Overview of tularaemia

The bacterium which causes tularaemia, Francisella tularensis (F. tularensis), is highly infectious and able to initiate disease in humans with exposure to as little as 10 to 50 micro-organisms.^1,2 Four subspecies of F. tularensis have been discovered, but only two of these are most commonly associated with human disease: F. tularensis subspecies tularensis (Type A) and F. tularensis subspecies holarctica (Type B).^2,3 The different subtypes are found at different prevalences in different areas: Type A, which is more virulent, is primarily found in North America, whereas Type B is found more widely globally.^1,2,3

Reservoirs and hosts of tularaemia^1,2,3

The bacterium F. tularensis can survive in a wide variety of environments, including soil, water, and decaying animal corpses, for extended periods, which renders it a versatile pathogen, and also one that is more difficult to prevent the spread of.^4,5

The vectors that are able to transmit the disease to humans become infected themselves through infected bites from ticks, ingested contaminated water or food, or through direct contact with the bacteria in the environment. Some of the most troublesome mammalian hosts or reservoirs for the spread of Tularaemia to humans include rabbits, rodents, and hares^3,4,5.

Rodents and hares^1,3,4

Rodents and hares are particularly susceptible to infections, and research has shown that a large outbreak of tularaemia in rodents and hares, which are more common in the summer, is often associated with an increase in human cases too.^3,4,5 When an infected rodent or hare dies, its corpse becomes a reservoir and source of F. tularensis. Their carcass can contaminate surrounding soil and water, which can then further perpetuate the spread and transmission to other hosts and vectors.^4,5

Tularaemia transmission to humans^1,2,3,4,5

Humans are able to contract tularemia through several different means, which makes it considered a multifaceted disease in regard to its transmission.

The various routes of exposure include:

Direct contact with infected animals (Living or dead)^1,2,3,4,5

One of the most common modes of transmission to humans is direct contact with infected animals. The bacteria can enter the body through minor cuts, abrasions on the skin, or cracked skin. This form of infection results in the ulceroglandular form of tularaemia, which is characterised by skin ulcers at the site of infection and swollen lymph nodes.

Alongside direct contact with the infected animal itself, direct contact with blood tissue or fluids from the infected animal can also transmit bacteria, particularly if protective measures aren’t taken.

From this, those most at risk are individuals who participate in activities such as hunting or skinning, or laboratory workers and veterinarians who handle bacterial cultures or animal tissue, respectively.

Insect bites: ticks, deer flies, and mosquitoes^1,2,3,4,5

Vector-borne transmission is another significant route of human infection. Various arthropods, including ticks, mosquitoes, and deer flies, can carry and transmit F. tularensis.

1. Ticks: Ticks are one of the most common vectors for tularemia in North America, particularly for the more virulent Type A strain. Ticks become ‘infected’ with the bacterium through feeding on infected animals, which then enables them to transmit it to humans during subsequent feedings. Tick-borne tularemia is often associated with outdoor activities such as hiking, camping, and working in forested areas, particularly in the spring and summer when tick populations are most active.
2. Deer Flies: In some regions, particularly the western United States, deer flies are significant vectors. They transmit the bacterium through bites, usually leading to the ulceroglandular form of the disease. 
3. Mosquitoes: Mosquitoes have also been implicated in the transmission of tularemia, particularly in Europe and Scandinavia, where mosquito-borne tularemia is more common. 

Inhalation of aerosolised bacteria^1,2,3,4,5

Inhalation of aerosolised F. tularensis particles can lead to pneumonic tularemia, which is the most severe and potentially fatal form of the disease. This transmission route is typically associated with activities that disturb contaminated soil or animal carcasses, such as farming, mowing, or construction. Inhalation-related tularemia can also occur in laboratory settings where the bacterium is handled.

If left untreated, Pneumonic tularemia is associated with a high mortality rate, making it of great concern. Inhalation can occur when bacteria are aerosolised through the disturbance of animal carcasses, soil, or even during laboratory procedures.

Ingestion of contaminated food and water^1,2,3,4,5

Another route of F. tularensis transmission is the consumption of contaminated food or water, although, in the modern day, this is less common than other modes of transmission. This form of tularemia, known as oropharyngeal tularemia, can occur when people drink untreated water from contaminated natural sources or consume undercooked meat from infected animals. The bacterium can survive in water sources contaminated by animal carcasses, making it a potential risk for people who rely on untreated water.

In areas where tularemia is endemic, public health authorities recommend boiling water or using water treatment methods to reduce the risk of infection.

Ocular transmission^1,2,3,4,5

Ocular tularemia is a mode of transmission whereby the bacterium enters through the eyes, usually through direct contact with contaminated fingers, water, or aerosols. This form of the disease, known as oculoglandular tularemia, presents as conjunctivitis and swollen lymph nodes around the eye. Although less common, it can occur when handling infected animals or coming into contact with contaminated water or dust.

Symptoms of tularaemia^1,2,3,4,5,6

The signs and symptoms of tularaemia can vary depending on the site of entry of F. tularensis:

• If exposure occurs via the eye, a symptom can be painful conjunctivitis (an eye condition entailing inflammation of the conjunctiva, which is the clear tissue that covers the white of the eye and the inner eyelid)
• If exposure occurs through the ingestion of contaminated food, water, or oral contamination from the hand, the symptoms include a painful throat, swollen lymph nodes in the neck, nausea, vomiting, diarrhoea, and abdominal pain
• If exposure occurs through a bite, i.e. via the skin, the symptoms include an ulcer at the site of the bite, with painful and swollen lymph nodes

Symptoms that can occur across all modes of exposure include:

• Headaches
• Fever and chills
• Rash
• Cough, shortness of breath, and chest pain
• Muscle and joint aches
• Painful and/or swollen lymph nodes

Diagnosis^2,4

Tularaemia is diagnosed through a combination of clinical evaluation, patient history and laboratory tests. Most commonly diagnosis is made via a series of blood tests. The bacteria can sometimes be grown from specimens, such as sputum or skin swab samples.

Prevention and control measures^2,4,5

Although F. tularensis is quite a versatile bacterium, reducing the risk of tularaemia is quite easy to do.

Ways to decrease the risk of F. tularensis infection:

• Never drink untreated water
• Ensure all meat is cooked to a safe temperature and areas of contact with uncooked meat are properly sanitised
• Wear gloves when handling animals (both living or dead)
• Supervise pets when they go outside to reduce the chances of bites from other infected animals
• Ensure pets get checked for ticks and have the proper preventative care

Summary

Although tularaemia is not a very common disease due to an increase in awareness and hence, better preventative care, it’s important to remember its dangers and risks to ensure that we can reduce the risk of ourselves or our pets being infected by F. tularensis.

The various routes of human infection include direct contact with infected animals, insect bites, ingestion of contaminated food or water, and inhalation of aerosolised bacteria. Common animal reservoirs include rabbits, rodents, and hares, while vectors like ticks and mosquitoes transmit the bacterium.