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Focus: lyme's disease
Parks, green spaces and the Sonian forest are the preferred locations for leisure activities and recreation. This contact with nature is undoubtedly beneficial for health and well-being... but it is not without risk. One of the risks is posed by coming into contact with ticks, who are carriers of various diseases ; the most well-known of which is Lyme's disease. However, although the risk of exposure to a tick bite, and consequently of becoming potentially infected, is present in the Brussels Region, it does not justify avoiding green spaces: observing the prevention advice allows the risk to be limited.
What is Lyme's disease?
In Brussels, many people visit parks, green spaces and the Sonian forest for recreational and leisure activities. This contact with nature is undoubtedly beneficial for health and well-being... but it is not without risk. One of the risks is posed by coming into contact with ticks, who are carriers of various diseases ; the most well-known of which is Lyme's disease.
Lyme's disease is an infection caused by the Borrelia Burgdorferi group of bacteria. These bacteria are transmitted to humans through the bite of an infected tick, primarily the species Ixodes ricinus.
Ticks are mites which require blood for their development. They especially like warm and damp places such as long grass, ferns, bushes and hedgerows, where they lie in wait for a host to feed on (Mersch, 2014). The hosts can be small rodents, large mammals, or more rarely, birds. As for humans, they are accidental hosts. If a first host is infected with the Borrelia bacteria, this can then be transmitted to the tick through the blood. Once contaminated, the tick will be able to infect a subsequent host. Most infections are due to nymph bites rather than from adult ticks. As such, they are in larger numbers and are more inconspicuous due to their small size.
The prevalence of the Borrelia bacteria in Ixodes ricinus is estimated to be slightly more than 10% in Belgium (Kesteman, 2010). It should be noted however that an infected tick does not necessarily transmit the disease, and not everyone infected will necessarily develop it.
The disease can progress through three successive stages if it is not treated.
In order to diagnose the disease, the symptoms observed must be directly connected with a risk of exposure, and all the more so if the patient does not recall being bitten by a tick. The appearance of an erythema migrans (1st stage, in 75% of cases) is sufficient to make a diagnosis and the disease can be treated directly with antibiotics, without the need for a serological test (which detects antibodies).
The subsequent stages, when the bacteria spreads throughout the body, are characterised by general complaints: attacks on the central nervous system (usually in the form of facial paralysis), joint pains (arthritis), double vision, or more rarely, heart rhythm disorders (stage 2) followed by joint problems, lesions to the skin, or in certain cases, chronic neurological problems (stage 3). If there is any doubt, a serological test will be requested by the doctor. If the result is positive or inconclusive, the confirmation test will be carried out (Scientific Institute of Public Health WIV-ISP).
Epidemiological data in Belgium
Lyme's disease is not included in the infectious diseases which must obligatorily be declared when the diagnosis is confirmed, but the Scientific Institute of Public Health (WIV-ISP) is monitoring the disease:
- A network of sentinel laboratories reports on the number of positive results from the serological analyses (detection of antibodies) carried out. These data are not exhaustive, but they make it possible to follow trends;
- data is also collected in terms of the number of people hospitalised for Lyme's disease (around 200 to 300 people annually);
- Two prospective studies carried out in 2003-2004 and in 2008-2009 made it possible to estimate the number of patients who consult a GP following a tick bite (18.6 patients/10,000 inhabitants, per year) or erythema migrans (8 to 9 patients/10,000 inhabitants, per year) (Vanthomme et al., 2012).
These data show that, in recent years, the number of tests carried out has increased sharply, due in particular to people giving the disease more attention. However, despite this increase, the rate of positivity (number of positive serological analyses/number of serological analyses carried out) has remained stable (Bapcoc, 2015).
The data from the Scientific Institute for Public Health also makes it possible to highlight the population at risk. The chart below shows that positive serology cases are reported at all age groups. However, the risk increases with age. From the age of 5-9 onwards, the reported cases are much more numerous. This relates in particular to the children taking part in forest activities, such as youth organisation activities, for example. The most affected age segment is that of 45-64 year olds. These are people whose exposure to bite risks is the highest, due to their professional or leisure activities.
Number of positive serological tests for borreliosis (per 100,000 inhabitants) carried out by the sentinel laboratories, per age segment, Belgium, 2014
Source: WIV-ISP (August 2015)
An increase in the incidence rate of Lyme's disease has been observed, both in Europe and the United States. This can be explained in particular by a better understanding of the disease, better diagnostic methods, and more effective information/awareness among doctors, but also by an increase in the population and (peri-)urban development, the fragmentation of natural habitats, evolutions in the management of natural spaces, changes to the recreational habits of the population (exterior activities, holidays, etc.), higher densities of ticks and/or climate changes (Vanthomme et al. 2012 ; Heyman et al., 2010 ; Hofhuis et al., 2010, Tack et al., 2012a and b).
In Belgium as a whole, according to data from the WIV-ISP, there seems to be no significant trend for the increase of the disease (Vanthomme et al., 2012 ; WIV-ISP, website).
