Human Infections

While invisible to the naked eye human and animal bodies are populated by vast numbers of microscopic life forms. Capnocytophaga canimorsus is one example of these bacteria/microbiota. “The word canimorsus is being derived from the Latin for ‘dog bite and C. canimorsus is commonly found in the mouths of people, dogs, and cats.” (1). But this bacterial pathogen is more commonly found in the saliva of healthy dogs and is transmitted to humans principally, by dog bites, licks, or even proximity to them.

C. canimorsus is formerly known as “dysgenic fermenter 2” and has low virulence in healthy individuals. C. canimorsus is a fastidious, non-spore-forming, facultative aerobe, slow-growing, and Gram-negative rod of the genus Capnocytophaga.

According to a Clinical Infectious Diseases article, “The culture media (chocolate agar) and culture conditions (temperature of 37 °C and CO2-enriched atmosphere) that are commonly used for CSF culture allowed the growth of C. canimorsus. The organism grows best at temperatures of 35 °C–37 °C in an aerobic atmosphere with 5%–10% CO2 or in an anaerobic atmosphere and on Columbia agar with 5% sheep’s blood or standard chocolate agar.

Capnocytophaga species cannot grow on MacConkey agar.” (2).

Although C. canimorsus generally has low virulence in healthy individuals, but it can cause severe illness in people with pre-existing conditions. Infection with C. canimorsus is more common in people who have a weakened immune system, especially those without a functional spleen (33% of cases) or those with a history of alcohol abuse (24% of cases). However, approximately 40% of patients with C. canimorsus septicemia are healthy individuals with normal immune systems.

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Other risk factors in some other cases are diabetes type one or two, being male, and being over 50 years of age. C. canimorsus can harm either human septicemia or meningitis, however, the mortality rate for C. canimorsus meningitis is very low when compared with the rate for C. canimorsus septicemia (5% Vs. 30%).

Dogs are the most common household in the United States. According to the 2017-2018 National Pet Owners Survey,” Sixty-eight percent of U.S. households, or about 85 million families, own a pet, and almost 50% of Americans will be bitten by a dog in their lifetime. (3). Recent estimates using polymerase chain reaction (PCR) techniques to amplify bacterial DNA are that C. canimorsus is in the mouths of 74% of dogs and 57% of cats. (3rd) However transmission to humans mostly occurs through dog (97%) or cat (3%) bites, scratches, licks, or simple contact. According to the Washington Post, “One in eight American adults, or 12.7 % of the U.S. population are alcoholics () which means that 12.7% of Americans have compromised immune system. (4). Hereupon it means that Americans have a high chance of getting infected by C. canimorsus because of having a weakened immune system and having a pet.

Because C. canimorsus does not have high virulence, only over two hundred cases have been reported since C. canimorsus was first identified in 1976.C. canimorsus is hard to grow in cultures because of their specific requirements for nutrients and long-term incubation to observe growth and colony formation. Usually, the microorganism cannot be detected by standard laboratory methods, this may underestimate the frequency of the infection and makes it an unreportable disease. Signs and symptoms of getting infected by C. canimorsus appear within 1–8 days after exposure to this pathogenic bacterium but usually present around the second day of infection. Symptoms span from flu-like symptoms to full-blown fulminant septicemia. Patients often complain of having diarrhea, abdominal pain, fever, vomiting, malaise, myalgia, dyspnea, headaches, and skin rashes. More severe cases of endocarditis disseminated intravascular coagulation, and meningitis has been reported. the infection can also lead to severe sepsis and fatal septic shock, and gangrene of the digits or extremities.

The first case was reported in 1976 by Bobo and Newton describing a man who developed septicemia and meningitis after experiencing multiple dog bites from two dogs on consecutive days. The organism had the same characteristics as C. canimorsus. And in 17 reported similar cases between 1961 and 1975, findings showed that, in all 17 of these cases, the same Gram-negative bacilli were seen. The most recent case was reported in 2018. Greg Manteufel went to the hospital on June 26 when he experienced a fever and weakness in his limbs. Doctors discovered Manteufel had contracted a blood infection caused by C. canimorsus. Before getting ill, he was playing with many dogs at a birthday party. He didn’t have any scratches, bites, or open sores on his hands. He probably touched one of the dogs or they have licked his hand and he rubbed his eye and that is the way the pathogen has entered Manteufel’s body. Manteufel’s symptoms started with a fever and pain in his legs. He eventually experienced delirium, bruises, and blemishes which were the result of sepsis. And because Manteufel’s body had decreased circulation to his limbs in response to the infection, doctors had to amputate Manteufel’s legs. Two weeks later, they removed his forearms, as well. (people). Even though it is rare to get infected by C. canimorsus, a similar case was documented in 2016 in London. Doctors said a 70-year-old woman was infected with C. canimorsus apparently after being licked by her Italian greyhound.

Many kinds of antibiotics are effective against C. canimorsus, but penicillin G and amoxicillin with or without clavulanate are the best choices of drugs that doctors usually use. “Although systemic administration of antibiotic therapy is debatable for healthy individuals, it is strongly recommended in cases of deep bite wounds and for patients with weakened immune systems, especially those with alcoholism or who have undergone splenectomy. The reason for this prompt antibiotic therapy is to avoid disseminated infection. For patients who develop meningitis after an animal bite, amoxicillin and a third-generation cephalosporin are recommended, pending the results of laboratory testing.” (1). Cefotaxime is one of the agents used in standard empirical therapy for bacterial meningitis, especially when gram-negative bacilli like C. canimorsus are found in CSF specimens. “Although it is reported that this pathogenic bacterium is susceptible to penicillin, imipenem, erythromycin, vancomycin, clindamycin, third-generation cephalosporins, chloramphenicol, rifampin, doxycycline, and quinolones, but it is reported to be resistant to aztreonam, polymyxin, metronidazole, trimethoprim, fosfomycin, and aminoglycosides.” (1).

Incidence of C. canimorsus infection is expected to increase over time because of a rising incidence of splenectomy due to injuries and accidents or to impaired immune responses due to a high prevalence of alcoholism, immunosuppressive therapies, aging of the human population, together with the popularity of pet ownership due to a public perception of their safety. Some recommended ways to decrease the chance of getting infected by C. canimorsus are education, immunoprophylaxis, and chemoprophylaxis for the prevention of infectious diseases. The potential seriousness of post-splenectomy sepsis and rapid time course of progression should be explained to patients who also have to be instructed to seek immediate medical attention in the event of any acute febrile illness. Further, patients should be educated about the risk of C. canimorsus through animal contact. Although a majority of C. canimorsus infections are in people with a predisposing condition, 40% of reported infections occurred in healthy individuals with no identifiable predisposing condition.

Human infections caused by C. canimorsus can develop following a bite or a scratch by a dog or cat. Severe fulminant sepsis with a fatal outcome is not uncommon; the main risk factor in the patient population is a present immune deficiency. Doctors should be aware of these signs and symptoms and any person at risk presenting to the hospital with a recent bite or scratch injury by canines or cats should be given adequate antibiotic treatment. C. canimorsus detection can be hard with standard laboratory cultures and the (PCR) method is useful to identify causative species of such infections.