1308 PART 5 Infectious Diseases

TABLE 168-3 Composition of World Health Organization

Reduced-Osmolarity Oral Rehydration Solution (ORS)a,b

CONSTITUENT CONCENTRATION, mmol/L

Na+ 75

K+ 20

Cl− 65

Citratec 10

Glucose 75

Total osmolarity 245

a

Contains (per package, to be added to 1 L of drinking water): NaCl, 2.6 g;

Na3

C6

H5

O7

·2H2

O, 2.9 g; KCl, 1.5 g; and glucose (anhydrous), 13.5 g. b

If prepackaged

ORS is unavailable, a simple homemade alternative can be prepared by combining

3.5 g (~1/2 teaspoon) of NaCl with either 50 g of precooked rice cereal or 6

teaspoons of table sugar (sucrose) in 1 L of drinking water. In that case, potassium

must be supplied separately (e.g., in orange juice or coconut water). c

10 mmol of

citrate per liter, which supplies 30 mmol of HCO3

/L.

hexose-Na+ co-transport mechanism to move Na+ across the gut

mucosa together with an actively transported molecule such as

glucose (or galactose); Cl–

 and water follow. This transport mechanism remains intact even when cholera toxin is active. ORS may be

made by adding safe water to prepackaged sachets containing salts

and sugar or by adding 0.5 teaspoon (i.e., a small spoonful) of table

salt and 6 level teaspoons (i.e., 6 small spoonfuls) of table sugar to

1 L of safe water. Potassium intake in bananas or green coconut

water should be encouraged. A number of ORS formulations are

available, and the WHO now recommends “low-osmolarity” ORS

for treatment of individuals with dehydrating diarrhea of any cause

(Table 168-3). If available, rice-based ORS is considered superior to

standard ORS in the treatment of cholera. ORS can be administered

via a nasogastric tube to individuals who cannot ingest fluid; however, optimal management of individuals with severe dehydration

includes the administration of IV fluid and electrolytes. Because

profound acidosis (pH <7.2) is common in this group, Ringer’s lactate is the best choice among commercial products (Table 168-4);

it must be used with additional potassium supplements, preferably given by mouth. The total fluid deficit in severely dehydrated

patients (>10% of body weight) can be replaced safely within the

first 3–4 h of therapy, half within the first hour. Transient muscle

cramps and tetany are common. Thereafter, oral therapy can usually

be initiated, with the goal of maintaining fluid intake equal to fluid

output. However, patients with continued large-volume diarrhea

may require prolonged IV treatment to match gastrointestinal

fluid losses. Severe hypokalemia can develop but will respond to

potassium given either IV or orally. In the absence of adequate staff

to monitor the patient’s progress, the oral route of rehydration and

potassium replacement is safer than the IV route.

Although not necessary for cure, the use of an antibiotic to which

the organism is susceptible diminishes the duration and volume

of fluid loss and hastens clearance of the organism from the stool.

Adjunctive antibiotics should therefore be administered to patients

with moderate or severe dehydration due to cholera. In many areas,

macrolides such as erythromycin (adults, 250 mg orally four times

a day for 3 days; children, 12.5 mg/kg per dose four times a day for

3 days) or azithromycin (adults, a single 1-g dose; children, a single

20-mg/kg dose) are the agents of choice. Increasing resistance to

tetracyclines is widespread; however, in areas with confirmed susceptibility, tetracycline (nonpregnant adults, 500 mg orally four times a

day for 3 days; children >8 years old, 12.5 mg/kg per dose four times

a day for 3 days) or doxycycline (nonpregnant adults, a 300-mg single

dose; children >8 years old, a single dose of 4–6 mg/kg) may be used.

Similarly, increasing resistance to fluoroquinolones is being reported,

but in areas with confirmed susceptibility, a fluoroquinolone such

as ciprofloxacin may be used (adults, 500 mg twice a day for 3 days;

children, 15 mg/kg twice a day for 3 days). Oral administration of

supplemental zinc is associated with decreased volume and severity

of diarrhea in young children, including in those with cholera. Children <6 months of age with cholera should be treated with 10 mg of

zinc daily for 10 days; children from 6 to <60 months of age should

be treated with 20 mg of oral zinc daily for 10 days.

■ PREVENTION

Provision of safe water and of facilities for sanitary disposal of feces,

improved nutrition, and attention to food preparation and storage in

the household can significantly reduce the incidence of cholera. In

addition, precautions should be taken to prevent the spread of cholera

via infected and potentially asymptomatic persons from endemic to

nonendemic regions of the world (as was probably the case in the outbreak in Haiti; see “Microbiology and Epidemiology,” above).

Much effort has been devoted to the development of an effective

cholera vaccine over the past few decades, with a particular focus on

oral vaccine strains. In an attempt to maximize mucosal responses, two

types of oral cholera vaccine have been developed: oral killed vaccines

and live attenuated vaccines. Currently, three oral killed cholera vaccines have been prequalified by the WHO and are available internationally. BivWC (ShancholTM; Shantha Biotechnics, Hyderabad, India)

contains both biotypes and serotypes of V. cholerae O1 and V. cholerae

O139 without supplemental cholera toxin B subunit. A related vaccine

is produced in South Korea (EuvicholTM, Euvichol-PlusTM; Eubiologics,

Seoul). WC-rBS (Dukoral®

; Valneva, Lyon, France) contains both

biotypes and serotypes of V. cholerae O1 supplemented with 1 mg of

recombinant cholera toxin B subunit per dose. The vaccines are administered as a two- or three-dose regimen, with doses usually separated

by 14 days. They provide ~60–85% protection for the first few months.

