Miguel Mejía - Academia.edu (original) (raw)

Papers by Miguel Mejía

Research paper thumbnail of Síndrome nefrótico y sus complicaciones en el hospital regional infantil Dr. Arturo Grullón

RESUMEN Con el objetivo de determinar la frecuencia de síndrome nefrótico ysus complicaciones en ... more RESUMEN Con el objetivo de determinar la frecuencia de síndrome nefrótico ysus complicaciones en los pacientes ingresa-dos en el hospital Regional InfantilDr.ArturoGrullón, San-tiago, República Dominicana, en el año 2002, se realizó un estudio descriptivo, transversal yretrospectivo, ...

Research paper thumbnail of Metabolic cues for puberty onset in free grazing holstein heifers naturally infected with nematodes

Theriogenology, 2001

Leptin is a new plausible candidate for the molecular link between nutritional status and the rep... more Leptin is a new plausible candidate for the molecular link between nutritional status and the reproductive axis. In previous studies we described that continuous natural nematode infections in heifers retarded growth and delayed the onset of puberty, and that the insulin-like growth factor I (IGF-I) was involved. In the present study we monitored the leptin levels during development in heifers naturally parasitized versus those chronically treated with ivermectin and we investigated whether growth hormone (GH) accounted for the differences in IGF-1 previously noted. Insulin levels were also measured. Prolactin hormone was recorded as an indicator of immune system activation. We found a direct correlation between leptin and body weight during development and a prepubertal surge of the hormone 2 weeks before the first progesterone peak that indicates the onset of puberty. This suggests that leptin may act as a signal for this event. Insulin did not vary during growth and prepuberty. On the other hand, GH was not responsible for diminished IGF-I levels in parasitized animals as levels were similar in both groups. The GH levels were high at birth and then diminished rapidly and remained constant during development and puberty. The last hormone studied, prolactin, followed seasonal changes of sunlight duration and presented sporadic bursts in infected animals. These were related to high nematode infection and are probably involved in the immune response of the host.

Research paper thumbnail of Effect of anthelmintics on reproductive performance and first-lactation culling rate in Holstein heifers

Female Holstein calves were treated with ivermectin from birth to first oestrus to study the effe... more Female Holstein calves were treated with ivermectin from birth to first oestrus to study the effect of parasitic burden and anthelmintic treatment on reproductive and productive performance. First oestrus, age at first service and age at calving were advanced by 30, 70 and 110 days, respectively (P<0.05), in ivermectin-treated animals compared with controls. No significant differences were observed in the conception rate, the number of services and the characteristics of the newborn calves and any problems at calving between the two groups. Daily milk yield, fat content in milk during first lactation, and the concentrations of growth hormone, insulin-like growth factor type 1, insulin and prolactin in serum were similar in both groups of cows. Culling during the first lactation was more common in untreated (47 per cent) than in treated (11 per cent) cows (P<0.05).

Research paper thumbnail of Gastrointestinal parasites presence during the peripartum decreases total milk production in grazing dairy Holstein cows

Veterinary Parasitology, 2011

Parasitism in cattle is known to impair growth and development. Recent findings suggest that prod... more Parasitism in cattle is known to impair growth and development. Recent findings suggest that productivity of adult animals is also affected, but little is known about the physiological mechanisms involved. Furthermore, development of nematode resistance to drugs makes imperative the search of management practices that avoid whole herd treatment. We undertook an epidemiological and endocrine study in a grass based dairy farm in Argentina to study the effect of parasites on milk production and the underlying mechanisms involved, and identify individual animals that would benefit from antiparasitic treatment. All the cows in the dairy were followed monthly for egg parasite output in feces. Samples were cultured for genera determination. Milk production and reproductive results were recorded and periodical bleedings for hormone determination were performed. Nematode egg output (EPG) was maximal in late Summer and Autumn and minimal in Spring in coincidence with the Ostertagia inhibition-disinhibition cycle as this genus had the highest prevalence in all the study. The highest proportion of positive samples was found in the high producing herd and maximal counts were found in the peripartal period. Milk production did not correlate with EPG mean values but, when cows were grouped by EPG positivity around parturition, a significant difference in total milk production between EPG null and positive cows was observed. Positive cows produced 7%, 12% or 15% less milk than null EPG cows, depending on the sampling month/s chosen for classification. The highest difference was seen when both prepartum and postpartum samples were taken into account. No difference in lactation length and a marginal effect on partum to first service interval were encountered. Endocrine studies revealed a decrease in serum growth hormone (GH), type I insulin-like growth factor (IGF-I) and prolactin during lactation in cows with positive EPG in the first postpartum sample with respect to null EPG cows at that time. GH levels decreased and prolactin and IGF-I levels increased in both groups of cows from month 0 to 6 in milk. Serum insulin levels remained stable throughout lactation and were similar in both groups of cows. In conclusion, EPG around parturition may be a useful tool for identifying cows that will have a decrease in productivity due to parasite effects and would possibly benefit from an antiparasitic treatment. Besides, our results suggest that detrimental effect of parasites on milk production may be mediated by GH, IGF-I and prolactin serum levels.

