Staphylococci

Coagulase-positive staphylococci are among the most commonly isolated pathogens. Staphylococci can survive in the environment and be cultured from dried clinical material after several months. They are relatively heat resistant. Staphylococci are found in the nasopharynx and skin and can contaminate any site on the skin and mucous membranes. Multiplying staphylococci can overcome local phagocytic defenses and gain access to the lymphatics and bloodstream. Staphylococcal bacteremia can lead to metastatic infections in the heart, lungs, and bones (see Chapter 34). Staphylococci secrete numerous enzymes and toxins that are implicated in their pathogenesis. Catalase can inhibit polymononuclear oxygen-free-radical-killing activity and their toxins damage cell membranes and cause cell lysis. Manifestations of their infection vary from trivial, as in some pyodermas, to overwhelming sepsis. Coagulase-positive Staphylococcus pseudintermedius (see Etiology, Chapters 34 and 84) have been common bacteria isolated from blood cultures of dogs with bacteremia alone, diskospondylitis, and endocarditis.^39,42 Common sources for bloodstream infection include abscesses, pyoderma, and wound infections. However, a source of infection is not always obvious. Physical manipulations of abscesses and cellulitis can increase tissue pressure and facilitate movement of bacteria into small veins and lymphatics. Staphylococci tend to spread from localized abscesses, wounds, and deep pyodermas into the bloodstream by invading blood vessels, producing septic thrombi, lymphatics, or incompetent lymph nodes. Metastatic foci of infection are common and often involve the spleen, kidneys, bones, joints, or heart valves.

Only a small percentage of local infections caused by S. pseudintermedius gain access to the bloodstream.⁴⁸ Nonetheless, enzymes such as staphylokinase, hyaluronidase, and protease can enable tissue invasion. Sepsis can result from enterotoxins that bind to T cells and macrophages, stimulating the production of cytokines. One of the targets of S. pseudintermedius is the endothelial cell.⁴⁸ Organisms bind to and are internalized by endothelial cells, where cytolysins are released that can disrupt the endothelium and allow access to tissue. They can survive inside of endothelial cells and phagocytic cells. This may explain their propensity to cause recurrent and refractory bacteremia.

S. pseudintermedius is normally present on canine hair and skin, and an association of cutaneous infections with staphylococcal bacteremia in dogs is to be expected. These microbes also may gain entry into the bloodstreams of dogs from foci such as osteomyelitis, diskospondylitis, septic arthritis, aspiration pneumonia, and genitourinary infection. Most S. pseudintermedius strains produce a β-lactamase that induces resistance to penicillin G and ampicillins. Stability of the β-lactam ring of methicillin is high and varies for cephalosporins. Staphylococci can become resistant to all β-lactam antibacterials and all cephalosporins. Resistance is increasing to quinolones, aminoglycosides, and macrolide antibacterials, such as clindamycin, as their indiscriminate use increases.

The coagulase-negative Staphylococcus epidermidis may be cultured from blood specimens but are often not fully identified in clinical microbiology laboratories. The majority of these are either contaminants or of no clinical importance. However, significant coagulase-negative staphylococcal bacteremia can occur in immunocompromised patients with indwelling intravenous catheters or profound neutropenia.

Intact skin and mucous membranes provide defense against staphylococcal invasion. When these defenses are breached, blood invasion can arise from any site, but most often from localized skin or soft tissue infections, surgical wounds, and catheters. Various conditions may predispose animals to staphylococcal bacteremia. Debilitation resulting from malignancies, renal failure, diabetes mellitus, or liver disease is an example, as is compromise of immune protection by glucocorticoids, cytotoxic drugs, or other immunosuppressive agents.

Sequelae to staphylococcal bacteremia are numerous. A life-threatening complication is valvular endocarditis. Whereas infection of the mitral valve is often curable, infection of the aortic valve is a virtual “death sentence” in dogs. Septic embolization of the kidneys, spleen, and other organs can occur with chronic staphylococcal bacteremia and is consistently present with valvular endocarditis of the left side of the heart. Renal infarcts can be detected by excretory urograms and ultrasonograms. Proteinuria and pyuria are often present, and immune-mediated glomerulonephritis can be an additional complication. Other sites of septic embolization or abscess formation are the brain, joints, and lungs. Bacteremia can predispose to pulmonary artery thromboembolism. Although uncommon, infective endocarditis (IE) of the tricuspid or pulmonic valve consistently causes septic embolization of the pulmonary circulation.

