Your eyes itch and burn, your throat is sore, you're congested and coughing from a constant tickle. Is it a cold? Allergies? Sorry to say, it's most likely red tide. It returned last week to South Marco Beach with a reading of 106,000 cells of Karenia brevis per liter that would register as medium and be sufficient to cause respiratory irritation and probable fish kills.

It was also detected earlier this month in a sampling taken approximately 40 miles offshore, west of Doctors Pass in Naples.

Under these conditions, people with emphysema or other chronic respiratory disease such as asthma should avoid going to the beaches where these illnesses would be aggravated. Onshore winds could bring discomfort such as itchy and burning eyes, dry cough or congestion.

Although red tide has been around for a long time, public awareness rose on a muggy fall morning in 1947, when a community in Venice, Fla., awoke to thousands of dead fish along its beaches and a stinging, choking irritant in the air. Scientists soon discovered the cause: red tide. Although this was the first scientific documentation along the Florida gulf coast, reports of similar incidents had been recorded in the mid-1800s.

Red tide is caused by the bloom of harmful algae, certain microscopic, single-celled plants that live in the sea. Most species of algae or phytoplankton are harmless, serving as energy producers at the bottom of the food chain to help sustain higher life forms on this planet. Occasionally, algae grow quickly, and the bloom intensifies into dense, visible patches near the water's surface.

A small number of species produce potent neurotoxins that can be transferred through the food web where they affect and even kill higher forms of life such as zooplankton, shellfish, fish, birds, marine mammals, and even humans that feed on them. The term harmful algal bloom (HAB) is preferred by scientists to refer to algae that contain toxins or cause negative impacts. HABs pose a serious and recurrent threat to human health, wildlife, marine ecosystems, fisheries, coastal aesthetics and our economy.

The manatee can be another victim of red tide when the one-celled organism that causes the bloom containing a breve toxin is released into the water when the algae die.

In the first three months of 2003, 120 manatees died, and of those, the Florida Marine Research Institute stated that 60 of the deaths along the Southwest Florida coast between Feb. 27 and April 15 were caused by red tide.

According to Tom Pitchford, a wildlife biologist with the Florida Fish and Wildlife Conservation Commission, "They can ingest the toxins when they eat, or they can inhale the toxins when they come to the surface to breathe. Once the toxin is in the animal, it affects its coordination and causes paralysis."

Scientists relate stories of six manatees that survived red tide when assisted by people holding their heads above water as they recovered from the effects of paralysis.

Nancy Richie, environmental specialist with the City of Marco Island said, "Red tide usually kills about five loggerhead turtles from Sarasota to Marco Island each year."

During a particularly long period of red tide, dogs became ill, possibly from drinking affected water or being near fish that had died from a HAB.

The most troublesome species in the Gulf of Mexico is Gymnodinium breve (G. breve, renamed Karenia brevis, or K. brevis) in honor of Dr. Karen Steidinger, a prominent red tide scientist from the Florida Marine Research Institute in St. Petersburg, Fla.

K. brevis in high concentrations might create a brownish red sheen on the water's surface, while in other instances it may look yellow green or be invisible. Some red tides have blanketed several hundred square miles of water. No one can predict when or where red tides will appear or how long they will last, since they are influenced by physical factors and variables such as weather, currents, winds and tides.

Readings are interpreted in the following ways:

* Present: Normal levels of 1,000 cells or less, little or no respiratory irritation.

* Very low a: More than 1,000 but less than 5,000 cells, possible respiratory irritation.

* Very low b: 5,000 to 10,000 cells, possible respiratory irritation and shellfish harvesting closures.

* Low a: More than 10,000 but less than 50,000 cells, respiratory irritation, but chlorophyll levels too low to be detected by satellites.

* Low b: 50,000 to less than 100,000 cells, respiratory irritation, possible fish kills and bloom chlorophyll probably detected by satellites.

* Medium: 100,000 to less than 1,000,000 cells, respiratory irritation and probable fish kills.

* High: Greater than 1,000,000 cells, respiratory irritation and probable fish kills plus discoloration.

Measurements are taken by the Collier County Health Department and Rookery Bay National Estuarine Research Reserve while the algae is in bloom. Constantly changing winds and tides can cause frequent changes in the intensity of the bloom's effect on land over a period of hours, if it is in the area. In previous years, when the effects of red tide continued, the Collier County Health Department opened a hotline to keep residents informed of daily conditions.

The next sampling was Nov. 15.