Anaerobic Bacterial Infections

Chapter 37

Anaerobic Bacterial Infections

Etiology and Epidemiology

An anaerobe is an organism that requires an environment that is reduced in oxygen for growth and does not grow in air. Anaerobic bacteria represent a significant component of the normal flora of the upper and lower gastrointestinal tracts of dogs and cats as well as their genital tracts; in addition, anaerobes play an important role in the regulation of the bacterial composition of the normal flora. Aerobes and facultative organisms that reside alongside anaerobes use oxygen present in these sites and reduce the local oxidation-reduction potential, which permits survival and growth of anaerobes. Sites that are protected from oxygen such as the gingival crevices also favor anaerobes. The metabolic activity of anaerobic bacteria prevents colonization of mucosal surfaces by potentially more pathogenic microbes (this is known as colonization resistance). Finally, anaerobes are important for the maintenance of normal mucosal barrier function.

Anaerobic bacteria can cause disease when they gain access to normally sterile sites. This occurs when mucosal barriers are disrupted, or when anaerobes gain access to subcutaneous tissues as a result of a contaminated penetrating wound. Anaerobic bacterial infections are often mixed infections owing to the concurrent presence of other anaerobes, as well as aerobes and facultative bacteria such as Pasteurella spp., Escherichia coli, streptococci, or staphylococci. Although there are hundreds of anaerobes that comprise the normal flora, the anaerobic bacterial species most often identified in disease are the gram-negative anaerobes Fusobacterium spp., Bacteroides spp., Prevotella spp., and Porphyromonas spp., and the gram-positive anaerobes Clostridium spp. and Peptostreptococcus spp. (Box 37-1).2 An accurate understanding of the epidemiology of anaerobic bacterial infections in dogs and cats has been hampered by difficulties associated with culture of anaerobes. The use of molecular methods such as PCR has begun to shed more light on this topic. The reader is referred to Chapters 48 and 54 for information on gastrointestinal Clostridium perfringens and Clostridium difficile infections, and disease caused by the toxins of Clostridium tetani and Clostridium botulinum, respectively.

Clinical Features

Signs and Their Pathogenesis

Anaerobes can cause disease in a variety of different organs. Often, their presence is associated with abscess formation and necrosis at the site of inoculation, which contains a mixed population of anaerobic as well as facultative bacteria. The possession of virulence factors by anaerobes contributes to their ability to cause disease after host barriers are disrupted. These include capsular polysaccharides, extracellular proteases, endotoxin (gram-negative anaerobes), and a variety of other toxins. Capsular polysaccharides play an important role in the stimulation of abscess formation by certain anaerobes such as Bacteroides, Porphyromonas, Fusobacterium, and Prevotella species. Just as aerobic bacteria potentiate the survival of anaerobes through local consumption of oxygen, anaerobic bacteria produce metabolites such as short-chain fatty acids, which inhibit phagocytosis and promote survival of co-infecting facultative organisms.

Anaerobes of the oral cavity are important contributors to periodontal disease, tooth root abscesses and mandibular osteomyelitis, retrobulbar abscesses, and bite wound infections. Anaerobes are frequently isolated from the lungs and pleural cavities of dogs or cats with aspiration pneumonia and pyothorax, respectively. Intestinal anaerobes may be isolated from the abdominal cavities of animals with septic peritonitis secondary to intestinal perforation. The female genital tract is also heavily colonized with anaerobic bacteria, and pyometra may sometimes be associated with mixed aerobic and anaerobic infections that include the anaerobic species Bacteroides, Prevotella, Fusobacterium, and Peptostreptococcus species.

Plant awn migration in dogs and cats can lead to abscess formation in a variety of tissues, including retroperitoneal tissues and the brain. These abscesses usually contain a mixed population of aerobes and anaerobes, especially Fusobacterium, Actinomyces, and Bacteroides species. Brain abscessation can follow direct extension of anaerobic bacterial infection from otitis or tooth root infection. Alternatively, hematogenous spread may occur (Figure 37-1).

Periodontal Disease

Periodontal disease is one of the most common disorders seen by small animal veterinarians and may result in chronic pain, decreased appetite, and loss of teeth. It begins with formation of dental plaque as a result of poor oral hygiene and other host factors. Dental plaque is a bacterial biofilm composed primarily of gram-positive facultative anaerobes. Subsequently there is proliferation of pathogenic bacteria, which include gram-negative rods and especially anaerobes. These secrete toxins and incite an inflammatory response. Anaerobes implicated in canine and feline periodontal disease (such as Porphyromonas gulae) differ from those implicated in human periodontal disease (Porphyromonas gingivalis).36 Periodontal disease is manifested as gingival erythema, edema, gingival bleeding, and halitosis, with accumulation of calculus, gingival recession, alveolar bone loss, and sometimes the presence of purulent exudate (Figure 37-2).

Retrobulbar Abscesses

Retrobulbar or orbital abscesses can result from tooth root abscessation, penetrating foreign bodies such as grass awns, or penetrating trauma to the orbit. Clinical signs include unilateral exophthalmos, protrusion of the nictitans, conjunctival hyperemia, serous to mucopurulent ocular discharge, inappetence, fever, and pain on opening the mouth (Figure 37-3). A fluctuant swelling or draining tract posterior to the last ipsilateral molar may be present on examination of the oral cavity. In one study, Bacteroides spp. and Clostridium spp. were the most frequent anaerobes isolated from 34 dogs with retrobulbar abscesses; other frequently isolated bacteria were Staphylococcus spp., Pasteurella spp., and Escherichia coli. In cats, the most frequent anaerobe was Bacteroides vulgatus, and the most common nonanaerobe was Pasteurella spp.7

Skin and Soft Tissue Infections

Cat bite abscesses are one of the most common reasons that cats are brought to small animal clinics. Clinical signs include fever, lethargy, inappetence, pain, and firm or fluctuant subcutaneous swellings that may rupture and drain purulent material. Oral anaerobes and Pasteurella multocida are frequently isolated from these abscesses.8 Anaerobic species involved include Peptostreptococcus anaerobius, Porphyromonas spp., Clostridium villosum, Fusobacterium spp., Bacteroides spp. (especially Bacteroides tectum), and Prevotella spp.9 Despite the widespread occurrence of cat bite abscesses, studies that use molecular methods to identify the types and relative prevalences of anaerobes in cat bite abscesses are lacking.

Other skin and soft tissue infections associated with a variety of anaerobes include foreign body penetrating wounds and dog bite wounds (see Chapter 57).

Pneumonia and Pyothorax

Anaerobes such as Fusobacterium spp., Peptostreptococcus anaerobius, Prevotella spp., and Porphyromonas spp. are frequently isolated from dogs and cats with aspiration pneumonia, foreign body pneumonia (especially that due to plant awn migration), and pyothorax.10 Dogs with plant awn migration develop multifocal necrotizing pneumonia with abscess formation. In some cases, aspiration pneumonia is followed by lung abscess formation (Figure 37-4). In human patients, a period of 6 to 7 days is required for necrosis and abscess formation to occur after aspiration, and so aspiration pneumonia with lung abscess formation is generally a chronic disease process.

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Jul 10, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Anaerobic Bacterial Infections

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