4.2.1.1. Circumstantial and historical evidence

Circumstantial evidence indicates that humans are moderately resistant to anthrax. Historical human:animal case ratios in handlers of anthrax carcasses suggest a relative resistance on the part of humans. Before vaccines and antibiotics became available, and at a time when understanding of industrial hygiene was relatively basic, workers in at– risk industrial occupations processing animal products were exposed to significant numbers of anthrax spores on a daily basis. In the United Kingdom, although case rates in terms of total numbers with apparently equal exposure are not known, it seems likely that the average of 40 cases of industrial anthrax a year during the 50-year period 1900–1949 (1979 cases total) (Anon., 1959) represents a very low infection: exposure ratio. In four mills in the USA, in which unvaccinated workforces, varying in size from 148 to 655, were “chronically exposed to anthrax”, annual case rates were only 0.6% to 1.4% (Brachman et al., 1962). In one mill, workers were found to be inhaling 600–1300 anthrax spores over an 8-hour shift without ill-effect (Dahlgren, 1960), and in two goat-hair mills, B. anthracis was recovered from the nose and pharynx of 14 of 101 healthy workers. It needs to be remembered that the vast majority of the cases that did occur in these at-risk occupations were cutaneous infections (section 4.4.1).

More recently, the analysis by Meselson et al. (1994) of the accidental release of anthrax spores from a microbiology facility in the former Soviet Union (the Sverdlovsk incident in 1979) showed that the large majority of human cases were mapped within a narrow zone approximately 4 km long downwind from the source, while cases in cattle and sheep occurred in a zone up to 50 km in the same downwind direction. In this event, the value of exposure:infection ratio estimates as an indicator of relative resistance to infection is limited by the fact that the effectiveness of the public health measures in preventing development of human cases is not known. While tabulated clinical cases numbered 77, some 59 000 persons were considered eligible by the public health authorities for “prophylactic immunization against anthrax”. The attack rate, in actual fact, may have been higher than 77 (Gumbel, 1991) (see also section 4.2.2.3).

4.2.1.2. Wildlife workers

Despite extensive exposure to anthrax carcasses, cases among workers in wildlife reserves are exceedingly rare (Quinn & Turnbull, 1998; de Vos and Turnbull, 2004). In three decades of specimen collection from hundreds of anthrax carcasses by unvaccinated researchers, rangers and wardens with minimal other personal protective equipment in the Etosha National Park, Namibia, there have been just two anecdotal, unconfirmed cases of cutaneous anthrax. Serology failed to detect evidence of past infection in seven rangers/researchers deemed to have had a high potential exposure to anthrax (Turnbull et al., 1992a). In the recent outbreak in the Malilangwe Trust, Zimbabwe (Clegg et al., 2006), two individuals dealing with carcass disposal developed cutaneous lesions on their hands. One did not seek treatment but recovered without complications and developed measurable antibodies; the other was given antibiotic treatment and did not measurably seroconvert (Turnbull, Clegg & Wenham, unpublished data).

Anthrax was first confirmed in northern Canada in 1952 when two employees of Wood Buffalo National Park were treated for the cutaneous form of the disease after handling a dead bison (Gates et al., 1995). In 1962, during the first recorded large-scale epizootic in bison, the biologist who discovered the first carcasses developed cutaneous anthrax after performing several postmortem examinations without any protective gear (Pyper & Willoughby, 1964). Later during the same outbreak, a backhoe operator helping to bury carcasses developed inhalation anthrax. He had repeatedly crawled under his machine to clean the blades of contaminated soil, hair and offal. The individual was a heavy smoker, which may have contributed to the development of disease. Prompt medical treatment resulted in his survival.

In the Kruger National Park, South Africa, “during three major epidemics in wildlife, many necropsies were performed, and large teams of workmen were used to track down, sometimes cut up and burn anthrax carcasses, but none contracted the disease” (de Vos & Turnbull, 2004). Other wildlife parks in central and southern Africa similarly report a negligible incidence in wildlife workers, despite the fact that levels of personal care probably drop somewhat during animal epidemics, which result in large numbers of carcasses and associated stress from pressure of work. Serological evidence of infection was detected in 12 of 24 persons in a village on the Luangwa River in Zambia who were believed to have eaten meat from hippos that had died of anthrax during the epizootics of 1987 and 1988, but there were no records of clinical cases (Turnbull et al., 1991).

