Mass dynamics of the spleen and other organs in geese: measures of immune relationships to helminths?

The spleen is an important organ of avian immune systems. We examined wheth er helminth loads were related to spleen mass in the lesser snow goose, Che n caerulescens caerulescens. On 27 collecting occasions, 744 geese were obt ained at 13 different locations in a south-north gradient in midcontinental North America. The masses of the spleen, caecum, small intestine, large in testine, pancreas, heart, and crop of all geese were determined, and intest inal and caecal helminths were counted. Seventy-eight percent of geese harb ored at least one helminth species. For analyses, helminths were grouped as cestodes (26% prevalence), trematodes (19% prevalence), and nematodes (70% prevalence). After sample location and time, host age, host sex, and host body size were controlled for in a multivariate analysis of covariance, nem atodes were the only helminth group associated with variation in organ mass es. Greater nematode loads were weakly associated with lower spleen, higher caecum, lower large intestine, and lower heart masses. When uninfected ind ividuals were excluded from the analysis, seater nematode loads were no lon ger associated with variation in spleen size but were associated with highe r crop mass, and greater cestode loads were associated with higher heart ma ss. In neither of these analyses were any other cestode-organ or trematode- organ associations significant. Geese carrying two or more helminth groups had lower spleen masses than did geese infected with no or one helminth gro up. When we interchanged response and explanatory variables from the preced ing analyses and retained the same covariates, the same organ mass - helmin th associations tended to remain significant. Nonetheless, the small variat ion in helminth loads explained by variation in spleen mass (or vice versa) provided only weak support for the hypothesis that intraspecifically, wild individuals with lower investment in immunity are more susceptible to nema tode infections.