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# Parasitized, non-parasitized, why? A study of factors influencing tick burden in roe deer neonates

Léa Bariod, Sonia Saïd, Clément Calenge, Stéphane Chabot, Vincent Badeau, G. Bourgoin

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Léa Bariod, Sonia Saïd, Clément Calenge, Stéphane Chabot, Vincent Badeau, et al. Parasitized, non-parasitized, why? A study of factors influencing tick burden in roe deer neonates. EvoLyon, Nov 2021, Lyon, France. . hal-03865224

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Submitted on 22 Nov 2022

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LBBE

# Parasitized, non-parasitized, why? A study of factors influencing tick burden in roe deer neonates.

Léa Bariod<sup>a,b,c</sup>, Sonia Saïd<sup>b</sup>, Clément Calenge<sup>b</sup>, Stéphane Chabot<sup>b</sup>, Vincent Badeau<sup>d</sup>, Gilles Bourgoin<sup>a,c</sup>

<sup>a</sup>École nationale vétérinaire VetAgro Sup – Marcy l'Étoile, France; <sup>b</sup>Office Français de la Biodiversité - Birieux, France; <sup>c</sup>Laboratoire de Biométrie et Biologie Évolutive – Lyon, France; <sup>d</sup>Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement – Champenoux, France

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OFB
OFFICE FRANÇAIS
DE LA BIODIVERSITÉ

INRAO

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## Context of study

* *Ixodes ricinus*, the most common tick in Central Europe, is an important vector of pathogens **threatening human and animal health**¹.

* Tick abundance and repartition increased during last decades with global environmental changes in the continent, which include **climate and habitat changes**, and **increase of deer densities**².

* Indeed, **cervid species** are identified as **key hosts to ticks**, especially **roe deer** (*Capreolus capreolus*) whose populations have increased in recent years in France and Europe³.

* Mammal **newborns** have a naïve and immature immune system and are therefore more vulnerable to tick infestations.

* Thus it is important to **improve knowledge of factors influencing the tick burden of neonates**. Here, we study roe deer fawns monitored during 27 years in a French population with a novel Bayesian model.

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## Study site:

« Territoire d'Étude et d'Expérimentation » of Trois-Fontaines (Grand-Est, France)

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An enclosed forest of
1360 ha (48°38'N, 4°54'E)

## Dataset available:

27 years
1992 → 2018

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965 newborn fawns

Captured during
breeding period of the
female roe deer (late
April to mid-June)

## Material & methods

### Bayesian model describing the infestation process of the roe deer fawns by ticks according to several factors

Instantaneous
rate of infestation

~

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Year
(random factor)

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package
nimble

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Tick burden measure on fawns (3 categories):

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* < 10 ticks
  (n=696)

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* 10-20 ticks
  (n=167)

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* > 20 ticks
  (n=102)

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* Age (in days)

* Body surface area (bsa, in m²)

→ Meeh's formula : bsa = k x W²/³

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Instantaneous rate of infestation (tick number)
per unit of body size area of fawn per days

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>5 days old: We considered a mean duration of tick blood meal of 5 days on fawns before falling to the ground.

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Average **temperature** (°C) & **humidity** (%) of the 6 days before capture (SAFRAN model, Meteo France)

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**Vegetation structure:** five categories of habitat characteristics

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**Annual roe deer density:** Number of adult roe deer in the population calculated each winter

## Results

### Bayesian model with the most probable combination of variables to explain the infestation rate of fawns

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Instantaneous rate of infestation

~

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Year
(random factor)

### Hypothesis 1

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The instantaneous risk of infestation increases the first 5 days of fawn life and after becomes equal to zero.

Est. of >5 days old: -17;7 [-45.75; -2.6]CI95%

### Hypothesis 2

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**Humidity** 6 days prior the fawn capture had **significant effects** on the tick burden of fawn.

**Temperature** 6 days prior the fawn capture had **no significant effects** on the tick burden of fawn.

Est. of Humidity:
**0.013 [-0.003; 0.03]**CI95%

Est. of Temperature:
-0.005 [-0.047; 0.037]CI95%

### Hypotheses 3 and 4

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**Vegetation structure**, where the fawn was found, had **no significant effect** on the tick burden of fawn.

