Livestock production is the main source of livelihood for most rural communities in Africa, including Namibia (1). However, the sector faces challenges from infectious diseases that cause losses in the productive and reproductive performance of livestock, such as brucellosis and Rift Valley fever (RVF). RVF and brucellosis are neglected tropical diseases (NTD) that are frequently under-diagnosed and under-reported in developing countries (2).

Brucellosis is caused by gram negative bacteria of the genus Brucella in humans and a wide variety of domesticated and wild animals (3). Infection of livestock causes abortions, stillbirth, reduced fertility, poor weight gain, and decline in milk production. Animal and animal product trade in affected regions is restricted. The disease is thus of economic importance (4). Bovine brucellosis is primarily caused by Brucella abortus and occasionally by Brucella melitensis and Brucella suis (5). It is transmitted among cattle through contact with or ingestion of food contaminated with aborted material such as fetuses, fetal membranes and vaginal secretions. Human infection is common among professionals who work with cattle and handle their products (meat, raw milk, and dairy products), such as farmers, farm workers, veterinary personnel, and butchers (6). The infection can manifest with mild non-specific clinical symptoms which are clinically indistinguishable from common human febrile diseases, like malaria and influenza.

Bovine brucellosis is endemic in Southern Africa. An epidemiological study conducted in South Africa from 2007 to 2015 revealed an overall seropositivity of 5.85% (44,687/764,276) for brucellosis in livestock (7). Bovine brucellosis prevalence at the wildlife-livestock interface in southern Africa is rarely reported. However, studies in Mozambique, Zimbabwe and Zambia have reported prevalence of 9.77%, 9.9% and 19% respectively (8–10). In Namibia, brucellosis has been reported in cattle, sheep, and goats on the basis of clinical signs and serological surveys (11–15). The seroprevalence of bovine brucellosis in livestock in Namibia ranges from 0.49% to 1.94%, while the prevalence of human brucellosis is estimated to be 11.64% (11, 16). Brucella disease control strategies in Namibia involve vaccination, surveillance and culling of infected animals. However, as bovine brucellosis is endemic in most areas of Southern Africa, the control of the disease is difficult to achieve, especially under conditions of nomadic or migratory husbandry practices where hygienic precautions are difficult to implement.

Rift Valley Fever (RVF) is a zoonotic mosquito-borne disease of sheep and goats that can also infect cattle and antelope species. Aedes spp. mosquitoes are the primary vectors of RVF virus (RVFV) in Africa, but Culex spp. mosquitoes can also transmit the virus (17). Mosquitoes transmit RVFV via blood-feeding to susceptible livestock and humans. The virus can also be transovarially transmitted to the next generation of Aedes mosquitoes (18). In southern Africa, the emergence of transovarially infected Aedes spp. mosquitoes causing RVFV outbreaks in animals and humans is linked to persistent heavy rainfall, flooding associated with the El Niño phenomenon and floods occurring at the end of the rainy season (19, 20). RVFV outbreaks are often followed by long inter-epidemic intervals that may last for many years. Climate change is predicted to expand the geographical range of RVFV and shorten the period between outbreaks (21). Human infection may also occur by direct contact with infected blood, organs, fetuses and raw meat, and to a lesser extent by drinking unpasteurized milk. RVFV causes high abortion rates in pregnant animals and typically high mortality in young animals (95%−100%), as well as non-specific, febrile influenza-like infections in humans (5) that can be mistaken for malaria and other febrile diseases. RVFV can also lead to hepatitis, encephalitis and retinitis in humans (22, 23). It is primarily an occupational disease of professionals such as butchers, farmers, farm workers, herders and veterinary personnel. In Africa, seroprevalence of RVFV in cattle and sheep can reach 100% (20). RVFV caused epidemics in livestock in the southern and western parts of Namibia in 2010 and 2011 (24). Despite its public health and socioeconomic impact, no studies investigating seroprevalence rates in cattle in Namibia have been conducted to date.

The tropical climate (hot and humid) and the human-wildlife-livestock interface areas in the Zambezi region present ideal conditions for the survival and transmission of zoonotic pathogens including Brucella and RVFV to resource-limited communities and domestic animal species inhabiting these areas. Seasonal flooding that occurs in the eastern Zambezi region is ideal for the proliferation of mosquito vectors of RVFV, which can lead to disease outbreaks.. Previous studies on brucellosis focused on commercial livestock farming systems, where animal disease control measures are better implemented, rather than on communal farming systems, where animal disease controls such as vaccination are not applied and herds share grasslands. Therefore, the objective of this study was to determine the seroprevalence of Brucella and RVFV in a rural communal area of the Zambezi region in Namibia and recommend appropriate prevention and control strategies for cattle and humans.

