Infection Related To Bites

Etiologic Agents

The organisms causing bite wound infections generally derive from the microbial flora of the biting animal’s mouth rather than the victim’s skin. Therefore, the infecting organisms vary by species. Bite wound infections should be considered polymicrobial infections; in a large, prospective study of infected dog and cat bites, cultures done at a central reference laboratory yielded a median of five bacterial isolates per culture. Mixed aerobic and anaerobic infection occurred in more than 50% of cases. Specimens sent concomitantly to local microbiology laboratories identified significantly fewer organisms, emphasizing the need for careful microbiologic analysis.

Table 87.1 shows the aerobic and anaerobic bacteria isolated from patients with infected dog, cat, and human bite wounds. ^, Pasteurella spp. were the most common isolates from dog and cat bites, with P. canis the predominant organism isolated from dog bites and P. multocida the most common from cat bites. Other species isolated from these patients included streptococci, staphylococci, and Moraxella and Neisseria spp. Common anaerobic isolates included Fusobacterium , Bacteroides , Porphyromonas , Prevotella , Cutibacterium (formerly Propionibacterium ) , and Peptostreptococcus spp . A number of other bacterial isolates previously had not been recognized as bite wound pathogens.

Other studies confirm the polymicrobial nature of bite wounds. ^, Pasteurella multocida is seen only in animal bites, more commonly from cats, whereas Eikenella corrodens and Streptococcus pyogenes are more closely associated with human bites, although they are sometimes seen with animal bites as well. Capnocytophaga canimorsus , ^, C. cynodegmi , Neisseria weaveri (formerly M-5), ^, Bergeyella zoohelcum , Neisseria canis , Staphylococcus intermedius , NO-1, and Paracoccus yeeii (EO-2) are all uncommon organisms to recover from general clinical specimens but are isolated from bite wound infections, particularly from dog bites.

The cause of infections resulting from bites of species other than humans, dogs, and cats is less well described. Simian bites appear to be similar to human bites in microbiology. Polymicrobial infections with Staphylococcus aureus , S. pyogenes , Pseudomonas aeruginosa , and Bacillus spp. have been reported with camel bites. Bites of large cat species such as leopards and tigers are associated with organisms typical of domestic cat bites, with isolation of P. multocida and N. weaveri . Snake bites do not routinely require antibiotic management unless there is necrosis, in which case aerobic gram-positive cocci and gram-negative bacilli are thought to predominate. A Pasteurella caballi infection was seen after a horse bite, Pasteurella aerogenes and a Chryseobacterium- like organism have been isolated from infected pig bites, ^, Actinobacillus spp. have been reported from horse and sheep bite wounds, and Halomonas venusta has been isolated from a fish bite. In bites occurring in marine settings, organisms associated with water, such as Vibrio spp., Aeromonas hydrophila , Plesiomonas shigelloides , and Pseudomonas spp., have caused infections from bites of catfish, eel, crocodile, and swan. Additionally, systemic diseases have been transmitted by bites, including tularemia from cats, dogs, and other mammals, ^, rat-bite (Haverhill) fever and sodoku ( Spirillum minus ) from rats, herpesvirus B infection from monkeys, hepatitis B from humans, and leptospirosis from dogs and rodents. A rare infectious entity acquired from contact with seals, known as seal finger, is likely caused by a marine mycoplasma. HIV appears to be difficult to transmit by human bite.

Epidemiology

Dog bites account for >90% of bite wounds that come to medical attention. The annual incidence of dog bites of children has been estimated at 1–3 per 1000 children per year ^, in resource-rich countries such as the US ( Fig. 87.1 ), to rates as high as 26 per 1000 per year in resource-poor countries. Dog-bite injuries alone account for 0.3%–0.4% of all emergency department (ED) visits and 1.1% of all injury related visits to the ED. ^{, ,} Incidence depends on age and sex. (see Fig. 87.1 ). The incidence and body part involved with dog bites vary by age. Children are more likely than adults to sustain a dog bite, with the highest risk in the second year of life and steadily decreasing each year thereafter (based on data from Austria) or with peak incidence in the 5–9-year age group (based on US data). ^{, ,} A recent study of patients receiving care for dog bites at a level 1 pediatric trauma center demonstrated a median age of 5 years, a male predominance, and a majority of dogs belonging to the patient’s immediate or extended family. Several studies have identified pit bulls as the most common breed involved in dog bites coming to medical attention. ^, Body site of dog bite injuries is age dependent ( Fig. 87.2 ). Injuries to the face, head, and neck are most common, accounting for two-thirds of dog-bite injuries in preschool-aged children (extremities accounting for 27%) (see Fig. 87.2 ). Injuries to the extremities (upper more than lower) become more common with increasing age, accounting for 55% beyond 14 years of age (at which age only 9% involve the head or neck). Overall, approximately 12% of patients have multiple bite wounds. Between 7%–25% of children sustaining dog bites require hospital admission, depending on use of general anesthesia for primary wound management and the rate of infectious complications. ^,

