Diagnosis of Bovine Brucellosis: RBPT Vis-A-Vis STAT

Priya P.M.*, Nithina K. Baburaj, Nikesh Kiran, Shimi Rajanand Jayesh V.

Dept of Veterinary Microbiology, College of Veterinary and Animal Sciences (COVAS), Pookot, Wayanad District, Kerala, India- 673576.

ABSTRACT:

Bovine brucellosis is found worldwide however, it has been eradicated from many countries it is one of the most serious diseases in developing countries. The highest prevalence is seen in dairy cattle. Conventionally, serological tests are used to screen for, or to confirm the disease. These screening tests are inexpensive, fast and highly sensitive but not necessarily highly specific. Serodiagnosis of bovine brucellosis was carried out in 64 sera samples of cattle using RBPT and STAT. The study revealed greater sensitivity of RBPT as compared to STAT. Considering sensitivity, specificity and ease in performing the test, it is suggested that a combination of RBPT and STAT can be used in the diagnosis of bovine brucellosis in order to control and eradicate the disease.

KEYWORDS: Bovine brucellosis, Brucella abortus, serological tests, RBPT, STAT, Kerala

INTRODUCTION:

Brucellosis is a contagious and most widespread zoonotic disease caused by bacteria of genus Brucella.¹It has a considerable impact on animal and human health, as well as wide socio-economic impacts, especially in countries in which rural income relies largely on livestock breeding and dairy products. The disease is endemic in India and the prevalence rate in cattle ranged from 0.3% in Himachal Pradesh ² to 56.2% in Assam ³ and is showing steady increase in the same territories of investigation compared to earlier studies.^{4, 5} The most specific diagnostic tests are bacterial culture and serological tests including the Rose Bengal Plate Test (RBPT), Standard Tube Agglutination Test (STAT), Mercaptoethanol Agglutination Test (MET), Complement Fixation Test (CFT) and Enzyme Linked Immuno Sorbant Assay (ELISA). Isolation of pathogen is considered as confirmatory test in bacterial infection, but in brucellosis bacterial cultures are time consuming and hazardous. The validity of single test for enduring the accurate diagnosis and estimation of the disease prevalence may prove fallacious, since tests have their own limitations and advantages in terms of sensitivity and specificity. Hence the present study was carried out to compare the sensitivity of RBPT and STAT as serological tests for brucellosis.

MATERIALS AND METHODS:

Collection of sera samples:

A total of 64 blood samples were collected from cattle slaughtered in the Municipal slaughter house, Kalpetta of Wayanad district of Kerala. Serum was separated and stored at -20⁰ C until use.

Coloured antigen for RBPT and Brucella abortus plain antigen for STAT were procured from Institute of Animal Health and Veterinary Biologicals (IAH&VB), Hebbal, Bangalore, India and all the samples were subjected to RBPT and STAT.

Rose Bengal Plate Test (RBPT): The test was performed according to the standard procedure.⁶ Briefly, 30 µl of sera sample was mixed with equal volume of antigen on a microslide and circled approximately 2 cm in diameter with a micro tip. The sample was gently mixed by rotating movement for 5 minutes at room temperature and then observed. Any sign of agglutination was considered as positive.

Standard Tube Agglutination Test (STAT): The STAT using plain antigen was done as per the method described.⁶ Due to special significance of 50% end point, a control tube was set with 0.75 ml carbol saline and 0.25 ml plain antigen. The degree of agglutination was determined by degree of clearing before shaking the tubes. The highest serum dilution showing 50% agglutination or more was taken as the end point or titre. Serum titres of 80 IU or above were considered as positive, 40 IU as doubtful and less than 40 IU as negative. Statistical analysis of the data was done. ⁷

RESULTS:

The study indicated that 9 out of 64 animals tested were positive to RBPT (14.1%) and 7 of the 64 animals were positive to STAT (11%) (Table-1).

DISCUSSION:

Though STAT detected less number of samples as positive, statistically there was no significant correlation exist with RBPT. The specificity of STAT was more than RBPT as it recorded less number of positive cases, which is in accordance with other workers.^8,9 Similar results were found on comparing ELISA, MRT (Milk Ring Test) and STAT for diagnosing brucellosis at herd level.¹⁰Diagnostic value may be questionable on individual basis because of cross reacting antibodies but for screening of herd these basic tests remain ideal.¹¹The low pH of 3.65 used in RBPT antigen prevents some agglutination by IgM and encourages agglutination by IgG1 thereby reducing non-specific interactions.¹²The milk based PCR had several advantages over the current microbiological methods for the diagnosis of brucellosis, including speed, safety, high sensitivity and specificity; however for evaluation of asymptomatic, exposed animals, the traditional methods might be superior, especially if the cost is taken into consideration.¹³

Hence, this study suggests that RBPT and STAT can be together employed in detecting bovine brucellosis more accurately in dairy units because of its ease, economy and easy adaptability to any laboratory condition.

Table-1 Comparison of RBPT and STAT for screening Bovine Brucellosis

Serological test employed	Total number of sera screened	Number positive	Percent positive
RBPT	64	9	14.1
STAT	64	7	11.0

ACKNOWLEDGEMENT:

The authors are thankful to The Associate Dean, College of Veterinary and Animal Sciences, Pookot for the facilities provided to carry out the work.

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Received on 18.09.2010

Accepted on 13.11.2010

Research J. Science and Tech. 2(6): Nov. -Dec. 2010: 145-146