Following the September 11, 2001, attack on the World Trade Center towers and the October 4, 2001, discovery of a case of inhalational anthrax in Florida, the practice of medicine in the United States changed significantly. Terms such as biological warfare and bioterrorism that were previously of historical or theoretical interest are now integrated into daily discussion and medical practice.

Medical Aspects of Bioterrorism

The traditional list of microorganisms and toxins associated with biological warfare includes dozens of agents that cause human, animal, and plant diseases. In 2000, the U.S. Centers for Disease Control and Prevention (CDC) reclassified the most important human pathogens into three categories, A, B, and C. Category A is the highest priority and includes the agents that are easiest to disseminate and transmit and cause the greatest public health crisis (Table 16). Category B agents are second-highest priority, are moderately easy to disseminate, and cause moderate morbidity and mortality. Category C includes emerging pathogens that could potentially be developed into bioweapons. Only Category A agents are reviewed in this syllabus.

Many of the diseases on the list are uncommon, making recognition difficult for most clinicians. Classic symptoms may also be altered because of different methods of dissemination or molecular engineering of the pathogen. Therefore, recognition of a bioterrorism event depends on prodromal surveillance and awareness by physicians and other health care providers. Epidemiologic clues include clusters of disease unusual for a season of the year or geographic location (for example, an urban tularemia outbreak).

Bioterrorism agents are odorless, colorless, and invisible. Dispersion can be by point source, such as letters sent in the mail that contain anthrax, or line source, such as crop dusters or other spraying devices. Rapid reporting of suspected bioterrorism events to local public health officials is crucial in order to initiate a prompt investigation and response. Any community undergoing an attack will need additional support from state and federal agencies.

Smallpox

Smallpox accounted for more deaths worldwide in the 20th century than deaths from all wars and epidemics during the same time period. The last naturally occurring case was in Somalia in 1977, and the world was declared disease free in 1980 (Breman and Henderson). The United States and Russia are known to have retained stock cultures of smallpox virus. However, it is not known whether other countries or groups also have access to smallpox cultures.

Smallpox is a member of the Poxviridae family of single-stranded DNA viruses in the same family as monkeypox and vaccinia. No natural non-human reservoir exists. Transmission is by respiratory droplets from an infected person to close contacts and can primarily occur after the person develops a rash. The incubation period is 7 to 17 days with a febrile prodrome characterized by severe headache and backache. After 2 to 3 days of fever, an oral enanthem occurs, followed within 1 day by macules over the face and extremities. The macules then evolve to papules and then to pustules. The discrete pustules measure 4 to 6 mm in diameter and are subepidermal with normal skin between lesions. The lesions occur in synchronous crops on each body area, and distribution is centrifugal or spread from the face to the extremities. The trunk is spared. Lesions often occur on the palms and soles. The pustular phase persists for 5 to 8 days, followed by crusting and eventual separation of the crust. The patient is contagious from the first appearance of the rash until the final sloughing of all crusts.

Diagnosis is made on clinical grounds. Confirmatory studies by electron microscopy, polymerase chain reaction, cell culture, and other techniques are available at the CDC.

No antiviral agents are known to be effective for treating smallpox. Cidofovir has been tested in an animal model with possible modest success, but its association with renal toxicity may limit this drug’s usefulness. Vaccinia immune globulin is not effective for treating active smallpox infection. Routine care is supportive and includes nutritional, hemodynamic, and volume support as well as prevention and treatment of secondary bacterial infections. Because of advances in nonspecific supportive care, it is difficult to predict the case fatality rate based on current medical practices.

Vaccination confers immunity in 95% of the population. Vaccination of the United States population ended in 1972, leaving all people under 30 years of age susceptible to smallpox. Persistence of immunity wanes after 5 to 10 years, leaving previously vaccinated people partially protected.

