In the past two years, reports of cases of leukemia/hemangioma that occurred in egg farms across the country after the start of production have generally risen significantly, and have spread widely on the Internet. Virus isolation and identification showed that the leukemia/hemangioma that occurred after the first two years of production at the layer farm was mainly caused by subgroup J avian leukosis virus, but at the same time, it was also caused by subgroup A and B leukemia infections. In fact, the J subgroup avian leukemia virus-induced myeloid cell tumor has been introduced into China as early as the 1990s. It first brought great losses to China's white broiler chickens, and further introduced the local strains of three yellow chickens and eggs. Use type chicken. The following is a brief exposition of chicken leukemia and its prevalence in China and its control measures.

1 Avian leukosis virus and its epidemic characteristics

1.1 General Characteristics of Avian Leukemia Virus (ALV)

ALV is a retrovirus with a single-stranded RNA genome that resembles human HIV but does not infect humans. Different birds may be infected with different ALVs. According to the antigenicity of envelope proteins specifically related to the virus and host cells, ALV can be divided into A, B, C, D, E, F, G, H, I, and J. Subgroups. However, only six subpopulations of A, B, C, D, E and J are naturally infected. Among them, subgroup J is the most pathogenic and infective, while subgroup E is non-pathogenic or has very weak pathogenicity.

1.2 Exogenous and exogenous viruses

One of the biggest differences between ALV and other viruses is that ALVs in chickens can be further divided into two major groups, exogenous ALV and endogenous ALV. Exogenous ALV in chickens refers to ALVs that do not transmit through the chromosomes of host cells, including subgroups A, B, C, D, and J. Chickens with strong pathogenicity belong to the exogenous virus. They can be transmitted in the form of complete virions between cells and cells like other viruses, or can be transmitted laterally through direct contact or contaminants between individuals and groups, and can also be vertically transmitted from chickens through chicken embryos in the form of intact virions. For future generations. Endogenous ALV refers to ALV in which the proviral cDNA can be integrated into the chromosomal genome of the host cell and thus can be transmitted vertically through the chromosome. It may be only an incomplete fragment of the genome, it does not produce an infectious virus, and it is generally not related to pathogenicity. However, it is also possible that the whole genome can produce infectious virus, but such viruses are usually very weak or not pathogenic. The endogenous ALVs currently found belong to the E subgroup. A specific site in the chicken cell genome contains (stably integrates) the entire genome of the E-subpopulation ALV that replicates the transmissible viral particles, such as the ev21 locus on sex chromosome Z, which is closely linked to the K gene that determines the fast and slow plumes. From this ev21 can produce the infectious virus EV21. E-subpopulations Endogenous ALVs are usually not pathogenic but interfere with the differential diagnosis of leukemia. The breeder flocks purify the ALV and, at this stage, it mainly purifies the exogenous virus. We understand whether or not chickens have ALV infection, and at this stage we only refer to exogenous viral infections.

1.3 Epidemic features and routes of transmission of ALV

Because the ALV's resistance in the outside world is very weak, the ALV's horizontal transmission ability is much weaker than other viruses. At the temperature inside the house, especially in the summer, the ALV in the environment is excreted, and even without any cleaning and disinfection measures, the virus will be completely inactivated. The virus is also very sensitive to various disinfectants. ALV is mainly propagated by the breeder chicken through the egg (embryo) to the next generation vertically, that is, the ancestral chicken farm is directly transmitted to the parental generation and the commercial generation, and is enlarged from generation to generation. Direct infection of chicks with the vertical infectious virus after hatching in the hatchery and transport box with high density in direct contact with other chicks can cause severe transversal infections. In addition, attenuated vaccines contaminated with ALV are also important routes of transmission. Therefore, on the issue of ALV transmission, breeder farms must take an important responsibility for the next generation of chicken farms.

2 chicken ALV infection, chicken leukemia and chicken leukemia tumors

Among the various known viral infections, the pathological changes and manifestations of ALV infection in chickens are the most diverse. Leukemia is not only manifested as visceral tumors or skin hemangiomas on the surface, but more chickens exhibit decreased egg production, immunosuppression, or growth retardation. In fact, the subclinical pathological effects of infection by ALV may be more economical than the loss from clinical manifestations of neoplastic death. Among them, some sub-clinical performances are difficult to make judgments on the chicken farm, unless the chicken farm maintains systematic production records and regular periodic serum test records for comparative studies. It is worth noting that not every infected chicken exhibited the same immunosuppression, growth retardation, and decreased egg production. This was closely related to the age of infection, the subpopulation and virulence of infected strains, and the heritability of chickens. However, in general, the more early infection, especially vertical infection, has a strong pathogenic effect, and tumors often occur when they are sexually mature. What is more important is that ALV can cause different tumors in different organs of different organs of chickens. Liver, spleen, kidney, heart, and ovary are all common organs. Some tumors show large tumor nodules, while others show diffuse small nodules. Some shape rules, some irregular shapes. In particular, the cell type of the tumor is not the same. Such as lymphoma, myeloid cell tumor, erythroblastoma, fibrosarcoma, vascular endothelial cell tumor, etc., which is mainly related to the different characteristics of the virus and hereditary chicken. However, in general, AB subpopulations induce lymphocyte tumors and form larger tumor masses, while J subpopulations frequently cause myeloid cell tumors, and many diffusely distributed white fine tumors appear in enlarged livers. Nodules. It also causes irregular tumors in other organs. From a group perspective, this can be used as one of the important basis for differential diagnosis.

