Larva
- Slovak term: Slovenský termín
- Worm-like stage of development of insects and helminths, which can move and seek food (WHO 1992).
Explanation
Invertebrate animals are naturally present in many water resources used as sources for the supply of drinking-water. Small numbers of adults or their larvae may pass through water-treatment works if the barriers to particulate matter are not completely effective. Many of these animals can survive and even reproduce within the supply network by deriving their food from the microorganisms and organic matter in the water or, more commonly, present in deposits on pipe and tank surfaces (WHO 1992).
The infective stages of many helminths, such as parasitic roundworms and flatworms, can be transmitted to humans through drinking-water. As a single mature larva or fertilized egg can cause infection, these should be absent from drinking-water.
Only animals that are aquatic for the whole of their life-cycle can colonise the distribution system and form breeding populations there. This excludes most insect larvae. Nevertheless, larvae of many species of chironomid may be present in the distribution system in appreciable numbers. Some pesticides are used for public health purposes, including the addition to water to control the aquatic larval stages of insects of public health significance (WHO 1992).
Example
“The life cycle of fleas has four stages: egg, larva, pupa and adult. Adult fleas are 1-4 mm long and have a flat narrow body. They are wingless with well developed legs adapted for jumping. They vary in colour from light to dark brown. The larvae are 4-10 mm long and white; they have no legs but are very mobile.” (WHO 1992)
