First, the parasitic characteristics of ringworms
1.1 Host specificity
Most species of ringworm parasitize fish species have obvious specificity and have a strong selective effect on the host, for example, the small sheath ringworm is the main pathogen of silver ringworm disease in China, the pathogen of carp ringworm disease is bad gill ringworm, and polygynode (a type of ringworm) parasitizes to catfish and split-bellied fish.
1.2 Distribution of ringworms on gills
The body length of the ringworm is generally less than 2 mm, and when the worm parasitizes, it parasitizes the secondary gill filament of the fish, the coarse posterior sucker is deeply inserted into the secondary gill filament of the fish, the central hook extends into the respiratory epithelial cells or pierces into the capillaries, the edge hook hooks the gill filament tissue, protrudes into the respiratory epithelial cytoplasm and tears the cells, resulting in gill filament epithelial ulceration and microhemorrhage, gill fusion, hyperplasia and necrosis.
The parasitic distribution of ringworms on each gill is uneven. Usually: the first gill piece (the gill piece located at the very tip of the gill cavity) is the least, followed by the fourth gill piece, and the second and third gills are the most. This inequity may be related to the large water flow at the second and third gills, the density of gills, and the easy attachment of ringworm larvae.
On the same gill piece, in mild infection, ringworms are mostly distributed near the free end of the primary gill filament; at high infection, distributed in the middle of the gill filament, at the base and even on the gill arch, the insect body is often clustered together in clusters of three or five.
The posterior suckers of some adult ringworms can be encased in a position by the host tissue that grows at the parasite site, and for such trematodes, their spatial distribution on the gills may be mainly randomly determined by their larvae during infection.
1.3 The position of the adult ringworm on the gill piece moves
The positional movement ability of various ringworm adults on the gills varies greatly. Cantonese eel gill parasitizing ring worms, once the larvae parasitize, the posterior sucker can be wrapped by the host tissue that grows in the parasitic site, the worm body can not fall off from the fixed position of the gill pieces, even if death occurs, the central hook will remain on the gills. When observed with a microscope, the gills parasitic ringworms in the Gills of Yancheng District, Jiangsu Province, will find that the insects are active between the gills, can flexibly migrate between the gills, and can be resettled in new locations. After taking the gill tablet, do a ulnar worm movement on the slide to observe the insect body, its body is not broken, and the rear suction device is complete. Comparing the two kinds of ringworms, (1) the body size of the ringworm parasitic ringworm in the Gill gill part of Yancheng District, Jiangsu Province is significantly smaller than that of the individual parasitic ringworm of the eel in Guangdong, the latter has a body length of more than 8 times that of the former, and the latter's posterior suction device is significantly larger than the former, forming a large anchoring force; (2) and the latter's posterior suction device is wrapped by the host parenchymal tissue and is difficult to fall off.
2. Life history of ringworms
Life history of ringworms: fertilized eggs hatch one larvae, one parasitize one adult, one fertilized egg, with an average lifespan of 1 month. Ringworm hermaphrodites, oviparous, few, large, oval-shaped. Adults continue to lay and hatch during the warm season, and after the self-fertilized eggs are released from the worm body, the eggs float on the surface of the water or attach to other objects or host gills and skin. When the eggs are at a water temperature of 22-26 ° C, the larvae hatch in 3 days, and the larvae emerge from the eggs and fall into the water. Free-swimming ciliated larvae are the only infectious period in the life history of monosozoans outside the host body. Ciliate larvae have phototropism, ciliate larvae just drilled out of the egg shell to show very active, sometimes in a rapid straight line or simple arc movement, sometimes in a very complex curve movement; at water temperature of 22 ° C, ciliate larvae individuals survive for 1 to 75 hours after hatching, the ciliate larvae in the temperature after hatching for 24 hours basically lose the ability to swim; at water temperature 12 ° C, in addition to the survival time is greatly extended, ciliated larvae until 60 hours of incubation also have the ability to swim. However, as ciliated larvae swim longer in the water body, they consume more energy and die without finding a suitable host. Ciliated larvae use their heads to identify hosts as they search for suitable hosts. When a suitable host is encountered, it attaches to the parasite, and after the insect body attaches, the cilia are removed, and various organs are formed one after another. If the water temperature is higher than 25 ° C, it can mature and lay eggs in 7 to 9 days. In general, the development of the larvae, the posterior sucker is completed before the reproductive organs. It grows and develops into an adult, lays eggs, and begins the next round of life history. After the death of the host, the parasitic ringworm will die in a short period of time, and its survival time is generally not more than 24 hours, and the death of the ringworm on the dead host may be caused by the decay of the host tissue.
