Blood worms could be found in swimming pools complex. Blood worms are larvae of an insect (midge) belonging to the family of Chironomidae. After mating, the female midge lays eggs on the water surface. Under favourable conditions, the eggs hatch in about two to three days. The larvae live about four weeks and pupation follows. Pupae usually last about two days before adults emerge. Most of the midge larvae are aquatic. They are red in colour because of the presence of haemoglobin in their blood, although a few species are green or yellow. Polluted water favours their multiplication and emergence. Most of the larvae are detritus feeders of tiny particles of organic matters and some algae, etc. The larvae are likely to be found where water is polluted by organic matters.

Adult midges do not need to be fed and do not bite. However, their breeding may cause nuisance to humans. To safeguard public health, the management of swimming pools can follow the guidelines on the prevention of the breeding of blood worms in swimming pools.

Personal Hygiene of Staff

• Staff should maintain good personal hygiene, including going through the shower and the foot bath before entering the pool.
• Upon entering the pool area, staff should only wear footwear or overshoes permitted within the pool area.

Environmental Hygiene of Swimming Pools

• The whole area and all the facilities of the swimming pool (including walls, floors, equipment, tables and chairs, stepways, handrails, diving boards, chutes, changing rooms, showers, foot baths, lockers and latrine fitment) should be kept clean. Regular cleansing and disinfection should be carried out at least once a day. Use diluted household bleach (e.g. adding 1 part of household bleach to 99 parts of water) when carrying out cleansing and disinfection, and rinse with water and mop dry.
• Inspection should be carried out once a week to the whole area and all the facilities of the swimming pool. Particular attention should be paid to the drains, planted green areas, trees (especially cracks and holes in the dead parts), and clefts between walls and floors. Defective ground surfaces as well as cracks and holes in trees, if any, should be filled up to prevent accumulation of stagnant water. Defective play equipment and installations in swimming pool has to be repaired promptly. Refuse that can hold water, such as lunch boxes and soft drink cans, should be immediately picked up and disposed of properly.

Supervision of Swimmers

• A notice in Chinese and English stipulating the requirements on personal hygiene for compliance by swimmers and staff should be conspicuously displayed at the entrance of the pool.
• Swimmers should go through the shower and the foot bath before entering the pool.
• Persons not wearing swimming suits should be prohibited from swimming in the swimming pool.
• Persons carrying articles that may contaminate pool water such as foods and drinks should be prohibited from entering the pool area.

Check for Midge Breeding

Since midges can breed in still and slow running water, inspection should be made to the swimming pool and its surrounding areas at least once a week to ensure that there is no breeding of midges. Attention should be paid to the following during inspection:

(i) Nullahs
(ii) Surface channels at the side of the pool for collecting overflowed pool water
(iii) Sand traps/Manholes
(iv) Cable trenches, cable draw pits and water tap stands near the pool
(v) Undersides of artificial turfs, especially the parts that are often wet or damaged;
(vi) Undersides of damaged floor tiles near the pool
(vii) Leisure facilities in the pool like water slides (particular attention should be paid to those parts that are often wet and under the shade)
(viii) Refuse collection points near the pool
(ix) Keyholes of manhole covers
(x) Discarded objects which may hold water
(xi) Places where midges have been found

Control of Midge Larvae and Pupae

Since larvae of midges live under water, they cannot be killed by malarial oil. Larvicides such as B.t.i. can be used to eliminate larvae in stagnant water that cannot be removed immediately.

Prevention and Control of Adult Midges

As adult midges have a lifespan of two to three days only, use of insecticidal aerosol is not recommended. However, if their presence becomes a nuisance to the public, adult midges hiding in trees, bushes, shaded manholes and indoor environments can be knocked down by insecticidal aerosol or light traps (insect killers).

Adult midges are attracted by light sources and become very active during sunset. Their nuisance can be abated by avoiding the use of unnecessary lights.

In case blood worms are found in your swimming pool, please deal with them immediately.