5 Periodontal disease – an introduction
Periodontal disease is the result of the inflammatory response to dental plaque, i.e. oral bacteria, and is limited to the periodontium. It is probably the most common disease seen in small animal practice, with the great majority of dogs and cats over the age of 3 years having a degree of disease that warrants intervention.
Periodontal disease is a collective term for a number of plaque-induced inflammatory lesions that affect the periodontium. It is a unique infection in that it is not associated with a massive bacterial invasion of the tissues. Gingivitis is inflammation of the gingiva and is the earliest sign of disease. Individuals with untreated gingivitis may develop periodontitis. The inflammatory reactions in periodontitis result in destruction of the periodontal ligament and alveolar bone. The result of untreated periodontitis is ultimately exfoliation of the affected tooth. Thus, gingivitis is inflammation that is not associated with destruction (loss) of supporting tissue – it is reversible. In contrast, periodontitis is inflammation where the tooth has lost a variable degree of its support (attachment) – it is irreversible.
Infection of the periodontium may cause discomfort to the affected animal. There is also strong evidence that a focus of infection in the oral cavity can cause disease of distant organs. Consequently, prevention and treatment of periodontal diseases is, contrary to common belief, not a cosmetic issue, but a general health and welfare issue!
The primary cause of gingivitis and periodontitis is accumulation of dental plaque on the tooth surfaces. Calculus (tartar) is only a secondary aetiological factor.
Classic experiments have demonstrated that accumulation of plaque on the tooth surfaces reproducibly induces an inflammatory response in associated gingival tissues, and that removal of the plaque leads to disappearance of the clinical signs of this inflammation.
Dental plaque is a biofilm composed of aggregates of bacteria and their by-products, salivary components, oral debris, and occasional epithelial and inflammatory cells. Plaque accumulation starts within minutes on a clean tooth surface. The initial accumulation of plaque occurs supragingivally but will extend into the sulcus and populate the subgingival region if left undisturbed.
The formation of plaque involves two processes, namely the initial adherence of bacteria and then the continued accumulation of bacteria due to a combination of bacterial multiplication and further aggregation of bacteria to those cells that are already attached. As soon as a tooth becomes exposed to the oral cavity, its surfaces are covered by the pellicle (an amorphous coating of salivary proteins and glycoproteins). The pellicle alters the charge and free energy of the tooth surfaces, which increases the efficiency of bacterial adhesion. Certain specific bacteria such as Streptococcus sanguis and Actinomyces viscosus can adhere directly to the pellicle. These bacteria produce extracellular polysaccharides, which then aggregate other bacteria that are not otherwise able to adhere.
The plaque associated with healthy gingiva is mainly comprised of aerobic and facultative anaerobic bacteria. As gingivitis develops, plaque extends subgingivally. Aerobes consume oxygen and a low redox potential is created, which makes the environment more suitable for the growth of anaerobic species. The aerobic population does not decrease, but with increasing numbers of anaerobes, the aerobic/anaerobic ratio decreases. The subgingival florae associated with periodontitis are predominantly anaerobic and consists of Porphyromonas spp., Prevotella spp., Peptostreptococcus spp., Fusobacterium spp. and spirochetes. High levels of Porphyromonas spp. and spirochetes are consistently associated with progressive periodontitis in the dog. The bacterial florae of cats with and without gingivitis and periodontitis are similar to those found in humans and dogs under similar conditions.
To summarize, the first bacteria to adhere to the pellicle are aerobic Gram-positive organisms. In dogs and cats, the main bacteria in supragingival plaque are Actinomyces and Streptococcus spp. As the plaque thickens, matures and extends further down the gingival sulcus, the environment becomes suitable for growth of anaerobic organisms, motile rods and spirochetes.
Dental calculus is mineralized plaque. However, a layer of plaque always covers calculus. Both supragingival and subgingival plaque becomes mineralized. Supragingival calculus per se does not exert an irritant effect on the gingival tissues. The main importance of calculus in periodontal disease seems to be its role as a plaque-retentive surface. This is supported by well-controlled animal and clinical studies that have shown that the removal of subgingival plaque on top of subgingival calculus will result in healing of periodontal lesions and the maintenance of healthy periodontal tissues.