Risk factors for introduction of BHV1 into BHV1-free Dutch dairy farms: A case-control study

In May 1998, a compulsory eradication programme for BHV1 started in the Net herlands. In December 1999 approximately 24% of Dutch dairy farms were cert ified BHV1-free (Animal Health Service (AHS)). Ninety-three certified BHV1- free dairy farms participated in a cohort study that investigated the proba bility of introduction of infectious diseases. The probability of introduct ion of BHV1 was determined from March 1997 until April 1999. Ninety of thes e farms remained BHV1-free and could be used as control farms. From January 1997 until March 1998, BHV1 was introduced into 41 BHV1-free dairy farms i n the Netherlands (case farms). Management data were collected for both cas es and controls and were complete for 37 case farms and 82 control farms. F or small data sets and for data in which both low and high frequencies were expected in the contingency tables, the asymptotic methods were unreliable . Our data set clearly resembled such a data set; the risk factors were rar e events because the BHV1-free farms were closed farms on which few direct animal contacts occurred. Therefore, an exact stratified modelling approach was most suitable for the data. The study showed that dairy farms should p revent cattle from escaping or mingling with other cattle and that professi onal visitors should always wear protective farm clothing.