Validating wing biopsies for blood-borne pathogen characterization in bats

While wing biopsies from common vampire bats show a lower probability of detecting blood-borne pathogens compared to blood samples, they remain a viable, less invasive method for initial pathogen discovery and characterizing infections, though they are less ideal for accurately estimating infection prevalence.

The Gist

Imagine you are a detective trying to solve a mystery: What invisible germs are living inside a colony of bats?

For a long time, the only way to catch these clues was to play "doctor" with the bats. Researchers had to catch them, hold them still, and prick their veins to draw blood. While effective, this is like trying to interview a nervous witness while they are being held down—it's stressful for the bat, time-consuming for the researcher, and often requires special training.

This paper asks a simple question: Can we solve the mystery without drawing blood?

The researchers decided to test a "backdoor" method. Instead of a blood draw, they took tiny, painless "snips" of the bat's wing membrane (like taking a tiny piece of a leaf). Since bat wings are full of tiny blood vessels, the idea was that these little snips might contain enough blood to reveal the germs hiding inside.

Here is the breakdown of their investigation, explained simply:

1. The Suspects (The Germs)

The team was looking for three specific types of "blood-borne" germs that can jump from animals to humans:

• Bartonella: Bacteria often spread by insects (like fleas).
• Hemoplasmas: Bacteria that love to hang out inside red blood cells.
• Trypanosomes: Parasites often spread by bugs (like kissing bugs).

2. The Experiment: The "Blood vs. Wing" Taste Test

The researchers caught 63 common vampire bats in Belize. For every single bat, they did two things:

1. The Gold Standard: They drew a tiny bit of blood.
2. The Shortcut: They took a tiny 2mm punch of the wing skin.

They then ran a high-tech DNA test on both samples to see if the germs showed up.

3. The Results: The "Faint Signal" Problem

Think of the blood sample as a loud, clear radio broadcast of the germs. The wing tissue sample was more like a faint signal on a walkie-talkie with static.

• The Bad News: The wing tissue was much worse at finding the germs than the blood. If a bat was infected, the blood test almost always caught it. The wing test missed it most of the time.
  • Why? When researchers cut the wing, they are careful to avoid the big blood vessels to keep the bat safe. So, they accidentally cut mostly "empty" tissue, missing the "juice" where the germs live.
• The Good News: The wing test did work sometimes! They found germs in the wings of at least two bats for each type of germ.
• The "Same Germ" Check: When they found the same germ in both the blood and the wing of the same bat, the DNA was a near-perfect match (like finding the same fingerprint in two different places). This proved that the wing wasn't just picking up random noise; it was actually detecting the real infection.

4. The Verdict: When to Use Which Tool?

The researchers concluded that wing tissue is not a perfect replacement for blood if you want to know exactly how many bats are sick (prevalence). If you use wings, you will likely underestimate the number of sick bats because the signal is too faint.

However, wing tissue is a fantastic "Scout."

Here is the best analogy:

Imagine you are looking for a rare bird in a massive forest.

• Blood sampling is like catching the bird, putting it in a cage, and studying it in detail. It's accurate but hard work and stressful for the bird.
• Wing sampling is like walking through the forest and looking for a feather on the ground. You might not find a feather every time the bird is there, but if you do find one, you know the bird is in the area.

Why This Matters

• Less Stress: Taking a wing snip is much easier and less scary for the bat than a blood draw.
• Free Data: Museums already have thousands of bat wings stored in jars from decades of research. This study suggests we can dig into those old jars and find new germs without ever catching a live bat again!
• The Strategy: Use the wing snips as a first filter. If the wing test finds a germ, then researchers know they need to do the harder work of blood sampling to get the full picture.

In short: Wing tissue isn't a perfect replacement for blood, but it's a brilliant, low-stress tool to help us discover where to look for dangerous germs in the wild.

Technical Summary

1. Problem Statement

Wildlife surveillance is essential for tracking emerging infectious diseases, particularly zoonotic blood-borne pathogens like Bartonella spp., hemotropic mycoplasmas (hemoplasmas), and trypanosomes. However, standard surveillance methods face significant barriers:

• Logistical and Ethical Constraints: Collecting blood from wildlife is time-consuming, requires specialized training, and induces significant stress on the animals.
• Lethal Sampling Limitations: While organ tissues (spleen, liver) contain blood-borne pathogens, collecting them requires lethal sampling, which conflicts with conservation goals and permitting.
• The Gap: Non-lethal vascular tissue samples (e.g., wing biopsies in bats) are routinely collected for other research (species ID, aging, population genetics) and stored in museums. However, it remains unclear whether these tissues contain sufficient pathogen load for reliable detection and genetic characterization compared to direct blood collection.

2. Methodology

The study utilized a paired-sample design to compare pathogen detection in blood versus wing tissue.