Disseminated intravascular coagulation (DIC) is another sequela of staphylococcal bacteremia, but its prevalence is probably less than that associated with gram-negative bacteremia. Prolonged activated coagulation time, prothrombin time, and activated partial thromboplastin time; thrombocytopenia; decreased plasma fibrinogen; and the presence of fibrin degradation products and D-dimers are consistent with the diagnosis. Results of thromboelastography are usually consistent with a hypercoagulable state with peracute DIC.

Staphylococcus aureus bacteremia in humans occasionally triggers a septic shock (toxic shock) syndrome that can be virtually indistinguishable from gram-negative bacillary endotoxic shock. The authors of this chapter (CAC and JDT) have observed a similar syndrome in dogs; however, staphylococcal endocarditis usually follows a subacute to chronic evolution without septic shock.

Streptococci

Systemic streptococcal infections are common in dogs and cats (see Chapter 33). Streptococcal bacteremia can originate from cutaneous sites and the upper respiratory tract. Streptococcal pneumonia may be associated with a high prevalence of subsequent bacteremia. In bacteremic dogs with various underlying illnesses, blood culture results may be positive for hemolytic Streptococcus canis, non-β-hemolytic Streptococcus viridans, and enterococci. Most β-hemolytic streptococci enter the bloodstream via the skin, whereas non-β-hemolytic streptococci usually enter via breaks in the mucous membranes. Non-β-hemolytic streptococci are normal skin commensals and occasionally may contaminate improperly collected blood culture specimens.

Group A streptococci (Streptococcus pyogenes) are enveloped in a hyaluronic acid capsule that retards phagocytosis by neutrophils and macrophages. Toxins produced by S. pyogenes are pyrogenic, are cytotoxic, and enhance susceptibility to the effects of endotoxin.⁶⁰ Group B streptococci can cause sepsis in dogs and cats and have been incriminated as a cause of bacteremia and death in the “fading puppy” syndrome. In humans, bacteremia caused by group D streptococci (enterococci), including Streptococcus faecium, Streptococcus bovis, and Streptococcus faecalis, usually originates from the urinary tract but also can develop after manipulation of the lower bowel. S. bovis was reported to cause IE in a small-breed dog with myxomatous valvular disease after routine dental prophylaxis.²¹⁶ Dogs infected with streptococci may be more likely to have mitral valve involvement and have a higher prevalence of neutrophilic polyarthritis.²¹¹ Enterococcal bacteremia, which has been reported in dogs, is especially serious because enterococci are resistant to many antibacterials.

Gram-Positive Aerobic Bacilli

Diphtheroids, a heterogenous group of bacteria including the genus Corynebacterium, are often interpreted as contaminants when isolated from blood culture because they normally inhabit skin and mucous membrane surfaces. Corynebacterium accounts for a minority of cases of bacteremia in dogs and has rarely been associated with endocarditis. Bacillus species are frequent blood culture contaminants. However, in the immunocompromised host, Bacillus cereus and Bacillus subtilis can gain access to the bloodstream. Corynebacterium and Bacillus should be isolated from multiple blood cultures before incriminating these agents as causing a bloodstream infection. Erysipelothrix tonsillarum strains have been isolated from the heart valves of dogs with endocarditis (see Chapter 33).²¹²

Gram-Negative Bacilli

The term gram-negative bacteremia is typically applied to a hematogenous infection caused by the Enterobacteriaceae and Pseudomonadaceae. Bacteremias resulting from gram-negative agents, such as Pasteurella, Brucella, Bartonella, Serratia, and Salmonella species, which can produce similar clinical manifestations, are usually considered discrete clinical entities. Salmonella enteritidis was the most common organism isolated from bacteremic cats in one study.⁶⁸ Gram-negative bacillary bacteremia usually represents a serious opportunistic infection that has developed subsequent to significant suppression of host immune defenses. Gram-negative bacteremia is often associated with high mortality, and the number of such infections has increased along with the use of invasive medical devices and immunosuppressive therapy for malignancies and inflammatory diseases.

Gram-negative bacilli are ideally suited for opportunistic infections. Ubiquitous in the environment, they are major components of the fecal flora, are normal skin inhabitants, and are present in all hospital environments. They tend to be relatively resistant to moisture, drying, and some disinfectants. Some may persist and multiply in water. These agents tend to develop antibacterial resistance to a greater degree than gram-positive bacteria. Although antibacterial exposure per se does not induce resistance, it does provide a selective reproductive advantage to bacteria that are resistant.

