Bacillus anthracis (Anthrax)

Epidemiology

Anthrax is a worldwide disease of domesticated and wild animals that may secondarily infect humans. Estimates of worldwide cases vary widely, but it is estimated by the World Health Organization (WHO) that there are 2000 to 20,000 human cases per year ( Fig. 207.1 ). Although cases occur worldwide, there is little genetic diversity among isolates. Examination of variable number tandem repeats loci identifies six major clones among two branches. Based on identification of variable number tandem repeats in different geographic areas, it appears that southern Africa has the greatest diversity of strains and is believed to be the geographic origin of B. anthracis. The actual number of anthrax cases worldwide has been difficult to ascertain owing to poor reporting, but anthrax in animals has been reported from 82 nations. It is significantly more common among grazing herbivores in some areas of the Middle East, Africa, and Latin America than in more developed countries. The enormous areas of savanna and large populations of ungulate herbivores in southern Africa provide an ideal environment for the development of anthrax. In 1923 in South Africa, it was estimated that 30,000 to 60,000 animals died of anthrax. In the last decade, isolates of Bacillus cereus strains possessing the virulence plasmids of B. anthracis and expressing the anthrax toxins and capsule have been obtained from diverse species of animals in tropical rainforests in sub-Saharan Africa and may threaten the survival of chimpanzees in that area. No human cases with these strains have been reported to date.

The largest human outbreak of anthrax occurred in Zimbabwe during the years 1979–85 with approximately 10,000 reported cases and 182 deaths. These cases, almost all of which were cutaneous, were associated with cattle ranching and lapsed veterinary control practices during the civil war that established the country. Anthrax remains enzootic in much of sub-Saharan Africa with continued cases in wildlife and livestock and more cases in humans than in most of the rest of the world combined.

In most of Europe and North America, human cases are rare, and animal cases are sporadic and uncommon. A single animal death usually is met with an intense veterinary public health response that mandates proper disposal of carcasses; decontamination of fields; and immunization of surviving, potentially exposed animals. A single human case in a nonagricultural, nonrural setting appropriately raises concern for an act of bioterrorism.

The natural cycle of anthrax infection in humans and animals is illustrated in Fig. 207.2 . Many animals that die of anthrax have a characteristic terminal hemorrhage from the nose, mouth, and anus that contains large amounts of anthrax bacilli. Animals are then infected when they graze on fields or grain contaminated with spores or through the bites of flies that have fed on infected carcasses. Seasonal variations in anthrax cases have been noted for decades. Heavy spring rains may serve to concentrate spores into low-lying areas, and if this is followed by a hot, dry period, animals grazing on these areas with high spore burdens may become infected. River beds that dry out after flooding and serve as pasture for animals have been repeatedly implicated. Additionally, in periods of drought, animals grazing on dried grasses close to the soil surface have increased oral abrasions from the dry vegetation. These abrasions provide areas for spore entry and germination and a subsequent increase in animal cases of anthrax. In 2016 anthrax reappeared in the remote Yamal Peninsula in Siberia for the first time in 75 years. As global climate change has melted permafrost, the carcasses of reindeer that had been frozen for decades have thawed and infected reindeer herds and local people with deaths of thousands of reindeer and one human. The Siberian experience demonstrates the resilience of anthrax spores in the right environmental settings. Naturally acquired human cases are usually associated with exposure to infected animals or contaminated animal products. Numerous products have been implicated in transmission to humans including wool, hair, bone and bone meal, meat, horns, and hides. The source may not be readily evident because the animal product may have been processed (e.g., shaving brushes, goat-skin drums, wool-based tapestries, and bone meal–based fertilizers). In 2013 an inhalational anthrax case occurred in an American who had traveled through four US states with endemic anthrax. Despite an extensive investigation, it appears his only risk factor was driving through herds of bison and burros a few days before developing symptoms. Transmission from flies has also been documented; biting flies may carry spores or vegetative forms from a carcass to another animal or human, and even nonbiting flies have been shown to carry B. anthracis in feces or vomit that they deposit onto vegetation. Birds such as vultures shed anthrax spores in their feces for up to 2 weeks after they ingest infected meat. In Europe, people who used injection drugs developed anthrax infections from injecting heroin contaminated with spores possibly acquired from the goat skin containers used in the transport of the drug from Turkey.