3. Examination of ringworm disease
The main feature of the ringworm is that there are four black eye spots on the back of the head, arranged in a square arrangement, and the mouth is located in a tube or funnel shape near the ventral eye point at the front end, which can be retracted, orally orally, with a short esophagus under the ingophagus, and a forked tube ring intestine under the esophagus, without an anus. A disc-shaped rear suction cup is visible at the rear end of the worm body, with a pair of large anchor hooks in the center of the disk, two supporting rods connecting the two hooks, and seven pairs of small hooks on the edge of the rear suction cup. Film observation: In addition to the above structures such as eye spots and posterior suction cups, it can also be seen that there is an ovary in the middle or posterior part of the body, and a sperm nest after the ovary, and the ovarian glands are developed and distributed on both sides of the body and around the intestinal tube. Take the gills suspected of ringworm infection, put it on the slide, add a drop of water, place it under the microscope for observation, the living ringworm is milky white, the insect body is finger-like flat and long, with the back suction cup fixed on the gills, the body is constantly stretching and moving, testing and looking for food, moving when making ulnar worm-like action, under the slide, the individual ringworm can stretch the body very thin and long, about 4 to 5 times the original length, which can be diagnosed. In the case of a small amount of ringworm infection, a few more gill pieces should be taken for observation, and the gills should be fully separated as much as possible to facilitate the observation of ringworms at the base of the gill filaments or ringworms with relatively small individuals. When the ringworm is examined for diseased and dead fish, there are no obvious abnormal changes in body color, mucus and nutritional status. The gills of the diseased fish are increased, all or part of the gills are pale, the gills are obviously swollen, anemic, and some are grayish-white, so the entire gills look gills- shaped, especially the first pair of gills next to the gill cover, and the gills are obviously gray-white. Due to the swelling of the gill filament, the gill cover is opened, and a large amount of mucus is secreted on the gill, and the gills and gills are adhered to each other, so that the gas exchange is affected. Some gill filaments have white lesions that are distributed in various parts of the gills, but are most distributed near the edge of the gill flap, and in severe cases, the entire gill flap can be covered. Scraping off the lesion a little and placing it under the microscope to observe, it was found that the gill tissue inside the lesion was destroyed and disintegrated by the insect body, the gill tissue was chaotic, and there were many ringworms gathering.
Fourth, ringworm hazards
The pathogenicity and damage of ringworms depend on the age of the host and the number and size of the parasites. Ringworms usually insert a hook on their posterior sucker (posterior fixer) into the parasitic site, causing lesions at the parasitic site, resulting in the following adverse consequences:
(1) After the ringworm fixer is attached to the gills, it is used as a ulnar oyster movement, destroying the epidermal cells of the gills, causing gill damage, bleeding, and increased mucus, stimulating the increased mucus secretion of gill epithelial cells and affecting respiration; a small number of parasitic ringworms can easily lead to poor fish feeding and solo swimming; a large number of parasitism leads to fish hunger strike, slow or guided fish growth
Causing the death of fish;
(2) Destroying the integrity of organs, in the process of parasitic host, shedding from the host and feeding, the epidermis is broken, which is easy to cause the invasion of pathogens (such as bacteria and viruses), causing inflammation and secondary diseases;
(3) Ringworm sucks fish blood and mucus, stimulates the host to produce a large amount of secretions, destroys normal physiological activities, causes electrolyte imbalance and pH imbalance in the main fluid, and causes a variety of complications:
(4) Parasitism can cause systemic hypoxia of the host, aggravate the appearance of extensive and progressive lesions in various organs, until the metabolism of each system is disturbed, causing multiple organ failure in fish.
