Understanding the Negative Spiral: How Helminth Infection and Malnutrition Promote Mortality in Soay Sheep

Abstract

Gastrointestinal helminth infections have the ability to cause morbidity and mortality in domestic animals worldwide, leading to economic loss. When helminth infection is paired with protein malnutrition, the two disease states synergize to promote further morbidity and mortality in animals. This is known as the negative spiral. The Soay sheep population, whose isolated nature provides an ideal system in which to study the negative spiral, periodically drops due to the synergistic nature of the negative spiral. These “crashes” tend to occur during periods of high population density and harsh weather conditions, eliminating up to 70% of the population. The objective of this study was to assess what factors were most significant in predicting overwinter survival. To do so, a collection of classic field data was combined with laboratory protein assays to build a regression model for overwinter survival. Individual weight, age, sex, serum creatinine concentration, the interaction between serum total protein concentration and strongyle count were significant predictors of overwinter survival. In addition, stained liver tissues from the dead sheep were used to quantify the degradation of the animal’s tissues as a result of the negative spiral. The dead Soay sheep liver tissue showed higher levels of irregularity, indicating tissue break down. The model results, combined with the tissue degradation analysis, indicate that the negative spiral mechanism is in effect in this population.