Capnocytophaga canimorsus

Epidemiology

Significant dog bites affect tens of millions of people globally each year, and cases of human infection following exposure to C. canimorsus have been observed worldwide. It is estimated that 2% of the U.S. population, 4.5–4.7 million people, are bitten by dogs each year. Cases have been reported in the United States, Canada, Europe, Australia and S. Africa. Symptoms may appear within 2–3 days after exposure, or up to 4 weeks later. Middle-aged and elderly persons are at greater risk for contraction of the disease; more than 60% of sufferers are 50 years of age or older. In addition, individuals who spend a greater portion of their time with canines and felines are also at higher risk. This includes veterinarians, breeders, pet owners, and keepers. Having certain pre-existing medical conditions exacerbates the risk. Chance of infection by any bacterial species after dog bites varies between 3 and 20%; for cats, it may be as high as 50%.

Morphology, culture, and isolation

C. canimorsus is a fastidious, Gram-negative, fermentative, encapsulated, nonspore-forming rod. Bacilli are usually 1-3 μm in length. After growth on agar plates, longer rods tend to have a curved shape. The bacteria do not have flagella, but move with a gliding motion, although this can be difficult to see. C. canimorsus requires the right medium for growth. The bacterium grows well on blood agar plates (heart infusion agar with 5% sheep or rabbit blood) and chocolate agar plates. Colonies may not be visible for up to 48 hours due to slow growth. At 18 hours, colonies are usually less than 0.5 mm in diameter, and are spotty and convex. At 24 hours, colonies may be up to 1 mm in diameter. After 48 hours, colonies are narrow, flat, and smooth, with spreading edges. At this time, colonies may appear white-grey or grey, rather translucent, and once they are scraped from the agar plate, they may have a slightly yellow tint.

Genome

The genome of C. canimorsus strain Cc5 consists of a single circular chromosome of 2,571,406 bp with a G+C content of 36.11%, and it encodes 2,405 open reading frames.

In animals

Members of the genus Capnocytophaga are found in the oral cavities of humans and animals. Most of these species are not found in humans. C. canimorsus is a commensal bacterium found in dogs and cats; it is not a member of the normal microbiota of humans. About 26% of dogs carry these commensal bacteria in their mouths. C. canimorsus rarely causes disease symptoms in animals. One case of C. canimorsus isolated from a dog bite wound on a small dog's head has been reported; the bacteria were localized to the wound, and the dog did not present with bacteremia. A few cases of infection have been reported in rabbits after being bitten by dogs. Clinical manifestations of C. canimorsus in rabbits causes a range of symptoms, including disseminated intravascular coagulation, cellular necrosis (tissue death), low blood pressure, gangrene, and kidney failure.

High-risk categories

In addition to those at higher risk of developing complications from C. canimorsus due to greater contact with felines and canines, certain pre-existing conditions place individuals in a critically high-risk category. Among these are those who have undergone a splenectomy, alcoholics, and individuals with immunosuppression due to the use of steroids such as glucocorticoids. Individuals with β-thalassemia and smokers are also listed as high-risk. These individuals, such as people with asplenia and alcohol use disorder, have increased levels of alimentary iron in their bloodstream. C. canimorsus requires large amounts of iron to grow, so these conditions are optimal for the bacillus.

Asplenia

Overwhelming post-splenectomy infection is associated with encapsulated organisms such as Capnocytophaga canimorsus.

Of the cases presented in literature, 33% occurred in asplenic individuals, who have decreased IgM and IgG production. They also have delayed macrophage assembly and produce less tuftsin. Tuftsin is responsible for the stimulation of phagocytosis, so its decrease in the presence of bacterial infection poses a problem. A functional spleen is important for the removal of pathogens. Because this particular pathogen seems to flourish in asplenic patients, both IgM antibodies and tuftsin may be critical in the process of marking this bacterium for destruction by phagocytosis. People with asplenia are 60 times more at risk of developing fatal clinical manifestations of the bacterium. Individuals with asplenia often experience symptom onset within a day of exposure. The infection rapidly progresses toward multiple organ system failures and finally death. The mortality rate in individuals with asplenia is much higher than in any other at-risk category for C. canimorsus infections.

Alcohol use disorder

People who struggle with alcohol addiction represent around 24% of individuals presenting with C. canimorsus infections. Alcoholism has been shown to result in decreased superoxide production in neutrophils, as well as declines in neutrophil elastase activity. This results in an increase in predisposition to bacteremia (bacteria in the blood). As a result, people suffering from alcoholism are more likely to suffer from the more dangerous aspects of C. canimorsus invasions. Finally, people with alcohol addictions are associated with increased blood iron content.

Immunosuppression

Immunosuppression resulting from an underlying illness or medical treatment is a risk factor for C. canimorsus infection. Immunosuppressants are often used to treat autoimmune diseases, such as systemic lupus erythematosus. These treatments, including glucocorticoids, reduce immune system activity and can impair the body's ability to fight infection. This reduces the body's ability to eliminate bacteria introduced through an animal bite, scratch, or contact with saliva. As a result, immunocompromised individuals may be more likely to develop severe infection after exposure to C. canimorsus than healthy individuals. In a review of reported cases, approximately 5% of patients presenting with C. canimorsus infection were immunosuppressed.

