Anthrax

From Ganfyd

Contents 1 Introduction 2 Occurrence 3 Clinical Features 3.1 Cutaneous Anthrax 3.2 Respiratory Anthrax 3.3 Gastrointestinal Anthrax 4 Diagnosis 5 Prevention and Prophylaxis 6 Related Links

Introduction

Anthrax is a bacterial infection caused by the aerobic, gram-positive spore-forming bacterium, Bacillus anthracis. It is primarily an infection of wild and domestic herbivores (cattle, sheep, goats, camels and other herbivores), but can also cause incidental infection in humans.

Occurrence

Whilst anthrax is an infection of herbivores who acquire infection from contact with soil-borne spores, humans can be infected by coming into contact with infected animals or their products. Human infection may also arise from the use of Bacillus anthracis as a biological warfare or bioterrorism agent. (Anthrax has been used as a weapon for many years: the deadliest anthrax epidemic on record occurred in 1770, when farmers in Saint Dominique, Hispaniola, deliberately sent infected meat to market.^[1])

Cases in the West are usually sporadic or occur in small outbreaks, and are almost always associated with occupational exposure to contaminated animal products such as wool, or direct contact with animals that are sick or have died from anthrax. In Africa there have been many epidemics of human disease associated with epizootics of anthrax in cattle, and herbivorous wildlife in Africa remains a reservoir where vaccination is not practical.

Clinical Features

Over 95% of cases of anthrax in the West are cutaneous. The majority of the remaining cases are respiratory. Gastro-intestinal outbreaks are regularly reported in Third World Countries.

Cutaneous Anthrax

Most often encountered on exposed skin such as arms, hands, face and neck. Infection begins as a pruritic area which develops into a pruritic papule resembling an insect bite. Over 24-48 hours the lesion enlarges and ulcerates. The ulcer is usually 10-35mm in diameter and is painless. The centre of the lesion evolves into a characteristic black eschar, with extensive surrounding oedema. Small secondary vesicles are sometimes seen. After 1-2 weeks the lesion dries and eventually the eschar falls off, leaving a permanent scar. Untreated infections may spread to regional lymph nodes and can lead to overwhelming septicaemia. Antibiotic therapy will not alter the natural progression of cutaneous disease, though it can decrease or inhibit systemic complications and surrounding oedema.

Differential diagnoses include Orf, Plague, Tularaemia and Pyoderma gangrenosum.

Respiratory Anthrax

Initial symptoms of respiratory anthrax are mild and non-specific. This is because B. anthracis spores do not secrete toxin or cause a significant inflammatory repsonse before germination. As they are transported to the hilar and mediastinal lymph nodes, the spores germinate into bacilli, where they cause profound haemorrhagic necrosis and mediastinal oedema. Initially respiratory anthrax manifests as malaise, low-grade pyrexia and a dry cough, sometimes with a feeling of mediastinal fullness.

As the infection progresses, mediastinal widening, tracheal pressure leading to stridor, bloody pleural effusions and cervical oedema may occur. The patient deteriorates, often rapidly with high fever, dyspnoea, tachycardia and stridor. If there is haematogenous spread to the gut, ulceration leading to haematemesis and melaena may occur. Fulminant septic shock with or without preceding delerium follows, usually leading to death. Respiratory anthrax may follow a more insidious course, though the outcome is generally the same. Because respiratory anthrax does not manifest itself early, antibiotic therapy is often of little use.

Gastrointestinal Anthrax

This manifestation may be found after ingestion of contaminated meat, and takes two forms: abdominal anthrax and oropharyngeal anthrax, depending on the site of the lesion. Abdominal anthrax initially causes nausea, vomiting, anorexia and fever, leading to abdominal pain, haematemesis, bloody diarrhoea and eventually septic shock and death. Oropharyngeal anthrax causes cervical oedema and tissue necrosis, manifesting as sore throat, dysphagia, fever and lymphadenopathy. Most affected patients die of sepsis.

Diagnosis

Diagnosis is usually by clinical picture and by a history of exposure to the infectious agent. A painless pruritic papule, with or without surrounding vesicles on an exposed part of the body should raise the suspicion of cutaneous anthrax. Respiratory anthrax has a more non-specific picture in the early stages, though mediastinal widening on chest X-ray along with severe respiratory distress in the later phase is strongly suggestive.

Bacilli are often found in large numbers in vesicular fluid, blood, cerebrospinal fluid, pleural effusion and other bodily fluids. These may be demonstrated microscopically by gram stain or McFadyean's methylene blue stain, or by culture on standard blood agar. Colonies grow readily in aerobic conditions, are greyish-white and non-haemolytic. Individual colonies often have a ground-glass appearance with irregular 'Medusa Head' projections at the edge. When teased with a loop, colonies may stand up like beaten egg white. The bacilli are catalase positive, non-motile, lysed by the gamma-bacteriophage and form endospores.

Immunohistochemical examination of fluids and biopsy specimens along with PCR and ELISA are valuable tools in the early detection of acute infection.

Prevention and Prophylaxis

Anthrax vaccine is available and is currently a cell-free filtrate. In the UK this vaccine is currently recommended only for those at risk from occupational exposure. Work is underway on a new vaccine that utilises recombinant protective antigen (rPA), a major component of anthrax exotoxin, to stimulate an immune response. Needle free vaccines (either applied as a skin patch or inhaled) are currently in the early stages of development.

Post-exposure prophylaxis with antibiotics can be very effective at reducing the risk of infection. Current recommended regimens include ciprofloxacin 500mg po bd for 28 days, or doxycycline 100mg po bd for 28 days.

Related Links

• Department of Health Anthrax Information (UK)
• Department of Health Deliberate Release of Anthrax Information (UK)
• DEFRA Antrax Information (UK)
• Health Protection Agency Anthrax Information (UK)
• HPA Porton Down Special Pathogens Reference Unit (UK)
• NHS Prodigy Guidelines - Deliberate Release (UK)
• CDC Emergency Preparedness & Response (USA)
• WHO Collaborating Centre for Anthrax Epidemiological Data (International)