Other Tick-Borne Illnesses: Tularemia, Colorado Tick Fever, Tick Paralysis

Ticks are vectors of viruses that cause diseases in humans and animals. Tick-borne viruses are maintained in nature through circulation between ticks and vertebrate hosts, but the molecular mechanisms by which viruses switch between infecting tick and vertebrate cells are largely unknown. During their blood meal, ticks counteract multiple host defense responses by a wide array of biologically active salivary molecules. Vertebrate host skin provides the first line of defense to injury and infection by viruses. The early interactions between host immunity and initial tick-mediated immunomodulation at the tick–host skin interface are suggested to be essential for transmission of viruses. Although promotion of transmission of viruses by tick saliva has been reported, the underlying molecular mechanisms are poorly understood, especially at the skin interface during early transmission. Future research should be aimed at the identification of the key tick salivary components enhancing virus transmission and the molecular mechanisms of the tick–host–virus interactions.

Several important human diseases worldwide are caused by tick-borne viruses. These diseases have become important public health concerns in recent years. The tick-borne viruses that cause diseases in humans mainly belong to 3 families: Bunyaviridae, Flaviviridae, and Reoviridae. In this review, we focus on therapeutic approaches for several of the more important tick-borne viruses from these 3 families. These viruses are Crimean-Congo hemorrhagic fever virus (CCHF) and the newly discovered tick-borne phleboviruses, known as thrombocytopenia syndromevirus (SFTSV), Heartland virus and Bhanja virus from the family Bunyaviridae, tick-borne encephalitis virus (TBEV), Powassan virus (POWV), Louping-ill virus (LIV), Omsk hemorrhagic fever virus (OHFV), Kyasanur Forest disease virus (KFDV), and Alkhurma hemorrhagic fever virus (AHFV) from the Flaviviridae family. To date, there is no effective antiviral drug available against most of these tick-borne viruses. Although there is common usage of antiviral drugs such as ribavirin for CCHF treatment in some countries, there are concerns that ribavirin may not be as effective as once thought against CCHF. Herein, we discuss also the availability of vaccines for the control of these viral infections. The lack of treatment and prevention approaches for these viruses is highlighted, and we hope that this review may increase public health awareness with regard to the threat posed by this group of viruses.