Introduction

Anthrax is an infectious disease caused by spore-forming bacteria called Bacillus anthracis (B. anthracis). Anthrax is a zoonotic disease which means the disease can be transmitted to humans from animals like cows, goats and sheeps Anthrax can cause severe illness in animals and humans due to the toxins produced by the bacteria which can be fatal when inhaled.¹

Anthrax (B. anthracis) spores are found in the soil worldwide and can exist as a dormant spore in an environment for decades before resurfacing when the soil is disturbed from flooding, rain or by landslides. Once the spores are inhaled by animals or humans, they germinate and replicate, producing toxins which make animals and humans very sick. There are many ways anthrax can be transmitted: 

• Insects infecting other animals
• Contaminated animal feed containing bone meal from infected animals
• Human contact with infected animals
• Laboratory accidents

It is important to understand the transmission of anthrax to prevent the spread of anthrax and to contain the outbreak to prevent further spread of transmission and to save lives. Knowing how anthrax spreads is important to protect public health, and prevent an anthrax outbreak. This article will be discussing how anthrax spreads.^1,2

Types of anthrax

Anthrax can be classified by how B. anthracis enters the body and causes infection. There is three routes anthrax spores can enter:

1. Through the skin (Cutaneous Anthrax)
2. Through consumption of infected meat (Gastrointestinal Anthrax)
3. Through the lungs (Inhalation Anthrax) ³

Cutaneous anthrax

Anthrax spores can enter the skin through cuts on the skin when an individual handles infected animal or contaminated animal products like its hair or wool. When the bacteria spores enter the skin, it multiplies at the site of the cut and starts producing toxins. This leads to patient developing the following symptoms:^1,2,3

• Itchy bumps on the skin which develop into a black sore
• Swelling around the black sore
• Headaches
• Muscle
• Fever
• Vomiting

This is the most common form of anthrax and is also the least dangerous of all anthrax infections. 

Gastrointestinal anthrax

Anthrax can enter the body when an individual eats raw or uncooked meat from an infected animal and this causes the spores to affect the gastrointestinal tract. Like with cutaneous anthrax, the spores will replicate, and cause symptoms similar to food poisoning but worsen to:^1,3

• Severe abdominal pain with stomach swelling,
• Vomiting of blood,
• Severe and bloody diarrhoea
• Fever and chills,
• Swelling of neck glands
• Headaches
• Fainting

Inhalational anthrax

Inhalational anthrax (pulmonary anthrax) is the rarest and most fatal form of human anthrax and is caused by inhaling the anthrax spores from the air into the lung alveolar space. The spores are then engulfed by immune cells such as macrophages and neutrophils which transport the engulfed spores to the lymph nodes. At this site, the bacteria germinate and replicate and start to produce the toxins which causes the following symptoms to appear:^2,3

• Fever and chills
• Chest pains, coughs and shortness of breath
• Confusion and dizziness
• Nausea, vomiting and stomach pains
• Extreme tiredness

Inhalation anthrax symptoms first appear as a common cold but can quickly progress into severe lung difficulties. As the infection continues, bloodstream infection can occur leading to septic shock and as a result, patients can show their symptoms suddenly and rapidly deteriorate.

All types of anthrax can be fatal and can lead to death if no medical treatment is given.^1,2,3 

Routes of transmission

Human anthrax infection can occur either by direct or indirect contact with infected animals or by inhaling the spores. Anthrax is only transmitted from animals and there are no cases reported of direct human-to-human transmission.^2,4

Direct contact

This can include:

• Handling infected animals through contact with its hair, wool or hide
• Contact with contaminated animal products like meat, blood, wool, bones or hides
• Contact with contaminated soil infected with B. anthracis spores

Direct contact transmission of anthrax occurs when an individual has direct contact with the source of anthrax source and the spores enter through cuts in the body leading to cutaneous anthrax infection.^1,5

Indirect contact: This can include:

• Handling contaminated equipment or tools used on contaminated animals

Indirect contact transmission of anthrax occurs when an individual comes into contact with objects that are contaminated with anthrax spores. This can be tools or equipment that have been used on infected animals, like tools used in hide-making or wool-shearing. 

Inhalation

Inhaling spores directly from the soil can also cause inhalation anthrax where the spores occupy the lung space, leading to lung infection. This is the most dangerous route of transmission, with inhalation anthrax being the most fatal anthrax infection.

Ingestion

Eating contaminated meat is another route where the spores can enter the body and cause anthrax infection. 

