DALLAS _ Last winter, when Dr. James Musser read about the new coronavirus circulating in China, he told his group to get ready.
Musser, a top infectious disease expert at Houston Methodist Hospital, wanted a test he could deploy quickly, because he was sure the virus would make its way to Texas. "Houston, like Dallas, has ties to essentially everywhere in the world," he said.
Houston Methodist set up a test for the novel coronavirus in February and deployed it in early March, when the hospital saw its first suspected cases of COVID-19. While Musser had made the test for patients, he also used it to lay the groundwork for an added benefit: reconstructing how the virus arrived in Texas.
As increasing numbers of patients tested positive, Musser and his team compared their virus samples with strains from around the world. What emerged surprised him. Unlike the West Coast, which received many of its first cases from China, and New York, which received many of its early cases from Europe, Houston samples of the virus came from a range of countries.
"The only reasonable interpretation is that there were multiple independent introductions of strains to Houston from multiple geographic areas," said Musser about his research, which he posted online in May. "Some of the strains matched strains from the far East, some matched strains from Europe, some matched strains from South America."
While Musser has not studied COVID-19 in North Texas, he said the virus likely followed a similar pattern here. Both urban areas are major travel hubs that serve large, diverse populations.
In the six months since the first COVID-19 case was detected in Texas on March 4, scientists have worked to understand how and when the virus first made its way here, and how it spread through our communities. Tracing its early path will help policymakers prepare for future pandemics by showing how well interventions such as travel restrictions and social distancing have worked.
Research shows the coronavirus likely arrived in Texas far earlier than its first documented case near Houston and spread quickly and silently. Its proliferation was aided by limited testing and the ability of people with no symptoms to transmit the virus.
Spencer Fox, who is part of a team of scientists who study the virus at the University of Texas at Austin, said person-to-person transmission in Texas likely started by early to mid-February, based on the timing of the state's first hospitalizations and deaths.
A JANUARY ARRIVAL?
On Jan. 27, Lewisville resident Leonard Seiple developed a fever and intense body aches. One moment he was shivering and the next he was burning up. He felt nauseated. At night, his lungs burned as he coughed and struggled to breathe.
Seiple, 57, went to see his doctor. She told him he probably had influenza, though she didn't test him. Ten days passed before Seiple, who had gotten his flu shot, started to improve.
In April, Seiple told his doctor he was sure he'd had COVID-19, so she recommended he take a coronavirus antibody test. He took the test, and it came back positive _ a sign that he had contracted COVID-19 and recovered from it.
Seiple had not left Texas. So if his January illness had been COVID-19, and not flu, could he have been an early case? Dallas County's first case was diagnosed on March 10, but researchers who have studied the outbreak in Wuhan, China, and across the U.S. say it's possible.
"Texas is one of the top 10 states in terms of the number of cases imported from abroad," said Alessandro Vespignani, a Northeastern University expert on travel patterns and virus transmission. He believes cases could have started arriving in Texas as early as January, seeding local person-to-person transmission.
Vespignani used a global air travel database and information about the contagiousness of SARS-CoV-2, among other measures, to model how the virus spread around the world. His estimates suggest the disease was widespread in the major travel hubs of New York City and San Francisco, likely by mid-February. Smaller outbreaks were underway in cities like Seattle, Chicago, Boston and Dallas by then as well, Vespignani's research suggests.
Other studies back up his estimates. Scientists at UT-Austin reported in an April study posted online that community spread in Dallas was probably well underway by the time the county identified its first case of COVID-19 in early March, and could have reached people like Seiple in late January.
WHERE DID THE VIRUS COME FROM?
Musser and his team at Houston Methodist decoded the genetic makeup of 320 samples of the SARS-CoV-2 virus collected from patients in Houston between March 5 and March 30.
The vast majority, the scientists found, were closely related to strains then circulating in Europe and Asia. His research appears online but has not yet been published in a peer-reviewed journal.
The team also found the virus raced through the city _ within days, genetically identical strains had shown up all over town. Musser says the most likely explanation is that the virus had spread where people gathered _ with family, at offices, during religious services or in bars and restaurants.
"It just reinforces what we now, unfortunately, know: that this virus has an uncanny ability to spread very, very rapidly," he said.
Texas also saw an influx of coronavirus strains from other states, including California, Washington, New York and Louisiana, said Joseph Petrosino, a virologist at Houston's Baylor College of Medicine.
Genetic studies of hundreds of samples taken from Baylor patients who got sick beginning in March suggest the virus also flowed in from New Orleans, Petrosino said. The timing matched up with the Mardi Gras festival, which also helped ignite a massive outbreak in Louisiana.
"Whether it was Mardi Gras or not, we can't be sure, but it's a good hypothesis," said Petrosino. Business travel between the two states is another possible explanation.
In Vespignani's studies, his team has pointed out that international travel restrictions are not as effective as early detection and breaking the chains of local transmission with measures like social distancing and quarantines.
Cases also reached Texas from an international conference in Boston held Feb. 26 and 27, according to researchers at Massachusetts General Hospital and the Massachusetts Institute of Technology.
By studying the genetic makeup of virus samples from Boston patients, the researchers found a strain of the virus that had infected a single person at the conference had then spread widely through the city. Later, they found, the same strain made its way to Texas and other states and countries. Since then, the original strain has spawned tens of thousands of additional cases.
"The take-home message is that the ripple effects of a single event can be enormous," said Dr. Jacob Lemieux, lead author of the study and an infectious-disease expert at MGH. "An individual decision about whether to attend a party without a mask doesn't just affect that person. It affects all of society."
Dr. Wendy Chung, the chief epidemiologist for Dallas County, said that, to her knowledge, genetic analysis has not been done on local strains of the virus.
Medical examiners are also getting involved in the research by examining tissue that's routinely saved during autopsies.
Dr. Pramod Gumpeni, a pathologist at the Harris County medical examiner's office, said his team is looking back at autopsies from January and February, from people who died at home with flu-like symptoms. Next, he said, he wants to have the tissue analyzed to see if it contains the COVID-19 virus.
Dallas County medical examiner Dr. Jeffrey Barnard said his office also maintains tissue samples that could be analyzed for early cases.
Reconstructing how the virus spread in the U.S. could help states prepare for future pandemics, Gumpeni said.
"This is not going to be the last pandemic we see," he said. "It's important as public health officials, as medical doctors or scientists, that we have proper safeguards."
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