Acta Veterinaria Scandinavica 2007, 49:10 http://www.actavetscand.com/content/49/1/10 SFaigmuprlieng4procedure Sampling procedure. Sedated horse resting its head on a support bar during a dental examination. SFaigmuprlieng5procedure Sampling procedure. Dental excavator3 inserted into a carious lesion. All S. mutans-like bacteria from P2 carious lesions identi- fied on the MSB agar showed the same pattern of growth and biochemical characteristics as the bacteria in the sam- ples from the first six horses sampled in 2002, which were described by Collins et al.,[16] and we therefore suggest that they belong to the same bacterial species. The aetiology of caries in horses is not known in detail but it is reasonable to regard it as multifactorial as is the case in humans. As mentioned above, in bacteria are regarded as one of the main causal factors for caries in humans. Bacteria that provided fermentable carbohydrates, sugars in particular, can colonise the oral cavity in contact with dental tissues. The present study shows that S. devriesei is associated with caries in P2 in Swedish horses. S. mutans and S. devriesei may have the same capability to adhere to the tooth surface, and given the right substrate e.g. sugars, they produce copious extracellular polysaccharides (Fig. 7) that provide the bacteria with a favourable environ- ment for multiplication. Moreover, the fermentation of S. devriesei can lower the pH to the critical level at which den- tal tissue is demineralised, resulting in a caries lesion. However, our result does not clarify the causal relation- ship between S. devriesei and dental decay. It can not be ruled out that the bacteria subsequently invade and colo- nise the lesion on account of a favourable environment in the caries cavity. We also found the bacteria, in four of the control animals, although in a much smaller number, which indicates that it can be present in the oral cavity of normal horses. Nevertheless, these horses may have been in the early stage of caries development without clinically visible demineralisation, or they may have been in the risk zone for developing carious lesions. S. mutans-like bacterial isolates was not found in samples taken from P2 carious lesions between 1990 and 1999 even though all the samples analysed in this study were collected in the same way and analysed at the same labo- ratory using the same methods. This makes it likely that S. devriesei was introduced into the Swedish Equidae family after 2000. Nevertheless, it cannot be ruled out that an unidentified factor, in the sample handling and process- ing, influenced the results from the 1990s. The numbers of horses and ponies have increased signifi- cantly in Sweden during the last 15 years, with this increase having been greatest around the major cities, and hence many horses live in urban surroundings with their foodstuff produced elsewhere. Within the same period of time, many new commercial equine food compositions have been introduced on the market, many of them being all-in-one feeds in the form of pellets, textured feeds, extruded nuggets and fortified hay cubes. In our experi- ence, several of these products become sticky during con- Page 6 of 9 (page number not for citation purposes)