Tularemia: Rabbit Fever as a Biological Weapon?

History

Tularemia was initially described in 1911 as a plague-like illness in rodents by McCoy and Chapin in California, USA. In 1912, the organism F. tularensis (called Bacterium tularense at the time) was cultured for the first time from squirrels in the region. Two years later, researchers Wherry and Lamb in Ohio described the first F. tularensis infection in humans [3,18]. 

In 1919, Dr. Edward Francis proposed “tularemia” to describe several clinical conditions caused by F. tularensis. He also deduced that wild rabbits were the source of infection. Tularemia is also known as rabbit fever for this reason. 

Simultaneously, other researchers were hard at work trying to understand tularemia. In 1923, Parker et al. made a significant discovery that ticks were the primary vectors of the disease. However, it took almost two decades later to isolate and name the causative agent. 

Dr. Francis also accidentally discovered that F. tularensis was highly infectious when he and his entire team contracted the disease. In honor of his dedicated work on Tularemia, the bacteria was named Franciscella Tularensis in 1947 after his death [3,5,18]. 

By 1960, around 85% of all cases of tularemia in the south-central US were found to be associated with tick exposure. Rabbits were the main contributors to the spread of tularemia to humans [16]. Eventually, it was discovered that tularemia could be spread through other vectors, including deer flies, which is why the disease is also known as deer fly fever. 

Tularemia as a Biological Weapon

Since the pathogen is infectious and can be aerosolized, many nations began exploring its potential as a biological weapon [18]. Japan started early research on using F. tularensis as a biological weapon in 1932 and performed experiments on prisoners of war. China also developed bioweapons using F. tularensis and other species. For instance, during the Second World War, over 10,000 people were subjected to bioweapon experiments involving F. tularensis and other contagious organisms [4].   

By the mid-twentieth century, the United States and the Soviet Union (including Russia) also began research using F. tularensis. They used genetic engineering techniques to create new strains of bacteria that are antibiotic-resistant and vaccine-subverting. These new versions were stockpiled to be used as biological weapons [2].

According to a former Soviet Union biological weapons scientist, most tularemia outbreaks in Eastern Europe during the Second World War resulted from bio-attacks. Modified strains of F. tularensis were used intentionally to cause infectious disease outbreaks. The Soviet Union continued to conduct research on biological weapons even in the 1990s [17].  

In the 1950s and 1960s, the US conducted voluntary human experiments using F. tularensis on inmates in penitentiaries and non-combatant soldiers [18]. However, the country later prohibited further attempts to develop such toxins. And after the Biological Weapons Convention of 1972 banned the development, production, and stockpiling of bacteriological and toxic weapons, all US stockpiles were destroyed [17].

Epidemiology

There are four different species of Francisella tularensis, but only two of them cause tularemia in humans: F. tularensis and F. holarctica. Infections caused by F. tularensis have a mortality rate of 5% to 6%, and F. holarctica-related tularemia has a mortality rate of less than 0.5% [6].  F. tularensis is more prevalent in the North American region, whereas F. holarctica is prevalent in Europe and Asia [7].

The rate of infection is usually high in summers. This is because ticks, vectors responsible for the spread of the infection, come out of hibernation and become active during the season. Children and people over the age of 75 years are the ones that are most affected by the disease [7]. Butchers, farmers, foresters, hikers, hunters, people living in the countryside, and veterinarians, to name a few, are among other people who are also at a higher risk of getting tularemia [8].

The incidence rate for Tularemia is higher in boys compared to girls. This may be because boys may be more exposed to vectors as they engage in outdoor activities like hunting and trekking during the summer [7]. Tularemia is mainly reported in countries located in the temperate regions, which are between the tropic and the polar regions. These include the USA, Canada, China, Russia, Korea, Japan, and nordic countries [5]. However, lately, the disease is re-emerging in countries that had not reported the disease before. These include Bulgaria, Turkey, Kosovo, Yugoslavia, Spain, Sweden, Switzerland, and even parts of China [20]. 

Notable Tularemia Outbreaks

Several Tularemia outbreaks occurred during the 20th century and the beginning of the 21st century. The first outbreak was reported in Tulare County, California, USA, in 1911 [6]. Decades later, the United States witnessed another notable Tularemia outbreak in 2002. The outbreak began in a massive pet trading facility in the State of Texas. The organization sold a variety of animal species to domestic and international distributors and was found to have had thousands of prairie dogs infected by F. tularensis [3]. 