4.2.1.3. Outbreaks in humans

Although the evidence indicates that humans are relatively resistant to anthrax, outbreaks and epidemics do occur in humans; sometimes these are sizeable. Among the most notable was the epidemic in Zimbabwe which began in 1979 and was still smouldering in 1984–1985. More than 10 000 persons were affected, albeit with a low (1%–2%) case-fatality rate (Turner, 1980; Davies, 1982; Kobuch et al., 1990; Pugh & Davies, 1990). In the epidemic in the Gambia reported by Heyworth et al. (1975), 448 cases of human cutaneous anthrax were diagnosed with just 12 known deaths. In 2000, apparently “hundreds” were affected in the Afar region of Ethiopia, many with oral and gastroenteric infections. Consistent among these cases was the skinning and butchering of sick and dead animals, handling contaminated meat and eating raw or inadequately cooked meat.

Mortality rates will have been reduced in these outbreaks by the availability of penicillin but occasionally case-fatality rates are substantial, such as in the Sverdlovsk incident in the former Soviet Union, with 66 known deaths in 1979 (Abramova et al., 1993; Meselson et al., 1994), and possibly many more (Gumbel, 1991). In the 2001 bioterrorist anthrax letter events in the USA, exposure rates are not known but it seems likely that the 22 clinical cases (11 inhalational [5 deaths] and 11 cutaneous) represented a small proportion of the numbers of persons exposed. The widespread prophylactic use of antibiotics is likely to have played a significant role in the final case rate also (Jernigan et al., 2001; Inglesby et al., 2002).

4.2.2.1. General

In general terms, human infectious doses have not been established. Clearly data were generated from human experimentation in units 731 and Ei 1644 in Japan during the Second World War, but the published subcutaneous and oral MID₅₀ doses of 10 and 50 mg (Harris, 1999), respectively, are hard to interpret without further information. On the basis that 1 mg dry weight of spores contains a little over 10⁹ cfu, the subcutaneous dose at least is so much higher than what would be expected from the historical and epidemiological evidence that only a small number of spores are needed to initiate cutaneous anthrax when infecting through a breach in the skin.

The issue of inhaled dose, greatly highlighted by the anthrax letter events in the USA, remains unresolved. Haas (2002) compares the curves developed by Druett et al. (1953) and Glassman (1966). Druett et al.’s data suggest an RID₂ of approximately 2300 spores in contrast to 9 spores on the probit slope published by Glassman. (RID₂ is the dose at which 2% of the population exposed to that dose via the respiratory route would develop a clinical infection, but not necessarily die.)

As a generalization, the severity of the resulting infection undoubtedly depends on several factors such as route of infection, nutritional and other states of health on the part of the infected person, and possibly also on the relative virulence of the infecting strain. For the purpose of risk assessment, dependency on information from animal tests is unavoidable. Some of the published data on infectious and lethal doses in animals are given in section 3.1.

The issue of the human MID for anthrax by various routes of exposure is an important consideration in remediation of contaminated sites, i.e. how safe are sites in which it may be impossible or unfeasible to eliminate low levels of contamination? Cutaneous and inhalational exposure are of primary concern, but gastrointestinal exposure may also be of concern, depending on the circumstances. Risk assessments following the 2001 anthrax letter events in the USA resulted in the conclusion that there is no safe level of contamination. It has long been anecdotal wisdom that it only requires a single spore reaching the correct site to initiate anthrax infection and, as noted in section 4.2.2.2, much depends on the probability of that spore reaching that site. Certainly the probability may be dependent in part (i.e. the chance of infection for a particular dose may be dependent in part) on the individual’s underlying disease(s) and immune status.

4.2.2.2. Cutaneous infections

A century’s experience of the natural disease in humans suggests that it probably does not take many spores to initiate a cutaneous infection once the necessary access to subepidermal tissues is achieved, despite the implication otherwise in the experimental data from Japanese Unit 731 (section 4.2.2.1). It is generally accepted that B. anthracis is non-invasive and that the spores must gain access to subepidermal tissue through a cut or abrasion before infection can occur, although this has been challenged: in a series of laboratory-acquired cutaneous anthrax cases, preceding trauma at the site of the lesion was noted by only 5 of the 25 patients (Ellingson, 1946).

The risk of infection reflects the chance of the spores gaining access and, in general, this is a low-probability event. This risk is greatly reduced in at– risk occupations by appropriate clothing and gloves, dressing of wounds, and other hygienic practices.

