Table 1.
Univariable and multivariable logistic regression analysis of potential risk factors for Rift Valley fever virus (RVFV) seropositivity in people working on farms in the study area of South Africa during 2015–2016.
| Variables 1 | RVFV Seropositive n/N (%) | Univariable Analysis | Multivariable Analysis 2 | ||
|---|---|---|---|---|---|
| Odds Ratio (CI95%) | p-Value (<0.2) | Odds Ratio (CI95%) | p-Value (<0.05) | ||
| Demographic characteristics | |||||
| Age (years) | |||||
| 16–29 | 5/196 (2.6%) | 1 (base) | - | 1 (base) | - |
| 30–39 | 12/196 (6.1%) | 2.7 (0.8–8.7) | 0.100 | 2.64 (0.76–9.18) | 0.126 |
| 40–49 | 13/114 (11.4%) | 5.9 (1.9–18.2) | 0.002 | 6.91 (2.15–22.2) | 0.001 |
| 50–63 | 20/106 (18.9%) | 12.6 (4.3–37.0) | <0.001 | 12.9 (4.15–40.0) | <0.001 |
| ≥64 | 7/40 (17.5%) | 16.9 (4.3–66.0) | <0.001 | 25.6 (5.50–119) | <0.001 |
| Working on farm (years) | Eliminated | ||||
| ≤5 | 16/299 (5.4%) | 1 (base) | - | ||
| 6–10 | 11/108 (10.2%) | 2.1 (1.0–4.5) | 0.53 | ||
| 11–20 | 14/128 (10.9%) | 2.5 (1.2–5.3) | 0.019 | ||
| 21–30 | 8/64 (12.5%) | 3.1 (1.2–7.9) | 0.016 | ||
| 31–40 | 5/37 (13.5%) | 4.0 (1.4–11.8) | 0.012 | ||
| >40 | 8/48 (16.7%) | 5.3 (2.0–13.6) | 0.001 | ||
| Working with animals (years) | Eliminated | ||||
| ≤5 | 10/240 (4.2%) | 1 (base) | - | ||
| 6–10 | 8/110 (7.3%) | 1.9 (0.8–4.7) | 0.166 | ||
| 11–20 | 15/145 (10.3%) | 3.0 (1.3–7.3) | 0.014 | ||
| 21–30 | 15/85 (17.6%) | 6.1 (2.5–15.2) | <0.001 | ||
| 31–40 | 6/51 (11.8%) | 4.5 (1.4–14.8) | 0.014 | ||
| >40 | 7/48 (14.6%) | 6.3 (2.1–18.6) | 0.001 | ||
| Job | |||||
| Farm worker/herdsman | 46/487 (9.5%) | 1 (base) | - | 1 (base) | - |
| Farm/livestock owner/manager | 15/173 (8.7%) | 0.9 (0.5–1.7) | 0.758 | 1.26 (0.42–3.80) | 0.678 |
| Family, domestic worker, driver | 1/24 (4.2%) | 0.4 (0.05–3.3) | 0.403 | 0.65 (0.06–6.72) | 0.718 |
| Activities in past | |||||
| Cleaning equipment | Eliminated | ||||
| Yes | 53/547 (9.7%) | 1.5 (0.8–2.9) | 0.176 | ||
| No | 9/137 (6.6%) | 1 (base) | - | ||
| Injection and collection of samples from animals | |||||
| Yes | 53/500 (10.6%) | 2.4 (1.2–4.7) | 0.014 | 2.33 (1.03–5.30) | 0.043 |
| No | 9/184 (4.9%) | 1 (base) | - | 1 (base) | - |
| Assisting with birth of animal | Eliminated | ||||
| Yes | 57/577 (9.9%) | 2.1 (0.7–6.3) | 0.172 | ||
| No | 5/107 (4.7%) | 1 (base) | - | ||
| Assisting with surgery | |||||
| Yes | 8/136 (6.0%) | 0.5 (0.3–1.1) | 0.097 | 0.38 (0.15–0.98) | 0.046 |
| No | 54/548 (9.9%) | 1 (base) | - | 1 (base) | - |
| Slaughtering of animals | |||||
| Yes | 58/563 (10.3%) | 3.3 (1.3–8.7) | 0.014 | 3.93 (1.20–12.88) | 0.024 |
| No | 4/121 (3.3%) | 1 (base) | - | 1 (base) | - |
| Burying dead animals | Eliminated | ||||
| Yes | 45/422 (10.7%) | 1.7 (0.9–3.0) | 0.079 | ||
| No | 17/262 (6.5%) | 1 (base) | - | ||
| Eating hooved animal found dead | |||||
| Yes | 42/342 (12.3%) | 3.4 (1.8–6.7) | <0.001 | 3.14 (1.49–6.61) | 0.003 |
| No | 20/342 (5.8%) | 1 (base) | - | 1 (base) | - |