Spores are the usual infecting form of anthrax. However, ingestion of either spore or vegetative forms of B. anthracis in contaminated meat may lead to gastrointestinal infection.

The world distribution of anthrax cases in humans and animals is tracked via a geographic information system and remote sensing by WHO as part of the World Anthrax Data Site. This includes an updated nation-by-nation breakdown of cases by species and year.

Microbiology

B. anthracis, the causative agent of anthrax, is a large (1–1.5 µm × 3–8 µm), gram-positive bacillus with rapid, nonhemolytic growth on blood agar that readily forms spores in the presence of oxygen. Colonies have a characteristic “Medusa's head appearance,” sometimes also referred to as a “comet tail,” appearing slightly curled at the periphery ( Fig. 207.3A ). The white or gray-white colonies are tenacious when attempts are made to remove them from agar, and this is often described as “a whipped egg white appearance” when a loop is passed through a colony ( Fig. 207.3B ). In culture the bacilli may form long chains with prominent central or paracentral oval spores that do not cause swelling of the bacilli ( Fig. 207.4 ). In infected tissue, bacteria occur singly or in short chains of two to three bacilli without spores. In the presence of carbon dioxide in the laboratory, or of bicarbonate in tissue, B. anthracis forms a prominent poly- d -γ-glutamic acid capsule important in the inhibition of phagocytosis of the vegetative bacilli. Catalase positivity and nonmotility of organisms are further characteristics that differentiate B. anthracis from other Bacillus spp. These basic identification techniques can typically be performed in nearly all microbiology laboratories, but definitive identification of B. anthracis requires further demonstration of lysis by γ phage, detection of the capsule by fluorescent antibody, and identification of toxin genes by polymerase chain reaction (PCR) assay, usually best performed at a reference laboratory.

In contrast to growth during in vitro cultivation, B. anthracis sporulation does not occur in viable tissues until they are exposed to atmospheric levels of oxygen, typically after an infected animal has died and the carcass is opened. The spores, although sensitive to prolonged ultraviolet radiation, are extremely hardy and may survive in certain soil conditions for decades. In the interior of buildings, typically shielded from ultraviolet light, spores may also remain viable for years. Although anthrax spores have demonstrated viability in soil and carcasses for decades, and even longer in bones from an archeological site, in most environments where the organism must compete with other soil-dwelling bacteria they typically survive only for months and rarely more than 4 years. Spores may also remain dormant, but viable, in living animals for a period of at least 100 days, as demonstrated by primate studies in which viable organisms were recovered at necropsy from the lungs of apparently healthy animals, a finding that has important therapeutic implications, as discussed later.

The two major virulence factors are the antiphagocytic poly- d -γ-glutamic acid capsule, encoded on the pX02 plasmid, and two exotoxins, encoded on a separate plasmid, pX01. The anthrax toxins have been intensively studied, and components of the toxins have important use in vaccines, in diagnostics, and as targets for new adjunctive therapeutics. A schematic representation of the main anthrax toxins in shown in Fig. 207.5 .

The pX01 plasmid encodes three toxin components, known as protective antigen (PA), edema factor (EF), and lethal factor (LF), each of which individually is biologically inactive. Studies in the 1950s and 1960s established that edema toxin, composed of PA combined with EF, produced local skin edema and that lethal toxin, composed of the same PA combined with LF, was highly lethal for experimental animals. The combination of all three components was the most lethal and produced many characteristics of an actual anthrax infection.

