Farm biosecurity measures to prevent hepatitis E virus infection in finishing pigs on endemically infected pig farms

Abstract

Hepatitis E virus (HEV) can be transmitted from pigs to humans and cause liver inflammation. Pigs are a major reservoir of HEV and most slaughter pigs show evidence of infection by presence of antibodies (ELISA) or viral RNA (PCR). Reducing the number of HEV infected pigs at slaughter would likely reduce human exposure, yet how this can be achieved, is unknown. We aimed to identify and quantify the effect of biosecurity measures to deliver HEV negative batches of pigs to slaughter. A case-control study was performed with Dutch pig farms selected based on results of multiple slaughter batches. Case farms delivered at least one PCR and ELISA negative batch to slaughter (PCR^-ELISA^-), indicating absence of HEV infection, and control farms had the highest proportion of PCR and/or ELISA positive batches (PCR⁺ELISA⁺), indicating high within-farm transmission. Data about biosecurity and housing were collected via a questionnaire and an audit. Variables were selected by regularization (LASSO regression) and ranked, based the frequency of variable selection. The odds ratios (OR) for the relation between case-control status and the highest ranked variables were determined via grouped logistic regression. Thirty-five case farms, with 10 to 60% PCR^-ELISA^- batches, and 38 control farms with on average 40% PCR⁺ELISA⁺ batches, were included. Rubber and steel floor material in fattening pens had the highest ranking and increased the odds of a PCR^-ELISA^- batch by 5.87 (95%CI 3.03-11.6) and 7.13 (95%CI 3.05-16.9) respectively. Cleaning pig driving boards weekly (OR 1.99 (95%CI 1.07-3.80)), and fly control with predatory flies (OR 4.52 (95%CI 1.59-13.5)) were protective, whereas a long fattening period was a risk. This study shows that cleaning and cleanability of floors and fomites and adequate fly control are measures to consider for HEV control in infected farms. Yet, intervention studies are needed to confirm the robustness of these outcomes.

Keywords: Foodborne; HEV; Mitigation; Risk factors; Within-farm transmission; Zoonosis.

Graphical abstract

Fig. 1

Selection criteria for high vs. low within-farm transmission farms and number of farms approached per step. Legend: Red boxes (): Farms with high transmission of HEV, i.e. a higher amount of batches that are either or both PCR or ELISA positive. Green boxes (): Farms with low transmission of HEV, i.e. at least 1 batch that is PCR and ELISA negative (PCR⁻ELISA⁻). The criteria for selecting farms had to be loosened twice because of insufficient willingness of farmers to participate, which is represented by the different green and red rectangle boxes below each other and number (N) of farms approached per selection criterion are shown in white hexagons (). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Barplot of the number of PCR⁻ELISA⁻ batches per farm, compared to the number of total batches sampled. Legend: Total number of batches are shown in grey, and number of PCR⁻ELISA⁻ batches in green. Results are ordered from lowest to highest number of batches sampled per farm. Bar labels give the proportion of batches sampled that is PCR⁻ELISA⁻. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Flowchart of the steps in data processing in three sections; data cleaning, missing and associated data and statistical analysis. Legend: Rectangles () represent the number of variables and farms at a certain step in data processing, while cylinders () represent steps to clean the data and parallelograms () steps in the statistical analysis. Q: questionnaire; A: audit; NA: non-available; MIDS: Multiple imputed dataset.

Fig. 4

Barplot of how often variables from the questionnaire and audit were selected by 500 bootstrapped LASSO regressions.