Coronavirus symptoms may follow a distinct order, new research shows — and diarrhea could signal more aggressive disease
  • Classic symptoms of COVID-19 follow a distinct order, a new study found.
  • Patients usually start out with a fever, followed by a cough. Some may develop gastrointestinal issues later.
  • Diarrhea may be an early sign of a more severe case, according to the study.
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The official list of COVID-19 symptoms keeps growing. The Centers for Disease Control and Prevention now associates 11 symptoms with the disease, though some patients have reported conditions that don't appear on that list, like hair loss, hiccups, and purple, swollen toes.

"I've never seen an infection with this broad range of manifestations," Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told Facebook founder Mark Zuckerberg last month.

That can make coronavirus cases difficult to characterize. But a recent study from the University of Southern California identified a distinct order of symptoms among nearly 56,000 COVID-19 patients.

The researchers found that most symptomatic patients start with a fever, followed by a cough. After that, they may experience a sore throat, headache, or muscle aches and pains, followed by nausea or vomiting, then, finally, diarrhea.

For the most part, the researchers found, patients with milder cases showed the same order of symptoms as those with severe cases. But diarrhea could be a sign of a more severe infection, they found.

"This report suggests that diarrhea as an early symptom indicates a more aggressive disease, because each patient in this dataset that initially experienced diarrhea had pneumonia or respiratory failure eventually," the researchers wrote.

The CDC has also found that some COVID-19 patients may have diarrhea prior to developing fever.

Still, diarrhea was relatively uncommon among the COVID-19 patients examined in the new study: under 4% of patients had it, compared to 88% who had a fever and 68% with a cough. Other, smaller studies have found diarrhea to be more prevalent among hospitalized COVID-19 patients: between 10% and 34%.

Defining a 'classic' case of COVID-19

Defining a typical progression of COVID-19 symptoms could help officials determine which public-health measures are particularly useful for preventing the virus' spread. 

"The order of the symptoms matters," Joseph Larsen, the new study's lead author, said in a statement. "Knowing that each illness progresses differently means that doctors can identify sooner whether someone likely has COVID-19, or another illness, which can help them make better treatment decisions."

For example, since COVID-19 patients are now understood to be most infectious at the start of their illness, a fever may be a sign that a person is contagious (though people can also be contagious before their symptoms start). That means temperature checks could be a useful tool.

"Our results support the notion that fever should be used to screen for entry into facilities as regions begin to reopen," the researchers wrote.

The new research also better enables medical professionals to differentiate between COVID-19, SARS, and MERS (which are all coronaviruses). With the latter two, patients also started off with a fever, followed by a cough, according to the study. But unlike COVID-19 patients, they tended to develop diarrhea before nausea or vomiting. 

COVID-19 also starts differently than seasonal influenza: The researchers found that people with the flu develop a cough before a fever.

There's more to learn about coronavirus symptoms

Of course, there are limitations to these findings. The researchers' data comes from Chinese cases reported by the World Health Organization in February. But cases confirmed that early in the pandemic tended to be more severe and involve hospitalization, so the data could be skewed. The authors stressed the need replicate their results for patients in the US and elsewhere.

The CDC also estimates that around 40% of coronavirus patients are asymptomatic — meaning a significant chunk of coronavirus cases don't follow the pattern outlined in the study. 

For that reason, there's still a lot to learn about what a "typical" case looks like.

"One of the things we've learned about the virus is to not underestimate it and not rule out that something is coronavirus just because it didn't fit our model of what coronavirus is," Dr. Nate Favini, the medical lead at Forward, a primary-care practice that's collecting data on coronavirus patients around the country, told Business Insider. "Given the amount of asymptomatic spread of the virus, it's almost like you need to act like someone could have coronavirus until proven otherwise."

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