4.2.2.3. Inhalational infections

Based on two large studies in monkeys, the estimated human LD₂ ranges from 9 to 2300 inhaled spores, depending on whether exponential dose-response or a log-probit regression analysis is used (Druett et al., 1953; Glassman, 1966; Haas, 2002) (LD₂ is the dose at which 2% of the target population receiving that dose by the specified route would die). In the Sverdlovsk accidental release incident, sheep reportedly died of anthrax in a village 54 km distant from the point of release at the research institute (Meselson et al., 1994). If the respiratory MID for sheep is 35 000 spores (see section 3.1) and the minute respiratory volume for sheep is 8 litres, a cloud passing the affected premises would have contained more than 2187 spores/litre of air, assuming that the cloud originated as a single puff and transited the premises in 2 minutes. The concentration could have been lower with a more prolonged release but, either way, this suggests that the inhaled dose for humans near the research institute may have been greater than Meselson et al.’s originally proposed 9 spores. As already covered in section 4.2.1.1, the account of Gumbel (1991) suggests that there may have been a higher death rate than that reported by Meselson et al. (1994). Many of those who succumbed had predisposing respiratory illness.

Recorded inhalation LD₅₀s in non-human primates range from 2500 to 760 000 spores (Meselson et al., 1994; Watson & Keir, 1994). The United States Department of Defense based its strategies on an estimate that the LD₅₀ for humans is 8000 to 10 000 spores (Meselson et al., 1994). This may date from work during the Second World War, or shortly thereafter, when it was seen that “something in excess of 10 000 inhaled spores were needed to kill most species” (Carter & Balmer, 1999). However the only direct, but semiquantitative, data on inhalation infectious doses in humans come from the studies in goat-hair processing mills referred to in section 4.2.1.1. It is well established that, at sizes above 5 µm, particles face increasing difficulty in reaching the alveoli of the lungs (Druett et al., 1953). The likelihood of inhaled spores penetrating far enough to induce inhalation anthrax therefore depends greatly on the size of the particles to which they are attached.

The overall conclusion from the available evidence is that, deliberate release scenarios apart, the risk of pulmonary anthrax outside industrial situations is very low. For example, addressing concerns over hazards in the vicinity of anthrax carcass sites, Turnbull et al. (1998b) analysed air samples 3–9 m downwind from disturbed dry dusty anthrax-carcass sites with soil contamination levels of 10⁴–10⁶ anthrax spores per gram in Namibia, and found that, even at the highest concentrations found in the air, it would require about 2.5 minutes for an average human undergoing moderate activity to inhale 1 cfu of spores. Clearly the dilution factor rapidly reduces the risk of infection.

In the anthrax letter events in the USA in late 2001, in which 11 persons contracted inhalational anthrax (Jernigan et al., 2001; Anon., 2002a), the exposure doses were not determined. It is possible, although unproven, that exposures were < 10 000 spores but that these individuals represented only a small proportion of persons actually exposed reflecting, again, the “low probability event” nature of anthrax infection and the historically low infection rate:exposure ratio discussed in sections 4.1, 4.2.2.1 and 4.2.2.2. In this case, however, evidence of subclinical infection, such as might be anticipated in at least a proportion of a large number of persons exposed to sublethal doses, surprisingly was not found by serological surveys (Quinn et al., 2002).

These events also underscored the problem of overdependence on experimental animal LD₅₀ data in risk assessments for human exposures to “weaponized” preparations of B. anthracis. Bioaggression centres around how many casualties can be inflicted on the opposing force, which requires focusing on LD₅₀ estimates. In contrast, the focus in public health, which emphasizes the prevention of any avoidable illness, is the MID by alternative routes of exposure. Age, underlying disease status, immunological deficiencies, etc. all need to be taken into account. Sizeable doses may be necessary to deliver the necessary number of respiratory LD₅₀s¹ to young healthy soldiers in an act of aggression, but the minimum respiratory infectious dose might be considerably less for the elderly or immunocompromised, and those with a poor lung-clearance efficiency. Those that died in the Sverdlovsk incident were mostly over 45 years of age, smokers with emphysema or suffering from welder’s lung, also with emphysema – conditions having a serious impact on lung-clearance capacity for inhaled particles (Hugh-Jones, personal communication, 2003).

4.2.2.4. Oral route infections

The oral MID₅₀ dose determined from human experimentation in units 731 and Ei 1644 in Japan during the Second World War is 50 mg, equivalent to approximately 10¹¹ spores (section 4.2.2.1). No other information on infectious doses in humans by the oral route was found, but what is true for the skin is probably largely true for the oropharyngeal and gastrointestinal epithelium, namely that the chance of infection is likely to be enhanced by, if not dependent on, the existence of a lesion in the epithelium through which spores can gain entry and establish an infection. Uptake of bacilli and/or spores through the tonsillar epithelium and M cells overlying Peyer’s patches in the small intestine may be possible routes of entry also.

Reports of oral LD₅₀s from tests in animals, even for species regarded as highly susceptible to infection, range from 10⁶ to 10⁸ cfu of spores with MIDs of approximately 1.5 to 5 x 10⁸ (see section 3.1). Illness with recovery and seroconversion may occur with non-lethal doses in both humans and animals (see section 4.4.1). The extent to which the outcome reflects activities (colonization, germination, toxin production, etc.) within the gut, as opposed to the point at which very large numbers of spores simply overcome natural intestinal barriers, is unknown at present.

