Epidemiology of brucellosis in urban and peri-urban cattle in Gulu and Soroti towns of Uganda

Abstract

Brucellosis is a zoonosis of major public health, animal welfare, and economic significance, and is endemic in livestock in Uganda. The transmission of this disease in cattle and humans involves a multitude of risk factors, with unprocessed animal products accounting for most of the human infections. Most of unprocessed livestock products, like milk, in Uganda are consumed by the urban population. To shed new light on the epidemiology of brucellosis in urban and peri-urban areas in Uganda, a cross-sectional study was carried out in Gulu and Soroti towns to estimate the sero-prevalence of brucellosis in cattle, identify factors associated with sero-positivity, isolate, biotype and molecularly characterize Brucella isolates from cattle milk and determine their antimicrobial susceptibility. Gulu and Soroti are the two rapidly growing towns in northern and eastern Uganda, respectively, with a high rate of rural to urban human migration and growing urban and peri-urban livestock farming. A total of 1007 sera and data on biologically plausible risk factors from 166 herds and their spatial locations, were collected from cattle. The sera were analyzed using indirect ELISA and sero-positive reactors confirmed by competitive ELISA. Multivariable models were used to investigate for risk factors. Milk samples were collected from lactating cattle at the time and cultured in Brucella selective media. The isolates were bio-typed and molecularly characterized and their antimicrobial susceptibility determined using the broth micro-dilution method. The overall animal-level and herd-level sero-prevalence was 7.5% (n = 1007, 95% CI: 6.15 – 9.4%) and 27.1% (n = 166, 95% CI: 20.9% - 34.3%), respectively. Herd-level sero-prevalence was significantly (p<0.001) higher in Soroti than Gulu. In Gulu town, sero-positivity increased with an increase in herd size (p=0.03) and age (p=0.002), and was higher in cattle brought in from western Uganda (p<0.0001). In Soroti town, introduction of new cattle into a herd was significantly (p=0.027) associated with herd sero-positivity. There was a geographically differential risk (clustering) of Brucella sero- positivity in herds in Soroti town. The data highlights differences in brucellosis risk factors in Gulu and Soroti towns and in turn calls for a tailored approach when designing control measures for prevention of transmission of bovine brucellosis in different areas. Brucella abortus without a biovar designation was isolated from eleven out of 207 milk samples from cattle in Gulu and Soroti towns of Uganda. These isolates had a genomic monomorphism at 16 variable number tandem repeat (VNTR) loci indicating that they may be from a common source of infection. The isolates showed in turn high levels of genetic variation when compared with other African strains or other B. abortus biovars from other parts of the world. Comparatively, the isolates had a closer genomic similarity with a Kenyan B. abortus strain-07-994-2411 (a former B. abortus biovar 7) than any other known strains from distant regions suggesting an epidemiological linkage. All the isolates were susceptible to tetracycline, streptomycin, gentamicin, ciprofloxacin, ceftazidime and cefotaxime, but had intermediate susceptibility to rifampicin. All isolates were resistant to ampicillin, sulfamethoxazole and trimethoprim, drugs that had shown activity in previous studies elsewhere. The isolates being resistant to rifampicin calls for a judicious use of streptomycin and tetracyclines to avoid development of resistance to them as well, since these remain the best alternatives to rifampicin in treating human cases of brucellosis