Seroprevalence and Risk Factors of Brucella Infection in Dairy Animals in Urban and Rural Areas of Bihar and Assam, India
Abstract
:1. Introduction
2. Materials and Methods
2.1. Data and Sampling Procedure
2.2. Biological Sample Collection and Laboratory Analysis
2.3. Data Analyses
2.4. Criteria Followed for Selection of the Variables for Multivariable Analysis
- For all pairs of variables within a group (e.g., FC, FM, etc.) in which both variables in the pair were either continuous or dichotomous, correlations were computed;
- For all pairs of variables within a group (e.g., FC, FM, etc.) in which one variable was continuous and the other was categorical (the continuous variable was considered as the outcome and the categorical variable was converted to a set of dummy variables), R2 and a p-value were computed using linear regression;
- For all pairs of variables within a group (e.g., FC, FM, etc.) in which one variable was categorical and the other variable was either categorical or dichotomous, a p-value was computed using a Chi-square test.
- A variable that did not have a plausible biological relationship to the outcome variable was excluded;
- A variable that did not have substantial variability (e.g., species of dairy animals) was excluded;
- A variable that had more than 10% missing values was excluded;
- If correlation between two variables was greater than 0.7 or R2 > 0.5 (as computed above), the following selection criteria were used to exclude one variable in favour of the other: (1) the variable that had a weaker association with outcome variable was excluded, (2) the variable that had higher missing value was excluded, (3) the variable for which it was relatively difficult to collect accurate data was excluded;
- A variable that had no significant association with the outcome variable (p > 0.3) was excluded.
2.5. Steps Followed for Model Building for Multivariable Analyses
- Separate models were built for each group of variables (i.e., PD, FC, CD, and FM).
- Following the causal diagram, initially, an analysis was conducted using variables in the PD group. This was followed by the FC, CD, and FM groups in sequence. For each group, significant variables from the antecedent groups (i.e., to the left of the group of interest) were retained to see if there was any confounding effect. No variables from subsequent groups were included, as these would be intervening (intermediate) variables. For example, when analysing variables in the CD group, significant variables from the PD and FC models were retained, but all variables in the FM group were excluded.
- During analyses, variables were manually eliminated one at a time. The variable for elimination was selected based on its p-value, absence of evidence of confounding effects (i.e., changes in the magnitude of other coefficients), and the plausibility of its causal association with Brucella seropositivity.
- A variable was dropped from the analysis if it had strong collinearity with another variable that had more plausible association with Brucella seropositivity.
- In the final model, only significant variables (p < 0.1) identified through multivariable regression analysis were kept, along with the pre-selected confounders, if any. A cut point of p < 0.1 was chosen in light of the relatively small sample size and low prevalence of Brucella seropositivity.
3. Results
3.1. Brief Profile of Farming System and Dairy Animals
