I was recently called about what antibiotics to give to a septic patient in the emergency department. I was a bit surprised about this as we have excellent sepsis guidelines and normally the doctors just crack on with managing the patient. The difference for this patient was that he had been bitten by his pet dog a few days earlier and the doctors quite rightly wanted to make sure he was on the correct antibiotics for severe sepsis following a dog bite.
The patient was a young man who had been playing with his Saint Bernard when they had clashed faces and the dog’s teeth had punctured the patient’s skin. The patient was terrified that someone would take his dog away or put the dog down and so had not come to the hospital when it had happened and had stayed at home feeling increasingly unwell, with worsening fevers, headaches, diarrhoea and abdominal pain. On arrival in the emergency department the patient was hypotensive and tachycardic, he had a spreading petechial rash on his chest. His blood tests showed a low total white blood cell count and platelets due to the onset of disseminated intravascular coagulation. He was transferred to the critical care unit and I advised starting IV Ceftriaxone and Metronidazole to ensure he was covered for bacteria that could be related to the dog bite.
There are over 30 million domestic pets in the UK spread over about 50% of all British households. Many of these pets are cats and dogs, and their owners (or servants in the context of cats!) are unaware that each pose a potential health risk to the other. I’m only to concentrate on bacteria passed from pets to humans in this blog, but maybe I’ll look at the other way around in a future blog… after all we also pass bacteria to our cats and dogs.
Bites are more common from dogs, about 60% of all animal bites compared to 20% from cats. The larger dog breeds can cause severe crush injuries to bone and soft tissue in addition to any risk of infection, whereas cats tend to cause small puncture wounds which because of their narrow openings and deep penetration are more likely to go on to form abscesses. Severe infections occur in about 20% of bites from dogs and cats.
The day after the patient’s admission his blood cultures were positive for a pleomorphic (variable shaped) Gram-negative bacillus. He was still on critical care however he was stable and starting to improve.
The most common bacteria involved in animal bite infections are from the normal oral flora of the animal doing the biting, as well as occasionally from the skin of the person who was bitten. The two bacteria most likely to go on to cause sepsis are Capnocytophaga canimorsus and Pasteurella spp. (both pleomorphic Gram-negative bacilli).
Capnocytophaga canimorsus
Capnocytophaga canimorsus is a pleomorphic Gram-negative bacillus which grows as a facultative anaerobe. It is considered fastidious or fussy as it can take many days to grow on blood agar in the laboratory, sometimes as long as 10-14 days, although blood cultures from septic patients tend to be positive quickly (within 48 hours) even if the bacterium is then difficult to sub-culture on solid agar. If you can get it growing it is both oxidase and catalase positive.
The original name for this bacterium was Centers for Disease Control group dysgonic fermenter 2 (DF-2) but fortunately for us it was given a more sensible name in 1989 which noted its association with dogs, Capnocytophaga canimorsus. I looked up DF-1, as I was curious...DF-1 Capnocytophaga spp. are part of the oral flora of humans but do not normally cause human infections...I wonder if DF-1 would be infectious to dogs?
Risk factors for the development of sepsis with Capnocytophaga canimorsus following a bite include chronic alcohol use and cirrhosis, asplenia and immunosuppression. In addition to sepsis and cellulitis, Capnocytophaga canimorsus has been described as causing infective endocarditis, meningitis and even fatal acute haemorrhagic adrenal insufficiency (Waterhouse-Friderichsen syndrome) although these are all very rare.
The incubation period for infection with Capnocytophaga canimorsus is between 1-7 days. Clinical features include fever, chills, muscle pains, vomiting, diarrhoea, abdominal pain, malaise, shortness of breath, confusion and headache. There is often a petechial rash on the trunk, lower limbs and mucous membranes. The clinical features can rapidly progress to disseminated intravascular coagulation, acute respiratory distress, multi-organ failure and death. Sepsis from Capnocytophaga canimorsus is associated with a mortality of up to 60% even with appropriate treatment.