Pre-exposure prophylactic vaccination is relatively contraindicated for persons with altered immune states, such as AIDS patients, transplant recipients, pregnant women, and persons with chronic eczema (Bartlett et al.). However, an immunocompromised state is not a contraindication to vaccination in the event of an actual exposure because the risk of complications is much less than the risk of disease. Complications include accidental inoculation caused by contact with virus shed from vaccinated individuals to susceptible persons, disseminated vaccinia, vaccinia necrosum, eczema vaccinatum, encephalitis, and death (Sepkowitz). Severe complications are treated with vaccinia immune globulin, which, at the time of this writing, is in short supply. Vaccination within 3 days of exposure to smallpox is highly effective in preventing or modifying the disease.

Anthrax

Anthrax causes both human and animal disease and is endemic worldwide, including North America. Cutaneous infection occurs most commonly. Inhalational infection (woolsorter’s disease) is rare. Only 18 cases were reported in the United States in the 20th century (Swartz). The gastrointestinal form is very uncommon and has never been diagnosed in the United States. Bioterrorism-related anthrax, such as the recent occurrence of contaminated letters, produces both a cutaneous infection (Freedman et al.) and a more lethal inhalational infection (Borio et al.; Bush et al.; Mayer et al.).

Anthrax is caused by a gram-positive, spore-forming rod, Bacillus anthracis. Inhalational anthrax occurs when spores measuring 3 to 5 μ are deposited directly into the alveoli, where they are phagocytized and carried to regional lymph nodes. High-grade bacteremia soon follows with the production of three toxins (protective antigen, edema factor, and lethal factor). Protective antigen binds to the target cell and facilitates endocytosis of edema factor and lethal factor. Virulence is also enhanced by an antiphagocytic capsule (Inglesby et al.).

Based on animal data, the estimated LD₅₀ (median lethal dose) for humans is 2500 to 55,000 inhaled spores. However, as few as one to three spores may be sufficient to cause disease in some patients, as is suspected to have occurred in the two fatal cases in New York City (Mina et al.) and Connecticut (Barakat et al.) in 2001.

The incubation period following inhalation of anthrax spores is typically 6 days but may vary according to the level of exposure. In animal studies and the human outbreak in Sverdlovsk (a town in the former Soviet Union where weaponized anthrax was accidentally released into the air), the incubation period has been as long as 100 days. Initial symptoms include fever, malaise, headache, dry cough, and abdominal discomfort (Jernigan et al.). Typical influenza-related symptoms such as rhinorrhea and sore throat are uncommon. Patients may transiently improve without therapy but soon develop rapid deterioration with respiratory difficulties and shock. The biphasic presentation associated with anthrax makes early diagnosis difficult based on clinical grounds alone.

During the recent anthrax outbreak in the United States, all patients had abnormal chest radiographs with airspace disease, a widened mediastinum, and pleural effusions (Earls et al.). Any patient with suspicious chest radiograph findings should have a follow-up CT scan of the chest, which may show hyperattenuation of mediastinal or hilar lymphadenopathy or mediastinal hemorrhage. After symptom onset, diagnosis may be confirmed by blood cultures, which are frequently positive in less than 12 hours if antibiotics have not been administered. Level A hospital laboratories can isolate the organism from routine cultures and provide presumptive identification. Confirmation tests are performed at Level B laboratories. Polymerase chain reaction and specific antigen staining assays are available at the CDC.

Naturally occurring anthrax strains are susceptible to many antibiotics, including penicillins, tetracyclines, and fluoroquinolones. All strains are resistant to cephalosporins, and some strains produce an inducible penicillinase. Empiric therapy should be started as soon as the diagnosis is suspected, as survival is related to the time from onset of symptoms to administration of antibiotics. In patients with significant symptoms, ciprofloxacin or doxycycline should be combined with clindamycin, which is a potent inhibitor of toxin production. Vancomycin or rifampin may be added to cover central nervous system infections, which may occur in up to 50% of patients. Specific antibiotic therapy should be based on antibiotic susceptibility testing (when available), because bioweapons may be genetically altered and have unusual drug susceptibilities. Historically, the case fatality rate has been 80% to 100%. However, during the recent United States outbreak, the rate was 45%, presumably because of rapid diagnosis and treatment.