Within the family Enterobacteriaceae, Escherichia coli is the most common bloodstream isolate from animals. E. coli is abundant in the lower GI tract, which often serves as a reservoir for infection of other body sites. Intestinal epithelial damage, necrosis, or sloughing, particularly in debilitated or immunocompromised patients, can quickly lead to overwhelming and lethal E. coli–induced sepsis. The urinary tract is another source. Oropharyngeal and fecal colonization with gram-negative bacilli can increase progressively in seriously ill hospitalized patients as their clinical status deteriorates. Gram-negative bacteria not only gain access to the blood from extravascular foci but also can originate from intravenous catheters, from urinary catheters, and from septic thrombophlebitic conditions. In extravascular infections, bacteria often gain access to the blood via lymphatics or invasion of small blood vessels within a site of infection. Other sources for bloodstream invasion include drainage tubes, contaminated intravenous fluids, and contaminated aerosolization devices; disrupted mucosal barriers (e.g., after dental procedures or endoscopic examinations); and decubital ulcers. In contrast to staphylococcal bacteremia, gram-negative bacteremia in dogs and cats seldom is associated with septic thrombosis and metastatic abscess formation but is often rapidly progressive and likely to result in sepsis (see Chapter 36).

Although common in the environment and occasionally present on mucosal surfaces, Pseudomonas species have rarely been isolated from deeper tissues of healthy patients. They have been more common in otitis externa, dermatitis, cystitis, and respiratory infections of dogs. Because Pseudomonas organisms are opportunists, their rapid colonization with subsequent development of bacteremia is much more likely to occur after disruption of host defenses, especially cutaneous barriers (surgery, intravenous catheters, and burns), and depletion of neutrophils, as occurs in patients who are receiving chemotherapy for cancer.⁶⁷ Extensive antibacterial use or contaminated intravenous fluids may also predispose patients to Pseudomonas bacteremia. Most reported cases of Pseudomonas bacteremia in dogs have been nosocomial.⁶⁷

Prevention is easier than treatment of gram-negative bacteremia. The use of intravenous and urinary catheters should be limited to instances in which they are absolutely necessary. They should be inserted and maintained under scrupulously sterile conditions and removed or changed within 3 to 5 days (see Chapter 90). Strict aseptic precautions in the management of wounds, use of tube drainage systems, prevention of decubitus, and limitation of prophylactic antibacterials are all important in preventing opportunistic infections for all patients, particularly those with weakened immune systems. The risk-benefit ratio of glucocorticoid administration must be carefully evaluated.

Anaerobic Bacteria

Anaerobic bacteria, particularly anaerobic gram-negative rods, are considered to be serious pathogens. Development of anaerobic bloodstream infections may be encouraged by the presence of periodontal disease, deep abscesses, granulomas, peritonitis, osteomyelitis, septic arthritis, and septic pleural effusion (see Chapter 39). Clostridium perfringens is the most common canine isolate, whereas Bacteroides and Fusobacterium are commonly isolated from cats. A mechanically correctable lesion (abscess, perforated bowel, necrotic tissue) is often the source of anaerobic bacteremia. Bacteroides may enter the bloodstream via intra-abdominal sources, such as GI and genital inflammatory diseases. Fusobacterium bacteremia often originates from infections of the respiratory tract. Actinomyces turicensis, a facultative anaerobic gram-positive bacterium, was isolated from endocarditis in a dog.¹³⁸ Characteristics of anaerobic bacteremia include fever, thrombophlebitis, and icterus, particularly with Bacteroides bacteremia. Sequelae to anaerobic bacteremia include metastatic abscess formation and endocarditis. Clostridial bacteremia tends to have a relatively insidious clinical course without obvious signs of sepsis, although septic shock occurs occasionally.

Bartonella

Bartonella spp. are gram-negative, fastidious bacteria adapted to one or more mammalian reservoir hosts and are associated with long-lasting endotheliotropic infection with relapsing intraerythocytic bacteremia (see Chapter 52). Arthropods, including biting flies, fleas, lice, and ticks, are suspected vectors of transmission. Bartonella spp. can contribute to both endothelial and intraerythrocytic infection and induce immune suppression through unknown mechanisms, and the bacteremia can be chronic, spanning months to years in duration. Arthritis, granulomatous inflammation, endocarditis, epistaxis, lymphadenomegaly, osteomyelitis, panniculitis, and vasoproliferative lesion are manifestations of Bartonella infections in humans and dogs.³³ In a study of IE, dogs infected with Bartonella spp. were significantly more likely to be afebrile, have involvement of the aortic valve, and have a higher prevalence of congestive heart failure when compared to dogs infected with other pathogens.²¹¹In a retrospective study of infective endocarditis in 9 dogs, DNA of B. henselae was amplified in 7 dogs and three of these had concurrent DNA of B. vinsonii ssp. berkhoffii.^78a