European countries have also had their share of Tularemia outbreaks. In the late 1990s, Spain witnessed its first outbreak, with over 500 cases reported. In 2000, another notable outbreak was reported in Kosovo, a country in southeast Europe, where over 300 cases were reported [3]. Bulgaria and Turkey have also experienced Tularemia outbreaks. In Bulgaria, 285 cases were reported from 1997 to 2005 [9]. Turkey witnessed several epidemics of Tularemia starting in 1936. As of 2008, 1300 cases were serologically confirmed in the country [10].

Tularemia was more common in the past. For example, while in 1950, the United States had just under 1000 cases, there were only 274 cases reported to the CDC in 2019 [19]. However, Sweden reported their largest Tularemia outbreak in 2019, with almost 1000 confirmed cases [21].

How is it Spread?

Although the pathogen is highly infectious, tularemia cannot be spread from person to person. Humans can get a Tularemia infection through: 

• Ticks: F. tularensis can be transmitted to humans through different ticks – mainly the American dog tick. In the US, the Lone Star tick, Rocky Mountain wood ticks, Pacific Coast ticks, and American dog ticks are the common vectors of F. tularensis. The infection is commonly spread in Europe through ornate cow ticks (ornate dog, meadow, and marsh ticks) and castor bean ticks [16].   
• Fleas and flies: Tularemia can also be transmitted through fleas or flies – including deer flies and mosquitos [2,11]. The disease is also known as ‘deer fly fever.’ 
• Animals: People can get infected if they eat the meat of an infected animal, handle the carcass of an infected animal, or are bitten by an infected animal [11]. The tularemia bacteria can infect over 200 mammals, including rabbits, coyotes, rats, and more.
• Water: Tularemia can be transmitted through water contaminated with F. tularensis since this particular bacteria can survive long periods in water [7].
• Air: Inhaling aerosolized airborne bacteria or agricultural or landscaping dust contaminated with F. tularensis can infect people [11]. 

Biology of the Disease

F. tularensis is a facultative intracellular gram-negative bacteria. Once it enters the human body, the pathogen can enter, proliferate and survive in various host cells. The first types of cells it interacts with are immune cells — macrophages, neutrophils, and dendritic cells are phagocytic cells. Phagocytes in our immune systems are meant to engulf bacteria and foreign particles and envelop them in phagosomes. After that, a phagosome is supposed to degrade and destroy foreign bodies. 

However, F.tularensis can break through a phagosome’s barrier, invade the host’s bloodstream and lymph nodes, and enter many other types of cells. Over time, the bacteria reach and infect various organs like the lungs, spleen, kidney, liver, and more. 

F. tularensis is stealthy, and its natural protections evade the host cell’s immune inflammation until the disease is in its later stages [5, 21]. This makes it hard to detect the infection early. 

Diagnosis

Tularemia can be hard to diagnose. Many symptoms are common with many other illnesses. It is also a rare condition, and not all clinicians may consider it in their differential diagnosis at the point-of-care. Different diagnostic methods are used to detect the presence of F. tularensis in the human body. The most common ways to diagnose tularemia are: 

Serology: Serology is one of the most widely used diagnostic methods to diagnose the disease. The serum of the person suspected of having the disease is examined for agglutinating antibodies. But, the process takes time as these antibodies cannot be detected until the second week of illness. This method is very useful in understanding the emergence of the disease in a specific region [3,7].

PCR: Polymerase Chain Reaction (PCR) is one of the methods to detect the presence of F. tularensis DNA in clinical specimens. The PCR assays are more accurate and help rapidly detect the organism’s presence in specimens [3]. 

Culture: An F. tularensis infection is detected by isolating the organism in blood, body fluids, or tissue. It is a slow-growing organism that can be cultured on artificial mediums such as enriched chocolate agar, cysteine heart agar, and buffered charcoal yeast extract. It takes 1 to 3 days for the bacteria to grow. Since F.tularensis is highly infectious, culture tests are done only in biosafety level 3 (BSL-3) laboratories [3,7].

16S rDNA sequencing: 16S rDNA sequencing is an advanced diagnostic method for identifying bacteria in patient specimens. It is particularly effective at detecting slow-growing bacteria like F. tularensis [3].

