Thursday, September 19, 2019

Disease Post: Anthrax

Something a little different: our first bacteria!


Anthrax is a disease caused by a bacteria called Bacillus anthracis.

What is bacteria? A bacterium is a single-celled organism. The cells are a little like ours, but a bit more primitive. They are called "prokaryotes" which means "primitive nucleus." (Our cells are "eukaryotes," meaning "true nucleus.") This means that they do not have a nucleus that holds the DNA like our cells do.

Bacteria come in all different sizes and shapes. Bacillus anthracis is in the shape of a rod and is large (relatively speaking). It is Gram-positive, which refers to the way bacteria are stained before looking at them under a microscope. Gram-positive means that they stain with crystal violet dye because they have a cell wall made of peptidoglycan. I won't go into too much detail about peptidoglycan, but it is made of sugars and amino acids and is important in how the bacteria functions and how the immune system interacts with the bacteria.
Anthrax bacillus. Credit: the Wellcome Collection. The bottom two images show the long, rod-shaped bacteria the best.

Another important thing is that this bacteria forms endospores. Endospores (or just "spores") are dormant bacterial cells that cannot reproduce and are very resistant to anything that would kill a normal bacterium. This means that anthrax can survive in bad conditions and in the environment for a long time (usually a few months to a few years, but some can survive for decades). Once a spore enters a host, it can enter the active phase and start reproducing again. Once they are exposed to the environment again, they can sporulate and wait in the dormant state until they can infect another host.

Anthrax bacteria can contain up to three genes in their DNA that codes for different toxins. It is the toxins that cause symptoms. There are particular ways that these genes can be present and they can be transferred between the bacteria, but I don't want to go into too much detail about that. Just know that there are important toxins that can make an anthrax infection worse if the bacteria have any of those genes.

How is it spread?

There are a few types of infection that anthrax can cause: cutaneous (skin), gastrointestinal (in the digestive tract), inhalational (infected through the lungs), and, rarely, meningitis (infecting the nervous system).

Anthrax is mostly a disease of animals and is naturally present in soil all over most of the world. Human to human transmission is very, very rare. We see it in domestic livestock like cattle and in wild animals. Humans are usually infected through the skin or the lungs, often from animals that have the disease or animal products that are contaminated with anthrax.


Let's break it down into the different types of infection.

Cutaneous: It takes about 1-7 days (incubation period) until a small papule forms on the skin at the infection site. It may become larger. They are usually painless. They can rupture easily and become ulcers and the base will become black, which is characteristic of cutaneous anthrax infection. As long as there are no complications and treatment is received, the lesions will heal just fine. This form is very rarely fatal and is one of the more common forms humans get.
Meat handler: skin lesion of anthrax. Credit: Royal Veterinary College and the Wellcome Collection

Gastrointestinal: This is often how animals are infected, by ingesting the bacteria, and it is an uncommon infection in humans. The incubation period is abut 1-5 days. There will be a fever and localized symptoms like nausea, vomiting, diarrhea, and/or abdominal pain. From here, it can spread to the blood stream (sepsis) and may cause secondary meningitis if it gets into the central nervous system. The gastrointestinal form can be treated, but is about 40% lethal without treatment.

Inhalational: This is the other infection type that humans get (relatively speaking, inhalational anthrax cases are actually quite rare). It is this form that makes anthrax so scary. Inhalational antrax infection is extremely dangerous and is nearly always fatal without treatment. And treatment needs to happen very quickly and aggressively. This type is about 92% fatal, even with treatment. Usually in this form, spores are inhaled and enter the lungs. They incubate for about 1-6 days. The first symptoms are flu-like (fever, tiredness, aches, headaches). As the disease progresses, there is a high fever, decreased lung function, respiratory distress, chest pain, and more. This form can also cause secondary sepsis and/or meningitis as the bacteria spread. While all forms of human cases of anthrax are rare, this one is the reason it is an agent for bioterrorism.

Meningitis: This form is rare and is mostly associated with gastrointestinal or inhalational infections. This happens when the bacteria infect the central nervous system via the blood stream.

Prevention and Treatment:

There is an anthrax vaccine, but it is not normally available to the public. People who usually have access are laboratory and military personnel and other people who may come in contact with infected animals. The vaccine can also be used after a person has been exposed, much like the rabies vaccine.

There are vaccines available for animals like livestock. Vaccinating these animals significantly reduces human exposure.

There are various antibiotics that can be used to treat the different types of infections. Doxycycline and Ciprofloxacin are commonly used, according to the CDC, but there are others. The treatment with antibiotics is very long and intensive to make sure that the bacteria and any spores are treated.

Medical Microbiology 6th Edition by Patrick R. Murray, Ken S. Rosenthal, and Michael A. Pfaller
Centers for Disease Control: Anthrax
WHO: Anthrax

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