International Journal of Molecular Veterinary Research 2024, Vol.14, No.1, 23-31 http://animalscipublisher.com/index.php/ijmvr 25 H5N1 viruses are also spread through direct contact with infected birds or their secretions, making the poultry farming environment a potential source of infection. Dense poultry farms and markets can easily form transmission chains and accelerate the spread of the virus. Therefore, it is particularly urgent to strengthen the monitoring, quarantine and health management of poultry breeding sites. The development of the H5N1 avian influenza virus epidemic is affected by multiple factors, among which climatic conditions, bird migration patterns and human activities are the three key factors. Meteorological factors such as seasonal changes, temperature and humidity directly affect the survival and spread of H5N1 viruses in the environment. The cold season and humid environment contribute to the stable existence of the virus and increase the risk of transmission (Salaheldin et al., 2018). The migration paths of poultry are closely related to the spread of H5N1 viruses, especially on an international scale. The long-distance migration of birds may serve as a bridge for viruses to cross geographical barriers, triggering regional or global outbreaks. Agriculture, trade and human behavior are important factors in the spread of the epidemic, especially in areas with intensive poultry farming. Man-made transportation and trade activities can spread the virus quickly, and the closer the poultry farms are to each other, the faster the spread. By in-depth analysis of the transmission patterns of the H5N1 avian influenza virus, researchers can more accurately predict the outbreak and spread trends of the epidemic. This provides an important basis for formulating scientific and targeted prevention and control strategies, and emphasizes the urgency of strengthening international cooperation, information sharing and monitoring systems on a global scale. 2 Threats to Chicken Flock Health The clinical symptoms and disease development caused by infection with H5N1 avian influenza virus pose a serious threat to the health of chickens. Understanding the mechanisms of these effects has far-reaching implications for effective prevention, control and management. 2.1 Clinical symptoms and disease course The clinical symptoms caused by H5N1 avian influenza virus infection are very obvious and widespread in chicken flocks. After infection, chickens often show a variety of typical symptoms, including shortness of breath, decreased appetite, and ruffled feathers (Letsholo et al., 2022). These symptoms often develop rapidly in a short period of time and directly affect the overall physiological state of the chicken. Shortness of breath indicates an impact on the respiratory system, while decreased appetite reflects disturbances in their overall metabolic and immune systems. Fluffy feathers may be due to physical discomfort caused by infection and the immune system's response to fight the virus. In cases of severe infection, high mortality is a common outcome. Even those that survive may suffer long-term effects from reduced productivity. This includes slowed growth, reduced egg production, etc., posing a direct threat to the poultry breeding industry. Therefore, timely identification and understanding of these clinical symptoms can enable effective prevention and control measures to be taken as soon as possible and reduce the losses caused by the epidemic. The disease development of avian influenza usually includes three stages: incubation period, acute stage and recovery stage. The incubation period refers to the time period during which the virus gradually reproduces in the chicken after infection. At this stage, no obvious clinical symptoms have yet appeared, but the virus has gradually spread in the chickens. This is a critical period, as early detection and isolation of infected chickens can effectively slow the spread of the epidemic. The acute phase is a stage in which viral replication and pathological changes increase dramatically. At this stage, clinical symptoms in the flock become apparent, including shortness of breath and decreased appetite. Due to the rapid replication and spread of the virus, the mortality rate also increases. Early diagnosis and isolation of infected chickens at this stage can help reduce mortality and reduce the economic burden on the breeding industry. The recovery period is the process in which an individual gradually recovers and returns to normal physiological state. During this stage, the flock's immune system gradually overcomes the virus and infected individuals begin to recover. A thorough understanding of the pathological changes and disease
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