A variety of creatures living in the sea and in estuaries may cause bites and stings. Some, such as the Blue Bottle are well known, while many are not.

Sometimes, users of Narrabeen Lagoon may notice an itchy dermatitis following swimming or wading in the lagoon. Last year, Pittwater Council sought expert advice on what caused the stings from Dr John Walker who was the Senior Parasitologist at Sydney's Westmead Hospital. Dr Walker said that many creatures may possibly cause stings, including larval jellyfish and flukes. More information was required in each situation to determine what was causing the stings, and thus what action can be taken.

Dr Walker stated that the blood fluke Austrobilharzia terrigalensis was the most common cause of the 'itch' in coastal waters in NSW, where the water temperature exceeded 24 degrees. The itch, variously referred to as Swimmers Itch, Pelican Itch, Terrigal Itch and Narrabeen Itch is caused by the parasite penetrating the skin and then dying. Narrabeen Itch can be contracted when wading or bathing in fresh or salt water in areas infested by parasitic schistosome flat worms.

The life-cycle of the flatworm is complex and involves two hosts - one a vertebrate, usually a bird such as a seagull and the other an invertebrate. At Narrabeen Lagoon, the invertebrate host is the water snail Batillaria australis.  Austrobilharzia terrigalensis was first discovered living in the blood vessels of a seagull from Terrigal. However, the organism is found more widely around the coast of Australia, wherever the seagulls and the intermediate host snails occur together on the shore. Some studies show about 75% of birds tested in the Narrabeen Lagoon are infected. Batillaria occurs in Narrabeen Lagoon in millions.

When the water temperature reaches at least 24 degrees, the parasite leaves the snail and moves to the surface of the water seeking contact with a bird - it is here that it sometimes accidentally encounters a human, buries into the skin but cannot reach blood and then dies. The life-cycle of the parasite is broken at this point. The immune response to the dead larva is the cause of the rash that develops some time later. It usually only becomes itchy if scratched or rubbed by clothing. If not scratched and infected, the lesions normally recede and disappear after two to three weeks. 

Dr Walker spoke about half a dozen other sting and itch causing animals likely in the Lagoon and stated that more information is needed from people who are stung so that the cause can be identified.

If stung, please present yourself to a Doctor or Chemist so samples of the itch causing organism may be identified, or the rash photographed. Knowledge of the cause assists in effective management. The professional advice was that opening the Lagoon more frequently would not assist the situation.

People should refrain from feeding seagulls, as this practice encourages them close to places where people use the lagoon for recreation.