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Ticks are responsible for spreading over 25 human and animal diseases. While you may be familiar with some of these – such as Lyme disease – there are many others you’ve probably never heard of, including some that have been discovered only in the past few years, such as wetland virus.
In a new case study, researchers in China have published details about wetland virus. It was first identified in 2019, after a patient bitten by a tick while visiting a wetland park in Inner Mongolia was admitted to hospital with symptoms of fever, headache and vomiting that progressed to multiple organ dysfunction.
To understand the cause of the patient’s illness, the researchers sequenced the genetic material extracted from a blood sample to find the virus responsible for the disease. They found a previously unknown virus – a close relative of other tick-transmitted viruses, including the dangerous Crimean–Congo haemorrhagic fever virus, which has a 30% mortality rate. Wetland virus is a member of the orthonairovirus genus in the Nairoviridae family.
The research team then screened other patients from across north-eastern China who had developed an acute fever within one month of a tick bite. This led to them identifying another 17 cases of wetland virus infection – confirming the virus was well established in the region.
Next, the team carried out a large-scale ecological survey to understand the source of wetland virus. They looked at ticks, as well as livestock and wild animals living in the region, as animals often play an important role in the spread of tick-borne pathogens. They found some sheep, pigs and horses were infected, suggesting livestock could be reservoirs for wetland virus.
So far, wetland virus has only been found in north-eastern China. However, the tick species implicated in transmitting it has a much wider distribution across Europe and Asia, so it’s likely the virus is far more widespread. Surveys of ticks and livestock, and even patients with unexplained fever, elsewhere in Asia and in Europe could confirm this.
Tick-borne diseases
So why does it seem like we are finding a lot of new tick-borne diseases at the moment?
First, many tick-borne diseases have non-specific symptoms such as fever, headaches and fatigue, making them easily mistaken for other common illnesses. Furthermore, tick-borne diseases are not contagious, so do not occur in dramatic epidemics like COVID or flu.
Taken together, these characteristics make spotting a tick-borne disease for the first time very difficult. However, new genetic sequencing techniques, including the one used to identify the wetland virus, has made spotting new pathogens easier.
Second, even if a new pathogen is detected in a patient, linking it to ticks is not always straightforward. Many people get bitten by ticks without knowing.
Unfed ticks are small and often attach to our skin in places that are hard to check, like the backs of our legs. Also, we don’t “feel” a tick bite in the same way as you would, say, a mosquito bite. So many patients with a tick-borne disease don’t have any recollection of being bitten.
Plus, for many tick-borne diseases, the onset of symptoms can be delayed, so linking them with a tick bite is not an obvious step. For instance, Lyme disease symptoms typically show up three to ten weeks after a bite.
Third, medical awareness of tick-borne diseases – particularly those that are emerging or rare – is patchy and, in many parts of the world, the resources needed to diagnose them often just aren’t there.
These shortfalls reflect the fact that most tick-borne pathogens have only been described relatively recently. Even the cause of Lyme disease wasn’t understood until the early 1980s. So scientific and medical understanding of tick-borne illnesses is still nowhere near as well established as it is for many other infectious diseases.
These factors make it difficult to determine if tick-borne illnesses are really on the rise, or whether we just need to improve surveillance and diagnosis.
In addition to discovering new tick-borne diseases, we’re also seeing the distribution of established tick-borne diseases changing. There are several reasons why this is happening, among which is climate change.
Tick activity is strongly affected by temperature and humidity, so changes in climate can affect when ticks are active and create conditions that allow ticks to thrive in areas that weren’t previously suitable for them.
This may be happening with tick-borne encephalitis virus. This virus has historically only been seen in parts of Asia and central and eastern Europe, but cases have recently been reported in the Netherlands and the UK, which is concerning given we’d previously assumed that the UK’s climate was unsuitable for this virus.
In temperate parts of the northern hemisphere, Lyme disease remains by far the most common tick-borne disease. However, other diseases are increasingly being reported. Human anaplasmosis is becoming more common in the US, and cases of Crimean-Congo haemorrhagic fever are on the rise in southern and eastern Europe.
To protect yourself from tick-borne diseases, you should wear long trousers and use repellent sprays when hiking and camping in grassy and wooded areas. Thoroughly check yourself (and your dog) for ticks when you get home.
If you are bitten, as soon as possible remove the tick carefully to avoid leaving the tick’s mouth parts stuck in your skin. If you develop a rash, fever or flu-like symptoms, go to your GP and tell them about the bite.
Richard Birtles currently receives funding from NERC, BBSRC, DEFRA and USDA to study tick-borne pathogen ecology.
Laura Brettell does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
