Shigella Species Test · Stool

If you have diarrhea that turns bloody, lasts more than a few days, or comes with high fever, the question is no longer whether you have a stomach bug. It is which bug. Shigella is one of the nastier answers on that list, and knowing it is present changes what you do next.

This test checks a stool sample for Shigella bacteria. A positive result means you have shigellosis, a dysentery-type illness that can spread through food, water, sex, and household contact. It also means antibiotics are usually indicated and that you should take steps to avoid passing it to others.

What This Test Actually Detects

Shigella is not a normal part of the gut. Healthy people have none of it. The test looks for the bacteria in stool using either culture or a molecular method such as PCR (polymerase chain reaction, a DNA-based detection technique) or LAMP (loop-mediated isothermal amplification, a faster rapid-test chemistry). A result is essentially detected or not detected, which is why the interpretation is different from a typical numeric lab value.

Four main species cause human illness: S. flexneri, S. sonnei, S. dysenteriae, and S. boydii. In wealthier countries, S. sonnei dominates. In lower-income regions, S. flexneri is more common. Some labs report Shigella together with enteroinvasive E. coli (EIEC, a closely related bacteria that looks nearly identical on genetic tests) because molecular assays cannot always separate the two.

Why Shigellosis Matters

Shigella causes roughly 165 million cases and around 1 million deaths worldwide each year, with about 70 percent of episodes and 60 percent of deaths in children under five. It is among the top causes of moderate-to-severe diarrhea in children across Africa, South Asia, and Latin America. In adults, it can cause disabling illness for a week or more and spreads easily in households, daycare settings, and through sexual contact.

In children under five, repeated or severe Shigella infections are linked to persistent diarrhea, slower linear growth, and long-term economic burden for families. Even infections that do not produce visible symptoms have been associated with growth faltering in some cohorts. Adults can develop reactive joint pain, prolonged diarrhea, and, rarely, bloodstream infection.

Antibiotic Resistance Is Changing the Picture

Shigella is not the infection it was 20 years ago. Resistance to older drugs like ampicillin and trimethoprim-sulfamethoxazole is now routine in many regions. In a San Diego cohort, 55 percent of Shigella samples were resistant to azithromycin, 23 percent to fluoroquinolones such as ciprofloxacin, 70 percent to ampicillin, and 83 percent to trimethoprim-sulfamethoxazole. In Bangladesh, over 96 percent of strains showed some form of antibiotic resistance.

Extensively drug-resistant S. sonnei has driven recent outbreaks in Europe and among networks of men who have sex with men. This matters for your result because it means a positive test should usually be followed by susceptibility testing. A Bangladesh study of drug-resistant shigellosis found that decreased susceptibility to azithromycin was linked to worse clinical outcomes, so the choice of antibiotic is not academic.

Complications and Long-Term Effects

Most healthy adults recover from shigellosis in five to seven days, with or without antibiotics. But complications do happen. Severe infection can cause dehydration, electrolyte imbalance, and in some cases sepsis, especially in young children, older adults, and people with weakened immune systems. A systematic review of children under five found that Shigella infection was associated with persistent diarrhea and slower linear growth over months to years.

Early-life Shigella infections have also been linked to cognitive effects years later. In a three-cohort study, systemic inflammation in early childhood was strongly associated with lower verbal fluency scores at school age. This is part of why treating and preventing Shigella in children carries weight beyond the acute illness.

How to Read Your Result

This is a qualitative test. Results come back as detected or not detected, sometimes with the species (S. sonnei, S. flexneri, etc.) identified. There is no optimal number to target because healthy people carry no Shigella at all.

The test performance depends heavily on the method used. A meta-analysis of rapid tests found that LAMP achieved roughly 100 percent sensitivity and 97 percent specificity compared with culture or PCR, while dipstick tests showed about 95 percent sensitivity and 98 percent specificity. Stool culture alone, historically the standard, misses a large share of molecular-confirmed cases. In the MAL-ED child cohort, culture detected only about 6.6 percent of Shigella infections that quantitative PCR identified.

What this means for you: a positive result should prompt a conversation about antibiotic choice based on local resistance patterns, not an automatic prescription of whatever is handy. It also means you should be careful about food preparation for others, hand hygiene, and avoiding sex until diarrhea has resolved.

When Results Can Be Misleading

• Recent antibiotics: taking antibiotics in the days before testing can suppress Shigella enough that stool culture misses it, even when PCR still detects bacterial DNA. If your result is negative but symptoms strongly suggest shigellosis, retesting with a molecular method is reasonable.
• Sample timing and handling: Shigella is fragile. Delayed transport, warm temperatures, or samples collected late in the illness can reduce detection on culture. Molecular tests are more forgiving.
• Positive PCR without active disease: in high-burden settings, some people carry Shigella DNA without clear symptoms. A positive molecular test in someone without diarrhea may reflect residual DNA or low-level carriage rather than active infection.
• Shigella reported together with EIEC: many PCR assays target a shared gene (ipaH) and cannot distinguish Shigella from enteroinvasive E. coli. The result is usually labeled Shigella/EIEC and treated clinically the same way.

Serial Testing and When to Retest

For most adults, a single well-collected stool test using a molecular method is enough to confirm or rule out Shigella during an acute illness. A follow-up test of cure is not routinely recommended unless you work in food service, healthcare, childcare, or another setting where documented clearance is required. Public health authorities in many regions mandate this before returning to work.

If symptoms persist after treatment, retesting is warranted, ideally with susceptibility testing if not already done. Resistance to the first-choice antibiotic is a common reason for treatment failure. If you are in a risk group with recurrent exposure, such as a household with an infected child, testing should be repeated whenever new symptoms appear rather than on a scheduled interval.

What To Do With a Positive Result

A positive Shigella test is not something to manage alone. The practical next steps are straightforward but worth doing in order. First, get susceptibility testing if your lab did not run it automatically, because treatment choice depends on which drugs still work against your specific strain. Second, pair this result with a broader stool workup if your symptoms are atypical, since coinfection with other pathogens is common. Third, talk with an infectious disease or gastroenterology clinician if you are in a risk group for severe illness, have bloody diarrhea that is not improving, or belong to a network where drug-resistant strains are spreading.

Household and sexual contacts should be informed. Shigella has a very low infectious dose, meaning even a small amount of contaminated material can spread it. Handwashing with soap, avoiding food preparation for others until cleared, and abstaining from sex until diarrhea has fully resolved are the main practical measures. Many health departments require reporting, so expect a follow-up call from public health after a positive result.