3.2. Laboratory Results of Serum Samples
3.3. Univariable Analysis
3.4. Multivariable Analysis of Risk Factors
4. Discussion
4.1. Sero-Prevalence
4.2. Risk Factors
4.3. Comments on Study Design
5. Conclusions
Supplementary Materials
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Variables | Particulars | Bihar | Assam | Total | p-Value |
---|---|---|---|---|---|
Mean Herd Size per Farm (± standard error (SE)) | 2.8 ± 0.2 | 4.1 ± 0.4 | 3.4 ± 0.2 | 0.009 | |
Breed of the animals surveyed | High-producing (exotic or crossbred) | 345/376 (91.8) * | 186/364 (51.1) | 531/740 (71.8) | <0.001 |
Non-descript indigenous | 31/376 (8.2) | 178/364 (48.9) | 209/740 (28.2) | ||
Species of the animals | Cattle | 354/376 (94.1) | 364/364 (100.0) | 718/740 (97.0) | <0.001 |
Buffalo | 22/376 (5.9) | 0/364 | 22/740 (2.9) | ||
Mean age of animals surveyed (± SE) | 4.7 ± 0.1 | 6.2 ± 0.1 | 5.4 ± 0.1 | <0.001 | |
Breeding method followed by households | Followed artificial insemination (AI) | 266/292 (91.1) | 128/242 (52.9) | 394/534 (73.8) | <0.001 |
Variables | Particulars | Kamrup (Metropolitan) | Golaghat | Baska | p-Value |
---|---|---|---|---|---|
Mean herd size of dairy animals | Per farm | 8.8 ± 1.1 | 1.7 ± 0.1 | 1.6 ± 0.1 | 0.01 |
Rearing system | Fully stall-fed | 141/178 (79.2) * | 9/105 (8.6) | 6/94 (6.4) | <0.001 |
Partly stall-fed | 37/178 (20.8) | 96/105 (91.4) | 88/94 (93.6) | ||
Breed | Improved | 148/176 (84.1) | 18/96 (18.7) | 20/92 (21.7) | <0.001 |
Non-descript | 28/176 (15.9) | 78/96 (81.2) | 72/92 (78.3) | ||
Adoption of AI | Yes | 152/178 (85.4) | 45/105 (42.8) | 34/94 (36.2) | <0.001 |
Animal movement | Yes | 39/178 (21.9) | 96/105 (91.4) | 90/94 (95.7) | <0.001 |
New animal introduced | Yes | 71/178 (39.9) | 6/105 (5.7) | 4/94 (4.2) | <0.001 |
Animals belonging to trained farmers | Yes | 41/178 (23.0) | 5/105 (4.8) | 5/94 (5.3) | <0.001 |
Animals belonging to farmers who had consultation with veterinarian | Yes | 158/178 (88.8) | 77/105 (73.3) | 70/94 (74.50) | 0.001 |
Use of disinfectants in cleaning the farms | Used | 52/176 (29.5) | 2/96 (2.1) | 4/92 (4.3) | <0.001 |
Variables | Description of the Variables | Sero-Positive/Total (%) | Coefficient of Unconditional Association with Brucella Sero-Positivity | p-Value of Unconditional Association with Brucella Sero-Positivity | Missing Value | Kept for Multivariable Model |
---|---|---|---|---|---|---|
Outcome | ||||||
ELISA results of Brucella infection | Positive | 58 | 13 | |||
Negative | 306 | |||||
Identifiers | ||||||
Districts | Kamrup (large farms) | 49/135 (18.9) | Ref. * | <0.001 | 0 | Yes |
Kamrup (small farms) | 3/41 (7.3) | −2.19 | ||||
Golaghat | 2/96 (2.1) | −2.77 | ||||
Baska | 4/92 (4.3) | −3.55 | ||||
Farm Characteristics (FC) | ||||||
Location of the farm in rural or urban areas | Rural CDB | 20/161 (12.4) | Ref. | 0.160 | 0 | Yes |
Urban CDB | 38/203 (18.7) | 0.43 | ||||
Category of farms | Small (1–3 dairy animals), | 8/223 (3.6) | Ref. | <0.001 | 0 | Yes |
Medium (4–10 dairy animals) | 19/81 (23.4) | 2.19 | ||||
Large (>10 dairy animals) | 31/60 (51.7) | 3.58 | ||||
Dairy animals in contact with goats | Yes | 9/106 (8.5) | −1.19 | 0.030 | 0 | Yes |
No | 49/258 (19.0) | Ref. | ||||
Type of floor | Concrete | 20/67 (29.8) | Ref. | <0.001 | 0 | Yes |
Earthen | 9/202 (4.4) | −2.42 | ||||
Others | 29/95 (30.5) | 0.10 | ||||
Farm Management (FM) | ||||||
