Prevention and Control of Diplotrematosis in Platyrhynchus platyrhynchus

Flat snout is a national first-class protected animal. In recent years, as the protection of this species has been paid more and more attention, its artificial reproduction has been included in the agenda of aquatic wildlife protection in * * Autonomous region. In 2004, in the course of the project "artificial reproduction of flat snout", * * Institute of Fisheries Science found that some of the collected and bred flatfish developed symptoms such as blindness and eyeball cataract, but had little effect on drug treatment, which affected the artificial breeding of flatfish. In order to find out the cause, the author made a definite diagnosis of double-hole trematodes by investigating the collection and cultivation of parent fish, sampling, examination and analysis, and took control measures, and gained some curative effect and experience in prevention and treatment. In this paper, the characteristics and control methods of double-hole trematodes in flat snout are introduced as follows:

I. morphological characteristics of pathogens

Double-hole trematodes belong to trematodes, double-hole families and double-hole trematodes, and their morphological types are mainly divided into miracidia, metacercariae, cercariae, metacercariae and adults. When parasitic fish body, its developmental morphology is metacercaria. Double-hole clonorchiasis is also known as cataract disease and clonorchiasis.

The shape of metacercaria is divided into anterior and posterior parts, transparent, flat and oval, with an oral sucker at the front, a lateral organ on each side, an oral sucker in the middle, the lower pharynx and the intestinal branch extending to the posterior end of the body. There is a ventral suction disk similar in size to the oral sucker in the posterior half of the body, an oval adhesive below, and a diamond-shaped excretory sac at the back end, which separates the excretory tube from the front of the sac, and many granular and shiny limebodies are distributed in the body.

When the cercariae are still in the water, they show a "T" shape and swim up and down continuously in the upper layer of the water, which has obvious phototaxis. The body is divided into body and tail, and the body is covered with fine spines. There is a head organ at the front of the body, a muscular pharynx below, and an intestinal tube divided into two forks; an abdominal sucker in the middle of the body, followed by two pairs of drill cells, and a tube leading to the head organ; there is an excretory sac at the end of the body. The tail is composed of tail trunk and tail fork, the tail trunk is well developed, there are 9 pairs of flame cells, and many caudate bodies and several tail hairs are distributed in the tail trunk.

II. Life history of pathogens

The two-hole trematode adult parasites in the gut of the gull bird, and the eggs fall into the water with the excrement of the gull bird. After 3 weeks, the miracidia are hatched (when the water temperature changes from 25 ℃ to 35 ℃, the incubation period becomes shorter with the increase of the water temperature). The cercariae swim in the water, select the radish snail and the vertebral snail as the first intermediate host, drill into the liver and the outer wall of the intestine to develop into cercariae, and the cercariae produce cercaria. The cercariae moved to the outer cavity of the snail, quickly escaped into the water body, and sank and floated in the water body, showing a regular intermittent movement. The fish is its second intermediate host. When the fish passes through the water layer of the cercaria, it is quickly stung by the cercariae. The cercariae remove the tail and drill into the muscles of the fish body, then drill into the nearby blood vessels from the muscles, and finally move to the heart, ascend to the head, enter the eyeball from the optic vessels, and develop into metacercaria in the crystal after a month. After the gull bird ate the fish with double-burrow trematodes, the metacercariae developed into adults in its intestines.

III. Symptoms and pathological changes of fish disease

The infection of double-hole clonorchiasis can be divided into acute and chronic. Acute infection has a large number of cercariae drilled into the fish body in a short period of time, due to cercariae invading the eyeball from the blood vessels, resulting in mechanical damage caused by the special expansion of the optic vessels in the process of migration, resulting in the destruction of the optic vessels, resulting in congestion in the brain and eyeballs of the fish, and then swimming slowly in the water, lying on the water, sometimes out of control, head down, tail up, turning back and forth in the water, and finally swimming slowly and dying. Chronic infection shows that the number of cercariae invaded each time is small, there are no acute symptoms, and the diseased fish do not die, and with the growth of the fish, pathogens accumulate in the eyes, causing eyeball lens opacity, local or complete whitening of the eyes, showing "cataract" symptoms, some diseased fish orbital congestion, and even lens shedding. In addition, cercariae drilling into the Gill can block Gill vessels; when cercariae migrate in blood vessels and heart, it can cause blood circulation disturbance; cercariae invade the nervous system, which can cause fish body bending, and it is more common in fish fry acute infection.

IV. Epidemic situation

Double-hole clonorchiasis is generally from May to August in spring and summer, and it is more prevalent in central China, where there are more gulls and conical snails. Silver carp, bighead carp, blunt snout bream, common carp and crucian carp can all be parasitized. Fish species die quickly during acute infection, with high mortality; chronic infection can cause "cataract" symptoms, which occur all year round, which is a serious fish disease. The disease is the first case of flat snout fish infection.

5. Diagnosis

A preliminary diagnosis can be made according to the symptoms of white turbidity in the eyes of diseased fish. When further examination, first dig out the fisheye, cut it, then take out the crystal and put it in normal saline or clear water, scrape down the surface layer of the crystal, and observe it with a microscope. If you find that there are double-hole trematodes, you can make a diagnosis of the disease. In the case of acute infection of fish fry and fry, the eyes generally do not turn white, and the number of parasites in the eyes is small, and it is necessary to take whether the head of the diseased fish is congested, whether the fish body is bent, whether the fish body activity is manic or swimming, and whether there are cone snails in the pond as a diagnostic reference.

VI. Prevention and control methods

Because the pathogen invasion, transfer pathway and parasitic site of double-hole clonorchiasis are special, so it is not easy to treat. At present, the main measures to prevent and control diseases are to cut off the life history of pathogens, such as driving out gulls, eliminating eggs, cercariae and intermediate hosts, supplemented by drug treatment. The main methods adopted are:

1. Remove too much silt from the pond and thoroughly disinfect it with quicklime or bleach. The dosage of quicklime is 100kg/ mu ~ 150kg/ mu when the water depth is 1.0m, or tea seed cake is used to clean the pond with water, and the dosage is 50kg/ mu when the water depth is 1.0m.

two。 The diseased pond can be sprinkled with 0.7mg/L copper sulfate throughout the pond and reused twice within 24 hours to kill intermediate hosts.

3. During the epidemic season of fish disease, crystal trichlorfon was sprinkled in the whole pond so that the concentration of medicine in the pond water was 0.5mg/L, which could kill cercariae.

4. The dipterex bait was fed with 15g ~ 20g trichlorfon per ten thousand fish feed, once a day for 5 days.