Abstract:
The co-occurrence of different pathogen species and their simultaneous infection of hosts are common, and may affect host
health outcomes. Co-infecting pathogens may interact synergistically (harming the host more) or antagonistically (harming
the host less) compared with single infections. Here we have tested associations of infections and their co-infections with
variation in growth rate using a subset of 455 animals of the Infectious Diseases of East Africa Livestock (IDEAL) cohort
study surviving to one year. Data on live body weight, infections with helminth parasites and haemoparasites were collected
every 5 weeks during the first year of life. Growth of zebu cattle during the first year of life was best described by a linear
growth function. A large variation in daily weight gain with a range of 0·03–0·34 kg, and a mean of 0·135 kg (0·124, 0·146;
95% CI) was observed. After controlling for other significant covariates in mixed effects statistical models, the results
revealed synergistic interactions (lower growth rates) with Theileria parva and Anaplasma marginale co-infections, and
antagonistic interactions (relatively higher growth rates) with T. parva and Theileria mutans co-infections, compared with
infections with T. parva only. Additionally, helminth infections can have a strong negative effect on the growth rates but this
is burden-dependent, accounting for up to 30% decrease in growth rate in heavily infected animals. These findings present
evidence of pathogen–pathogen interactions affecting host growth, and we discuss possible mechanisms that may explain
observed directions of interactions as well as possible modifications to disease control strategies when co-infections are
present.
