Seasonal differences in the incidence of infection with Fasciola gigantica in Cambodian cattle.

Farmer's cattle were treated with triclabendazole and used as tracer animals to detect new infections with Fasciola gigantica in three villages located on the bank of the Bassac River (a major tributary of the Mekong River) and in a fourth village located on farmland away from the river, from April 1999 until January 2001. The month of infection was estimated by subtracting 4 months from the date when eggs of F. gigantica were detected in faeces. Farmers were interviewed each month to record the nature of the agricultural and animal husbandry activities that occurred during the previous month, especially events that might have exposed cattle to infection with F. gigantica. Results support the conclusions that infection of cattle in riverbank villages acquired from about August or September until November originated from herbage and water in irrigation canals and dams on the riverbank, and that the progressively increasing monthly incidence from December until April (up to 87% per month in April 2000) was derived from herbage and water in recently harvested rice fields and lakes adjacent to the riverbank. The abrupt cessation of new infection in riverbank villages in May coincided with flooding of low-lying land, the movement of cattle to land above flood height on the riverbank, and a change of diet to dry-land crop residues, stored dry rice stalks, and herbage and water that were unlikely to contain metacercariae. It was concluded that snails in dams and canals on the riverbank became infected with E gigantica after cattle were moved to the riverbank in May, and cercariae shed from these snails provided the new infections that occurred in cattle in August and September. In the village located away from the river, infection of cattle between September and March coincided with the rice harvest, supporting the conclusion that feeding of fresh rice stalks and stubble after the rice was harvested was the main source of infection. The low monthly incidence observed (up to 6.4% per month) was consistent with the hypothesis that snails did not survive in the dry rice fields between crops and that few snails would have been available from the small number of aquatic refuges that persisted through the dry season to recolonize rice fields during the wet season. Between April and August there was no opportunity for new infection because cattle were fed forage from around houses and headlands, and on dry-land crop residues and stored dry rice stalks. Control of fasciolosis was proposed using a single treatment of cattle with triclabendazole in riverbank villages in May when cattle were moved to the riverbank, and after harvest of the last rice field in villages located away from the river.

Histamine directly stimulates gonadotropin-releasing hormone secretion from GT1-1 cells via H1 receptors coupled to phosphoinositide hydrolysis.

It is unclear whether the central stimulating effect of histamine on GnRH secretion is exerted directly on GnRH neurosecretory neurons or indirectly via multisynaptic pathways, and controversy exists about the nature of the receptors involved. The current studies were undertaken to examine whether GnRH secretion from immortalized GnRH cell lines is directly regulated by histamine and, if so, to determine the identity of the receptors and the signaling pathways coupling this action. Histamine stimulated GnRH release from GT1-1 cells in a sustained and reversible manner and in a dose-dependent fashion. This effect was blocked by the selective H1 histamine receptor antagonist, mepyramine, but not by the H2 or H3 antagonists, ranitidine or thioperamide, respectively. Saturable and specific binding sites for [3H]mepyramine were demonstrated in GT1-1 cells, showing high affinity (apparent Kd, 37.8 nM) and density (apparent binding capacity, 279 fmol/mg protein) comparable to respective values in brain tissue. Competition of [3H]mepyramine binding was achieved with mepyramine at concentrations 3 orders of magnitude lower than those of ranitidine. Histamine also increased the production of inositol phosphates in GT1-1 cells in a dose- and time-dependent manner. This response was mimicked by the selective H1 receptor agonist 2-thiazolylethylamine and blocked by the H1 antagonists mepyramine, chlorpheniramine, and triprolidine. In contrast, histamine did not alter the formation of cAMP in GT1-1 cells. The present results show a direct action of histamine on immortalized GnRH neurons, suggesting that histamine may stimulate the reproductive axis by activation of H1 receptors on the surface of GnRH neurons coupled to the formation of inositol phosphates.