Polymicrobial Bacteremia

Bloodstream infection with multiple species of bacteria occurs in up to 20% of dogs and 30% of cats with positive blood culture results. Anaerobic bacteria, especially Bacteroides and Clostridium, are often components of polymicrobial infection in dogs. Clinical implications of polymicrobial compared with monomicrobial bacteremia are not clearly established, and higher mortality rates have not been observed in dogs or cats. Factors that can predispose patients to polymicrobial bacteremia include neutropenia, GI and urogenital tract obstruction and infection, bowel perforation and surgery, and prostatic surgery.

Sources of Infection and Risk Factors

Although not always identified, the most common sources of bacteremia include infections of the integumentary, GI (including biliary tract), genitourinary, and respiratory systems. Intravenous catheter–associated infections also occur.39,42 Bacteremia can be a complication of parenteral nutrition, and the catheter should always be considered a source of infection. Infection often begins locally in the catheter wound when the patient’s cutaneous flora invade the tract during catheter insertion and thereafter. Hub contamination is also a source of infection. Catheter-related bacteremia is confirmed when catheter and blood culture results yield the same microbe. The most common organisms are staphylococci. Catheters should always be removed and cultured immediately if bacteremia or thrombophlebitis is suspected.

Catheter-related bacteremia originates from the migration of bacteria from the venipuncture into the catheter tract and along the external surface of the catheter to the intravascular tip, which becomes colonized.¹⁰⁸ Contamination of the hub can allow migration within the catheter lumen. Teflon and polyurethane are more resistant to bacterial colonization than polyethylene or polyvinyl chloride. Some bacteria, such as staphylococci, can bind to host fibronectin that is deposited on catheters. Peripheral venous catheters have a lower risk of bloodstream infection than central venous catheters.¹⁰⁸ Catheters impregnated with chlorhexidine or silver sulfadiazine are resistant to infection. Sterile gloves should be worn during catheter placement, and the area should be sterile. Insertion and maintenance of intravenous catheters by inexperienced staff members increase the incidence of catheter-related bacteremia. In cats, pyothorax, septic peritonitis, GI tract disease, pneumonia, endocarditis, pyelonephritis, osteomyelitis, pyometra, and bite wounds are all sources of bacteremia.²⁶

Various factors (Box 86-1) have been cited as predisposing patients to developing bacteremia. When considering mortality from bacteremia and sepsis, the single most important factor influencing outcome after infection is the severity of the patient’s underlying disease. Death from bacteremia is much less likely to occur if the animal was healthy before the bacteremia developed.

Neutropenic patients are very susceptible to sepsis. Cancer chemotherapy–induced neutropenia is particularly dangerous because the destruction of GI crypt epithelial cells can be a concomitant problem. Enteric bacteria gain access to the blood via the damaged mucosal barrier, and the neutropenia renders phagocytosis of these bacteria ineffective. To complicate matters, fever may be absent or minimal in a bacteremic patient because neutrophils are a component of the inflammatory process. The febrile neutropenic patient is a medical emergency. Patients who have had a splenectomy are also predisposed to bacterial sepsis because the spleen is an important component of the host’s defense mechanisms and in eliminating bloodborne microorganisms.

Time Course

Relating the time course of bacteremia to the infecting organism is not always possible. Peracute bacteremia, which develops over several hours, often in debilitated or immunosuppressed patients, can be the result of either gram-positive or gram-negative infection. Acute bacteremia develops over 12 to 24 hours and is usually the result of gram-negative or staphylococcal infection. Subacute bacteremia develops and persists for several weeks or longer and is often the result of gram-positive but occasionally anaerobic infections. Chronic bacteremia, lasting weeks to months, can result from infections with microorganisms of low toxicity (e.g., Brucella canis, S. pseudintermedius, and Bartonella spp.); sequestration of bacterial colonies on heart valves in bone or in intervertebral disk spaces; abscess formation in the liver, spleen, kidneys, or muscles; or partial response to antibacterial therapy.