Booster immunizations of WC-rBS are recommended after 2 years for

individuals ≥6 years of age and after 6 months for children 2–5 years of

age. For BivWC, which was developed more recently, no formal recommendation regarding booster immunizations exists. However, BivWC

was associated with ~60% protection over 5 years among recipients

of all ages in a study in Kolkata, India; the rate of protection among

children ≤5 years of age approximated 40%. In outbreak situations,

even a single dose of BivWC can provide some protection: 40% and

63% adjusted protection for 6 months for all and severely dehydrating

cholera, respectively; although there was no evidence of protection in

children younger than 5 years of age. Models predict significant herd

immunity when vaccination coverage rates exceed 50%. The killed

vaccines have been safely administered among populations with high

rates of HIV infection.

Oral live attenuated vaccines for V. cholerae O1 are also in development. These strains have in common their lack of the genes encoding

cholera toxin. One such vaccine, CVD 103-HgR (VaxchoraTM; PaxVax,

Redwood City, CA), is approved by the U.S. Food and Drug Administration for use in travelers to cholera-endemic regions. The vaccine

was 90 and 80% efficacious against severe cholera after experimental

infection of North American volunteers 10 days and 90 days after

vaccination, respectively. Vaxchora is approved for use in individuals

2–64 years of age; no recommendations concerning the timing or need

for booster vaccinations are currently available. Other live attenuated

vaccine candidate strains have been prepared from El Tor and O139 V.

cholerae and have been tested in studies of volunteers. An advantage

of live attenuated cholera vaccines is that they may induce protection

after a single oral dose. Conjugate and subunit cholera vaccines are also

being developed.

TABLE 168-4 Electrolyte Composition of Cholera Stool and of

Intravenous Rehydration Solution

SUBSTANCE 

CONCENTRATION, mmol/L

NA+ K+ CL− BASE

Stool

Adult 135 15 100 45

Child 100 25 90 30

Ringer’s lactate 130 4a 109 28

a

Potassium supplements, preferably administered by mouth, are required to replace

the usual potassium losses from stool.

1309CHAPTER 168 Cholera and Other Vibrioses

Recognizing that it may be decades before safe water and adequate

sanitation become a reality for those most at risk of cholera, the WHO

has recommended incorporation of cholera vaccination into comprehensive control strategies and has established an international stockpile

of oral killed cholera vaccine to assist in outbreak responses. A global

strategy on cholera control was launched in 2017. This countryby-country approach aims to reduce cholera deaths by 90% and to

eliminate cholera in as many as 20 countries by 2030. Integral components of this strategy are advancing water, sanitation, and hygiene

(WASH) programs, as well as use of cholera vaccine. From 2016−2020,

>64 million doses of cholera vaccine have been requested from the

Global Vaccine Stockpile, and >33 million doses have been shipped to

requesting countries for use in control programs.

OTHER VIBRIO SPECIES

The genus Vibrio includes several human pathogens that do not cause

cholera. Abundant in coastal waters throughout the world, noncholera

vibrios can reach high concentrations in the tissues of filter-feeding

mollusks. As a result, human infection commonly follows the ingestion of seawater or of raw or undercooked shellfish (Table 168-5).

Most noncholera vibrios can be cultured on blood or MacConkey agar,

which contains enough salt to support the growth of these halophilic

species. In the microbiology laboratory, the species of noncholera

vibrios are distinguished by standard biochemical tests. The most

important of these organisms are Vibrio parahaemolyticus and Vibrio

vulnificus. Vibriosis causes an estimated 80,000 illnesses and 100 deaths

in the United States every year.

The two major types of syndromes for which these noncholera

vibrios are responsible are gastrointestinal illness (due to V. parahaemolyticus, non-O1/O139 V. cholerae, Vibrio mimicus, Vibrio fluvialis,

Vibrio hollisae, and Vibrio furnissii) and soft tissue infections (due to

V. vulnificus, Vibrio alginolyticus, and Vibrio damselae). V. vulnificus is

also a cause of primary sepsis in some compromised individuals.

■ SPECIES ASSOCIATED PRIMARILY WITH

GASTROINTESTINAL ILLNESS

V. parahaemolyticus Widespread in marine environments, the

halophilic V. parahaemolyticus is the leading seafood-borne bacterial

cause of enteritis worldwide. This species was originally implicated

in enteritis in Japan in 1953, accounting for 24% of reported cases in

one study—a rate that presumably was due to the common practice

of eating raw seafood in that country. In the United States, commonsource outbreaks of diarrhea caused by this organism have been linked

to the consumption of undercooked or improperly handled seafood

or of other foods contaminated by seawater. Since the mid-1990s, the

incidence of V. parahaemolyticus infections has increased in several

countries, including the United States. Serotypes O3:K6, O4:K68,

and O1:K-untypable, which are genetically related to one another,

account in part for this increase. The enteropathogenicity of V. parahaemolyticus is associated with its ability to cause hemolysis via a

thermostable direct hemolysin (Vp-TDH). Although the mechanisms

by which the organism causes diarrhea are not fully defined, most

V. parahaemolyticus genomes encode two type III secretion systems,

which directly inject toxic bacterial proteins into host cells. The activity

of one of these secretion systems is required for intestinal colonization

and virulence in animal models. V. parahaemolyticus should be considered a possible etiologic agent in all cases of diarrhea that can be

linked epidemiologically to seafood consumption or to the sea itself.

The incidence of V. parahaemolyticus infection in the United States

may be increasing, with this species accounting for almost half of all

Vibrio isolates reported in this country in 2014.

