Some patients with COVID-19 experience gastrointestinal symptoms, particularly diarrhea , as the first sign of illness, according to a new study.

Among this subset of patients — who have mild disease overall — respiratory symptoms show up only later in the illness, and some never develop respiratory symptoms at all, the authors said.

The findings are important because those without classic symptoms of COVID-19 — such as cough, shortness of breath and fever — may go undiagnosed and could potentially spread the illness to others, the researchers said.

Still, they note that digestive problems are common overall and don't necessarily mean that a person has COVID-19. But doctors should recognize that sudden digestive symptoms in people with a possible COVID-19 contact "should at least prompt consideration of the illness," the authors wrote in their paper, published ahead of print Monday (March 30) in The American Journal of Gastroenterology. "Failure to recognize these patients early and often may lead to unwitting spread of the disease."

The study is not the first to report digestive symptoms as a sign of COVID-19. For example, a study posted March 18 in the same journal found that, among about 200 COVID-19 patients at three hospitals in Wuhan, China, around 50% reported at least one digestive symptom, and 18% reported diarrhea, vomiting or abdominal pain. However, that study and others have tended to focus on patients with severe illness, rather than those with mild disease.

In the new study, the researchers analyzed information from 206 patients at Union Hospital, Tongji Medical College in Wuhan, which was designated as a hospital for COVID-19 patients. To be included in the study, patients needed to have a mild illness, without difficulty breathing or low blood oxygen levels.

Overall, 48 patients (23%) were admitted with digestive symptoms only, 89 (43%) with respiratory symptoms only and 69 (33%) with both respiratory and digestive symptoms.

Among all patients with digestive symptoms (117 patients), about 67 (58%) had diarrhea, and of these, 13 (20%) experienced diarrhea as the first symptom of their illness. Patients' diarrhea lasted from one to 14 days, with an average duration of five days, the report said. About one-third of patients with digestive symptoms never experienced a fever.

Patients with digestive symptoms tended to seek health care later than those with respiratory symptoms, an average of 16 days from the start of their symptoms, compared with 11 days for those with respiratory symptoms, the study found. Those with digestive symptoms also took longer to clear the virus from their body (test negative for COVID-19), taking about 41 days on average, compared with 33 days for those with respiratory symptoms only.

Finally, those with digestive symptoms were much more likely to have the new coronavirus, SARS-CoV-2, detected in their stool, with about 73% having positive stool samples, compared with 14% of those with respiratory symptoms only. This finding suggests, but does not definitely confirm, that that virus infects the gastrointestinal tract, the authors said.

Overall, "these data emphasize that patients with new-onset diarrhea after a possible COVID-19 contact should be suspected for the illness, even in the absence of cough, shortness of breath, sore throat or even fever," the authors concluded. "Optimally, testing for COVID-19 should be performed using both respiratory and stool samples, if available."

The authors note that their study was relatively small, and larger studies are needed to further describe digestive symptoms in patients with mild COVID-19.

Originally published on Live Science.