More than 30 people have fallen sick and four have died in Guatemala after eating shellfish.

The National Commission for the Surveillance and Control of Red Tide in Guatemala has extended an alert because of the presence of saxitoxins above the limits on the coasts of Tiquisate, Escuintla, Retalhuleu and San Marcos.

Public health officials said that since the end of April, 34 people had been affected and three children and one adult had died.

Authorities urged the public not to eat bivalve mollusks such as mussels, clams and oysters.

A warning was issued earlier this month after monitoring and analysis detected high concentrations of saxitoxins above the limits.

Saxitoxin is a toxin responsible for paralytic shellfish poisoning (PSP). Harmful algal blooms are often called red tides.

Saxitoxin affects the nervous system, with symptoms usually occurring within 30 minutes. Severity depends on the amount of toxin consumed. Initial reactions are tingling of the lips and tongue, which spreads to the face, neck, fingertips, and toes, followed by a headache, dizziness, and nausea. Muscular paralysis and respiratory difficulty may occur within five to 12 hours.

Officials said people with any of the symptoms mentioned above should go immediately to the nearest health center to receive treatment. Patients were told not to self-medicate as all services are free.

Affected shellfish will taste and appear no different than safe shellfish, and cooking does not destroy the toxin.