3. Clinical Aspects of Anthrax
Laboratory confirmation of anthrax cases
Incident response to possible deliberate release of anthrax
Anthrax is an acute infection caused by B. anthracis. The incidence of disease, in order of decreasing frequency in naturally acquired cases, is: cutaneous (>95%), gastrointestinal and inhalational, although this order varies in different parts of the world. Meningitis has been reported as a secondary complication in cutaneous, inhalational and intestinal infection. Photographic images of these presentations are available at:
Centre for Disease Control - Public Health Image Libary
Centre for Disease Control - Emergency Preparedness and Response
- with clinical signs and symptoms where anthrax should be considered in the differential diagnosis (see below); or
- or Asymptomatic and seeking post exposure prophylaxis as the result of potential exposure due to a ‘suspicious substance incident’.
If the patient has symptoms of anthrax, travel, occupational and recreational histories should be taken to establish a possible epidemiological link with the disease. People who work with susceptible animals (herbivores such as cattle, sheep and goats) or their products (such as wool and hides) are at risk of the disease, particularly cutaneous anthrax.
Symptoms of cutaneous anthrax begin with the appearance of a small papule at the infection site. After 1 to 2 days, the papule develops into a vesicle which ruptures to form the painless ulcer with the central necrotic dry black scab— the typical anthrax eschar. Its size ranges from about 1 cm to several centimetres across, and there is usually little or no pain, despite its ugly appearance. The eschar becomes painful only if a secondary infection (generally staphylococcal or streptococcal) occurs. The eschar is surrounded by oedema and numbers of haemorrhagic vesicles. Oedema affecting the head and neck, which is usually more extensive than on other parts of the body, may become life-threatening. Malignant oedema is a rare complication which may progress to toxaemic shock.
Findings that are strongly suggestive of cutaneous anthrax include:
- absence of pain associated with the eschar;
- extensive oedema which is out of proportion to the size of the eschar; and
- the rarity of polymorphonuclear leukocytes on Gram stain.
In uncomplicated cases, the eschar begins to resolve about 10 days after appearance of the initial infection. Surgical intervention is not indicated. Irrespective of treatment, the eschar will continue to develop, and will take 2 to 6 weeks to resolve completely. Scarring is rare. Swabs of lesions should be taken for culture and sensitivity testing (see Appendix 2).
Infectious doseThere are no data on the infectious dose for cutaneous anthrax.
Incubation periodThe incubation period for cutaneous anthrax is usually 2 to 5 days , with a range of 12 hours to 14 days. Of 11 cases of cutaneous anthrax associated with the deliberate release of anthrax spores in the USA in 2001, the incubation periods ranged up to 10 days . There are a few reports of up to 12 days [4, 5] and one report of 14 days . The shortest incubation periods recorded have been one day [2, 3, 7-9] and one report of a pustule developing 12 hours after an abrasion caused by an infected horse brush .
Case fatality rateThe case fatality rate of untreated cutaneous anthrax is variable but low, unless complications arise. Untreated, approximately 20% of cases are fatal due to either secondary septicaemia or respiratory distress caused by cervical or upper thoracic oedema. In one case series (N=101), the case fatality rate was 3% . In another series (N=27), the case fatality rate was 11% .
Initial symptoms usually resemble those of a viral upper respiratory tract infection, and include fever, chills, malaise, myalgia and non-productive cough. Some patients may experience a brief period, of perhaps a few days, when the symptoms appear to be resolving. This is a dangerous phase as toxin accumulates. It is followed by fulminant illness, characterised by dyspnoea, stridor, cyanosis, chest pain, nausea, vomiting, prostration and shock, followed by death . The duration of the fulminant phase is less than 24 hours. A recent study indicated that non-headache neurological symptoms (such as confusion, dizziness or loss of consciousness), dyspnoea, nausea or vomiting, and any finding on lung auscultation were suggestive of anthrax rather than an upper respiratory tract infection in the context of a suspected deliberate release of anthrax . Chest X-rays show a widened mediastinum early in the course of the disease. All 11 cases of inhalational anthrax seen following the 2001 bioterrorist attacks in the USA had abnormal chest X-rays on initial presentation. Seven had mediastinal widening, seven had infiltrates and eight had pleural effusion. Mediastinal widening is considered highly suggestive of inhalational anthrax (but see also ‘Gastrointestinal anthrax’ below).
