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Worries About Unusual Botulinum Toxin Prove Unfounded

A culture of <em>Clostridium botulinum</em>, stained with gentian violet.
A culture of Clostridium botulinum, stained with gentian violet.

Remember that worrisome new form of botulinum toxin we told you about in late 2013, the one that supposedly had to be kept secret out of fear it could be used as a bioweapon that would evade all of our medical defenses?

Well, as it turns out, it's not that scary after all. The antitoxin stored in the government's emergency stockpile works and would neutralize the toxin just fine.

That's the conclusion of some recent studies done by folks at the Centers for Disease Control and Prevention, which after two years of waiting finally received the strain of Clostridium botulinum bacteria from the public health department that identified it.

Botulinum toxin is one of the most poisonous substances known, and for a long time scientists knew of seven different types. Then researchers at the California Department of Public Health announced that they'd discovered the first new form of the toxin to turn up in over 40 years.

They called it "type H" and described their find in a medical journal. But here's the unusual and worrying part: The editors of the journal allowed the researchers to withhold key genetic details that would allow others to make or study this toxin. The reason given was that "no antitoxins as yet have been developed to counteract the novel C. botulinum toxin."

As NPR reported, however, other botulism researchers soon raised serious concerns. They said the California lab wasn't sharing the genetic information, and was also keeping the novel strain under lock and key. No one could independently confirm the claims that this was a new and potentially lethal toxin — or consider how to protect the public, if the concerns were valid.

When other scientists finally got the strain, they were relieved. "We don't think it poses a new, novel threat," says Robert Tauxe of the CDC's division of foodborne, waterborne and environmental diseases. "It appears to be a hybrid, that is a naturally occurring combination of two other existing toxins."

Tauxe explains that the botulinum toxin is a protein with two different parts. "I think of it as being like the scissors on a Swiss Army knife," he says. "There is the scissors part that actually does cut a piece of protein on a nerve cell. And that is held in a larger handle. And the larger handle is really how the immune system recognizes the toxin and reacts to it."

He says the newly discovered hybrid toxin has a "scissors" part that cuts like a type F toxin, but the "handle" is very similar to a type A toxin.

"What this means is that our medical countermeasures, our antisera that we have produced in quantity and have stocked in the national stockpile and that we use to treat botulism routinely, contain antibodies that protect against this novel toxin," says Tauxe.

At the CDC, scientists did experiments with mice showing that the current antitoxin offers protection. Other studies using cultures of nerve cells came to the same conclusion.

NPR asked the California Department of Public Health how it responds to criticism over the decision not to make the genetic sequence and strain available to others earlier, given that other labs were able to quickly ascertain that the current defenses would work. In reply, a spokeswoman sent a statement from James Watt, chief of its division of communicable disease control.

He said that discovery of the new strain was unusual for the department:

"The department does not typically identify agents that are potentially novel and/or of national security significance. Since there was no immediate threat, we chose to proceed carefully and deliberately to prevent any possible threat to public health and security."

The department has transferred governance over the new strain to the feds, he said, adding, "any decision regarding both releasing the sequence and the strain will be made by them now."

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Nell Greenfieldboyce is a NPR science correspondent.