Anthrax: causes, symptoms, diagnosis, treatment, prognosis, and prevention

Anthrax is an acute infectious anthropozoonosis caused by bacillus anthracis, mostly in herbivores such as bovine, sheep, goat, and horse. After exposure to infected animals with anthrax, humans can be infected. Main manifestations are cutaneous anthrax, followed by pulmonary anthrax and intestinal anthrax. Secondary anthracemia and meningitis may be present, with a high mortality rate.


Causes

Bacillus anthracis is the largest Gram-positive bacillus, bamboo-shaped in light microscopy, 1 - 1.5μm x 3 - 5μm in size, with cateniform arrangement. In humans or animals, or under specific environmental conditions such as in carbonate agar medium containing 5% - 25% CO2, the bacterium can generate capsules, and transparent rings around the bacterium can be seen in india ink capsule stain, and the capsules are red in basic methylene blue stain. Spores can be formed in environments in vitro, and the unreleased spores are in the center of the bacterium. The bacteria have no flagella and are nonmotile.

The bacterium grows well in common media, and the optimum culture temperature is 35 °C - 37 °C. The bacteria can be cultured in both common incubators and CO2 incubators, and can grow under anaerobic conditions. The colonies formed in the common nutrient agar are rough, with moist surfaces, and some strains form caudal prominences on one side of the colonies. After 15 - 24 hours of culture on blood agar plates, the well separated colonies are 2 - 5mm in diameter. The flat and slightly raised colonies are irregularly round, with undulating edges and smooth surfaces. The culture medium is viscous, and a raised caudal protrusion can be formed when picked up with an inoculating loop. Bacillus anthracis does not cause β-hemolysis. However, in the long-term fusion, weak hemolysis may be observed in the overgrowth zone, which should not be confused with β-hemolysis. In low power microscopy, the edges of the colonies are curled. The bacterium is flocculent in the broth culture medium, and the broth culture medium is not turbid.

The biochemical reaction of bacillus anthracis is not active, egg yolk reaction, nitrate reduction, and gelatin liquefaction and catalase are positive, and the bacteria cannot decompose starch and mannitol. The antigenic structure can be divided into two major groups of bacterial antigens and exotoxins. Bacterial antigens include capsular polypeptides and bacterial polysaccharides. The capsule has an anti-phagocytic effect, which is related to virulence. When capsules cannot be formed due to bacterial variation, the pathogenicity is absent. Polysaccharide antigens are not associated with virulence. The exotoxin complex consists of edema factors, lethal factors, and protective antigens.

The fertility and resistance of the bacterium are the same as those of common bacteria, but the spore resistance is strong. Conventional disinfectants, such as carbolic acid, cresol solution, bromogeramine, and other quaternary ammonium salts, have poor disinfection effects, whereas peracetic acid, formaldehyde, epoxyethane, 0.1% iodine solution, and chlorine-containing preparations have a good effect. The bacteria can survive in high pressure at 121 °C for 30 minutes and in dry heat at 140 °C for 3 hours.

Humans are mainly infected by exposure to the fur and meat of livestock, and carnivores can also get infected through herbivores and then transmit to humans. Persons in frequent contacts with livestock, such as pastoralists, veterinarians, and butchers, have more chances of infection. Furs contaminated with bacillus anthracis entering the processing enterprises, or infected meat entering the market, may cause epidemic outbreaks.

Pathogenesis

The pathogenicity of bacillus anthracis is related to its exotoxin and polypeptide capsules, which are encoded by two plasmids of pX01 and pX02. Capsules have anti-phagocytosis effect. The pX02-deficient strains cannot produce capsules, are easily phagocytized and killed by leukocytes, and are not pathogenic.