Specimens should be taken for diagnostic testing as outlined in Appendix 2, but therapy should begin immediately, and not be delayed until after results are available. Every effort should be made to get specimens before treatment is started, which should not cause significant delay.
Infectious doseEstimates of the infectious dose for inhalational anthrax in humans are imprecise. Based on studies in monkeys, one author made a conservative estimate of LD50 in humans of 4000 spores . This figure should not be regarded as precise for various reasons, including the fact that humans are generally regarded as being more resistant to anthrax than monkeys, and the proportion of an inoculum which reaches the lungs is dependent on particle size. Other authors give an estimate of the LD50 in humans of 2500 to 55 000 spores, based on non-human primate data (quoted in ). Recent estimates of the human lethal dose, based on published studies in non-human primates is LD10 = 50-98 spores, and LD1 = 1-3 spores . Modelling, based on the anthrax release in Sverdlovsk in the former Soviet Union in 1979, suggests that the infectious dose in humans was possibly as low as 2 spores . Clearly, given the range of these estimates, any person who has been exposed to aerosolised B. anthracis spores should be regarded as at risk.
Incubation periodThe incubation period of inhalational anthrax is usually 1 to 5 days , with a range up to 43 days [3, 5]. Following the deliberate release of anthrax spores in the USA in 2001, 11 people were diagnosed with inhalational anthrax. The incubation period ranged from 4 to 6 days in this series . In a series of cases following an atmospheric release of anthrax in the former Soviet Union in 1979, one patient fell ill with the disease 43 days after the incident . Because of the limited data available, the upper limit is not known precisely.
Case fatality rateUntreated, the case fatality rate of inhalational anthrax approaches 100% and, even with aggressive therapy, is about 80%. In a series of cases following an atmospheric release of anthrax (N=77), the case fatality rate was 86% . Following the deliberate release of anthrax spores in the USA in 2001, five of 11 patients with confirmed inhalational anthrax died, a case fatality rate of 45% . Regardless of the intervention, cases which had progressed to the fulminant stage prior to commencement of treatment died .
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Naturally occurring intestinal anthrax is associated with the consumption of contaminated meat. The symptoms begin insidiously with mild fever, malaise and gastrointestinal disturbance, including diffuse abdominal pain with rebound tenderness, and constipation or diarrhoea. These symptoms last for up to a few days, and are followed by the sudden onset of fever, chills, prostration and shock. Nausea and vomiting are common and, due to ulceration of the gastrointestinal tract, vomitus is often blood-tinged or has a ‘coffee grounds’ appearance. Stools may be either melaenic or blood-tinged. Ascitic fluid, which ranges in appearance from clear to purulent, develops 2 to 4 days after onset of symptoms, accompanied by a reduction in the severity of abdominal pain. Mediastinal widening on chest x-ray has rarely been reported in intestinal anthrax.
Oropharyngeal anthrax is also associated with consumption of contaminated meat, but is less common than intestinal anthrax. Initial symptoms include dysphagia and dyspnoea due to cervical oedema and lymphadenopathy, secondary to necrotic ulcerations in the oropharynx. Some lesions may appear pseudomembranous .
Infectious doseThere are no data available on the infectious dose for gastrointestinal anthrax.
The incubation period of intestinal anthrax is usually 2 to 5 days , with a range of 15 hours to 10 days. In one outbreak of 143 cases, the incubation period was 15 to 72 hours . There is one report of an incubation period of 10 days . There are few data on the incubation period of oropharyngeal anthrax. In one series of cases, the range was reported as 2 to 144 hours .
