Introduction 

Tularemia is categorised as an acute febrile zoonotic illness caused by the highly infectious Gram-negative organism, Francisella tularensis. The symptoms can demonstrate variability due to a dependence on the route.

What are zoonotic diseases?

Zoonotic diseases or zoonosis (zoonotic disease or zoonoses -plural) are infectious diseases that are transmitted between species from animals to humans (or from humans to animals).

Overview of tularaemia 

• Pathogen: Francisella tularensis bacteria
• Transmission Methods: Inhalation, ingestion, direct contact with infected animals, or bites from ticks, flies, etc. 
• Symptoms and Severity: Sudden fever, skin ulceration, swollen lymph glands, pneumonia in severe cases
• Geographic Prevalence: Common in North America, Europe, and parts of Asia

Key zoonotic diseases for comparison 

Brief descriptions of other zoonotic diseases: 

• Rabies: Viral disease; transmitted through the saliva or mucus of infected animals, usually through bites or scratches
• Lyme Disease: Bacterial disease, transmitted through the bite of an infected tick, usually a deer tick, also known as a black-legged tick
• Plague: Bacterial disease, transmitted by flea bites or by coming into contact with infected animals
• Brucellosis: A Bacterial disease, transmitted through direct contact to humans by infected animals or contaminated animal products
• Leptospirosis: Bacterial disease, also known as Weil's disease, is transmitted through the urine of infected animals

Analysis and symptoms

Tularemia is a zoonotic disease that is capable of causing a variety of symptoms, including sudden fever, chills, severe headache, diarrhoea, muscular aches, joint pain, dry cough, progressive weakness, skin ulcers, and swollen, painful lymph glands. 

Symptoms have an appearance 3 to 5 days following bacterial exposure, but can take as long as 14 days for expression. The incubation period can range anywhere between 1-21 days, depending on the mode of infection and the infective dose or titre. 

Tularemia is most often found in rodents such as rabbits, hares, squirrels, etc. People and animals most commonly get afflicted with tularemia due to a bite by an infected tick or fly, contact with an infected animal, or Infected pets or wildlife. 

Sources of infection and routes of transmission

Tularaemia can be transmitted to humans via multiple routes:

• Arthropod bites (predominantly tick or mosquito bites), which probably cause 10 – 90% of tularaemia cases in regions like Europe 
• Direct transmission from an animal reservoir, which might have an occurence through handling an infected animal (especially a hare), ingestion of undercooked meat prepared from an infected animal, or an animal bite (especially by small rodents, cats, and dogs)
• Spread through direct contact or ingestion of contaminated water and soil
• Inhalation of infective aerosols, e.g. during farming activities

Clinical manifestations

The incubation period of tularaemia is usually 3 – 5 days, and can extend up to 21 days. The clinical presentation depends on the entry site and titre of the bacteria, virulence of the F. tularensis strain, as well as the immune status of the host. 

Clinical presentation of tularaemia may range from asymptomatic to severe, with progression to sepsis and a fatal outcome if left untreated, particularly in immunocompromised patients.

Six different clinical forms occur depending on the site of transmission, involving oculoglandular, oropharyngeal, glandular, typhoidal, ulceroglandular and respiratory tularemia. However, it has been observed that the initial clinical presentation is very similar in all of them. 

From the site of infection, bacteria spread through the lymphatic system to draining lymph nodes, where their replication takes place. The bacteria may then disseminate to various organs, including the spleen, liver, lungs, kidneys, central nervous system, and skeletal muscles. During this time, infected humans express or present with flu-like symptoms, including the rapid development of fever with chills, headache, fatigue and general body aches. Within a few days of the onset of fever, the patient usually perceives a regional lymph node enlargement.

Besides fever and nonspecific symptoms, lymphadenopathy is a major reason for drawing the attention of clinicians.⁴ 

Diagnosis

Diagnosis of tularemia comes with the requirement of a high degree of clinical suspicion due to the difficulty in obtaining a confirmation with laboratory testing. It is important to recognise that early testing might demonstrate a negative result as antibodies have not yet had time to form. Therefore, it is essential to keep in mind that initial negative serology does not rule out tularemia infection.³

Treatment/management

To date, there hasn’t been a randomised controlled trial to obtain a definition of the optimal antibiotic management for tularemia infection. Currently, most sources have a recommendation of treatment with intravenous (IV) gentamicin for 7 days to a fortnight, depending on the illness severity.

Fluoroquinolones, such as ciprofloxacin, or tetracyclines should be used cautiously for the treatment of tularemia, as these agents are bacteriostatic and are associated with a high rate of rebound or relapse following discontinuation of treatment. In some cases, incision and drainage of the affected lymph nodes may also serve as a requirement.

Differential diagnosis

• Chlamydial infections
• Diphtheria
• Endocarditis, bacterial
• Endocarditis, fungal
• Legionella infection
• Lyme disease
• Malaria
• Psittacosis
• Q-fever

Prognosis

If tularemia goes untreated, it carries a mortality rate of 10 - 50%. Typhoid and tularemia carry the highest mortality. Even the patients who manage to survive are left with residual scars, lung and kidney damage, accompanied by muscle loss.

Complications

• Lung abscess, pneumonia
• ARDS
• Rhabdomyolysis
• Renal failure
• Meningitis
• Peritonitis

Summary

Tularemia is an acute, potentially severe zoonotic disease caused by the bacterium Francisella tularensis. It is transmitted to humans through arthropod bites (ticks, flies), direct contact with infected animals, ingestion of contaminated food or water, or inhalation of infected aerosols. The disease primarily affects rodents and lagomorphs but can spread to humans, often through tick bites or handling infected animals.

Symptoms vary based on transmission route and include sudden fever, lymph node swelling, skin ulcers, pneumonia, and in severe cases, sepsis. Six clinical forms of tularemia exist, depending on the entry route. Diagnosis can be difficult due to nonspecific symptoms and delayed antibody formation, requiring high clinical suspicion.

Treatment typically involves IV gentamicin; however, relapse can occur with other antibiotics like fluoroquinolones or tetracyclines. If untreated, tularemia has a mortality rate of 10–50%. Complications can include pneumonia, meningitis, renal failure, and lasting organ damage.

Compared to other zoonotic diseases like rabies, Lyme disease, plague, brucellosis, and leptospirosis, tularemia is distinct for its multiple transmission routes, diverse clinical forms, and high potential for systemic complications.