The Tortoise and the Tick: Unraveling a Viral Mystery in Tunisia's Tortoises

Investigating the role of tortoises and ticks in Crimean-Congo haemorrhagic fever transmission

Introduction: A Deadly Virus and an Unlikely Host

Tortoise with ticks
Figure 1: Spur-thighed tortoise (Testudo graeca) with ticks

Crimean-Congo haemorrhagic fever (CCHF) ranks among the world's most lethal tick-borne diseases, with fatality rates reaching 50% in humans. As climate change expands tick habitats, understanding this virus's ecology becomes critical. Enter an unexpected player: the unassuming spur-thighed tortoise (Testudo graeca) and its dedicated parasite, the tick Hyalomma aegyptium. In Northern Tunisia, where 91% of tortoises carry these ticks, scientists set out to solve a puzzle: Could this reptile-tick duo be silently spreading a deadly virus? 1 2

Key Concepts: Viruses, Vectors, and Vertebrates

CCHF Virus: A Global Threat
  • CCHF virus (CCHFv) is a Nairovirus transmitted primarily by Hyalomma ticks.
  • Humans contract it through tick bites or contact with infected animal blood.
  • Endemic across Africa, Asia, and Europe, it causes fever, hemorrhage, and organ failure. Tunisia reported human seroprevalence as early as 2014 4 .
Ticks with a Specialty
  • Hyalomma aegyptium is a three-host tick uniquely adapted to tortoises:
  • Larvae/Nymphs: Feed on birds or small mammals.
  • Adults: Depend almost exclusively on Testudo graeca tortoises 2 7 .
  • In Tunisia, this tick infests 66-91% of tortoises, with intensities up to 8.5 ticks per host 1 .
The Tortoise Paradox
  • Tortoises were suspected as viral reservoirs after CCHFv was found in H. aegyptium in Algeria and Turkey 5 .
  • Yet their low body temperature (20-30°C) seemed incompatible with viral replication. Could ticks bypass this barrier?

The Key Experiment: Hunting a Virus in Tunisia's Wilderness

Methodology: From Field to Lab

In 2017, researchers launched a surveillance study across Northern Tunisia:

  1. Tortoise Capture: 147 wild Testudo graeca were collected from humid/subhumid regions (March-April 2017).
  2. Tick Harvest: 1,174 ticks were manually removed from tortoises.
  3. Species ID: 120 ticks (10%) were morphologically identified as H. aegyptium adults.
  4. Viral Screening: 1,054 ticks were pooled (46 pools) and tested via:
    • RNA Extraction: Using Qiagen Viral RNA Mini Kit.
    • Real-Time RT-PCR: Targeting the CCHFv S-segment with primers CCHFV.S122F/R 1 2 .
Table 1: Tortoise Infestation in Northern Tunisia
Metric Value
Tortoises Examined 147
Infestation Prevalence 91% (134/147)
Mean Ticks per Tortoise 7.8
Total Ticks Collected 1,174

Results: The Absence That Mattered

  • All ticks were identified as adult H. aegyptium—confirming strict host specificity.
  • Zero pools tested positive for CCHFv RNA, despite large sample sizes 1 6 .
Table 2: CCHFv Screening Results
Sample Type Pools Tested Positive Pools
Engorged Ticks 12 0
Non-Engorged Ticks 34 0

Scientific Significance

This absence suggests:

  • H. aegyptium tortoise ticks are unlikely players in CCHFv's ecology in Tunisia.
  • The virus may rely on other Hyalomma species (e.g., H. marginatum) feeding on livestock 4 .
  • Tortoises' low body temperatures might inhibit viral replication, preventing reservoir establishment 2 .

Geographic Contrast: Why Turkey's Story Differs

While Tunisia's tortoises were virus-free, a 2020 Turkish study revealed a stark contrast:

  • 9.5% of Testudo graeca blood samples and 5.3% of questing H. aegyptium in Thrace tested PCR-positive for CCHFv 5 .
  • This hints at a cryptic transmission cycle where tortoises support viral persistence.
Table 3: Tunisia vs. Turkey CCHFv Findings
Region Tortoise Host CCHFv in Ticks Potential Role in Transmission
Tunisia Testudo graeca 0% (0/1,054) Unlikely
Turkey Testudo graeca 5.3% (2/38) Possible cryptic cycle
Map showing Tunisia and Turkey
Figure 2: Geographic comparison between Tunisia and Turkey

The Scientist's Toolkit: Essentials for Tick-Borne Virus Research

Table 4: Key Research Reagents and Tools
Reagent/Tool Function
Real-Time RT-PCR Kit Detects CCHFv RNA in tick homogenates
Qiagen Viral RNA Mini Kit Extracts viral RNA from samples
Morphological Identification Keys Identifies Hyalomma species (e.g., Hoogstraal's keys)
Tortoise Handling Permits Legal authorization for protected species
PreCellys Homogenizer Tubes Grinds tick tissues for RNA extraction

Unanswered Questions and Future Directions

  1. Why the Discrepancy?
    • Do Tunisian ticks lack exposure to livestock? Or do local viral strains lack tortoise adaptability?
  2. One Health Approach: With Tunisia reporting CCHFv antibodies in slaughterhouse workers, integrated animal-human surveillance is critical 4 .
  3. Climate Change Impact: As temperatures rise, could H. aegyptium expand its host range to mammals?

Conclusion: Silent Tortoises, Loud Implications

Tunisia's tortoises, while heavily infested, appear to be dead-end hosts for CCHFv. This absence redirects focus toward livestock-associated ticks but also highlights ecology's complexity: the same tick species may play vastly different roles across regions. As tick-borne diseases spread, such nuanced insights become vital shields in our global health arsenal.

"In nature's puzzle, even empty pieces reveal the bigger picture."

References