Infections with V. parahaemolyticus can result in two distinct

gastrointestinal presentations. The more common of the two presentations (including nearly all cases in North America) is characterized

by watery diarrhea, usually occurring in conjunction with abdominal

cramps, nausea, and vomiting and accompanied in ~25% of cases by

fever and chills. After an incubation period of 4 h to 4 days, symptoms

develop and persist for a median of 3 days. Dysentery, the less common

presentation, is characterized by severe abdominal cramps, nausea,

vomiting, and bloody or mucoid stools. V. parahaemolyticus also causes

rare cases of wound infection and otitis and very rare cases of sepsis.

Most cases of V. parahaemolyticus–associated gastrointestinal illness, regardless of the presentation, are self-limited. Fluid replacement

should be stressed. Antimicrobial agents may be of benefit in moderate or severe disease. Doxycycline, fluoroquinolones, macrolides, or

third-generation cephalosporins are usually used. Deaths are extremely

rare among immunocompetent individuals. Severe infections are associated with underlying diseases, including diabetes, preexisting liver

disease, iron-overload states, or immunosuppression.

Non-O1/O139 (Noncholera) V. cholerae The heterogeneous

non-O1/O139 V. cholerae organisms cannot be distinguished from V.

cholerae O1 or O139 by routine biochemical tests but do not agglutinate in O1 or O139 antiserum. Non-O1/O139 strains have caused

several well-studied food-borne outbreaks of gastroenteritis and have

also been responsible for sporadic cases of otitis media, wound infection, and bacteremia. Generally, non-O1/O139 V. cholerae strains do

not produce cholera toxin and do not cause large epidemics of diarrheal disease. Like other vibrios, non-O1/O139 V. cholerae organisms

are widely distributed in marine environments. In most instances,

recognized cases in the United States have been associated with the

consumption of raw oysters or with recent travel. The broad clinical

spectrum of diarrheal illness caused by these organisms is probably due

to the group’s heterogeneous virulence attributes.

In the United States, about half of all non-O1/O139 V. cholerae

isolates are from stool samples. The typical incubation period for gastroenteritis due to these organisms is <2 days, and the illness lasts for

~2–7 days. Patients’ stools may be copious and watery or may be partly

formed, less voluminous, and bloody or mucoid. Diarrhea can result

in severe dehydration. Many cases include abdominal cramps, nausea,

vomiting, and fever. Like those with cholera, patients who are seriously

dehydrated should receive oral or IV fluids; the value of antibiotics is

not clear.

Extraintestinal infections due to non-O1/O139 V. cholerae commonly follow occupational or recreational exposure to seawater.

TABLE 168-5 Features of Selected Noncholera Vibrioses

ORGANISM VEHICLE OR ACTIVITY HOST AT RISK SYNDROME

Vibrio parahaemolyticus Shellfish, seawater Normal Gastroenteritis

Seawater Normal Wound infection

Non-O1/O139 Vibrio cholerae Shellfish, travel Normal Gastroenteritis

Seawater Normal Wound infection, otitis media

Vibrio vulnificus Shellfish Immunosuppresseda Sepsis, secondary cellulitis

Seawater Normal, immunosuppresseda Wound infection, cellulitis

Vibrio alginolyticus Seawater Normal Wound infection, cellulitis, otitis

Seawater Burned, other immunosuppressed Sepsis

a

Especially with liver disease or hemochromatosis.

Source: Table 161-3 in Harrison’s Principles of Internal Medicine, 14th edition.

1310 PART 5 Infectious Diseases

Around 10% of non-O1/O139 V. cholerae isolates come from cases of

wound infection, 10% from cases of otitis media, and 20% from cases

of bacteremia (which is particularly likely to develop in patients with

liver disease). Extraintestinal infections should be treated with antibiotics. Information to guide antibiotic selection and dosing is limited,

but most strains are sensitive in vitro to tetracycline, ciprofloxacin, and

third-generation cephalosporins.

■ SPECIES ASSOCIATED PRIMARILY WITH SOFT

TISSUE INFECTION OR BACTEREMIA

(See also Chap. 129)

V. vulnificus Infection with V. vulnificus is rare, but this organism

is the most common cause of severe Vibrio infections in the United

States. Like most vibrios, V. vulnificus proliferates in the warm summer

months and requires a saline environment for growth. In the United

States, infections in humans typically occur in coastal states between

May and October and most commonly affect men >40 years of age. V.

vulnificus has been linked to two distinct syndromes: primary sepsis,

which usually occurs in patients with underlying liver disease, and primary wound infection, which generally affects people without underlying disease. (Vulnificus is Latin for “wound maker.”) Some authors have

suggested that V. vulnificus also causes gastroenteritis independent of

other clinical manifestations. V. vulnificus is endowed with a number

of virulence attributes, including a capsule that confers resistance to

phagocytosis and to the bactericidal activity of human serum as well

as a cytolysin. Measured as the 50% lethal dose in mice, the organism’s

virulence is considerably increased under conditions of iron overload;

this observation is consistent with the propensity of V. vulnificus to

infect patients who have hemochromatosis.