Other than the general aspects of treatment for bites below, Capnocytophaga canimorsus is sensitive to a number of different antibiotics:
Pasteurella spp.
Pasteurella spp. are also Gram-negative bacilli which grow as facultative anaerobes. They are catalase and oxidase positive, however unlike Capnocytophaga canimorsus they grow very well on blood agar within 24 hours. The most common strain isolated from patients after animal bites is Pasteurella multocida. Pasteurella multocida is more commonly related to cat bites rather than dog bites, although other animals also carry Pasteurella spp. (including Komodo Dragons which have a particularly virulent strain in their oral flora which they put to good effect in killing their prey!).
Pasteurella spp. can cause a number of serious infections including severe soft tissue infection, septic arthritis, osteomyelitis, sepsis and meningitis. Severe infections are more common in the very young or old as well as immunodeficient patients. Underlying liver disease is also a common feature predisposing to severe infection. The mortality from sepsis due to Pasteurella spp. is up to 25% even with appropriate treatment.
Pasteurella spp. are usually sensitive to penicillins although rare instances of resistance have been described. Other active antibiotics include:
Management of cat and dog bites
Wounds should be irrigated with sterile water and any foreign material removed e.g. broken teeth. If possible, do not initially close the wound as this can worsen anaerobic infection. Consider waiting 24-48 hours or allow healing by secondary intention.
Bites are more common from dogs, about 60% of all animal bites compared to 20% from cats. The larger dog breeds can cause severe crush injuries to bone and soft tissue in addition to any risk of infection, whereas cats tend to cause small puncture wounds which because of their narrow openings and deep penetration are more likely to go on to form abscesses. Severe infections occur in about 20% of bites from dogs and cats.
The day after the patient’s admission his blood cultures were positive for a pleomorphic (variable shaped) Gram-negative bacillus. He was still on critical care however he was stable and starting to improve.
The most common bacteria involved in animal bite infections are from the normal oral flora of the animal doing the biting, as well as occasionally from the skin of the person who was bitten. The two bacteria most likely to go on to cause sepsis are Capnocytophaga canimorsus and Pasteurella spp. (both pleomorphic Gram-negative bacilli).
Capnocytophaga canimorsus
Capnocytophaga canimorsus is a pleomorphic Gram-negative bacillus which grows as a facultative anaerobe. It is considered fastidious or fussy as it can take many days to grow on blood agar in the laboratory, sometimes as long as 10-14 days, although blood cultures from septic patients tend to be positive quickly (within 48 hours) even if the bacterium is then difficult to sub-culture on solid agar. If you can get it growing it is both oxidase and catalase positive.
The original name for this bacterium was Centers for Disease Control group dysgonic fermenter 2 (DF-2) but fortunately for us it was given a more sensible name in 1989 which noted its association with dogs, Capnocytophaga canimorsus. I looked up DF-1, as I was curious...DF-1 Capnocytophaga spp. are part of the oral flora of humans but do not normally cause human infections...I wonder if DF-1 would be infectious to dogs?
Risk factors for the development of sepsis with Capnocytophaga canimorsus following a bite include chronic alcohol use and cirrhosis, asplenia and immunosuppression. In addition to sepsis and cellulitis, Capnocytophaga canimorsus has been described as causing infective endocarditis, meningitis and even fatal acute haemorrhagic adrenal insufficiency (Waterhouse-Friderichsen syndrome) although these are all very rare.
The incubation period for infection with Capnocytophaga canimorsus is between 1-7 days. Clinical features include fever, chills, muscle pains, vomiting, diarrhoea, abdominal pain, malaise, shortness of breath, confusion and headache. There is often a petechial rash on the trunk, lower limbs and mucous membranes. The clinical features can rapidly progress to disseminated intravascular coagulation, acute respiratory distress, multi-organ failure and death. Sepsis from Capnocytophaga canimorsus is associated with a mortality of up to 60% even with appropriate treatment.