Virtually nothing is known about the relative infectivity of vegetative cells as compared to spores and the possible importance of this to oral infectious dose. (It is not always appreciated that it is almost impossible to prepare spore-free vegetative cell preparations with which to study the infectivity of vegetative cells experimentally.) In pigs, however, the pharyngeal form of ingestion anthrax generally results from consumption of meat and bones from infected carcasses, which would contain both vegetative cells and spores, while the intestinal form is associated with consumption of spore-contaminated mineral supplements (section 3.4.4). By analogy, in humans the oropharyngeal form has been reported in persons eating undercooked meat (presumably containing vegetative cells) from water buffalo (Sirisanthana et al., 1984). Intestinal manifestations presumably result from the presence of spores which survive the gastric juices.

4.2.2.5. Treatability

The fact that anthrax is readily treated if diagnosed at a sufficiently early stage of infection also needs to be taken into account when assessing risks. Awareness of the likelihood of exposure having taken place is clearly an important part of the equation. Of the 11 patients diagnosed as having inhalational anthrax following the deliberate-release anthrax letter events of late 2001 in the USA, 6 were successfully treated (Anon., 2002a) proving that even this most dangerous form of the disease is susceptible to timely intervention. Antibiotic combinations were thought to provide a therapeutic advantage over single antibiotics (see section 7.3.1.6).

In evaluating the prognosis, the pathogenic role of the anthrax toxin complex needs to be kept in mind (see section 5.5.3) In cutaneous anthrax, progressive evolution of the lesions continues for about 24 hours after bacteriological cure is achieved (Ellingson et al., 1946); similar progression of disease is probable in the other clinical forms of the disease. As long ago as the 1950s (Keppie et al., 1955) it was noted that, in guinea-pigs, once bacteraemia had reached ± 3 x 10⁶ chains/ml (approximately 8 hours before death), termination of the bacteraemia and removal of the infection by streptomycin treatment delayed but did not prevent death. Similarly, in both monkeys and rats challenged with sterile anthrax toxin, antiserum is effective in preventing death if given early but not late in the post-challenge period (Lincoln et al., 1967).

4.4.2.1. Differential diagnosis

A history of exposure to contaminated animal materials, occupational exposure and living in an endemic area is important when considering a diagnosis of anthrax. A painless, pruritic papule, surrounding vesicles and oedema, usually on an exposed part of the body, is suspicious. Clinical diagnosis is confirmed by the demonstration of Gram-positive encapsulated bacilli from the lesion and/or positive culture for B. anthracis from the lesion and/or positive specialist tests as described in section 4.4.2.2.

The differential diagnosis of the anthrax eschar includes a wide range of infectious and non-infectious conditions: boil (early lesion), arachnid bites, ulcer (especially tropical); erysipelas, glanders, plague, syphilitic chancre, ulceroglandular tularaemia; clostridial infection; rickettsial diseases; Rhizomucor infections, orf, vaccinia and cowpox (Lewis-Jones et al., 1993), rat-bite fever, leishmaniasis, ecthyma gangrenosum or herpes. Generally these other diseases and conditions lack the characteristic oedema of anthrax. The absence of pus, the lack of pain, and the patient’s occupation may provide further diagnostic clues. The outbreak of Rift Valley Fever, referred to in section 3.5.7 and initially thought to be anthrax in livestock, also affected numerous humans.

In differential diagnosis of the severe forms, orbital cellulitis, dacrocystitis and deep tissue infection of the neck should be considered in the case of severe anthrax lesions involving the face, neck and anterior chest wall. Necrotizing soft tissue infections, particularly group A streptococcal infections and gas gangrene, and severe cellulitis due to staphylococci, should also be considered in the differential diagnosis of severe forms of cutaneous anthrax. Gas and abscess formation are not observed in patients with cutaneous anthrax. Abcess formation is only seen when the lesion is infected with other bacteria such as streptococci or staphylococci.

4.4.2.2. Immunological and other tests

The rapid hand-held, on-site, immunochromatographic detection and diagnostic devices that have been developed in recent years are discussed in section 6.2.

