Anaerobic bacterial infections and response to treatment in dogs and cats: 36 cases (1983-1985)

Anaerobic bacteria have been increasingly implicated as important pathogens in animals. To determine the prevalence of anaerobic bacterial infection, the results of anaerobic bacteriologic culture of 599 specimens obtained from dogs and cats hospitalized at the Colorado State University Veterinary Teaching Hospital were reviewed. Obligate anaerobic bacteria were isolated from 35% of properly submitted specimens; Bacteroides spp and Fusobacterium spp were the organisms most commonly isolated. Infections most often containing anaerobes were abscesses, pleuropulmonary infections, and abdominal infections. A complication rate of 28% was observed with anaerobic bacterial infections; failure to initially treat with an effective antibiotic increased the rate of recurrence of infection.     

Occurrence of anaerobic bacteria in diseases of the dog and cat.

A survey for anaerobic bacteria was conducted in 314 clinical specimens from dogs and cats. A total of 187 anaerobic isolates in pure and mixed culture were isolated from 111 of the specimens that contained anaerobic bacteria. Common isolated included Actinomyces (9.1%), Clostridium perfringens (19.3%), other Clostridium spp (11.2%), Peptostreptococcus anaerobius (7.5%), Bacteroides melaninogenicus (13.4%), other Bacteroides spp (17.6%), and Fusobacterium necrophorum (5.3%). Anaerobic bacteria were involved in serious lesions that often were life threatening to the animals. Antibiotic susceptibility data indicated that the lincomycin family, the penicillin family, chloramphenicol, and cephaloridine are preferred drugs for treatment of anaerobic infections. Data from the survey were used in formulation of a table to aid practitioners in clinical diagnosis of disease caused by anaerobes. Clostridium perfringens was isolated in large numbers from five of six dogs with a clinical diagnosis of canine hemorrhagic gastroenteritis and from one cat with hemorrhagic diarrhea. Experimental infections were induced in rats, using caine feces as inoculum. Induced lesions contained aerobic and anaerobic bacteria similar to those bacteria isolated in the clinical survey, indicating that feces may serve as a major source of these bacteria in clinical infections of the dog.     

Aerobic and anaerobic bacterial isolates from horses with pneumonia or pleuropneumonia and antimicrobial susceptibility patterns of the aerobes

Frequency of aerobic and anaerobic isolates in 327 aspirates and in 123 pleural fluid samples from 327 horses with pneumonia or pleuropneumonia and antimicrobial susceptibility patterns of the aerobes were reported. Of the 327 horses, 75% survived, 20% were euthanatized, and 5% died. Tracheobronchial aspirates or pleural fluid specimens from 25 of the horses did not yield growth. Of the remaining 302 horses, 221 had only aerobic organisms isolated, whereas only anaerobes were isolated from 6 of the 302 horses. The remaining 75 horses had mixed aerobic and anaerobic bacterial infections. The survival rates for horses with aerobic only isolates was twice that of horses with anaerobic isolates. The aerobic bacteria most frequently isolated were beta-Streptococcus spp, Pasteurella spp, Escherichia coli, and Enterobacter spp. The anaerobic species most frequently isolated were Bacteroides spp and Clostridium spp.     

Assessment of infectious organisms associated with chronic rhinosinusitis in cats

OBJECTIVE: To determine detection rates for feline herpesvirus type 1 (FHV-1), Mycoplasma spp, fungi, and bacteria in flush samples and biopsy specimens from the nasal cavities of cats with and without chronic rhinosinusitis (CRS). DESIGN: Prospective study. ANIMALS: 10 CRS-affected cats and 7 cats without signs of respiratory tract disease. PROCEDURES: Nasal flush samples and biopsy specimens were collected from all cats for bacterial (aerobic and anaerobic), fungal, and mycoplasmal cultures; additional biopsy specimens were collected for virus isolation and polymerase chain reaction (PCR) assay (to detect FHV-1 DNA). RESULTS: Aerobic bacteria were detected in flush samples from 5 of 7 control cats; culture of flush samples from CRS-affected cats yielded aerobic bacteria (9/10 cats), anaerobic bacteria (3/10), and Mycoplasma spp (2/10). No fungal organisms were isolated from any cat. Potential pathogens were isolated significantly more often from CRS-affected cats than from control cats. Bacterial culture of biopsy specimens yielded aerobic bacteria (2/7 control cats and 4/10 CRS-affected cats) and anaerobic bacteria (2/10 CRS-affected cats). Although FHV-1 was not detected in nasal biopsy specimens from control or CRS-affected cats, FHV-1 DNA was detected via PCR assay in specimens from 4 of 7 control cats and 3 of 10 CRS-affected cats. CONCLUSIONS AND CLINICAL RELEVANCE: Compared with findings in control cats, anaerobic bacteria, Mycoplasma spp, and a variety of potentially pathogenic organisms were detected more commonly in samples from cats with CRS. In both groups, FHV-1 was detected via PCR assay as a nonviable organism or in noncultivable amounts.     

