In Ethiopia, camels are multipurpose animals. Over two million Camelus dromedarius live in pastoralist and agro-pastoralist areas, mainly in Afar regional state, in Ethiopian Somali, and eastern and southern Oromia (2) . However, camel brucellosis has gained little attention. Despite a few seroprevalence studies, genotyping has not been carried out for brucellosis-causing Brucella species in camel populations. In this study, RBPT and PCR were used for the detection of Brucella spp. in serum and lymph node samples taken from apparently healthy camels slaughtered at the Akaki abattoir, Ethiopia. The PCR assay was found to be more sensitive than the rapid immunological method RBPT. This immunological method only shows the presence of anti-Brucella antibodies, as mentioned by many researchers (21) . The seropositive samples were confirmed by PCR. The outcomes of the current investigation revealed that healthy camels can be an important reservoir for the transmission of this zoonotic disease to humans, themselves, and other domestic animals in the study area. Previously, the seroprevalence of brucellosis in camels was determined by some investigators (4, 10, 11, 22-27) . who reported a seroprevalence of 3.2%-12.44% using RBPT, a complement fixation test (CFT), and an enzyme-linked immunosorbent assay (ELISA). These findings support the results obtained in our study. As shown in Table 2, out of a total of 100 serum samples tested for RBPT, 5 (5%) were positive for brucellosis. Of these, 5 (100%) were B. abortus upon further confirmation by PCR. Our serological findings are fairly consistent with those of (11, 23) . who reported 5.4% and 5.7% individual seroprevalence in camel herds, respectively. An investigation conducted by (16) identified B.abortus from ruminant serum in Afar areas. This study also confirmed the presence of B.abortus in camel serum originating from the same areas (data not included). These two confirmatory findings indicate that pastoralists who are in close contact with their animals and who consume raw milk are vulnerable to brucellosis (Figure 3). (21) in their studies, human infection caused by Brucella from camels is known to occur typically through the consumption of raw milk. The consumption of Brucella-contaminated milk and meat from camels has led to a high number of human brucellosis cases and serious public health problems.
Although camels are not known to be primary hosts for Brucella, they are very susceptible to B. abortus and B. melitensis (8) . The degree of brucellosis incidence in camels relies on the infection rate of primary hosts (28) . As mentioned previously [16], B.abortus was detected in the serum of both large and small ruminants in the Afar region. The authors suggested that interspecies transmission could be caused by mixed farming, in which small and large ruminants share the same pasture. According to the authors, the eco-diversity and multipathogenicity of Brucella allow it to cross the species barrier, with Brucella being a preferential host between large and small ruminants. The cohabitation of camels with small ruminants is common in the Afar region (11) [Figure 4]. The spillover of B.abortus from cattle to small ruminants and its subsequent spread to camels is expected based on our findings. B.abortus in camels can also be directly sourced from cattle brucellosis. (8) . Investigation by a species-specific real-time PCR system showed that only B. abortus was found in the serum of camels. These authors suggested that a spillover of B.abortus from cattle was the origin of their findings in the absence of Brucella-positive sheep and goats. Cattle are the main source of B.abortus for camel infection (8) .
PCR is one of the most accurate, widely used molecular methods. It has proven to be an invaluable diagnostic method (1, 8) . It is faster and more sensitive than phenotypic methods. To the best of our knowledge, PCR has not been used for the diagnosis of camel brucellosis in Ethiopia. This study targeted Brucella species detection using Brucella genus-specific primers and B. abortus- and B.melitensis species-specific primers. A total of 105 (5 seropositive and 100 lymph node) samples were subjected to PCR amplification targeting a Brucella-specific primer pair. Brucella PCR-positive samples were further subjected to PCR amplification with B. abortus- and B.melitensis species-specific primers. Of the 100 lymph node samples tested, 35 (35%) were positive for Brucella (223 bp)and B. abortus (156 bp), but B.melitensis was not detected. This finding is consistent with (29) who tested 481 samples from 296 dairy cattle, comprising vaginal discharge, placental tissue, milk, blood, vaginal swabs, and fetal tissues. Among these specimens, 41.6% and 30.4% were positive for brucellosis according to the I-ELISA and RBPT, respectively. Our PCR findings also agreed with those of (30) who investigated 2225 camel milk and serum samples using serological tests. The authors obtained an overall seroprevalence of 37.5%.
On the other hand, in our study, the molecular detection rate in retropharyngeal lymph nodes was seven times greater than the seroprevalence of RBPT in the same camels and greater than previous individual camel seroprevalence levels reported by (3, 4, 10, 11, 23-27) . The high percentage of positive camels detected by PCR could be credited to the high diagnostic sensitivity of PCR. This assay detected as little as 23 femtograms (fg) of Brucella DNA per reaction at 95% confidence (8) . The low detection rate in our RBPT may be due to camels being kept in the lairage or quarantined for a considerable time before slaughtering and acquiring acute infection from infected camels. Acute infections might not reach detectable antibodies or chronically infected camels for which antibody levels are not yet detectable. Serology relies upon the increasing and decreasing titers of antibodies during the different phases of brucellosis. Serological diagnostic kits against camel brucellosis have not been validated, and studies have revealed that antibody concentrations are lower in camels than in cattle (28) .However, PCR directly detects Brucella DNA that persists in the lymphoid tissue throughout the life of camels (26) . Our findings demonstrated that conventional PCR was seven times more sensitive than RBPT for detecting Brucella species in camels. However, all RBPT-negative serum samples should be checked to confirm that PCR has superior sensitivity to RBPT. Otherwise, there may be a sample type factor. Retropharyngeal lymph nodes located in the camel’s head drain bucco-pharyngeal lymphatic vessels where Brucella organisms may be predominantly localized. This suggestion is supported by (8) , who reported that B. abortus was recovered mainly from the lymph nodes of the camel’s head rather than from other organs.