Viruses such as West Nile and Zika that target the nervous system in the brain and spinal cord also can disrupting bowel movement and causing intestinal blockages, a study has found.

Researchers at the Washington University School of Medicine in St. Louis found that these blood viruses can kill neurons in the guts of mice and could be explain the reason why some people suffer unpredictable bouts of abdominal pain and constipation. "There are a number of people who are otherwise healthy who suddenly develop bowel motility problems, and we don't understand why," said Thaddeus S. Stappenbeck, professor at the varsity. 

"But now we believe that one explanation could be that you can get a viral infection that results in your immune cells killing infected neurons in your gut," Stappenbeck added. For the study, published in the journal Cell, the team studied mice infected with West Nile virus -- a mosquito-borne virus that causes inflammation in the brain -- when he noticed something peculiar. 

The intestines of some of the infected mice were packed with waste higher up and empty farther down, as if they had a blockage. They found that not only West Nile virus but its cousins Zika, Powassan and Kunjin viruses -- all of which target the nervous system like West Nile -- caused the intestines to expand and slowed down transit through the gut. 

In contrast, chikungunya virus, an unrelated virus that does not target neurons, failed to cause bowel dysfunction. The infected mice's digestive tracts gradually recovered over an eight-week time span. But when the researchers challenged the mice with an unrelated virus or an immune stimulant, the bowel problems promptly returned. This pattern echoed the one seen in people, who cycle through bouts of gastrointestinal distress and recovery. The flare-ups often are triggered by stress or illness, but they also can occur for no apparent reason.