Spatiosurvival analysis of mortality on smallholder dairy farms in Tanga and Iringa regions of Tanzania.

A combination of survival and spatial analyses was applied to a dataset containing cattle mortalities on smallholder dairy farms in two separate regions (Tanga and Iringa) of Tanzania. Cattle mortality data for the year 1998 were collected retrospectively during the period January to April 1999. The objectives of the current study were, firstly, to quantify the hazard of mortality and, secondly, to investigate whether there was a spatial pattern in the variation that was unaccounted for in the hazard of mortality. A Cox regression model with farm as a frailty term was used to quantify the hazard of mortality. Geostatistical techniques were used to explore the first- and second-order spatial distribution of the farm frailty term. A total of 1,790 cattle from 400 randomly sampled smallholder dairy farms were included in the study. The overall mortality incidence rate for the study was 11.8 per 100 animal years (95% CI 10.0-13.8). The hazard of mortality for male cattle was 2.4 (95% CI 1.7-3.4) times higher than that of female cattle. There was evidence of a first-order spatial pattern, i.e., a tendency for farms with higher frailty to aggregate.