The obligate and facultatively anaerobic bacterial flora of the normal feline gingival margin

Samples from the gingival margins of 14 cats considered normal on clinical examination were cultured for facultative and obligate anaerobic bacteria. All mouths were free from any gingival marginal inflammation and tartar build-up; all cats were between 6 and 12 months of age. A mixed growth was obtained from all samples. The mean number of bacterial species per sample was 10.7 with a range of 7-16 isolates. Of the 150 isolates processed, 109 (72.66%) were obligate anaerobes. Of the facultatively anaerobic species, Actinomyces (including A. viscosus, A. hordeovulneris and A. denticolens) comprised 12%, Pasteurella multocida 9.33% of isolates and Propionibacterium species 6% of all isolates. Gram-negative bacilli belonging to the genera Bacteroides and Fusobacterium were isolated from 12 of the 14 samples, and comprised 77% of the obligate anaerobes isolated. Clostridium villosum comprised 10.1% of obligately anaerobic isolates, Wolinella species made up 6.42%, while 4.58% were Peptostreptococcus anaerobius. The most commonly isolated obligately anaerobic species was C. villosum and the most commonly isolated facultatively anaerobic species was P. multocida. These findings show a bacterial flora of the normal feline mouth which is very similar in composition to that of cat fight abscesses and feline pyothorax.     

Oral associated bacterial infection in horses: studies on the normal anaerobic flora from the pharyngeal tonsillar surface and its association with lower respiratory tract and paraoral infections

Two hundred and seventy bacterial isolates were obtained from the pharyngeal tonsillar surface of 12 normal horses and 98 obligatory anaerobic bacteria were characterised. Of these, 57 isolates belonging to 7 genera (Peptostreptococcus (1); Eubacterium (9); Clostridium (6); Veillonella (6); Megasphera (1); Bacteroides (28); Fusobacterium (6)) were identified, and 16 of these were identified to species level (P. anaerobius (1); E. fossor (9); C. villosum (1); B. fragilis (1); B. tectum (2); B. heparinolyticus (2)). Three hundred and twenty isolates were obtained from 23 samples from horses with lower respiratory tract (LRT) or paraoral (PO) bacterial infections. Of the 143 bacteria selected for detailed characterisation, obligate anaerobes accounted for 100 isolates, facultative anaerobes for 42 isolates and obligate aerobes for one isolate. Phenotypic characterisation separated 99 of the isolates into 14 genera. Among the obligately anaerobic species, Gram-positive cocci including P. anaerobius comprised 25% of isolates, E. fossor 11% and other Gram-positive rods (excluding Clostridium sp.) 18% of isolates. The Gram-negative rods comprised B. fragilis 5%, B. heparinolyticus 5%, asaccharolytic pigmented Bacteroides 3% and other Bacteroides 13%, while a so-far unnamed species of Fusobacterium (7%), and Gram-negative corroding rods (3%) were isolated. Among the facultatively anaerobic isolates, S. equi subsp. zooepidemicus accounted for 31% of isolates, followed by Pasteurella spp. 19%, Escherichia coli 17%, Actinomyces spp. 9%, Streptococcus spp. 9%. Incidental facultative isolates were Enterococcus spp. 2%, Enterobacter cloaceae 2%, Actinobacillus spp. 2% and Gram-negative corroding rods 5%. On the basis of the similarities (as determined by DNA hybridization data and/or phenotypic characteristics) of some of the bacterial species (e.g. E. fossor and B. heparinolyticus) isolated from both the normal pharyngeal tonsillar surfaces and LRT and PO diseases of horses, it is considered that the most likely source of bacteria involved in these disease processes is flora from the oral cavity.     

