Ichthyophthiriasis (White spot disease)

 

Definition:

It is one of the most prevalent disease affecting  mostly cultured and aquraium fishes caused by Ichthyophthirius multifiliis, characterized by presence of white spots allover the external body surface (skin ,fins, gills and body surface).

Ichthyophthiriasis comes in front of all parasitic diseases, as it is widespread in all types of intensive culture. Also, outbreaks of this disease lead to mass mortality and reduce the growth rate of fish.

In Egypt disease was recorded in numerous fish species such as tilapia carp and ornamental gold fish.

 

Etiology of Ichthyophthiriasis:

Disease caused by Ichthyophthirius multifilliis. It is round to oval in shape. The body surface covered with cilia. The full grown parasitic form grows to 0.5-1.5 mm, which is quite large for unicellular organisms. Both macronucleus and micronucleus are present. During the parasitic stage, the micronucleus, which is responsible for the generative processes lies on the concave side of the macronucleus, which is horseshoes-shaped and controls vegetative functions  The parasite needs 3 day to 5 weeks to complete its life cycle depends on water temperature. The parasite have three phase to complete its life cycle:

1.     Adult phase, it is embedded in the skin or gills of the fish, causing irritation  and the appearance of small white nodules . As the parasites grows it feeds on red blood cells and skin cells. After a few days, it bores itself out of the fish and falls to the bottom of the aquarium.

2.     Cyst phase: after falling  to the bottom ,the adult parasite forms into a cyst with rapid cell divisions occurring.

3.     Free swimming phase, after the cyst phase, about 1000 free-swimming  young swim upwards looking for a host. If a host is not found within 2 to 3 days, the parasite dies . Once a host is found the whole cycle begins anew.

 

Stress factors:

Stress can bring about an outbreak of disease in a fish population, these include:

1.     Overcrowding of fish.

2.     Low dissolved oxygen.

3.     Chemical pollutants in the water.

4.     High temperature, and spawning activities( 15- 25 C outbreak occur).

 

Susceptible species:

Ichthyophthirius.multifiliis appears to parasitize all fish. There are no records of species with natural resistance.

 

Mode of transmission:

1.     The disease is transmitted through direct and indirect contact with infected fish.

2.     Water act as vehicle for spreading the infection.

3.     The disease is easily introduced to fish culture by adding new infected fish or contaminated aquatic plants.

 

Clinical signs:

The disease is characterized by appearance of white spots on the skin, gills, fins and cornea of the eye. The white spots appeared as white specks on their skin as though they were sprinkled with salt.  The fish react to the infestation of parasites by irritation, flashing and rubbing on the bottom, swimming violently and holding the fins close to the body. More severe attacks result in skin detachments. When the gills become infected, they appeared pale in color and swollen, respiration become difficult and the fish aggregate at the water inlet and die.

 

Diagnosis:

1.     Clinical signs of the disease.

2.     Microscopical examination of tissue from gill arch, tail fin or body surface is necessary to make a definitive diagnosis of Ichthyophtheriasis. The causative agent is easily seen in unstained wet mounts, with a distinctive horseshoe- shaped nucleus that is a pathognomonic signs of Ichthyophtherius infection.

 

Treatment and Control:

When we approach to treat a case of white spot disease in fish, three main steps must be carried out, side by side, as the following:

1.     Proper management:

a.     decrease of stocking rate of fish in pond must be done.

b.     increase of water inflow to the pond to take theronts away.

c.      dead fish should be removed as soon as they are found, the immature theront die, but the mature trophonts begin to drop off as tomonts and encyst immediately.

d.     fish fry must be transferred from the spawning ponds to the nursery ponds (within one week), they will not be attach by the following stages of the parasites.

e.      introduction of new infested or carrier fish, plants, stones and snails obtained from other aquaria must be prevented and placed in quarantine for at least seven days at a water temperature from 24 to 28C. Thus, white spots will become visible on fish and theronts will emerge finding no host.

f.       wild fish should be prevented from entry through water inlets.

g.     the ponds should be drained and dried after an outbreak, then treated with quicklime for killing cysts.