Research paper thumbnail of Comparison of three methods for gastrointestinal nematode diagnosis determination in grazing dairy cattle in relation to milk production

Research paper thumbnail of Endocrine studies in ivermectin-treated heifers from birth to puberty1

Research paper thumbnail of Endometritis treatment with a PGF 2α analog does not improve reproductive performance in a large dairy herd in Argentina

Theriogenology, 2005

In Argentina, most dairy cows with endometritis are treated with prostaglandin (PGF 2a or its ana... more In Argentina, most dairy cows with endometritis are treated with prostaglandin (PGF 2a or its analogs) and insemination is withheld until there are no signs of endometritis. The objective of the present study was to evaluate if this method of managing endometritis enhances reproductive performance. Three experiments were conducted over 4 years in a large farm in the west of Buenos Aires province. In Experiment 1, half of the cows diagnosed with endometritis (>1.5-fold difference in diameter of uterine horns, as determined by rectal palpation) received standard endometritis management (treatment with tiaprost, a PGF 2a analog, rectal palpation every 20 days, and withholding of AI until endometritis apparently resolved) and the other half was untreated, with AI at the first estrus after the voluntary waiting period. Untreated cows were inseminated and conceived 20 days earlier than treated cows, and the pregnancy rate by Day 90 postpartum was higher in the untreated group. In Experiment 2, cows with endometritis were divided into four groups according to the severity of symptoms; within each group, cows were allocated to treatment or control, as in Experiment 1. Although first service conception rate decreased as endometritis severity increased, reproductive performance in treated versus control cows was similar to that of Experiment 1 (with no interaction due to degree of endometritis). Re-evaluation of the treated cow (to confirm uterine ''normality'') may have been responsible for the delay in conception in both experiments. The objective of Experiment 3 was to determine the effects of tiaprost treatment on clinically normal postpartum cows (no evidence of endometritis). Tiaprost treatment reduced the interval from calving to conception in multiparous cows, but it delayed conception and reduced the conception rate in primiparous cows. In conclusion, treatment with tiaprost impaired reproductive performance in primiparous cows (in the absence of endometritis). Furthermore, the standard treatment for endometritis (treatment with a prostaglandin analog and withholding insemination until clinical signs abated) impaired reproductive performance and increased costs. #

Research paper thumbnail of Síndrome nefrótico y sus complicaciones en el hospital regional infantil Dr. Arturo Grullón

RESUMEN Con el objetivo de determinar la frecuencia de síndrome nefrótico ysus complicaciones en ... more RESUMEN Con el objetivo de determinar la frecuencia de síndrome nefrótico ysus complicaciones en los pacientes ingresa-dos en el hospital Regional InfantilDr.ArturoGrullón, San-tiago, República Dominicana, en el año 2002, se realizó un estudio descriptivo, transversal yretrospectivo, ...

Research paper thumbnail of Metabolic cues for puberty onset in free grazing holstein heifers naturally infected with nematodes

Theriogenology, 2001

Leptin is a new plausible candidate for the molecular link between nutritional status and the rep... more Leptin is a new plausible candidate for the molecular link between nutritional status and the reproductive axis. In previous studies we described that continuous natural nematode infections in heifers retarded growth and delayed the onset of puberty, and that the insulin-like growth factor I (IGF-I) was involved. In the present study we monitored the leptin levels during development in heifers naturally parasitized versus those chronically treated with ivermectin and we investigated whether growth hormone (GH) accounted for the differences in IGF-1 previously noted. Insulin levels were also measured. Prolactin hormone was recorded as an indicator of immune system activation. We found a direct correlation between leptin and body weight during development and a prepubertal surge of the hormone 2 weeks before the first progesterone peak that indicates the onset of puberty. This suggests that leptin may act as a signal for this event. Insulin did not vary during growth and prepuberty. On the other hand, GH was not responsible for diminished IGF-I levels in parasitized animals as levels were similar in both groups. The GH levels were high at birth and then diminished rapidly and remained constant during development and puberty. The last hormone studied, prolactin, followed seasonal changes of sunlight duration and presented sporadic bursts in infected animals. These were related to high nematode infection and are probably involved in the immune response of the host.

Research paper thumbnail of Effect of anthelmintics on reproductive performance and first-lactation culling rate in Holstein heifers

Female Holstein calves were treated with ivermectin from birth to first oestrus to study the effe... more Female Holstein calves were treated with ivermectin from birth to first oestrus to study the effect of parasitic burden and anthelmintic treatment on reproductive and productive performance. First oestrus, age at first service and age at calving were advanced by 30, 70 and 110 days, respectively (P<0.05), in ivermectin-treated animals compared with controls. No significant differences were observed in the conception rate, the number of services and the characteristics of the newborn calves and any problems at calving between the two groups. Daily milk yield, fat content in milk during first lactation, and the concentrations of growth hormone, insulin-like growth factor type 1, insulin and prolactin in serum were similar in both groups of cows. Culling during the first lactation was more common in untreated (47 per cent) than in treated (11 per cent) cows (P<0.05).