PA, which was originally identified as an antigen that induced protective immunity in experimental animal models, attaches to the anthrax toxin receptors—tumor endothelial marker 8 (TEM8) and capillary morphogenesis protein 2 (CMG2)—on the cell surface and is cleaved by a cell surface protease into PA ₆₃ , which forms a heptamer to which up to three EF and/or LF molecules may attach. CMG2 appears to be the major toxin receptor in the mouse model in which neutrophils contribute to resistance to infection. PA may also be cleaved by a serum protease with formation of toxins in the circulation. When the PA heptamer complexes with EF, it forms edema toxin, and with LF it forms lethal toxin. The toxins are then taken into the cytosol, where they mediate cellular damage. LF is a calmodulin-dependent zinc metalloprotease that cleaves and inactivates multiple mitogen-activated protein kinases and interferes with signal transduction, whereas EF is an adenylate cyclase that increases intracellular cyclic adenosine monophosphate concentrations and interferes with cell function. The toxins have been shown in vitro to impair cell functions associated with innate immunity including neutrophil chemotaxis; phagocytosis; superoxide production; and macrophage, T and B lymphocyte, and dendritic cell function and likely affect many other cells possessing toxin receptors including endothelial cells resulting in increased vascular permeability and cells of the cardiovascular system and liver. Studies with isolated toxins suggest they both cause hypotension in experimental animals and are additive, but shock has not been a prominent finding in patients presenting with inhalational anthrax. Edema toxin was so named because of its ability to cause edema in experimental animals, and lethal toxin has also been shown to increase vascular permeability. Lethal toxin demonstrates effects on the intestinal epithelium that may allow for secondary infections with enteric pathogens. However, there are discrepancies between effects of toxins observed in cells in vitro and in vivo and findings observed in experimental infections. The cell targets and exact role and mechanisms of toxin-induced host dysfunction during infection remain under investigation, and numerous other bacterial factors contribute to virulence, although they are of less importance than the capsule and toxins.

Clinical Manifestations and Diagnosis

The clinical manifestations of anthrax in animals and humans have been well described since the 1800s, when cases were relatively common in many areas of the world. The three primary forms of anthrax are dependent on the route of exposure: cutaneous, gastrointestinal, and inhalational. In the past decade an additional form of cutaneous anthrax has been described among people who inject heroin. Heroin contaminated by B. anthracis spores may cause severe systemic infection similar to that seen in advanced inhalational anthrax. Bacteremia secondary to any of the primary forms of anthrax may lead to seeding of any site including the central nervous system (CNS), with the resulting hemorrhagic meningoencephalitis being nearly always fatal.

Online resources for clinicians considering a diagnosis of anthrax are readily available. Frequently updated and extensively referenced anthrax websites for both naturally occurring and bioterrorism-associated disease are provided by the Centers for Disease Control and Prevention (CDC) ( www.cdc.gov/anthrax/ ), the European Centre for Disease Prevention and Control ( https://ecdc.europa.eu/en/anthrax ), and the Center for Infectious Disease Research and Policy ( http://www.cidrap.umn.edu/infectious-disease-topics/anthrax ). Additional resources are provided by WHO at www.who.int/csr/disease/Anthrax/resources/en/ . The WHO site features a comprehensive handbook of WHO guidelines for anthrax in animals and humans that includes information on handling of carcasses, disinfection, and control of infections. Laboratory guidelines and guidelines for shipping and handling of clinical specimens are continually revised by the American Society for Microbiology and are available at https://www.asm.org/index.php/guidelines/sentinel-guidelines .

Cutaneous Anthrax

Naturally occurring anthrax infections in humans cause cutaneous disease in more than 95% of cases. Series of cutaneous anthrax cases from the 19th century and early 20th century that were untreated demonstrated 16% to 39% mortality. As anthrax immune serum was used for treatment in cutaneous case series in the years 1903–41, mortality decreased to 0% to 28%, although no controlled studies were reported. With penicillin treatment available, mortality rates have generally decreased to less than 5%. In a series from 1955, Gold carefully reported the findings of 117 cases of anthrax he observed near a Philadelphia goat-hair mill over 20 years. All but one case was cutaneous anthrax, and the only fatalities were the single pulmonary case and one cutaneous case. The Chinese Center for Disease Control and Prevention reported 120,111 human cases of anthrax for the years 1955–2014 with 4341 fatalities (3.6%). Of 258 confirmed Chinese anthrax cases for the years 2005–14, 98% were cutaneous anthrax, and only 2% of all anthrax cases were fatal.