2.     Physical control: This type of control is used to break the cycle of infection in different stages:

a.     Flushing method: It is used mainly in hatcheries or tank. The fish are held in containers with perforated bottom and sides for two to three weeks and maintained in running water. This process may lead to the disappearance of all stage of the parasite. As the parasites leave the infected fish are flushed out with water, this will interrupts their life cycles.

b.     Transferring method: It is used for ornamental fishes as the infected ones are transferred to parasite free aquaria every day or 12 hours. The tomonts fall off into the first container developing into theronts. The repeated transfer of fish to parasite free aquaria is continued for one week.

c.      Heat method: This method depends on mainly raising water temperature up to 30 to 32C. It should be raised slowly and not more than 1C hourly and maintained for 5 to 7 days then lowered slowly. It must be used only for tropical fishes especially in some ornamental and labyrinth fishes .By putting the infected fishes especially in some ornamental and labyrinth fishes. By putting the infected fishes in a tank or aquarium without aquatic plants with high temperature, the parasites encyst dividing by simple direct divisions or even die from lack of oxygen while labyrinth fish can stand this treatment is not the reason for killing the parasites but it acts only indirectly by the lack of water oxygen, which can result from it. In addition, the higher temperature the quicker the parasite will grow to mature size and leave the fish to the unfavorable surrounding water.

3.     Chemical method: The free-swimming phase is the best time to treat with chemical. There is no chemical available that kills the trophonts in the skin of the infected fish without killing the fish too. This is due to the fact that the layer of epidermis protects the trophonts and chemicals cannot penetrate directly to it. Thus, the treatment goes forward to all unprotected stages of the parasite in water. So, the following drugs can eliminate only theronts and tomonts immediately after separation from the fish prior to encyted. It is advisable from my field experiment, especially in aquarium fishes, to raise the water temperature during the drug therapy except in case of formalin.

a.     Sodium chloride, it has a dual purpose as its use kills the free-swimming tomonts and therontds. In addition, it compensates the decrease of serum sodium caused by the epithelial damage and helping the infected fish to maintain the immune defense. Infected fish must be tested for the sensitivity to NaCl, so euryhaline fishes can be used successfully. Basically, it should be only used in treatment of aquarium fishes by short baths at a level of 1.5 to 2.5% for 10 to 30 minutes/ 7 days.

b.     Potassium permanganate, it must be used in freshly prepared solutions. In ponds, the does is 2 ppm for scaleless fishes and 2 to 5 ppm for scald ones as continuous treatment. It must be remembered that organic matter reduces its potency. Ornamental fishes can be treated with 15 ppm for indefinite time.

c.      Formalin, firstly, the treated fish should be tested for their sensitivity to formalin especially in warm water. fishes in ponds are treated with 15 to 25 ppm while aquarium fishes are treated with 25 ppm on alternate days until the infection is cleared.

d.     Malachite green, ponds fishes are treated with 0.1ppm at three to four days intervals by spraying of entire pond area. The aquarium fishes are treated with same dose and time. Malachite green zinc-free oxalate is the best choice for treatment of Ich disease because of its ability penetrate skin. For infected aquarium fish combination of Malachite green, (0.05 ppm) with formalin (15 ppm) is applied as a prolonged  bath on three alternate days as both agents are synergistic.

e.      Acriflavin, in a neutral state or Acriflavin hydrochloride is used as a long bath at 10 ppm for 3 to 20 days.

f.       Acetic acid, it is used as a bath  (1%) solution in five minutes for aquarium fishes.

g.     Methylene blue, it is recommended at 2-3 ppm as a permanent bath in aquaria at a temperature between 21 and 26 C daily. In Egypt, it is considered as the most common and effective treatment for white spot disease in ornamental fishes. It should be added directly to the aquarium water .The water becomes deep blue coloured but it fades gradually as bacterial processes and reactions with substances present in water reduce the dye. It hinders the reproduction forcing the parasite to simple division.