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Disinfectants and Disinfection Methods

             Disinfection is an integral part of healthy and successful silkworm rearing. It aims at the total destruction of disease causing pathogens. Several diseases caused by bacteria, viruses, fungi and protozoa affect the silkworms. These pathogens released by diseased silkworms easily accumulate and spread in the rearing environment through different routes. They are  not easily destroyed and can persist / survive for long periods under congenial conditions. The spores of the pathogens, especially those of fungi are light and can easily be drifted by air current resulting in easy spread of diseases. There are no curative methods for any of the silkworm diseases and they are best prevented than cured. This is achieved by adoption of proper and effective methods of disinfection and stepwise maintenance of hygiene during rearing. To realize the benefit of disinfection (mass) and rearing at village or block level considering them as one unit.

Chemical Disinfectants available for use in Sericulture

Formalin

            It is commercially available as 36% formaldehyde in solution form. A mixture of 2 % formalin + 0.05 % detergent is an effective solution that can be used for disinfection purpose as spray. Formalin is effective only in rearing houses, which can be made airtight and it is faster and more pronounced at temperature above 25 0 C and humidity more than 70 %.

Bleaching powder

            It is white amorphous powder, with a pungent smell of chlorine. For effective disinfection, a high-grade bleaching powder with an active chlorine content of 30 % must be used. It should be stored in sealed bags, away from moisture, failing which it will be rendered ineffective. The action of bleaching powder is optimal under wet and contact conditions and therefore the surfaces of equipment and walls should be drenched with this solution. A 2% bleaching powder in 0.3 % slaked lime solution is used for disinfection as spray.

Slaked lime

            A very useful bed disinfectant in sericulture. especially against viruses. It absorbs moisture and can be used to regulate bed humidity and maintain hygiene. Application of lime dust in combination with bleaching powder in and around rearing houses and premises improves hygiene in the environment.

Chlorine dioxide

            Chlorine dioxide marketed as sanitech is an ideal disinfectant available at 20,000 ppm concentration is a strong oxidizing agent, effective at broader pH  range and at 2.5 % concentration in combination with 0.5 % slaked lime is effective against all silkworm pathogens. It is stable and may be activated at the time of its use. it possesses tolerable odour and is least corrosive at the suggested concentration.

Material required for disinfection

            Disinfectants, detergent, sprayer – Rocking or Power sprayer, buckets, measuring jar, weighing scales, gas masks, metal pans, room heaters, slaked lime powder, hand gloves and muslin cloth.

Technology for Prevention / Control of Diseases in Silkworm

            The technology for prevention / control of diseases in Silkworm rearing is as sequential steps.

Cleaning of rearing house and appliances

(after completion of silkworm rearing and cocoon disposal)

            Immediately after the disposal of cocoons, collect at one place inside the rearing house, all diseased and dead larvae, pupae, floss, left over mulberry, bed refuse, silkworm faeces, dust, dirt, etc., and disinfect by sprinkling 5 % bleaching powder solution and dispose off by burying at 2 ft.depth or burning.

Disinfection of rearing house and appliances

(with appliances inside the rearing house)

            Measure the length and breadth of the rearing house including leaf preservation room, mounting room/place, etc., and calculate the floor area for disinfection

Estimation of the quantity of disinfectant required

            The disinfectant required for disinfection of rearing house is 2lt/sq.m floor area or 185 ml/sq.ft. floor area.

            To estimate the quantity of disinfectant solution required, multiply the floor area in square meter by 2.0 or the floor area in sq.ft. by 0.185 which gives the actual disinfectant required in litres for disinfection of inside of the rearing house.

Disinfection

            Disinfect the rearing house using 2 % bleaching powder in 0.3 % slaked lime, 2.5 % Chlorine dioxide in 0.5 % slaked lime or 2 % formalin + 0.05 % detergent solution.

Disinfection of rearing house, appliances, etc., inside the rearing house

            Spray using powerful jet sprayer, the required quantity of disinfectant
(@2.0 lt/m2 floor area of rearing house +25 % of disinfectant solution for appliances + 10 % for outside of rearing house) uniformly to drench all parts of rearing house inside appliances and outside. Keep the rearing house closed for a minimum period of 6 to 10 hours.  After minimum period of 18-24 hours, shift the rearing appliances out of rearing house and sundry for 10 – 12 hours.

             Silkworm rearing using the mulberry shoot do not require the rearing trays but uses the platform. Rearing trays are the main source of infectious agent and most difficult and expensive to achieve disinfection. To avoid this, changing over to rearing silkworm on shoot is advantageous. This method does not require additional disinfectant for appliances as only the rack and nylon nets are used.

Disinfection prior to brushing

            Disinfection process should start 4-5 days prior to brushing. However, the eggs may be incubated in separate disinfected incubation room.

Five days before brushing

            The rearing house and appliances are cleaned, washed in water. if trays and other appliances namely, basins, Vinyl sheet,leaf basket, etc., are used, conduct additional disinfection by dipping them in disinfectant or spraying disinfectant.

Dipping in disinfectant

            Disinfect the rearing appliances that could be disinfected with 2 % bleaching powder in 0.3 % slaked lime solution by dipping them for 10 minutes in the solution in a disinfection tank. A tank of 2 feet depth and 4 feet diameter is suitable for disinfection. Prepare the disinfectant solution to fill half of the height of the tank. To determine the quantity of disinfectant solution to be prepared in the tank, calculate the volume of the tank using the formula 22/7 * r2 h (where r is the radius and h is the height of the tank)

Disinfection by spraying

            Disinfection of appliances by dipping in disinfectant is the best method. However, where the facility of disinfection tank is not available, the following method is advised.

            Disinfect the appliances using 2 % formalin + 0.05 % detergent solution or 2.5 % of Sanitech in 0.5 % slaked lime or with 2 % bleaching powder in 0.3 % slaked lime solution. Spray disinfectant @ 35 ml/sq.ft. surface area or 700 ml for a tray of 3.6 feet diameter. After the spray, put them together and keep completely covered with vinyl sheet for a minimum period of 6 hours. Disinfect the mountages also, following the above method.

Four days before brushing

            Sun dry the appliances. If the prevalence of viral diseases (Grasserie and flacherie) were high during the previous crop, spray 0.3 % slaked lime in water
(3 g/lt) to the rearing house and appliances @ 2 lt/sq.m floor area + additional requirements. Sun dry the appliances after 1-2 hour of spray.

Three days before brushing

            Conduct second disinfection of rearing house and appliances. Shift all the disinfected appliances into the disinfected rearing house and arrange in the room. Disinfect the rearing house and appliances by spraying 2 % bleaching powder in 0.3 % slaked lime solution or 2.5 % Sanitech in 0.5 % slaked lime or 2 % formalin + 0.05 % detergent solution. The quantity of disinfectant solution required is calculated and sprayed as per the first disinfection. After disinfectant spray, keep the room closed preferably for 24 hours. The second disinfection may follow the first if the gap between the two is not much.

Two days before brushing

            Dust 5 % bleaching powder in slaked lime powder @ 200 g/sq.m at the entrance of the rearing house and the passage to it. Sprinkle water @ 1 lt/sq.m floor area. Open the windows of the rearing house and ventilate to drive off all the odour of disinfectant. Prevent contamination of rearing house and appliances.

One day before brushing

            Arrange appliances for chawkie rearing and maintenance of hygiene. Keep the rearing room and mounting hall closed and open only one day before the larvae are to be shifted.