Case fatality rateThe case fatality rate of intestinal anthrax is variable. One reviewer quoted a case fatality rate of greater than 50% . However, some authors report a much lower mortality rate. In three case series from Uganda  (N=143), Thailand  (N=72) and Lebanon  (N=6), the case fatality rates were 6%, 4% and 17% respectively, with a weighted mean of 6%. If intestinal anthrax progresses to septicaemia, the mortality rate approaches 100%. Although data are limited, it appears that the case fatality rate of intestinal anthrax is higher in children. In two case series, all the deaths recorded were in children [18, 22].The case fatality rate from oropharyngeal anthrax is generally reported as being lower than for intestinal anthrax, although in one series reported from Thailand, there were three deaths from 24 cases of oropharyngeal anthrax, a case fatality rate of 13% .1 This guideline may be of use to medical laboratory staff where responsibilities in the Chemical, Biological and Radiological setting may include the acceptance of and/or processing of suspicious substance specimens for biological analysis, and should be read in conjunction with this document.
Collection of clinical specimensThe microbiological laboratory should be contacted for advice on the collection and preparation of specimens for transport, and advice given as to when they are likely to arrive. The clinical context should be provided with the specimen when it is sent to the laboratory—e.g. whether the patient has symptoms (and their nature) or is well with possible environmental exposure. Staff taking and handling specimens should also ensure that chain of custody of specimens is maintained to ensure they are admissible as evidence in court proceedings.
Clinical samples from a patient with suspected anthrax should not be sent to the usual diagnostic laboratory for routine culture. They should be sent directly to a specified reference laboratory able to do culture on selective media, if necessary, and properly validated PCR on suspicious specimens. Based on the clinical signs and symptoms, specimens to be taken from cases may include:
- blood for culture
- swabs from cutaneous lesions
- swabs from oropharyngeal lesions
- cerebrospinal (CSF) fluid2
Post-mortem specimensUnless necessary for coronial or other reasons, full post-mortem examination should not be performed, due to the risk of dissemination of spores into the environment. If a full post mortem is undertaken, infection control practitioners should be consulted and specimens of mediastinal lymph nodes, pleural fluid, pleura, lung and spleen taken for diagnostic purposes.
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When a full post mortem is not performed, the following post-mortem specimens could be taken to assist in diagnosis:
- venous blood
- nasal swabs
- swabs of haemorrhagic exudate from orifices
- aspirated pleural fluid
- samples from the centre and the periphery of the eschar
- wedge biopsy of both lungs
- biopsy of obviously involved lymph nodes
- tru-cut biopsy of solid organs
Specimen handling and transportClinical and environmental specimen handling and transport is subject to Commonwealth, state and territory legislation governing the transport of Biological agents (such as the Australian Dangerous Goods Code for Road and Rail and the Civil Aviation Safety Regulations for Air Transport).
Anthrax is a Tier 1 SSBA, a biological agent of the highest security concern to Australia. Handling of Tier 1 SSBA agents is subject to the National Health Security (NHS) Act 2007, NHS Regulations 2008 and the Security Sensitive Biological Agents (SSBA) Standards , including requirements for reporting to the Department of Health and Ageing (See Security Sensitive Biological Agents web site).
If you are handling or sending suspected SSBAs for confirmatory testing you must:
- comply with Commonwealth, state and territory legislation governing the transport of Biological agents (such as the Australian Dangerous Goods Code for Road and Rail and the Civil Aviation Safety Regulations for Air Transport)
- ensure that the receiving entity will accept the suspected SSBA prior to dispatch of the agent (a record must be maintained of the acceptance)
- notify the receiving entity of the shipment details
- report the transfer within two business days of the transfer occurring 
Specific protocols should be developed by each jurisdiction for the packaging and transport of specimens, including labelling as a possible/confirmed Risk Group 3 pathogen. Clinical specimens should be transported to the on-site laboratory according to routine protocols with the addition of chain-of-custody requirements. Specimens should be taken directly to the laboratory. The laboratory must be notified that specimens are being taken and when they will arrive to minimise handling and simplify chain-of-custody procedures.
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Specimens or culture isolates transported to the reference laboratory by air must be packed in biohazard containers according to Civil Aviation Safety Regulations for Air Transport. The reference laboratory must be notified that samples are en route and chain-of-custody requirements must be followed. Samples must be taken directly to the laboratory. Blood culture and swab specimens can be transported at room temperature. Other body fluids, sputum and faeces should be refrigerated during transport.