Primary sepsis most often develops in patients who have cirrhosis or hemochromatosis. However, V. vulnificus bacteremia can also

affect individuals who have hematopoietic disorders or chronic renal

insufficiency, those who are using immunosuppressive medications

or alcohol, or (in rare instances) those who have no known underlying disease. After a median incubation period of 16 h, the patient

develops malaise, chills, fever, and prostration. One-third of patients

develop hypotension, which is often apparent at admission. Cutaneous

manifestations develop in most cases (usually within 36 h of onset)

and characteristically involve the extremities (the lower more often

than the upper). In a common sequence, erythematous patches are

followed by ecchymoses, vesicles, and bullae. In fact, sepsis and hemorrhagic bullous skin lesions suggest the diagnosis in appropriate settings. Necrosis and sloughing may also be evident. Laboratory studies

reveal leukopenia more often than leukocytosis, thrombocytopenia,

or elevated levels of fibrin-split products. V. vulnificus can be cultured

from blood or cutaneous lesions. The mortality rate approaches 50%,

with most deaths due to uncontrolled sepsis (Chap. 304). Accordingly,

prompt treatment is critical and should include empirical antibiotic

administration, aggressive debridement, and general supportive care.

V. vulnificus is sensitive in vitro to a number of antibiotics, including

tetracycline, fluoroquinolones, and third-generation cephalosporins.

Data from animal models suggest that either a fluoroquinolone or the

combination of a tetracycline and a third-generation cephalosporin

should be used in the treatment of V. vulnificus septicemia.

V. vulnificus–associated soft tissue infection can complicate either

a fresh or an old wound that comes into contact with seawater; the

patient may or may not have underlying disease. After a short incubation period (4 h to 4 days; mean, 12 h), the disease begins with swelling,

erythema, and (in many cases) intense pain around the wound. These

signs and symptoms are followed by cellulitis, which spreads rapidly

and is sometimes accompanied by vesicular, bullous, or necrotic

lesions. Metastatic events are uncommon. Most patients have fever and

leukocytosis. V. vulnificus can be cultured from skin lesions and occasionally from the blood. Prompt antibiotic therapy and debridement

are usually curative.

V. alginolyticus First identified as a pathogen of humans in 1973,

V. alginolyticus occasionally causes eye, ear, and wound infections.

This species is the most salt-tolerant of the vibrios and can grow in salt

concentrations of >10%. Most clinical isolates come from superinfected

wounds that presumably become contaminated at the beach. Although

its severity varies, V. alginolyticus infection tends not to be serious

and generally responds well to antibiotic therapy and drainage. Cases

of otitis externa, otitis media, and conjunctivitis due to this pathogen

have been described. Tetracycline treatment usually results in cure.

V. alginolyticus is a rare cause of bacteremia in immunocompromised

hosts.

■ FURTHER READING

Domman D et al: Integrated view of Vibrio cholerae in the Americas.

Science 358:789, 2017.

Islam MS et al: Environmental reservoirs of Vibrio cholera. Vaccine

38(Suppl 1):A52, 2020.

Qadri F et al: Emergency deployment of oral cholera vaccine for the

Rohingya in Bangladesh. Lancet 391:1877, 2018.

Qadri F et al: Efficacy of a single-dose regimen of inactivated wholecell oral cholera vaccine: Results from 2 years of follow-up of a randomised trial. Lancet Infect Dis 18:666, 2018.

Weill FX et al: Genomic history of the seventh pandemic of cholera in

Africa. Science 358:785, 2017.

World Health Organization: Cholera vaccines: WHO position

paper. Wkly Epidemiol Rec 92:477, 2017.

■ DEFINITION

Brucellosis is a bacterial zoonosis transmitted directly or indirectly to

humans from infected animals, predominantly domesticated ruminants and swine. The disease is known colloquially as undulant fever

because of its remittent character. Although brucellosis commonly

presents as an acute febrile illness, its clinical manifestations vary

widely, and definitive signs indicative of the diagnosis may be lacking.

Thus the clinical diagnosis usually must be supported by the results of

bacteriologic and/or serologic tests.

■ ETIOLOGIC AGENTS

Human brucellosis is caused by strains of Brucella, a bacterial genus

that was previously suggested, on genetic grounds, to comprise a single

species, B. melitensis, with a number of biologic variants exhibiting

particular host preferences. This view was challenged on the basis of

detailed differences in chromosomal structure and host preference.

The traditional classification into nomen species is now favored both

because of these differences and because this classification scheme

closely reflects the epidemiologic patterns of the infection. The nomen

system recognizes B. melitensis, which is the most common cause of

symptomatic disease in humans and for which the main sources are

sheep, goats, and camels; B. abortus, which is usually acquired from

cattle or buffalo; B. suis, which is generally acquired from swine but has

one variant enzootic in reindeer and caribou and another in rodents;

and B. canis, which is acquired most often from dogs. B. ovis, which

causes reproductive disease in sheep, has not been clearly implicated

in human disease, while rare human infections have been reported

with B. neotomae, which is found in desert rodents. Two relatively

new species, B. ceti and B. pinnipedialis, have been identified in marine

mammals, including seals and dolphins. At least one case of laboratory-acquired human disease due to one of these species has been

described, and several cases of natural human infection have been

reported. As infections in marine mammals appear to be widespread,

more cases of zoonotic infection in humans may be identified. Other

newly reported species include B. microti (isolated from field voles),

169 Brucellosis

Nicholas J. Beeching

1311CHAPTER 169 Brucellosis

B. suis in several countries, including Australia. Family members of

individuals involved in animal husbandry may be at risk, although it

is often difficult to differentiate food-borne infection from environmental contamination under these circumstances. Laboratory workers

who handle cultures or infected samples also are at risk. Travelers and

urban residents usually acquire the infection through consumption

of contaminated foods. In countries that have eradicated the disease,

new cases are most commonly acquired abroad. Dairy products, especially soft cheeses, unpasteurized milk, and ice cream, are the most

frequently implicated sources of infection; raw meat and bone marrow

may be sources under exceptional circumstances. Infections acquired

through cosmetic treatments using materials of fetal origin have

been reported. Person-to-person transmission is extremely rare, as is

transfer of infection by blood or tissue donation. Although brucellosis

is a chronic intracellular infection, there is no evidence for increased

prevalence or severity among individuals with HIV infection or with

immunodeficiency or immunosuppression of other etiologies.

