甲砜霉素在红笛鲷体内的组织分布和药代动力学研究

研究腹腔注射和口灌给药方式下甲砜霉素在红笛鲷体内的组织分布和药代动力学特征.甲砜霉素单剂量10 mg/（kg· bw）分别腹注和口灌红笛鲷,给药后取血浆、肌肉、肝脏和肾脏,各组织中药物浓度用HPLC-MS/MS测定,所得药时数据用DAS3.0软件分析.结果显示：两种给药方式下红笛鲷血浆药时数据均符合一级吸收二室模型;血药达峰时间（tp）分别为2.39和4.51 h,血药浓度峰值（Cmax）分别为5.80和5.06 μ.g/mL,消除半衰期（t1/2β）分别为7.25和50.09 h.腹注和口灌给药肌肉、肝脏和肾脏的Cmax分别为2.504和2.678 μ g/g、4.612和4.393 μg/g、12.464和7.509 μg/g,tp分别为6.0和4.0h、2.0和4.0h、1.5和4.0h;甲砜霉素在红笛鲷体内消除速度较慢,肌肉、肝脏和肾脏的t1/2β分别为37.168和33.519 h、22.499和33.649 h、19.672和8.673 h.结果表明：腹注给药方式下甲砜霉素在红笛鲷体内的吸收快于口灌给药,在血浆和肝脏中的消除快于口灌给药,在肌肉和肾脏中的消除则慢于口灌给药.     

诺氟沙星盐酸小檗碱预混剂在眼斑拟石首鱼体内的药代动力学和残留消除规律研究

研究诺氟沙星盐酸小檗碱在眼斑拟石首鱼体内的药代动力学和残留消除规律。在水温28±2℃、盐度28条件下,将诺氟沙星(Norfloxacin,NFLX)盐酸小檗碱(Berberine hydrochloride,BBH)预混剂按450 mg/kg的剂量口灌眼斑拟石首鱼后,其血浆、肌肉、肝脏和肾脏中的药物浓度采用HPLC-MS/MS测定,药时数据用DAS3.0软件分析。结果显示:单剂量口灌给药后,眼斑拟石首鱼血浆中NFLX和BBH的药时数据均符合一级吸收二室模型;血药达峰时间(tp)分别为1.20、1.79 h,血药浓度峰值(Cmax)分别为358.40、144.89μg/L,药时曲线下面积(AUC0-∞)分别为8252.80、6454.52μg/(L·h),消除半衰期(t1/2β)分别为43.26、27.30 h;NFLX和BBH在鱼体各组织中的分布较广,灌服给药后,肌肉、肝脏和肾脏中NFLX和BBH的Cmax分别为418.05、230.76μg/kg;1745.94、901.09μg/kg;1143.45、997.09μg/kg,tp分别为1.5和1.5 h、1.5和2.0 h、1.5和2.0 h,AUC0-∞分别为7682.00、5596.30μg/(kg·h);31236.90、19096.85μg/(kg·h);22593.93、37509.17μg/(kg·h);NFLX和BBH在眼斑拟石首鱼体内消除速度较慢,灌服给药后肌肉、肝脏和肾脏的t1/2β分别为33.41、61.81 h;20.44、15.04 h;30.28、23.43 h。按450 mg/kg剂量连续5 d口灌给药后,眼斑拟石首鱼血浆、肌肉、肝脏、肾脏中的NFLX和BBH残留消除半衰期(t1/2)分别为30.24、33.33 h;40.76、61.60 h;38.68、36.96 h;43.77、59.40 h。以10μg/kg为最高残留限量,肌肉作为食用靶组织,在本试验条件下,建议休药期不得少于12 d。     

两种氟苯尼考粉剂在肉鸡体内的药动学比较

健康黄羽肉鸡(公母各半)20只随机分为A、B 2组,分别单剂量灌服19.91%氟苯尼考粉(受试品)和10%氟苯尼考粉(对照品),给药量均为15 mg/kg,进行药动学比较研究。给药后按预定时间采集血样,采用高效液相色谱法(HPLC)法测定血浆中药物含量。实测血药浓度-时间数据,采用Win Nonlin 5.2.1药动学分析软件处理。结果显示:A组平均消除半衰期(T_(1/2β))约为9.158 h,达峰时间(T~(max))和峰值浓度(C~(max))分别为0.600 h和5.786μg/m L,平均曲线下面积(AUC)为26.474 h·μg/m L,平均滞留时间(MRT)4.357 h;B组平均T_(1/2β)约为7.513 h,T~(max)和C~(max)分别为1.900 h和5.106 mg/L,AUC为25.749 h·μg/m L,MRT 5.695 h;相对生物利用度约为93.979%。结果表明,19.91%氟苯尼考粉T~(max)比10%氟苯尼考粉提前(P0.01),其他药动学参数无明显差异(P0.05)。     

