The application of avermectins in China and factors that can influence the pesticide effect
Avermectin can also be called abamectin, which is a kind of sixteen-carbon ring macrolides antibiotics and antiparasitic agent that is extracted in the metabolite fermented by streptomyces avermitilis. This kind of medicine was firstly separated in Kitasato, Japan in 1975. A few years later, Merck & Co Inc studied and found that it has good effect on repelling nematodes and arthropods. In recent ten years, abamectin pesticide has been more and more widely used in the insect repellent of animals and killing pests of plants.
Avermectins includes abamectin and synthetic derivatives (ivermectin and doramectin). The mechanisms of abamectin are as follows: it can stimulate the release of a kind of neurotransmitter called gamma-aminobutyric acid (GABA) that exists in arthropods and nematodes, thus it can paralyze the parasites. Parasites and higher animals that have this kind of medium will both paralyzed by abamectin. However, abamectin will have no effect on the parasites that are not carried with the medium. Therefore, abamectin insecticide can have repelling and killing effect on nematodes and epizoa of higher animals, but it will have no effect on flukes and tapeworms. This can be regarded as a defect of abamectin.
Abamectin is not a kind of single substance, however, it is the mixture of a series of chemical substances that have same structures and are fermented by streptomyces avermitilis. Currently, abamectin products mainly include 8 kinds of compositions, which are A1a, A1b, A2a, A2b, B1a, B1b, B2a, B2b and so on. Among them, abamectin B1 is the main composition, in which the main composition of abamectin B1 is B1a.
In the fermentation process, the metabolites of streptomyces avermitilis not all including abamectin, but also including many other substances. Currently, research on these kinds of substances is still not deep enough as most of them have not been identified clearly yet. Some of the substances will do harm to higher animals and human beings. Among them, at least two kinds of compositions are toxic. One of them is a kind of material that can cause animal suffocation, while the other of them is a kind of material that can cause cancer. Therefore, in the extraction process of abamectin, materials except for abamectin should all be removed.
Abamectin is a kind of white or light yellow crystalline powder, which is tasteless, almost insoluble in water but is slightly soluble in alcohol and is freely soluble in chloroform. Abamectin preparation is not stable enough, which is easy to oxidize and will decline the vitality fast, especially for the powder and premix. However, as the injection will not directly contact with oxygen in the air, it will have good stability. At present, abamectin products that have been developed in the market mainly include ivermectin and doramectin. In addition, methylamine abamectin is also being developed in China but has not been launched.
Ivermectin is generated after the artificial structure transformation of abamectin B1a, while abamectin B1a is the precursor of ivermectin. By hydrogenizing the double bond between the 22nd and 23rd carbon atoms of abamectin B1a macrolide structure, it can be changed to ivermectin. In China, it is required that ivermectin should contain more than 80 percents of dihydro abamectin B1a and less than 20 percents of dihydro abamectin B1b.
Compared with abamectin, ivermectin will be in saturation state after double bond hydrogenation reduction, thus it will have good stability and oxidation resistance as well as will have more reliable pesticide effect. Besides, as the toxicity of ivermectin is only half of abamectin B1, it will have high safety, however, its price is more than 1 time higher of abamectin. Ivermectin can also be called ivomec.
Doramectin is generated by connecting six-member ring structure on carbon chain C25 of abamectin B1a, thus it will have good stability and pesticide effect as well as high safety. This kind of medicine was developed and produced in America, but it has not been promoted in China.
Ivermectin and doramectin both have same mechanism and insecticide spectrum with abamectin, which can be used in repelling and killing the nematodes in the animals as well as mites, louse and fly on the animal body surface.
Abamectin and ivermectin preparations that are produced in China mainly include powder, premix, capsula, tablet and injection. Recently, oral liquid, paste, pour on solution and ointment also have been developed, but they have not been widely used. As every dosage form will both have advantages and disadvantages, they should be used according to different animal species. Considering the bioavailability, injection will have best effect, followed by pour on solution and oral liquid. Injection and premix are suitable for being used on pigs and rabbits. After abamectin and ivermectin injections are injected on the subcutaneous, they will be easy to combine with subcutaneous fat, generate sustained release and extend the effect time. However, oral liquid will not have this kind of effect. Therefore, injection will have best effect on pigs, but it will have high labour intensity in the operation process and will disturb pigs to some extent. Besides, the cost is rather high. If we consider these kinds of factors, premix can be used. In order to fatten pigs, 2mg/kg premix can be used for 7 consecutive days or continuously using 1mg/kg premix for 14 days. Besides, 4mg/kg premix can be continuously used on saw and boar for 10-14 days. Regarding cattle and sheep, it should choose appropriate dosage forms according to local feeding model and drug using habit.
