Which new viruses could be transmitted to humans through tick bites? Recent research conducted in China, published in the Med journal, tried to answer this question.
Tick-borne viruses (TBVs) can cause diseases ranging from mild illness, such as fever, to very serious conditions, such as encephalitis and haemorrhagic fever. For that reason, they’re the subject of growing public health interest.
In recent years, advanced technologies have enabled the rapid analysis of genetic material and scientists have discovered that ticks carry a wide variety of viruses, many previously unknown.
Diseases transmitted by ticks
After mosquitoes, ticks are the main vectors of transmission of viruses and bacteria. These small parasites, belonging to the acarus group, attach themselves to the skin of the host - human or animal - and feed on its blood; if infected during a previous feed on an animal, they may also transmit that infection to humans.
Ticks are found in most parts of the world. In Switzerland, Ixodes ricinus is the most common of the approximately twenty tick species present, and can transmit two infectious diseases to humans: Lyme borreliosis, caused by the bacterium Borrelia burgdorferi, and tick-borne encephalitis (TBE).
Prevention and protection: what we can do
Prevention remains one of the most effective tools for reducing the risk of infection. Bodies such as the World Health Organisation and the Center for Disease Control and Prevention (CDC) underline the importance of taking simple precautions, especially in areas at risk.
When visiting natural environments such as woods or grassland, it's advisable to wear protective clothing that covers the skin and to use specific repellents. Afterwards, it's vital to carefully check one's body, since a tick bite is often painless and may go unnoticed. Early identification reduces the risk of transmission of certain pathogens.
If bitten, the tick must be removed with tweezers as soon as possible, without squashing it, and that area of the body must be disinfected. As indicated by the Federal Office of Public Health, it's also important to monitor any symptoms during the following week and to consult a doctor if there are any unusual marks.
Within a context in which new viruses are emerging with the potential for transmission to humans, the combination of scientific monitoring and individual prevention remain essential to limit the risk to public health.
How the research into emerging viruses was conducted
Although not all ticks are infected and the risk of transmission is relatively small, knowledge of the viruses that could be passed on to humans is vital for public health.
In recent years, information on the types of TBVs and the areas where they are present has improved, but important aspects are yet to be understood, such as their geographic distribution and actual capacity to infect humans (potential zoonotic disease).
In light of this, researchers have reviewed the scientific literature on TBVs, with three objectives:
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to collect all available information on emerging viruses that can be transmitted by ticks
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to develop a model based on the analysis of viral genetic material to understand which could present a risk to humans
- to verify if any of those deemed to be most hazardous are actually able to cause infections in humans.
An analysis was conducted of over 52,000 studies, published between 1990 and 2024, which enabled the identification of emerging viruses and the mapping of 2,376 locations worldwide where they have been reported.
An overview of viruses associated with ticks
This analysis of the last thirty years identified 230 emerging viruses, belonging to 23 viral families and 15 orders, spread over 53 countries and across all continents. Of these, 19 have been classified as able to infect humans, especially those belonging to the Nairoviridae and Flaviviridae families.
The results show that the viruses are most widespread in China, where the highest number of human infections was recorded, followed by Japan and the US. The most prevalent viral family is the Phenuiviridae, while the Flaviviridae show the greatest capacity to infect several hosts, including humans. The tick species Haemaphysalis longicornis hosts the greatest number of viruses (47) while, in animals, sheep are the most frequently infected host.
Which viruses could infect humans?
To assess the potential for infection of the 128 viruses not yet associated with human infections, the researchers developed a model based on their genetic information. RNA polymerase is an enzyme that enables the replication of viruses within cells; the properties of this “molecular machine” were used as an indicator of the viruses’ behaviour, and to identify those that could adapt themselves to humans. This approach led to the identification of 25 viruses with high zoonotic potential.
The results were then verified practically with an epidemiological investigation: in the final stage, 896 patients with fever and a history of tick bites were analysed, recruited from hospitals in north-east China. Three viruses, previously only identified in ticks, were also found in patients: Sara tick phlebovirus (STPV), Dabieshan tick virus (DBTV) and Shanxi tick virus 2 (SXTV2), were detected in 28 (3.13%), 3 (0.33%) and 6 (0.67%), respectively, of the patients examined.
In conclusion, the study showed that numerous tick-borne viruses remain little known, and some already have the capacity to infect humans. In light of these results, the researchers recommend that monitoring is strengthened, especially of TBVs belonging to the Flaviviridae, Phenuiviridae and Nairoviridae families, in order to identify other new viruses that may be of importance for human health.
