An outbreak of babesiosis has been identified for the first time in this country, and experts say it will be impossible to contain.
Two dogs in Essex have died and three others needed blood transfusions.
American Bulldog Ollie is thought to be the first dog in the UK to be diagnosed with the disease.
Owner Julie Newman, from Harlow, said she is "hugely relieved" her pet survived the illness following a transfusion.
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"It was really touch-and-go if he would make it.
"He was really lethargic, he normally has so much energy but he just lost it, he went really strange and we noticed he had blood in his urine," she said.
She said it was "really lucky" that vets spotted the symptoms, because it can easily be mistaken for other less dangerous diseases.
Vet Clive Swainsbury, who has treated several of the dogs, confirmed this problem, saying it is "easy to miss" because "we aren't used to looking for this disease on a regular basis."
The ticks carrying the parasite have been found in fields in Harlow. The local council has put up a sign advising dog walkers not to enter.
What is Babesiosis?
However, one of the dogs that has died is understood to have lived in Epping, suggesting the disease could already have started to spread.
Two government agencies are investigating the outbreak: the Animal and Plant Health Agency and Public Health England.
Mr Swainsbury said: "We are doing everything we can but every tick will make up to 1000 eggs. The only solution is to kill the ticks quickly.
"Some of the tick products available will kill the tick quick enough to prevent the tick spreading the disease to the dog, because the tick needs to be feeding for 24 hours at least before it transmits the disease."
A dog owner says her pet is "lucky to be alive" after contracting a deadly tick-borne disease that is expected to spread around the UK.