A cross-sectional study of paratuberculosis in cattle from selected districts of Western Uganda and analysis of potential virulence of selected strains

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 within herd 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 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; umaA1, 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 paratuberculosis as an 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.