Sero-prevalence of brucellosis in sheep and springbok (Antidorcas marsupialis) in the Karas Region of Namibia

Abstract

Game farming developed in Namibia over the years as a result of constraints associated with livestock farming such as diseases and profitability. The development of this industry has brought livestock and game species into close contact. In the Karas Region, a major sheep producing area, sheep and springbok are reared together on commercial farms. The rearing of these species in close proximity may result in crosstransmission of zoonotic diseases such as brucellosis, enabling such diseases to enter the human population through meat and other livestock products. Game species may complicate the control of brucellosis by acting as reservoirs of infection after the disease has been controlled in sheep. Brucellosis due to B. melitensis has been reported in Namibia as a cause of reproductive failure in sheep. An outbreak of brucellosis occurred xvi in 2009 affecting sheep, goats and humans on a farm in the adjacent Hardap Region. Brucellosis outbreaks in sheep have the potential to disrupt Namibia’s foreign currency earning as the sheep industry contributes greatly to the economy of the country. This aim of the study was to estimate the prevalence of Brucella (B. melitensis, B. abortus, B. ovis).infections in sheep and springbok in the Karas Region and to find out if the outbreak of brucellosis which occurred in the Hardap Region in 2009 had spread to the Karas Region. Two experimental designs were used in this study. The first was a retrospective analysis of brucellosis testing results from 2008-2010 to indicate probable prevalence and to identify positive farms for follow-up sampling in sheep and springbok. Serological testing results of sera (n=22994) collected from 762 farms between 2008 and 2010 were analyzed and used to estimate apparent brucellosis prevalence. A total of 472 sheep sera and nine springbok sera were collected from eight farms that tested positive for Brucella antibodies between 2008 and 2010. The second part of the study was a prospective serological study in sheep and springbok reared together; sheep in the Tses and Berseba communal areas and in culled ewes at the regional abattoir. Sexually mature sheep and springbok were selected for the prospective serological study because they are more likely to show serological responses than younger animals. Prior to the serological study, eleven questionnaires were completed on the farms (n=11) that reared sheep and springbok together to gather information about farm management and risk factors for brucellosis. In the serological prevalence study, 332 sheep and 345 springbok sera were collected from the eleven commercial farms and 664 sheep sera were taken from the two communal areas. At the abattoir, 2302 sheep sera were collected from 40 farms in the region using the sample size for determining the absence or presence of disease. All sera were tested for Brucella (B. melitensis, B. abortus) antibodies using the RBT as a screening test and the CFT as a confirmatory test. B. ovis antibodies were tested for in sera from commercial farms only using the CFT test. Results from the retrospective study revealed an apparent sheep brucellosis prevalence of 0.14% (95% CI: 0.1%-0.2%) over the three years and an annual brucellosis prevalence of between 0.05% and 0.19%. At district level, apparent prevalence was between 0% and 0.49%. The prevalence of positive farms was between 0.72% and 1.82%. When apparent prevalence was adjusted for CFT sensitivity and specificity, the prevalence was zero in all cases, suggesting that the prevalence detected in this study may be due to false positive reactions. However, some of the positives serological reactions were from suspected brucellosis clinical cases which were also confirmed by the PCR test. At district level, brucellosis prevalence was shown to be rising in the Karasburg district and decreasing in the Keetmanshoop and Bethanie districts. However, statistical analysis of the data using Fisher’s exact test showed that the differences in brucellosis prevalence between districts was not significant, but that the differences in brucellosis prevalence between the three years was significant. All trace back sera collected in 2011 (using the sample sizes for proving disease freedom) from sheep (n=472) and springbok (n=9) on previously positive farms (n=8) identified by the retrospective study, tested negative for Brucella (B. melitensis, B. abortus, B. ovis) antibodies. The negative results provided strong evidence that brucellosis control measures implemented on the farms following the outbreak were effective and that these farms were now free of brucellosis. Results of questionnaire interviews showed that sheep and springbok were the main species on the farms and that the two species came into close proximity throughout the year especially at watering points in the summer. The interviews also revealed that the study population was naïve because farmers did not vaccinate sheep against brucellosis. All sera collected in the serological study on commercial farms (sheep and springbok), in the two communal areas (sheep) and at the abattoir (culled ewes) tested negative for Brucella antibodies (B. melitensis, B. abortus). The prevalence of B. ovis antibodies in rams on one farm was 10% (3/30). B. ovis antibodies were not detected in springbok. The role of springbok in the epidemiology of sheep brucellosis could not be inferred due to the negative results recorded in both species. Results of the retrospective and prospective serological studies confirmed that apparent brucellosis prevalence in sheep in the Karas Region was low. These results provided evidence that sheep and springbok reared together on the eleven commercial farms were not infected with Brucella. It was surprising that no positive reactors were found in sheep in the communal areas because the intermingling of sheep from different flocks enhances the spread of brucellosis. The absence of positive reactors at the abattoir confirms that the chances of contracting human brucellosis at the abattoir were low and confirms that the forty farms tested were free of Brucella infections.