Isolation and characterisation of bacteria from pyothorax (empyaemia) in cats

Samples from 19 cases of feline empyaemia were examined. All cases had no prior treatment and specimens were collected by thoracocentesis after preparation of the skin as for aseptic surgery. Of the 87 bacterial strains isolated, 70 (80.5%) were anaerobes and 17 (19.5%) were facultative anaerobes. Fourteen cases contained mixtures of anaerobes and facultative anaerobes, and five contained anaerobes only. Bacteroides was the most commonly isolated genus (42.5% of all isolates). Clostridium villosum (16.1%) was the most frequently isolated bacterial species followed by Peptostreptococcus anaerobius and Pasteurella multocida (each 12.6%). The most commonly isolated anaerobic species was Clostridium villosum sp.nov. (20% of anaerobic isolates and 16.1% of all isolates) and Pasteurella multocida was the most commonly isolated facultatively anaerobic species (64.7% of facultative isolates and 12.6% of all isolates).     

Recovery and identification of anaerobes in veterinary medicine: A 2-year experience

The examination of 300 specimens over a 2-year period in a veterinary diagnostic laboratory (1975–1977) revealed that of the specimens yielding bacteria, 56.3% contained anaerobes while 71.8% yielded facultative bacteria. Of the total specimens 26.6% proved to be sterile and 4.6% were unsuitable. The anaerobes which were isolated with the highest frequency from diseased animals belonged to the following genera: Clostridium, 46.0%; Bacteroides 15.1%; Fusobacterium, 14.3%; Actinomyces, 11.1%; and Propionibacterium, 5.6%. The culture method used in this study was based upon reducible solid media for primary isolation. Its efficiency compared favorably with what is commonly accepted as an effective recovery rate for anaerobes. However, the relative proportion of anaerobic genera obtained from veterinary clinical specimens was in direct contrast with specimens obtained in medical diagnostic laboratories. Because of this difference and the kinds of anaerobes most frequently encountered in veterinary medicine, it is believed that the culture method used in this study is more suitable to veterinary medicine than other methods more commonly employed in medical diagnostic laboratories.     

Anaerobic bacteria in 21 horses with pleuropneumonia

Anaerobic bacteria are important and overlooked bacterial pathogens of the lower respiratory tract in horses. Twenty-one of 46 horses with pleuropneumonia had anaerobic bacteria isolated from pleural fluid or from tracheobronchial aspirate. Bacteroides oralis and B melaninogenicus were the anaerobes most frequently isolated. Survival was significantly less for horses from which anaerobes were isolated than for horses from which anaerobes were not isolated. Putrid odor was associated with the pleural fluid and/or breath in 62% of the horses from which anaerobes were isolated. In these horses, the survival rate was significantly less than for horses from which odoriferous specimens were not isolated.     

Changes in prevalence and susceptibility of obligate anaerobes in clinical veterinary practice

Of 3,133 clinical specimens obtained from domestic animals, 26% contained species of bacteria that were obligate anaerobes. Members of the genera Bacteroides, Fusobacterium, and Peptostreptococcus accounted for 77% of the isolates. On the average, 2 species of obligate anaerobes were found in each specimen, usually admixed with facultatively anaerobic bacteria. Of those specimens containing an obligate anaerobe, 20% contained one isolate that was resistant to penicillin, ampicillin, and cephalothin. These resistant isolates belonged to the genus Bacteroides, the most common being Bacteroides fragilis. Approximately one third of these penicillin-resistant isolates was resistant to tetracycline as well. All isolates were susceptible to chloramphenicol, tetracycline (excluding penicillin-resistant Bacteroides), metronidazole, and clindamycin.     

The bacteriology and antimicrobial susceptibility of infected and non-infected dog bite wounds: fifty cases

Dog bite wounds are a common reason for dogs requiring veterinary care, but there is surprisingly little data on the bacteriology of bite wounds. A prospective study was performed on dogs with various grades of bite wound to identify the bacteria present in these wounds. Swabs were collected from all wounds for bacterial culture and cytology. All swabs were cultured aerobically and anaerobically and all aerobic cultures were evaluated for antibiotic susceptibility using the Kirby Bauer disk diffusion test. Fifty dogs with 104 bite wounds, inflicted within the previous 72h, were included. The victims were predominately intact male small breed dogs. Of the 104 wounds, 21 were judged by cytology to be infected and 83 non-infected. Infected wounds were significantly more likely to culture positive (p=0.02). Sixteen percent of wounds showed no growth. Sixteen percent grew aerobes, 1% anaerobes and 67% a mixture of aerobes and anaerobes. Pasteurella canis and pyogenic streptococci were common in infected wounds, whereas Bacillus spp., Actinomyces spp. and the oral streptococci were usually found in contaminated wounds. Three anaerobic genera were cultured, namely, Prevotella, Clostridium and Peptostreptococcus. One case represented the first isolation of Capnocytophaga canimorsus in an infected dog bite wound. Although no single antibiotic therapy was considered to be effective against all the bacteria, amoxycillin plus clavulanic acid, 1st and 3rd generation cephalosporins ampicillin or amoxycillin and potentiated sulphonamides gave the best in vitro sensitivity results.     