An acellular vaccine containing protective antigen has been licensed in the United States since 1970 for pre-exposure prophylaxis. The vaccine may have mild local side effects in as many as 63% of recipients and severe side effects in less than 1% of recipients (Pittman et al.). The usual pre-exposure regimen is six injections over an 18-month period with subsequent annual booster immunization. Animal studies have shown that a three-dose regimen, combined with antibiotic administration for 30 days, provides effective post-exposure prophylaxis. Without vaccination, post-exposure antibiotics are required for 60 to 100 days. However, scientific support of these recommendations is difficult because of limited animal studies, clinical experience, and controlled human studies.

No person-to-person transmission has been reported to date. Standard precautions are adequate for all patients, regardless of the type of anthrax syndrome. Prophylaxis of close contacts is not required. Doxycycline and ciprofloxacin are approved for post-exposure prophylaxis, and amoxicillin may be used for children and pregnant women.

Plague

Pandemics of bubonic plague or the “black death†have occurred several times throughout the centuries, with mortality rates approaching 50% to 60%. Plague no longer occurs as pandemics but persists as enzootics with rodent reservoirs. Fewer than 20 cases are reported annually in the United States, and most occur in the southwestern states (Inglesby et al., 2000). Naturally occurring plague primarily develops after a person is bitten by an infected flea. The bite is followed by the development of regional necrotizing lymphadenitis or “buboes.†Bubonic plague may then progress to secondary septicemia and pulmonary involvement. The only person-to-person transmission is by respiratory droplets from patients with pulmonary involvement. Primary pneumonic plague from aerosol exposure is highly contagious. Any case of pneumonic plague should be considered a potential bioterrorism event.

The usual incubation period for bubonic plague is 1 to 6 days, whereas pneumonic plague may develop in only 2 to 4 days. Symptoms of pneumonic plague include productive cough, chest pain, dyspnea, and hemoptysis. Gastrointestinal symptoms, such as abdominal pain, will likely occur, but buboes do not develop. Acral gangrene may also occur.

The causative organism is Yersinia pestis, a gram-negative coccobacillus that may be seen on Gram-stained sputum specimens. Y. pestis exhibits bipolar or “safety pin†staining on Wright, Giemsa, and Wayson stains. Diagnosis is confirmed by culture and identification of the organism. Rapid tests, such as polymerase chain reaction, antigen detection, and immunoassays, are available at state reference laboratories and the CDC.

There are no clinical trials to guide the treatment of pneumonic plague. In vitro antibiotic susceptibility testing and previous anecdotal experience indicate that streptomycin and gentamicin are the primary antibiotics. Tetracyclines and fluoroquinolones have also been effective in animal studies. The case fatality rate approaches 100% without treatment and 5% to 14% after administration of streptomycin.

Doxycycline administered for 7 days may be used for post-exposure prophylaxis. Patients require droplet isolation for at least the first 48 hours of antibiotic therapy. No vaccine is available.

Tularemia

Tularemia is a worldwide, sporadic, endemic zoonosis associated with exposure to small animals or ticks. Although tularemia is thought to be underdiagnosed, approximately 200 cases are reported annually in the United States, primarily from the south central and western states (Dennis et al.). Naturally occurring tularemia has different presentations. The most common forms are ulceroglandular and typhoidal infections. Pneumonic tularemia is very uncommon, and bioterrorism should be suspected if the pneumonic form is encountered.

The causative organism is Francisella tularensis. As few as 10 organisms are sufficient to cause infection by either the cutaneous or the aerosol route. A nonspecific febrile illness occurs 3 to 5 days after exposure. Symptoms include fever, fatigue, chills, headache, and malaise. Bioterrorist-related infection would be associated with pneumonic tularemia from aerosol deposition, but ocular or ulceroglandular tularemia may also occur if contact with the conjunctivae or with broken skin occurs.

Diagnosis is usually established by growth of F. tularensis on cysteine-enriched culture media at a Biological Safety Level 3 (BSL-3) facility or by acute and convalescent serologic studies. Positive blood cultures are rare. Fluorescent antibody assay or immunochemical stains of sputum may also confirm the diagnosis.

Treatment is streptomycin or gentamicin for 10 days. Tetracycline and chloramphenicol are also effective but require a minimum of 14 days to prevent relapses. Fluoroquinolones have been shown to be effective in vitro and in animal studies but are not approved by the U.S. Food and Drug Administration for this purpose. The case fatality rate is 30% to 60% for untreated patients and less than 2% for those receiving treatment.