Brucellosis may be acquired by ingestion, inhalation, or mucosal or

percutaneous exposure. Accidental injection or ingestion of the live

vaccine strains of B. abortus (S19 and RB51) and B. melitensis (Rev 1)

can cause disease. B. melitensis and B. suis have historically been developed as biological weapons by several countries and could be exploited

for bioterrorism (Chap. S3). This possibility should be borne in mind

in the event of sudden unexplained outbreaks.

■ IMMUNITY AND PATHOGENESIS

Exposure to brucellosis elicits both humoral and cell-mediated

immune responses. The mechanisms of protective immunity against

human brucellosis are presumed to be similar to those documented in

laboratory animals, but such generalizations must be interpreted with

caution. The response to infection and its outcome are influenced by

the virulence, phase, and species of the infecting strain. Differences

have been reported between B. abortus and B. suis in modes of cellular entry and subsequent compartmentalization and processing.

Antibodies promote clearance of extracellular brucellae by bactericidal

action and by facilitation of phagocytosis by polymorphonuclear and

mononuclear phagocytes; however, antibodies alone cannot eradicate

infection. Organisms taken up by macrophages and other cells can

establish persistent intracellular infections. The key target cell is the

macrophage, and bacterial mechanisms for suppressing intracellular

killing and apoptosis result in very large intracellular populations.

Opsonized bacteria are actively phagocytosed by neutrophilic granulocytes and by monocytes. In these and other cells, initial attachment

takes place via specific receptors, including Fc, C3, fibronectin, and

mannose-binding proteins. Opsonized—but not unopsonized—bacteria

trigger an oxidative burst inside phagocytes. Unopsonized bacteria are

internalized via similar receptors but at much lower efficiency. Smooth

strains enter host cells via lipid rafts. Smooth lipopolysaccharide (LPS),

β-cyclic glucan, and possibly an invasion–attachment protein (IalB) are

involved in this process. Tumor necrosis factor α (TNF-α) produced

early in the course of infection stimulates cytotoxic lymphocytes and

activates macrophages, which can kill intracellular brucellae (probably

mainly through production of reactive oxygen and nitrogen intermediates) and may clear infection. However, virulent Brucella cells can

suppress the TNF-α response, and control of infection in this situation

depends on macrophage activation and interferon γ (IFN-γ) responses.

Cytokines such as interleukin 12(IL-12) promote production of

IFN-γ, which drives TH1-type responses and stimulates macrophage

activation. Inflammatory cytokines, including IL-4, IL-6, and IL-10,

downregulate the protective response. As in other types of intracellular

infection, it is assumed that initial replication of brucellae takes place

within cells of the lymph nodes draining the point of entry. Subsequent

hematogenous spread may result in chronic localizing infection at

almost any site, although the reticuloendothelial system, musculoskeletal tissues, and genitourinary system are most frequently targeted. Both

acute and chronic inflammatory responses develop in brucellosis, and

the local tissue response may include granuloma formation with or

without necrosis and caseation. Abscesses may also develop, especially

in chronic localized infection.

B. papionis (from baboons), B. vulpis (from foxes), and B. inopinata

(from a patient with a breast implant). Additional novel strains have

been described in diverse species, including frogs, bats, and various

rodents, and the genus likely will expand further in forthcoming years.

Moreover, it has become apparent that Brucella is closely related to the

genus Ochrobactrum, which includes environmental bacteria sometimes associated with opportunistic infections. Genomics-based studies are beginning to elucidate the pathway of evolution from free-living

soil bacteria to highly successful intracellular pathogens.

All brucellae are small, gram-negative, unencapsulated, nonsporulating rods or coccobacilli. They grow aerobically on peptone-based

medium incubated at 37°C; the growth of some types is improved

by supplementary CO2

. In vivo, brucellae behave as facultative intracellular parasites. The organisms are sensitive to sunlight, ionizing

radiation, and moderate heat; they are killed by boiling and pasteurization but are resistant to freezing and drying. Their resistance to

drying renders brucellae stable in aerosol form, facilitating airborne

transmission. The organisms can survive for up to 2 months in soft

cheeses made from goat’s or sheep’s milk; for at least 6 weeks in dry

soil contaminated with infected urine, vaginal discharge, or placental

or fetal tissues; and for at least 6 months in damp soil or liquid manure

kept in cool dark conditions. Brucellae are easily killed by a wide range

of common disinfectants used under optimal conditions but are likely

to be much more resistant at low temperatures or in the presence of

heavy organic contamination.

■ EPIDEMIOLOGY

Brucellosis is a zoonosis whose occurrence and control are closely

related to its prevalence in domesticated animals. Its distribution is

worldwide apart from the few countries where it has been eradicated

from the animal reservoir. The true global prevalence of human brucellosis is unknown because of the imprecision of diagnosis and the

inadequacy of reporting and surveillance systems in many countries.

