As biologists with the Maine Department of Marine Resources continued testing coastal waters for red tide contamination on Thursday, one of the four people hospitalized after eating tainted shellfish in Washington County earlier this week remained in critical condition at a Bangor hospital.

Randy Beal of Harrington and his wife, Brenda Beal, along with two other family members who have not been identified, became ill Tuesday night after eating a batch of contaminated mussels.

According to a spokesperson for Eastern Maine Medical Center, Brenda Beal’s condition was upgraded Thursday to fair and her husband’s to serious.

The two other family members were last listed in fair condition, one at EMMC and the other at Down East Community Hospital in Machias.

The family apparently became sick after eating shellfish that Randy Beal found attached to a barrel floating in ocean waters near Jonesport, according to officials.

The batch of mussels was tested and found to have high levels of the marine biotoxin known to cause paralytic shellfish poisoning. The biotoxin is found in certain algae that make up red tide. Shellfish consume the algae and become contaminated.

Shellfish that can be contaminated include mussels, oysters and clams, which filter their food from seawater that may contain toxins. Lobsters and crabs are meat eaters and are not affected by red tide.

It was the state’s first documented case of human red tide poisoning at least since 1980, according to the Maine Bureau of Health, but officials are not concerned about an outbreak.

“We did some extra sampling [Wednesday] in the Jonesport area and many other areas,” DMR’s Deputy Commissioner David Etnier said Thursday afternoon. “The tests are coming up clean. We have no reason to believe that this is not isolated.”

Etnier said DMR biologists constantly test shellfish samples as part of its red tide monitoring program. Testing methods include injecting laboratory mice with a mixture of hydrochloric acid and blended shellfish and observing the effects, Etnier said.

Most of the testing Wednesday and Thursday was being done at the DMR’s biotoxin monitoring lab at Lamoine State Park.

“We have the barrel there. It looks like [Beal] was just harvesting somewhere he shouldn’t have been,” Etnier said.

The natural toxin that causes paralytic shellfish poisoning works at the cellular level, blocking the movement of sodium particles essential to transmission of nerve impulses, according to a Bangor physician who has been involved in the care of the family.

“It prevents the electrical conduction of nerves from stimulating the muscles to contract,” explained Dr. Thomas Kandra, a pulmonary and critical care specialist who practices at Eastern Maine Medical Center. Symptoms can emerge within minutes of eating contaminated shellfish, he said Thursday, and may progress rapidly from tingling of the lips and fingertips to an inability to move the arms and legs to – most deadly – the inability to breathe, which is caused by the paralysis of the muscles that control the action of the lungs.

Victims also may experience difficulty swallowing and loss of eyesight. In rare cases, the heart muscle may be affected, Kandra said.

The extent of the damage depends on many factors, including how much toxin is ingested, the age and health of the victims, and how long it takes them to reach medical care.

Victims may need to be placed on mechanical ventilation until they recover, Kandra said. Patients rarely have their stomach pumped; since the poison acts so quickly, it’s often too late for such a measure to be effective and needlessly increases discomfort. Sometimes, he said, victims are dosed with liquefied charcoal, which helps prevent further absorption of the toxin into the bloodstream.

Patients typically are monitored for heart and brain abnormalities and may be given medications to decrease anxiety and discomfort. Health care providers work to prevent complications of inactivity such as pneumonia and blood clots.

“Basically, we’re giving the patients time to clear the toxins themselves,” Kandra said. In most cases, paralysis subsides as the body rids itself of the poison, he said. Full recovery within a few days is the norm, though some victims may experience residual weakness and require physical therapy or other rehabilitation.

Dr. Dora Anne Mills, head of the Maine Center for Disease Control and Prevention, said Thursday that the mussels ingested by the Beal family contained extremely high levels of the red tide toxin. The state’s threshold for closing shellfish harvesting is 80 micrograms of toxin per liter of test solution; the mussels that poisoned the Beal family contained levels of about 16,000 micrograms per liter, or 200 times the closure threshold.

Mills said the warm, sunlit surface of the Bay of Fundy provides a perfect environment for the toxin-producing algae, and floating on the barrel in an open current would have allowed the mussels to filter more water than they would in a sheltered bay.

Mills reiterated her praise for the state’s response to the isolated incident and her confidence in the Department of Marine Resources’ shellfish monitoring program. She urged anyone planning to harvest shellfish for personal consumption to check first for red tide or other closures on the DMR’s Web site, www.maine.gov/dmr/index.htm, or call toll free 800-232-4733.