5. Pathological damage of ringworms
Mild infection: mildly infected fish with abundant gill filament tissue is damaged, causing local mechanical damage to the gill filament, causing gill valve defects, bleeding, necrosis and tissue hyperplasia. Moderate infection: the gill filaments parasitic of the insect are pale in color, and local anemia occurs at the parasite. At the place where the large hook is pierced, a large number of white blood cells are infiltrated, some of the gill blood vessels are hyperemic, slight swelling occurs, the respiratory epithelium of adjacent two or more secondary gills undergoes cell infiltration, the epithelial cells slightly proliferate, and close to each other, until the cells fuse into a tight structure, and then gradually uplift from the fissure, and then form a cell plate composed of several layers of respiratory epithelium, which seriously affects the respiratory function of the gill filament, causing suffocation and death. Severe infection: in severe infection, damage to gill filaments is characterized by symptoms of both mild and moderate infection, but the scope of damage is much wider, that is, total gill. Observed by the naked eye, the gill filament mucus of the diseased fish increased significantly, and all of them were pale. The gills are visibly puffy , and the gill flap is covered with many white spots densely packed with insect bodies. The range of epithelial cell fusion is expanded compared with moderate infection, a large number of cell infiltration occurs in the parasite of the insect body, while the gill filament epithelial cells are seriously hypergrown and hypertrophied in a large area, the respiratory epithelium and capillaries are seriously detached, and there are serious pathological changes such as gill filament swelling, fusion and other inflammatory reactions or necrosis and disintegration, resulting in fish death. Changes in the type or degree of lesions in fish host tissues On the one hand, blood is closely related to the number of parasitic worm bodies, and the parasitic time of insect bodies in gill filament tissue also plays a very important role. When the ringworm is infected in a small number and the lesion area is not large, the insect body often falls off on its own, the symptoms of the diseased fish can be healed, and the fish form an effective immunity to the ringworm, and the insect body is difficult to infect for a period of time.
Second, the current situation of prevention and control
Ringworm disease is a common disease in aquatic products, mainly transmitted by eggs and larvae, mostly prevalent in late spring and early summer, suitable water temperature of 20 ~ 25 ° C, many species of ringworms can cause serious diseases of fish, causing major economic losses. At present, the prevention and control of ringworm disease still relies on special insecticides, and there are many basic drugs for the treatment of the disease, mainly including: one is benzimidazole mebendazole, flubendazole, fenbendazole, albendazole, trichlorobendazole, thibenedazole, non-bantyl; the second is organophosphorus diopteran, octylthion, halesone, etc.; the third is quinone iquinone, pyroquinone, methyl triazinone caprylic acid; fourth, antibiotics ivermectin, avermectin, mebemycin, doramycin, moxyrmectin, etc.; other piperazine, thiodichlorophenol, hexachloro-p-xylene , sodium antimony and so on. However, due to the long-term use of the above drugs, coupled with the frequent circulation of aquatic products, farmers abuse of drugs and other factors, the formation of insect resistance, resulting in a decrease in medicinal properties, or even ineffective, some have been banned. Therefore, in recent years, many fishery medicine manufacturers and research institutes have increased the research and development of new drugs, and have also made certain achievements, such as Chinese herbal extracts such as neem, but there are expensive raw materials (such as neem, 6000 yuan / kg), high technical requirements for preparations, difficulty in registering new drugs and other factors, and the current promotion and use area is not large.
At present, there are more imidazole drugs used for the control of ringworms in China, especially mebendazole has a good effect and a large amount of use, and this variety is included in the "National Standard for Veterinary Drugs - National Standard for The Rise of Local Standards for Veterinary Drugs" published in March 2005, which can predict that the drug will be the preferred drug for the control of ringworms in aquatic animals for a period of time in the future.