Enteric Clostridium perfringens infection associated with parvoviral enteritis in dogs: 74 cases (1987-1990).

Parvovirus infection was confirmed by fluorescent antibody staining of or viral isolation from specimens of small intestine in 181 (17%) of 1,110 dogs necropsied between July 1, 1987 and Dec 31, 1990. Clostridium perfringens was isolated from 74 (69%) of 108 dogs with parvovirus infection from which specimens of jejunum also had been obtained for culture of anaerobic bacteria. Gram-positive bacilli in association with focal to diffuse necrosis of the superficial portions of the villi were observed in histologic sections of specimens of small intestine from 56 (98%) of 57 dogs from which parvovirus and C perfringens had been identified. These findings indicate that C perfringens frequently proliferates in dogs with parvovirus infection.     

Bacterial infection of the lower respiratory tract in 34 horses

OBJECTIVE: To investigate associations between the bacteriology and aspects of history, clinical presentation, outcome and pathology of lower respiratory tract disease of 34 horses. PROCEDURE: Detailed aerobic and anaerobic bacteriological investigations were performed on clinical specimens from horses with pneumonia, lung abscessation and necrotic pneumonia with or without pleurisy in an attempt to identify those bacteria that might contribute to the initiation and progression of infection. RESULTS: Bacteria were cultured from 33 of the 34 horses. In ten cases, only aerobic/facultatively anaerobic isolates were cultured while aerobic/facultatively anaerobic bacteria and obligately anaerobic bacteria were isolated in the other 23 cases. Moderate to large numbers of anaerobic bacteria were isolated only when the estimated duration of illness was at least five days. Bacteria were not cultured from 12 of the pleural fluid samples but were always cultured from pulmonary samples (either transtracheal aspirates from live horses or pulmonary lesions at necropsy). Streptococcus equi subsp zooepidemicus was isolated in the three cases where only one bacterial species was cultured. In the other 30 cases, multiple species were isolated. These included most often and in greatest numbers, Streptococcus equi subsp zooepidemicus, Pasteurellaceae, Escherichia coli, anaerobic cocci, Eubacterium fossor, Bacteroides tectum, Prevotella heparinolytica, Fusobacterium spp, and pigmented members of the genera Prevotella and Porphyromonas. Aerobic/facultatively anaerobic organisms were isolated from 97% of horses, while obligately anaerobic organisms were cultured from 68% of horses. CONCLUSION: There was no association between the isolation of any specific bacterium and the outcome of disease. However, obligately anaerobic bacteria (such as anaerobic cocci, Bacteroides tectum, P heparinolytica and Fusobacterium spp) and the facultatively anaerobic species Escherichia coli, were recovered more commonly from horses that died or were euthanased than from those that survived. There was an association between failure of horses to recover from pleuropneumonia and delay in diagnosis and initiation of treatment.     

Normal ocular features, conjunctival microflora and intraocular pressure in the Canadian beaver (Castor canadensis)

