17. Incidence of Lyme Disease in Humans
S Subak

Number of cases of Lyme disease reported in England & Wales each year shown in relation to average summer temperature (June, July and August) in Central England

Specific Indicator
Lyme disease is caused by a bacterium (Borrea burgdorperi), which is spread by ticks (Ixodes ricinus) and is associated with warm weather. The number of cases in the UK has increased dramatically in recent years. It is not a notifiable disease, so many cases go unreported, but the Communicable Disease Surveillance Centre (CDSC) keeps a record of all cases diagnosed in National Health Service and Public Health Laboratory Service Laboratories. The specific indicator is the number of cases of Lyme disease reported to the CDSC in the UK each year.
[Source: CDSC, Abton House, Wedal Rd., Cardiff CF4 3QX]

Relevance
Warm weather may be expected to increase the incidence of vector-borne diseases. However, the link with UK temperatures is tenuous in the case of, say, Q-fever (a respiratory disease) because it is spread by dust as well as ticks, and diseases such as malaria are all imported. Lyme disease, on the other hand, is spread entirely by ticks and is a growing problem, especially in warm years. Several hundred cases are reported each year. About 10% of cases involve the more serious symptoms of arthritis and 5% experience facial paralysis, in addition to the skin rash that is characteristic of the disease. There are no human vaccines although antibiotics can be successfully used in treatment.

Sensitivity to climatic and other factors
In warmer weather, people are more likely to visit the parks and areas where ticks are present. Also, ticks are more active in mild weather, and warmer winters can shorten the dormant period in the tick life cycle. Warmer weather can also affect the active range. Currently, over half of reported cases appear to have been acquired in the south-western regions of the country with very few cases reported from exposure to ticks in northern England and Scotland. This pattern could change as northern regions experience milder winters. The tick life cycle requires, during its first year, a rodent host, which may increase in numbers following warmer winters, leading to more opportunities for tick-host contact. Extremely hot dry weather, however, is believed to put a check on tick populations because the insect needs moist conditions.

Positive correlations are found between the number of cases of Lyme disease reported each year and summer (June-August) temperatures in Central England. Indeed, nearly half of these Lyme disease cases are diagnosed in the months of August and September when people spend more time in the countryside, parks and gardens where they can come into contact with the infected tick. The relationship with summer temperature seems to have become closer in recent years. There is no significant relationship between the number of reported cases and temperatures in winter, spring and autumn.

Change over time
Cases of Lyme disease diagnosed and reported by UK laboratories have increased tenfold since records began in 1986. Since the late 1990s the number of cases has almost doubled. The period 2000 - 2002 has seen record levels of more 250 cases per year. Overall, there has been a dramatic increase in recent years.

In the future, it is expected that, with warmer year-round temperatures, the UK would see more cases of Lyme disease related to all of the factors described above, including more outdoor recreation, changes in the tick numbers and activity.