Recently, there has been increased recognition of the high incidence

of brucellosis in India, Pakistan, Sri Lanka and parts of China, and of

importations to countries in Oceania, such as Fiji, and in Asia, such

as Thailand and Vietnam. In Europe, the incidence of brucellosis in

a country is inversely related to gross domestic product, and, in both

developed and less well-resourced settings, human brucellosis is related

to rural poverty and inadequate access to medical care. Failure of

veterinary control programs due to conflicts or for economic reasons

contributes further to the emergence and re-emergence of disease, as

seen currently in some eastern Mediterranean countries.

Even in well-resourced settings, the true incidence of brucellosis in

domesticated animals may be 10–20 times higher than the reported

figures. Bovine brucellosis has been the target of control programs

in many parts of the world and has been eradicated from the cattle

populations of much of northern Europe, Australia, New Zealand,

and Canada, among other nations. Its incidence has been reduced to

a low level in the United States and most western European countries,

with a varied picture in other parts of the world. Efforts to eradicate

B. melitensis infection from sheep and goat populations have been

much less successful. These efforts have relied heavily on vaccination

programs, which have tended to fluctuate with changing economic

and political conditions. In some countries (e.g., Israel), B. melitensis

has caused serious outbreaks in cattle. Infections with B. melitensis

still pose a major public health problem in Mediterranean countries;

in western, central, and southern Asia; and in parts of Africa and

South and Central America. Infections with B. abortus are common

in cattle-rearing communities in African countries such as Kenya and

Uganda. Canine infection with B. canis is present on most continents—

the incidence appears to be increasing in North America and in

several European countries, often associated with importation of dogs

from an endemic area.

Human brucellosis is usually associated with occupational or

domestic exposure to infected animals or their products. Farmers,

shepherds, goatherds, veterinarians, and employees in slaughterhouses and meat-processing plants in endemic areas are occupationally exposed to infection. Feral pig hunters are at risk of infection with

1312 PART 5 Infectious Diseases

The determinants of pathogenicity of Brucella have not been fully

characterized, and the mechanisms underlying the manifestations of

brucellosis are incompletely understood. The organism is a “stealth”

pathogen whose survival strategy is centered on several processes

that avoid triggering innate immune responses and that permit

survival within monocytic cells. These processes include evasion of

intracellular destruction by restricting the fusion of type IV secretion

system–dependent Brucella-containing vacuoles with lysosomal compartments, inhibition of apoptosis of infected mononuclear cells, and

prevention of dendritic cell maturation, antigen presentation, and activation of naïve T cells. The smooth Brucella LPS, which has an unusual

O-chain and core-lipid composition, has relatively low endotoxin

activity and plays a key role in pyrogenicity and in resistance to phagocytosis and serum killing in the nonimmune host. In addition, LPS

is believed to play a role in suppressing phagosome–lysosome fusion

and diverting the internalized bacteria into vacuoles located in endoplasmic reticulum, where intracellular replication takes place. Specific

exotoxins have not been isolated, but a type IV secretion system (VirB)

that regulates intracellular survival and trafficking has been identified.

In B. abortus this system can be activated extracellularly, but in B. suis

it is activated (by low pH) only during intracellular growth. Brucellae then produce acid-stable proteins that facilitate the organisms’

survival in phagosomes and may enhance their resistance to reactive

oxygen intermediates. A type III secretion system based on modified

flagellar structures also has been inferred, although not yet confirmed.

Virulent brucellae are resistant to defensins and produce a Cu-Zn

superoxide dismutase that increases their resistance to reactive oxygen

intermediates. A hemolysin-like protein may trigger the release of brucellae from infected cells.

■ CLINICAL FEATURES

Brucellosis almost invariably causes fever, which may be associated

with profuse sweats, especially at night. In endemic areas, brucellosis

may be difficult to distinguish from the many other causes of fever.

However, two features recognized in the nineteenth century distinguish brucellosis from other tropical fevers, such as typhoid and

malaria: (1) Left untreated, the fever of brucellosis shows an undulating

pattern that persists for weeks before the commencement of an afebrile

period that may be followed by relapse. (2) The fever of brucellosis is

associated with musculoskeletal symptoms and signs in about one-half

of all patients.

The clinical syndromes caused by the different nomen species are

similar, although B. melitensis tends to be associated with a more acute

and aggressive presentation and B. suis with focal abscess induction.

B. abortus infections may be more insidious in onset and more likely to

become chronic. B. canis infections are generally regarded as less severe

but, like other species, can cause serious disease such as endocarditis.

The incubation period varies from 1 week to several months,

and the onset of fever and other symptoms may be abrupt or insidious. In addition to experiencing fever and sweats, patients become

increasingly apathetic and fatigued; lose appetite and weight; and have

nonspecific myalgia, headache, and chills. Overall, the presentation of

brucellosis often fits one of three patterns: febrile illness that resembles

typhoid but is less severe; fever and acute monoarthritis, typically of the

hip or knee, in a young child; and long-lasting fever, misery, and lowback or hip pain in an older man. In an endemic area (e.g., much of the

Middle East), a patient with fever and difficulty walking into the clinic

would be regarded as having brucellosis until it was proven otherwise.

Diagnostic clues in the patient’s history include travel to an endemic

area, employment in a diagnostic microbiology laboratory, consumption of unpasteurized milk products (including soft cheeses), contact

with animals, accidental inoculation with veterinary Brucella vaccines,

and—in an endemic setting—a history of similar illness in the family

(documented in almost 50% of cases). Focal features are present in the

majority of patients. The most common are musculoskeletal pain and

physical findings in the peripheral and axial skeleton (~40% of cases).

Osteomyelitis more commonly involves the lumbar and low thoracic

vertebrae than the cervical and high thoracic spine. Individual joints

that are most commonly affected by septic arthritis are the knee, hip,

sacroiliac, shoulder, and sternoclavicular joints; the pattern may be one

of monoarthritis or polyarthritis. Osteomyelitis may also accompany

septic arthritis.

