Empty Nets?: Fishing for Hard Facts on Pfiesteria.

Pfiesteria piscicida and related toxic dinoflagellates were implicated in numerous fish kills in Atlantic coast estuaries in the 1990s, raising concerns about possible threats to public health. In 1997, Maryland watermen reported health effects from environmental exposure to toxic Pfiesteria, including memory loss and confusion. North Carolina also recorded many Pfiesteria-related fish kills, but researchers there had not systematically studied health effects among people who were exposed to waterways where Pfiesteria and related toxic dinoflagellates could occur.

In this issue, Marian Swinker of the East Carolina University School of Medicine and colleagues report on the first comprehensive examination in North Carolina of people with long-term contact with waterways where they may encounter Pfiesteria [EHP 106:21-26]. Conducted at the request of the state health agency in November 1997, the study found no pattern of abnormalities that could be attributed to possible exposure to Pfiesteria and related dinoflagellates.

It is difficult to assess whether someone has experienced contact with Pfiesteria. This dinoflagellate, which inhabits estuaries, spends nearly all of its time in nontoxic life stages. Moreover, the dinoflagellate attacks fish suddenly and then retreats, returning quickly to a nontoxic form on the estuary bottom. To confuse the issue, toxic algae such as Pfiesteria are not the primary cause of fish kills in estuarine waters; instead, low oxygen concentrations commonly kill large numbers of fish in shallow estuaries during warm months. As a consequence of these factors, biologists have had difficulty tracking the dinoflagellate and determining whether it actually is the cause of many fish kills, even in places where the organism has been found.

In their study, the North Carolina team used the 1997 Centers for Disease Control and Prevention case description for estuary-associated syndrome (EAS). This case description covers anyone who complains of persistent health effects such as memory loss, confusion, or acute skin burning after recent or remote contact with fish kills, fish with lesions, or affected waterways. For the North Carolina study, the term "affected waterways" could mean one of three things: any estuarine areas with conditions conducive to a Pfiesteria outbreak, any area where diseased fish were reported from June to September 1997, or any area where Pfiesteria had been seen in the past.

The team examined 22 licensed commercial fishermen and state employees who had worked in such waterways. Seventeen of the fishermen in this group reported exposure to a fish kill or to fish with lesions--possibly, but not conclusively, caused by Pfiesteria. For controls, the team examined 21 watermen and state employees who worked in the ocean, where Pfiesteria cannot survive. Eleven of those in the control group reported exposure to a fish kill or to fish with lesions that could not have been caused by Pfiesteria.

The team's examinations included a medical, occupational, and environmental history; general medical, dermatologic, and neurologic examinations; vision testing; and neuropsychologic evaluations. One subject in each group had had symptoms of EAS in the past, but neither subject had significant and current neuropsychologic impairment when examined.

The team found no pattern of abnormalities in these evaluations, with one exception. Watermen who worked on affected waterways had a significant reduction in a visual contrast sensitivity test--a measurement of the ability to detect visual patterns. Neurotoxins can affect vision, including the ability to detect visual patterns. But certain chemicals, drugs, alcohol, and several developmental and degenerative conditions can have the same effect. Moreover, scientists have not determined whether visual contrast sensitivity is affected by known dinoflagellate toxins. The researchers point out that there is no evidence that a relationship exists between potential environmental exposure to Pfiesteria or related toxic dinoflagellates and a reduction in visual contrast sensitivity, but that such reduced sensitivity should be considered in future studies to assess whether it might act as a marker of toxic dinoflagellate exposure.