Introduction

Anthrax is a rare but serious disease caused by Bacillus anthracis. This bacterium forms tough spores that can survive for a long time in harsh conditions. These spores produce toxins that result in a high lethality rate. Anthrax is transmitted from ruminant animals such as cows, sheep, and goats to humans. The spores can be either from the infected animal or contaminated animal products.

The severity of Bacillus anthracis is due to several factors. Its shape allows it to evade the immune system, it produces powerful toxins, and it can also replicate quickly. The main toxins that it produces are the oedema toxin and the lethal toxin. A cell-binding protein called protective antigen helps these two toxins to enter target cells. Oedema toxin causes severe swelling, while lethal toxin triggers a serious immune response, leading to the sudden death often seen in people with anthrax infection.

There are three main types of anthrax: cutaneous, inhalational, and gastrointestinal. Cutaneous anthrax is the most common and occurs when the spores enter through a cut or scrape on the skin, causing a painless sore with a black centre. Gastrointestinal anthrax comes from eating contaminated meat and affects the digestive system, causing serious stomach issues.

Inhalational anthrax happens when spores are breathed in and are the most dangerous form of anthrax, leading to severe lung problems and often being fatal if not treated quickly. All three types of anthrax require quick and appropriate medical evaluation and treatment, which can greatly improve survival rates and help prevent serious complications. This article will explore the tests and methods used for anthrax diagnosis.^1,2

Types of anthrax and their initial clinical assessment

The person’s history and symptoms are important to diagnosing a potential anthrax infection. Risk factors for anthrax include contact with animals or animal products, travelling in regions where anthrax is more common, or potential occupational exposure to contaminated materials. 

Cutaneous (skin) anthrax is the most common form of anthrax. This form of anthrax is usually contracted when anthrax spores come into direct contact with an open cut or abrasion on the skin surface. An itchy bump forms, which then rapidly turns into the characteristic black sore. Some people also experience headaches, muscle aches, fever, and vomiting. 

Gastrointestinal anthrax occurs after eating meat from an infected animal. The initial symptoms are similar to food poisoning, but will eventually increase in severity to severe abdominal pain, vomiting of blood, and severe diarrhoea. 

For inhalation (pulmonary) anthrax, the symptoms usually develop within one week of exposure but can take up to two months to appear. In the initial stages of illness, people experience fever, chills, and fatigue. Sometimes this is accompanied by coughing, shortness of breath, chest pain, and feelings of nausea, which looks similar to influenza and pneumonia. Symptoms rapidly progress to extreme shortness of breath, altered mental states, and shock.

More recently, there is an additional, less common form of anthrax known as injection anthrax. This form of anthrax occurs when anthrax contaminates a needle that is then used on an individual. This has happened in heroin users in  Northern Europe, and the symptoms are described as similar to cutaneous anthrax but with a deeper infection.^1,2

Laboratory tests

Anthrax is not commonly seen in countries such as the United States and the United Kingdom, although outbreaks have occurred. The only way to confirm a suspected case of anthrax is to measure the antibodies and toxins in the individual’s blood and to test for the presence of the bacteria in the blood and various other clinical samples.

Serological tests are tests that check for the presence of antibodies in the blood. In the case of anthrax, doctors will look for the presence of antibodies against Bacillus anthracis. This is because the presence of antibodies means that the body has already been exposed to the bacteria and that the body has begun an immune response to fight it.

Blood can also be used to look for the presence of the bacteria itself. Blood samples are taken to allow any bacteria that may be present in the sample to be grown in a laboratory setting on special media like blood agar. This means that if Bacillus anthracis is present it will grow during this process because blood agar helps Bacillus anthracis to grow, separating it from other bacteria in the sample. 

In non-blood samples, a common technique to classify bacteria is gram staining. This is where samples from other parts of the body such as skin or sputum are stained with a special dye and examined with a microscope. This technique can allow Bacillus antracis to be identified visually.  

Once the bacteria grow, they can be confirmed using techniques like polymerase chain reaction (PCR). PCR is a quick and very sensitive test that detects the genetic material of Bacillus anthracis. This means that the bacteria can be rapidly identified even if there are only small amounts present.^3,4

Imaging studies

Because inhalation anthrax affects the lungs, doctors may use chest X-rays or CT scans to help diagnose it. In these scans, they will be looking for typical signs of infections such as a widened area in the middle of the chest (widened mediastinum) or fluids around the lungs (pleural effusion).^3,4

Advanced diagnostic methods

Some advanced diagnostic methods can be used to help diagnose anthrax. Some of the advanced diagnostic methods for anthrax are mass spectrometry and next-generation sequencing (NGS). Mass spectrometry is a technique that rapidly identifies Bacillus anthracis proteins, and is both quick and accurate at providing results. NGS is a more detailed genetic analysis of bacterial samples, which helps to identify specific strains of Bacillus anthracis and any genetic variations.^3,4

Differential diagnosis

Certain conditions can have symptoms similar to those of anthrax. For inhalation of anthrax, conditions such as community-acquired pneumonia, influenza, and respiratory syncytial virus have similar symptoms like cough, fever, and breathing difficulties. Similarly, cellulitis, a bacterial skin infection, can cause redness, swelling, and pain, which might be confused with the skin sores of cutaneous anthrax. 

Distinguishing anthrax from these and other similar infections is very important because anthrax requires specific treatments and can have severe complications if not treated properly.¹

Summary

Anthrax is a rare but serious disease caused by the bacterium Bacillus anthracis, which forms hardy spores capable of surviving in harsh conditions and producing deadly toxins. It is transmitted to humans from infected ruminant animals like cows, sheep, and goats or contaminated animal products.

The virulence of Bacillus anthracis is due to its ability to evade the immune system, produce powerful toxins (oedema toxin and lethal toxin), and replicate rapidly. A cell-binding protein called protective antigen aids these toxins in entering target cells, causing severe swelling and triggering a serious immune response that can lead to sudden death.

There are three main types of anthrax: cutaneous, inhalational, and gastrointestinal. Prompt medical evaluation and treatment for all types of anthrax can significantly improve survival rates and prevent serious complications. Doctors confirm suspected cases of anthrax by measuring antibodies and toxins in blood and clinical samples.

Serological tests check for antibodies against Bacillus anthracis, while blood samples can be grown in labs to isolate it. Gram staining classifies bacteria in non-blood samples, and PCR techniques can detect genetic material in small amounts. Doctors can also use imaging techniques like X-rays and CT scans to confirm diagnosis.