University of Jyväskylä

Dissertation: 3.11.2017 The wild life of tick-borne pathogens (Cayol)

Start date: Nov 03, 2017 12:00 PM

End date: Nov 03, 2017 03:00 PM

Location: Seminaarinmäki, Seminarium S212

Claire Cayol Picture: Florence G.
DVM, MSc Claire Cayol defends her doctoral dissertation in Ecology and Evolutionary Biology "Eco-epidemiology of tick- and rodent-borne pathogens in boreal forests". Opponent Professor Annapaola Rizzoli (Centro Ricerca e Innovazione, Italy) and custos Docent Tapio Mappes (University of Jyväskylä).


Ticks are strange creatures

At first glance, the life of a tick can look lazy: they hatch from an egg, climb up the nearest grass stalk, wait for a passerby host, feed three times in their life, have sex one time while eating, reproduce and die. The reality is more complicated. Excessive dry or hot weather can impair the life of a tick. When the conditions are unfavorable, ticks can remain inactive in the soil for up to several years. They rely on their host for food and dispersal: from lizards to large mammals such as deer, including small rodents, which are favored hosts for juvenile stages. During a blood meal, ticks can transfer a wide range of parasites, bacteria or virus, acquired from a previous host. This includes bacteria from the Borrelia burgdorferi sl. group, which causes Lyme disease. These pathogens naturally circulate in nature between several wild host species and man can become infected during a tick bite. Because ticks are highly efficient parasites and because of current global changes that favor their survival in high latitude and altitude, ticks are now spreading northwards, and Northern Europe is one of the places where these changes are the most visible.

This thesis examined the natural circulation of ticks and pathogens carried by ticks and rodents in the wild, in Central Finland. The ultimate aim is  to characterise the risks these pathogens pose to humans. The research focused on the bank vole, which is the most abundant rodent species in Central Finland and an important host for ticks and pathogens. The study show that bank voles from forests in central Finland are parasitized with two tick species: the sheep tick Ixodes ricinus, a generalist tick that can also bite human and the vole tick, Ixodes trianguliceps, which bite only rodents. The sheep tick’s distribution is extremely patchy, being found commonly in urban forests nearby open water bodies such as lakes and rivers. The tick abundance was also highly seasonal, with the highest risk periods for tick bites on humans in May–June, and September. The presence and abundance of sheep tick is prerequisite for the circulation of the bacteria causing Lyme disease in nature. Moreover, Borrelia-infected male bank voles showed lower reproductive performance and they moved less than uninfected males, when kept in a low population density. Furthermore, the cost of infection with Borrelia was bigger in large males than in small individuals. Surprisingly, infected females reproduced earlier than uninfected ones. Bank voles were frequently infected with several parasites at the same time, and the risk of infection with Puumala virus, which causes nephropathia epidemica in human, was reduced in individuals previously infested with ticks and fleas, probably because the immune response observed against ticks and fleas might be effective in reducing infection risk with PUUV.

Altogether, these findings show the complex interplay between rodents, pathogens and parasites and the importance of small mammals for the circulation of human pathogens in the highly seasonal boreal environment. Consequently, this thesis provides crucial information on the risks of infectious diseases at the animal-human-ecosystem interface.

Claire CAYOL,, Tel: +358 (0)40 484 3717

Dissertation URL:

Claire Cayol, DVM, MSc

 Personal history:

DVM (Doctor of Veterinary Medicine, Vet school Oniris, Nantes, France, 2006),

MSc in Public health (Vet School of Maison Alfort, University Paris Est and Paris Sud, France, 2011),

Former work places:

-          French national agency for wildlife and hunting, ONCFS (2006-2009),

-          Veterinarian practitioner (2009-2012).


Cayol, Claire

Eco-epidemiology of tick- and rodent-borne pathogens in boreal forests

Jyväskylä: University of Jyväskylä, 2017, 54 p.

(Jyväskylä Studies in Biological and Environmental Science ISSN 1456-9701; 336)

ISBN 978-951-39-7205-9 (nid.)

ISBN 978-951-39-7206-6 (PDF)

Yhteenveto: Puutiaisten ja jyrsijöiden levittämien taudinaiheuttajien ekoepidemiologia

boreaalisissa metsissä



Infectious diseases are amongst the ten major causes of human mortality worldwide, 60% of them being animal-borne. Variations of abiotic and biotic conditions are likely to modify the transmission of parasites and pathogens within reservoir species, and, as a consequence, alter the zoonotic risk for human. My thesis aims at elucidating the dynamics and mechanisms of the maintenance of ticks, tick-borne pathogens (TBPs) and the Puumala hantavirus (PUUV) in the reservoir host, the bank vole (Myodes glareolus, BV). In Northern Europe, tick-borne diseases are growing in importance to human because of the latitudinal expansion of the tick Ixodes ricinus. Field monitoring revealed that I. ricinus was the only species found in the vegetation in Central Finland. The abundance of immature I. ricinus in nature was positively associated with the BV abundance. The highest risk periods for tick bites on humans were May– June and September. Ixodes ricinus was positively associated with open water coverage and human density, which might offer suitable moisture conditions and anthropogenic modifications favouring the species. The infection of BV with the zoonotic B. burgdorferi s.l. was associated with the abundance of I. ricinus at the site, indicating that this tick species was required for the transmission and persistence of this pathogen. An experiment revealed, for the first time, that B. afzelii can modify the behaviour and the breeding success of its host, and these effects are both sex- and size-specific and density-dependent. Space-state modelling of longitudinal field data revealed that PUUV infection likelihood was the lowest in BV previously infested with vectors in comparison to Anaplasma phagocytophilum infected BV, or individuals without any previous infections. Altogether, this study shows how seasonality, co-infecting pathogens and host population density influence the risk of tick-borne pathogens and the zoonotic risk in Central Finland.

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Claire Cayol
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