Preventive Veterinary Medicine 97 (2010) 29–36 Contents lists available at ScienceDirect Preventive Veterinary Medicine journal homepage: www.elsevier.com/locate/prevetmed Age at which dairy cattle become Mycobacterium avium subsp. paratuberculosis faecal culture positive Maarten F. Weber a,∗ , Jan Kogut b , Joop de Bree b , Gerdien van Schaik a , Mirjam Nielen c a GD Animal Health Service, P.O. Box 9, 7400 AA Deventer, The Netherlands b Central Veterinary Institute of Wageningen UR, P.O. Box 65, 8200 AB Lelystad, The Netherlands c Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80151, 3508 TD Utrecht, The Netherlands article info Article history: Received 6 July 2009 Received in revised form 6 July 2010 Accepted 8 July 2010 Keywords: Cattle Paratuberculosis Faecal culture Age Survival analysis abstract Age at which cattle become faecal culture positive for Mycobacterium avium subsp. paratu- berculosis (Map) can be used as a proxy parameter for age at onset of faecal shedding, which is an important parameter in the control of Map in cattle herds. To investigate the age at becoming faecal culture positive, survival analysis methods were applied. The analyses were carried out on asynchronous interval censored data of faecal culture results of sam- ples collected from 18,979 female Holstein-Frisian cattle in 353 Dutch herds between 1996 and 2002. The data were analysed with a Weibull proportional hazards model. The results indicate that the distribution of age at onset of faecal shedding in Holstein- Frisian dairy cattle in infected herds is associated with the within-herd prevalence. In higher classes of apparent prevalence, cattle started to shed Map at younger age on average. In herds with an apparent prevalence <0.05, 0.05–0.1, 0.1–0.2 and ≥0.2, the proportion (95% CI) of cattle with onset of faecal shedding before 2 years of age was estimated at 1% (0.5%; 2%), 4% (3%; 5%), 8% (5%; 10%) and 20% (11%; 32%), respectively. This study indicates that a considerable proportion of young stock is shedding Map, espe- cially in high prevalence herds. Therefore, infectious young stock should be a major concern in the control of paratuberculosis. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Paratuberculosis (or Johne’s disease) in cattle is an infectious disease caused by Mycobacterium avium subsp. paratuberculosis (Map). The disease is widespread world- wide and causes significant economic losses. Control programmes for paratuberculosis are based on improve- ment of farm management (e.g., separation of young stock from adult cattle) and on early culling of infected cattle to reduce transmission of Map within a herd. ∗ Corresponding author at: Ruminant Health Department, GD Animal Health Service, P.O. Box 9, 7400 AA Deventer, The Netherlands. Tel.: +31 570 660379; fax: +31 570 660190. E-mail address: m.weber@gddeventer.com (M.F. Weber). In the development of control programmes, it has fre- quently been assumed that young stock do not become infectious. Provided that adult cattle are highly resistant to infection, this assumption means that effective sepa- ration of young stock from adult cattle largely prevents postnatal infection. However, this assumption is in contrast with observed faecal shedding of Map in experimentally infected young stock (Rankin, 1959, 1961; McDonald et al., 1999; Waters et al., 2003), young stock exposed to exper- imentally infected cattle (Rankin, 1961; van Roermund et al., 2007) and naturally infected young stock (Kalis et al., 1999; McDonald et al., 1999; Waters et al., 2003; Antognoli et al., 2007). Moreover, calf–calf transmission has experi- mentally been demonstrated (van Roermund et al., 2007). The age at onset of faecal shedding depends on the progression of the infection within an infected individual. 0167-5877/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2010.07.004