Research paper thumbnail of Gastrointestinal parasites presence during the peripartum decreases total milk production in grazing dairy Holstein cows

Veterinary Parasitology, 2011

Parasitism in cattle is known to impair growth and development. Recent findings suggest that prod... more Parasitism in cattle is known to impair growth and development. Recent findings suggest that productivity of adult animals is also affected, but little is known about the physiological mechanisms involved. Furthermore, development of nematode resistance to drugs makes imperative the search of management practices that avoid whole herd treatment. We undertook an epidemiological and endocrine study in a grass based dairy farm in Argentina to study the effect of parasites on milk production and the underlying mechanisms involved, and identify individual animals that would benefit from antiparasitic treatment. All the cows in the dairy were followed monthly for egg parasite output in feces. Samples were cultured for genera determination. Milk production and reproductive results were recorded and periodical bleedings for hormone determination were performed. Nematode egg output (EPG) was maximal in late Summer and Autumn and minimal in Spring in coincidence with the Ostertagia inhibition-disinhibition cycle as this genus had the highest prevalence in all the study. The highest proportion of positive samples was found in the high producing herd and maximal counts were found in the peripartal period. Milk production did not correlate with EPG mean values but, when cows were grouped by EPG positivity around parturition, a significant difference in total milk production between EPG null and positive cows was observed. Positive cows produced 7%, 12% or 15% less milk than null EPG cows, depending on the sampling month/s chosen for classification. The highest difference was seen when both prepartum and postpartum samples were taken into account. No difference in lactation length and a marginal effect on partum to first service interval were encountered. Endocrine studies revealed a decrease in serum growth hormone (GH), type I insulin-like growth factor (IGF-I) and prolactin during lactation in cows with positive EPG in the first postpartum sample with respect to null EPG cows at that time. GH levels decreased and prolactin and IGF-I levels increased in both groups of cows from month 0 to 6 in milk. Serum insulin levels remained stable throughout lactation and were similar in both groups of cows. In conclusion, EPG around parturition may be a useful tool for identifying cows that will have a decrease in productivity due to parasite effects and would possibly benefit from an antiparasitic treatment. Besides, our results suggest that detrimental effect of parasites on milk production may be mediated by GH, IGF-I and prolactin serum levels.

Research paper thumbnail of Comparison of three methods for gastrointestinal nematode diagnosis determination in grazing dairy cattle in relation to milk production

Research paper thumbnail of Endocrine studies in ivermectin-treated heifers from birth to puberty1

Research paper thumbnail of Endometritis treatment with a PGF 2α analog does not improve reproductive performance in a large dairy herd in Argentina

Theriogenology, 2005

In Argentina, most dairy cows with endometritis are treated with prostaglandin (PGF 2a or its ana... more In Argentina, most dairy cows with endometritis are treated with prostaglandin (PGF 2a or its analogs) and insemination is withheld until there are no signs of endometritis. The objective of the present study was to evaluate if this method of managing endometritis enhances reproductive performance. Three experiments were conducted over 4 years in a large farm in the west of Buenos Aires province. In Experiment 1, half of the cows diagnosed with endometritis (>1.5-fold difference in diameter of uterine horns, as determined by rectal palpation) received standard endometritis management (treatment with tiaprost, a PGF 2a analog, rectal palpation every 20 days, and withholding of AI until endometritis apparently resolved) and the other half was untreated, with AI at the first estrus after the voluntary waiting period. Untreated cows were inseminated and conceived 20 days earlier than treated cows, and the pregnancy rate by Day 90 postpartum was higher in the untreated group. In Experiment 2, cows with endometritis were divided into four groups according to the severity of symptoms; within each group, cows were allocated to treatment or control, as in Experiment 1. Although first service conception rate decreased as endometritis severity increased, reproductive performance in treated versus control cows was similar to that of Experiment 1 (with no interaction due to degree of endometritis). Re-evaluation of the treated cow (to confirm uterine ''normality'') may have been responsible for the delay in conception in both experiments. The objective of Experiment 3 was to determine the effects of tiaprost treatment on clinically normal postpartum cows (no evidence of endometritis). Tiaprost treatment reduced the interval from calving to conception in multiparous cows, but it delayed conception and reduced the conception rate in primiparous cows. In conclusion, treatment with tiaprost impaired reproductive performance in primiparous cows (in the absence of endometritis). Furthermore, the standard treatment for endometritis (treatment with a prostaglandin analog and withholding insemination until clinical signs abated) impaired reproductive performance and increased costs. #