Pasteurellosis, infection with Pasteurella sp, is a zoonosis–a disease that humans acquire from exposure
to infected animals or products made from infected animals. While systemic (septicemia, arthritis, endocarditis, osteomyelitis, meningitis) and pneumonic forms are possible, cutaneous infection, frequently resulting from animal bites, is the most common form.
These wounds can become infected with Pasteurella because these organisms often reside in the respiratory tract and oral cavity of birds and mammals. Cutaneous infections can quickly progress resulting in inflammation and exudate production. While Pasteurella infections can result from a variety of animal bites, they often
occur as the result of feline bites.
At least 17 species of Pasteurella have been identified; however, based on DNA hybridization, several species are most closely related to other genera such as Actinobacillus. Pasteurella multocida is the most frequently isolated species and includes three subspecies:
multocida, septica, and gallicida. Pasteurella multocida also consists of five serogroups (A, B, D, E, and F) defined by capsular antigens. Pasteurella canis (associated with dogs), and Pasteurella stoma tis and Pasteurella dagmatis (both associated with dogs and cats) are also isolated from humans.
Pasteurella are gram-negative, nonmotile, facultative, anaerobic coccobacilli appearing ovoid, filamentous,
or as rods. Bipolar staining (safety pin appearance when the poles of the cells are more intensely stained)
is frequently observed. In biochemical tests, these bacteria are catalase and oxidase (most isolates) positive and ferment glucose with weak to moderate acid production without gas. In TSIA, a weak glucose fermentation reaction appears.
All Pasteurella spp. will grow on SBA and CHOC, producing grayish colonies (Figure 19-24). Conversely,
MacConkey agar will not support the growth of most Pasteurella spp. Growth on SBA in the absence of
satellitism or in pure culture combined with bipolar staining may differentiate Pasteurella from Haemophilus
spp. P multocida produces nonhemolytic colonies that may appear mucoid followed by the production of a narrow green to brown halo around the colony on SBA plates after 24 and 48 hours of incubation at 37° C,
respectively. Table 19-5 lists the various growth characteristics and biochemical reactions that can be useful in the differentiation of Pasteurella spp. associated with human infections.
Pasteurella bettyae, formerly HB-5, has been obtained from placenta, amniotic fluid, blood, rectal sites,
abscesses, and urogenital specimens. Isolates are fastidious, capnophilic coccobacilli and rods that are facultatively anaerobic. They ferment glucose and fructose and are catalase positive, oxidase variable, and indole positive. They may grow on MacConkey agar and are nonmotile.
Brucellosis, infection with bacteria from the genus Brucella, is an important zoonotic disease throughout the world. Due to the potential application in bioterrorism, Brucella spp. are considered category B Select
Biological Agents by the Centers for Disease Control and Prevention (CDC). Category B agents are easy to
disseminate and cause moderate morbidity but low mortality. U.S. Federal Code 42CFR72.6 describes the
requirements for possession and transfer of select biological agents. For more information about biological
threat agents, see Chapter 30.
Brucellosis has been described by both the disease course (undulant fever) and geographic locations where cases have occurred (Mediterranean, Crimean, and Malta fevers). Certain subpopulations are at higher risk of contacting brucellosis, such as those exposed to animals and animal products (e.g., veterinarians and
hunters) and laboratory workers. Brucella infections are acquired through aerosol, percutaneous, and oral
routes of exposure. While direct person-to-person transmission is considered rare, cases resulting from
sexual contact and breast-feeding have been reported.
Brucellosis is a CDC reportable disease because diagnosis can have public health implications.
The resulting clinical presentation appears to be similar for all exposure routes. The three clinical stages
of brucellosis are acute, subchronic, and chronic.
Symptoms of acute infection are nonspecific (fever, malaise, headache, anorexia, myalgia, and back pain)
and usually occur within 8 weeks of exposure. Brucellosis is a systemic, deep-seated disease resulting in
various long-term sequelae. The subchronic or undulant form of the disease typically occurs within a year
of exposure and is characterized by undulating fevers (characterized by normal temperatures in the morning
followed by high temperatures in the afternoon and evening), arthritis, and epididymoorchitis (inflammation
of the epididymis and testis) in males. The chronic manifestation commonly presents a year after exposure
with symptoms such as depression, arthritis, and chronic fatigue syndrome.
The four species that are most commonly associated with human illness are Brucella melitensis,
Brucella abortus, Brucella suis, and Brucella canis. Two other species are Brucella ovis and Brucella neotomae; in recent years, additional species have been isolated from marine mammals. Brucellae are small gramnegative,
aerobic, nonmotile, unencapsulated bacteria that do not form spores and may appear as coccobacilli
or rods. Under optimal growth conditions on agar, smooth, raised, and translucent colonies will appear (Figure 19-25). These bacteria are facultative intracellular pathogens that can reside within phagocytic cells.