As described earlier, a multitude of contaminated animal sources have been implicated in naturally acquired cutaneous anthrax in humans. After the introduction of anthrax spores into the skin, often with just trivial trauma, there is an incubation period of 1 to 19 days (more commonly 2–5 days), leading to the development of a small, pruritic papule at the inoculation site. Most lesions are on exposed areas of the head, neck, and extremities. Owing to the associated pruritus, patients (and clinicians) often attribute these painless lesions to an insect or spider bite and ignore them.

A day or two after the formation of the papule, vesicles form around the lesion and may become 1 to 2 cm in diameter. The vesicles contain a clear to serosanguineous fluid, and Gram stain reveals numerous bacilli but a paucity of leukocytes. Culture of vesicular fluid will readily yield B. anthracis in most cases in which antibiotics have not been administered. There is no purulence, and the lesions remain painless unless secondarily infected. The vesicles are thin roofed and easily rupture, leading to formation of a dark brown eschar that turns black at the base of a shallow ulcer. The ulcer is typically surrounded by an area of induration, and in some cases nonpitting edema may be marked ( Fig. 207.6 ). Most other organisms causing skin infections are not typically associated with extensive induration around a skin lesion or with frank edema, so these findings may be the first clue to the diagnosis of anthrax. As the ulcer matures, its base becomes characteristically black and is the source of the name anthrax, which derives from the Greek word anthrakis, meaning “coal.”

In uncomplicated cases (i.e., without secondary spread), lesions slowly heal over a period of 1 to 3 weeks, and the eschar loosens and falls off, typically without leaving a scar. Antibiotics do not affect the evolution of the skin lesions. In most cases, patients report associated headache, malaise, and low-grade fever even if the infection does not progress to bacteremia.

Multiple lesions may occur in some cases, probably representing multiple inoculation sites, but at other times small satellite lesions may appear around an initial isolated lesion. Serious cutaneous disease may be marked by extensive edema that involves an entire extremity or the trunk from neck to groin. This has been described as “malignant edema” and may be associated with inflammation of the overlying skin with pain, signs of toxemia, and subsequent secondary seeding of other sites as bacteremia develops; cutaneous anthrax with severe edema has been more commonly seen in injectional anthrax (see later discussion).

Untreated cutaneous disease in humans has been associated with a fatality rate of 10% to 20%, whereas treated cutaneous disease (before the onset of secondary bacteremic spread) is rarely fatal. Repeat infections are rarely reported and tend to be milder, implying some degree of acquired immunity.

Clinical characteristics that should place cutaneous anthrax high in the differential diagnosis include a painless lesion (during initial stages), the presence of edema out of proportion to the size of the lesion, and a Gram stain of vesicular fluid or ulcer swab with gram-positive rods but rare white blood cells.

Differential Diagnosis of Cutaneous Anthrax

The differential diagnosis of the unusual skin lesions associated with cutaneous anthrax includes some diagnoses uncommonly encountered by most clinicians. Brown recluse spider bites, which may also cause a black eschar and some associated edema, may be confused with cutaneous anthrax lesions. The key difference is the significant pain associated with a recluse spider bite and the absence of pain in anthrax lesions (although there may be tender adenopathy in association with an anthrax skin lesion). The differential diagnosis of cutaneous anthrax also includes tularemia, scrub typhus, rat-bite fever, blastomycosis, cutaneous fungus acquired from animals, and mycobacterial infection with Mycobacterium marinum.

Laboratory Diagnosis of Cutaneous Anthrax

Diagnostic procedures for cutaneous anthrax should preferably be performed before initiation of antibiotics because vesicular fluid and biopsy material are quickly rendered noninfectious after initiation of antibiotics. Appropriate samples for Gram stain and culture include vesicle fluid, either in a syringe or on a swab; a specimen from swabbing the edge of the base of an eschar; and material from a full-thickness punch biopsy of the edge of a vesicle and/or the center of an eschar ( Table 207.1 ). A Bacillus species in a culture specimen can be confirmed as B. anthracis by demonstration of bacterial lysis in the presence of the γ phage, detection of the capsule by direct fluorescent antibody (DFA), and identification of toxin genes by PCR assay as described later (all generally available in reference laboratories).

