Almost a third of dogs checked at random across the UK were found to be carrying a tick, researchers say.
The finding comes from the largest survey of ticks in dogs.
Researchers also found that the risk of an animal picking up a tick is as great in urban areas as in rural ones.
Ticks can carry a range of diseases including Lyme disease, and also a parasite discovered in the UK for the first time earlier this year that is potentially fatal to dogs.
Lyme disease has the potential to cause serious health problems, such as meningitis and heart failure.
In the most serious cases, it can be fatal.
Almost 15,000 dogs from across the UK were examined in the study, which was carried out by Bristol University last year.
Just under a third (31%) of these dogs checked at random during a visit to the vet were found to be carrying a tick.
The researchers found that the arachnids are present right across the UK, with the highest risk areas being in Scotland, East Anglia and the South West. There can be just as many in urban areas as in rural areas.
Launched in April 2015, the project asked participating vets to examine dogs in their practice for each week and complete a questionnaire relating to the clinical history of each dog. The species, life-cycle stage, sex and location of origin and whether it was carrying any pathogens were recorded.
Prof Richard Wall, who led the Big Tick Project team at the University of Bristol, said: "The work that we have carried out shows that ticks are extremely widely dispersed. The records that we have got appear to show that we have had an increase in tick numbers right across the country.
"What we are primarily concerned about is the diseases that ticks carry. In the UK, we have relatively low rates of the prevalence of these pathogens at the moment and, in contrast, in continental Europe they have much higher rates of disease. As there seems to be a rise in tick numbers, we need to be concerned and be aware of the potential for increasing problems."
Rural vs urban risk
Prof Wall said pet owners should be aware of the risk in woodland or areas of long grass, but urban areas were also affected.
Reacting to the new data, conservationist Chris Packham said the "good solid hard data" which was a "tremendously significant project" had revealed "some very shocking and surprising things about the distribution, the population and potential that ticks have to give diseases to our pets and ourselves".
It was surprising ticks were not just found in isolated parts of the UK, but all over the UK, he said.
Ticks don't jump or fly, they climb on to clothes if a person brushes against something that the tick is holding on to. Typically they can be found in woods, urban parks, heathland and fields. They can also be found in gardens.
Lyme disease is transmitted by the bite of a tick infected with Borrelia burgdorferi bacteria.
The tick acts as a vector. In the same way that a mosquito transmits malaria when it sucks blood from a person, the tick does the same to an animal or a person.
According to Public Health England, it's estimated that there are 2,000 to 3,000 new confirmed cases of Lyme disease in England and Wales each year, although not all cases are confirmed by laboratory testing. About 15% of cases are in people who have returned from abroad.
One initial symptom can be a red circular rash around the bite - called a "bullseye" rash. But this isn't always present and so can't be relied upon as a warning signal. Victims can develop flu-like symptoms along with muscle and joint pain.
Lyme disease can be treated with antibiotics. But if left untreated it can seriously damage a person's health, including affecting the nervous system, causing meningitis or heart failure.
The threat of Lyme disease (Lyme borreliosis) in the UK is still thought to be small compared with other countries - some 3% of ticks carry the bacteria.
However, given the results of this study showing the high numbers of ticks across the country and with the serious potential consequences of Lyme disease, scientists are urging people to be aware of the risks and do their best to avoid being bitten.
The Big Tick Project has launched an online tool that people can access to find out where ticks are most prevalent.
Lyme disease - Sarah's story
Sarah Bignell was a very fit and active vet before she went on a walking holiday in Aberfeldy in Scotland in 2011. "There was a small note in the cabin saying this was a Lyme disease hotspot but I didn't think anything of it. I had no idea," she says.
Six weeks later, she started suffering pain in her bones, joints and nerves. Doctors put her symptoms down to other conditions, including multiple sclerosis or a brain tumour. However, Sarah's background meant that she knew to push for a Lyme disease test. She had been bitten by a tick - and she had been completely unaware of it. She also had no "bullseye rash".
It became agony "virtually overnight" for Sarah to deal with sound or light. She couldn't swallow, and when she could, it was agony. She had developed encephalitis. "I spent eight-and-a-half months locked in a room," she explains. She needed constant care and could no longer look after her dog and cat.
Over the next four-and-a-half years, Sarah had many close calls. "I was in danger of dying. I am very lucky," she says.
"The pain used to be so severe and my quality of life so awful, at that point I was living hour to hour. If I had been well enough I would have ended up in Switzerland and I wouldn't be here now - I am a vet and I am used to ending suffering."
With extremely strong treatment, Sarah says she now only has one or two really bad days a month. "I still get bouts of extreme fatigue. Days when I think - having a shower or putting the washing out - which one should I do today? Again I am lucky, there are loads of people not having the same treatment."
So what does she think should be done about Lyme disease? It's not particularly about investment or research, Sarah says. "It is a very cunning and specialised bacteria. It is so complicated and mimics so many other diseases and each effect needs individual treatment. It's about being aware and not getting bitten," she explains.
"I'm not saying avoid the outdoors. The first thing I did again when I could get myself up on sticks was to go to the woods with my dog Matty. I walk him every day but I wear wellies even in the sunshine. I also wear trousers and long-sleeved shirts if I'm walking in long grass. Always light-coloured so the ticks show up. I live in Kent and one day this year when I got back to the car I found five ticks."
Sarah is working on her fitness and says she hopes to go back to part-time work as a vet next year.
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