Halitosis, Bad Breath

Have you noticed recently that your dog's breath has somewhat offensive aroma? Bad breath should never be a reason of trivial concern such as "It does not affect my relationship with my pet," because it is often an indication of an underlying health problem that may present a risk for your dog, yourself and your family.

Halitosis or bad breath is a common finding in dogs of different breeds and ages. Halitosis can be of several origins: type 1 (oral), type 2 (airway), type 3 (gastroesophageal), type 4 (blood-borne) and type 5 (subjective); however, it has been suggested that halitosis could be potentially considered the sum of these types in any combination. Several animal studies have pointed out that the nasal chamber is one of most involved systems where tartar, stomatitis, and halitosis are quite often present. Furthermore, bacteria that cause halitosis can be transmitted from animal companions to humans. More than 80% of patients belonging to the halitosis group reported having pets in childhood and over 70% still owned a pet.1

Oral cavity microbiota of humans and animals is made up of a wide variety of yeasts and bacteria. These microorganisms can be responsible for different oral infections and may as well be involved in systemic diseases. Oral cavities of dogs with halitosis usually have a large number of yeast microorganisms involved in dental biofilm and tartar formation. Trichosporon asahii yeast has been isolated from oral cavity and bronchi of healthy dogs and from the external ear of healthy cats. This fungus is the most important species causing systemic disease in immunocompromised human patients, while inhalation of its spores is the most important cause of summer-type hypersensitivity pneumonitis (an immunologically induced lung disease) in healthy humans.2 Species of Trichosporon cause superficial fungal infections of the hair (White Piedra infection). Trichosporon is the second most common cause, after Candida species, of yeast blood poisoning in patients with blood cancer resulting in high mortality rates.

Dogs kiss and lick their owners to express friendly emotions. This raises the possibility of transferring bacteria from dogs to their human owners. In addition, periodontal diseases such as gingivitis and periodontitis are the most common diseases in dogs. Studies have demonstrated between 44% and 63.6% of dogs are affected. Several reports have described the companion animal-to-human passage of oral bacteria, including Pasteurella multocida and Tannerella forsythia, which are linked to local and systemic human infections.


A single dead yeast particle being phagocytized by a macrophage
Credit: National Institute of Allergy and Infectious Diseases (NIAID)/CDC

Among the common bacteria isolated from oral cavities of dogs are:

  • Pasteurella multocida, a toxin-producing bacteria. Pasteurella Multocuda Toxin (PMT) interferes with proper functioning of human dendritic cells, one of the principal initiators of immune responses and patrol agents for danger signals.4,5
  • Streptococci and Staphylococci. These two as well as Escherichia coli, Klebsiella pneumoniae, Bacteroides and Enterococci are most common pathogens of hepatic abscess in human patients.
  • Moraxella species; these are Gram-negative aerobic cocci that colonize the nasopharynx and occasionally cause meningitis, blood poisoning, pus-forming infections, pericarditis and pneumonia.

  • Corynebacterium pseudotuberculosis and Corynebacterium ulcerans; these are able to produce diphtheria toxin and cause diphtheria-like disease in humans. Both can be transmitted between humans and their companion pets such as cats and dogs.7
  • Gemella morbillorum bacteria; even though Gemella morbillorum infection (GMI) is rare in humans, it may nevertheless, cause endocarditis, meningitis, brain abscess, pleural empyema, nephritis, mediastinitis, and occasionally, liver abscess. Liver abscesses caused by Gemella morbillorum have a high mortality rate.
  • Neisseria species; Neisseria meningitidis as well as Escherichia coli, Klebsiella species and Bacillus species are common causes of bacterial endophthalmitis, an ocular inflammation resulting from the introduction of an infectious agent into the eye. During infection, irreversible damage to delicate photoreceptor cells of the retina frequently occurs. Despite aggressive therapeutic and surgical intervention, endophthalmitis generally results in partial or complete loss of vision, often within a few days of inoculation.8 Oral-to-oral transfer of Neisseria shayeganii from dogs to humans may result in periodontal disease.9 Neisseria weaveri and Neisseria zoodegmatis are opportunistic pathogens in human peritonitis, lower respiratory tract infections, wound infections, septicemia, and periodontitis.
  • Porphyromonas canigingivalis, one of the "red complex" infectious agents involved in periodontal tissue distruction; can be transferred from dogs to humans.
  • Tannerella forsythia, a major pathogen involved in periodontal disease; referred to as one of the "red complex" infectious agents; can be transferred from dogs to humans.10
  • Actinomyces species; the causative agents of actinomycosis, periodontal disease, and endocarditis; one of the most frequently identified organisms in the canine oral cavity.
  • Porphyromonas gulae, a major cause of periodontitis in dogs, closely resembles Porphyromonas gingivalis, the primary periodontal pathogen in humans.

The transfer of bacteria from dogs to humans is more influenced by oral-to-oral contact than by oral-to-skin and skin-to-skin contact. Of course, most bacteria transferred from dogs to humans fail to colonize the human mouth. There are several reasons for that. First, dog saliva is less acidic (pH 8.5–8.65) than that of human saliva (pH 6.5–7.5) and most bacteria in the dog oral cavity may not survive well in the human oral environment. Second, most humans brush their teeth every day. During tooth brushing, most of the supra-gingival bacteria are removed and are then regenerated. When canine oral bacteria are transmitted to the human oral cavity, they have to compete with human oral microbiome constituents and are removed by frequent tooth brushing.

Although oral bacteria are not contagious, the canine oral cavity might harbor potential zoonotic pathogens. The dog owners and people who are involved in the pet industry should be aware of potential hazard of dog oral bacteria. To reduce the numbers of potential pathogens and possibility of accidental infection, regular tooth brushing and dental scaling are recommended.

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