TITLE: 24 life stages add to mystery of the microbe
BYLINE: Traci Watson
EST. PAGES: 2
DATE: 09/19/97; 06:23:15
SOURCE: USA TODAY; USAT
EDITION: FIRST; SECTION: NEWS; PAGE: 04A
(Copyright 1997)
No screenwriter, no novelist could have invented something
as fantastic as Pfiesteria piscicida. This one-cell microbe can
change its shape, spit out a powerful venom and stun a large-mouth
bass senseless -- all before breakfast. Thanks to these skills,
the puny Pfiesteria has become the bully of the Eastern Seaboard,
killing millions of fish and possibly sickening dozens of humans.
Pfiesteria is neither a virus nor a bacterium. It belongs instead
to a group of weird organisms known as dinoflagellates, named
for their long, whip-like tails, or flagella. Other dinoflagellates
harm fish and humans, but they can only spread their poison when
the cells are broken open. Pfiesteria is more malignant, deliberately
squirting its poison into the water to catch prey.
With at least 24 possible life stages, Pfiesteria is the microbe
of a thousand faces. Sometimes it takes the form of a cyst that
sits dormant at the bottom of a river. Other times it becomes
an amoeba, a starfish shape with long arms. It also can sprout
a tail to propel itself through the water.
No matter its form, Pfiesteria usually subsists benignly on
bacteria and bits of algae. But sometimes, for unknown reasons,
the microbe gets a taste for sashimi. It waits until it detects
chemicals given off by fish and their excrement. Then it releases
poisons into the water near its prey.
Scientists now believe that Pfiesteria produces at least two
powerful venoms. One, acting within minutes, stuns the fish, probably
by affecting the nervous system. The other poison produces abrasions,
the telltale bloody spots that are the most obvious sign of Pfiesteria's
presence. The microbe feeds on the blood and skin sloughed off
from the sores. One of the poisons seems to weaken the fish's
immune system, at least in the short term.
Every species tested in the lab succumbs to Pfiesteria attack,
from blue crabs to eels to mullet. But the most common victims
in the recent outbreaks in the Chesapeake Bay region have been
menhaden, a small bait fish.
Scientists suspect that menhaden are at high risk because they
are oily and swim in large schools, giving Pfiesteria more cues
to attack. They also are thin-skinned, says Pat Tester, a marine
biologist at the National Marine Fisheries Service, and ``once
(the skin) is broken, infection by bacteria or fungi is more likely.''
Pfiesteria is so toxic that even humans who have encountered
the microbe have suffered from symptoms such as vomiting and liver
problems, though no direct link between exposure to the microbe
and illness has been proved. But humans are not the microbe's
target, says Donald Schmechel, a neurologist at Duke University
Medical Center. ``It just wants to have a fish dinner.''
While researchers don't know how or whether Pfiesteria attacks
humans, they do know that only a tiny amount of its poison is
enough to kill a fish. Yet when toxicologist Patricia McClellan-Green
of Duke University and her research team examined fish found near
the site of a North Carolina fish kill, they found that the vast
majority of the specimens were poison-free. Only one species --
menhaden -- showed any trace of toxin. The rest, which included
spot, mullet and flounder, were perfectly clean. Fish are constantly
on the move, says McClellan-Green; those tested might have never
stayed in one spot long enough to get an injurious dose of poison.
Despite intense research, scientists do not know what, exactly,
transforms Pfiesteria from an algae-eater into a fish-killer.
They do know that both the amoeba and the long-tailed version
of Pfiesteria can become toxic when fish are near. So can the
cyst, which can change into a mobile form and emit poison within
a matter of hours. But the trigger that gives the microbe a taste
for fish is unknown.
Nor it it understood why Pfiesteria, an ancient species, should
suddenly go on so many mass-murder sprees. North Carolina State
University scientist JoAnn Burkholder, who discovered the organism,
has linked Pfiesteria blooms to high water levels of nutrients,
such as nitrogen, found in sewage and animal manure. But other
scientists say the link is far from proven. They point out, for
example, that the Rappahannock River in Virginia is one of the
cleanest rivers in the state, yet it had an outbreak of injured
fish this month.
Soon scientists might have more Pfiesteria-like mysteries to contend with. Florida Department of Environmental Protection scientists have found a Pfiesteria-like microbe in both the Maryland river where thousands of fish died and a Florida river where fish with lesions were found. The new microbe might not be toxic. But it will take much research before Pfiesteria and the new microbe can be coaxed into giving up their secrets.