In the case of retrospective diagnosis of anthrax, serology can be supportive (sections 3.5.6, 4.4.1). Purified protective antigen and lethal factor (see section 5.5.3) are available commercially from List Laboratories).²

Turnbull et al. (1992a) detected seroconversion in 17 of 38 (44.7%) patients with bacteriologically-confirmed cutaneous anthrax, and a further 5 patients in whom bacteriological confirmation was not successful. The percentage of seroconverters (71%) was much higher in the 14 persons for whom paired sera were examined than in the remaining 24 persons (29%). Similarly, seroconversion was only found in 5 of 21 (24%) individuals from whom blood was obtained ≤ 7 days after the first appearance of lesions as compared with 15 (83%) of the 18 persons bled ≥ 8 days after the appearance of lesions. The failure to seroconvert by the 21 individuals with bacteriologically confirmed anthrax and 19 other persons clinically diagnosed as anthrax cases was interpreted as indicating that treatment early in the course of the infection prevented elaboration of sufficient antigen to induce a detectable antibody response. Studies in non-human primates showed that early antibiotic treatment after a known challenge with B. anthracis spores abrogated a detectable antibody response (Friedlander et al., 1993). Negative results should therefore be interpreted with caution and in the light of the full patient history.

Quinn et al. (2004) analysed sera from 16 individuals with confirmed or suspected cutaneous or inhalation anthrax resulting from the 2001 anthrax letter releases in the USA, and one laboratory worker with laboratory-acquired cutaneous anthrax also associated with that event. In 6 patients surviving inhalation anthrax, anti-PA (anti-protective antigen) IgG was detected 11–22 days after the onset of symptoms (15–28 days after likely exposure). Anti-PA IgG was also detectable in the serum from 10 of the 11 patients with bioterrorism-associated cutaneous anthrax and the one patient with laboratory-acquired infection. In these cutaneous cases anti-PA IgG was detectable at 12 days after the onset of symptoms (24 days after estimated exposure). One cutaneous anthrax patient was seronegative at day 18 after the onset of symptoms but had detectable anti-PA IgG at day 34. In one cutaneous anthrax patient, anti-PA IgG was not detectable at 4, 5, 47 and 253 days after the onset of symptoms. Anti-PA IgG was detectable 8–16 months post-symptoms in all 6 survivors of inhalation anthrax and in 7 of 11 persons suffering from the cutaneous form of the disease. There was a positive correlation between serum toxin neutralizing activity and anti-PA IgG levels.

Immunohistochemistry, noted by Fritz et al. (1995) in experimental inhalation anthrax in rhesus monkeys to have diagnostic value, proved to be an invaluable aid to confirmation of diagnosis in the anthrax letter events in the USA in 2001, with the particular advantage over other diagnostic tests of being able to detect anthrax-specific antigens in tissues regardless of treatment (Shieh et al., 2003). In one patient, definitive diagnosis retrospectively depended on this technique being applied to a skin biopsy taken as long as 9 days after treatment had begun. At present, however, the method is confined to specialist laboratories with access to appropriate specific antibodies.

In the Russian Federation, a skin test utilizing Anthraxin^T,³ first licensed in the former Soviet Union in 1962, has become widely used for retrospective diagnosis of human and animal anthrax and for vaccine evaluation (Shylakhov et al., 1997). This is a commercially produced heat–stable protein–polysaccharide–nucleic acid complex without capsular or toxigenic material, derived from oedematous fluid of animals injected with the vaccine STI-1 or the Zenkowsky strains of B. anthracis. It is sterilized by autoclaving. The test, which is still used in the Russian Federation (Cherkasskiy, personal communication, 2003), involves intradermal injection of 0.1 ml of Anthraxin^T. A positive test is defined as erythema of ≥ 8 mm with induration persisting for 48 hours (Shlyakhov et al., 1997). This delayed-type hypersensitivity is seen as reflecting anthrax cell mediated immunity and was reportedly able to diagnose anthrax retrospectively some 31 years after primary infection in up to 72% of cases (Shlyakhov et al., 1997). It was used with success in a retrospective investigation of a series of cases occurring in a spinning mill in Switzerland where synthetic fibres were combined with goat hair from Pakistan (Pfisterer, 1990). The diagnostic reliability of Anthraxin^T, like Ascoli test antigen (Annex 1, section 11.1), depends on the nature of anthrax rather than on the specificity of the antigens involved.

With the same provisos as given in section 3.5.5, the PCR (section 6.3.2; Annex 1, section 10.7.4) has now become accepted as a sensitive method for detecting anthrax-specific DNA in clinical samples (Ellerbrok et al., 2002; Shieh et al., 2003).