• Study Subjects: Common vampire bats (Desmodus rotundus) captured in Orange Walk District, Belize (November 2021 and April–May 2022).
• Sample Collection:
  • Blood: Collected via lancing the propatagial vein into heparinized capillary tubes and stored on Whatman FTA cards.
  • Wing Biopsies: Two 2 mm punches taken from the lower wing membrane (avoiding visible blood vessels) and stored in DNA/RNA Shield.
  • Sample Size: 63 individual bats with paired samples (60 screened for Bartonella and trypanosomes; 62 for hemoplasmas).
• Laboratory Protocols:
  • DNA Extraction: Two different kits were used across institutions (Quick-DNA/RNA Viral Magbead Kit vs. Quick-DNA Miniprep Plus Kit). The study controlled for extraction method differences in statistical analysis.
  • Pathogen Screening: Nested PCR was used to target specific genes:
    • Bartonella: gltA gene.
    • Trypanosomes: Small subunit rRNA (SSU rRNA) gene.
    • Hemoplasmas: 16S rRNA gene.
  • Sequencing: Amplicons were Sanger sequenced, and consensus sequences were assigned to genotypes using NCBI BLASTn.
• Statistical Analysis:
  • Binomial Generalized Linear Mixed Models (GLMMs) were used to compare infection detection probabilities between sample types, accounting for individual bat variation and extraction method.
  • Sample size calculations were performed to determine the number of wing biopsies needed to detect a single infection or estimate prevalence with 80% power and 95% confidence.

3. Key Results

• Detection Probability: The probability of detecting infection was consistently and significantly lower in wing tissues compared to blood for all three pathogens:
  • Bartonella: 98% less likely in wing tissue (OR = 0.02).
  • Hemoplasmas: 92% less likely in wing tissue (OR = 0.08).
  • Trypanosomes: 95% less likely in wing tissue (OR = 0.05).
• Positive Cases: Despite lower sensitivity, wing tissues successfully detected infections in at least two individuals for each pathogen.
  • Bartonella: 49 total positives (48 blood, 3 wing); 1 bat was positive only in wing tissue.
  • Hemoplasmas: 38 total positives (33 blood, 9 wing); 4 bats were positive only in wing tissue.
  • Trypanosomes: 26 total positives (24 blood, 2 wing); 1 bat was positive only in wing tissue.
  • Coinfection: 14 bats were coinfected with all three pathogens; 2 of these would have been missed without wing tissue screening.
• Genetic Concordance:
  • For bats positive in both blood and wing tissue, Bartonella sequences were 100% identical.
  • Hemoplasma sequences showed high similarity (98.66% ± 0.54%), with most showing lineage matches.
  • Trypanosome sequences could not be compared for concordance as no single bat was positive in both sample types for this pathogen.
• Sample Size Requirements: To detect a single infection in wing tissue with 80% power, the required sample sizes were estimated at 31 (Bartonella), 10 (hemoplasmas), and 32 (trypanosomes).

4. Key Contributions

• Validation of Non-Lethal Sampling: The study confirms that wing biopsies, while less sensitive than blood, are a viable source for detecting blood-borne pathogens in bats.
• Discovery of "Hidden" Infections: The research demonstrated that relying solely on blood collection would miss specific infections (6 total infections in this study were detected only via wing tissue).
• Genetic Fidelity: The study proved that when pathogens are detected in wing tissue, the genetic sequences largely match those found in blood, validating the use of wing tissues for pathogen characterization and diversity studies.
• Methodological Robustness: The study showed that different DNA extraction kits yielded comparable results for most pathogens, suggesting flexibility in laboratory protocols for retrospective studies using museum collections.

5. Significance and Implications

• Surveillance Strategy: The authors conclude that wing biopsies are not ideal for estimating precise infection prevalence due to the low probability of detection compared to blood. However, they are highly effective for initial pathogen discovery and assessment.
• Resource Optimization: Utilizing wing tissues allows researchers to screen large numbers of bats with minimal stress and effort. This is particularly valuable for:
  • Retrospective Studies: Leveraging existing museum collections (cryogenically stored or ethanol-preserved) to screen for historical pathogen presence without new fieldwork.
  • Targeted Surveillance: Using wing tissues to identify which pathogens are circulating in a population, thereby guiding more intensive, targeted blood-sampling efforts for prevalence estimation and zoonotic risk assessment.
• Conservation: This approach supports "One Health" surveillance by reducing the physiological stress on wildlife while expanding the scope of pathogen monitoring.

Conclusion: While blood remains the gold standard for prevalence estimation, wing biopsies serve as a critical, non-lethal tool for expanding the known diversity of blood-borne pathogens in wildlife and guiding future surveillance priorities.