| Measures against mosquito bites | |||||
| Yes | 22/305 (7.2%) | 0.7 (0.4–1.1) | 0.114 | 0.52 (0.29–0.90) | 0.021 |
| No | 40/379 (10.6%) | 1 (base) | - | 1 (base) | - |
| Working on farm with primarily domestic or wild animals | Eliminated | ||||
| Wild | 3/16 (18.8%) | 2.3 (1.2–4.4) | 0.011 | ||
| Domestic | 59/669 (8.8%) | 1 (base) | - | ||
| Working on farm with private or communal land use 3 | |||||
| Communal | 3/44 (6.8%) | 0.8 (0.3–2.4) | 0.656 | 0.75 (0.09–5.93) | 0.784 |
| Private | 59/641 (9.2%) | 1 (base) | - | 1 (base) | - |
| Working on farm that kept cattle | |||||
| Yes | 47/581 (8.1%) | 0.5 (0.2–1.0) | 0.059 | 0.35 (0.14–0.88) | 0.025 |
| No | 15/104 (14.4%) | 1 (base) | - | 1 (base) | - |
| Manmade dam(s) on farm | |||||
| Yes | 54/532 (10.2%) | 2.5 (1.1–5.7) | 0.032 | 2.68 (1.04–6.89) | 0.041 |
| No | 6/139 (4.3%) | 1 (base) | - | 1 (base) | - |
| New animals are quarantined | Eliminated | ||||
| Yes | 7/142 (4.9%) | 0.4 (0.2–1.1) | 0.069 | ||
| No | 55/543 (10.1%) | 1 (base) | - | ||
| Main purpose of farming 3 | |||||
| Meat | 40/367 (10.9%) | 1 (base) | - | 1 (base) | - |
| Dairy | 1/24 (4.2%) | 0.3 (0.03–3.6) | 0.373 | 1.00 (0.13–7.56) | 0.999 |
| Meat-wool | 15/194 (7.7%) | 0.7 (0.4–1.2) | 0.186 | 0.67 (0.34–1.29) | 0.225 |
| Other (milk, bartering, wealth, ceremonial, resale, tourism) | 6/100 (6.0%) | 0.5 (0.2–1.3) | 0.169 | 0.17 (0.03–0.93) | 0.041 |
| Animals are slaughtered on farm | Eliminated | ||||
| Yes | 47/443 (10.6%) | 1.8 (0.8–4.2) | 0.158 | ||
| No | 12/198 (6.1%) | 1 (base) | - | ||
| Animals vaccinated against RVFV in the past | Eliminated | ||||
| Yes | 41/357 (11.5%) | 1.9 (1.1–3.3) | 0.027 | ||
| No | 19/300 (6.3%) | 1 (base) | - | ||
| RVF on farm in past incl. participants working there 4 years or more only | Not included | ||||
| Yes | 19/149 (12.8%) | 1.3 (0.6–2.7) | 0.498 | Checked for confounding | |
| No | 28/272 (10.3%) | 1 (base) | - | ||
| Drinking milk | Not included | ||||
| Pasteurised/boiled | 19/233 (8.2%) | 1 (base) | |||
| On occasion raw | 5/56 (8.9%) | 1.2 (0.4–3.6) | 0.762 | ||
| Raw | 27/292 (9.2%) | 1.1 (0.5–2.2) | 0.817 | ||
1 The variables with univariable p-value < 0.2 were included in the multivariable analysis. The variables with multivariable p-value < 0.05 were kept in the multivariable model. 2 Eliminated means that the variable was first included in the model and then it was omitted due to the fact that its p-value in the model was ≥ 0.05. 3 Most communal farmers farm with mixed purpose while private land-owned farms usually specialize in one or dual production motive. When landownership variable was eliminated from the model the odds for seropositivity in mixed production purpose gave unrealistic results and for this reason landownership was retained as potential confounder in the model despite its statistical insignificance. Sex was not significant by univariable analysis (p ≥ 0.2). Sex was not considered as potential confounder and most farm workers were male.