Organophosphates (OPs) as fish disease treatments

Organophosphates are used to treat monogenean flukes, Gyrodactylus (skin fluke) and Dactylogyrus (gill fluke), as well as leeches and crustacean ectoparasites Argulus (fish lice) and Lernaea (anchor worm). Organophosphates, work by interfering with the nervous system and thus affect vital physiological processes. OPs are potentially dangerous to both fish and humans and, for a variety of reasons, their use in fish disease control has been banned in many countries. There are currently safer alternatives being developed, but these are liable to be very expensive for hobbyists.

The two most commonly used OPs are dichlorvos (DDVP) and trichlorfon (marketed as masoten, dipterex, and neguvon). The concentrations of active ingredients vary in the different trichlorfon preparations. When trichlorfon is added to water it degrades to DDVP, which then slowly degrades to less toxic by-products.

Using OPs

A major consideration in deciding the necessary dosage is the rate of breakdown of OPs from DDVP to less toxic by products. The quicker it breaks down, the higher the initial dose needs to be. The rate at which it degrades depends on various factors: light, high temperatures, aeration and high pH all speed up degradation. In mild temperatures and moderate pH the half-life can be several days, which means that it is active for longer. In alkaline conditions and high temperatures the half-life can be as short as a day.

Usually 2 –3 doses a week apart (longer at cooler temperatures) are needed in order to deal with the emerging larvae and juveniles. The recommended rates for trichlorfon are:

• 0.25 mg trichlorfon per litre in temperatures below 27^oC
• 0.50 mg trichlorfon per litre in temperatures above 27^oC.

A popular organophosphate used by koi keepers in the in the UK is masoten (Bayer). The recommended doses for this product are (Peter Waddington):

• 1 gram to 330 gallons (UK) [0.7 mg/ litre] at temperatures above 13^oC
• 1 gram to 600 gallons (UK) [0.4 mg/ litre] at temperatures below 13^oC

This product also comes in liquid form containing 20% active trichlorfon and is used at the rate of:

• 1ml to 88 gallons (UK)[1ml per 499 litres] at temperatures above 13^oC
• 1 ml to 170 gallons (UK) [I ml per 770 litres] at temperatures below 13^oC

There is no doubt that OPs are extremely effective against parasites such as Argulus and Lernaea; and in almost all cases they will quickly clear a pond of these nasties. In addition to experiments to find alternatives or ‘safer’ organophosphate products there have been some experiments using chitin-inhibitors such as dimilin against these crustacean parasites. Because crustaceans larva, such as Argulus and Lernaea, undergo several molts before reaching maturity these products can interfere with the formation of new exoskeletons (an outer skeleton made mainly of a hard substance called chitin). There are mixed results with these products.

More about organophosphates

	Do not use with Orfe, Rudd or Tench
	Dilute the required dose in 10 litres of water and add slowly to the pond to avoid ‘hot spots’
	Repeated doses can lead to a build-up in the fish’s tissues and could eventually prove fatal or injurious
	Handle with extreme care as these are neurotoxins and possibly tetatrogenic
	Masoten must be stored in a cool, dry place in an airtight container. Masoten readily absorbs moisture and becomes toxic to fish. Do not use if it becomes damp or lumpy (toxic)