Transmission mechanisms

Transmission mechanisms of anthrax can be by:

1. Animal-to-human transmission
2. Environmental contamination
3. Bioweapons

Animal-to-human transmission

Anthrax bacteria is found naturally in the soil and it affects livestocks and wild animals such as:²

• Sheep
• Goat
• Cattle
• Wild deer
• Antelope

Human infection occurs through direct and indirect contact with infected animals leading to an anthrax infection. Anthrax can be transmitted by animals to humans as the bacteria is zoonotic and there are no reports of direct human-to-human transmission.²

Environmental contaminationAs described earlier, anthrax is a soil-dwelling bacteria that is found in the environment naturally. However, the bacteria spores are released into the environment when the soil is disrupted by flooding, extensive rain or by landslides. This then causes the spores to be either ingested by grazing animals which then become infected and can pass on the infection to humans. It can also cause direct inhalation of spores by humans, leading to the dangerous inhalation anthrax.¹

Bioweapons

Anthrax is a dangerous pathogen that has the potential to be used as a bioweapon and is classified as a Category A pathogen by the Centers for Disease Control & prevention. Anthrax has already been used in a bioterrorism attack in 2001 in the anthrax letter events in the US where anthrax spores were sprayed on letters leading to more than 800 infections within two weeks. ^2,4

Risk factors for transmission

Anthrax is not a pathogen that is commonly experienced and there are certain risk factors that increase transmission of the bacteria. Overall, interaction either direct or indirectly with infected animals will increase risk of transmission.

Occupation

As described earlier, anthrax can be transmitted by contact with infected animals or through exposure to bacteria spores in the environment. Therefore certain occupations are hot spots for anthrax transmission. 

• Farmers
• Butchers
• Veterinarians 
• Individuals involved in processing bone, wool and hair of infected animals

People working in laboratories working with anthrax pathogens are also at risk of developing anthrax infection.^2,5

Geographic location

Geographical location is an important factor when considering the transmission of anthrax. Individuals in rural areas are at higher risk of anthrax infection compared to people living in the cities or urban settings as they have more contact with livestock and anthrax spores contaminating the soil. 

Anthrax is endemic to agricultural areas in Sub-Saharan Africa, Central and South America, Southern and Eastern Europe and central and Southwestern Asia. Therefore tourists in these areas should take extra caution as they are at an increased risk of anthrax infection.⁶

Exposure to infected animals or their products

Anthrax's main route of transmission is by animals to humans. More exposure to infected animals or to infected animal products can increase the risk of transmitting anthrax.⁶

Prevention and control measures

There are many ways to prevent transmission of anthrax infections.

Vaccination

Vaccines are available for both humans and animals to prevent anthrax infection. Veterinary vaccines are given to animals to prevent and control anthrax infection, especially within livestock.¹ This will then prevent anthrax transmission from animals to humans, also reducing anthrax infection in humans. Human vaccines are only given to individuals who have high exposure to anthrax and that includes farmers, scientists, military personnel and vets.^1,6

Hygiene practice

It is important to avoid contact with livestock especially if livestock is not vaccinated against anthrax.⁷ Washing hands regularly is important and must be implemented when dealing with animals and animal products. Following the protocols for handling, processing and disposing of animal products is important to reduce transmission of anthrax.⁷ When an animal is infected, the correct disposal of the animal and decontamination of the site must be implemented to prevent further transmission of anthrax.

Surveillance and early detection

Early detection of anthrax is crucial to prevent further transmission of anthrax. Detecting where anthrax has originated from, the site of infection, and who has been in contact with the infected animal can help medical officers determine who needs treatment against anthrax. 

FAQ’s

How does anthrax spread?

Animals grazing on soil may ingest anthrax spores and become infected, and when products of these animals are used by humans, it leads to indirect transmission. Biting flies which are vectors of anthrax may also spread the bacteria by biting livestock and/or humans.

How is one exposed to anthrax?

Anthrax can be exposed by contact of spores with the skin, eating meat infected with anthrax spores, touching or inhaling anthrax spores in the environment and through open injuries on the skin.

Summary

Anthrax is an infectious disease caused by spore-forming bacteria called Bacillus anthracis (B. anthracis). Anthrax can cause severe illness in animals and humans due to the toxins produced by the bacteria which can be fatal when inhaled. There is three routes anthrax spores can enter the body and cause infection: through the skin (Cutaneous Anthrax), by eating uncooked infected meat (Gastrointestinal Anthrax) and through inhalation (Inhalation Anthrax).¹
Human anthrax infection can occur either by direct or indirect contact with infected animals or by inhaling the spores. Anthrax is only transmitted from animals and there are no cases reported of direct human-to-human transmission. Anthrax is not a pathogen that is commonly experienced and there are certain risk factors that increase transmission of the bacteria. Interacting with sick animals and having exposure to anthrax spores makes you more likely to develop anthrax infection. Anthrax is very rare in developed countries and is more common in developing countries but anthrax still remains a major concern as its potential to be used as a bioweapon.^2,4