Sero-Prevalence and Epidemiology of Brucellosis in Camels , Sheep and Goats in Abu Dhabi Emirate

Brucellosis is a disease of animals caused by Brucella species and is transmissible to humans. This study was undertaken to determine the sero-prevalence of the disease in livestock including sheep and goats and camels, in different regions of Abu Dhabi emirate and to identify factors associated with the epidemiology of the disease. A serological study using 6126 blood samples from livestock were obtained from 267 farms (Izaba) during the period from January 2009 to December 2010. The Rose Bengal Plate Test and competitive ELISA were used as screening and confirmatory tests, respectively. The overall sero-prevalence of Brucella antibodies was 8.00% and 7.00% detected by the RBPT by c-ELISA respectively. Brucella prevalence was 8.3, 5.9 and 4.7% in Alain, Abu Dhabi and Western region. The prevalence of the disease was higher (8.4%) in sheep and goats than (4.4%) in camels respectively. The result showed that, the prevalence of brucellosis was significantly higher in females than male (p<0.04) Out of the 267 farms sampled in the study, 147 (55.1%) were infected with Brucella. There was strong correlation between herd size and prevalence of the disease, very large herds had significantly higher prevalence when compared with small ones. The study revealed light of a size able prevalence among livestock in Abu Dhabi Emirate and the results reflect the necessity of a control program of the disease is needed to be adopted.


INTRODUCTION
Brucellosis is an infectious bacterial disease caused by the genus Brucella and affecting a number of animal species (Hirsh and Zee, 1999).It is a worldwide zoonotic disease that is recognized as a major cause of heavy economic losses to the livestock industry and poses serious human health hazard (Ocholi et al., 2005).In animals, it is a significant cause of reproductive losses usually caused by B. abortus in cattle, B. melitensis and B. ovis in small ruminants, B. suis in pigs and B. canis in dogs (Khorasgani et al., 2008).
The disease is endemic particularly in Mediterranean, Middle East and the Arabian Gulf, parts of Africa, Europe and Latin America.Consumption of contaminated foods and occupational contact remain the main sources of infection (Annapurna et al., 2012).The disease is reported in domestic animals and humans in Oman, Qatar, Kuwait and Saudi Arabia which share border with United Arab Emirates (Yasmin and Remya, 2011).
In Abu Dhabi emirate, where a large number of animals are kept closed as mixed flocks, there is a relatively lack of information about the epidemiology of brucellosis in livestock, accompanied with an absence of control measures and programs especially in the non commercial farms.Earlier studies carried out in 1989 revealed that the average Brucella prevalence was 6.4% in goats, 5.4% in sheep and 1.5% in camels and then degreased in 1990 to 3.4, 2.0 and 2%, respectively (Refai, 2002).
All the previous studies conducted in Abu Dhabi Emirate were either in a limited area and/or period and did not reflect the exact impact of the prevalence of brucellosis in the emirate.Moreover, up to date, no systemic research on the epidemiology of brucellosis that cover all the regions of Abu Dhabi Emirate was carried out to best of our knowledge, therefore, the objective of this study was to investigate the seroprevalence of the disease in large number of different animal species including camel, sheep and goats distributed randomly in Abu Dhabi emirate and to elucidate the role of risk factors in the spread of the disease among livestock.This in turn will provide a base for the designation of the suitable control program of the disease.

Animals and samples:
The study was conducted during the period from January 2009 to December 2010 in three main regions of Abu Dhabi emirate, Abu Dhabi region, Alain region and Western region.The number of animals sampled was determined according to Pfeiffer (2002) and Robinson (2003).Sampling was conducted in two stages, randomly selected herds then randomly selected animals within those herds based on the animal populations.The regions were stratified into three regions: Abu Dhabi, Alain and Western region; sex of animals into two groups: males and females; and the herd size into four categories: small herd (<50), medium herds (50<150), large herds (150<500) and very large herds (>500).
A total of 6126 blood samples of both sexes of sheep, goats and camels were randomly collected from 267 mixed farms (izba) located in three regions of Abu Dhabi emirate.They were 664 males and 3649 females of sheep and goats from 257 herds; 170 males and 1643 female of camels collected from 188 herds.About 5-10 mL of blood was drawn from a jugular vein of each apparently healthy animal using a plain vacutanier tube (without EDTA).Tubes were incubated over night at 4ºC and serum was then separated by centrifugation.The collected sera were lapelled and stored at-20ºC until used.
Animal experiments were conducted in accordance with the guidelines provided by the Ministry of Environment and Water, United Arab Emirate.All experiments complied with article number 12 of animal welfare federal law number 16 for 2007.

Rose Bengal Plate Test (RBPT):
All serum samples were screened by the RBPT for the presence of antibodies against Brucella antigens.The test was performed on round-bottom-welled plastic plates according to Alton et al. (1988) using the antigens, positive and negative control provided by the Veterinary Laboratory Agency (VLA), Weybridge, UK.Before testing, both sera and the antigen were left at room temperature to reach about 25 • C. Equal volumes of test serum and antigen (30µL) were placed on the plate test and mixed thoroughly by stirrer.The plate was then shaken on a rocker for 4 min.Later, all degrees of agglutination reaction were read and recorded as positive if there were visible agglutinations or as a negative result if there were no visible agglutinations at the end of the agitation period (OIE, 2008).

