Dog study warns of rising Lyme disease threat
25 January 2012 by Tom Marshall
Lyme disease is more common than we thought, a survey of the nation's dogs suggests.
Scientists asked a group of English, Scottish and Welsh vets to check a randomly-chosen sample of the dogs brought to their surgeries for ticks, which are responsible for spreading the infection. Where they found them, the vets then tested the bloodsucking arachnids for the bacterium that causes the disease.
Of 3,534 hounds examined between March and October 2009, 739 had ticks, and of these 17 (around 2·3 per cent) were infected with Lyme disease. Hence, researchers would expect to find infected ticks on around 0·5 per cent of dogs, or alternatively that for every 100,000 dogs there will be 481 infected ticks.
Lyme disease appears to be a rapidly growing problem in the UK.
It's not certain how many of the animals carrying infected ticks went on to develop Lyme disease, but they were all at risk of it. And because pet dogs generally go to the same places as their owners, the study's authors think the figures provide a reasonable index of the risks humans face.
These figures are much higher than previous estimates, but they may nevertheless underestimate the true scale of the problem, as by definition dogs that are brought to the vet are relatively well cared for compared to the dog population as a whole. And the disease is only likely to grow more common as the climate warms; ticks usually become dormant during the coldest part of the year, but lately there have been cases of dogs picking up disease-bearing ticks in midwinter.
"Lyme disease appears to be a rapidly growing problem in the UK with important health and economic impacts in terms of loss of working hours and potential decrease in tourism to tick hotspots," says Faith Smith from the University of Bristol, lead author of the paper.
She recommends that people be aware of the symptoms of the disease and seek treatment quickly if they think they may have been infected, while also making sure to check their dogs for ticks after they've been out in potentially risky areas. These include habitats like long grass, moorland, heaths and forest, although infected ticks also been detected even in urban settings like public parks.
As well as people and dogs, Lyme disease infects many other kinds of mammal, including sheep and deer. Human infections are comparatively rare; the disease generally survives in reservoirs in wild animal populations.
But rates of infection have been growing steadily and have more than quadrupled since the beginning of the century. In 2000, just 0·38 cases were reported per 100,000 people; in 2009 that number had risen to 1·79. The following year there were 953 reported cases in total, but it's likely that many more people became infected but didn't report it; the Health Protection Agency thinks the true figure may be somewhere between 2000 and 3000.
While some of the increase may be due to better surveillance and reporting, it's very unlikely this accounts for all the growth. Doctors aren't obliged to report all cases of Lyme disease, so many of those infected with Lyme disease may just have been given antibiotics and sent on their way. Others again may never have received treatment.
Caring for sufferers costs an estimated £331,000 a year in Scotland alone, where they are thought to account for between 1·25 and 16·5 individuals per 100,000 people. And these figures don't take into consideration other possible costs, such as loss of income from tourism or of working hours.
Dog infections also seem to be rising; after the animal charity PDSA started actively looking for signs of Lyme disease, it reported an increase of around 150 per cent in suspected cases. Smith notes that the rules of the Pet Passport scheme have recently been relaxed so that people bringing their dogs back into the country no longer have to treat them for ticks. This may mean we end up with more new species of tick entering the country, and perhaps even with more cases of disease transmission, she warns.
Once someone's contracted Lyme disease after being bitten by a tick that's infected with the borrelia burgdorferi bacterium, they suffer a range of symptoms including fever, headaches and distinctive bullseye-shaped skin rashes. If not treated promptly with antibiotics, the infection can cause long-term neurological problems and arthritis. Dogs are harder to diagnose because their symptoms are less specific, often overlapping with those of more common health problems.
The NERC-funded research appears in Comparative Immunology, Microbiology and Infectious Diseases. An earlier study by Smith based on the same dog survey results, published in 2011, showed that a new, invasive species of tick, capable of transmitting Lyme disease, had taken up residence in southern England. The research is a collaboration between University of Bristol scientists and researchers from multinational company Merial Animal Health .
'Estimating Lyme disease risk using pet dogs as sentinels',Faith D Smith, Rachel Ballantyne, Eric R Morgan, Richard Wall. Comparative Immunology, Microbiology & Infectious Diseases, Available online 16 January 2012, ISSN 0147-9571, 10.1016/j.cimid.2011.12.009.