Although E-subgroup ALV infection does not necessarily cause tumors, chickens infected with ALV-E congenitally or after hatching often have higher mortality rates and are more susceptible to exogenous ALV infection. This may be related to the easy generation of immune tolerance to ALV after early infection with ALV-E, so that it is not easy to produce corresponding specific antibodies after exogenous ALV infection.

3 The history of the occurrence and epidemic of chicken leukemia in China

Chicken leukaemia has been found in chickens for more than 100 years. It was originally a classic A, B, C, D subset of leukemia. As a result of strict purifying measures, exogenous chicken leukemia virus has been declared purified from large breeder chicken companies in developed countries after 1987. However, our country has never taken any purifying measures for ALV, so there may be classic chicken leukemia in our local strains of chickens. Only due to the small scale of the breeding, no harm can be seen. In 1988, the United Kingdom discovered a new J-subpopulation ALV from white-feathers, and its tumor incidence was much higher than classic ALV. In a few years spread throughout the world almost all white chickens. In China, the introduction of the J subgroup ALV was also introduced in the introduction of white feather meat breeders. After 2004, several major breeder chicken companies in the world have basically purified ALV-J. Since 2005, ALV-J tumors have not become a serious problem in the production of white broiler chickens in China. During this period, however, due to improper introduction and improper management of breeding farms during the breeding process, J subgroup ALV has also been brought into China's self-breeding yellow feather broilers (with 2 to 3 billion feathers raised annually). ) and egg feeding chickens with an annual breeding capacity of 12-15 million feathers. The prevalence of leukemia in laying hens has become one of the most important diseases currently affecting the chicken industry, especially the layer industry.

Over the past ten years, we have isolated and identified more than 60 strains of ALV-J from tumors in various types of chickens. At the same time, subgroups A, B, and C were also isolated, but only about 10 were added together. Most of the other domestic units also reported ALV-J. Obviously, in the past decade or so, chicken leukemia in China has mainly been caused by ALV-J. This is not only the major problem of white feather broilers before 2005, but also the main problem in our local strains. In these two years, it has also become a problem in egg-type chickens.

4 Current status of ALV infection and epidemic in chickens in China in recent years

4.1 The overall status of ALV infection in different types of chickens across the country

In 2007, during the implementation of the project on epidemiological investigation and prevention and control of chicken leukemia, which was approved by the Department of Science and Technology and the Veterinary Bureau of the Ministry of Agriculture, we have implemented different types of different regions in China. A serological investigation was performed on the status of ALV infection in the flock. Our results showed that 237/468 (50.6%) populations were positive for ABV subgroups of ALV antibodies, and 1 201/28 853 (4.2%) individuals were individuals with positive ALV infections in subgroup J. Accounting for 231/476 (48.5%) individuals and 1,817/29 914 individuals (6.1%). This result shows that ALV infection is quite common in chickens in China, but with different types of chickens, the infection rate varies greatly.

4.2 J Subgroup Avian Leukosis in Chinese Flock

J-ALV is a new subpopulation of leukemia viruses discovered and isolated from broilers for the first time in the late 1980s in the United Kingdom. It initially occurred in meat-type chickens. Although hens can be infected, the natural incidence is very low. In China, J-ALV was isolated and isolated from chickens in broiler and broiler farms for the first time in 1999, and the prevalence of J-ALV was determined in several provinces.

The state of ALV-J infection in China is as follows: (1) White broiler breeder chickens, which were common before 2003, are now significantly reduced. In 2005, ALV-J was not isolated again. This is because after more than ten years of strict purification, foreign countries The large-scale breeder farms have basically not been infected with ALV-J, but the white-feathered broilers in China have all been imported. As long as they keep their biological safety, they will not be reinfected. (2) Egg-type chickens used to be mainly concerned with classical leukemia, but they are In the 4th to 5th years, ALV-J was often isolated; (3) ALV-J was the most commonly isolated tumor in local strain chickens, Sanhuang chickens in Guangdong and Guangxi, and local strain chickens in Shandong and other provinces.

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