Secondary arthropathies, glomerulopathies, embolic abscesses or thrombi, and splenomegaly are more often associated with IE than with bacteremia alone (Fig. 86-1; Table 86-2). Metastatic infection can result in life-threatening complications. Virtually all dogs with IE of the left side of the heart experience multiple continuous embolizations and renal infarctions, which may lead to renal failure. Subacute and chronic bacteremia can result in sustained antigenic stimulation of the immune system and increased circulating immunoglobulin. Circulating immune complexes (CICs) may be deposited in many tissues, leading to development of polyarthritis, myositis, vasculitis, and glomerulonephritis. Young, growing animals can develop metaphyseal embolization with resultant hypertrophic osteodystrophy (see Chapter 85).²⁰³ In humans, bacteremia has been found to result in hemolysis, presumably because of modified erythrocyte antigens that develop as a result of the circulating organism. A similar association of bacteremia with immune-mediated hemolytic anemia was not found in dogs.¹⁵³ Bacteria such as intraerythrocytic Bartonella species and hemotropic mycoplasmas can induce hemolytic anemia (see Chapters 52 and 31, respectively), as do many protozoan erythroparasites.

Cats

In cats with bacteremia, with or without endocarditis, common clinical signs are anorexia, pyrexia, and shifting leg lameness.⁴⁴ Heart murmurs can be found in those with endocarditis. In the cat, respiratory failure can occur early in the course of sepsis. In some cats, endocarditis can be more gradual, permitting the infected valves to become dystrophic.^139,164 With valvular lesions, although signs of sepsis have varied, cats can develop signs of right- or left-side congestive heart failure (CHF).

Dogs

The leukograms of dogs with bacteremia alone and with bacterial endocarditis are similar.⁴² A neutrophilic leukocytosis with an appropriate left shift and monocytosis are present in most dogs with gram-positive or anaerobic bacteremia and in virtually all dogs with chronic IE at some point during the course of disease. Leukopenia with inappropriate left shift has been more common with bacteremia alone, usually in association with peracute and acute gram-negative infections.

A normocytic, normochromic, nonregenerative anemia and thrombocytopenia are common with subacute or chronic bacteremia in dogs or cats.26,42 With sepsis, hemoconcentration occurs after fluid losses from the intravascular space. Serum total solids tend to decrease because of the loss of protein from the intravascular compartment.

Serum chemistry abnormalities are common.⁴² Hypoalbuminemia (less than 2.5 mg/dL), a twofold or greater elevation of alkaline phosphatase (ALP) activity and/or bilirubin concentration, and hypoglycemia (less than 80 mg/dL) are consistent with bacteremia (also known as the “septic triad”). Hyperglycemia occurs during the early phase (hyperdynamic phase) of septic shock. Analysis of serial serum chemistries can reveal trends as described earlier. Hospitalized, stressed dogs should have a blood glucose concentration above 100 mg/dL.

The liver is an important site of removal of bacteria from the blood. Increased serum ALP activity is associated with gram-positive and gram-negative infections, and bacterial toxins are associated with impaired bile metabolism and cholestasis. Hyperbilirubinemia, bilirubinuria, and icterus are the hallmarks of reactive hepatopathy of sepsis (see Chapters 36 and 89).

Hypoalbuminemia is a common manifestation of most types of bacteremia.⁴² Subacute and chronic bacteremia can result in transcapillary leakage as a result of immune-mediated or embolic vasculitis or bacterial toxins. Sepsis also has been associated with reduced hepatic synthesis of albumin, and as many as 50% of bacteremic dogs can have increased Bromsulphalein retention, suggesting either reduced hepatic function or reduced hepatic arterial blood flow. Bile acid levels can also be increased, suggesting hepatic insufficiency. Hypocalcemia is often observed and usually attributed to hypoalbuminemia.

The mechanism of hypoglycemia involves the effects of bacteria or bacterial toxins on the intermediary metabolism of glucose. In contrast, hyperglycemia has been correlated with a higher postoperative mortality in dogs than normoglycemia or hypoglycemic septicemic dogs.⁴¹ This finding can be related to the fact that hyperglycemia has been seen in patients with early, severe septicemia, and hypoglycemia develops in more chronically affected cases.

Hypercoagulability leading to DIC is a common sequela of bacteremia. Evidence of DIC (low fibrinogen, prolonged prothrombin time and activated partial thromboplastin time, increased fibrinogen degradation products and D-dimer levels) is consistent with advanced sepsis wherein major organ failure, including cardiovascular collapse, can be imminent.113,130,130

Septic patients often have a metabolic acidosis with respiratory compensation. The carbon dioxide tension can decrease severely to compensate for acidemia, but hypoxemia is uncommon in conscious patients.

