Seroprevalence of brucellosis and associated factors among livestock slaughtered in Oko-Oba abattoir, Lagos State, southwestern Nigeria

Introduction Brucella infection, a neglected tropical zoonosis, poses public health threat to abattoir workers in developing countries including Nigeria. Oko-Oba abattoir is one of the largest abattoirs in the country that collects livestock from different parts of the country. This study determined the prevalence and factors associated with seropositivity of brucellosis among livestock slaughtered at Oko-Oba abattoir. Methods A cross-sectional study was conducted from January to May 2018. A total of 473 serum samples were collected from livestock at the abattoir and tested for antibodies to Brucella species using the Rose Bengal Plate Test (RBPT) and indirect Enzyme Linked Immunosorbent Assay (iELISA). Data were analyzed using descriptive statistics and chi square test (p < 0.05). Results Overall seroprevalence values were 15.3% (RBPT) and 16.3% (iELISA) among the livestock slaughtered at the Oko-Oba abattoir. Seroprevalence of 17.2% (RBPT) and 15.8% (iELISA) in cattle; 15.1% (RBPT) and 14.5% (iELISA) in goat; and 8.3% (RBPT) and 23.3% (iELISA) in sheep were obtained. Higher seroprevalence were recorded among females in cattle (18.8% iELISA) and sheep (23.1% iELISA) while male goats had average value higher (14.7% iELISA) than the female (p > 0.05). Conclusion Presence of Brucella infection among slaughtered livestock was confirmed at Oko-Oba abattoir, Lagos State, Nigeria and poses a threat to abattoir workers and public health. Control of the disease in livestock and use of personal protective gear is recommended.


Introduction
Brucellosis occurs worldwide but is better controlled in developed countries by routine screening of domestic animals and vaccination.
The disease is a major neglected zoonosis of developing countries including Nigeria [1]. However, the prevalence varies in time and space among different livestock [2]. Brucellosis is endemic in sub-Saharan Africa, with significant effects on economic and social conditions of people in this region [3,4]. The incidence is influenced by management factors, herd size, population density, type of animal breed and biological features such as herd immunity [5,6]. McDermott and Arimi [7] made a summary of data in cattle prior to 2001 which varied from 7.5% to 40% for pastoralists in arid and semiarid areas, 0.3-25.4% for cash/subsistence crops with livestock in sub-humid areas, 1.5-16.2% for crop-livestock in tropical highlands and 2.4-45.0% for crop with small-scale livestock production in humid areas.
Studies reported seroprevalence of 2.9% with iELISA in argentine Creole sheep, 12.0% in Nepal and 4.1% to 6% in Costa Rica among cattle, respectively [8][9][10]. However in Ethiopia, 1.6% was reported in pastoral goats [11]. It is evidenced that the distribution and seroprevalence of brucellosis differs with species and location.
In Nigeria, several serological investigations have shown that Brucella infection is endemic in livestock population. In cattle, prevalence of 8.6% in Lagos State [12]; and 37.0% in three northern states (Kaduna, Kano and Adamawa) [13] have been reported. Also, another study in north-central Nigeria reported prevalence of 16.1% in cattle and most recent abattoir study in Ibadan, south-west Nigeria recorded a prevalence of 7.8% among cattle slaughtered at the abattoir [14].
Most of the investigation were done in cattle, although few documented evidence shows that the disease also exist in small ruminants, with prevalence of 0.86%, 14.0% and 25.80% reported in south-west, north-east and north-central Nigeria, respectively [15,16]. Ogugua [18]. The projected population of Lagos State from Nigeria´s 2006 National census is estimated to be 15 million [18]. About 75% of the population lives in urban areas.
Oko-Oba abattoir ranked the largest abattoir in Nigeria and biggest in West Africa [19]. About 3000 livestock are slaughtered daily. It has an automated slaughtering line, rendering units and waste treatment plant [20]. The abattoir accounts for 50% of meat consumed in Lagos State.
Study subjects: the study population was made up of all livestock that were slaughtered at the Oko-Oba abattoir. Cattle, goats and sheep at the abattoir at the time of sample collection were included in the study while all cattle, goats and sheep at the abattoir whose owner did not consent at the time of sampling were excluded.  Data analysis: the age of the animal was determined using estimation of age by teeth appearance [22]. The body conditions were assessed by clinicians through examination and grading based on automatic grading of livestock [23]. Data were analyzed using Epi-Info version 7.2 software. Demographic characteristics were measured as the independent variables while the outcome of the serological test was measured as the dependent variables. We determined frequencies, proportions, and odds ratios. Chi square test was used to test level of significance in the values of the odds ratios at p < 0.05.