The principle of action of mebenimidazole in the prevention and control of ringworms: mebendazole blocks the operation of klycrazole glucose, leads to the depletion of glycogen and adenosine triphosphate in the body or prolongs the retention of cellular hydrolases and accelerates corticolysis. The principle of action of mebendazole on energy circulation determines that the ring worm needs to die for a long time under the action of mebendazole, not all insects can die within 24 hours, not in the next day the examination found that there are still insect parasitic eels that the effect of the drug is not satisfactory, usually it is recommended that the user use the earth pond for 5 days before changing the water to ensure the effect of the drug. Foreign experiments have shown that 0.5mg/L and 1.0 mg/L medicinal baths for 24 h, the insect body to 48 h after a large reduction, to 72 h all the larvae died, a small number of adults survived, 96h adults also died.
When mebendazole is used to control ringworms, the ringworms decompose rapidly after death, and the drug deworming effect can only be judged from the number of living insects. This is similar to the same as people taking "intestinal insect clearance" to kill intestinal parasites, after children take "intestinal insect clearance", there is no whole parasite in the excrement, only fragments of parasites.
In terms of control timing, try to choose the use of ringworm disease at the beginning of the occurrence, when the number of worm mouths is not large; sub-adult ringworms are more sensitive to mebenzidazole than adults, which is easy to kill; reduce the occurrence of other complications.
Third, new ideas for prevention and control
1. Life history blocking method
Some scholars have carried out experiments on the effect of potassium permanganate, dimethalar, methylene blue, sodium chloride and other drugs on the hatching of ciliate larvae and eggs of pseudo-ringworms, and the results show that dimethodium is not suitable for controlling pseudo-ringworm infection and is prone to drug resistance, methylene blue has little effect on ciliated larvae, but can effectively reduce the hatching rate of worm eggs, potassium permanganate and 3% sodium chloride solution treatment for 5 h has a certain insecticidal effect on ciliate larvae. The high-salt solution in this study seems to be able to block the development of insect eggs, providing a new idea for the prevention and control of ringworm disease, we can continue to screen some chemical drugs that can completely block the development of insect eggs and are harmless to farmed fish, in combination with conventional insecticides, both kill adult insects and prevent the development of insect eggs, thus overcoming the shortcomings of chemical drugs that are only effective for adults and ineffective for eggs in the past.
In addition, Xia Xiaoqin and Wang Weijun (1997) and other studies have shown that the newly hatched ciliate larvae have obvious phototropism, which is in line with the need for ciliate larvae to disperse and find a host, so it is possible to use phototropism to trap ciliate larvae and provide new ideas for the prevention and control of ringworm disease.
2. Environmental control law
Adult worms in adverse environmental conditions or changes in environmental conditions, etc., will accelerate the rate of spawning to maintain population reproduction. This phenomenon suggests that in the process of clinical treatment, after the insecticide is sprinkled in the cultured water body, the adult insects are likely to be stimulated and accelerate egg laying, at this time, increase the amount of water exchange, try to discharge these newborn eggs out of the cultured water body, may help to control pseudo-ringworm disease.
3. New drug development
On the one hand, we must speed up the development of new drugs, on the other hand, how to apply modern advanced preparation technology to existing drugs is also an important new idea. Such as: (1) the selection and use of synergists, synergists are one of the important measures to overcome pest resistance and improve the killing effect; (2) the use of water-based microemulsions, the so-called water-based microemulsions, is based on the theory of microemulsions, water-based solvents, do not use or less use organic solvents, under the action of surfactants, auxiliary surfactants, the original drugs insoluble in water are formulated into a stable microemulsion, droplet particle size is 0.01-0.1um. The product has high stability and high transfer efficiency, has a strong solubilization effect, green environmental protection, very suitable for the use of aquaculture water; (3) the use of diffusion infiltration system, through the study of the secondary dispersion system of drugs in the water body, especially the dispersion movement, distribution form and change law of pharmaceutical particles, establish a scientific diffusion infiltration system, to achieve the purpose of uniform distribution in the water body instantaneously and rapid penetration of insect bodies. Its system can be selected from silicone additives, nitrogenone, thiothione, eucalyptin, penetrant T to add one or more scientifically added to the formulation.