ANTHRAX

The anthrax bacillus

Bacillus anthracis is an aerobic, Gram-positive rod which forms spores on contact with oxygen. On culture in the laboratory long chains of square-ended bacteria are produced, giving colonies on solid media an appearance of swirling hairs (medusa-head colonies). Cultures held at 37C show the gradual appearance of one spore per bacterium, bring in the centre of the cell over a period of 12-24 hours. Spore formation is slower at lower temperatures.

Anthrax naturally infects many species of grazing mammals, can also affect animals such as pigs and badgers, which cat a mixed diet by grazing and scavenging pasture, and also eating insects and small animals. Carnivores are also occasionally affected. Infection is usually by ingestion of food from spore-contaminated pasture, but is occasionally by inoculation or exchange of infected discharges from sick animals, or by consumption of heavily-infected meat.

Herbivores are very susceptible to anthrax, and suffer overwhelming sepsis with invasion of the bloodstream.


Anthrax bacteria filling a capillary in fatal infection
(electron micrograph X1250)


When affected animals die, their bodies contaminate pasture, as the bacteria which they release will produce spores. Spores are resistant to a wide range of climatic conditions and can remain on contaminated ground for many years.

Epidemiology

Human infection is acquired from infected animals, and is therefore usually an occupational disease of farmers, slaughterers, skinners, hide workers, tanners and woolworkers. In these cases the infectious spores in the animal's hair, meat, bones or secretions are the source of the disease. In over 95% of cases the infection is cutaneous, due to inoculation of spores into small abrasions on the skin.

Where anthrax is common. and meat is scarce, humans may eat animals which have died of the disease, but adequate cooking much reduces the infectiousness of the meat. Skinning and boning the carcase, however, are hazardous procedures. Ingestion of large loads of bacteria or spores can result in intestinal infection.

Although humans are less susceptible to anthrax than most herbivores, large inhaled doses of spores can produce serious pneumonia, and lead to devastating blood-borne infection. In natural conditions, airborne spread of spores is usually on dust or moisture particles from heavily infected wool, hair or bone meal. Particles between one and five microns in diameter can be inhaled and lodge in the lungs larger particles are trapped in the nose and pharynx, and smaller particles are expelled with the breath).

Clinical features

Cutaneous anthrax

Anthrax of the skin is the commonest form of the disease. Typically, it is noticed about three days after exposure, when a raised, inflamed pimple appears. This may be slightly itchy, and it quickly acquires a small halo of clear vesicles. The adjacent, draining lymph nodes may be swollen. In the following days, the halo of blisters expands, with the formation of a tough, adherent, black scab in the centre, but there is still only minimal pain and tenderness. A striking feature is the very extensive oedema which accompanies the lesion; swelling often affects the whole of a limb, the head or the upper body adjacent to the lesion, with a varying degree of redness surrounding the vesicles and scab. This helps to differentiate anthrax from the tender lesions of cellulitis and the eschars of tick-borne diseases, which are not surrounded by oedema.


A typical fully developed lesion on the skin

The skin lesion tends to heal, even without treatment, but active diagnosis should be attempted, as there is a risk of infection spreading to the bloodstream in untreated cases.

Pulmonary anthrax (anthrax pneumonia, wool-sorters disease)

Illness usually occurs two or three days after exposure, though longer incubation periods can follow mild degrees of exposure. For the first few hours there are influenza-like symptoms with aches and pains, fever and increasing cough and shortness of breath. The symptoms then progress rapidly to severe cough, collapse and respiratory failure, often with a fatal outcome in two or three days.

Despite the dramatic severity of the disease, patients do not excrete large numbers of organisms. This is because the main lesions are in the lymph nodes and surrounding tissues of the mediastinum, and toxin production by the rapidly multiplying bacteria contributes largely to haemodynamic collapse and pulmonary damage. Person-to-person spread of anthrax pneumonia is not a significant risk.

Anthrax septicaemia

This is a rare development in cutaneous anthrax, but is much more common with pneumonia. Anthrax bacteria multiply in the bloodstream, reaching enormous concentrations. They have a capsule which resists phagocytosis, and a toxin which can enter and damage host cells by means of it's so-called protective antigen, a component of anthrax toxin which attaches to the host-cell membrane.

The septicaemic illness is characterised by high fever, collapse and vascular thrombosis, leading to tissue hypoxia, multi-organ failure and early death.

Gastrointestinal anthrax.

This is rare, as ingestion of sufficient spores to cause infection is unlikely. It takes the form of severe watery diarrhoea, with the excretion of numerous sporing bacteria.

Diagnosis

Anthrax bacteria are easily demonstrated by Grain's stain (or McFadyean's methylene blue stain, used in veterinary practice). In swab specimens or smears from skin lesions, diarrhoea or lung samples the typical-shaped bacteria can be seen, hying singly or in pairs. In terminal disease, direct stains of blood films, may show numerous bacteria.


B anthracis in infected lung tissue


The bacteria grow well on blood agar, and in standard blood culture media. Staining of the colonies shows typical bacteria, with the progressive formation of central spores.











Chains of anthrax bacteria from a laboratory culture

Treatment

B anthracis is susceptible to common antibiotics. Benzyl penicillin is the usual treatment of choice, but oral tetracyclines, or ciprofloxacin may be useful in cutaneous disease. By the time that pneumonia or septicaemia are recognised, treatment may not arrest the disease before a fatal outcome.

Prevention

Vaccines for human use have been developed. In the UK they are kept by the Public Health Laboratory Service for issue to workers at occupational risk. Three doses of the UK licensed vaccine are given over six weeks with a booster after six months. Good protection develops by the third dose. Other vaccines are available in several Western countries, with varying dosage regimens.

For accidental exposure, vaccine prophylaxis alone is too slow. Chemoprophylaxis is a good alternative. Useful drugs are tetracyclines, including doxycycline, and ciprofioxacin. After heavy exposure, spores can persist in the tissues for long periods, so chernoprophylaxis should be discontinued cautiously. The drugs are relatively safe to take for four to six weeks, though neither are recommended for young children (tetracycline may stain and weaken developing teeth; ciprofloxacin may cause damage to developing bones, or weaken the ligaments of the legs). Vaccination during antibiotic prophylaxis may be appropriate for some heavily exposed individuals, to give continuing protection after stopping drug treatment.

Public health

Anthrax is a notifiable or reportable disease in most Western countries, including the UK.

It is a major hazard to animals. Effective spore-based vaccines for farm animals are available. Animals dying with suspected anthrax are investigated, and those with anthrax are disposed of by incineration or by burial in lime at or near the site of death. Contaminated pasture can sometimes be chemically treated.

Anthrax spores can be killed by heat. The preferred method is autoclaving, but boiling for 10 minutes will substantially reduce the viability of most spores. This probably prevents a considerable number of food-borne infections in areas where anthrax is common.

Formaldehyde and glutaraldehyde are effective disinfectants for dealing with local contamination and spillages Formaldehyde in water vapour can be used for fumigation of equipment hides and fabrics, but is hazardous, and should only be used by trained personnel. In most Western countries, imported hides, bones, bone meal and wool from endemic areas must be disinfected by heat or fumigation to destroy spores.