静脉和肌内注射后氟苯尼考在麻鸭体内药动学的初步研究

试验研究了麻鸭单次静脉注射和肌内注射氟苯尼考后的药动学,给药剂量均为20 mg/kg体重。麻鸭给药后,定点采血,分离血浆,然后以高效液相色谱法测定血浆中的药物浓度,并利用房室分析法计算两种不同给药途径下氟苯尼考的药动学参数。结果显示:静脉注射氟苯尼考表观分布容积(V_β)为(8 388.45±850.43)m L/kg,消除较缓慢,消除半衰期(t_(1/2β))为(6.61±0.83)h;肌内注射氟苯尼考峰浓度(C_(max))为(1.42±0.16)μg/m L,达峰时间(t_(max))为(1.60±0.19)h,绝对生物利用度为71.59%。结果证实氟苯尼考在麻鸭体内具有优异的药动学特征,分布迅速、广泛、消除较缓慢,肌内注射吸收迅速且较完全。结合氟苯尼考对鸭疫里默氏杆菌、沙门菌及大肠杆菌的MIC数据,计算得出对如上3种细菌感染的治疗,静脉或肌内注射20 mg/kg氟苯尼考较难达到良好的治疗效果,应适当增加给药剂量。     

磺胺二甲嘧啶在黑鲪体内的药物代谢动力学和残留消除研究

在(14±1)℃水温条件下,对黑鲪单次口灌100 mg/kg体重的磺胺二甲嘧啶,进行药物代谢动力学研究。在(20±2)℃水温条件下,按照《中华人民共和国水产行业标准磺胺类药物水产养殖使用规范》推荐剂量对黑鮶连续5天口灌给予磺胺二甲嘧啶,研究其在黑鮶体内的残留消除规律。血浆、肌肉和肝脏样品采用高效液相色谱检测,DAS2.0药物代谢动力学软件对数据进行处理分析。结果表明磺胺二甲嘧啶在黑鲪血浆、肌肉和肝脏中均符合一室模型,肝脏、血液和肌肉中药物达峰时间分别为6 h,8 h和10 h;峰值浓度分别为26.45μg/g、25.57μg/g和31.15μg/g;连续多次给药后,黑鮶血液、肌肉、肝脏中药物浓度分别在给药后12d、14d、15d后小于最大残留限量要求(0.1mg/kg)。     

牛蒡子粉中牛蒡苷元在仔猪体内的药物动力学研究

为研究牛蒡子粉中牛蒡苷元在仔猪体内的药物动力学特征,了解其在仔猪体内的吸收、分布、转化和排泄规律,为新兽药的研发和临床用药提供理论参考依据。选取健康仔猪8头(30.0±5.0 kg),以1.0 g/kg·bw的牛蒡子粉灌胃给药,不同时间点前腔静脉采血,采用HPLC法对猪血浆中牛蒡苷元的浓度进行分析。牛蒡子粉灌胃给药后,符合有吸收二室模型,主要药物动力学参数为:吸收半衰期(t_(1/2 ka))为0.274±0.102 h,分布半衰期(t_(1/2α))1.435±0.725 h;消除半衰期(t_(1/2β))63.467±29.115 h;表观分布容积(V_d)1.680±0.402 L/kg;清除率(CL_b)0.076±0.028 L/(h·kg);达峰时间(t_(max))为0.853±0.211 h,峰浓度(c_(max))为0.430±0.035μg/mL,药时曲线下面积(AUC)14.672±4.813μg·h/mL。试验表明:牛蒡子粉口灌后牛蒡苷元在仔猪体内吸收迅速、分布广泛、代谢消除缓慢,能够较长时间发挥药理作用。     

