Classification of Bacteria

Bacteria are most commonly classified by their cell wall type and their ability to grow in the presence or absence of oxygen (aerobic/anaerobic). The majority of pathogens can be classified according to cell wall type with a Gram stain; however, some microbial genera, such as Mycoplasma and Chlamydophila, stain poorly. Other staining agents help to identify agents more specifically such as the Ziehl-Neelsen acid-fast stain for mycobacteria. Knowledge of predilections of bacterial families to colonize particular body sites is useful in empirical selection of antimicrobial agent(s) for initial therapy (Table 15-1).

TABLE 15-1 Examples of common expected normal flora by body site

Working with a Diagnostic Microbiologist

Frequently, it is desirable to obtain a definitive bacteriologic diagnosis (e.g., when empiric therapy is failing, infections are recurrent, and specific knowledge of an infectious agent is required for prognostication and/or duration of treatment). Despite the fact that culture and identification from animal specimens can be performed by human microbiology laboratories, it is in the best interest of practitioners and their clients to work with a veterinary laboratory diagnostician; doing so will save time, money, and frustration and yield better clinical outcomes. The veterinary microbiologist is able to guide the laboratory bacterial workup so that the bacteriologic differential diagnoses are covered efficiently and that isolates selected for antimicrobial susceptibility testing are appropriate. Moreover, state and university veterinary diagnostic laboratories make available consultant microbiologists via telephone at no charge.

General Neonatal Septicemia and Bacteremia

Septicemia is a major cause of mortality in puppies and kittens during the first 3 weeks of life. At parturition, puppies and kittens are in an immunocompromised state; they are susceptible to infection by bacteria that are regarded as “commensal” (i.e., normal microbial colonizers of the mucosal surfaces and skin). Despite some transplacental passage of immunoglobulins in dogs and cats, to successfully defend themselves against infection, neonates depend on passive immunity via colostrum imbibed in the first 1 to 2 days of life. Also important is the innate immune mechanism, which is afforded in large part by maintaining healthy mucosal surfaces. Puppies and kittens must be assured unfettered nursing access and kept in a room with proper ambient temperature so energy can be spent growing rather than trying to thermoregulate. In the first week of life, ambient temperature should be 85° to 90° F (29° to 32° C) and approximately 80° F (26.5° C) for the second through fourth weeks of life.

Puppies are most susceptible to septicemia during the first week of life. Septicemia is also a common sequela to viral enteritis, which occurs most commonly from 8 to 16 weeks of age. The most common organisms implicated in these infections belong to the family Enterobacteriaceae, notably Escherichia coli, Klebsiella spp., and Proteus spp. To a lesser extent, other gram-negative pathogens, such as Pseudomonas spp., may be associated with sepsis. Of the gram-positive bacteria, β-streptococci (usually group G or B) have the potential to cause systemic infection during this time frame. These organisms are normal colonizers of mucosal surfaces of the dam, particularly the vagina and anus.

E. coli is the most frequent agent involved; they are numerically dominant in feces (among the facultative organisms), and many E. coli serotypes that comprise the normal commensal microflora have intrinsic complement resistance caused by long lipopolysaccharide residues (O-antigens). These organisms typically gain access to the bloodstream via translocation across the intestinal mucosa and/or by a compromised umbilicus.

Kittens are also susceptible to neonatal sepsis by commensal Enterobacteriaceae during the first few weeks of life; however, bacteremic Pasteurella multocida infection is also common in the feline neonate. More common than in puppies, kittens are particularly at risk for systemic group G streptococcal infection. This agent is a ubiquitous inhabitant of the vaginal mucosa of queens.

Because these diseases progress rapidly in neonates and sudden death often occurs, the problem is frequently diagnosed postmortem. Typical histopathologic findings include bacterial emboli in multiple organs, and if culture is performed on fresh tissues, one bacterial species typically dominates as a pure culture in high numbers. Expired animals submitted for histopathology should be refrigerated (not frozen) and sent to the diagnostic laboratory intact within 1 day. If animals are too large to send in their entirety, they should be necropsied as soon as possible with fresh and fixed tissues collected for histopathologic examination and ancillary testing (Table 15-2).

TABLE 15-2 Samples for postmortem diagnosis of the acutely dead neonate

History and physical examination findings are the basis for the clinical suspicion of systemic bacterial infections. Sepsis should be a differential diagnosis for a puppy or kitten with any of the following general signs: inappetence, inactivity, excessive crying, hypothermia, failure to thrive, and diarrhea. Any seizure activity also merits sepsis on the differential diagnosis list. Clinicopathologic results are generally nonspecific, although they may reveal failure of a specific organ. The leukogram may reflect evidence of inflammation such as a neutrophilia with a left shift or a normocytic, normochromic anemia. Hypoglycemia is a common finding on the chemistry profile of patients with bacteremia, although it is not pathognomonic for systemic bacterial infections. Immediate empiric antimicrobial therapy and supportive care should be instituted in these patients rather than seeking an etiologic diagnosis; these are emergencies.

If obtaining cultures does not significantly delay initiation of empiric antimicrobial therapy, the following is recommended:

Identification of the organism(s) and their susceptibility data will guide the antimicrobial regime in the patient. If littermates are also affected, obtaining clinical microbiologic data on each patient is recommended, since the bacteria involved in each puppy or kitten may be different.

Methods to prevent neonatal septicemia include the use of pooled adult serum administered to colostrum-deprived puppies and kittens to attempt to increase serum immunoglobulin concentrations. In the puppy, 22 ml/kg of pooled adult dog serum can be administered subcutaneously. Approximately 15 ml/kitten of pooled cat serum can be administered subcutaneously or intraperitoneally to kittens to correct a suspected immunoglobulin deficiency.

Prevention of some infections can be achieved by administering antimicrobial agents to queens in the peripartum period (Table 15-3), if there is a known or suspected genital colonization by β-hemolytic streptococci. Proper sanitation of the birthing area is also essential in the prevention of both umbilical infections and neonatal septicemia. Antiseptic solutions, such as povidone-iodine, should also be applied to the umbilicus of the neonate to help prevent umbilical infections.

TABLE 15-3 Prophylaxis of β-streptococcal infections in cats at parturition

Treatment of neonatal systemic infections involves antimicrobial agents and supportive care. Balanced electrolyte solutions, such as lactated Ringer’s solution or Normosol R, should be used for fluid therapy. Some patients may require dextrose or potassium chloride supplementation as part of their fluid therapy. Patients that are profoundly hypoglycemic should receive a bolus (1 to 2 ml/kg) of 50% dextrose that should be diluted to make a 10% to 25% solution. Thermal support, oxygen supplementation, and nutrition are also important elements of supportive therapy for the bacteremic patient (see chapter 10).

Oral drug absorption is unreliable in neonates and decreased perfusion of peripheral capillaries in septic shock renders subcutaneous administration of antimicrobial agents of little value, especially in dehydrated patients. Therefore antimicrobials should be administered either intravenously or intraosseously. Second- and third-generation cephalosporins, such as cefoxitin (30 mg/kg intravenous [IV] every 8 hours [q8h]) and cefotaxime (25 to 50 mg/kg IV q8h), respectively, are good initial empiric therapies because they provide broad-spectrum antimicrobial coverage against gram-positive, gram-negative, and anaerobic bacteria. If there is concurrent renal compromise, try to use a cephalosporin that is not renally cleared; however, if renally cleared cephalosporins are all that are available, use the following formula: