Halitosis, also called bad breath, foul breath, malodor, foetor ex ore, and fetor oris is defined as an offensive odor emanating from the oral cavity. Bad breath is a common pet odor complaint. Causes may be oral (most common) or extraoral (rare).

The sour milk odor accompanying periodontal disease may result from the bacterial population associated with plaque, calculus, unhealthy tissues, decomposing food particles retained within the oral cavity, or the rotten meat odor emanating from tissue necrosis. Contrary to common belief, neither normal lung air or stomach aroma contributes to halitosis.

The most common cause of halitosis is periodontal disease caused by plaque (bacteria). Bacteria is attracted to the pellicle (an acellular film formed from the precipitation of salivary glycoproteins). In the freshly cleaned and polished tooth a glycoprotein layer forms over the tooth as soon as the patient starts to salivate. Bacteria attaches to the pellicle within 6-8 hours. Within days, the plaque becomes mineralized producing calculus. As plaque ages and gingivitis develops into periodontitis (bone loss), the bacterial flora changes from a predominantly non-motile gram-positive aerobic coccoid flora to a more motile, gram-negative anaerobic population including: Bacteroides, Fusobacterium, and Actinomyces species.

Calculus’s rough surface attracts more bacteria while irritating the free gingiva. As the inflammation continues, the gingival sulcus is pathologically transformed into a periodontal pocket. The pocket accumulates putrefied food debris, bacterial breakdown products, and resorbing bone leading to halitosis. The primary cause of malodor is gram negative anaerobic bacterial putrefaction causing the generation of volatile sulfur compounds (VSC), such as hydrogen sulfide, methyl mercaptan, and dimethyl sulfide. The volatile sulfur compounds also may play a role in periodontal disease affecting the integrity of the tissue barrier allowing endotoxins to produce periodontal destruction.

Most patients suffering from halitosis have oral causes, the remaining are caused by, dermatologic, metabolic, respiratory, or gastrointesinal disease.