Other than the general aspects of treatment for bites below, Capnocytophaga canimorsus is sensitive to a number of different antibiotics:
- Penicillins
- Beta-lactamase inhibitor combinations
- Ceftriaxone and Cefotaxime
- Carbapenems
- Clindamycin
- Tetracyclines
- Chloramphenicol
Pasteurella spp.
Pasteurella spp. are also Gram-negative bacilli which grow as facultative anaerobes. They are catalase and oxidase positive, however unlike Capnocytophaga canimorsus they grow very well on blood agar within 24 hours. The most common strain isolated from patients after animal bites is Pasteurella multocida. Pasteurella multocida is more commonly related to cat bites rather than dog bites, although other animals also carry Pasteurella spp. (including Komodo Dragons which have a particularly virulent strain in their oral flora which they put to good effect in killing their prey!).
Pasteurella spp. can cause a number of serious infections including severe soft tissue infection, septic arthritis, osteomyelitis, sepsis and meningitis. Severe infections are more common in the very young or old as well as immunodeficient patients. Underlying liver disease is also a common feature predisposing to severe infection. The mortality from sepsis due to Pasteurella spp. is up to 25% even with appropriate treatment.
Pasteurella spp. are usually sensitive to penicillins although rare instances of resistance have been described. Other active antibiotics include:
- Beta-lactamase inhibitor combinations
- Ceftriaxone and Cefotaxime
- Carbapenems
- Tetracyclines
- Fluoroquinolones
Management of cat and dog bites
Wounds should be irrigated with sterile water and any foreign material removed e.g. broken teeth. If possible, do not initially close the wound as this can worsen anaerobic infection. Consider waiting 24-48 hours or allow healing by secondary intention.
Click for larger image
Total Duration
10-14 days
Other considerations
Tetanus booster if soil has contaminated the wound or there is devascularised tissue.
Consider rabies vaccination in returned traveller.
Cat and dog bites are common reasons for patients to attend emergency departments in the UK and abroad. Management of the patient involves thorough cleaning and debridement of damaged tissue as well as prophylactic antibiotics. If the patient develops sepsis then they should be managed as a septic patient (see sepsis blog or sepsis webpage) BUT their antibiotic therapy should be specifically targeted to the bacteria associated with animal bites.
The patient above made a full recovery going home 2 weeks after admission. His dog had been looked after by a friend during his admission and there was no consideration to put the animal down, as they essentially bumped heads and the dog’s teeth won. It took considerably longer than 2 weeks to identify the bacterium in his blood cultures which was Capnocytophaga canimorsus as expected. Clinical diagnosis is therefore essential; microbiology results are sometimes only there as confirmation of a diagnosis. Waiting for results can be dangerous. To know how long laboratories take to process specimens please see the A-Z of microbiology specimens section in the book Microbiology Nuts & Bolts. Buy a copy of Microbiology Nuts & Bolts here
10-14 days
Other considerations
Tetanus booster if soil has contaminated the wound or there is devascularised tissue.
Consider rabies vaccination in returned traveller.
Cat and dog bites are common reasons for patients to attend emergency departments in the UK and abroad. Management of the patient involves thorough cleaning and debridement of damaged tissue as well as prophylactic antibiotics. If the patient develops sepsis then they should be managed as a septic patient (see sepsis blog or sepsis webpage) BUT their antibiotic therapy should be specifically targeted to the bacteria associated with animal bites.
The patient above made a full recovery going home 2 weeks after admission. His dog had been looked after by a friend during his admission and there was no consideration to put the animal down, as they essentially bumped heads and the dog’s teeth won. It took considerably longer than 2 weeks to identify the bacterium in his blood cultures which was Capnocytophaga canimorsus as expected. Clinical diagnosis is therefore essential; microbiology results are sometimes only there as confirmation of a diagnosis. Waiting for results can be dangerous. To know how long laboratories take to process specimens please see the A-Z of microbiology specimens section in the book Microbiology Nuts & Bolts. Buy a copy of Microbiology Nuts & Bolts here
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