A total of 371 serum samples from cattle were collected from 18 herds in the communal areas of Kabbe South Constituency. The majority of these samples were from females (80.1%, 297/371) and cattle of breeding age, i.e., ≥2 years (95.7%, 355/371). Samples were drawn from the following cattle breeds: Sanga (80.6%, n = 299), Brahman (12.1%, n = 45), Simbra (0.5%, n = 2), Limousine (0.3%, n = 1), Swiss (0.3%, n = 1), and crossbreeds (6.2%, n = 23). Most cattle (69%, 256/371) were 4 years old.

Brucellosis and Rift Valley fever are important zoonotic diseases in southern Africa, affecting human and livestock health and can have a great economic impact in regions where livelihoods depend on livestock farming. Their epidemiology is poorly understood, especially in rural regions at the wildlife-livestock interface where diagnostic resources are limited. In this study, we aimed to determine the exposure of cattle to Brucella spp. and RVFV in Northern Namibia. We found a relatively high seroprevalence of Brucella spp. and RVFV in apparently healthy communal pastoral cattle. To our knowledge, this is the first report of RVFV seroprevalence in cattle in Namibia.

Our study confirms that brucellosis is endemic in cattle managed under a communal pastoral system at the human-wildlife-livestock interface, with a relatively high individual animal and herd seroprevalence of 22.6% and 83.3%, respectively. The individual animal prevalence was higher than the approximately up to 2% in cattle drawn mainly from commercial farms reported previously in the southern drier part of Namibia (11, 14, 15), where preventive and control measures for brucellosis are strictly implemented. The elevated prevalence found in this study is consistent with a higher brucellosis prevalence of 10.3% recorded previously in communal pastoral cattle in Namibia (15). A relatively high prevalence of Brucella reactors (15.6%) has also been reported in communal cattle in the Kwa-Zulu Natal province of South Africa (29) and from several surveys in communal pastoral cattle in Africa (30–32). In contrast, findings from other neighboring countries reported lower seroprevalence rates of 8% in cattle from Zambia (33), 6% in cattle from South Africa (7) and 6% in buffalo from Botswana (34). The high individual animal and herd prevalence in this study point to a relatively increased level of zoonotic disease burden in cattle and risk to human health, as the prevalence of positive reactors within herds was as much as 70%. Moreover, elevated Brucella seroprevalence values in livestock, as recorded in our study, have been linked to seropositivity in the human population (35). The high prevalence of Brucella reactors in communal areas can be ascribed to a communal pastoral system of cattle management that promotes intermingling between cattle herds; the sharing of pastures, water, and breeding bulls; and a lack of awareness of the disease, all of which promote disease transmission. Direct and indirect contact between cattle herds and between cattle and wild ruminants at the wildlife-livestock interface brings infected aborting and birthing animals into close contact, increasing the risk of Brucella transmission (36). Identified risk factors for brucellosis persistence and failure of eradication in developing countries comprise inefficient surveillance and control programs that become evident in the inappropriate disposal of aborted materials, the lack of knowledge and awareness in medical professionals, veterinarians, and in traditional livestock owners, reluctance of vaccination, uncontrolled animal movement and lack of differential diagnostics that include biotyping methods (37). The fact that our sample consisted mainly of cows kept in herds for longer periods may have contributed to higher exposure and prevalence of Brucella infection (38). In addition, contact with potential wild ruminant reservoirs of Brucella abortus such as buffalo (39) and wildebeest (40) in this interface area could have contributed to the high proportion of Brucella reactors seen in cattle. Our results point to the possibility of high reproduction losses due to infertility, abortions, and neo-natal mortalities associated with Brucella and RVFV infections in communal cattle and a high risk for zoonoses in the human population. The I-ELISA assay used in this study detects IgG anti-Brucella antibodies. Therefore, we can conclude that positive reactors were animals that carried chronic infections or that had recovered from previous infection.

