Situación Chilena Caligus

Caligus rogercresseyi: current situation in Chile.

1. Caligus biology:

The Caligidosis is a disease caused by Caligus rogercresseyi, commonly called sea lice, which corresponds to an ectoparasitic copepod that inhabits seawaters of Chile. It parasite farmed salmon causing economic losses mainly explained by the mortality of fishes (SRS and ISA vector), increased susceptibility to secondary infections, the treatment costs, lengthening the growing cycle product of decreased feed conversion efficiency and the cleanliness of the carcasses during processing.

Of salmon species reared in net pens in Chile the most susceptible host are the Atlantic salmon and Rainbow trout, where in severe outbreaks the level of infestation can reach over 200 parasites per fish. In contrast, coho salmon has been shown to be resistant to this parasite. The lice are unable to reach the adult stage of coho salmon.

The life cycle of Caligus rogercresseyi is comprised of eight developmental stages: two planktonic nauplius stages, one infective copepodid stage, four attached chalimus stages and one adult stage, without differences in size between males and females. In contrast to the Lepeophtherius species (North hemisphere lice), preadult stages are not developed in Caligus species.

Figure 1. Caligus rogercresseyi life cycle

The length of time it takes a sea louse to develop greatly depends on the temperature in the water. Under laboratory conditions, the life cycle of C. rogergresseyi is completed in about 45 days when the temperature is 10° C, 32 days at 12° C and 26 days at 15° C. The lowest temperatures in seawater in Los Lagos and Aysén are about 8° C and the reach 15° C in summer.

2. Control of Caligus rogercresseyi:

In Chile, the main way of sea lice control is through treatment. The main problem in the control of the parasite is its short cycle of life, characteristic that provides a high speed of the acquisition of resistance to several drugs used to control this disease. As if that were not enough, in Chile, aquaculture can handle about 5 drugs approved by SAG (Agricultural and Livestock Service, the organization that gives permission to the use of drugs in animal production in Chile). The latter, combined with the poor performance of treatments have allowed a relative resistance to different drugs, some use today.

There are two way of administration: orally and through immersion.

Figure 2. Ways of treatment of Caligus rogercresseyi in Chile.

Administration Chemical

Oral Emamectin benzoate

Diflubenzuron

Immersion Deltamethrin

Cypermethrin

Azametiphos

Hydrogen peroxide

a) Oral treatment:

Emamectine benzoate is a drug used orally and handled only in specific moments. This drug is used at the beginning of the production cycle due to its withdrawal period (1800 UTA), this means, when the farms are near harvest, the only option are drugs with little time in muscle and tissues. Emamectine its an avermectin produce by Streptomyces avermitilis. This drug blocks the mechanism of neurotransmission, setting the gamma amino butyric acid (GABA), causing immobility of the parasite. Besides this, it has been demonstrated resistance of the ectoparasite to emamectin benzoate, one of the drugs most widely used in Chilean aquaculture against caligus.

Diblubenzuron is a benzoylurea-type insecticide of the benzamide class. The mechanism of action of diflubenzuron involves inhibiting the production of chitin which is used by an insect to build its exoskeleton, preventing molting. It’s an expensive drug and it has been shown that this drug does not leave residue, so its effect is only when fed. These features make companies to avoid using it.

b) Immersion treatment:

Deltamethrin and cypermethrin are pyretroids, therefore have an effect on the central and peripheric nervous system of the parasites. Pyrethroids stimulate constantly nerve cells, through the sodium channels, resulting in a paralysis. These products are administered through immersion (bath), using tarpaulin or wellboat. Once the fish are found within the immersion, the drug is administrated and maintaining immersion for 30 to 40 minutes to deltamethrin and cypermethrin, respectably. It should not be varied bath time, because the Service’s audits are based on the drug manufacturer's recommendations.

Both have a dose of 0.3 ml/m3 and should be used at depth of 4 meters. This is equivalent to a practical dose of 1080 ml / cage, for cages of 30x30 meters2.

Azametiphos is a recently approved drug for immersion treatment. It is an organophosphate insecticide that acts by inhibition of cholinesterase activity in the node. This is responsible for the breakdown of acetylcholine, not destroyed, there is over activity, ending with the death of the parasite (paralysis).

The dose is 0.2 g/m3 and should be used at depth of 4 meters. This is equivalent to a practical dose of 720 g/cage, for cages of 30x30 meters2. The immersion must be for 30 minutes.

Hydrogen peroxide needs SAG’s approval. There are permissions of Sernapesca, for handle the peroxide as a drug against the parasite, and of the Chilean Navy for transporting it, but no one of them with a register product. There are companies working in the register of this product as a drug.

There is no concern in the effectiveness of this drug. The have been proved there is only detachment of the lice, produced by a momentary mechanical paralysis, which does no kill the parasite. Some researchs says there

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