Blood lactate levels are often increased in septic patients. Hemoconcentration and shock cause decreased oxygen delivery to the tissues, resulting in anaerobic metabolism. Cellular oxidative respiration is inhibited by an endotoxin or a mediator.¹⁹⁹ Septic patients tend to be in a hypermetabolic state, requiring increased oxygen delivery to tissues.

Proteinuria, occult hematuria, and pyuria may occur in association with bacteremia or, more commonly, with bacterial endocarditis in which renal infarction, glomerulonephritis, and renal microabscess formation are common sequelae. Proteinuria with acellular urinary sediment suggests glomerular or tubular renal proteinuria and can occur with renal injury from immune-complex deposition caused by bacteremia. Urinary tract infections can be either a cause or a result of bacteremia, and urine cultures should be submitted regardless of whether abnormalities are detected on urinalysis.

Test results for immune-mediated diseases, such as antinuclear antibody titer (ANA), rheumatoid factor (RF), and lupus erythematosus cell preparation, occasionally are positive in bacteremic patients, particularly those with IE.

Bacteremia is only occasionally diagnosed by direct microscopic examination of leukocytes in blood smears. Direct Gram stains of peripheral blood are usually unrewarding because the number of microorganisms present is often much lower than the 10⁵/mL necessary for detection. Wright stains of buffy-coat smears can increase the rate of detection. Acridine orange is more sensitive than Gram stain, because organisms can be detected at 10⁴/mL concentrations. Slides must be handled carefully to prevent inadvertent contamination, suspected when extracellular bacteria are seen.

A simple point-of-care method of performing leukocyte smears involves placing one drop of freshly collected, nonclotted venous blood on a clean glass coverslip and incubating it in a moist Petri dish for 25 minutes at 37° C (98.6° F). The clot on the coverslip is gently washed off with normal (0.9%) saline, and the coverslip with attached leukocytes is immersed in fixative (methanol or glutaraldehyde) before Giemsa staining and microscopic examination.

Cats

In the most severe cases, cats have had prehepatic jaundice, associated with erythrocyte destruction, and neutrophilia with a left shift.²⁶ Cats with sepsis often exhibit hypoxemia, hypercapnia, and metabolic acidosis. In addition, they often have hypoalbuminemia, low serum ALP activity, and hyperbilirubinemia.²⁶

Blood Culture

The definitive diagnosis of bacteremia requires compatible clinical signs and laboratory data and the isolation of the offending microbe from blood cultures. Primary or secondary sites of infection, urine, and joint fluid may also contain the organism. Culture of joint fluid is usually unrewarding because the arthropathy is usually immune-mediated rather than septic. Preferably, the bacteria should be isolated from more than one blood sample. In addition, a source of infection should be sought and attempts made to isolate an organism from that site. Negative blood culture results in bacteremic patients can occur as a result of prior antimicrobial therapy, chronic low-grade infections such as those associated with diskospondylitis and endocarditis, Bartonella infections (requiring specialized handling), intermittent shedding of the organism, causative agents other than bacteria, uremia, and right-sided endocarditis. Clinicians should test all patients with negative blood culture results for Bartonella spp. (see Chapter 52).

Polymerase chain reaction (PCR) performed on blood samples can prove useful in the identification of bacteremia. There has been some success in dogs with IE using panbacterial PCR primers targeted to amplify the 16s ribosomal bacterial DNA.¹⁰¹

When an etiologic diagnosis is established with positive blood culture results, more appropriate and effective antibacterial therapy can be used. Clinicians often have the opinion that blood culture attempts are unrewarding. However, a common cause of negative blood culture results is the absence of bacteremia. When blood culture samples are taken from patients with a high likelihood of bacteremia, and when the samples are handled properly, positive results are common. Timing, volume, number of specimens and proper laboratory processing are important for positive results. Prior antibacterial therapy, fastidious microbes, intracellular microbes, and failure to culture for anaerobes are factors leading to false-negative results. Adhering to recommended guidelines for obtaining and processing blood culture specimens can make the diagnostic yield from blood cultures rewarding. Although empiric multiple-antibacterial therapy often has been instituted in critically ill patients before obtaining culture results, the effort and expense required for blood culture have been justifiable because improper treatment may increase mortality from bacteremia. Furthermore, prior administration of antimicrobials appears to slow but not prevent bacterial isolation.⁶⁸

Technique

Before venipuncture, thorough skin antisepsis, as for surgery, is the most effective means of avoiding culture contamination (Fig. 86-2, A).¹⁰⁹ Small numbers of bacteria may persist inside hair follicles and sweat and sebaceous glands, which may be penetrated by the needle. If the vein must be palpated after skin disinfection, a sterile glove should be worn. To minimize the risk of contamination, blood samples should not be drawn through an indwelling intravenous catheter unless it is a recently and appropriately placed jugular catheter. Arterial blood specimens offer no advantage over venous blood specimens. Finding the same organism at two different surgically prepared sites reduces the likelihood that the organism is a contaminant, especially when sampling intervals are close.