4.4.2.3. Precautions

Surgical tools should be sterilized without delay after use, and dressings should be incinerated (Annex 1, sections 7.8 & 7.9). The wearing of surgical gloves by medical staff and orderlies is recommended, but risks to these staff are not high. Direct human–to– human transmission is exceedingly rare (see section 4.3.2); standard contact precautions for management of patients are recommended with any form of anthrax (Ashford et al., 2000). For the most part, this involves wearing disposable gloves and gown or laboratory coat while taking specimens or dressing lesions, changing gloves after the relevant action and before touching anything else and properly disposing of the gloves, thorough handwashing at the end of procedures, ensuring decontamination and disinfection of gown/laboratory coat, bedding and other items that may have become contaminated from the patient’s infected site and ensuring that other appropriate procedures for cleaning and disinfection are in place (Garner, 1997; see also Annex 1, sections 7.8 & 7.9). Vaccination of medical staff and orderlies is not necessary.

4.4.3.1. Signs and symptoms

There are two clinical manifestations of anthrax that may result from ingestion of B. anthracis in contaminated food or drink – oropharyngeal anthrax and gastrointestinal anthrax. The oropharyngeal form is the less commonly seen.

The suspicion of alimentary canal anthrax depends largely on awareness and alertness on the part of the physician as to the patient’s history and to the likelihood that he/she has consumed contaminated food or drink. Response to treatment can be good (Van den Bosch, 1996) and there is evidence that mild undiagnosed cases with recovery occur (Ndybahinduka et al., 1984; Turnbull et al., 1992a; Oh et al., 1996; Centers for Disease Control and Prevention, 2000; see also sections 4.2.2.4 and 4.4.1).

The incubation period is commonly 3–7 days. The spectrum of disease ranges from asymptomatic to severe, terminating in sepsis, septic shock and death. In developing countries, mild cases with gastroenteritis attract little attention and the patients with severe infections, leading to death within 2–3 days, may never reach a medical facility. In endemic areas, all physicians should be aware of gastrointestinal anthrax.

4.4.3.2. Confirmation of diagnosis

Diagnosis of oropharyngeal anthrax is covered in section 4.4.3.1. For the gastrointestinal form, see section 4.4.4.2.

4.4.3.3. Differential diagnosis

In the differential diagnosis of oropharyngeal anthrax, diphtheria and complicated tonsillitis, streptococcal pharyngitis, Vincent angina, Ludwig angina, para-pharyngeal abscess, and deep-tissue infection of the neck should be considered.

The differential diagnosis in gastrointestinal anthrax includes food poisoning (in the early stages of intestinal anthrax), acute abdomen owing to other reasons, and haemorrhagic gastroenteritis caused by other microorganisms, particularly necrotizing enteritis caused by Clostridium perfringens and dysentery (amoebic or bacterial).

4.4.4.1. Symptoms and course of the disease

The term “inhalational anthrax” has largely replaced the older names for this form of the disease, the most common of which was “pulmonary anthrax”, reflecting the fact that active infection occurs in the lymph nodes, rather than the lung itself, and that bronchopneumonia does not occur. The inhaled spores are carried by macrophages from the lungs, where there is no overt infection, to the lymphatic system where the infection progresses. Germination and initial multiplication begin within the macrophages while in transit to the lymph nodes (Hanna & Ireland, 1999). The vegetative cells kill the macrophages and are released into the bloodstream where they continue to multiply and lead to fatal septicaemia (see also section 5.2).

Symptoms prior to the onset of the final hyperacute phase are nonspecific, and suspicion of anthrax depends on the knowledge of the patient’s history. Analysis of 10 of the 11 inhalational cases associated with the anthrax letter events of 2001 in the USA (Jernigan et al., 2001; Inglesby et al., 2002) revealed a median incubation period of 4 days (range 4–6 days) and a variety of symptoms at initial presentation including fever or chills (n=10), sweats (n=7), fatigue or malaise (n=10), minimal or nonproductive cough (n=9), dyspnoea (n=8), changes in mental state including confusion (n=5) and nausea or vomiting (n=9). All patients had abnormal chest X-rays with infiltrates (n=7), pleural effusion (n=8) and mediastinal widening (n=7). Mediastinal lymphadenopathy was present in seven cases. In the previously best– documented set of five case reports of inhalational anthrax, Plotkin et al. (1960) also recorded headache, muscle aches and development of a cough in four patients and mild pain in the chest in one. Jernigan et al. (2001) drew attention to profound sweating as a prominent feature in their patients not emphasized in previous reports.

In contrast to the median incubation period of 4 days found in the anthrax letter inhalation cases, Brookmeyer et al. (2001) estimated it to have been 11 days in the Sverdlovsk outbreak. One consideration that should be kept in mind is the possibility that reflux of spores from the respiratory tract into the alimentary tract may occur with the development of lesions there, and that this may affect time of onset of symptoms. However, while high exposure may lead to swallowing as well as inhaling spores, it is the alternative view that enteric manifestations result from toxic action being carried to the gastrointestinal tract via the bloodstream rather than from concurrent ingestion anthrax.

