How scientists are using family trees to track a deadly virus in boas and pythons.
Published on: October 15, 2023
Imagine a python, a creature of immense power and ancient grace, lying listless in its terrarium. It refuses food, its head tilts strangely, and it starts to vomit. For reptile veterinarians and breeders, this is a terrifying sight—the hallmark of Inclusion Body Disease (IBD), a devastating and often fatal illness. For decades, the cause was a mystery. Now, scientists have identified the prime suspects: a family of viruses known as arenaviruses. But how do you track an invisible enemy? The answer lies in building a family tree—not of the snakes, but of the viruses themselves.
Key Insight: A recent scientific paper, "Updated Phylogenetic Analysis of Arenaviruses Detected in Boid Snakes," provides a major leap forward in this detective story. By mapping out the genetic relationships between these viruses, researchers are not just identifying the culprits; they are uncovering their origins, their movements, and ultimately, paving the way to stop them.
To understand why this discovery is so significant, we need to start with the basics.
Traditionally, arenaviruses were famous (or infamous) for causing serious diseases in humans, like Lassa fever. Their natural hosts are almost always rodents. A mouse or rat can carry the virus for life without getting sick, silently shedding it in its urine and feces. This is a classic viral survival strategy: keep your host alive and mobile to spread far and wide.
The discovery of arenaviruses in boas and pythons was a shock. It broke all the rules. How did a "rodent virus" jump to snakes? And more puzzlingly, why does it cause such a severe, often fatal disease in its new reptilian host? This is the core mystery that scientists are trying to solve .
Understanding this jump is crucial, not just for saving pet snakes, but for understanding the fundamental rules of how viruses evolve and cross species barriers—a topic with immense implications for human and animal health worldwide .
The key to unraveling this mystery is phylogenetic analysis. In simple terms, it's the science of building evolutionary family trees based on genetic data.
Analogy: Think of it like this: if you compared the recipe for chocolate chip cookies from your grandmother, your mother, and yourself, you would see small changes. Maybe your mom uses margarine instead of butter, and you add sea salt on top. By analyzing these differences, you could reconstruct who learned from whom. Phylogenetics does the same with genes.
Scientists take a specific gene from a virus found in a sick boa constrictor in California and compare it to a virus from a python in Germany and another from a rat in Africa. The more similar their genetic sequences, the more closely related they are, and the more recently they shared a common ancestor.
This simplified phylogenetic tree shows the relationship between different arenavirus clades, illustrating how snake viruses form distinct branches separate from rodent viruses.
Let's break down the core experiment that fuels this phylogenetic research.
Researchers collected tissue samples (often from the brain, liver, or kidney) from boas and pythons showing clear signs of Inclusion Body Disease, as well as from seemingly healthy snakes.
Inside the lab, they use chemical kits to break open the cells and isolate all the genetic material (RNA and DNA) present.
Since the viral RNA is mixed with millions of times more snake RNA, they use a technique called Polymerase Chain Reaction (PCR). This acts like a genetic photocopier, using custom-designed "primers" that only latch onto and amplify the unique genetic sequence of the arenavirus, making billions of copies of just the viral gene.
The amplified viral gene is then fed into a DNA sequencer, a machine that reads the exact order of the genetic "letters" (A, C, G, U) in the sample.
The newly read genetic sequences from the snake viruses are lined up against a library of known arenavirus sequences from around the world. Sophisticated computer programs then calculate how they are all related and draw the most likely family tree.
| Research Tool | Function in the Experiment |
|---|---|
| RNA Extraction Kit | A set of chemicals and filters used to purify and isolate viral RNA from complex snake tissue samples without degrading it. |
| Reverse Transcriptase | A special enzyme that converts the virus's single-stranded RNA into complementary DNA (cDNA), which is more stable and easier to work with in the lab. |
| PCR Primers | Short, custom-designed pieces of DNA that act as "search probes" to find and bind specifically to the arenavirus genes, marking them for amplification. |
| DNA Polymerase | The "workhorse" enzyme used in PCR to make billions of identical copies of the targeted viral DNA segment. |
| DNA Sequencer | A sophisticated instrument that determines the exact order of nucleotides (A, T, C, G) in the amplified DNA, providing the raw data for the family tree. |
The updated phylogenetic analysis confirmed several critical hypotheses:
Snake arenaviruses are not a single group. They form two separate, deep-branching lineages, which scientists call "clade A" and "clade B." This suggests that the jump from rodents to snakes may have happened not once, but twice in evolutionary history .
There are strong hints that certain types of viruses might be associated with specific types of snakes (e.g., one clade more common in boas, another in pythons).
The virus found in a snake in the United States is often more closely related to one in Europe than to other U.S. strains. This points to the global pet trade as a major vector for spreading the disease.
| Virus Clade | Example Virus | Primary Natural Host | Disease in Snakes? |
|---|---|---|---|
| Old World | Lassa virus | Multimammate Mouse (Rodent) | No |
| New World | Junin virus | Drylands Vesper Mouse (Rodent) | No |
| Snake (Clade A) | University of Helsinki virus | Boa Constrictor | Yes (IBD) |
| Snake (Clade B) | California Academy of Sciences virus | Ball Python | Yes (IBD) |
| Symptom | Description |
|---|---|
| "Stargazing" | The snake holds its head and neck in an unnatural, upright position, often wobbling or unable to right itself. |
| Regurgitation | The snake vomits its food shortly after eating, leading to rapid weight loss. |
| Lethargy | A profound lack of energy and response to stimuli. |
| Paresis | Partial paralysis or weakness, especially in the rear part of the body. |
| Skin Infections | Secondary bacterial infections become common as the immune system fails. |
The updated phylogenetic analysis is more than just an academic exercise. It has immediate, real-world implications. By knowing exactly which viral strain is present, veterinarians can better understand disease progression. For breeders, this genetic map is a powerful tool for biosecurity. Quarantining new snakes is no longer just a best practice; it's a necessity to prevent introducing a new viral lineage into a collection.
Broader Implications: This research illuminates a dark corner of the natural world, showing us that the rules of viral ecology are constantly being rewritten. Every time a virus jumps to a new host, it's a natural experiment in evolution. By carefully studying these events in boa constrictors and ball pythons, we are not only learning how to save them from a devastating disease—we are gathering the clues to predict and prevent the next global pandemic, wherever it may come from .