Adoption of AI | Yes | 46/225 (20.4) | 1.16 | 0.020 | 0 | Yes |
No | 12/139 (8.6) | Ref. | ||||
Introduction of new animals | Introduced | 22/79 (27.8) | 1.54 | 0.006 | 0 | Yes |
Not introduced | 36/285 (12.6) | Ref. | ||||
Animal movement | Animal moved | 9/213 (4.2) | −2.67 | <0.001 | 0 | Yes |
Not moved | 49/151 (32.4) | Ref. | ||||
Use of disinfectant in cleaning farms | Used disinfectant | 35/160 (21.9) | 1.06 | 0.020 | 0 | Yes |
Not used disinfectant | 23/204 (11.3) | Ref. | ||||
Producer Demographics (PD) | ||||||
Education of farmers | No education | 17/66 (25.7) | Ref. | 0.14 | 0 | Yes |
Class I–V | 11/49 (22.4) | −0.15 | ||||
Class VI–X | 18/149 (12.1) | −1.16 | ||||
Class XI and above | 12/100 (12.0) | −1.13 | ||||
Age of farmers | 20–40 years | 21/89 (23.6) | Ref. | 0.10 | 0 | Yes |
41–60 years | 26/191 (13.6) | −1.15 | ||||
60 years and above | 11/84 (13.1) | −1.15 | ||||
Training completed by farmers | Completed | 15/50 (30.0) | 1.52 | 0.02 | 0 | Yes |
Not completed | 43/314 (13.7) | Ref. | ||||
Interaction had with the veterinarians | Had interaction | 55/297 (18.5) | 1.85 | 0.005 | 0 | Yes |
No interaction | 3/67 (4.5) | Ref. | ||||
Cow Demographics (CD) | ||||||
Breed of animal | Non-descript indigenous | 7/178 (3.9) | Ref. | <0.001 | 13 | Yes |
Improved/CB/pure | 51/186 (27.4) | 2.66 | ||||
Age of animals | With Brucella seropositive | 6.83 ± 0.33 | 0.03 | 13 | Yes | |
With Brucella sero-negative | 6.09 ± 0.13 |
Odds Ratio | Standard Error | p-Value | 95% Confidence Interval | |
---|---|---|---|---|
Producer Demographics (PD) | ||||
District | ||||
Kamrup (large farms) | Ref. * | |||
Kamrup (small farms) | 0.14 | 0.10 | 0.007 | 0.030–0.590 |
Baska | 0.07 | 0.04 | <0.001 | 0.020–0.250 |
Golaghat | 0.03 | 0.03 | <0.001 | 0.006–0.160 |
Cow Demographics (CD) | ||||
Age of the dairy animals, in years | 1.23 | 0.10 | 0.008 | 1.050–1.440 |
Farm Management (FM) | ||||
Artificial insemination adopted | 0.33 | 0.20 | 0.070 | 0.100–1.100 |
Variables | Odds Ratio | Standard Error | p-Value | 95% Confidence Interval |
---|---|---|---|---|
District | ||||
Kamrup (large farms) | Ref. * | |||
Kamrup (small farms) | 0.11 | 0.09 | 0.007 | 0.020–0.540 |
Baska | 0.03 | 0.02 | <0.001 | 0.005–0.150 |
Golaghat | 0.01 | 0.01 | <0.001 | 0.002–0.100 |
Age of dairy animals, in years | 1.24 | 0.10 | 0.007 | 1.060–1.440 |
Artificial insemination adopted | 0.33 | 0.20 | 0.072 | 0.100–1.100 |
Random effect of village | 0.40 | 0.49 | ||
Random effect of household | 0.51 | 0.69 |
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Deka, R.P.; Shome, R.; Dohoo, I.; Magnusson, U.; Randolph, D.G.; Lindahl, J.F. Seroprevalence and Risk Factors of Brucella Infection in Dairy Animals in Urban and Rural Areas of Bihar and Assam, India. Microorganisms 2021, 9, 783. https://doi.org/10.3390/microorganisms9040783
Deka RP, Shome R, Dohoo I, Magnusson U, Randolph DG, Lindahl JF. Seroprevalence and Risk Factors of Brucella Infection in Dairy Animals in Urban and Rural Areas of Bihar and Assam, India. Microorganisms. 2021; 9(4):783. https://doi.org/10.3390/microorganisms9040783
Chicago/Turabian StyleDeka, Ram Pratim, Rajeswari Shome, Ian Dohoo, Ulf Magnusson, Delia Grace Randolph, and Johanna F. Lindahl. 2021. "Seroprevalence and Risk Factors of Brucella Infection in Dairy Animals in Urban and Rural Areas of Bihar and Assam, India" Microorganisms 9, no. 4: 783. https://doi.org/10.3390/microorganisms9040783