The mild initial phase of nonspecific symptoms is followed by the sudden development of dyspnoea, cyanosis, disorientation with coma, and death. In Plotkin et al.’s cases, treatment was unsuccessful in four of the patients, and death occurred within 24 hours of onset of the hyperacute phase.

The typical clinical course of this form of the disease is consistent with the lesion development within the mediastinal lymph nodes before the development of bacteraemia. The assault on the lung appears to be two-pronged. In the initial phase, the blockade of the lymphatic vessels develops, in association with symptoms such as a sensation of tightness of the chest. Lymphatic stasis is associated with oedema, which may be apparent above the thoracic inlet, and pleural effusion. Histological sections of the lung may reveal bacilli within the lymphatic vessels. In the acute phase, damage associated with septicaemia occurs. This is manifested morphologically by the changes described by Dalldorf et al. (1971). Occasional patients do not develop the mediastinitis which usually typifies this form of the disease. Mediastinal widening has been found to be a relatively frequent manifestation of other diseases, leading to the recommendation for computerized axial tomography (CAT) scans to demonstrate lymph node involvement.

Recent findings using computerized tomography (CT) scans combined with autopsy observations have enhanced clinical interpretation of early inhalational anthrax evolution (Galvin et al., 2001). The earliest detectable specific finding pointing to inhalational anthrax is mediastinal widening on posteroanterior (PA) chest X-rays. However, mediastinal widening is not a rare finding in a series of patients presenting at a hospital emergency department. Imaging in inhalational anthrax patients using a non-contrast spiral CT will reveal hyperdense lymph nodes in the mediastinum associated with oedema of mediastinal fat. The hyperdensity of the lymph nodes represents haemorrhage and necrosis, following spore germination and vegetative growth with exotoxin elaboration. Lymphatic stasis resulting from the damaged lymph nodes leads to dilatation of pulmonary lymphatics which originate in the pleura and drain towards the hilum, following interlobular septa in association with blood vessels. The stasis manifests as an early onset pleural effusion and peripheral infiltrates, representing thickened bronchovascular bundles, detectable on chest X-ray. These findings mark fully developed initial stage illness. Ultimately, the bacteria escape from the damaged lymph nodes and invade the blood stream via the thoracic duct. Once the bacteraemia and associated toxaemia reach a critical level, the severe symptoms characteristic of the acute phase illness are manifest. During the acute phase illness, damage of the lung tissue becomes apparent on X-ray. This damage results from the action of anthrax toxin on the endothelium of the lung’s capillary bed (Dalldorf et al., 1971). Primary damage of the lung is not normally a feature of the initial phase illness and primary pulmonary infection is an uncommon presentation (see also section 5.2).

The X-ray picture of the lung appears to be a very sensitive diagnostic aid with multiple abnormalities, including mediastinal widening, paratracheal fullness, pleural effusions, parenchymal infiltrates and mediastinal lymphadenopathy (Jernigan et al., 2001).

As stated in section 4.4.1, the number of recorded cases of inhalational anthrax in history is lower than might be perceived from the high profile given to this manifestation, and it has long been suspected, with some supportive evidence, that undiagnosed low-grade inhalational infections with recovery may occur in at-risk occupations.

4.4.4.2. Confirmation of diagnosis (inhalational and ingestion anthrax)

Clinical diagnosis is dependent on a high index of suspicion resulting from knowledge of the patient’s history (see sections 4.4.3.1 and 4.4.4.1). Early symptoms are nonspecific and flulike, with mild upper respiratory tract signs in inhalational anthrax or resembling mild food poisoning in intestinal anthrax. Where the history has not led to suspicion of anthrax, confirmatory diagnosis of pulmonary or gastrointestinal anthrax will usually take place after the patient has died or, if correct treatment is initiated early enough, after recovery.

Albrink & Goodlow (1959) demonstrated in monkeys with inhalational anthrax that detectable bacteraemia occurred before the fulminant clinical phase set in. However, treatment resulted in negative cultures after just one or two doses of antibiotics. In the 2001 anthrax letter cases in the USA, blood culture was positive in all patients who had not received prior antibiotic therapy (Jernigan et al., 2001). Sputum for staining and culture was not consistently collected; however, positive isolation from sputum would supply a definitive diagnosis. In general, visualization of the Gram-positive, capsulating bacilli and verification by culture should be attempted with pulmonary effusions, CSF, or other body fluids or tissues of suspected anthrax cases. Similarly, isolation from vomitus, faeces and ascites in intestinal anthrax may not always be successful, especially if the patient has been given antibiotics, but would be definitive when positive. The conclusion from analyses of the October-November 2001 events in the USA was that nasal swabs were not useful as clinical samples (Inglesby et al., 2002). See also Annex 1.