TABLE 207.1

Collection and Transport of Laboratory Specimens for Diagnosis of Anthrax

Modified from Center for Infectious Disease Research and Policy. Anthrax: Clinical Laboratory Testing. http://www.cidrap.umn.edu/infectious-disease-topics/anthrax#overview&1-5 .

TYPE OF ILLNESS	SPECIMEN COLLECTION AND TRANSPORT	COMMENTS
Cutaneous anthrax	All stages: Collect two swabs, one for Gram stain and culture and one for PCR assay.	Swabs: Moisten with sterile saline or water; transport in sterile tube at 2°C–8°C.
	Vesicular stage: Perform Gram stain, culture, and PCR assay of fluids from unroofed vesicle (soak two dry sterile swabs in vesicular fluid). Note: Gram stain is most sensitive during vesicular stage.	Transport swabs for PCR assay only at −70°C. Do not use transport medium. Tissue, fresh: ≥5 mm 3 ; store and transport at 2°C–8°C (≤2 h) or frozen at −70°C (>2 h).
	Eschar stage: Perform Gram stain, culture, and PCR assay of ulcer base or edge of eschar without removing it.	Tissue, preserved in 10% buffered formalin: 1.0 cm 3 ; store and transport at room temperature.
	Ulcer (no vesicle or eschar present): Swab base of ulcer with premoistened sterile saline.	Obtain biopsy specimen of lesions for histopathology, preserved in 10% buffered formalin: 0.3 mm diameter; store and transport at room temperature.
	A punch biopsy for IHC testing and a second biopsy for culture, Gram stain, PCR assay, and frozen tissue IHC if patient has not received antibiotics should be obtained on all patients with suspected cutaneous anthrax. Include skin adjacent to papule or vesicle. If vesicles and eschars are both present, separate biopsy specimens should be obtained.	Freeze serum after separation at −20°C or colder, ship on dry ice. Ship part of sample (>1.0 mL) and retain part in case of shipping problems.
	Serum: Collect acute serum within first 7 days of symptom onset and convalescent serum 14–35 days after symptom onset. Collect blood for culture and PCR assay and serum for LF detection with evidence of systemic involvement.	Obtain blood for culture per local protocol. Collect blood for PCR assay in EDTA (purple top) tube. Ship at room temperature (≤2 h transport) or 2°C–8°C (>2 h transport). Assay for serum LF toxin and presence of capsule available at CDC.
Inhalational anthrax	If sputum is being produced, collect sputum specimen for Gram stain and culture ( note : inhalational anthrax does not usually result in sputum production).	Sputum: Transport at room temperature in sterile, screw-capped container (<1 h transport time) or at 2°C–8°C (>1 h transport time).
	Obtain blood for smear, culture, and PCR assay and serum for LF detection.	Blood cultures: Obtain appropriate blood volume, number, and timing of sets per laboratory protocol; transport at room temperature.
	If a pleural effusion is present, collect a specimen for culture, Gram stain, PCR assay, and LF detection.	Blood for PCR assay: 10 mL in EDTA (for pediatric patients collect volumes allowable). Transport directly to laboratory at room temperature (2°C–8°C if transport ≥2 h).
	Collect CSF if meningeal signs are present or meningitis is suspected for culture, Gram stain, PCR assay, and LF detection.	Pleural fluid: Collect >1 mL in sterile container. Store and transport at 2°C–8°C.
	Serum: Collect acute serum within first 7 days of symptom onset and convalescent serum 14–35 days after symptom onset.	CSF: Transport directly to laboratory at room temperature, or 2°C–8°C if transport ≥2 h.
	Biopsy material: Bronchial or pleural biopsy material can be evaluated if available.	Transport serum or citrated plasma (separated and removed from clot) at 2°C–8°C (transport <2 h) or freeze at −20°C or colder (transport ≥2 h); ship on dry ice. Ship part of sample (>1.0 mL) and retain part in case of shipping problems.
		Preserve biopsy specimens in 10% buffered formalin, and transport at room temperature.
Gastrointestinal anthrax	Obtain stool specimen for culture (>5 g). Obtain rectal swab from patients unable to produce stool (insert swab 1 inch beyond anal sphincter).	Stool: Transport in sterile container unpreserved (≤1 hr transport time) or at 2°–8°C in Cary-Blair medium or equivalent (>1 hr transport time); specimen >5.0 g.
	Obtain blood for smear and culture (and possibly PCR testing and LF detection). Blood cultures most likely to yield Bacillus anthracis if taken 2–8 days postexposure and before administration of antibiotics.	Blood: Transport at room temperature.
	If ascites is present, obtain a specimen for Gram stain and culture (and possibly PCR testing and LF detection).
Anthrax meningitis	Obtain CSF specimen for Gram stain, culture, PCR assay, and LF detection. Obtain blood for Gram stain, culture, and PCR assay, and serum for LF detection.	See comments above for collection and transport of blood and CSF for Gram stain, culture, PCR assay, and LF detection.

