Scientists Use Virus to Trace Assault Suspect

The prosecution turned initially to Michael Metzker, an expert in molecular genetics and phylogenetic analysis (evolutionary history) of HIV at the Baylor College of Medicine in Houston. Metzker, lead author of the report in the Proceedings of the National Academy of Sciences, began piecing together the history of the evolution of the virus from Trahan and the blood found in Schmidt's office.

But to guard against any chance of contamination, or bias, the prosecution asked Mindell to conduct the same tests. David Hillis, another expert on HIV at the University of Texas, was selected to compare the results of both series of tests.

Blood was drawn from 28 AIDS patients in Louisiana to be incorporated into the test so that the researchers could compare similarities between the Trahan sample and others.

"We compared the DNA sequences" from all the samples, Mindell says, "looking for shared characters," or common traits.

"It is really like a family tree," he adds. The researchers were looking for what they call the "maximum likelihood" that each sample belonged on a particular tree. It's a painstaking process because "there are hundreds of thousands of possible trees."

They were looking for, quite literally, blood brothers. And in the end they found it. Graphics showing the DNA sequences for all the samples revealed two that "leaped out" at the researchers. After looking at the results of both research efforts, and ruling out any chance for contamination, Hillis testified that two of the samples were "closely related." So close, he said, that no two samples could be any closer.

One of the two came from Trahan. The other came from the AIDS patient whose blood was found in Schmidt's office.

Age of New Evidence

It's worth noting that the evolutionary evidence isn't what put Schmidt in the slammer. There was other circumstantial evidence, including a boast from Schmidt that he would infect his nurse with AIDS if she dropped him as a lover.

But the scientific evidence showed that Schmidt could not be ruled out as a suspect. And the two viruses were so closely related that the virus Schmidt shot into his Trahan's arm almost certainly came from the vial of blood found in his office. Thus the evolutionary history became a key in the prosecution's case. Even the expert witness for the defense ended up admitting on the stand that the two samples came from the same evolutionary tree, so they were very closely related.

That was the first, and as far as could be determined, the only time that evolutionary history has been used in a court in this country to convict a criminal. But it probably won't be the last.

Prosecutors have a new and powerful forensic tool in their hands, and that's only part of the story.

Mindell says the technique could be used to trace almost any biological pathogen, like those that could be released during biological warfare, back to its source. Investigators would need samples of what they suspect to be the source, of course.

"It doesn't mean you identify who was involved, but if you have an alleged source, or some material you suspect might be the source, this can be a powerful way to test that hypothesis," he says.

Equally important, it might rule out a suspected source, thus reducing the chances that some country will bomb the wrong suspect.

Although testing for the Schmidt trial took months, Mindell says it could be done much more quickly in the event of biological warfare.

"This could be done in a week," he says.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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