Rodents are a risk factor for the spreading of pathogens on farms

In a recent paper Messens et al. (2009) investigate the genetic diversity of thermotolerant Campylobacter in commercial broiler flocks and in the environment of broiler farms in Belgium. One of their conclusions is that although rodents can serve as vectors and reservoirs of Campylobacter (Meerburg et al., 2006) these animals only impose a limited risk for introducing Campylobacter in broiler houses. This conclusion is based on the frequent presence of on-farm rodent control programs as previously described (Berndtson et al., 1996). Unfortunately, I cannot agree with Messens et al. on this point. Farmers generally apply rodent control programs to prevent economic losses, as rodents can cause considerable feed losses or structural damage (e.g. gnawing on insulation). However, farmers only do so when rodent densities exceed a certain subjective threshold (Meerburg and Kijlstra, 2007). In Denmark, it was found that mice were regularly observed on 69% of the farms but that their presence was rarely considered a problem by the farmers: only 9% thought it to be problematic (Leirs et al., 2004). This subjectivity means that farmers will act at different absolute numbers of rodents on their premises. The question remains at what absolute number rodent presence becomes a risk factor, but unfortunately exact quantification of this risk is extremely difficult (Berends et al., 1996). Potential contamination of livestock with pathogens of rodents is not yet seen as a problem by farmers. Furthermore, although many farmers claim to apply rodent control programs, these are often poorly operated, mostly due to lack of time. This attitude imposes a potential risk factor for the introduction of a number of different pathogens within livestock production and may cause food safety problems later in the chain. In case of Campylobacter, it was demonstrated that mice are able to carry this pathogen for a prolonged time and can act as reservoirs on the farm (Berndtson et al., 1994). Moreover, the presence of rats on farms has been associated with an increased risk of Campylobacter introduction into broiler houses (Kapperud et al., 1993), a conclusion shared by others (Kasrazadeh and Genigeorgis, 1987) who found that 87% of rat fecal samples tested were positive for C. jejuni. In the study of Berndtson et al. (1996) which is also mentioned by Messens and colleagues, it was shown that in flocks where mice were noticed, the Campylobacter frequency was 40% compared with 23% positive flocks were no mice were proven, although an estimation of the real number of mice was difficult in this study. Additionally, rodents were also identified as potential contamination source for other pathogens, such as Salmonella (Henzler and Opitz, 1992 D.J. Henzler and H.M. Opitz, The role of mice in the epizootiology of Salmonella enteritidis infection on chicken layer farms, Avian Dis. 36 (1992), pp. 625–631. View Record in Scopus | Cited By in Scopus (90)Henzler and Opitz, 1992) and more recently the protozoan parasite Toxoplasma gondii (Kijlstra et al., 2008). Thus, the conclusion that rodent presence can be considered a limited risk factor seems incorrect. Instead, the importance of proper on-farm rodent management for food safety purposes should be stressed in order to minimize this potential risk factor. Rodent management consists of a combination of three elements: prevention, monitoring and control. Good understanding of ecology of the pest species should be the main focus. Farmers should be advised that it is not only important to apply rodent management to prevent economic losses but also from a veterinary perspective. This is important, as agriculture in general is changing to more open production systems due to public demands (as is the case with organic farming), which may result in more interaction between livestock and wild fauna.