We further found a relatively high individual animal (41%) and herd seroprevalence (100%) for RVFV, in line with previous reports of RVF being an endemic disease in Africa with a prevalence of up to 100% (20). The overall prevalence of RVFV in cattle was similar to a prevalence of 42.9% in a vaccinated population in South Africa (41), but higher than the 12.37%−33.3% reported in other African countries (42–46). Notably, the RVFV prevalence in cattle in the current study was comparable to that reported previously in springbok (Antidorcas marsupialis) (35%) and black-faced impala (Aepyceros melampus petersi) (62.5%) in a national park in Namibia (47), perhaps a reflection of the potential role of wildlife in the transmission cycle. Wild animals in the study area, such as buffalo, waterbuck, elephant, eland, and wildebeest, may have acted as maintenance hosts for RVFV, from which spillover infection was transmitted to domestic animals through infected mosquitoes (48). With a 100% herd prevalence, our results suggest a high burden and circulation of RVFV in cattle at the wildlife-livestock interface in the Kabbe South constituency. These findings reveal an animal and public health risk, as seroprevalence rates in domestic animals correlate positively with the risk of infection in humans (49). Interestingly, no RVFV outbreaks have ever been reported in this part of the country, despite the presence of established animal disease surveillance structures; occurrence of favorable ecological and climatic conditions for the breeding of Aedes or Culex mosquito vectors in the form of frequent episodes of heavy rainfall, seasonally flooded areas, wetlands/dambos, and permanent water bodies (50). The detection of seropositive cattle suggests the current circulation of the virus or the long-term persistence of anti-RVFV antibodies due to previous infections. Of note, the cattle of this study were not vaccinated against RVFV. The silent circulation of RVFV in animals has also been documented by previous studies (51–53). Cattle are more attractive to RVFV mosquito vectors than other livestock species (54), hence an increased prevalence is often seen in these species. The high RVFV seroprevalence observed in cattle could also be due to spillover of infection from infected wild animals that frequently mingle with and share grazing and water resources with cattle. Previous studies in Namibia have provided serological evidence of the role of wildlife such as springbok (Antidorcas marsupialis), wildebeest (Connochaetes taurinus), and black-faced impala (Aepyceros melampus petersi) in the epidemiology of RVFV (47). Moreover, the study area falls within the Kavango-Zambezi Transfrontier Conservation Area (KAZA-TCFA), an area where wild animals such as buffalo and wildebeest roam freely across five international borders (Botswana, Zambia, Zimbabwe, Namibia, and Angola).

It is noteworthy that several cattle (10.8%, n = 40) were seropositive for both Brucella and RVFV, suggesting successive or co-exposure to the two pathogens. Either way, our findings point to significant reproductive challenges in the affected herds and a public health risk in the respective communities.

In this study, established risk factors for Brucella infection, that is, age, sex and breed (55), were not significantly associated with seropositivity, although seropositivity was higher in females than males, and in sexually mature (4 years and older) than younger animals. Our findings are consistent with previous reports which noted a higher Brucella seroprevalence among sexually mature cattle (36, 56) and in cows (57, 58). In contrast to the findings in Malawi (46) and Madagascar (42), our study established a higher RVFV seroprevalence in females than males (p = 0.004), perhaps due to the larger number of female cattle that were recruited in the study. Male cattle are often culled, resulting in reduced exposure time, while cows stay longer and dominate the herds (58). RVFV seropositivity occurred across all age groups of cattle tested, with higher but not significant prevalence rates in adults than in younger animals, in agreement with prior studies (46). As expected, the Sanga breed, the major cattle breed in the study area, had a higher seroprevalence of Brucella and RVFV than other breeds.

Test agreement between Brucella CFT and I-ELISA was poor (kappa value of 0.2322). Previous studies have confirmed that I-ELISA has a higher sensitivity and specificity than CFT (27), which explains our results. In our study, I-ELISA detected Brucella antibodies in younger cattle (≤ 2 years), while the CFT did not, and all cattle herds that tested positive on the CFT, also tested positive on the I-ELISA, confirming its superiority over the CFT. We therefore recommend that the Central Veterinary Laboratory (Namibia) adopt the I-ELISA as a confirmatory test (after RBPT screening) in place of the CFT.

Our findings highlight the need to prioritize zoonoses in the country and to establish a One Health policy for effective surveillance, prevention and control of zoonotic pathogens such as Brucella and RVFV, as has been implemented elsewhere in Africa (59–61). This is because Brucella and RVFV infections have multidimensional determinants of disease epidemiology that include human, domestic animal, wildlife, climatic, and ecological factors. The One Health approach should foster collaboration between professionals working in different sectors such as human health, animal health and production, entomology, social sciences, wildlife, and the environment (62). In particular, the public health and animal health sectors need to establish regular communication, collaboration and information sharing in the surveillance, prevention, and control of zoonotic infections. We recommend that the public health sector increase awareness of these infections among health professionals to ensure effective surveillance and detection in high-risk populations at the interface. Vaccination of cattle against brucellosis has been shown to reduce infection, transmission and disease burden. It should therefore be implemented in communal areas. Further studies on mosquito vectors, humans, and domestic and wild ruminants are recommended, using serological, molecular, culture, and virus isolation techniques to characterize circulating strains of Brucella and RVFV.

The inability to identify Brucella spp. infecting animals in Namibia may reduce the effectiveness of control measures that are currently implemented in cattle and humans, as all smooth Brucella spp. can cause infection singly or in co-infections. Therefore, the characterization of Brucella spp. infecting cattle should be the focus of future studies.

Our study determined a relatively high seroprevalence of Brucella spp. and RVFV antibodies in cattle raised in a communal pastoral system at the wildlife-livestock interface in Northern Namibia. Sex was a significant risk factor for RVFV, with females having a higher likelihood of infection. In contrast, age, sex and breed were not significant predictors of Brucella spp seropositivity. The detection of these zoonotic infections has serious animal and public health implications.