The exotoxin of bacillus anthracis consists of three proteins of protective antigen (PA), lethal factor (LF), and edema factor (EF). PA binding to cell surface receptors serves as a binding site for the other two factors. The PA receptor complex promotes the entry of LF and EF into the cells. LF combined with PA forms lethal toxin (LT), and EF combined with PA forms edema toxin (ET). LT inactivates major mitogen activated protein kinases in cells, interferes with intracellular information transmission, releases oxygen free radicals and proinflammatory cytokines, causes cell death, and destroys the vascular barrier. ET serving as a calmodulin-dependent adenylate cyclase can cause a sharp elevation in intracellular cAMP levels, leading to equilibrium damage, inhibiting neutrophils, so that human body is more susceptible to bacillus anthracis, and local infection and edema can occur. These two toxins are associated with septicemia, which can lead to death due to multiple organ failure in severe cases.

After the propagules or spores enter the human body and are engulfed by phagocytes, the spores resuscitate, produce exotoxin, and form anti-phagocytic capsules. Exotoxin directly causes local tissue edema, hemorrhage, and necrosis, and can cause systemic toxic symptoms. The anti-phagocytic capsule helps transmission of the bacteria, causing lymphatic hemorrhage and necrosis, and even invades the bloodstream, causing sepsis. The invasion of meninges can cause meningeal hemorrhage and edema.


Signs and Symptoms

The incubation period is generally 1 - 5 days, minimally 12 hours, maximally 2 weeks. According to the different portals of entry, the disease is mainly divided into cutaneous anthrax, inhalation anthrax, and ingestive anthrax. Some patients can develop severe diseases such as sepsis and meningoencephalitis, with poor prognosis.

Cutaneous anthrax

Cutaneous anthrax is the most common anthrax, accounting for 95% - 98%. Lesions are more common in the exposed areas such as hands, feet, face, neck, and shoulders. Initially, papules or maculae occur in the damaged skin. On the 2nd day, vesicles occur on the top of the rash, with light yellow fluids, with swollen surroundings. On the 3rd to 4th day, hemorrhagic necrosis and depression occur in the center of the skin lesions, which are surrounded by clustered vesicles, and the edema areas continue to expand. On the 5th to 7th day, diabrosis in the necrotic area results in superficial ulcers, the bloody exudates form solid and dark eschars, and granulations are formed beneath the crusts. The diameter of the ulcers varies from 1cm to 5cm, and the cutaneous infiltration and edema around the ulcers are large, up to 5 - 20cm in diameter. Because the local peripheral nerves are damaged, obvious pain and tenderness is absent. Mild pruritus is present, and abscess is absent, which are the clinical features of cutaneous anthrax. Subsequently, the edema subsides within 1 - 2 weeks, healing slowly. Most cases are solitary, but multiple vesicles may appear due to scratches in few patients, resulting in autoinfection. The duration of the disease is about 1 - 6 weeks.

During cutaneous anthrax, fever, headache, joint pain, general malaise, local lymph nodes enlargement, and splenomegaly may occur.

In a small number of patients, vesicles and eschars are absent, but pale red or pale swelling expands rapidly, mostly in the loose tissues such as eyelids, neck, thighs, and hands. Systemic symptoms such as high fever, headache, nausea, and vomiting are severe, and delayed treatment leads to poor prognosis.

Inhalation anthrax

Inhalation anthrax is caused by exposure to spores or inhalation of dust contaminated with spores. Sudden onset is present. Most patients develop low fever, fatigue, and precordial oppression 2 - 5 days after exposure. 2 - 3 days after onset, the symptoms are suddenly aggravated, and the manifestations are chest tightness, chest pain, fever, cough, and bloody mucoid sputum in mild patients, and chills, high fever, respiratory distress, dyspnea, stridors, cough, cyanosis, and bloody sputum, with or without pleural effusions in severe patients.

The signs and symptoms of the lungs often do not match the condition. Only scattered small moist rales, friction sounds, or decreased breath sounds can be heard in auscultation. X-ray examination reveals widened mediastinum, pleural effusion, and infiltrative shadows. Sepsis and meningitis often occur. If without prompt treatment, patients often die of toxic shock, respiratory failure, or circulatory failure 1 - 2 days after acute symptoms.

