As if it weren’t enough that the new coronavirus can steal away your ability to breathe and make your immune system turn against you, now we know this fearsome pathogen can also literally curdle your blood. News of the “bizarre, unsettling” complication—one that’s been killing young and middle-aged patients with Covid-19—made headlines last month. “ It crept up on us,” one doctor told The Washington Post for a story published on April 22. “We are scared,” said another.

Other outlets quickly added to the terrifying coverage. Vox cited a hematologist who called the disturbing new outcome “unprecedented … This is not like a disease we’ve seen before.” AFP described the “mysterious” clotting phenomenon as the coronavirus’s “latest lethal surprise.” The New York Times wondered if it might explain another unexpected symptom seen in patients: swollen, red and purple “Covid toes.” There was coverage of a 41-year-old Broadway star, hospitalized with the virus, who’d had to have his leg amputated due to a clot. Yet for all the seeming strangeness of these cases, it’s stranger still that so many people would be acting so bewildered. In fact, researchers have long known about the link between infectious diseases and blood clotting. There’s even data to suggest a heightened risk of fatal heart attacks—a related complication—among those who get plain old influenza.


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The study of disease-induced clotting stretches back more than a century. Writing in 1903, pathologists described the same phenomenon in typhoid fever. Adam Cunningham, an immunologist at the University of Birmingham, notes that many common bacteria, such as Helicobacter pylori and Escherichia coli, have also been associated with an increased risk of blood clots. If this fact has mostly been forgotten, it may be on account of our success at treating such infections. “One of the things that probably made a big difference was the introduction of the antibiotic era, so many of the pathogens didn’t get that severe,” Cunningham says.

In 2006, I wrote up a large-scale study finding that patients suffering from respiratory or urinary tract infections were at doubled risk of developing deep vein thrombosis, a potentially fatal complication of abnormal clotting. Then I, too, forgot about it. (The actual magnitude of this risk has not been pinned down.) That’s just one of many such findings, though. Other viruses associated with clotting complications include hepatitis, measles, and HIV. There are similar reports in cases of H1N1, also known as swine flu: Canadian doctors looked at the records of 119 patients hospitalized during the 2009 pandemic outbreak of that disease and found that seven had experienced major clots. These occurred everywhere from the patients’ lungs to their arms.

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Although there are many examples of this in the research literature, clotting still isn’t thought of as a typical outcome from viral lung infections. “It’s not the first thing you expect of a respiratory disease,” says Nonantzin Beristain Covarrubias, a research fellow and collaborator of Cunningham’s at the University of Birmingham. But that same literature reveals that blood clots have been linked to other coronaviruses. Clotting was found in the small veins of Chinese patients struck by severe acute respiratory syndrome (SARS), a coronavirus illness that hit multiple countries in 2003. In Singapore, a handful of critically ill SARS patients developed the complication in their brains, lungs, and other organs. Local medical researchers called for “increased vigilance” against stroke in future SARS-CoV outbreaks.

Another life-threatening condition related to blood clots was seen in the past among both SARS patients and those infected with the coronavirus that causes Middle East respiratory syndrome (MERS). When blood clots form, they can use up the body’s available platelet cells. Since these platelets are crucial for stopping bleeding under normal circumstances, this can lead to a dangerous problem known as thrombocytopenia. As many as one-third of patients with MERS in one Saudi Arabia study developed thrombocytopenia. The same condition has also been observed in patients with Covid-19. (Low platelet counts are also linked to a scary condition in which blood clots spread throughout a person’s blood vessels, though there isn’t yet consensus on whether this risk is present in the current pandemic.)

Many different pathogens are linked to blood disorders, but the specific clotting mechanisms may vary. These details matter: If we know exactly how a particular infection leads to blood clots, we can make a better guess at which drugs might be most useful as a treatment.