The relaxation of rules designed to protect the UK against rabies has opened the way to the entry of other animal diseases.
By Alex Kirby
BBC News Online environment correspondent
The Pet Travel Scheme (Pets) allows owners of companion animals to take them in and out of Britain.
Faithful friend: And a potential risk
But animals coming from exotic disease areas abroad are now bringing new problems with them.
Veterinary surgeons say more checks are needed to see what risk they represent.
They report their concerns in the Veterinary Record, the journal of the British Veterinary Association (BVA).
A team of UK and French vets analysed diagnostic data from imported animals which were found to be sick between April 2001 and July 2002.
They also reviewed data from animals quarantined during that time (quarantine is obligatory for animals from countries not included in the Pets scheme).
The team tested samples from 67 dogs and three cats, most of which had entered the UK under Pets: 14 of the dogs and one cat had been in quarantine.
High infection incidence
Blood samples were tested for three exotic diseases: babesiosis, leishmaniasis and ehrlichiosis.
Babesiosis is a severe tick-borne disease caused by various types of babesia, a microscopic parasite that infects red blood cells.
The first dog with his own passport
In humans it is rare, but potentially fatal. Babesiosis is a common infection in animals.
Leishmaniasis is a potentially contagious disease in dogs and cats. It is caused by a parasite transmitted by sandflies. It can affect humans, but most people are unlikely to become ill.
Ehrlichiosis, another tick-borne disease affecting dogs, can cause anaemia, decreased resistance to disease, infection and abnormal bleeding.
None of the cats showed signs of any of the diseases, but of the dogs 17 of the 53 that had entered under Pets and seven of the 14 that entered the UK through quarantine had one or more of the three pathogens.
Babesiosis was the most commonly diagnosed illness, and a total of ten dogs was confirmed with one of two strains of the disease. Several dogs showed evidence of more than one pathogen.
Threat from elsewhere
Five had travelled from France under Pets, two more under Pets from Thailand through Germany and from Japan, and three had been quarantined, two from Hong Kong and one from South Africa.
Five of the six dogs with ehrlichiosis had come from countries not included in the Pets scheme.
The study found no infected cats
Of the dogs with leishmaniasis, four had travelled from Spain: the vets say this probably underestimates the number of infected animals entering the UK under Pets.
They say their findings "indicate the significance of these infections as a health and welfare issue for both the animals and their owners.
"Should introduction of the appropriate vectors occur or alternative methods of transmission evolve, there is risk of extension of disease into dog, cat, human or wildlife populations which have not travelled.
"Larger targeted surveillance schemes are required to further evaluate and monitor the risk involved."
The Pets scheme was introduced in February 2000. It allows animals from an approved list of countries to avoid undergoing six-month quarantine periods.
It applies at the moment to animals travelling from most of Europe, from North America and from a number of long-haul rabies-free countries.