Ingestive anthrax

Ingestive anthrax is caused mostly by ingestion of undercooked meat products contaminated with bacillus anthracis, occasionally by water or milk contaminated with bacillus anthracis, and persons who take foods with the patient may develop the disease successively. Clinically, ingestive anthrax can be divided into oropharyngeal anthrax and gastrointestinal anthrax.

Oropharynx anthrax is manifested by severe throat pain, obvious submandibular and cervical edema, local lymphadenopathy, dysphagia caused by compression of the esophagus, and dyspnea caused by compression of the trachea.

Gastrointestinal anthrax is manifested by nausea, vomiting, abdominal pain, diarrhea, stools without blood, and subtle rectal tenesmus in mild patients, and abdominal pain, abdominal distension, diarrhea, and bloody stools in severe patients. Sepsis and toxic shock may occur, and failure to treat promptly can often lead to death.

Other clinical manifestations

Sepsis in anthrax

Sepsis can be secondary to inhalation anthrax, gastrointestinal anthrax, and severe cutaneous anthrax. In addition to aggravation of local symptoms, the clinical manifestations are severe septicemia, high fever, chills, and failures.

Meningitis in anthrax

The cases secondary to cutaneous anthrax are less than 5%, and very few cases can be secondary to inhalation and gastrointestinal anthrax. Clinical manifestations are purulent meningitis, rapid onset, severe headache, vomiting, coma, convulsions, obvious meningism, mostly bloody cerebrospinal fluids, merely yellow cerebrospinal fluids, increased intracranial pressure, elevated white blood cell count, and neutrocytophilia. The disease develops rapidly, and patients often die 2 - 4 days after onset due to delayed treatment resulting from misdiagnosis.


Diagnosis

Specimens for bacterial culture and Gram staining can be obtained from the lesions including skin lesions, pleural effusion, cerebrospinal fluids, ascites, and feces. Sputum examination and Gram staining in the absence of respiratory symptoms cannot be used for the diagnosis of inhalation anthrax. Polymerase chain reaction (PCR) and immunohistochemistry may be helpful in the diagnosis.

Nasal swab test is not recommended for patients who may be with inhalation anthrax, because the diagnostic value is unclear.

Chest X-ray or computed tomography (CT) examination should be performed if pulmonary symptoms occur. Widened mediastinum due to hemorrhagic lymphadenopathy and pleural effusion can be clearly revealed. Pulmonary infiltration is less common.

Lumbar puncture should be performed when meningitis or altered mental status occur.

Enzyme-linked immunosorbent assay (ELISA) can be used, and the antibody titer in the recovery phase fourfold more than that in the acute phase can assist in the definitive diagnosis.


Treatment

Early diagnosis and intensive supportive treatments, including mechanical ventilation, rehydration, and use of vasopressors, can reduce the mortality rate. Delayed treatment, usually due to misdiagnosis, can result in death.

Antibiotics

Cutaneous anthrax

If there is no significant edema or systemic symptoms, one of the following antibiotics can be used:

  • Ciprofloxacin 500mg in adults and 10 - 15mg/kg in children orally once every 12 hours
  • Levofloxacin 500mg orally once every 24 hours
  • Doxycycline 100mg in adults and 2.5mg/kg in children po q12h

If the infection is considered to be naturally acquired, amoxicillin can be used, and the dose is 500mg once every 8 hours.

Cutaneous anthrax without significant edema, systemic symptoms, or risk of inhalation exposure, the antibiotic treatment persists for 7 - 10 days. If accompanied by inhalation anthrax, the duration of treatment should be 60 days.

Although children and pregnant or lactating women should not use ciprofloxacin or doxycycline, these persons should be given one of these drugs in this case. However, if long-term treatment is required and the bacteria are sensitive to penicillin, the medication can be switched to amoxicillin 500mg in adults and 15 - 30mg/kg in children orally three times a day after 14 - 21 days of initial treatment. Death is rare after treatment, but the skin lesions can progress into eschars.

Inhalation anthrax and other anthrax, including cutaneous anthrax with significant edema or systemic symptoms, require a combination of 2 - 3 antibiotics. Antibiotic treatment should include one or more antibiotics with bactericidal activity and one or more protein synthesis inhibitors to block toxin production.