Immunohistochemical staining of pulmonary effusions or of bronchial biopsies (if available) proved valuable in the diagnosis of treated inhalational anthrax patients in the anthrax letter events in the USA in 2001 (Guarner et al., 2003; Shieh et al., 2003). As stated in relation to cutaneous anthrax, however, the method is currently confined to specialized laboratories with access to appropriate specific antibodies. Also as with cutaneous anthrax, direct PCR on clinical specimens from suspected inhalational anthrax cases is also now regarded as an acceptable diagnostic procedure (Ellerbrok et al., 2002; Shieh et al., 2003).

Specialized laboratories may be able to demonstrate anthrax toxin in fluid specimens (serum or oedematous fluid), or show these to be positive by PCR. This would permit an earlier diagnosis than culture and should still be positive in advanced cases where treatment has rendered culture negative. In the case of patients who survive, antitoxin antibodies may be demonstrable in convalescent sera (section 4.4.2.2). The rather older Anthraxin^T hypersensitivity test referred to in section 4.4.2.2 may also be applicable.

Belated treatment can sterilize the blood and tissue fluids while still not preventing toxin-induced death (section 4.2.2.5). Among the 11 cases of inhalational anthrax resulting from biological terrorism in 2001, 4 (36%) died despite antibiotic therapy. Post mortem, if the sterilizing effect of treatment has not occurred, the capsulated B. anthracis will usually be visible in capsule-stained smears of these fluids, and should be easily isolated from them by bacteriological culture. Again, tests for toxin or PCR on these fluids would still be positive when treatment has rendered them smear- or culture-negative.

Guidelines for confirmation of diagnosis of oropharyngeal anthrax are given in section 4.4.3.1. With regard to gastrointestinal infection, confirmation of diagnosis by culture may not be possible before death in the absence of a known prior event raising the suspicion of anthrax, at least in the index case(s). In those who survive, retrospective diagnosis by serology may be supportive. Where it is considered that anthrax is the cause of gastrointestinal symptoms, examination of ascetic fluid by smear, culture and/or toxin tests and/or PCR may again be used to confirm the diagnosis. Where anthrax has not been suspected prior to death and post mortem, characteristic signs of intestinal anthrax are dark haemolysed unclotting blood, enlarged haemorrhagic spleen, petechial haemorrhages throughout the organs, and a dark oedematous intestinal tract, ulcerated or with areas of necrosis. With inhalational anthrax, the haemolysed unclotting blood, enlarged haemorrhagic spleen and petechial haemorrhages throughout the other organs are again seen and the mediastinal lymph nodes are always affected with haemorrhagic necrotizing lymphadenitis.

Differentiation between inhalational and gastrointestinal anthrax at autopsy may be difficult, and the decision as to how the disease was contracted may have to be based, at least in part, on the patient’s history. The problem arises when varying sized (petechial to ecchymotic) haemorrhages occur in the gastrointestinal tract wall secondary to generalized spread of the infection (septicaemia/bacteraemia). Such haemorrhages occur in a number of diseases secondary to septicaemia/bacteraemia and are not indicative of primary infection via the gastrointestinal tract, reflecting instead irregularly distributed damage to the vascular bed caused by microorganisms and/or their toxins circulating in the bloodstream. Staging the lesions found at various locations within the body is an important means of determining the primary route of initial infection. The most advanced lesions will be located in the area of initial infection. For example, in gastrointestinal anthrax, the presence of a thickened, oedematous zone within the intestinal or stomach wall with an ulcerative lesion on the mucosal surface is typically present. The haemorrhages due to bacteraemia/septicaemia, in contrast, will not have marked thickening of the gut wall associated with them. Also, mesenteric adenopathy and ascites are usually present in gastrointestinal anthrax. Although, with gastrointestinal anthrax, the lungs may show damage similar to that found in patients dying of inhalational anthrax, the mediastinal lymph nodes, if affected at all, will have relatively less advanced pathological changes than those in the mesenteric lymph nodes. Microscopic study of the lesions by an experienced pathologist may be needed to determine their relative stage of progression.

4.4.4.3. Differential diagnosis (inhalational anthrax)

Alternative diagnoses to be considered are mycoplasmal pneumonia, legionnaires’ disease, psittacosis, tularaemia, Q fever, viral pneumonia, histoplasmosis, coccidiomycosis, malignancy.

4.4.4.4. Retrospective diagnosis

Quinn et al. (2004) showed that seroconversion can be expected in persons who recover from inhalational anthrax (see section 4.4.2.2).