Fatalities from bites occur primarily from massive blood loss after mauling by large dogs, particularly Rottweiler and pitbull-type dogs, or as the result of intracranial injury, particularly in smaller children. Children <5 years of age are particularly vulnerable to attack and are more likely to sustain injury from smaller dogs. Overall, the relative risk for attack is significantly higher from German shepherd, Doberman, pitbull, and Rottweiler breeds than from Labrador/retriever or cross-bred dogs, but because cross-bred dogs are so much more common, they represent the majority of bite injuries. Approximately 20 deaths occur each year in the United States as a result of dog bite attacks. Deaths due to infection after dog bites are uncommon but can occur as the result of septicemia or intracranial infection. In developing countries, rabies remains a significant late cause of death from dog bites. The estimated risk for infection in the otherwise healthy child depends on the location and extent of the dog bite injury and the initial management but is estimated to be 5%–15%.

Cat bites are spread more evenly across age groups and cause less overall initial injury because cats have smaller mouths and less biting force, which results in less tearing of the tissues. However, their thin, sharp teeth produce small, deep wounds that are difficult to cleanse and are more likely to result in clinical infection, estimated to occur after 10%–50% of bites.

Human bites may be sustained somewhat differently than bites of other animals and generally are defined as any disruption in the skin as a result of contact with the mouth. These can be self-inflicted (e.g., thumb-sucking resulting in skin breakdown, or self-injury as with autism), the result of a punch to the mouth of another person that results in a laceration of the hand, or the result of a typical bite but resulting from sexual activity or abuse that might be concealed from the medical provider. One study of human bite wounds in children seen at a single institution over a 6-year period identified 322 patients, representing 0.2% of all ED visits. Infection was seen in 9%, and 2%, overall, were hospitalized for treatment of infection.

Table 87.2 displays the rate of infection of bite wounds from any source by the body site of the bite. ^, The two studies enrolled subjects at the time of initial evaluation in the outpatient setting. Bites to the hand are significantly more likely to become infected compared with bites at other sites. Wounds to the face are the least likely to become infected, probably because of its extensive vascularity. In one of these studies, 12% of the wounds became infected, and the risk for infection differed on the basis of the biting species. Only 4% of 80 dog bites became infected, but 16% of 37 human bites and 50% of 22 cat bites became infected. Cat bites also can transmit Bartonella henselae and Sporothrix schenckii . ^, The paucity of reports regarding infection after rodent or rabbit bites makes the rate difficult to estimate, but it appears to be small, perhaps related to smaller and less penetrating wounds. Rat bites are associated with Streptobacillus moniliformis infections, hamster bites have caused Francisella tularensis and Pasteurella aerogenes infections, ^, and guinea pigs have caused Haemophilus influenzae infections. Bat bite is important because it can transmit rabies virus. Infections transmitted by biting insects are addressed in other chapters.

Pathogenesis

Bite wounds become infected by inoculation of microorganisms into subcutaneous or deeper tissues. Modifying factors include the type and depth of inoculation, the amount of crush injury and devitalized tissue, the involvement of infection-prone structures, the ability to cleanse and debride the wound, and the pathogenicity of the infecting microorganism. Most series show a higher infection rate in bite wounds to the hand and especially in clenched-fist punch to the mouth ( Fig. 87.3A ). Cat bites have the highest infection rates because of deep inoculation and high prevalence of P. multocida (see Fig. 87.3B ). Dog bites are more likely to cause lacerations or avulsion injuries that are easier to clean and debride. However, these wounds can become infected with S. pyogenes or Staphylococcus aureus in colonized victims (see Fig. 87.3C and D).

Most infections manifest as localized cellulitis or simple abscess. Infection can extend to adjacent areas, particularly if the teeth penetrate structures such as bones, joints, or the cranium. Regional lymphadenopathy, lymphangitis, fasciitis, toxic shock syndrome, septicemia, and shock also can develop. Septicemia occurs primarily in immunocompromised hosts, particularly asplenic individuals, who are prone to fulminant septicemia and shock with infection caused by C. canimorsus or Eubacterium plautii after dog bites.

Clinical Manifestations, Differential Diagnosis, And Clinical Approach

The history and physical examination readily lead to the likely organisms of most bite wound infections ( Box 87.1 ). If infected, signs and symptoms (erythema, pain, tenderness, and swelling) almost always become apparent within 24–48 hours after the bite. Infections caused by P. multocida tend to have a more rapid onset, often within 12–18 hours. There may be purulent or serosanguineous discharge. Most patients are afebrile, although fever can occur in patients with severe cellulitis or in the rare instance of bacteremia. Eschars at bite sites in individuals who appear ill may indicate C. canimorsus infection. Differentiating uninfected but severe (see Fig. 87.3E ) or unusual bites (see Fig. 87.3F –H) can be challenging.