Est. of habitat1:
-0;07 [-0.32; 0.15]CI95%

Est. of habitat2:
0.06 [-0.16; 0.29]CI95%

Est. of habitat 3:
-0.10 [-0.33; 0.11]CI95%

Est. of habitat 4:
0.11 [-0.12; 0.37]CI95%

Est. of habitat 5:
-0.01 [-0.24; 0.22]CI95%

**Roe deer density** had **no significant effect** on the tick burden of fawn.

Est. of Roe deer density:
0.002 [-0.001; 0.005]CI95%

### Random effect of the year

Vertical red lines delimit three periods according to the **Lothar hurricane** (December 1999):

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Standard deviation of the year random effect for each period:

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Est.: 0.427
[0.024; 1.112]CI95%

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Est.: **0.807**
**[0.410; 1.516]**CI95%

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Est.: 0.160
[0.01; 0.442]CI95%

## Conclusion

* The level of tick burden became constant when fawns were 5 days old and more. This result could be explained by the **depletion of questing ticks in the habitat of fawns** and the **turnover of tick feeding on fawns**. Indeed, ticks take their blood meal on hosts for a few days before falling to the ground, where they molt or lay eggs during few weeks.

* **Humidity** was significant but no effects of temperature, vegetation structure or roe deer density were found on the tick burden of fawn. However, we noticed a **strong heterogeneity of tick burden between years** suggesting two explanations:

1. Others variables could be involved, such as **local density of various host species** or **habitat characteristics** of fawn, which may be important for tick activity.

2. The environmental variables used in this study affected the infection process at **other spatial or temporal scales**.

* This would require **additional data on ticks in the environment**, at different times of the year, which would allow a more precise study in Trois-Fontaines to improve our understanding of tick infection in ungulate newborns.

## Corresponding author

Léa Bariod

PhD student at « École nationale vétérinaire VetAgro Sup », « Laboratoire de Biométrie et de Biologie Évolutive » and « Office Français de la Biodiversité, Direction recherche et Appui scientifique »

E-mail: bariod.lea@orange.fr

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linkedin.com/in/léa-bariod-39047312b/
 researchgate.net/profile/Lea_Bariod

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## Bibliography

1. de la Fuente, J., Estrada-Pena, A., Venzal, J. M., Kocan, K. M., & Sonenshine, D. E. (2008). Overview: ticks as vectors of pathogens that cause disease in humans and animals. *Front Biosci*, 13(13), 6938-6946.

2. Vor, T., Kiffner, C., Hagedorn, P., Niedrig, M., & Rühe, F. (2010). Tick burden on European roe deer (*Capreolus capreolus*). *Experimental and Applied Acarology*, 51(4), 405-417.

3. Kiffner, C., Lödige, C., Alings, M., Vor, T., & Rühe, F. (2010). Abundance estimation of *Ixodes* ticks (Acari: *Ixodidae*) on roe deer (*Capreolus capreolus*). *Experimental and Applied Acarology*, 52(1), 73-84.

4. Taylor, M. A., Coop, R. L. & Wall, R. L. (2015). Veterinary Parasitology. John Wiley & Sons.

5. Medlock, J. M., Hansford, K. M., Bormane, A., Derdakova, M., Estrada-Pena, A., George, J. C., ... & Van Bortel, W. (2013). Driving forces for changes in geographical distribution of *Ixodes ricinus* ticks in Europe. *Parasites & vectors*, 6(1), 1-11.

6. Tack, W., Madder, M., Baeten, L., Vanhellemont, M., Gruwez, R., & Verheyen, K. (2012). Local habitat and landscape affect *Ixodes ricinus* tick abundances in forests on poor, sandy soils. *Forest ecology and management*, 265, 30-36.

7. Ruiz-Fons, F., & Gilbert, L. (2010). The role of deer as vehicles to move ticks, *Ixodes ricinus*, between contrasting habitats. *International journal for parasitology*, 40(9), 1013-1020.

## Acknowledgments

This study was conducted in the national veterinary school VetAgro Sup and the French Office for Biodiversity (OFB) under partnership agreement CNV20-0947. We are also grateful to the volunteers and technicians for participating in the fawn captures and data collection at Trois-Fontaines (France).