Molecular subtyping: Molecular subtyping is an advanced methodology to detect the presence of all F. tularensis subspecies in the absence of culture. In addition to the subspecies tularensis (type A) and holarctica (type B), this particular diagnostic method is instrumental in also tracing out the other two subspecies of F. tularensis as well [3].

Types of Tularemia and Symptoms

The incubation period of tularemia is usually 1 to 21 days (2 to 5 days on average) [7]. There are six major clinical forms of tularemia depending on the bacteria’s mode of entry and the host’s immunity level. They are glandular, oculoglandular, oropharyngeal, pneumonic, typhoid, and ulceroglandular tularemia [12].

Ulceroglandular Tularemia: This is one of the common clinical forms of tularemia. It usually occurs through an insect bite (tick, flea, or fly) or even by handling an infected animal. Skin ulcers may appear at the site where the bacteria enters the body [13]. It accounts for 42% to 75% of cases of tularemia. Some of the common symptoms include:

• Painful or swollen papules
• Swollen lymph glands
• Fever
• Tiredness [7,12]

Glandular Tularemia: Glandular tularemia represents 15% to 44% of all cases of tularemia. The entry point for bacteria into the host is assumed to be the skin. Some of the common symptoms include:

• Tender lymphadenopathy
• Fever
• Tiredness [7,12]

Oropharyngeal Tularemia: This type of tularemia represents less than 5% of all cases. It is caused when people ingest food and water contaminated with F. tularensis. Common symptoms include:

• Fever
• Sore throat
• Mouth ulcers
• Swollen lymph glands in the neck region [7,12,13]

Oculoglandular Tularemia: This type of tularemia represents less than 4% of all cases. In oculoglandular Tularemia, the bacteria enters through the eyes. Individuals can get infected if they wash their eyes with water contaminated with F. tularensis or touch their eyes after handling contaminated meat [13]. Some of the common symptoms include:   

• Irritation and inflammation of the eye
• Swollen lymph glands [7,12,13]

Typhoidal Tularemia: Typhoidal tularemia occurs when F. tularensis enters the body through the nose or oral cavities. Some of the common symptoms include:

• Fever
• Headaches
• Muscle pain
• Throat pain
• Diarrhea [7,12]

Pneumonic Tularemia: Pneumonic tularemia is one of the most serious clinical forms of tularemia [13]. It is caused when an individual inhales aerosolized or agricultural or landscaping dust contaminated with F. tularensis [12,13]. Some of the common symptoms include:

• Fever
• Cough
• Chest pain 
• Difficulty in breathing [7,12,13]

Treatment

Since tularemia is a bacterial infection, antibiotics are the best treatment. Antibiotics such as streptomycin, doxycycline, ciprofloxacin, and gentamicin are commonly used [14]. Treatment for tularemia will have to start as soon as a person is suspected of having the disease rather than waiting for serological test reports. This is because more complications will arise as the treatment gets delayed [7]. Depending on the patient’s clinical condition, treatment may last 10 to 21 days. However, patients may continue to have symptoms for several more weeks even if they recover from the disease [14].

Prevention

Unfortunately, there is no approved vaccine for tularemia, particularly in the United States. Since F. tularensis is a top priority biothreat — 1000 times more infectious than Anthrax — there is an urgent need for one. Until now, the only vaccine against tularemia was a Russian-made live attenuated vaccine strain used to immunize people in the former USSR. However, the vaccine is not approved in many other countries, including the United States [15].

Research to find an effective tularemia vaccine is underway; hopefully, an approved vaccine will be available. Until then, a few basic preventive measures may help:

• Avoid touching sick animals or animal carcasses without gloves and protective equipment for the eyes.
• Stay away from sick animals and tick-infested places. 
• Avoid drinking water that is not adequately sterilized.
• Thoroughly cook meat before eating.
• Wear full-length clothes covering most of your body, particularly when outdoors. This way, you can prevent yourself from insect bites (ticks, flies, fleas, etc.).
• Apply medically-approved insect repellents to the body frequently.
• Check children for ticks (including their hair). Use tweezers to pull out ticks instead of using bare hands. 
• Wash hands well after handling ticks.
• In case of travel to regions known to be endemic to tularemia, mention it to your physician or pediatrician [7].