Mortality in East African shorthorn zebu cattle under one year : predictors of infectious-disease mortality
Thumbi, Samuel Mwangi; Bronsvoort, Barend Mark de Clare; Kiara, Henry; Toye, Philip G.; Poole, Jane; Ndila, Mary; Conradie, Ilana; Jennings, Amy; Handel, Ian Graham; Coetzer, Jacobus A.W.; Steyl, Johan Christian Abraham; Hanotte, Olivier
BACKGROUND: Infectious livestock diseases remain a major threat to attaining food security and are a source of
economic and livelihood losses for people dependent on livestock for their livelihood. Knowledge of the vital
infectious diseases that account for the majority of deaths is crucial in determining disease control strategies and in
the allocation of limited funds available for disease control. Here we have estimated the mortality rates in zebu
cattle raised in a smallholder mixed farming system during their first year of life, identified the periods of increased
risk of death and the risk factors for calf mortality, and through analysis of post-mortem data, determined the
aetiologies of calf mortality in this population. A longitudinal cohort study of 548 zebu cattle was conducted
between 2007 and 2010. Each calf was followed during its first year of life or until lost from the study. Calves were
randomly selected from 20 sub-locations and recruited within a week of birth from different farms over a 45 km
radius area centered on Busia in the Western part of Kenya. The data comprised of 481.1 calf years of observation.
Clinical examinations, sample collection and analysis were carried out at 5 week intervals, from birth until one year
old. Cox proportional hazard models with frailty terms were used for the statistical analysis of risk factors. A
standardized post-mortem examination was conducted on all animals that died during the study and appropriate
RESULTS: The all-cause mortality rate was estimated at 16.1 (13.0-19.2; 95% CI) per 100 calf years at risk. The Cox
models identified high infection intensity with Theileria spp., the most lethal of which causes East Coast Fever
disease, infection with Trypanosome spp., and helminth infections as measured by Strongyle spp. eggs per gram of
faeces as the three important infections statistically associated with infectious disease mortality in these calves.
Analysis of post-mortem data identified East Coast Fever as the main cause of death accounting for 40% of all
deaths, haemonchosis 12% and heartwater disease 7%. CONCLUSION: The findings demonstrate the impact of endemic parasitic diseases in indigenous animals expected to
be well adapted against disease pressures. Additionally, agreement between results of Cox models using data from
simple diagnostic procedures and results from post-mortem analysis underline the potential use such diagnostic
data to reduce calf mortality. The control strategies for the identified infectious diseases have been discussed.