Capnocytophaga Canimorsus Infection in Cats

Abstract

Overview:

Capnocytophaga canimorsus and Capnocytophaga cynodegmi are part of the normal bacterial flora of the oral cavity of dogs and cats. C canimorsus is more pathogenic and causes more severe infections in humans.

Infection:

Disease is less frequently seen after a cat bite, scratch or close contact than after dog contacts. Serious disease has been reported in people, especially associated with immunocompromise and alcoholism. Disease in cats is not well documented; two cases of respiratory infection have been associated with the presence of these bacteria.

Diagnosis:

Diagnosis is based on culture in specific media, but these are slow growing bacteria; polymerase chain reaction and sequencing may aid in diagnosis and species identification.

Treatment:

Penicillin or beta-lactams are the treatment options of choice.

Zoonotic potential:

Based on incidence surveys, the zoonotic potential is low. The risk may be higher for immunocompromised persons, where dog and cat ownership must be discussed.

Bacterial properties

Capnocytophaga canimorsus is a fastidious, slow-growing, capnophilic, facultative anaerobic, Gram-negative, rod-shaped bacterium displaying gliding mobility that belongs to the normal flora of the oral cavity of dogs and cats.^1,2 Another species, Capnocytophaga cynodegmi, is also present in the normal flora. ³ Both can cause wound infections in humans after a bite, C canimorsus being associated with more severe infections. ³

Epidemiology and pathogenesis

C canimorsus was first cultured in 1976 from the blood and spinal fluid of a dog-bite patient, hence its name (canis = dog, morsus = bite). Infections occur worldwide. Many dogs and cats carry C canimorsus in their oral cavity. In one study, C canimorsus was cultured from 26% of dogs and 15% of cats sampled. Using a species-specific polymerase chain reaction (PCR) that distinguishes between C canimorsus and C cynodegmi, much higher prevalences were found in Japan; the former species was detected in 74% of the dog and 57% of the cat samples. ⁴ In the Netherlands, a recent survey documented C canimorsus in 21% of the cats. ⁵

Human infections with C canimorsus are associated with dog and cat bites (54%), scratches (8.5%) or close animal contact (27%). ⁶ One case of keratitis reportedly followed a corneal injury caused by a fractured cat’s tooth during extraction. ⁷ Cat bites and scratches or contact with cats have been reported in fewer than 10% of cases. ⁶ In general, cat bites cause less tissue damage than dog bites, which might create less favourable conditions for bacterial growth. ⁸

Capnocytophaga species infection is quite uncommon after dog and cat bites. In surveys from Denmark and the Netherlands, a yearly incidence of sepsis due to C canimorsus was calculated to be 0.5 and 0.63 per million population, respectively.^9,10

Clinical presentation

Capnocytophaga species infections after bite traumas have not been reported in cats, and only two cases of a possible pathogenic role of the bacterium have been documented. In one instance, Capnocytophaga species was isolated from a case of chronic sinusitis and rhinitis, and – based on DNA sequencing – a strain closely related to C canimorsus and C cynodegmi was identified from nasal discharge. Since no other causes for chronic nasal discharge were found, the role of Capnocytophaga species was considered likely. ¹⁷ Another case report describes the isolation of C cynodegmi from bronchoalveolar lavage samples from a cat with lower respiratory tract infection and pulmonary carcinoma. Clinical signs and bacterial colonisation resolved after treatment with enrofloxacin. ¹⁸

Diagnosis

In humans, diagnosis of a C canimorsus infection is usually based on bacterial culture of blood or other body fluids; isolation from bite wounds is rare. The bacterium grows slowly on special media (chocolate agar or heart infusion agar with 5% rabbit blood, incubated in a 5% CO₂ atmosphere). ⁶ PCR methods can distinguish between C canimorsus and C cynodegmi. ⁴

Treatment and prevention

In humans, the first choice antibiotic is penicillin G or potentiated penicillins as amoxicillin–clavulanate acid. Immediate cleaning and disinfection of bite wounds and scratches is important, as is antibiotic prophylaxis [EBM grade III].^8,11

Key Points

• Capnocytophaga species are present in the normal oral flora of dogs and cats.

• Disease in humans is more often associated with dog bites or contact than with those of cats.

• While disease in humans is rare, mortality can be high in immunocompromised individuals if septicaemia occurs.

• Immunocompromised cat owners need to be alerted to the risk of bite or scratch wounds and the consequent bacterial infections.