What are the risks in the Brussels-Capital Region?
The Brussels parks and the Sonian forest are the preferred locations for leisure and recreational activities, and this need is growing, given the increases in urbanisation and the population. These are therefore areas where the risk of tick bites is the largest. To a lesser degree, this risk is also present in gardens, particularly in the outer suburbs (in areas with more dispersed habitats and close to forests).
For example, the Sonian forest is an important place for the Brussels population to engage in recreational activities. The numerous developments and facilities which are present make it a rather fragmented place, which has resulted in a change in the dispersion and distribution of species, as well as in the increase of predators and parasites. The abundance of small rodents, which are the preferred hosts of nymphs, tends for example to be larger in smaller plots than in bigger ones. The deer (host of the adult tick) also prefers the edges of forests, which provide a good alternative of food and shelter. Consequently, the abundance of ticks, and therefore the risk of infection, can increase with the fragmentation of the forest (Pfaffle et al., 2013; Tack et al., 2012 a and b, 2013).
With regards to the epidemiological data for the Brussels Region, these are not sufficiently representative to be the subject of thorough analysis.
Although the risk of exposure to a tick bite, and consequently of potentially becoming infected, is present in the Brussels Region, it does not justify avoiding green spaces: observing the prevention advice allows the risk to be limited. During activity in a risk area, it is therefore advisable to:
- wear bright clothing that covers the body as much as possible;
- stay on the indicated paths, and avoid contact with long grass and bushes. The same goes for dogs;
- systematically check their whole body (including hairs) at the end of the activity and remove any tick(s) which are present.
In the event of a bite, keep an eye on it and consult a doctor if there are any symptoms.
As a reminder, an infected tick does not necessarily transmit the disease, and not everyone infected will necessarily develop it. After having been bitten, the risk of developing the disease will be around 1 to 2% (WIV-ISP website).
Let's not deny ourselves the benefits of leisure activities in green spaces, and make sure that we act appropriately!
Studies and reports
ISP-WIV, 2015. "Zoonoses et maladies à transmission vectorielle : surveillance épidémiologique en Belgique, 2013 et 2014". 111 pages (.pdf, in French and Dutch only).
BAPCOC. 2015. "Recommandation pour la prévention, le diagnostic et le traitement de la borréliose de Lyme ". 25 pages (.pdf, in French only)
HEYMAN P., COCHEZ C., HOFHUIS A., VAN DER GIESSEN J., SPRONG H., PORTER S. R., LOSSON B., SAEGERMAN C., DONOSO-MANTKE O., NIEDRIG M., PAPA A., 2010. "A Clear and Present Danger: Tick-borne Diseases in Europe" (.pdf)
HOFHUIS A., HARMS M.G., VAN DER GIESSEN J.W.B., SPRONG H., NOTERMANS D.W., VAN PELT W. April 2010. "Ziekte van Lyme in Nederland 1994-2009 – Aantal huisartsconsultenten blijft toenemen. Is voorlichting en curatief beleid genoeg?". Infectieziekten Bulletin, Year 21 Number 3, pp. 84-87 (in Dutch only)
KESTEMAN T., ROSSI C., BASTIEN P., BROUILLARD J., AVESANI V., OLIVE N., MARTIN P., DELMEE M., 2010. "Prevalence and genetic heterogeneity of Borrelia burgdorferi sensu lato in Ixodes ticks in Belgium". Acta Clinica Belgica. Volume 65(5), pp. 319-322. (.pdf)
MERSCH W., 2014. "Services écosystémiques des forêts - Analyse exploratoire des risques et des bénéfices pour la santé. Application dans 3 sites forestiers belges". Université Catholique de Louvain, Final year project, 75 pages
PFAFFLE M., LITTWIN N., MUDERS S.V., PETNEY T.N., November 2013. "The ecology of tick-borne diseases". International Journal for Parasitology., Volume 43, Issues 12–13, pp. 1059-1077 (.pdf)
TACK W., MADDER M., BAETEN L., VANHELLEMONT M., GRUWEZ R., VERHEYEN K., 2012a. "Local habitat and landscape affect Ixodes ricinus tick abundances in forests on poor, sandy soils". Forest Ecology and Management, Volume 265, pp. 30–36
TACK W., MADDER M., BAETEN L., DE FRENNE P., VERHEYEN K., 2012b. "The abundance of Ixodes ricinus ticks depends on tree species composition and shrub cover". Parasitology, Volume 139, number 10, pp. 1273-1281 (.pdf)
TACK W., MADDER M., BAETEN L., VANHELLEMONT M., VERHEYEN K., 2013. "Shrub clearing adversely affects the abundance of Ixodes ricinus ticks". Experimental and Applied Acarology, Volume 60, number 3, pp. 411-420
VANTHOMME K., BOSSUYT N., BOFFIN N., VAN CASTEREN V., 2012. "Incidence and management of presumption of Lyme borreliosis in Belgium: recent data from the sentinel network of general practioners ". European Journal of Clinical Microbiology & Infectious Diseases., Volume 31, pp. 2385-2390