A parasite may be defined as any organism living in or on another species where
it obtains it's nourishment (plant or animal) and perhaps protection.
Disease is any departure from health; for our purposes as related to infection
or infestation by other organisms, i.e. parasites. Limited reference will be made to
infectious diseases "caused" by viruses, bacteria, fungi, algae and parasitic
diseases caused by protozoa, helminths, arthropods and other metazoa.
Types of controls may be divided into biological, physical, and chemicals
Physical/Mechanical controls- includes such things as increasing flow rate,
filtration, sonic vibrations, photoperiod and strength, pH, UV light sterilization,
surgery (brood fish) and temperature manipulation. There are often differences
between ideal temperatures and ranges for hosts and parasites, such that by
raising or lowering one may be selected for. Anthony (1969) observed that
raising temperature from 12 to 20 C. resulted in the disappearance of
Gyrodactylus elegans on the goldfish, Carassium auratus. According to Lom
(1969), the protective capacity of the mucus of carp infected with trichodinids is
manifested only at elevated temperatures.
The mechanical prevention and control of pathogens is often overlooked, but is
long established and has proven to be effective under many different
circumstances. Obvious examples include the use of flea/knit combs to reduce
the burden of fleas and head lice, mosquito nets to prevent malaria and barrier
contraceptives such as condoms to avoid the transmission of many sexually
transmitted diseases. Less well known examples also exist to control parasitic
infections found in aquaculture. Control of infections by species of Argulus
Müller, 1785 in carp culture has been achieved through the use of removable
substrates upon which the parasite lays its eggs. These substrates are removed
and cleaned before the parasite can hatch, thus reducing recruitment to the
system ([Bauer, 1970] and [Hoffman, 1977]). Control of species of eye fluke,
Diplostomum von Nordmann, 1832 and Tylodelphys Diesing, 1850 by
mechanical filtration and electrical grids has also been described by Larsen et al.
(2005) and Schäperclaus (1992) respectively. Another example is through the
dislodgement and removal of mobile stages of sea lice, Lepeophtheirus salmonis
(Krøyer, 1837) and Caligus elongatus von Nordmann, 1832, from salmon cages
through the use of pump systems (Anon, 1996).
Ichthyophthirius multifiliis (Fouquet, 1876) is the greatest cause of disease
attributed mortality in the UK trout industry, accounting for an estimated 2–5%
loss in annual production (c.360–900 tons), amounting to £2 million in lost
revenue (British Trout Association, personal communication). This ciliate
protozoan is highly pathogenic and can cause the disease “white spot” in both
wild and cultured freshwater fish (Matthews, 2005). The parasite has a
temperature dependant direct life-cycle. An infective theront stage penetrates the
epidermis of the fish before turning into the feeding trophont stage. The parasite
exits the host as a tomont which encysts on a suitable substrate within the
environment prior to undergoing binary fission and the release of the next
generation of theronts.
In order to prevent stress and damage to the host fish, the most obvious points in
the life-cycle of I. multifiliis to target by mechanical means are the free-living
stages. Heinecke and Buchmann (2009), in laboratory trials, used a combination
of water filtration and chemotherapy to control the free-living stages of I.
multifiliis. Tomonts were removed from the water column using 80 μm mesh and
then a 60 min treatment of 16–32 mg/l sodium percarbonate (11–22 °C) was
used to kill all the theronts. The mesh would not, however, prevent theronts
which measure 28.6–57.4 μm (length) × 20.0–28.6 μm (wide) (Aihua and
Buchmann, 2001) from entering systems. As most current, licensed
Concrete raceways are a commonly used system of rearing trout in the UK. Fish
farmers often brush the bottom of raceways on a regular basis in an attempt to
remove I. multifiliis cysts and settled waste from the system. However, due to the
porous nature and rugose finish of concrete, it is likely that numerous surface
pockets exist in which the tomonts can encyst and develop, thus casting doubt
on the effectiveness of brushing raceways to reduce the burden of the parasite.
This paper aims to assess the potential of controlling I. multifiliis infections using
engineering solutions. Specifically this study evaluates whether a mechanical
control system could significantly reduce the abundance of the parasite and
mortality attributed to it in raceway systems, by reducing parasite encystment,
and using a suction device to remove the parasite from the system.
The primary mechanical device consisted of a special suction head connected to
a pump that was used to vacuum the bottom of raceways to remove parasite
stages (Fig. 1). The device was submerged and then pushed, via a rod, towards
the water inlet and then dragged back to the outlet screen. A wiper the width of
the raceway trailed the raceway bottom behind the suction head. Any material
that was not drawn up the suction head was drawn to the outlet screen where it
was either drawn away by the water current or removed as part of normal
husbandry. In addition to the suction head, low-adhesion polymer sheeting was
used to line three test concrete raceways (6 m long × 1 m wide × 1 m deep) in a
commercial hatchery. The sheets of polymer were cut to size, the joints welded
together to give a watertight seal and then bolted to the concrete above the water
line using stainless steel fittings.
(A) The mechanical device used for the removal of I. multifiliis cysts from
commercial-scale rainbow trout raceways. The device operates over a smooth
coating applied to the internal face of the raceways. The arrow indicates the
direction of the device being pulled towards the operator. a = vacuum head is
pushed or drawn along the smooth raceway base by a pole attached to the top of
one of the wiper blades (attachment point not shown); h = hose connection to the
pump and then to waste; m = water, cysts and waste from the bottom of the
raceway are sucked up through an interchangeable mesh and through the hose
connection; w = wipers positioned at either end ensure that cysts are dislodged;
situated at either end of the vacuum head enables them to be engaged in either
a pulling or pushing action. (B) Polymer sheets are cut to size to fit the raceway.
The sheets are fixed to the walls of the raceway with stainless steel fittings above
the water line. The joints of the polymer sheets are welded to form a 100%
watertight seal. (C) Use of the mechanical device in the raceways. The device is
pushed towards the water inlet and then drawn back towards the outlet. I.
multifiliis cysts are sucked up and pumped away to waste. c = raceway with I.
multifiliis cysts on the base; d = mechanical device; e = empty raceway.
SUBMITTED TO PROF.DR. ASHRAF SB
SUBMITTED BY M SHAKIR
FISHERIES & AQUACULTURE
UVAS RACVI CAMPUS PATTOKI