Seroprevalence of brucellosis and risk factors associated with its seropositivity in cattle, goats and humans in Iganga District, Uganda

Introduction The burden of brucellosis among smallholder farmers is poorly-documented in Uganda. The disease burden is likely to be high, given the high levels of endemicity, lots of exposures and due to lack of control measures. In order to designate appropriate control measures, the magnitude and risk factors for brucellosis need to be known. We established the burden of and risk factors for Brucella seropositivity in cattle, goats, and humans in Iganga district, eastern Uganda. Methods A cross-sectional study was conducted in in Kigulamo Parish, Iganga District. We enrolled 226 households and administered a structured questionnaire to heads of households to capture data on socio-demographic characteristics, human brucellosis-related risk factors, and livestock farming practices. Human, cattle, and goat blood samples were collected and tested serologically using commercial indirect-ELISA kits manufactured by USDA, USA. Results Of 451 human blood samples, 20 (4.4%) were positive. Among 345 cattle blood samples, 4 (1.2%) were positive and among 351 goat blood samples, one (0.3%) was positive. Persons who reported consuming locally-made dairy products had 4 times higher odds of Brucella seropositivity (OR = 4.0, CI = 1.14-14.03, p = 0.031) than those who did not. None of the risk factors we asked about were significantly associated with seropositivity in cattle and goats. Conclusion The seroprevalence of brucellosis in humans in smallholder households in Kigulamo was relatively low and associated with consumption of locally made dairy products. No risk factors were significantly associated with seropositivity in livestock, likely due to the small number of seropositive animals. We recommend a One Health approach to control brucellosis simultaneously in animals and humans needed to sustainably reduce the burden of brucellosis in Uganda and beyond.


Introduction
Brucellosis is a zoonotic disease caused by Brucella species [1], and infects both humans and animals [1,2]. It exists worldwide, except from countries where eradication of bovine brucellosis has been achieved [3]. The disease is multiple species: those commonly implicated in domestic animals include Brucella melitensis, Brucella abortus, Brucella canis and Brucella suis, with their preferential hosts as sheep/goats, cattle, dogs, and pigs, respectively [4]. In humans, the implicated species include Brucella melitensis and Brucella abortus; thus, goats and cattle are associated with human infection.
The prevalence of brucellosis varies widely in animals and humans by country [5]. Globally, approximately 500,000 humans are infected per year [6]. The worldwide economic losses due to brucellosis are extensive not only in animal production but also in human health. In sub-Saharan Africa, the prevalence of brucellosis in humans ranges from 5-55% [6] in different countries, while in domestic animals it is between 8-46% [7]. Infection in humans is attributed to consumption of contaminated animal products such as undercooked meat or unboiled milk and through use of other animal products which are not well-aged or pasteurized [6,8]. In animals, risk for infections is associated with management factors such as herd size, population density, and herd immunity [8,9]. Consumption of unboiled milk was significantly (p = 0.004) associated with seropositivity in Mbarara District, however no association was reported among seropositivity with age, sex and awareness of human brucellosis in this study [10].
Mixed livestock or breeds have a higher possibility of infection as opposed to single breed herds [11] and mixing goats and sheep in Eastern and Western Uganda was a probable risk factor [12].
According to Silva et al. extensive grazing, large herd sizes and free grazing are some of the risk factors associated with Brucellosis [13].
Transmission of the disease can also be due to livestock movement from one geographical region with infection to another as well as hygiene factors [1]. In a study carried out by [14] age, entry of purchased animal on the farm, type of breed and sensitization of farmers were outlined as other important risk factors. Brucellosis has been studied previously in different areas of Uganda, where it is known to be endemic. In a study carried out in 2015 in South-western Uganda, the prevalence of brucellosis seropositivity was 14% in cattle, 17% in goats, and 11% in humans [15], and the seroprevalence of brucellosis among exposed cattle-keepers in Mbarara and consumers of unpasteurized milk in Kampala Districts was 5.8% and 9%, respectively [10]. Tumwine et al. [16]  The prevalence of brucellosis is higher in the pastoral grazing areas than in the urban and peri-urban areas [9]. In animals, specifically in cattle population in central and southern Uganda, the individual animal seroprevalence range from 8% to 75.9%, while in goats it was between 4% and 10% by various serological tests [18,19] Data collection: a list of households with at least one herd of cattle or goat in Kigulamo Parish was obtained from local officials and a total of 2 persons per household consented to participate in the study. The sample size for cattle and goats for the study was separately estimated using a sample determination formula described by Thrusfield, 1995 based on 95% confidence level and historical prevalence of 5.5% [9,26]. Table 1 and Table 2 (Table 5) were considered for inclusion in multivariable regression and analysed for being indicators of brucellosis (Table 6).
In multivariate analysis, only consumption of locally-made milk products (OR = 4.0, CI = 1.14-14.03) was associated with brucellosis seropositivity. Persons who reported consuming locally-made milk products were 4 times more likely to be seropositive than those not consuming. No risk factor was associated with brucellosis in cattle.
Only sex and age were associated with Brucella seropositivity in goats (Table 7). There were no risk factors found to be significant in multivariabe analysis.

Discussion
The  probably because animals in Uganda are more confined compared to other Sub-Saharan countries. At multivariable regression analysis, consumption of locally made milk products significantly increased perceived risk of brucellosis infection. Respondents who consumed the locally made dairy products were believed to be 4 times more likely to test positive to Brucella antigen than those who did not. This agreed with previous studies that revealed that consumption of unboiled milk was significantly associated with brucellosis seropositivity in Mbarara District [10] and Nigeria [33]. No significant statistical difference was found in prevalence between males and females. In this study we found out that more males assisted in animal deliveries and this probably explains why there was a tendency for males to test more seropositive than females.Although no significant differences were found between cattle of age groups below 1 year, 1-2 years and those aged above 3 years, the odds ratio (OR = 2.9) suggested older cattle above 3 years were more likely to test positive for brucellosis than younger cattle. This finding could be due to increased likelihood of exposure with age of cattle. This agrees with previous studies which found age to be a significant risk factor for brucellosis infection in livestock [34,35]. The tendency to have older animals testing more seropositive than the younger ones disagrees with the findings of Makita [9].

Conclusion
The seroprevalence of brucellosis in humans in smallholder households in Kigulamo was relatively low. The seroprevalence of brucellosis in cattle and in goats was also low compared to that previously reported in other parts of Uganda. No perceived risk factors were associated with Brucella seropositivity in cattle and goats.
However consumption of locally made milk products was the major risk factor associated with Brucella seropositivity in humans.
Nonetheless its impact on public health cannot be underestimated.
There is need for a One-health approach to reduce sustainably the burden of brucellosis and other zoonotic diseases in Uganda and beyond.  Table 1: sero-prevalence of brucellosis among humans by village            Page number not for citation purposes 10