CDC, Centers for Disease Control and Prevention; CSF, cerebrospinal fluid; EDTA, ethylenediaminetetraacetic acid; IHC, immunohistochemistry; LF, lethal factor; PCR, polymerase chain reaction.

Since the anthrax attacks in the United States in 2001, a two-component DFA assay that uses fluorescein-labeled monoclonal antibodies (MAbs) specific to the B. anthracis cell wall and capsule has been developed. Rapid PCR assays are also now available. Testing may be obtained through the CDC Laboratory Response Network (local Sentinel laboratories, city or state Reference laboratories, or CDC and military National laboratories) and some hazardous material teams. DFA and real-time PCR can be used for rapid and definitive identification of culture isolates and for presumptive identification of B. anthracis directly from clinical specimens and, in some cases, environmental samples. Caution must be used in interpreting these results because false-positive and false-negative findings may occur. In even the most experienced laboratories, cross-contamination is always a risk with PCR, and false-positive results can lead to considerable confusion.

Because blood cultures are frequently positive in cases that have progressed to sepsis, consideration should be given to obtaining blood cultures early in the evaluation, especially if there are any systemic symptoms. Automated blood culture systems commonly used in hospitals readily support growth of B. anthracis.

Culture of B. anthracis remains the gold standard for diagnosis of anthrax infections. Table 207.1 outlines diagnostic specimen preparation, handling, and testing. Despite development of molecular diagnostics for anthrax, there is still a role for serology. Three of the cutaneous cases in 2001 had no culture or PCR evidence of disease, but acute and convalescent serology demonstrated an anti-PA antibody response that confirmed the diagnosis. Acute and convalescent serum samples should be obtained for serology at 0 to 7 days of illness and at 14 to 28 days. A rapid enzyme-linked immunosorbent assay (ELISA) that measures total antibody to PA has been approved. A number of anthrax-PA ELISA kits have been approved by the US Food and Drug Administration (FDA) and can be used on serum to diagnose all types of anthrax or to demonstrate seroconversion after immunization. Retrospectively, anthrax PA antibody was detected by ELISA in 100% of both cutaneous and inhalation cases from 2001. However, ELISA is not positive early in disease; PA antibody is not detected until approximately 1 week after symptoms begin.

Anthrax diagnostics for both environmental and clinical samples continue to be developed. A number of lateral flow devices (handheld assays) designed for environmental samples are available. Although easy to perform and more rapid than other diagnostics, they are typically not as sensitive or specific as more traditional methods and should not be used on clinical specimens.

