Hypergastrinaemia, abomasal bacterial population densities and pH in sheepinfected with Ostertagia circumcincta

Serum gastrin and pepsinogen concentrations, food intake, abomasal pH and a bomasal aerotolerant and anaerobic bacterial populations were measured in s heep infected with Ostertagia circumcincta to search for links between hype rgastrinaemia, food intake and changes in the abomasal environment. Abomasa l pH and serum gastrin and pepsinogen concentrations were elevated in each of five sheep infected via abomasal cannulae with 150 000 exsheathed larval stage three, followed 11 days later by 100 000 sheathed larvae given intra ruminally. Unparasitised abomasa contained aerotolerant bacterial populatio n densities of between 10(3) and 10(6) cells ml(-1) and these did not chang e significantly following parasitism. In contrast, anaerobic bacterial popu lation densities increased markedly by about 10(4)-fold following parasitis m. Anaerobic numbers changed rapidly when abomasal pH increased from 2.5 to 3.5. At pH 4 and above, anaerobic bacterial numbers approached levels expe cted in rumen contents but parameters other than pH did not relate to bacte rial numbers. Brief periods when serum gastrin was lower than expected, coi nciding with raised abomasal pH, were not explicable by increased bacterial numbers. Food intake, which decreased for a variable period from around Da y 5 p.i., correlated poorly with serum gastrin concentration, suggesting hy pergastrinaemia is not the sole cause of anorexia in parasitised animals. T he survival of substantial numbers of rumen bacteria in the abomasum at onl y slightly raised pH may significantly lower the bacterial protein availabl e to the sheep. (C) 1999 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.