Objective The aim of the study was to assess the ocular features, normal conjunctival bacterial and fungal flora, and intraocular pressure (IOP) in the Canadian beaver (Castor canadensis). Sample population Sixteen, apparently healthy beavers with no evidence of ocular disease, and live‐trapped in regions throughout Prince Edward Island. Procedures The beavers were sedated with intramuscular ketamine (12–15 mg/kg). Two culture specimens were obtained from the ventral conjunctival sac of both eyes of 10/16 beavers for aerobic and anaerobic bacterial and fungal identifications. The anterior ocular structures of all beavers were evaluated using a transilluminator and slit lamp biomicroscope. Palpebral fissure length (11/16 beavers), and horizontal and vertical corneal diameters (10/16 beavers) were measured. IOPs were measured in both eyes of 11/16 beavers using applanation tonometry. Both eyes of 3/16 beavers and one eye of 1/16 beavers were dilated using topical tropicamide prior to sedation to effect timely maximal dilation. Culture specimens and IOPs were not evaluated in these four animals. Indirect ophthalmoscopy was performed on 7/8 eyes of these four beavers. Results Conjunctival specimens from all eyes cultured positively for one or more isolates of aerobic bacteria. The most common isolate was Micrococcus spp. (five beavers; 9/20 eyes). Other isolates included a Gram‐positive coccobacilli‐like organism (four beavers; 7/20 eyes), Aeromonas hydrophila (three beavers; 4/20 eyes), Staphylococcus spp. (three beavers; 4/20 eyes), Gram positive bacilli (one beaver; 2/20 eyes), Enterobacter spp. (two beavers; 2/20 eyes), Streptococcus spp. (two beavers; 2/20 eyes), aerobic diphtheroids (one beaver; 1/20 eyes), and Pseudomonas spp. (one beaver; 1/20 eyes). Clostridium sordellii (one beaver; 1/20 eyes) and Peptostreptococcus spp. (one beaver; 1/20 eyes) were the sole anaerobic bacteria isolated. All conjunctival specimens were negative for growth of fungi. Ophthalmic examinations revealed the normal beaver eye and ocular adnexa included dorsal and ventral puncta, a vestigial third eyelid, and a circular pupil. Average palpebral fissure length was 9.36 mm (SD = 1.00) for both eyes. Mean horizontal and vertical corneal diameters of both eyes were 9.05 mm (SD = 0.64) and 8.45 mm (SD = 0.69), respectively. Mean IOP for the right and left eyes were 17.11 mmHg (SD = 6.39) and 18.79 mmHg (SD = 5.63), respectively. Indirect ophthalmoscopic examinations revealed normal anangiotic retinas. Conclusions Gram‐positive aerobes were most commonly cultured from the conjunctival sac of normal beavers, with Micrococcus spp. predominating. The overall mean IOP in ketamine‐sedated beavers was 17.95 mmHg. The beaver, an amphibious rodent, has an anangiotic retina.     

Rapid presumptive diagnosis of anaerobic infections in animals by gas-liquid chromatography

The detection of volatile fatty acids (VFA) by gas chromatography of 85 purulent specimens from abscesses or pyogenic infections in cats, dogs, rodents, and ruminants was compared with the results of bacteriologic culturing, and proved to be a rapid means of presumptively diagnosing anaerobic infections. Of 83 bacteriologically positive specimens, 52 (61%) yielded obligate anaerobes and in 50 specimens, 1 or more VFA (butyric acid, isobutyric acid, valeric acid, isovaleric acid, caproic acid, or isocaproic acid) was detected. Forty-six specimens were positive for culturing of anaerobes and for detection of 1 or more of these VFA. By contrast, pus from infections caused by (facultative) aerobic microorganisms contained no VFA or only acetic and/or propionic acid.     

Comparison of the bacterial and fungal flora in the pharynx of normal horses and horses affected with pharyngitis

A total of 43 horses were used for the study of the pharyngeal bacterial flora. The median value of the number of bacteria in the group of 19 normal horses was 3.8 × 10⁴ cfu/g of secretions. This value was 6.4 × 10⁴cfu/g in horses with grade I pharyngitis, 1.3 × 10⁵ cfu/g in horses with grade II pharyngitis and 3.5 × 10⁶ cfu/g in horses affected with grades III and IV pharyngitis. Corynebacterium spp, coagulase-negative staphylococci, Nocardia spp, Moraxella spp and Enterobacter spp were the most frequently encountered bacteria in the normal animals as well as in horses affected with pharyngitis of grades I or II. Moraxella spp were isolated in 87.5% of the horses with pharyngitis of grades III and IV, followed by Streptococcus zooepidemicus, Pseudomonas aeruginosa, coagulase-negative staphylococci and Enterobacter spp. No fastidious bacteria, nor strict anaerobes were isolated from any of the 43 horses. None of the microorganisms were found in 100% of the animals and the majority of the isolates were opportunistic bacteria. These results demonstrate that the isolation of Moraxella spp and S. zooepidemicus in large numbers is frequent in horses with lymphoid follicular hyperplasia grades III and IV. Fungi were isolated in small numbers from two or three horses in each group.     