Symptom onset and clinical manifestations

Symptoms generally develop within 1–8 days after exposure to C. canimorsus but onset of symptoms most commonly appears on the second day. Initial symptoms vary depending on whether the infection remains localized or spreads systemically. Local symptoms at the site of a bite or scratch may include redness, swelling, pain, blisters, cellulitis, or pus. When the infection spreads beyond the site of exposure, it may cause symptoms resembling an influenza-like illness. Reported symptoms include fever, chills, vomiting, fatigue, diarrhea, malaise, abdominal pain, myalgia, headaches, and skin rashes such as exanthema. Neurological symptoms, including confusion and altered mental status, may develop as the infection progresses. Respiratory symptoms such as dyspnea may also occur.

If C. canimorsus enters the bloodstream, it can cause bacteremia and fulminant sepsis, which can be fatal. Severe infections may also result in septic shock, multiorgan failurem endocarditis, disseminated intravascular coagulation, or meningitis. Prior treatment with methylprednisolone has been associated with prolonged bacteremia, which contributes to the development of endocarditis.

Differential diagnosis

Diagnosing infections with C. canimorsus can be difficult. Common practice for culturing isolates is to keep agar plates for one week; sometimes, cultures of C. canimorsus are not visible at that point due to slow growth or inappropriate media. C. canimorsus requires very specific culture media and conditions; enriched media are necessary. C. canimorsus displays enhanced growth in high concentrations of carbon dioxide, so culturing the bacteria in candle extinction jars or carbon dioxide incubators is necessary. To diagnose this bacillus, certain reactions may be tested. The bacterium should test positive for catalase and oxidase, arginine dihydrolase, maltose, and lactose. It should test negative for nitrate reduction, urease, and H₂S production. C. canimorsus can be distinguished from other Gram-negative bacteria by testing negative for inulin and sucrose.

Due to the relatively slow growth of this bacterium, diagnosis often relies upon the clinician knowing that the patient was previously in contact with a canine or feline. Once aware of this, clinicians can request that agar plates be kept longer than one week to ensure proper isolation of the bacterium. Sometimes, even these methods fail. Cases have been noted where cultures repeatedly came up negative for C. canimorsus, only to determine its presence with 16S rRNA gene sequencing. PCR assays of species-specific genes may also be beneficial. For individuals presenting with meningitis, C. canimorsus can be diagnosed with a cerebrospinal fluid Gram stain. These methods are more costly, but are the best way to ensure species-level identification. Isolates are usually obtained from blood cultures (88% of the time) and less frequently from bite wounds. In incidents where the patient is in full septic shock, whole blood smears may be effective.

Treatment

Immediate cleansing of wounds caused by canines and felines can be successful in keeping C. canimorsus infections at bay. Irrigation and cleansing of wounds with soap and water for 15 minutes is recommended, and individuals are encouraged to seek medical help for the administration of antibiotics. Antibiotics are recommended if wounds are deep or for individuals with immune deficiency. Antibiotics that contain beta-lactamase inhibitors (i.e., oral Augmentin or parenteral Unasyn) cover C. canimorsus, as well as other organisms common in bites.

Penicillin G is the drug of choice, although some isolates have been found to show resistance. C. canimorsus is susceptible to ampicillin, third-generation cephalosporins, tetracyclines, clindamycin, and chloramphenicol. It has shown resistance to gentamicin. Treatment is recommended for a minimum of three weeks. Hospitalization is required in more severe infections. For cases of sepsis, high doses of penicillin are required. Third-generation cephalosporins are often given before diagnosis because they cover a broad range of Gram-negative bacteria. After diagnosis, provided the strain is not beta-lactamase-producing, medication should be switched to penicillin G. Presumably, penicillin G could be given with a beta-lactamase inhibitor combination, such as Unasyn, for patients with a beta-lactamase-producing strain.

Mortality of meningitis-related infections is much lower than the mortality associated with sepsis. Because C. canimorsus induces fulminant sepsis, earlier diagnosis is associated with greater survival.

Evasion of the immune system

C. canimorsus has been observed to multiply in the presence of mouse J774.1 macrophages. Macrophages recognize and kill pathogens by engulfing them. They also secrete cytokines necessary to begin the immune pathway cascade. C. canimorsus bacteria are not internalized by macrophages; in fact, macrophage monolayers break down in their presence, suggestive of a cytotoxin. In the presence of C. canimorsus, cytokine activity is greatly downregulated, because the macrophages fail to produce TNF-α, IL-8, IL-6 and IL-1α, interferon-γ, and nitric oxide. In addition, toll-like receptor 4 (TLR4) normally recognizes pathogens and begins a signalling cascade to induce production of proinflammatory cytokines via the NF-κB pathway. In cells infected with C. canimorsus, TLR4 did not activate the signalling pathway, so did not elicit an inflammatory response by the immune system. Because this species does not elicit a strong inflammatory response, the bacteria have ample time for replication before detection by the host immune system.

Electron micrographs of J774.1 monolayers infected with C. canimorsus have shown cells of the bacteria within the macrophage's vacuoles, surrounded by bacterial septa. This suggests that C. canimorsus replicates intracellularly in macrophages. C. canimorsus cells also show resistance to killing by complement and killing by polymorphonuclear leukocytes (PNMs). C. canimorsus, when in the presence of PMNs, obtains nutrients by deglycosylating host glycans. In fact, in the presence of PMNs, C. canimorsus experiences robust growth. C. canimorsus can evade these necessary immune functions, and, therefore, must be taken seriously. Greater knowledge about the pathogenesis of this bacterium is required to prevent and treat the diseases associated with it.