Compared with other antiparasitic agents, abamectin and ivermectin both have following advantages.
a. They have broad spectrum. They can repel and kill the imago of three kinds of insects (nematoda, insecta and arachnida) and other 73 genera parasites as well as at least one phase of polypide in the development process.
b. They have high efficiency. By using therapeutic dose, it can repel 100 percents of insects.
c. They have low toxicity. By using less than 10 times of abamectin therapeutic dose on cattle and sheep, they will not cause poisoning.
d. The dosage is very low. The therapeutic dose is generally 0.3mg/kg and can take effect by using it only one time.
e. They are easy to take orally. These kinds of medicine have no special smell and will not cause apastia when used on gastrointestinal tract.
f. They will not be easy to have drug resistance.
g. They can improve the fur quality of animals.
Currently, abamectin and ivermectin have been widely applied no matter in the international or in China’s animal health care product markets. Among the pesticides that used in repelling mites and ectoparasites in the abroad, abamectin and ivermectin are most widely used currently, besides, the dosage is still expanding. With the continuous development of production process technology, the price of ivermectin will drop continuously. In China, ivermectin will replace abamectin in more and more aspects.
The application of abamectin and ivermectin can be counted as a big progress of cultivation technology and is also a great contribution to animal husbandry.
However, in the using process of avermectins, many users reflect that the effect is not very ideal and it will even cause poisoning. Especially that the using effect of abamectin is not very stable. After carefully investigating and analyzing, we found that below factors may lead to these problems.
Firstly, as the chemical structure of abamectin is unstable, it will be easy to oxidize and reduce activity. This kind of activity attenuation process will occur in drug raw materials, tablet, powder, premix and capsula.
Secondly, as abamectin is a group of compounds, it will have complex compositions and molecular structure. Therefore, if the quality is not controlled well in the production process, it will have great influence on the drug quality.
Thirdly, some abamectin manufacturers sell streptomyces avermitilis mycelium that has not been purified and refined or the crude extract instead of abamectin, which will not have proper effect and may cause poisoning of livestock and birds or even cause deaths.
Fourthly, abamectin and ivermectin injection that produced by different manufacturers will contain different solvent and auxiliary (such as cosolvent, antioxidant, analgesic and so on), which will cause different pesticide effect.
Fifthly, abamectin and ivermectin both have good repelling and killing effect on imago, but they have poor effect or even no effect on larvae and worm egg. If they are not continuously or repeatedly used in the oral medication, insects in the larvae or egg stage will develop into imago and will cause repeated infection.
Sixthly, if drug is used in repelling insects without coordinating with housing cleaning disinfection measures, it will cause repeated infection of worm egg and polypide in the environment.
In view of the above situations, plantchemical.com suggests that below points should be paid attention to when purchasing and using abamectin and ivermectin.
Firstly, choose products that are produced by manufacturers having high credibility so as to ensure good quality.
Secondly, do not purchase preparations that are made by mycelium or crude extract. The qualified abamectin and ivermectin oral preparations will not appear to be ash black and are also tasteless. If the powder or tablet appears to be black or grey and smells bad, they must be made by mycelium or crude extract and should not be purchased.
Thirdly, if the conditions are permitted, choose ivermectin preparations as many as possible instead of choosing abamectin preparations.
Fourthly, choose products that have latest batch number as many as possible, especially when choosing abamectin.
Fifthly, make sure that the dosage and duration are enough. If it is necessary, use the drug repeatedly.
Sixthly, choose appropriate dosage form according to the feeding status and livestock breeds. If the operation condition is permitted, use injection as many as possible.
Seventhly, when using drug, pay attention to environmental sanitation cleaning and disinfection at the same time so as to kill the polypide and worm egg in the environment.