The culture bottle diaphragm is disinfected with alcohol or iodine before sample inoculation (Fig. 86-2, B). Blood is inoculated immediately and directly into culture media using a syringe and new needle or a blood transfer set (Fig. 86-2, C). Only commercial culture bottles packed under vacuum and fitted with rubber diaphragms should be used for routine blood cultures to minimize the risk of contamination (Table 86-3). Air must not be allowed to enter into vacuum bottles during blood injection. The blood should be dispersed in the culture medium by gently inverting the bottle two or three times. Blood culture bottles should be inoculated and can be maintained at room temperature to avoid killing temperature-sensitive bacteria; however, incubation at 37° C is often used in the laboratory.

TABLE 86-3

Commercially Available Blood Culture Systems

Indicated Use	Collection Media	Manufacturer
Aerobic and anaerobic	BBL SEPTI-CHEK with trypticase soy broth	Becton Dickinson Microbiology Systems, Franklin Lakes, NJ
Pediatric patients	BBL SEPTI-CHEK with brain heart infusion	Becton Dickinson Microbiology Systems, Franklin Lakes, NJ
Antimicrobial therapy	BBL SEPTI-CHEK with resins culture bottle	Becton Dickinson Microbiology Systems, Franklin Lakes, NJ
Centrifugation lysis	ISOLATOR SYSTEM	Wampole Laboratories, Princeton, NJ

One-half of each blood sample should be inoculated into an aerobic broth culture medium and the other half into an aerobic broth. These media should be capable of supporting growth of fastidious bacteria and, when appropriate, should contain a resin to remove antibacterials.

For catheter-related bacteremia, the catheter should be removed aseptically with a sterile forceps after local antisepsis of the insertion site with a swab soaked with 70% alcohol. A blood sample can be taken at the same time for culture. After removal of the catheter, the distal segment should be cut off and placed in a sterile, dry tube to be sent to the laboratory.

Liquid Media

Commercial multipurpose nutrient broth media, such as tryptic soy broth, trypticase soy broth, Columbia broth, and brain-heart infusion broth (Difco Laboratories, Detroit, MI) are all suitable for qualitative recovery of aerobic and anaerobic bacteria. Thioglycolate and thiol broth (Difco), intended for anaerobic blood culture only, should not be relied on for multipurpose cultures because aerobic (especially Pseudomonas) and facultative anaerobic bacteria are not reliably isolated. Most liquid media are bottled under vacuum with carbon dioxide added and usually are suitable to support the growth of clinically important anaerobes. The use of special anaerobic broth is rarely necessary.

Most commercial blood culture media contain 0.025% to 0.05% sodium polyanethol sulfonate (SPS), a polyanionic anticoagulant that also inhibits complement and lysozyme activity, interferes with phagocytosis, and inactivates therapeutic serum levels of aminoglycosides. Dilution of blood with liquid media is essential to neutralize serum and cellular antimicrobial properties. Even at 1 : 10 dilutions, serum may be bactericidal to coliforms, an effect that is counteracted by the addition of SPS. However, SPS is inhibitory to mycoplasmas and should not be in media intended for their isolation. Dilution of blood in culture broth usually lowers therapeutic concentrations of antibacterials to noninhibitory levels. When high levels of lactam antibacterials are present in blood before culture, β-lactamase can be added to the culture media. para-Aminobenzoic acid, available in some commercial blood culture media, competitively antagonizes the actions of sulfonamides and increases the blood culture yields from patients receiving sulfonamide drugs.

Bacterial growth can be suppressed in blood culture vials containing blood from patients receiving antibacterials. Anionic and cationic resins have been incorporated into blood collection vials for antibacterial removal before broth culture. The effectiveness of removal varies.58,194

In the laboratory, culture bottles are examined daily for evidence of microbial growth, which includes turbidity, hemolysis, gas production, or colony formation. In aerobic culturing, the broth should appear turbid within 24 hours of inoculation. In 95% of all instances wherein bacteria are isolated from blood culture media, isolation occurs within 7 days. Longer incubation may be necessary for specimens from patients who previously received antibacterial therapy or those with endocarditis caused by fastidious organisms such as Bartonella (see Chapter 52). In addition to visual inspection, routine (blind) subcultures onto solid culture media usually are performed for antimicrobial susceptibility testing between 7 and 14 hours after blood collection and again after 48 hours of incubation.