In addition to the usual causes of vertebral osteomyelitis or septic

arthritis, the most important disease in the differential diagnosis is

tuberculosis. This point influences the therapeutic approach as well

as the prognosis, given that several antimicrobial agents used to treat

brucellosis are also used to treat tuberculosis. Septic arthritis in brucellosis progresses slowly, starting with small pericapsular erosions. In the

vertebrae, anterior erosions of the superior end plate are typically the

first features to become evident, with eventual involvement and sclerosis of the whole vertebra. Anterior osteophytes eventually develop, but

vertebral destruction or impingement on the spinal cord is rare and

usually suggests tuberculosis (Table 169-1).

Other systems may be involved in a manner that resembles typhoid.

About one-quarter of patients have a dry cough, usually with few

changes visible on the chest x-ray, although pneumonia, empyema,

intrathoracic adenopathy, or lung abscess can occur. Sputum or pleural

effusion cultures are rarely positive in such cases, which respond well

to standard brucellosis treatment. One-quarter of patients have hepatosplenomegaly, and 10%–20% have significant lymphadenopathy; the

differential diagnosis includes glandular fever–like illness such as that

caused by Epstein-Barr virus, Toxoplasma, cytomegalovirus, HIV, or

Mycobacterium tuberculosis. Up to 10% of men have acute epididymo-orchitis, which must be distinguished from mumps and from surgical problems such as torsion. Prostatitis, inflammation of the seminal

vesicles, salpingitis, and pyelonephritis all occur. There is an increased

incidence of fetal loss among infected pregnant women, although teratogenicity has not been described and the tendency toward abortion is

much less pronounced in humans than in farm animals.

Neurologic involvement is common, with depression and lethargy

whose severity may not be fully appreciated by either the patient or

the physician until after treatment. A small proportion of patients

develop lymphocytic meningoencephalitis that mimics neurotuberculosis, atypical leptospirosis, or noninfectious conditions and that

may be complicated by intracerebral abscess, a variety of cranial nerve

deficits, or ruptured mycotic aneurysms.

Endocarditis occurs in ~1% of cases, most often affecting the aortic

valve (natural or prosthetic). Any site in the body may be involved in

metastatic abscess formation or inflammation; the female breast and

the thyroid gland are affected particularly often. Nonspecific maculopapular rashes and other skin manifestations are uncommon and are

rarely noticed by the patient even if they develop.

■ DIAGNOSIS

Because the clinical picture of brucellosis is not distinctive, the diagnosis must be based on a history of potential exposure, a presentation

TABLE 169-1 Radiology of the Spine: Differentiation of Brucellosis

from Tuberculosis

BRUCELLOSIS TUBERCULOSIS

Site Lumbar and others Dorsolumbar

Vertebrae Multiple or contiguous Contiguous

Diskitis Late Early

Body Intact until late Morphology lost early

Canal compression Rare Common

Epiphysitis Anterosuperior General: upper and lower

disk regions, central,

subperiosteal

Osteophyte Anterolateral (parrot

beak)

Unusual

Deformity Wedging uncommon Anterior wedge, gibbus

Recovery Sclerosis, whole-body Variable

Paravertebral abscess Small, well-localized Common and discrete

loss, transverse process

Psoas abscess Rare More likely

1313CHAPTER 169 Brucellosis

consistent with the disease, and supporting laboratory findings.

Results of routine biochemical assays are usually within normal limits, although serum levels of hepatic enzymes and bilirubin may be

elevated. Peripheral leukocyte counts are usually normal or low, with

relative lymphocytosis. Mild anemia may be documented. Thrombocytopenia and disseminated intravascular coagulation with raised

levels of fibrinogen degradation products can develop. The erythrocyte

sedimentation rate and C-reactive protein levels are often normal but

may be raised.

In body fluids such as cerebrospinal fluid (CSF) or joint fluid, lymphocytosis and low glucose levels are the norm. Elevated CSF levels of

adenosine deaminase cannot be used to distinguish tubercular meningitis, as they may also be found in brucellosis. Biopsied samples of

tissues such as lymph node or liver may show noncaseating granulomas

without acid/alcohol-fast bacilli. The radiologic features of bony disease develop late and are much more subtle than those of tuberculosis

or septic arthritis of other etiologies, with less bone and joint destruction. Isotope scanning is more sensitive than plain x-ray and continues

to give positive results long after successful treatment.

Isolation of brucellae from blood, CSF, bone marrow, or joint fluid

or from a tissue aspirate or biopsy sample is definitive, and attempts

at isolation are usually successful in 50%–70% of cases. Blood culture

using modern nonradiometric or similar signaling systems (e.g., Bactec)

usually become positive within 7 days. Clinicians should alert the

laboratory to the possibility of brucellosis if suspected, as all cultures

should be handled under containment conditions appropriate for dangerous pathogens. Brucella species may be misidentified as Agrobacterium, Ochrobactrum, or Psychrobacter (Moraxella) phenylpyruvicus

by the gallery identification strips commonly used in the diagnostic

laboratory. In recent years, matrix-assisted laser desorption ionization

time-of-flight mass spectrometry (MALDI-TOF MS) has emerged as a

powerful tool in bacterial identification. The relative homogeneity of

classical Brucella species makes identification beyond the genus level

by routine approaches challenging, although further improvements

may facilitate discrimination at the species level, particularly in reference laboratories. The place of this technique in routine diagnostic

practice will depend on further refinements. Meanwhile, the author

is aware of cases in which blood culture isolates have been identified

incorrectly using MALDI-TOF MS.