It can be difficult to diagnose brucellosis through direct examination of a clinical sample, most often blood or bone marrow, and the ability for direct isolation and culture can vary between acute and chronic
manifestations. Whereas 50% to 80% of acute cases yield positive blood cultures, only 5% of chronic cases
produce positive cultures. As a result, serologic tests are frequently used, in conjunction with patient history
and disease status, to diagnose brucellosis. In biochemical tests, brucellae are oxidase and catalase
positive and are urease positive within 2 hours.
As shown in Table 19-6,Brucella spp. can vary in their
requirement for CO2and production of a positive result for hydrogen sulfide using the lead acetate method.
These tests can also differentiate brucellae from similar organisms (e.g., Bordetella bronchiseptica,
Acinetobacter spp., and H. influenzae) when combined with growth on SBA, colony morphology, specimen source, and testing for X and V factor requirements (Table 19-7).
Due to the aerosol mode of transmission, Brucella organisms should be handled under biosafety level 3 conditions by an appropriately equipped laboratory, such as a reference laboratory. Approximately 2% of all reported cases of brucellosis are acquired in the laboratory, illustrating the risk of infection to clinical laboratory personnel. The chance of acquiring an infection from a laboratory exposure varies from 30% to 100%and depends on a range of factors.
According to the current taxonomic classification, there are two species in the genus Francisella: Francisella tularensis and Francisella ph i/omiragia, but only F tularensis has been implicated in human disease.
F tularensis has four subspecies or biovars: subsp. tularensis (type A), subsp. holarctica (type B), subsp. mediasiatica, and subsp. novicida. F. tularensis subsp. tularensis causes the most severe disease while F. tularensis subsp. novicida is an opportunistic pathogen, primarily causing disease in immunocompromised individuals. F. tularensis subsps. holarctica and mediasiatica produce a similar disease to F. tularensis subsp.
tularensis, but infections are rarely fatal.
Tularemia, infection with bacteria from the genus Francisella, is also a zoonotic disease and has many other names, including rabbit, deerfly, lemming fever, and water rat trappers’ disease. Tularemia can be contracted
through ingestion, inhalation, arthropod bite (e.g., ticks and biting flies), or contact with infected tissues. Clinical presentation can assume a variety of forms and is influenced by the route of bacteria exposure. The most common clinical form is ulceroglandular in which an ulcer forms at the site of inoculation and is followed by an enlargement of the regional lymph nodes. Tularemia can also occur in pneumonic (contracted via the inhalation route), glandular, oropharyngeal, oculoglandular, and typhoidal forms. In the postantimicrobial era, the case-fatality rates for F tularensis type A have decreased from 5% to 10% to 1% to 2%. However, mortality rates for untreated pneumonic and typhoidal forms before the introduction of antimicrobial agents were 30% to 60%.
F tularensis is a CDC category A Select Biological Agent. Category A agents are described as posing a
risk to national security because they can be spread through person-to-person contact or are easily disseminated and result in high mortality rates, leading to a potentially great public health impact and public panic.
Francisella spp. appear as small, nonmotile, nonspore- forming, gram-negative rods or coccoid bacteria and are strictly aerobic. Similar to Brucella spp., Francisella are also facultative intracellular pathogens. Francisella spp. may initially grow on SBA, but they are fastidious and require supplementation with cysteine, cystine, or thiosulfate for growth on successive passage. Chocolate, Thayer-Martin, and buffered charcoal yeast extract agars and Mueller-Hinton and tryptic soy broths may be used. MacConkey and eosin methylene blue agar will not support F. tularensis growth. Due to slow growth rates, F. tularensis colonies may not be visible before 48 hours of incubation at 3r C. Once visible, gray-white, raised colonies with a smooth appearance will be seen (Figure 19-26).
In biochemical tests, F tularensis is oxidase, urease, and satellite or X and V test negative, and weakly positive
for catalase and r~-lactamase activity. F. tularensis subsp. tularensis may be differentiated from F philomiragia
and F tularensis subsp. novicida by poor growth at 28° C. Additionally, clinical symptoms, patient exposure to geographic locations where tularemia is endemic, and serology are used in diagnosis of Ftularemia infections. Like brucellosis, tularemia is also a CDC reportable disease.
F. tularensis is a highly infectious agent with as few as 50 organisms causing an infection through the cutaneous (ulceroglandular form) or inhalational (pneumonia) routes and has been the cause of many laboratory-acquired infections. In a recent incident, three Boston University scientists became infected through work unknowingly performed with a fully virulent strain. Laboratory personnel should utilize appropriate laboratory safety techniques and precautions.
Biosafety level 3 conditions should be implemented when working with suspected F tularensis samples.