Shigella | Virulent bacteria

Stained and observed under a microscope, Shigella is a captivating sight. The rod-shaped bacteria resemble fat, hairy cartoon caterpillars. In reality, however, this gram-negative, contagious bacterium that causes shigellosis can be remarkably dangerous. Shigella is back in the news after an outbreak in Kerala that was first detected in late March 2026 and continued into June. According to the Kerala Health Department, 132 confirmed cases and about 75 probable cases of shigellosis have been reported till June 12. This year, three deaths have been linked to the disease, two of which were children under the age of five.

Shigellosis is a diarrheal disease characterized by fever, abdominal cramps and bloody diarrhea (dysentery). It was bacteriologist Kiyoshi Shiga who isolated shigella dysenteriae in 1897 after Japan experienced a severe dysentery epidemic. The clan eventually adopted his name.

Transmission is mostly via the fecal-oral route, where people ingest small amounts of faeces through contaminated food, water or hands. As few as 10-100 bacteria can cause an infection and even an outbreak. Shigella is estimated to cause 80–165 million infections annually and about 6,00,000 deaths worldwide, mainly in children under five years of age in sub-Saharan Africa and South Asia.

In an article in the Indian Journal of Medical Research, Neelam Taneja and Abhishek Mewara write about the epidemiology of Shigella in India. “Although humans and primates are the primary reservoir of shigella, it has been isolated from a variety of sources—water bodies (rivers, surface water, and coastal water), free-living amoebae, insects, birds, and wild animals.”

Taneja and Mewara explain that several water bodies in India have been found to show the presence of Shigella. Thus, fish could be a potential source of infection if harvested from sewage-contaminated water. Even ingesting small amounts of contaminated water while swimming or bathing, or eating crops grown in Shigella-contaminated soil/water, can cause serious infection.

The paper claims that while no individual can be considered immune to shigellosis, “some individuals are at increased risk. Globally, the incidence of shigellosis is highest in children under the age of five. It has been reported that the incidence of shigellosis increases steadily after the age of 40.”

An outbreak in Kerala

In late March 2026, Kerala reported an outbreak of shigellosis in Kuttikkattoor, Kozhikode, where a three-year-old girl died and more than 60 residents (mostly children) fell ill. Soon clusters appeared in Wayanad, Malappuram and Kannur. A large cluster appeared at a Wayanad school where over 300 children were hospitalized.

So far, experts have traced infections to contaminated water and food sources, along with poor hygiene practices in communal settings such as schools.

Explained: What is a Shigella infection?

This is not Kerala’s first brush with shigellosis. In 2009, more than 300 people were reported to have contracted foodborne shigella in the state. In December 2020, an outbreak broke out, again in Kozhikode, killing an 11-year-old and infecting 40 others; in May 2022, shigella was thought to be behind mass food poisoning in Kasaragod, where 30 people were hospitalized and a 16-year-old girl died.

While mild shigella infections usually clear up on their own with hydration, severe cases require antibiotics to shorten the duration of the illness, reduce severity and prevent complications, according to the US Centers for Disease Control. However, some antibiotics are not effective against certain types of Shigella. Health care providers must order lab tests to determine which antibiotics are likely to work.

In their article, Taneja and Mewara point out that “there is a nationwide presence of multidrug-resistant Shigella that rapidly develop resistance to most available antibiotics. Therefore, judicious use of antibiotics is among the most important measures in the fight against shigellosis.”

This requires continuous and rigorous surveillance of antibiotic resistance across the country for regular updates of local antibiograms so that clinicians can effectively identify the correct antimicrobial drug to use.

Published – 14 Jun 2026 01:32 IST