氟苯尼考环糊精包合物的药物代谢动力学研究

为研究氟苯尼考环糊精包合物的药代动力学及生物利用度,将健康白羽肉鸡24只,随机分为两组,每组12只,分别以20 mg/kg的剂量单次灌服20%普通氟苯尼考粉(A组)和20%氟苯尼考环糊精包合物(B组),并于给药后不同时间点从翅下静脉采血,采用已建立的UPLC-MS/MS测定血浆中的药物浓度,用WinNonlin 5.2.1药动学分析软件的非房室模型拟合血药浓度-时间数据。结果显示:A组达峰时间(T_(max))和达峰浓度(C_(max))分别为1.563±0.755 h、1043.15±391.42 ng/mL,平均消除半衰期(T_(1/2λz))约为4.814±3.058 h,平均曲线下面积(AUC_(last))为4283.53±2406.81 h·ng/mL;B组T_(max)、C_(max)分别为1.417±1.683 h、4691.95±1597.28 ng/mL,T_(1/2λz)约为2.106±1.476 h,AUC_(last)为14911.70±2976.22 h·ng/mL;相对生物利用度约为348.12%。试验表明,采用环糊精包合工艺的氟苯尼考粉的生物利用度显著高于普通工艺的氟苯尼考粉。     

阿莫西林在藏系羊体内的药物动力学研究

为研究阿莫西林在藏系羊体内的药物动力学特征,了解阿莫西林在藏系羊体内的吸收、分布、转化及排泄规律,以期为牧区兽医临床用药提供依据。本试验选取8只成年藏系羊,阿莫西林口灌给药,不同时间点采集藏系羊血液,利用高效液相色谱法测定血浆中药物浓度。结果表明,阿莫西林经藏系羊口灌给药(15 mg/kg.B.W)后,其主要药动学参数为:t_(1/2α)为(0.773±0.097)h,t1/2ka为(0.156±0.021)h,t_(1/2β)为(3.787±0.973)h,AUC为(8.249±1.023)μg·h/m L,T_(max)为(0.497±0.036)h,C_(max)为(2.667±0.198)μg/m L。表明阿莫西林口灌给药后,在成年藏系羊体内吸收迅速,消除较快。     

恩诺沙星及其代谢物在1日龄鸡体内的药动学研究

研究不同剂量恩诺沙星(ENR)及代谢产物环丙沙星(CIP)在雏鸡体内药动学特征。240只刚出壳黄羽肉鸡分2个剂量组,20 mg/kg和40 mg/kg皮下注射恩诺沙星,高效液相色谱法测定药物浓度,药时数据用WinNolin8.0软件处理非房室模型分析。结果表明,增加剂量后恩诺沙星以下药动参数发生变化:消除半衰期(t_(1/2))由7.37 h降至6.53 h,达峰时间(T_(max))由8 h降到4 h,最大血药浓度(C_(max))由3.75μg/mL增至4.92μg/mL,药时曲线下面积(AUC_(last))由40.45 h·μg/mL增至78.2 h·μg/mL,平均滞留时间(MRT_(last))由为8.6 h增至11.42 h;消除率(Cl)分别为0.49 L/h/kg和0.5 L/h/kg,表观分布容积(Vz_(obs))分别为5.22 L/kg和4.77 L/kg。不同剂量恩诺沙星给药后都都表现出分布迅速、广泛的特点,代谢产物环丙沙星产生与消除迅速。     

磺胺嘧啶钠在淮南麻鸭体内药物动力学试验

为了研究磺胺嘧啶钠在淮南麻鸭体内代谢过程,16只淮南麻鸭随机分成2组,每组8只,按100 mg/(kg·bw)的剂量分别口服和静脉注射磺胺嘧啶钠,评价其在血浆中的药动学特征。结果显示:本试验所建立的标准曲线相关性好,相关系数达0.99以上,日内变异系数小于5%,日间变异系数小于10%。磺胺嘧啶钠在淮南麻鸭体内吸收迅速,分布广泛,房室模型分析表明,药时数据均符合二室开放模型,口服给药下的主要药物动力学参数为:分布半衰期(t_(1/2α))为(4.61±0.94)h,消除半衰期(t_(1/2β))为(6.66±0.74)h,药时曲线下面积(AUC)为(395.06±24.03)(μg/mL)·h,达峰时间(T_p)为(3.71±0.84)h,峰浓度(C_(max))为(49.97±5.66)μg/mL。静脉注射条件下的主要药物动力学参数为:分布半衰期(t_(1/2α))为(0.62±0.08)h,消除半衰期(t_(1/2β))为(3.21±0.98)h,药时曲线下面积(AUC)为(385.14±13.52)(μg/mL)·h。     