Competitive Enzyme-Linked Immunosorbent Assay (c-ELISA):
The positive or doubtful samples with RBPT were further confirmed by the c-ELISA (CVL, New Haw, Weybridge, surrey KT15 3NB UK) according to the manufactory instruction.The cut-off for determining sero-positivity was calculated as ≥60% of the mean of the Optical Density (OD) of the 4 conjugates control wells and antibody titres were calculated as binding ratio using the formula defined by the c-ELISA kit manufacturer as follow: Banding ratio = Mean of 6 negative control wells Mean of 6 positive control wells Analysis of data: Data was statistically analyzed by Chi-square and t-test for independence comparison test using the statistical software program (SPSS for Windows version 15, USA).Differences were considered significant at p<0.05.

RESULTS
Screening of animals for Brucella antibodies: RBPT results showed that, out of the 6126 livestock sera tested, 506 (8.30%) were positive by RBPT.Prevalence of brucellosis was higher (9.5%) in sheep and goats than that observed (5.4%) in camels (Table 1).Highest prevalence of Brucella antibodies in sheep and goat was observed in Alain region (11.5%)followed by lower prevalence's of (7.5%) and (7.3%) in Abu Dhabi and the western regions respectively.In comparison the highest prevalence of Brucella antibodies in camel was also observed in Alain region (5.6%) followed by lower prevalence's of (5.2%) and (4.9%) in Abu Dhabi and the western regions respectively (Table 2).

Prevalence of brucellosis in animals:
To obtain the real sero-positive reactors, the positive or doubtful samples with RBPT were further confirmed by the c-ELISA.Out of the 506 positive sera tested by RBPT, 441 samples which represent 7.00% of the total samples (6126) used in the study were found to be positive by c-ELISA.The overall prevalence of brucellosis detected by c-ELISA remains higher in sheep and goats (8.4%) followed by low prevalence's of (4.4%) observed in camels (Table 3).With regard to regions under study the highest prevalence of brucellosis calculated from c-ELISA results in different species of livestock was 8.3% in Alain followed by 5.9% in Abu Dhabi were as the lowest prevalence was 4.7% observed in the Western region.Similarly in sheep and goats Brucella antibodies detected by c-ELISA decreased to (10.7%), (6.52%) and (5.26%) in Alain, Abu Dhabi and western regions respectively.The decrease in antibody prevalence in camel was (4.66%), (4.42%) and (3.15%) in Alain, Abu Dhabi and western regions respectively (Table 4).
According to livestock species, the study pointed that, prevalence of brucellosis confirmed by c-ELISA was higher in sheep and goats (8.4%) than that of camels (4.4%).The same phenomenon of higher prevalence of Brucella in sheep and goats was observed in all regions of Abu Dhabi Emirate.Sheep and goats represent the large number of animal's population in the emirate.These animals are kept overcrowded and reared in open system in which animals in different ages ; aborted and pregnant ones; males and females are housed together in high stocking density.In the traditional sector, infected animals with Brucella are usually kept for breeding despite the fact that congenital infection is a major epidemiological means of spread of the disease and it is well known that as high as 20% of calves born by to infected heifer's could be found persistently infected with brucellae (Nielsen and Duncan, 1990).
In contrast the prevalence of brucellosis in sheep and goats in this study was higher than those reported by a survey done in 1989 and 1990 in Abu Dhabi emirate (Refai, 2002).Also the sero-prevalence of brucellosis in camels in observed here was found to be higher than that reported by Afzal and Sakkir (1994), Moustafa et al. (1998) and Refai (2002).These differences in sero-prevalnces may be due to the difference in number of samples collected and the difference of diagnostic techniques used each study.
In this study the of brucellosis was significantly higher in females than male (p<0.04).This is in accordance with Hussein et al. (2005) and Solmaz et al. (2004).In all herds sampled, females were higher in number than males.In addition, females are comparatively under greater physiological stress during pregnancy and lactation which make them more susceptible to infection (Walker, 1999).Radostits et al. (2007) have shown that erythritol, a polyhydric acid found in higher concentration in the placenta and fetal fluids of females than in seminal vesicles and testis of males can be responsible for females being more susceptible than males.
Variability in prevalence of brucellosis between animal farms was observed in the three regions under this study.Highest (61.8%) prevalence of disease in livestock farms was recorded in Alain region.This might be attributed to the fact that more than 50% of the Abu Dhabi emirate's total livestock is in Alain region.Moreover, the absence of disease control measures especially in the traditional section, methods of husbandry including farming systems and practices, livestock movements, mixing and trading of animal and sharing of grazing grounds might have played a role in the increased prevalence in the region (Kabagambe et al., 2001).In contrast, the comparatively lower (47.8%)prevalence of the disease in the Western region farms could be due to the systems of animal husbandry practices, where animals range in low accumulation density in vast areas and the fact that chances of close contact between animals is low, with a resultant of decreased exposure.Also the climate of the area is arid with absolute temperature above 40 °C from March to November and yearly average of 9.6 h sunlight a day.Dryness and sunlight during long summer seasons are expected to decrease the survival chances of the organisms because of the sensitivity of brucellae to ultraviolet radiation and heat (Deutcher, 2006).
The results obtained here revealed that, herd size appears to be a major risk factor for brucellosis compared with other factors (p = 0.000).As herd size increases, the chance of contact between animal increases too, which leads to more chances of infection (Abbas and Agab, 2002) especially during calving or abortion when most of brucellosis contamination occurs (Gameel et al., 1993) and together with poor management are directly related to infection rate (Abou-Eisha, 2000;Wernery and Kaaden, 2002).
In conclusion, the prevalence of brucellosis in Abu Dhabi emirate was found to be high in both sheep and goats and camel.Higher prevalence of the disease was in females and large herd sizes and than males and small herds sized respectively.The prevalence was higher in Alain region followed by Abu Dhabi and western region.Results reflect the necessity of a control program of the disease is needed to be adopted.

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Table 1 :
Positive reactors for brucellosis with RBPT in different