Brucellosis, an infection caused by gram-negative bacteria of the genus Brucella, is an economically important zoonotic disease of humans, domesticated animals, marine mammals, and wildlife. Six main species of Brucella are recognized, four of which are identified as pathogenic to man. Brucella melitensis, which predominantly affects goats and sheep, is the most common cause of human brucellosis, whereas B. аbortus, found mainly in cattle, buffalo, elk, yaks, and camels, is the second most common cause of human infection. B. suis, which infects domestic pigs and rodents, and B. canis in canines are increasing in importance as sources of human brucellosis.1–3 The Food and Agriculture Organization (FAO), the World Health Organization (WHO), and the World Organisation for Animal Health (OIE) consider brucellosis to be one of the most widespread zoonosis causing substantial morbidity in both livestock and human populations globally.4 Brucella is also a potential agent for bioterrorism because of its propensity for airborne transmission and induction of a chronic debilitating disease that requires combined and lengthy antibiotic therapies.5

Human brucellosis is mainly transmitted from animal reservoirs through consumption of unpasteurized dairy products and undercooked meat products, inhalation of contaminated dust and contact with infected animal body fluids or tissues.6,7 Airborne transmission of Brucella to humans has also been documented in clinical laboratories and abattoirs.4 Owing to its low infectious dose, brucellosis poses an occupational risk for farmers, veterinarians, abattoir workers, laboratory personnel, and others who work with animals and consume their products.8,9 Clinically, human brucellosis presents as a febrile disease with varied clinical manifestations that are dependent on the stage of disease or the organs and systems involved. This feature makes the disease an important differential diagnosis for other important febrile diseases including malaria, tuberculosis, and typhoid. Brucellosis is rarely fatal but can be severely debilitating and disabling with a tendency for chronicity and persistence. Acute brucellosis has been estimated to have disability comparable to that of acute malaria, whereas chronic, localized brucellosis has been estimated to have disability comparable to osteoarticular disease.10 Brucellosis requires prolonged treatment with a combination of antibiotics, and can lead to permanent and disabling sequelae with considerable financial and economic consequences.8,11

In animals, although specific Brucella species predominantly infect cattle, sheep, goats, and camels, some Brucella species are multi-host and it is common, for example, for the highly virulent B. melitensis and B. suis to become established in cattle. Transmission of Brucella in animals is mainly through animal contact with infected aborted material, ingestion of contaminated pastures or milk. Sexual transmission can occur through natural mating or artificial insemination. The economic burden arising from brucellosis in animals is associated with productivity losses (longer calving intervals, reduced growth, increased incidences of abortion, infertility, and calf mortality), and restrictions on the trade and export of animals and their products.12

The control of brucellosis has been achieved in many developed countries. However, for other parts of the world including Latin America, the Middle East, Spain, parts of Africa, and western Asia brucellosis remains an endemic disease that causes more than 500,000 human infections each year.2,13 Most data on brucellosis in Africa have been reported from north Africa, although these reports likely underestimate the actual incidence. In Kenya, data on human brucellosis are scarce even though it is considered a priority disease.14 A study on patients presenting at a district hospital with flu-like symptoms reported 13% prevalence of brucellosis.15 Other studies have documented brucellosis exposure in animals in Kenya at varying levels.16,17

Despite the public health importance of brucellosis, its incidence and prevalence in animals and humans, as well as its socioeconomic impact, remain poorly understood in Kenya. In addition, despite its zoonotic transmission, few studies have investigated the burden and risk factors for animal and human brucellosis simultaneously among people and livestock living together. Taking into consideration different animal production systems, our objective was to determine the seroprevalence of Brucella exposure through the measurement of immunoglobulin G (IgG) antibody levels, and the risk factors for brucellosis among human and animal populations from the same household.

This study used a “One Health” approach to simultaneously investigate the prevalence of brucellosis in humans and livestock living in the same households. Although animals are the main reservoirs for human brucellosis, studies on brucellosis have mainly been conducted separately in humans and animals.4,17,23 The uniqueness of this study design was the simultaneous study of brucellosis in people and animals living in the same households, allowing for identification and quantification of risk factors for brucellosis transmission at the household level. The study also provides a more complete epidemiological picture and deepens the understanding of infection patterns at the animal–human interface.24 The approach has the added value of enhancing closer cooperation between human and animal health practitioners.

Our study found that the odds of human seropositivity were six times higher in households with a seropositive animal compared with those without. Similar findings were reported in a study in Kyrgyzstan.25 This association was highest if the seropositive livestock was a camel or a goat consistent with the hypothesis that human risk of brucellosis is a function of the herd prevalence and depends both on the livestock species reared and human-livestock contact.17,26,27 Conversely, it contrasts results from a study in Togo and Mongolia where no association was found between human and animal seropositivity.24,28 It is notable, however, that sampling in Togo was done at the village level as opposed to household level. The study in Mongolia found no correlation between cattle and human seropositivity and only found a significant correlation with sheep, but not for goats. In that study, sampling was done at village level and livestock and human samples were not necessarily collected from the same households.24 We think that the difference in sampling level could account for the differences observed with our study.