The peripheral blood–based polymerase chain reaction (PCR)

has enormous potential to detect bacteremia, to predict relapse, and

to exclude “chronic brucellosis.” This method is more sensitive and

is certainly quicker than blood culture, and it does not carry the

attendant biohazard risk posed by culture. Nucleic acid amplification

techniques are now quite widely used, although no single standardized

procedure has been adopted. Primers for the spacer region between

the genes encoding the 16S and 23S ribosomal RNAs (rrs-rrl), various

outer-membrane protein–encoding genes, the insertion sequence

IS711, and the protein BCSP31 are sensitive and specific. Blood and

other tissues are the most suitable samples for analysis. The clinical

significance of prolonged PCR positivity, commonly seen in blood after

successful treatment, remains controversial.

Serologic examination often provides the only positive laboratory

findings in brucellosis. In acute infection, IgM antibodies appear early

and are followed by IgG and IgA. All these antibodies are active in

agglutination tests, whether performed by tube, plate, or microagglutination methods. The majority of patients have detectable agglutinins

at this stage. As the disease progresses, IgM levels decline, and the

avidity and subclass distribution of IgG and IgA change. The result

is reduced or undetectable agglutinin titers. However, the antibodies

are detectable by alternative tests, including the complement fixation

test, Coomb’s antiglobulin test, and enzyme-linked immunosorbent

assays. There is no clear cutoff value for a diagnostic titer. Rather,

serology results must be interpreted in the context of exposure history

and clinical presentation. In endemic areas or in settings of potential

occupational exposure, agglutinin titers of 1:320–1:640 or higher are

considered diagnostic; in nonendemic areas, a titer of ≥1:160 is considered significant. Repetition of tests after 2–4 weeks may demonstrate

a rising titer.

In most centers, the standard agglutination test (or a derivative such

as the microagglutination test) is still the mainstay of serologic diagnosis. In an endemic setting, >90% of patients with acute bacteremia

have standard agglutination titers of at least 1:320 at the time of clinical

presentation. Some investigators rely on the Rose Bengal test, which

has been only partially validated for human diagnostic use but can be

used for screening. Dipstick assays for anti-Brucella IgM have been

developed but are uncommonly utilized. Other near-patient or pointof-care tests are still in developmental stages.

Antibody to the Brucella LPS O chain—the dominant antigen—is

detected by all the conventional tests that employ smooth B. abortus

cells as antigen. Because B. abortus cross-reacts with B. melitensis

and B. suis, there is no advantage in replicating the tests with these

antigens. Cross-reactions also occur with the O chains of some other

gram-negative bacteria, including Yersinia enterocolitica O:9, Escherichia coli O157, Francisella tularensis, Salmonella enterica group N,

Stenotrophomonas maltophilia, and Vibrio cholerae. Cross-reactions

do not occur with the cell-surface antigens of rough Brucella strains

such as B. canis or B. ovis; serologic tests for these nomen species must

employ an antigen prepared from either one. Similarly, the live B.

abortus vaccine strain RB51 does not elicit antibody responses in serologic tests that use smooth antigens, and this fact must be taken into

account if serologic tests are employed in attempts to identify or follow

the course of infections in persons accidentally exposed to the vaccine.

TREATMENT

Brucellosis

The broad aims of antimicrobial therapy are to treat and relieve the

symptoms of current infection and to prevent relapse. Focal disease

presentations may require specific intervention in addition to more

prolonged and tailored antibiotic therapy. In addition, tuberculosis must always be excluded, or—to prevent the emergence of

resistance—therapy must be tailored to specifically exclude drugs

active against tuberculosis (e.g., rifampin used alone) or to include

a full antituberculous regimen.

Early experience with streptomycin monotherapy showed that

relapse was common; thus dual therapy with tetracyclines became

the norm. This is still the most effective combination, but alternatives may be used, with the options depending on local or national

policy about the use of rifampin for the treatment of nonmycobacterial infection. For the several antimicrobial agents that are

active in vivo, efficacy can usually be predicted by in vitro testing.

However, numerous Brucella strains show in vitro sensitivity to a

whole range of antimicrobials that are therapeutically ineffective,

including assorted β-lactams. Moreover, the use of fluoroquinolones remains controversial despite the good in vitro activity and

white-cell penetration of most agents of this class. Low intravacuolar

pH is probably a factor in the poor performance of these drugs.

For adults with acute nonfocal brucellosis (duration, <1 month),

a 6-week course of therapy incorporating at least two antimicrobial agents is required. Complex or focal disease may necessitate

≥3 months of therapy. Adherence to the therapeutic regimen is very

important, and poor adherence underlies almost all cases of apparent treatment failure; such failure is rarely due to the emergence of

drug resistance, although increasing resistance to trimethoprimsulfamethoxazole (TMP-SMX) has been reported at one center.

There is good retrospective evidence that a 3-week course of two

agents is as effective as a 6-week course for treatment and prevention of relapse in children, but this has not yet been investigated in

prospective studies.

The gold standard for the treatment of brucellosis in adults is IM

streptomycin (0.75–1 g daily for 14–21 days) together with doxycycline (100 mg twice daily for 6 weeks). In both clinical trials and

observational studies, relapse follows such treatment in 5%–10% of

cases. The usual alternative regimen (and the current World Health

Organization recommendation) is rifampin (600–900 mg/d) plus

doxycycline (100 mg twice daily) for 6 weeks. The relapse/failure rate