Antibiotics with bactericidal activity include:

  • Ciprofloxacin 400mg in adults and 10 - 15mg/kg in children intravenously once every 12 hours
  • Levofloxacin 750mg intravenously once every 24 hours
  • Moxifloxacin 400mg intravenously once every 24 hours
  • Meropenem 2g intravenously once every 8 hours
  • Imipenem 1g intravenously once every 6 hours
  • Vancomycin intravenously with a dose of maintaining a serum concentration of 15 - 20μg/ml
  • Penicillin G 4,000,000U IV q4h for penicillin-sensitive strains
  • Ampicillin 3g intravenously once every 4 hours for penicillin-sensitive strains

Antibiotics inhibiting protein synthesis include:

  • Linezolid 600mg intravenously once every 12 hours
  • Clindamycin 900mg IV q8h
  • Doxycycline 200mg IV followed by 100mg q12h
  • Chloramphenicol 1g intravenously once every 6 - 8 hours
  • Linezolid should be used with caution in patients with myelosuppression, because it has neurological side effects. Therefore, it cannot be used for a long time.
  • Chloramphenicol has good central nervous system permeability and has been successfully used to treat anthrax.

Although rifampicin is not a protein synthesis inhibitor, it can also be used because it has a synergistic effect with the main antibiotics.

If meningitis is suspected, meropenem in combination with other antibiotics should be used, because meropenem has good central nervous system permeability. If there is no meropenem, imipenem and cilastatin can be substitutes, which have the same effect. The initial intravenous combination therapy should be at least 2 weeks and more or until the clinical condition is stable. If patients have been exposed to aerosolized spores, the treatment should last for 60 days to prevent recurrence. The ungerminated spores may still survive in their lungs after exposure to aerosolized spores.

Once the intravenous combination therapy is completed, a single oral antibiotic should be used.


Prognosis

Cutaneous anthrax has a good prognosis. The mortality rate is <1% after appropriate treatment. If serious complications occur, the mortality rate can reach 15% - 25%. Pulmonary anthrax is often complicated with bacteremia, shock, and meningitis, and patients may die 1 - 2 days after respiratory failure. Although promptly treated, the mortality rate is still as high as 80% - 100%. Intestinal anthrax is susceptible to septic shock, patients usually die 3 - 4 days after onset, and the mortality rate is 25% - 75%. The mortality rate of anthracaemia is more than 90%. Early diagnosis and prompt treatment may improve the cure rate.


Prevention

Anthrax vaccines composed of cell-free culture filtrates can be used in persons at high risks, such as military, veterinarians, laboratory technicians, and textile mill workers processing sheep wool.

Limited data suggest that cutaneous anthrax does not produce acquired immunity, especially in patients who are early treated with effective antibiotics, and limited data suggest that inhalation anthrax survivors can produce specific immunity.

Postexposure prophylaxis

Asymptomatic persons exposed to inhalation anthrax, including pregnant women and children, should be given oral antibiotics for 60 days:

  • Ciprofloxacin 500mg in adults and 10 - 15mg/kg in children once every 12 hours
  • Doxycycline 100mg in adults and 2.5mg/kg in children q12h
  • Levofloxacin 750mg once every 24 hours
  • Moxifloxacin 400mg once every 4 hours

If there are contraindications to ciprofloxacin and doxycycline, and bacillus anthracis is sensitive to penicillin, amoxicillin 500mg in adults and 25 - 30mg/kg in children orally 3 times a day can be administered.

Live spores can be detectable in the lungs 60 days after exposure to aerosolized spores. Persons exposed to aerosolized bacillus anthracis spores may be considered to be at risk of inhalation anthrax, as the ungerminated spores remain in their lungs after initial exposure, so antibiotic treatment should last for 60 days to remove germinating microorganisms.

It is recommended that patients exposed to anthrax spores should use vaccine in combination with antibiotics. For vaccinated patients, the duration of antibiotic treatment after exposure should be extended to 100 days.