Abstract:

Paratuberculosis (Johne’s Disease) caused by Mycobacterium avium subsp. paratuberculosis
(MAP) is a devastating disease in many animals but most especially dairy cattle with serious
economic losses in production. The disease occurs worldwide and is highly suspected to be
zoonotic. In dairy cattle, paratuberculosis is characterised by chronic granulomatous enteritis
marked by chronic or intermittent diarrhoea, leading to progressive weight loss, unthriftiness and
cachexia which culminates into death. Losses associated with paratuberculosis include: reduced
milk production, body wasting, increased veterinary costs, and early culling or eventual death of
the affected animal. The occurrence of the disease and associated risk factors is not fully known in
Uganda, and the virulence of the different local strains has not been documented yet these are
important to design strategies for the management of paratuberculosis (PTB). To determine the
prevalence of PTB, a survey was carried out in six districts of western Uganda in which 1,814 sera
and faecal samples were collected from 93 herds of cattle. Serum was tested with ELISA, whereas,
MAP DNA from faeces was detected by a molecular Recombinase Polymerase Amplification
assay. A true cow-level sero- and molecular prevalence of 4.9% and 3% respectively and a withinherd
sero-prevalence of 3.8 ± 2.1% were obtained. Herd-level prevalence of 43% and 41% were
got based on ELISA and RPA respectively. The high herd-level prevalence is an indication of
active PTB transmission among herds. Prolonged dry seasons were significantly associated with
MAP occurrence which may be attributed to stress leading to increased susceptibility to infection.
These results show the existence of PTB in the surveyed areas and the disease may become
endemic in the country if no control measures are instituted. Mycobacterium avium subsp.
paratuberculosis isolation and characterisation were also done to evaluate the potential virulence
of the different MAP isolates using an in vitro infection model. Significant differences in terms of
cytokine and virulence gene expression as well as the viability and quantity of MAP inside RAW
xvii
264.7 macrophages at different time points were observed among isolates, an indication of variable
virulence. Phylogenetic analysis of the putative virulence genes of MAP exhibited differences
when compared with the reference MAP K-10 strain for genes including; umaAl, katG, kdpC,
fabG2_2, papA2 and impA. These polymorphisms may explain the variable virulence observed in
the in vitro infection study. The function of these genes and others requires further inquiry to
establish their possible role in MAP pathogenesis in an in vivo model. This is the first study
involving MAP strain virulence analysis in Uganda. The study is useful in addressing the plight of
emerging infection, especially in the design of strategies for its
management. However, full data on MAP occurrence throughout the country and its possible
involvement in human diseases are needed to inform policy on measures for PTB control and
prevention.