白术、微米白术和白术多糖对断奶仔猪生长性能和肠道形态及微生态区系的影响

试验选用96头平均体重14.82 kg左右的杜×长×大断奶仔猪,随机分成4组,每组3栏,每栏8头(公母各半)。对照组饲喂基础日粮,试验1、2、3组分别添加1%80目白术、0.2%白术多糖和1%微米白术。试验期30 d。结果表明:在生长性能方面,与对照组相比,1%微米白术添加组可显著提高日增重(P0.05)、降低饲料增重比和腹泻率,而且效果优于1%80目白术组和0.2%白术多糖组,在肠道形态和肠道微生态区系方面,与对照组相比,日粮添加1%80目白术、0.2%白术多糖、1%微米白术均可不同程度的提高十二指肠和空肠的绒毛高度,加深十二指肠和空肠的隐窝深度,并且增加肠道微生态区系的多样性,其中以1%微米白术添加组的效果最佳。     

Prevalence and seasonal incidence of nematode parasites and fluke infections of sheep and goats in eastern Ethiopia

Sissay, M.M., Uggla, A. and Waller, P.J., XXXX. Prevalence and seasonal incidence of nematode parasites and fluke infections of sheep and goats in eastern Ethiopia. Tropical Animal Health and Production, XXXX. A 2-year abattoir survey was carried out to determine the prevalence, abundance and seasonal incidence of gastro-intestinal (GI) nematodes and trematodes (flukes) of sheep and goats in the semi-arid zone of eastern Ethiopia. During May 2003 to April 2005, viscera including liver, lungs and GI tracts were collected from 655 sheep and 632 goats slaughtered at 4 abattoirs located in the towns of Haramaya, Harar, Dire Dawa and Jijiga in eastern Ethiopia. All animals were raised in the farming areas located within the community boundaries for each town. Collected materials were transported within 24 h to the parasitology laboratory of Haramaya University for immediate processing. Thirteen species belonging to 9 genera of GI nematodes (Haemonchus contortus, Trichostrongylus axei, T. colubriformis, T. vitrinus, Nematodirus filicollis, N. spathiger, Oesophagostomum columbianum, O. venulosum, Strongyloides papillosus, Bunostomum trigonocephalum, Trichuris ovis, Cooperia curticei and Chabertia ovina), and 4 species belonging to 3 genera of trematodes (Fasciola hepatica, F. gigantica, Paramphistomum {Calicohoron} microbothrium and Dicrocoelium dendriticum) were recorded in both sheep and goats. All animals in this investigation were infected with multiple species to varying degrees. The mean burdens of adult nematodes were generally moderate in both sheep and goats and showed patterns of seasonal abundance that corresponded with the bi-modal annual rainfall pattern, with highest burdens around the middle of the rainy season. In both sheep and goats there were significant differences in the mean worm burdens and abundance of the different nematode species between the four geographic locations, with worm burdens in the Haramaya and Harar areas greater than those observed in the Dire Dawa and Jijiga locations. Similar seasonal variations were also observed in the prevalence of flukes. But there were no significant differences in the prevalence of each fluke species between the four locations. Overall, the results showed that Haemonchus, Trichostrongylus, Nematodirus, Oesophagostomum, Fasciola and Paramphistomum species were the most abundant helminth parasites of sheep and goats in eastern Ethiopia.     

Phase I evaluation of CCNU (lomustine) in tumor-bearing cats

1-(2-Chloroethyl)3-cyclohexyl-1-nitrosourea (CCNU) is an alkylating agent in the nitrosourea subclass. A prospective evaluation of CCNU was done to determine the maximally tolerated dosage of CCNU in tumor-bearing cats. Response data were obtained when available. Twenty-five cats were treated with CCNU at a dosage of 50-60 mg/m3 body surface area. Complete hematologic data were available for 13 cats. Neutropenia was the acute dose-limiting toxicity. The median neutrophil count at the nadir was 1,000 cells/microL (mean, 2,433 cells/microL; range, 0-9,694 cells/microL). The time of neutrophil nadir was variable, occurring 7-28 days after treatment, and counts sometimes did not return to normal for up to 14 days after the nadir. Based on these findings, a 6-week dosing interval and weekly hematologic monitoring after the 1st treatment with CCNU are recommended. The nadir of the platelet count may occur 14-21 days after treatment. The median platelet count at the nadir was 43,500 cells/microL. No gastrointestinal, renal, or hepatic toxicities were observed after a single CCNU treatment, and additional studies to evaluate the potential for cumulative toxicity should be performed. Five cats with lymphoma and 1 cat with mast cell tumor had measurable responses to CCNU. Phase II studies to evaluate antitumor activity should be completed with a dosing regimen of 50-60 mg/m3 every 6 weeks.     