Identification and characterisation of B. anthracisB. anthracis is a non-motile, non-haemolytic, Gram positive bacillus. Whenever B. anthracis is suspected on the basis of these characteristics, the laboratory should institute appropriate containment and disinfection methods. If anthrax was not suspected and the initial characterisation performed by a clinical laboratory, specimens and cultures should be forwarded immediately to the appropriate public health reference laboratory for diagnosis and detailed characterisation. Further identification and characterisation should not be undertaken by the clinical laboratory. When the identity of the organism is confirmed by standard laboratory methods, antibiotic sensitivity studies should be undertaken by the reference laboratory as a matter of urgency. The reference laboratory should undertake testing for the presence of virulence genes by PCR methods, and independent assessment of antibiotic sensitivities.
Antibiotic sensitivityIsolates should be tested for sensitivity to antibiotics, including benzyl penicillin, amoxycillin, ciprofloxacin, doxycycline, rifampicin and vancomycin. Results of sensitivity testing should be conveyed immediately to the treating doctor by telephone, followed by written confirmation.
Laboratory waste disposal and spills clean-upEnvironmental surfaces that may have been exposed to B. anthracis spores should be cleaned with 0.5% hypochlorite solution, by staff using standard infection control precautions, including appropriate personal protective equipment (PPE). Buckets, mops and other equipment should be rinsed thoroughly in tap water, and waste water disposed of in accordance with State/Territory regulations. Spills of laboratory cultures of B. anthracis should be absorbed on to paper towels and disposed of as clinical waste. The contaminated surfaces should be treated with 2.0-2.5% sodium hypochlorite, left for one hour, and cleaned again with paper towels that are disposed of as clinical waste.
Occupational health and safety for laboratory staffB. anthracis is a Risk Group 3 pathogen, and laboratory studies should only be undertaken in a Physical Containment Level 3 facility using a Class 1 or 2 safety cabinet, and should comply with the appropriate standards. There is no indication for antibiotic prophylaxis for laboratory staff unless there is a laboratory accident in which staff are likely to have been exposed to aerosolised spores, swallowed viable B. anthracis, or suffered a parenteral inoculation with the organism. Most clinical samples contain the vegetative form of anthrax which is not easily transmissible to laboratory workers .
Subject to the availability of a vaccine, staff who work with cultures of B. anthracis may be considered for vaccination. However, as there is currently no anthrax vaccine on the Australian Register of Therapeutic Goods, this could only be undertaken on a case-by-case basis, with the informed consent of the person to be vaccinated, and with the approval of the Therapeutic Goods Administration (TGA). Laboratory managers who wish to consider offering vaccination to their staff should contact the relevant public health unit for further advice (see Appendix 10: Contacts). Approval for vaccination of staff is unlikely to be given unless a Response Code 1 or higher alert is in place.
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Australia’s health approach to a possible deliberate release of anthrax spores is to maintain vigilance, with early detection of cases and exposed persons, and vigorous post exposure prophylaxis and treatment where indicated. Vaccination of high risk groups may also be considered, subject to availability of vaccine.
Rapid and effective deployment of response teams and prophylactic agents and the establishment of dedicated pre-exposure prophylaxis and treatment centres, and possibly vaccination centres, may be necessary. Where one or more human cases of anthrax are identified with no obvious occupational or other epidemiological link, skilled epidemiologists may be required to investigate the source of infection. Public concern would make effective communication strategies essential.
If a deliberate release is suspected, relevant jurisdictional police should be notified immediately and procedures instituted, to the extent possible and consistent with public health imperatives, to ensure that forensic evidence is identified and preserved. The plan incorporates actions to be taken at five response levels (Codes 0 to 4), shown in the following box.
Table 1: Australian response codes for anthrax
- Response Code 0: No credible threat of release
- Response Code 1: Credible threat of release
- Response Code 2: Release imminent
- Response Code 3: Overt release or suspected covert release
- Response Code 4: Multiple releases
1. Document is available on request. Contact CBRN.
2. This can be extremely helpful if anthrax meningitis is present as there are a lot of large Gram positive bacilli in cerebrospinal fluid which can lead to rapid presumptive diagnosis with consistent clinical information.