A live, attenuated vaccine derived from an avirulent strain is available as an investigational new drug. The vaccine has been used to protect laboratory workers. Immunity develops over 2 weeks but does not completely protect against inhalational exposure, and the vaccine is inappropriate for post-exposure prophylaxis.

The recommended post-exposure prophylaxis is doxycycline or ciprofloxacin for 14 days. If exposure is not confirmed, persons with possible infection should begin a “fever watch,†taking their temperature twice daily for 2 weeks and monitoring themselves for flu-like symptoms. Close contacts of patients with documented tularemia do not require prophylaxis, as person-to-person transmission has not been reported.

Routine F. tularensis microbiologic procedures performed in hospitals may be hazardous to laboratory workers. Specimens should therefore be forwarded to Level B or BSL-3 reference laboratories for confirmation and other studies.

Viral Hemorrhagic Fever

Hemorrhagic viruses were weaponized by both the former Soviet Union (Marburg, Ebola, Lassa, and Junin viruses) and the United States (yellow fever and Rift Valley fever). The hemorrhagic fever viruses are all small RNA viruses with lipid envelopes. These viruses belong to four families, only two of which have been associated with possible bioterrorism — Filoviridae (Ebola and Marburg viruses) and Arenaviridae (Lassa and New World or South American viruses). These viruses are among the most feared and least understood viruses in the world (Borio et al.).

Natural transmission of most hemorrhagic viruses is by contact with the excreta of infected rodents or arthropod vectors. Humans are incidental hosts. The reservoir and vector for Ebola and Marburg viruses are unknown. Infected patients transmit the viruses to close contacts, primarily family members and health care providers. Dispersion of viral agents is difficult. However, if hemorrhagic viruses were successfully dispersed for bioterrorism, the targeted population would be uniformly susceptible, and only limited treatments and vaccines are available.

Although the clinical presentation of infection in primates is the same following either parenteral or aerosol exposure, it is uncertain whether this is also true for humans. Differentiation of the various viral hemorrhagic fevers on clinical grounds may not be possible. All are characterized by high fever, headache, arthralgias, myalgias, and abdominal pain. Incubation ranges from 2 to 21 days. Conjunctivitis and pharyngitis develop, along with petechiae and purpura of the mucosae and conjunctivae. Gastrointestinal and urinary tract hemorrhages also occur. Multiorgan failure, shock, and death soon follow. All viral hemorrhagic fevers may be confused with meningococcemia.

Leukopenia, hemoconcentration or blood loss anemia, thrombocytopenia, and elevated liver enzyme values occur. Disseminated intravascular coagulation is common. Hospital laboratories are not equipped to confirm the diagnosis of viral hemorrhagic fever. Only the CDC and the United States Army Medical Research Institute of Infectious Diseases (USAMRIID) are Level D facilities with the diagnostic capacity to do these tests. As no viral hemorrhagic fevers are endemic in North America, any suspicion of such an exposure should immediately be reported to public health officials.

Treatment is primarily supportive. Intramuscular injections, antiplatelet drugs, and anticoagulant agents should be avoided. Corticosteroids are not beneficial. There are no antiviral agents or vaccines for treatment or prevention of the filoviruses. Junin is the only arenavirus for which a vaccine (live-attenuated) has been developed, but the vaccine is still classified as an investigational new drug in the United States. Many of the arenaviruses (Junin, Lassa, Machupo) are susceptible to ribavirin if given within 7 days of onset. Case fatality rates vary by virus and range from 0 to 90%.

Barrier and contact precautions must be meticulous. Respiratory isolation is indicated for all patients, although aerosol transmission of hemorrhagic fever viruses is infrequent. Patients should be placed in a negative-pressure room. Use of N-95 masks is appropriate. Prophylaxis following exposure to arenavirus may be attempted with ribavirin. Potentially exposed individuals should be instructed to check their temperature twice a day and monitor themselves for symptoms.

Various diseases that can be caused by biological attack are shown in Table 16(Arnon et al.).