenon: the relationship between the time of lambing and the commencement of the rise in faecal worm egg counts. N.Z. vet. J., 15: 35–40. [Taylor & Francis Online] , [Google Scholar]; Arundel and Ford, 1969 Brunsdon, R. V. 1970. The spring-rise phenomenon seasonal changes in the worm burdens of breeding ewes and in the availability of pasture infection. N.Z. vet. J., 18: 47–54. [Taylor & Francis Online] , [Google Scholar]).

The post-parturient rise: a comparison of the pattern and relative generic composition of faecal strongyle egg counts in ewes and wethers.

Abstract Extract The regular ,occurrence of a seasonal increase in the nematode egg output of breeding ewes has been well recognized for many years(for reviews see Crofton, 1963 Crofton, H. D. 1963. Nematode parasite population in sheep and on pasture, Tech. Comm. No. 35 104–104. St. Albans: Commonwealth Bureau of Helminthology. [Google Scholar]; Dunsmore, 1965 Dunsmore, J. D. 1965. Ostertagia spp. in lambs and pregnant ewes. J. Helminth., 39: 159–184. [Google Scholar]; Connan1968a Connan, R. M. 1968a. The post-parturient rise in faecal nematode egg count of ewes: its aetiology and epidemiological significance. World. Rev. anim. Prod., 4: 53–57. [Google Scholar]).

The post-parturient rise: a comparison of the pattern and relative generic composition of strongyle egg output from lactating and non-lactating ewes.

Abstract Extract The literature relating to the aetiology of the post-parturient rise in strongyle faecal egg counts of ewes has been recently reviewed and discussed by Dunsmore (1965) and Connan (1968a, b).

Pre-mating anthelmintic treatment of ewes and its effect on lambing performance.

Abstract Extract The study of helminth infections of breeding ewes has been concentrated particularly on the post-parturient rise in faecal egg counts (Connan, 1968). New Zealand studies on this phenomenon including economic returns from anthelmintic treatment of ewes prior to lambing and after lambing have been conducted by Brunsdon (1964, 1966a, b, c, 1967, 1970), Lewis and Stauber (1969) and Leaning et al. (1970).

A field study of nematode parasite populations in the lactating ewe.

A typical postparturient rise of parasite egg output occurred in a group of lactating ewes. At parturition these animals carried few inhibited fourth-stage larvae and the rise was associated with a net increase in parasite populations, which did not occur in non-lactating ewes. Lactating ewes whose established parasite populations were effectively removed just prior to parturition showed a similar rise derived entirely from infection acquired during lactation, which was of much greater magnitude than that seen in similarly treated non-lactating animals. Circumstantial evidence of increased fecundity of T. colubriformis females in lactating ewes was also obtained.It was concluded that the postparturient rise of egg output may derive from an increase in adult parasite populations by an increased rate of establishment of newly ingested infective larvae which develop to the adult stage without inhibition, as well as by the resumption of development of previously inhibited fourth-stage larvae. In addition, both newly acquired and established adult female parasites may show increased fecundity.Ewes whose lambs were removed at birth or after a period of lactation showed a rapid fall in faecal egg counts associated with rejection of part of their existing worm burden and a pronounced increase in resistance to new infection.All of the differences observed between lactating and non-lactating ewes in the behaviour of their parasite populations were found to be qualitatively the same as those that have been frequently reported in susceptible and resistant non-reproductive sheep. The hypothesis is suggested, therefore, that the postparturient rise derives from a pronounced but temporary relaxation of immunological control of gastro-intestinal nematode infection associated with the endocrine status of the lactating ewe.