Discussion
The study confirmed that Brucella infection is present in cattle, goats and sheep slaughtered for consumption at Oko-Oba abattoir, Lagos and that seroprevalence varied across species, breed, sex and age of animals. In this study, two serological tests were adopted to confirm the seroprevalence of the livestock using serum samples. This method was also adopted by Kashiwazaki et al. [24] [26]. A seroprevalence of 17.2% and 15.8% was observed among slaughtered cattle based on RBPT and iELISA, respectively. This is higher than the prevalence of 8.6% earlier reported among cattle in Lagos by Cadmus [12] and much higher than 4.9% reported by Ogugua et al. [27] in South Western Nigeria and 5.45% recorded by Bwala et al. [19] in Ibadan municipal abattoir Nigeria. Although, it was lower than 36.6% recorded among cattle in three northern states [13]. However the seroprevalence obtained in this study is much higher compared to other reports from developing countries which reported 2.21% in South Africa [28], and 2.90% in Ethiopia [29]. The difference in prevalence between our study and other previous studies could be partly due to the methodology used, in other previous studies only samples positive based on RBPT were tested with ELISA while our study tested all the samples with iELISA, the sampling frame, this frame was based on the number of livestock seen at time of sampling and may not depict the actual sampling frame of livestock slaughtered at the abattoir and also the study location the abattoir is the largest in the south west Nigeria and collects livestock from various locations across the country. Another important issue is the difference in sensitivity and specificity of serological tests used for screening. This factor contributes to the variations in results among researchers [30]. In small ruminants, seroprevalence of 15.1% and 14.5% was recorded in goat using RBPT and iELISA, respectively, this is quite high compared to previous reports which recorded 2.8% across four States in Nigeria [17]. It is also much higher than prevalence of 2.8% and 1.6% recorded in Somalia and Ethiopia respectively [11]. The likely reasons for the differences could also be due to the fact that the study was conducted in an abattoir which collects livestock from various locations across the country. Seroprevalence of 8.3% and 16.3% was observed among sheep using RBPT and iELISA, respectively. This is higher compared to prevalence of 3.6% [21] and 0.0% [15] reported by these authors.
Most of these studies were conducted among herd in the farm, while The explainable reason for higher seroprevalence recorded in the slaughtered livestock could likely be due to the fact that most of the animals slaughtered in the abattoir are gotten from the livestock market and farmers are known to sell animals that are sick or unproductive [31]. More so, there is no prevention and control strategy adopted in Nigeria, contributing to the higher seroprevalence when compared to other studies in other climes where control strategy exists. Higher prevalence of the disease was demonstrated in cattle than in the small ruminants, though the variation is not statistically significant. Similar results were found in Nigeria in which the seroprevalence of the disease was higher in cattle than in goats [15].
The results are also similar to a study carried out in Tanzania which demonstrated higher prevalence in cattle than goats [26]. Although bacteriological analysis was not conducted to determine the strain of Brucella species that was detected in these livestock, the serological test with RBPT was B. abortus and B. melitensis antigen for cattle and small ruminant respectively, B. melitensis is the most pathogenic [32].
However, iELISA test is regarded as the confirmatory serological test as documented by Lopez et al. [33]. The seroprevalence recorded in this study suggests that the risk of transmission of brucellosis to abattoir workers is very high. The seroprevalence was more in female cattle than male; however there was no association with Brucella infection. Although most goat sampled were male, the study found high seropositive cases among female goats than male by RBPT; however, there was no association. By iELISA, male goats had higher seroprevalence than the female. In sheep higher prevalence was recorded in female than male by RBPT, but by iELISA, male sheep had higher seroprevalence than female, however this association was not statistically significant. This can be partly explained in terms of sample distribution, most of the sheep sampled were female. Males are kept separately where they are fed well for market value. Their market value is much higher than that of females and they are usually sold when there is a need for cash or are slaughtered during religious ceremonies [34]. Interestingly, it is of importance to note that earlier reports have shown that female animals are more susceptible to Brucella infection and also represents a greater risk of spreading infection [35,36] because they stay longer in the farm and are only culled when they become unproductive. Therefore, having more female animals in the abattoir predisposes the workers to Brucella infection as farmers are known to sell off their female animals which are not doing well, and the most important indicator of not doing well in female animals are reproductive failure and low milk production [37], which are clinical signs of brucellosis in animal.
In this study, the seroprevalence was higher in the adult than the young animals with both serological tests, although this was not statistically significant. This finding is similar to the report of Cadmus et al. [38], and Assenga et al. [26], but in contrast to Ogugua et al. [17] who reported high seroprevalence in young animals. This could be due to increase in concentration of sex hormone which stimulate multiplication of Brucella organism [36]. Again it could also be as a result of the younger animals being resistant to infection, and being able to clear this infection frequently, although re-infection could occur at older age similar to the findings of Ogugua et al. [17].
Again, body condition could be a contributing factor to the differences in seroprevalence in livestock [31], on the contrary, this study found no statistical significant association in all the species surveyed. This study also highlights the variations observed in breed specific seroprevalence, in cattle, cross breed recorded the highest prevalence using RBPT, while Adamawa Gudali had the least. With iELISA, the highest breed specific was recorded in Ambala, this was in contrast to the findings of Ogugua et al. [38] who reported highest prevalence in Kuri and the reports of Cadmus et al. and Junaidu et al. [38,39] where the highest prevalence was reported in Bunaji and Sokoto Gudali, respectively. Although one of the factors implicated in conferment of resistance and tolerance is the genetic factor [40]. However, in cattle, the Ambala breed was eight times more likely to be Brucella positive by iELISA compared to the Adamawa Gudali, this shows that breed is associated with brucellosis among cattle in Nigeria [31]. Furthermore, the study found no significant association in both serological tests among goat and sheep breeds. Though, the highest seroprevalence was recorded among Red Sokoto and Uda using RBPT and iELISA, respectively; this is consistent with the reports of Ogugua

Competing interests
The authors declare no competing interests.

Acknowledgments
We would specially acknowledge the NFELTP Facilitators, Dr. C.D.
Umeokonkwo, Prof. Junaidu Kabir and a host of others for their intellectual contributions to the success of this work.