Abstract
It is well known that disease adversely affects the production of milk and meat
in domestic livestock and therefore it would be surprising if certain diseases did not
also affect the work output of draught animals. There is however no direct evidence
of this, although there is strong evidence from research with humans and laboratory
animals that exercise depresses immune responses. This effect may be exacerbated by
undernutrition. Draught animals are required to work hard for short periods during
the year, these periods often coincide with seasonal food shortages and sometimes
with outbreaks of disease. The combination of work, disease and malnutrition may
have serious adverse effects on the ability of draught animals to complete cultivation
tasks. In countries with short rainy seasons this can have a potentially disastrous
effect on subsequent crop yields.
The interactions of disease, work and undernutrition in draught animals were
examined in three separate studies. The first study carried out in Indonesia
investigated the effects of Trypanosoma evansi on the work output of twelve swamp
buffalo in two groups. The second undertaken in the UK, used four groups of six
sheep as a model to examine the effects of exercise and plane of nutrition on cellular
and humoral immune responses in the absence of a pathogen. The final study in The
Gambia, looked at the effects of work and undernutrition on the trypanotolerance of
32 N'Dama cattle in four groups challenged with T. congolense. In the first
experiment work output was monitored for 11 weeks in infected and uninfected
animals using a cross-over design. In the second laboratory based experiment the
immune responses of sheep challenged with two foreign antigens Brucella abortus
and ovalbumin, were measured for 11 weeks after challenge in a factorial design
involving two planes of nutrition and two levels of exercise. In the third study the
same factorial design was used to examine the effects of work and undernutrition on
the trypanotolerance of N'Dama cattle challenged with T. congolense. The results of
all three studies were analysed using either parametric and non-parametric statistical
tests as appropriate.
In the first experiment carried out in Indonesia, after a protracted period of
work (5 weeks pre-infection and 4 weeks post-infection), the parasitaemias of some
infected buffalo increased dramatically and at the same time work output declined.
However because of the experimental design and because a similar effect was not seen
in the first period of the experiment, it was not possible to prove that trypanosomosis
had caused this fall in output. In the second study with sheep, immune responses
varied markedly between individuals but there were no significant differences between
groups, with one exception, the speed of the primary antibody response to ovalbumin
was significantly faster in sheep on the high plane of nutrition than in those on the low
plane. In the final study in The Gambia work caused significant increases in animal
parasitaemias and reductions in blood packed cell volumes. Some of the working
cattle became so severely anaemic (PVC's < 15%) that they were unable to complete
the normal daily work programme and had to be retired early, the first after only
seven weeks work.
In conclusion it appears that in some circumstances work can affect the course
of a disease and conversely disease can reduce work output. It is however very
difficult to quantify these effects because of large differences in response between
individual animals and differences in pathogenicity between different diseases and
different strains of the same disease.
