Bovine tuberculosis in Ethiopian local cattle and wildlife : epidemiology, economics and ecosystems

Ethiopia has recently been focusing on intensive dairy cattle farming in order to supply
the growing demand for milk. For this purpose Holstein Frisians have been imported,
bred and distributed in farms especially around Addis Ababa. However, Holstein (Bos
taurus) although giving more milk than traditional zebus (Bos indicus) seem to be more
susceptible to bovine TB (BTB). Ethiopia ranks worldwide 8th in the number of newly
diagnosed TB cases, and the incidence of extra pulmonary TB was shown to be high.
Therefore, the following research questions have been raised: What is the prevalence of
BTB in cattle? What is the contribution of M. bovis in the national TB prevalence? Which
strains of M. bovis are present in cattle and humans? A consortium funded by the
Wellcome Trust (UK) and involving institutions from the UK, Switzerland, Ethiopia,
Ireland and Kenya was established in 2005 study BTB in Ethiopia and to address these
questions. The overall ultimate goal of the project is to measure the cost of BTB by
assessing its impact on the livestock and public health sector and to suggest the most
profitable intervention strategies. This thesis contributes to the overall BTB project by
providing baseline epidemiological and economical data needed for the development of a
SIR animal-human transmission model, and for the estimation of the economical impact
of BTB in the livestock sector in Ethiopia.
We assessed field prevalence of BTB using the comparative intradermal test (CIDT) in
6194 cattle in 5 different Woredas (districts) from Amhara, Oromia, and SNNPR regions.
Four of our study sites were located in the Ethiopian Highlands and were characterized
by extensive mixed crop-livestock farming with predominantly local zebu breeds,
whereas the last study site was a pastoralist area in the lowlands of south Omo (SNNPR).
We performed a repeated cross-sectional study in cattle in 3 of the Woredas, over 3 years.
In comparison to Central Ethiopia, which is characterized by the presence of a higher
number of exotic breeds and more intensive farming practice, apparent prevalence of
BTB in our study sites was very low (when using the official OIE definition for positive
reaction), with a minimum of 0% in the Bale Mountains and a maximum of 1.3% in Bako-Gazer and South Omo. We discussed the use of different cut-offs for the
interpretation of the skin test results.
In addition we interviewed in those study sites 450 farmers whose cattle were tested for
BTB in order to assess the risk for skin test positivity in cattle and TB diagnosis in
humans. Purchase of cattle and the presence of livestock other than cattle were a
significant risk factor for skin positivity in cattle. None of the classical risk factors such
as consumption of raw animal products, and close contact with animals were significantly
linked to the presence of human TB cases in households.
Since wildlife has been shown to be a potential reservoir for BTB in other countries we
started the first BTB survey in Ethiopian wildlife, in close collaboration with the
Ethiopian Wildlife Department and professional and recreational hunters. We sampled
specimens from 133 animals and performed serology in order to try to validate the rapid
test (RT), as well as culture (gold standard) followed by molecular typing of lymph node
samples. So far, no M. bovis were isolated from the 28 mammal species sampled (but
molecular analysis is still pending for a number of samples). However, 23% of the tested
animals, including flagship endemic rare species were sero-positive. Since we also serotested
live animals, culture could not be performed nor diagnosis confirmed in these
animals. Nearly half of the culture of samples yielded environmental Mycobacteria, their
role has still to be assessed.
In an additional investigation, 684 farmers were included in two independent surveys in
our study sites. This investigation highlighted the delicate balance between livestock,
cereal cropping and natural resources, the trend towards unsustainable use of natural
resources, fuelling a decrease in grazing land, human conflicts, and encroachments on
wildlife habitats and the need of a more holistic approach to secure future sustainability
of natural resources. Herd structure analysis showed that 52% of the total animals were
males, among them ¼ were oxen. Since oxen are intricately associated with agriculture
(e.g. ploughing, harvesting, threshing), farmers need to maintain a minimum herd size in
order to secure at least 2 draft animals, at all times. A disease such as BTB is therefore
likely to have a major impact on draft animals rather than on milk production in rural areas. Diseased draft animals are very likely to work less in the fields, leading to
decreased cropping yields, thus contributing to poverty and famine.
Some studies assessing the economical impact of BTB on the livestock sector were
started during this PhD but will go beyond the timeframe of the thesis. Economical
analysis of the impact of BTB to the society is not part of this thesis but the approach will
be discussed. We assessed the baseline productivity of cattle kept under traditional
extensive husbandry practice by following 21 farms (700 animals) over a period of four
years and keeping for each farm a herd book on productivity parameters (weight, milk,
fertility, entry, exit).In addition a long term study was started in collaboration with other
members of the consortium, to assess the impact of M. bovis on live animal and carcass
weight in six abattoirs.
Finally the data collected during the thesis suggest that the final analysis of BTB
(economical impact, transmission model, and intervention strategies) should be
performed on two distinct levels: 1) the urban and peri-urban level characterized by
intensive dairy farms, high numbers of exotic breeds and their cross breeds, high milk
production but also high BTB prevalence and 2) the rural level characterized by extensive
farming, local zebu breeds, low BTB prevalence and probable impact of the disease on
draft power rather than on milk.