Bourbon virus is an RNA virus in the genus Thogotovirus of the family Orthomyxoviridae, which is similar to Dhori virus and Batken virus. It was first identified in 2014 in a man from Bourbon County, Kansas, United States, who died after being bitten by ticks. The case is the eighth report of human disease associated with a thogotovirus globally, and the first in the Western Hemisphere. The lone star tick (Amblyomma americanum) is believed to play an important role in transmitting the virus to humans.
In May 2015, a case was discovered in Stillwater, Oklahoma. The patient fully recovered. Relatively little is known about the virus, however it is primarily vectored by ticks. No specific treatment or vaccine is specifically approved for Bourbon virus infection, and avoidance of bites is recommended to reduce the risk of infection. Individuals infected with the Bourbon virus may experience symptoms such as fever, headaches, muscle pain, fatigue, nausea, and skin rashes. In June 2017, a 58-year-old female Missouri State Park employee died from an infection of the Bourbon virus after it had been misdiagnosed for a significant period.
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Discovery
The virus was discovered in 2014 by Olga Kosoy, Amy Lambert, and colleagues from the Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado, in a sample of blood from the case patient. Tests had previously ruled out a wide range of tick-borne diseases including anaplasmosis, babesiosis, ehrlichiosis, Q fever, Rocky Mountain spotted fever and tularemia. During tests for Heartland virus, a recently discovered Phlebovirus known to be transmitted by ticks, prominent plaques, or areas where the cells were affected by virus infection, were observed on one-cell-thick cultures of African green monkey kidney cells.
The plaques did not resemble the effects of Heartland virus, and the researchers hypothesized that they were the work of another virus. Recently developed "next-generation" sequencing techniques were employed to find novel viral RNA sequences in cell culture supernatants, similar to viruses of the genus Thogotovirus, family Orthomyxoviridae. Lambert, who worked on the sequencing, explained that these "state-of-the-art" techniques could be used to identify pathogens that older technologies could not detect.
Next, real-time reverse transcription–polymerase chain reaction was used to confirm that this novel virus originated in the patient's blood sample. Finally, examining supernatants from the cell cultures under the electron microscope revealed virus particles of different shapes, including filaments and spheres. The virus was named "Bourbon virus" for the county in which it originated.
The identification of Bourbon virus represented an important development in the study of tick-associated viruses in North America. Genetic analysis showed that the newly discovered virus belonged to the genus Thogotovirus, although its genetic characteristics distinguished it from previously identified members of the group. Before the discovery, thogotoviruses associated with human illness had primarily been documented in other parts of the world. The identification of Bourbon virus in Kansas expanded scientific understanding of the geographic distribution of these viruses and demonstrated the importance of advanced genetic testing in identifying previously unrecognized pathogens.
Virology
Structure and Morphology
Bourbon virus is a type of thogotovirus, which is in the RNA virus family Orthomyxoviridae. The virus particles show different morphologies, including filamentous and roughly spherical forms, and have nucleoprotein and glycoprotein projections from the surface. The spherical virions have a range of diameters, with broadly 100–130 nm being common. Virus was present at high levels in the extracellular space, with occasional particles being observed in the process of endocytosis.
Genome and Viral Proteins
Like other members of the Orthomyxoviridae, the Bourbon virus genome is 10 to 11-kb, single-stranded, negative-sense RNA, which is segmented, or divided into several separate pieces. It has at least six segments. The segments include glycoprotein (GP), nucleoprotein (NP), matrix protein (MP), polymerase acidic protein (PA), polymerase basic protein 1 (PB1), and polymerase basic protein 2 (PB2). These genome segments help describe how the virus is organized and which proteins are important to its structure and replication.
Related Thogotoviruses
The Bourbon virus is most similar in RNA sequence to Dhori virus and the closely related Batken virus, both members of the genus Thogotovirus, but has diverged substantially from these viruses. Dhori and Batken viruses have only been reported in the Eastern hemisphere. This relationship helps place Bourbon virus withing the larger Thogotovirus family while showing how distinctive it is from the other members within the group.
Epidemiology
The Bourbon virus was first identified in a previously healthy man aged more than 50 years from Bourbon County, Kansas, United States, who worked as a farmer. He died in June 2014. Dana Hawkinson from the University of Kansas Hospital, where the patient was treated, has speculated that the virus had been previously present undetected. There have been five confirmed cases since 2014.
The CDC is developing investigational diagnostic tests for Bourbon virus infection. In December 2014, CDC and the Kansas Department of Health and Environment were planning to test blood samples from people in Kansas who have recently experienced similar unexplained symptoms. Blood testing conducted in mammals in the regions where the virus has been identified have revealed that several mammalian species, including domestic dogs, eastern cottontail rabbits, horses, raccoons, and white-tailed deer can all be infected.
The case patient reported multiple tick bites, including a blood-filled tick in the shoulder area that was observed a few days before the onset of symptoms. The virus is known to be tick-borne, and has been isolated in lone star ticks. A study performed in 2022 also detected the virus in populations of the Asian longhorned tick. Most thogotoviruses are transmitted solely by ticks, but the similar Batken virus has also been isolated from mosquitoes. Studies to establish the prevalence of Bourbon virus in tick and insect populations were planned in December 2014.
Signs and symptoms
The patient had high fever, headache, decreased appetite, muscle aches, joint pain, fatigue, malaise, nausea, vomiting, diarrhea, and a maculopapular rash on the abdomen, chest, and back. Late in the course of the illness, he experienced shortness of breath, which developed into acute respiratory distress syndrome. He died from multiple organ failure 11 days after the earliest symptoms.
Laboratory abnormalities observed included a decrease in the patient's white cell and platelet counts, considered to be caused by bone marrow suppression, and an increase in liver enzyme levels. These symptoms and alterations in blood cells are similar to those seen with tick-borne illnesses, including ehrlichiosis and disease caused by the Heartland virus.
The degree to which the Bourbon virus contributed to the individual's illness and death remains unknown as of February 2015, as does whether these symptoms and laboratory abnormalities represent the typical course of infection with the Bourbon virus. Hawkinson has speculated that there have been previous undiagnosed cases with milder symptoms from which the infected patients recovered.
Thogotoviruses rarely cause disease in humans. The case in Bourbon County is only the eighth report of human disease associated with a thogotovirus globally. Thogoto virus caused disease in two people from Nigeria, one of whom died. Dhori virus caused disease in five laboratory workers infected accidentally. Although another member of the genus Thogotovirus, Aransas Bay virus, has been found in seabird-associated ticks in the United States, this is the first reported case of human disease apparently associated with a thogotovirus in the Western hemisphere.
Treatment and prevention
No routine diagnostic test is yet available. There is currently no specific treatment or vaccine for the virus; supportive therapy is recommended. Antibodies effective at neutralizing the virus have been discovered in human blood serum samples, even from patients with no previously reported exposure. Since the virus is transmitted by a tick or insect, the main prevention method recommended is the avoidance of tick and insect bites. In mice models, Favipiravir has been shown to be beneficial both therapeutically and prophylactically.