Bacteriological findings for endodontical and apical molar dental diseases in the horse

In most cases the diagnosis of any molar dental disease in horses is made at an advanced stage, so that permanent restoration of the diseased teeth is not feasible. Complications such as bacteraemia and septicaemia due to infections as a result of dentogenous sinusitis and following dentosurgical procedures have been described in human medicine and in veterinary medicine. Twenty patients were available for examination from the Clinic for Horses of the School of Veterinary Medicine Hannover with molar dental disease in upper or lower jaw. As a result of this disease the infected tooth had to be removed surgically. The aim of this study was to determine the presence of and to identify microbes in 20 patients. Swab samples were taken from infected pulpa, from dental abscesses and from involved nasal sinuses. The samples were examined microbiologically and tested for aerobes and anaerobes at the same time. Infectious agents were found in 19 of 20 horses. In all, 27 different species of infectious agents were isolated, including both aerobic and anaerobic microbes. Fifteen patients (75%) showed a mixed flora. Further differentiation indicated a preponderance of the group of gram-negative obligatory anaerobic agents isolated from a total of 17 horses. In all these samples there was a high concentration of infectious agents of these genera, the most common of which were Prevotella spp (n = 16) and Fusobacterium spp. (n = 15). Pre-surgical antibacterial therapy is recommended to reduce the risk of intra- and/or post-surgical bacteraemia and its serious consequences. In light of these microbiological results and considering the high degree of resistance among all anaerobic microbes, all patients in this study were treated with Amoxicillin.     

A bacteriological investigation of subcutaneous abscesses in cats

Ten out of eleven subcutaneous abscesses in cats harboured anaerobic bacteria. Seven of the ten abscesses contained a mixture of obligate anaerobes and facultative anaerobes or microaerophilic bacteria. All of the anaerobic isolates were sensitive to the antibiotic metronidazole, but were resistant to streptomycin. Three isolates from one abscess were resistant to penicillin and ampicillin. Clinical resolution of subcutaneous abscesses was poor when procaine penicillin was used alone as treatment.     

Pyothorax in 26 cats: clinical signs, laboratory results and therapy (2000-2007)

Common historical findings in 26 cats with pyothorax were dyspnea (85%), lethargy (65%), and anorexia (62%), common clinical findings were dyspnea (89%), tachypnea (73%), fever (39%), and hypothermia (27%). Frequent laboratory abnormalities were leukocytosis (68%) with a left shift (100%), anemia (65%), hypoalbuminemia (91%), hyperglobulinemia (86%), hyperbilirubinemia (60%) as well as azotemia (52%). The thoracic effusion was purulent in all cats, the thoracic fluid evaluation (n = 19) met the criteria for an inflammatory exudate (protein 32-63 g/l, median 44; cell count 54.4-390 x 10(9)/l, median 100). Cytological analysis revealed bacteria in 21 of 24 cats. From 16 cats 13 different genera were isolated, 5 cultures were without bacterial growth. In 12.5% of the cats a single population of bacteria was identified, and in 87.5% a mixture of 2-4 different bacterial species were cultured. Obligate anaerobic bacteria were the most common isolates (70%) followed by facultative anaerob (22.5%) and aerob growing (7.5%) bacteria. Bacterial culture most often yielded Fusobacteria spp., Prevotella spp., Pasteurella spp., Porphyromonas spp. and Bacteroides spp. Three cats were euthanized at the day of presentation. A thoracic lavage was performed in 22 cats; mechanical complications with the chest tubes occured in 4 cats. Thoracotomy was performed in 2 cats. Of the 23 treated cats 17 survived (74%).     

Comparison of bacteriologic culture of blood and necropsy specimens for determining the cause of foal septicemia: 47 cases (1978-1987)

Diagnosis of bacterial septicemia was confirmed by results of bacteriologic culture of antemortem blood samples and/or necropsy specimens obtained from 47 foals up to 8 days old. Gram-negative bacteria were isolated from all 47 foals, and mixed infections with more than one organism were involved in 26 (55%). Escherichia coli, Actinobacillus spp, and Klebsiella pneumoniae were the most frequent isolates, infecting 55, 34, and 23% of foals, respectively. Gram-positive bacteria and anaerobic bacteria were isolated only from foals with mixed infections with gram-negative organisms. Clostridium perfringens was the only anaerobe isolated. In 38 (81%) of 47 foals with confirmed septicemia, blood samples were culture-positive. Thirty-two septicemic foals subsequently died, allowing a comparison to be made between the species of bacteria isolated by culture of blood with those recovered by culture of internal organs at necropsy. Blood failed to yield any gram-negative organisms in 12 (37.5%) of 32 foals from which a gram-negative pathogen was isolated at necropsy. Forty-three percent of the gram-negative bacteria, including 59% of the E coli, and 10% of the gram-positive bacteria found in septicemic foals at necropsy were not detected earlier by results of bacteriologic culture of blood.