Injectional Anthrax

Injectional anthrax is an uncommon form of cutaneous anthrax that has been described in people who use injection drugs who introduced contaminated heroin either into the skin or intravenously. Both Clostridium and Bacillus spp. (usually B. cereus ) are spore-forming organisms that have been previously associated with infections in people who use injection drugs. In 2009 a few sporadic injectional anthrax cases occurred in Europe; in 2009–10 there were 119 cases among people who injected heroin in the United Kingdom, mainly in Scotland. Injectional anthrax is difficult to diagnose because skin infections are common among people who use injection drugs, but the clinical presentations of injectional anthrax cases were atypical. Skin lesions were not typical cutaneous anthrax lesions; rather, patients presented with advanced localized soft tissue infections accompanied by disproportionate edema, often with less pain than is typically associated with other serious soft tissue infections such as necrotizing fasciitis. Fever was not a prominent feature. Some patients had no localized injection-related lesions but presented with features of systemic anthrax infection; deteriorated rapidly; and died with meningitis, sepsis, and multiorgan failure. Patients were treated with conventional antibiotics, and some required extensive surgical débridement for necrosis associated with deep infections and subsequent reconstructive surgery; this is in contrast to typical cutaneous anthrax, for which surgical débridement is not recommended. Fourteen of the patients also received intravenous (IV) therapy with anthrax immune globulin (AIG).

Inhalational or Pulmonary Anthrax

Even a single case of inhalational anthrax should raise the possibility of a deliberate spread of spores because naturally occurring inhalational disease is currently extraordinarily rare. Inhalational anthrax is an exceptionally dangerous type of B. anthracis infection that in the preantibiotic era was nearly uniformly fatal. In a review of 82 reported cases of inhalational anthrax in the years 1900–2005, there was an overall 92% mortality rate despite treatment with anthrax antiserum or antibiotics or both in the majority of cases. During the 2001 anthrax attacks in the United States, 5 of the 11 (45%) patients with inhalational anthrax died despite aggressive intensive care unit (ICU) management and appropriate antibiotics. Early diagnosis, initiation of antibiotics, and aggressive management of inhalational anthrax are crucial to survival.

During the 19th century, inhalational anthrax (woolsorter's disease) occurred among factory workers handling hair, wool, or hides contaminated with anthrax spores, with studies demonstrating that as many as 50% of samples of raw materials were contaminated with spores. In the Bradford district of England, 23 cases of inhalational anthrax were reported during the year 1880. Much of our experience with naturally acquired inhalational anthrax was gained in the preantibiotic era. Studies in the 1950s revealed that during an 8-hour period mill workers inhaled hundreds of spores smaller than 5 µm, and some had positive nasal or pharyngeal cultures, and yet inhalational anthrax remained extraordinarily uncommon. In a serologic study of unvaccinated mill employees, nearly 15% had antibodies to anthrax. It is evident that there is some threshold number of spores that can be destroyed through the innate immune response even in the absence of prior immunization. As safeguards were built into the process so that wool was decontaminated before handling by workers and ventilation was improved in factories, the number of annual cases in developed countries in the second half of the 19th century decreased significantly; with the addition of vaccines and respirators in the 1950s and 1960s, cases dropped nearly to zero.

In 2005 Lucey proposed a modified three-level staging system for inhalational anthrax characterized by an early prodromal stage leading to the intermediate progressive stage followed by the late fulminant stage that has generally become accepted as reflecting the course of both terrorist-associated and recent naturally occurring inhalational anthrax and is used here. As spores are inhaled, those larger than 5 µm are captured in the upper airways and transported out of the airways via the mucociliary elevator to the mouth. Spores in particles smaller than 5 µm may reach the terminal bronchioles and alveoli, where they are quickly phagocytized by alveolar phagocytic cells and transported to draining lymph nodes and then to mediastinal lymph nodes. More recent studies have suggested that spores may be transported to lymphatics through alveolar epithelial cells more commonly than phagocytes. This early prodromal stage is a clinically silent incubation period and is the presymptomatic stage of inhalational anthrax occurring 1 to 6 days after initial exposure. Although it has been extremely rare to see inhalational cases develop more than 1 week after natural exposure, significant controversy exists about potential incubation periods of 60 days or longer after very-low-dose exposure.