Household and herd seroprevalence ranged from 5% to 73% and 6% to 68%, respectively, in the three counties with seroprevalence highest in Marsabit. These results are similar to those from another study conducted in Kenya, which reported a cattle seroprevalence ranging from 2% (agricultural high potential area) to 15% (semiarid, pastoralist area).16 Similar differences based on predominant production system have also been reported in Ethiopia and Uganda.29–31 The livestock seroprevalence (13.5%) observed among pastoralist communities in our study was lower than the 23% and 35% reported among pastoralist communities in Jordan and Egypt, respectively.32,33 Similarly, a higher livestock seroprevalence (6.5%) was found in an agro-based community in Uganda34 compared with Kiambu (1.2%).

The high seroprevalence in humans and livestock could reflect the endemicity of brucellosis in some parts of Kenya. Differences in human seroprevalence between the counties are likely due to the predominant production systems. Practices that promote brucellosis transmission such as drinking raw milk, nomadic movements, and use of common grazing and drinking areas for livestock are more likely in the pastoralist communities than agro-based communities.17

In this study, the seroprevalence among livestock species was fairly similar within a county, though slightly higher in goats or sheep. This finding could suggest that animal husbandry practices applied in different production systems influence transmission of brucellosis in livestock. In addition, there is possible cross-transmission of multiple Brucella species among different livestock species at the household and community level. Studies from Togo and Chad have found low human and small ruminant seroprevalence compared with that in cattle.28,35 In the Togo study, cattle seroprevalence was 9%, and < 1% among humans, whereas Brucella antibodies were not detected in sheep or goats. In our study, the seroprevalence in humans, goats, or sheep was proportionately higher compared with the studies in Togo and Chad, consistent with observations that B. melitensis that predominantly affects sheep and goats is important in brucellosis transmission to humans.2 Alternatively, the predominant Brucella species in the two countries could be different from those in Kenya where our study shows seropositivity in all species of livestock. In a study conducted in central Kenya, both B. arbotus and B. melitensis were isolated from bovine milk and aborted fetus samples.36

The human individual prevalence of 16% is higher than reported in other community level studies in Egypt (2%),37 Chad (4%),35 and Kyrgyzstan (9%).25 Seroprevalence studies in high-risk populations reported 8% among abattoir workers in Nigeria,38 22% among slaughter house workers in Pakistan,39 and 40% in high-risk groups in Libya.37 These findings indicate high seroprevalence in humans in Kenya in the general population. This varying prevalence between counties is likely driven by the seroprevalence in livestock species, which also varies between counties. In our study, goat had the highest prevalence in Kajiado and Marsabit Counties, whereas sheep had the highest prevalence in Kiambu County. It is also likely related to different practices among the pastoralist communities in Marsabit and Kajiado, which promote transmission of brucellosis.

At the herd level, independent risk factors included keeping goats, keeping sheep, and use of calving pens. Goats and sheep often feed near homesteads where abortions due to brucellosis are likely to happen and could likely be exposed to contaminated environment for longer periods compared with the cattle and camels, increasing the likelihood of infection. Alternatively, goat and sheep could be more susceptible to Brucella. Farmers who use calving pens may not dispose the products of conception following calving, resulting in concentration and contamination of the environment and transmission of brucellosis within the herds. However, in Kiambu County, use of calving pens was found to be protective. This could be due to different practices in this high-potential agricultural production system compared with the pastoral production system including proper disposal of placentae and separating animals in peuperium from the rest of the herd for a longer time.

At the individual human level, risk factors included increasing age by decade, being male, regularly ingesting raw milk, exposure to goats (herding, milking, and feeding), and handling animal hides. These findings are consistent with findings from other studies, which indicate that the risk of human brucellosis is related to transmission through direct contact with animals or their products or indirectly through consumption of their products.24,40,41 The age-related increase in seroprevalence is consistent with an endemic pattern of infection in humans. In an endemic situation (where infection is continuously present), persons are increasingly likely to have become exposed with time and hence show increasing positivity with age. Male household heads are likely to propagate some cultural practices, which promote Brucella transmission, for example, not boiling milk before drinking. On the other hand, female household heads are likely to adhere to safer practices because of more frequent exposure to health workers while attending antenatal care or child welfare clinics. Persons who handle hides often have direct contact with raw meat and carcasses of infected animals, and through which infection probably occurs through cuts and wounds to bare hands, or through splashing of infected blood or other fluid to the conjunctiva.

This study has several limitations. First, the exclusion of children < 5 years of age limits the generalizability of our data to the entire population. Second, our testing methods report apparent rather than true prevalence. However, given the validity of the assays used, this difference is likely minimal. The ELISA tests used detected only antibodies against IgG but not IgM. It is possible that some participants could have had acute infection and be IgM positive but IgG negative, with the potential effect that the prevalence stated could be an underestimate. However, a study on diagnostic methods for brucellosis where IgG and IgM antibodies were measured simultaneously found little difference in the assays.42,43 Third, our use of pan-Brucella test kits limited our ability to distinguish antibodies against different Brucella species. Fourth, given the cross-sectional nature of this survey, we could not assess temporal variations in seroprevalence if these existed.

Our study gives evidence of a strong association between human and animal seropositivity at household level. In particular, goat and camel seropositivity was strongly associated with human seropositivity. There was higher prevalence of brucellosis in human and livestock in the predominantly pastoralist communities. To estimate the burden of brucellosis and identify appropriate interventions, it will be necessary to conduct further research to estimate the incidence of brucellosis with molecular typing of Brucella strains circulating among humans and livestock. Confirmation of transmission chains by molecular analysis of strains isolated from humans and different livestock species is warranted to determine the dominant brucella species.44 A socioeconomic study would give a societal perspective burden of the disease and help determine control measures to be undertaken in different settings.