Lysis-Centrifugation

Although broth-based blood culture methods are sensitive, they provide no quantitative information about the number of microorganisms present. In contrast, inoculation of blood on solid agar allows for colony enumeration. Direct plating of freeze-thawed specimens of anticoagulated blood improves the sensitivity by freezing intracellular and phagocytized organisms. Only small volumes (1 mL) of blood can be directly plated. Lysis-centrifugation methods, with commercially prepared tubes (e.g., Wampole Isolator-tubes, Carter-Wallace, Cranbury, NJ), allow for lysis of cellular elements followed by microbial concentration through centrifugation. These methods may also shorten the time to isolation of bloodborne organisms, but they are associated with an increased risk of bacterial contamination. Lysis methods also enhance the ability to recover intracellular organisms such as Brucella, Mycobacterium, Histoplasma, and Bartonella.

Urine Culture

Culture of urine has been commonly used to detect organisms associated with bacteremia and diskospondylitis. The presumption has been that these positive culture results represent the source of infection. In contrast, bacteriuria is likely a consequence of bloodborne bacteria reaching the kidney and being filtered by the glomerulus and entering the renal tubule and urine. Bacteria are found in urine within minutes of intravenous infusion. Bacteremic animals with positive results for urine culture usually have no lower urinary tract signs, suggesting that the infection did not come from ascending infection.

Antibacterials

For additional information on specific drugs discussed next, see Chapter 30 and the Drug Formulary in the Appendix. The penicillin family of antibacterials includes the narrow-spectrum penicillin G; intermediate-spectrum ampicillin; and the extended-spectrum carbenicillin, ticarcillin, ticarcillin-clavulanate, and piperacillin (Table 86-5). A common misconception is that ampicillin is a broad-spectrum antibacterial. Most coagulase-positive staphylococcal isolates are resistant to penicillin and ampicillin, and these are poor empiric choices for serious and life-threatening infections. Penicillin G should be restricted to streptococcal and some gram-positive anaerobic infections.

TABLE 86-5

Antimicrobial Dosages for Bacteremia with or without Endocarditis in Dogs and Cats

Druga	Species	Doseb	Route	Interval (hours)	Duration (days)c
Penicillin	B	20–40 × 103 U/kg	IV	4–6	7–14
Imipenem	B	10 mg/kg	IV	8	7–14
Carbenicillin	B	40–50 mg/kg	IV	6–8	7–14
Piperacillin	B	30 mg/kg	IV	6	7–14
Ampicillin	B	20–40 mg/kg	IV, SC	6–8	7–14
Amoxicillin-clavulanate	B	20 mg/kg	PO	12	Follow-up
Ticarcillin	D	50 mg/kg	IV, SC	8	7–14
Ticarcillin-clavulanate	D	50 mg/kg	IV	8	7–14
	C	40 mg/kg	IV	6	7–14
Cefazolin (first generation)	B	20–30 mg/kg	IV, SC	8	7–14
Cefapirin (first generation)	B	15–30 mg/kg	IV, SC	8	7–14
Cefoxitin (second generation)	B	30 mg/kg	IV	6–8	7–14
Cefuroxime (second generation)	D	15–30 mg/kg	PO, IV	8	7–14
Cefotaxime (third generation)	B	20–80 mg/kg	IV	8	7–14
Ceftiofur sodium (third generation)	B	2.2–4.4 mg/kg	SC	12	Follow-up
Gentamicind	B	4–6 mg/kg	IV	24	7–14
Amikacind	B	7–10 mg/kg	IV	24	7–14
Trimethoprim–sulfonamidee	D	15 mg/kg	IV	8–12	7–14
	B	30 mg/kg	SC, PO	12–24	Follow-up
Metronidazole	B	8–15 mg/kg	IV, PO	8	5–7
Clindamycin	B	10 mg/kg	IV	8–12	7–14
	B	10–11 mg/kg	PO	12	Follow-up
Chloramphenicol	D	15–25 mg/kg	IV	6–8	7–14
	C	10–15 mg/kg	IV	6–8	4–7
Ciprofloxacinf	B	10–15 mg/kg	PO	12	14
Enrofloxacinf	D	5–7 mg/kg	IV	24	4–7
	D	5–15 mg/kg	PO	24	Follow-up
	C	5 mg/kg	PO	24	Follow-up
Azithromycin	B	5–10 mg/kg	PO	12	Follow-up

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