Internal Parasites and Haematological Values in Cattle Slaughtered in Buea Subdivision of Cameroon

Tropical Animal Health and Production -     

Chemotaxis of macrophage in inflammation

Our particular attention in this article was given to natural mediators for macrophages isolated from the sites of tissue injury. A number of chemotactic factors, which may satisfy many criteria making them acceptable as inflammatory leucocyte chemotactic factors, has been separated. Among them, our laboratory has isolated three macrophage (monocyte) chemotactic factors (MCF-a, -b and -c). Their purification, characterization and functional specificity are discussed.     

Prevalence of gastric lesions in racing Alaskan sled dogs

Human and equine athletes are reported to have a high prevalence of gastric disease, and anecdotal evidence suggests a similar phenomenon applies to racing sled dogs. To investigate the prevalence of gastric disease in racing sled dogs, we conducted 2 gastroscopy studies on dogs competing in the annual Iditarod Sled Dog Race. A pilot study of dogs that were either dropped from the 2000 Iditarod Sled Dog Race because of illness or that finished the race indicated that, approximately 5 days after competing, 10 of 28 dogs (35%) had endoscopic evidence of gastric ulceration, erosion, or hemorrhage. The next year, an endoscopic study of 73 dogs participating in the 2001 Iditarod race was performed in order to evaluate a larger population of dogs. Data from 70 of these dogs could be used; 34 (48.5%) had ulceration, erosion, gastric hemorrhage, or some combination of these findings. When this group of 70 dogs was compared retrospectively to a control group of 87 dogs presented to the Texas A&M University (TAMU) Veterinary Medical Teaching Hospital, the Iditarod sled dogs had a significantly higher prevalence (P = .049) of gastric lesions. These findings suggest that, similar to athletes of other species, elite canine athletes have an increased prevalence of gastric disease compared to the canine population at large.     

Furosemide continuous rate infusion in the horse: evaluation of enhanced efficacy and reduced side effects

Continuous rate infusion (CRI) of furosemide in humans is considered superior to intermittent administration (IA). This study examined whether furosemide CRI, compared with IA, would increase diuretic efficacy with decreased fluid and electrolyte fluctuations and activation of the renin-angiotensin-aldosterone system (RAAS) in the horse. Five mares were used in a crossover-design study. During a 24-hour period, each horse received a total of 3 mg/kg furosemide by either CRI (0.12 mg/kg/h preceded by a loading dose of 0.12 mg/kg IV) or IA (1 mg/kg IV q8h). There was not a statistically significant difference in urine volume over 24 hours between methods; however, urine volume was significantly greater after CRI compared with IA during the first 8 hours ([median 25th percentile, 75th percentile]: 9.6 L [8.9, 14.4] for CRI versus 5.9 L [5.3, 6.0] for IA). CRI produced a more uniform urine flow, decreased fluctuations in plasma volume, and suppressed renal concentrating ability throughout the infusion period. Potassium, Ca, and Cl excretion was greater during CRI than IA (1,133 mmol [1.110, 1,229] versus 764 mmol [709, 904], 102.7 mmol [96.0, 117.2] versus 73.3 mmol [65.0, 73.5], and 1,776 mmol [1,657, 2.378] versus 1,596 mmol [1,457, 1,767], respectively). Elimination half-lives of furosemide were 1.35 and 0.47 hours for CRI and IA, respectively. The area under the excretion rate curve was 1,285.7 and 184.2 mL x mg/mL for CRI and IA, respectively. Furosemide CRI (0.12 mg/kg/h) for 8 hours, preceded by a loading dose (0.12 mg/kg), is recommended when profound diuresis is needed acutely in horses.     

Y‐chromosomal variation of local goat breeds of Turkey close to the domestication centre

Genetic variations in chromosome Y are enabling researchers to identify paternal lineages, which are informative for introgressions and migrations. In this study, the male‐specific region markers, sex‐determining region‐Y (SRY), amelogenin (AMELY) and zinc finger (ZFY) were analysed in seven Turkish native goat breeds, Angora, Kilis, Hair, Honaml?, Norduz, Gürcü and Abaza. A SNP in the ZFY gene defined a new haplotype Y2C. All domestic haplogroups originate from Capra aegagrus, while the finding of Y1A, Y1B, Y2A and Y2C in 32, 4, 126 and 2 Turkish domestic goats, respectively, appears to indicate a predomestic origin of the major haplotypes. The occurrence of four haplotypes in the Hair goat and, in contrast, a frequency of 96% of Y1A in the Kilis breed illustrate that Y‐chromosomal variants have a more breed‐dependent distribution than mitochondrial or autosomal DNA. This probably reflects male founder effects, but a role in adaptation cannot be excluded.     

Changes in proportion of canine urinary calculi composed of calcium oxalate or struvite in specimens analyzed from 1981 through 2001

The objective of this report was to characterize 20-year changes in proportion of calcium oxalate (CaOx) calculi and struvite calculi in dogs, and associations with breed, age, and sex. In this retrospective study, results of analysis of urinary calculi from dogs were reviewed for specimens received between July 1, 1981, and December 31, 2001. Breed, sex, age, year of submission of the specimen, and mineral type(s) were analyzed statistically. CaOx or Struvite or both were contained in 18,966 of 20,884 (91%) specimens. For both sexes, a 20-year statistically significant increase was observed in the proportion of calculus specimens that contained CaOx. The increase in this proportion was greater in females (1% to 31%) than in males (18% to 82%). From 1998 to 2001, when proportions may have plateaued, the odds of specimens containing CaOx were markedly higher in 18 breeds, markedly lower in 5 breeds, and not significantly different in 13 breeds compared with crossbreds. For both sexes, a 20-year statistically significant decrease was observed in the proportion of calculus specimens that contained struvite. This decrease in proportion was greater for males (79-16%) than for females (97-68%). From 1998 to 2001, when proportions plateaued, the odds of calculi containing struvite were markedly lower in 20 breeds, markedly higher in 1 breed, and not significantly different in 15 breeds when compared with crossbreds. Breed, age, and sex were associated statistically with CaOx or struvite urolithiasis. In conclusion, there appears to have been a long-term increase in the proportion of specimens of canine urinary calculi that contain CaOx as well as a long-term decrease in the proportion of specimens of calculi that contain struvite for both male and female dogs. The rate of change appeared to begin leveling off in the period 1998 to 2001. The recent proportion of dogs with either CaOx- or struvite-associated urolithiasis may depend on breed, age, and sex, and on interactions among these 3 factors.     

The Epidemiology of Gastrointestinal Nematodes of Dairy Cattle in Central Kenya

The epidemiology of H. placei and of other gastrointestinal nematodes in yearling dairy cattle was examined on two farms in Kiambu District, central Kenya during each of 13 one-month periods from April 1993 to April 1994. On each farm, 32 newly weaned dairy calves were given a single dose of albendazole and then placed on experimental pastures. Twelve of the animals were designated for bi-monthly slaughter (n = 2) and analysis of worm population characteristics and 20 were designated for blood and faecal collection and for weighing. Two parasite-free tracer calves were grazed alongside the weaner calves each month throughout the study period and were also slaughtered for analysis of worm populations. Faecal egg counts, haematological and serum pepsinogen determinations, herbage larval counts, and animal live weight changes were recorded monthly. The study revealed that Haemonchus placei, Trichostrongylus axei, Cooperia spp. and Oesophagostomum radiatum were responsible for parasitic gastroenteritis and that H. placei was the predominant nematode present in the young cattle on both farms. Faecal egg counts from resident cattle and necropsy worm counts revealed that pasture larval levels were directly related to the amount of rainfall. The total worm burdens in the animals were highest during the rainy season (March–June and October–December) and lowest during the dry seasons (July–September and January–February). The very low recovery of immature larvae of H. placei from the tracer calves indicated that arrested development is not a feature of the life cycle of this parasite in central Kenya. The maintenance of the parasite population depended on continuous cycling of infection between the host and the pasture. The agroclimatic conditions of the study area were such that, in general, favourable weather conditions for the development and survival of the free-living stages of gastrointestinal nematodes existed all year round.