Date Published: March 28, 2007
Publisher: BioMed Central
Author(s): Torbjörn S Lundström, Gunnar G Dahlén, Ove S Wattle.
Swedish equine dental practices have empirically found that the prevalence of infundibular caries as a primary disorder in the first permanent premolar teeth (P2) of the horse upper jaw has increased during the last 10 years. A previously unknown bacterial species, Streptococcus devriesei (CCUG 47155T), which is related to Streptococcus mutans, has recently been isolated from these carious lesions. To understand the aetiology of caries in horses, it is essential to elucidate the relationship between S. devriesei and P2 infundibular caries.
The anterior infundibulum of maxillary P2, or the occlusal surface at the site of the infundibulum, in 117 horses and ponies, 77 with and 40 without caries in this tooth, was sampled for bacteriological analyses between 1990 and 2004. Samples were transported in VMGA III medium and then inoculated onto MSB agar. The approximate number of bacteria was counted in each sample and the isolates were characterised biochemically, using a commercial kit.
All 50 samples taken from carious lesions after 2002 were positive for an S. mutans-like strain, i.e. S. devriesei. The bacteria were also found in four of the control animals, but were much less numerous than in samples from caries-affected horses. None of the swabs taken prior to 2002 were positive for this bacteria.
Our results demonstrate that S. devriesei can colonise the infundibulum of P2 of the horse upper jaw, which can be fatal for the dental tissue. We conclude that S. devriesei is strongly associated with P2 caries in horses.
The development of dental caries in humans has been discussed in terms of an interaction between three main factors: bacteria, substrate, and teeth . Owing to their ability to produce extracellular polysaccharides (polyglucans) from sucrose, certain bacterial species, e.g. streptococci, can adhere more easily to the tooth surface . Members of the group of mutans streptococci (e.g. S. mutans and S. sobrinus) are unique in this sense, since their polyglucans are more water insoluble and become sticky when produced in dental plaque . Tooth defects in the form of small fissures or enamel cracks, on the occlusal surface facilitate bacterial colonisation. In a favourable environment, such as the presence of abundant sugars within a tooth fissure, these bacteria produce lactic acids [3,4] in a manner that decreases the pH below the critical levels for demineralisation of cement, (pH < 6.7)  and enamel (pH ~ 5.5) [5,6]. A decrease in pH appears to cause very similar damage in human and equine enamels . The subsequent removal of the tooth matrix by proteolysis leads to the so-called cavity of decay. The equine premolar tooth differs from its human counterpart in that it expresses all three types of dental hard tissue on its occlusal surface (Fig. 1). Necrosis of infundibular cementum, infundibular cemental hypoplasia, and micro fractures have all been suggested as factors that predispose carious lesions in equine teeth . A total of 117 horses and ponies that had attended either the equine clinic at the Swedish University of Agricultural Sciences or the animal dental clinic in Söderköping, Sweden, for an oral examination as a part of a normal health check-up or after showing symptoms of an oral disorder were included in the study. Carious lesions were defined in this study as progressive decalcification and destruction of the cementum, enamel, and dentin in the infundibulum of the permanent P2 in the upper jaw (Fig. 2). Using the scale of Dacre,  this corresponds to grade 3 infundibular caries; that is, clinically the enamel ridge of the infundibulum is completely or partly missing and the decay feels sticky when an investigation probe is inserted into it. There was no growth of S. mutans-like bacteria in samples taken from carious lesions during the years 1990 – 1999. On the contrary, the results of incubation on MSB agar and biochemical characterisation showed that all samples taken from carious lesions after 2002 were positive for the S. mutans-like strain, S. devriesei. Of these 50 animals with caries, 24 had carious lesions bilaterally in P2s of the upper jaw. In 20 of the 30 animals with caries sampled as an internal control, S. devriesei was also found in samples from the occlusal surface of the mandibular P3 on the opposite side (Table 1). The presence of S. devriesei colonies was significantly smaller in the control animals than in the samples taken from horses and ponies with carious lesions (p < 0.0001, 3 df). Among the control animals, four were positive for S. devriesei at the P2 position and of these horses three also had positive samples taken at the internal control site. The samples from the remaining 36 control animals were all negative for S. mutans-like bacteria. On the MSB agar plate, the S. devriesei isolates showed a copious production of polysaccharides (Fig. 7). Carious lesions are reported to mainly occur in cheek teeth of the upper jaw . The gross anatomy differs between the upper and lower jaws with respect to the infundibula, which are absent in the mandibular cheek teeth. The infundibula in upper cheek teeth are usually incompletely filled by infundibular cementum and both Baker  and Kilic et al.  reported a high incidence of severe caries in this area. The incomplete filling with cementum could of course be looked upon as a typical predilection site, that is a damaged or incomplete tooth surface, where bacteria can easily adhere and colonise compared to the intact surface. Baker  suggested that Honma et al.  misnamed cemental hypoplasia as a carious lesion. However, it may be difficult to clinically define the difference between hypoplasia and caries of the cementum. In our experience, the caries definition used in the present study, – a progressive decalcification and destruction of the cementum, enamel and dentin in the infundibulum -, is more practical for clinical use. We suggest that S. devriesei forms part of the normal equine oral bacterial flora and that under certain conditions it colonises sites such as the infundibulum of the cheek teeth or traumatised dental hard tissues. It is obvious that such colonisation can be fatal for dental tissue. Further research into the normal function of saliva, food and immunological factors is needed to fully elucidate the aetiology of caries disease in horses. The author(s) declare that they have no competing interests. TL collected the samples, drafted the manuscript and participated in the design of the study, laboratory analysis and analysis of data. GD carried out the laboratory analysis and participated in the design of the study, analysis of data and drafting the manuscript. OW conceived the study, and participated in its design and coordination, analysis of data and helped to draft the manuscript. All authors read and approved the final manuscript Source: http://doi.org/10.1186/1751-0147-49-10