The first symptoms occur in the early prodromal stage with a flulike illness characterized by low-grade fever, malaise, fatigue, and myalgias usually without upper respiratory tract symptoms. Headache may be prominent, fatigue may be profound, and blurred vision and photophobia occur in some cases. Dry cough and mild precordial discomfort are also seen in some patients. Patients may experience a biphasic illness during which they feel somewhat improved after the 2 to 3 days of the prodromal illness, whereas others progress directly to the intermediate progressive stage associated with high fever, declining pulmonary status, respiratory distress, dyspnea, marked diaphoresis, pleuritic chest pain, and confusion or syncope. Blood cultures are typically positive in this stage, and mediastinal widening and pleural effusions are noted radiographically. Diagnosis during this stage and treatment with appropriate multiple antibiotics as well as antitoxin therapies (AIG and/or MAbs) coupled with intensive supportive care are associated with survival in most cases.

With or without therapy patients may progress to the late fulminant stage (often referred to in older literature as the fulminant acute phase). These patients have some combination of respiratory failure requiring intubation, sepsis, meningitis, and multiorgan failure associated with overwhelming bacteremia/toxemia. Death frequently occurs within 24 hours. In addition to aggressive antibiotics and intensive care management, these patients should be considered for treatment with antitoxin therapy such as AIG and anthrax MAbs.

Inhalational anthrax is a mediastinal process and not a primary airspace disease. Although the majority of inhaled spores are believed to germinate into vegetative organisms while being carried to (or after arrival in) the mediastinal lymph nodes, studies in nonhuman primates have demonstrated that some spores remain dormant for weeks to months. As the vegetative bacilli destroy and burst out of the cell that transported them across the alveoli, they become encapsulated and are released into the systemic circulation, leading to seeding of multiple organs including the meninges. Vegetative bacteria reach high levels in the blood and may be visible on staining of the buffy coat. Levels of the lethal toxin may become high enough terminally that a bacteria-free serum sample may contain enough toxin to kill another animal. The initial signs and symptoms of inhalational anthrax are not very specific, and discriminating between early inhalational anthrax and influenza can be quite difficult, although the characteristic upper respiratory tract symptoms found with influenza are usually absent in anthrax.

Chest radiography, or more commonly computed tomography (CT), reveals a widened mediastinum and often bilateral pleural effusions. The progression of inhalational anthrax with chest radiographs and CT from a 2001 bioterrorism case is seen in Fig. 207.7 . Before the bioterrorist-associated anthrax cases in 2001, it was generally accepted that pulmonary infiltrates or consolidation were not typically prominent in inhalational anthrax (because it is not a primary parenchymal lung disease), but 7 of 10 inhalational cases in the 2001 attacks were noted to have pulmonary infiltrates. However, it was found that areas of pulmonary infiltrate on chest radiography actually corresponded to pulmonary edema and hyaline membrane formation at necropsy, not pneumonia with bacterial infiltration of the alveoli. Pleural effusions are seen in most cases and are typically serosanguineous or hemorrhagic. They may rapidly reaccumulate after thoracentesis, requiring drainage with tube thoracostomy. Adequate pleural fluid drainage is important to achieve because it was associated with a significant survival advantage in the meta-analysis of 82 inhalational cases.

Diagnosis of Inhalational Anthrax

Table 207.1 outlines guidelines for diagnostic specimen preparation, handling, and testing, which are generally identical to guidelines described for cutaneous anthrax earlier. Although inhalational anthrax is typically not associated with a cough productive of sputum, if sputum is produced, it should be sent for Gram stain, culture, and PCR analysis. Pleural fluid is more frequently present and should be sent for diagnostic testing because it is much more likely to yield bacilli on staining, culture, or PCR assay. Much more commonly than in cutaneous anthrax, the diagnosis of inhalational anthrax is made by finding positive blood cultures, and these should be obtained before any antibiotics are administered. Especially in patients who have received antibiotics, blood samples should be sent for PCR assay and antigen detection. Buffy coat smears can also be examined for the presence of bacilli, an ominous sign that the patient has entered the late fulminant stage of anthrax. Immunohistologic studies of tissue specimens for the presence of bacillus cell wall and capsule antigens may be of particular value in treated patients because results may be positive when culture, Gram stain, and PCR are negative.