Highly pathogenic avian influenza virus infection

　　Highly pathogenic avian influenza virus infection is the disease caused by avian influenza virus infection in humans. Avian influenza was first reported in Italy in 1878, then called fowl plague. The etiological agent was first discovered in 1900 and was considered a filterable virus, known as true fowl plague virus. It was not until 1955 that it was confirmed as avian influenza virus (avian influenza virus) by serological methods.

　　1. Shape and genome of avian influenza viruses

　　Avian influenza viruses belong to the genus A influenza viruses of the family Orthomyxoviridae, and their common shape is spherical, with a diameter of 80 to 120 nm, averaging 100 nm. They have an envelope. Newly isolated or passaged viruses are mostly filamentous, with varying lengths, up to 4000 nm, and have segmented single-stranded negative-sense RNA genomes.

　　2. Classification and virulence of avian influenza viruses

　　Avian influenza viruses have many subtypes due to the differences in antigenicity of their hemagglutinin (HA) and neuraminidase (NA) proteins. So far, 15 HA subtypes (H1 to H15) and 9 NA subtypes (N1 to N9) have been identified from birds. Especially the H5 and H7 subtypes have a high pathogenicity to birds and can cause outbreaks of severe avian influenza. The next most pathogenic are the H9 and H4 subtypes. Since each influenza pandemic in humans has been associated with H1 to H3 and N1, N2 subtypes, it has been believed that avian influenza viruses have no pathogenicity to humans. Historically, there have been no reports of avian influenza virus infection in humans during outbreaks, including the two major outbreaks in the United States in 1983 and Mexico in 1995. This is because for a specific organism, a virus needs specific genes to produce surface proteins to successfully bind to proteins within the organism's body, thereby causing infection. Different viruses infect different organisms, and crossing the species barrier is not easy. However, under frequent contact between humans and animals, some strains may mutate and become capable of infecting humans. In May 1997, a strain of avian influenza virus was isolated from a 3-year-old boy in Hong Kong who died of unknown causes due to multiple organ failure. The virus was subsequently identified as H5N1 avian influenza virus by the Dutch National Influenza Center and the U.S. Centers for Disease Control and Prevention (CDC) in August of the same year. This was the first confirmed case of human infection with the avian influenza virus H5N1. Subsequently, there have been reports of human infections with H9N2 (1999 Hong Kong) and H7N7 (2003 Netherlands) subtypes. In December 2003 and at the end of 2004, avian influenza outbreaks occurred in Southeast Asia, with Thailand and Vietnam reporting cases of highly pathogenic avian influenza virus transmission to humans. In Asia, 44 people have been infected with H5N1 avian influenza virus, of whom 32 have died. A virus antigen surface gene fragment was obtained from a patient in Vietnam, confirming that it was identical to the H5N1 subtype, suggesting that H5N1 subtype infections are severe with a high mortality rate.

　　3. Specificity and variability of avian influenza virus

　　Avian influenza virus has many serotypes, strong infectivity, wide distribution, certain host specificity, and rapid mutation. The mutation of amino acids in the HA receptor binding center of the viral envelope may change its host specificity. Like other influenza A viruses, the main modes of mutation of avian influenza virus are antigenic drift (antigenic drift) and antigenic shift (antigenic shift). Recently, through the analysis of 18 strains of avian influenza virus H5N1 isolated in Hong Kong in 1997 and H9N2 isolated in 1999, it was found that none of them contained gene fragments of mammals such as humans and pigs, indicating that they have not undergone genetic recombination. This means that avian influenza virus can be transmitted directly to humans. If avian influenza virus and human influenza virus infect human cells and recombine, obtaining human gene fragments and having a preference for human cells, this virus may cause a global influenza pandemic.

　　4. Stability of avian influenza virus

　　Avian influenza virus is sensitive to organic solvents such as ether, chloroform, and acetone. Common disinfectants such as oxidizers, dilute acids, lauric sulfate sodium, halogen compounds (such as bleaching powder and iodine), and others are also easy to inactivate the virus. Avian influenza virus is sensitive to heat, and can be inactivated by heating at 56℃ for 30 minutes, 60℃ for 10 minutes, heating for several minutes at 65 to 70℃, or boiling (100℃) for 2 minutes. Direct sunlight for 40 to 48 hours or direct ultraviolet light exposure can quickly destroy its infectivity. In natural conditions, the virus existing in the oral cavity, nasal cavity, and feces is protected by organic matter and has great resistance. For example, the virus can survive for a week in feces and a month in water, and in pH

　　Most mild cases of highly pathogenic avian influenza virus infection have a good prognosis and do not leave sequelae. Certain cases (especially H5N1 infected cases) may develop rapidly, presenting with severe pneumonia, acute respiratory distress syndrome, pulmonary hemorrhage, pleural effusion, decreased blood cell count, multiple organ failure, sepsis, shock, and Reye syndrome, among other complications, which can lead to death. In 1997, among the 18 cases in Hong Kong, 8 had mild upper respiratory tract infection, 4 developed severe pneumonia, and all recovered after respiratory support. Six patients still died of various complications after monitoring.

　　The incubation period of highly pathogenic avian influenza virus infection has not been accurately reported yet. Currently, it is estimated to be within 7 days, usually 1 to 3 days. H5N1 virus infected patients often present with acute onset, early symptoms similar to common influenza, mainly fever, with body temperature usually above 39℃, lasting for 1 to 7 days, usually 3 to 4 days, and may be accompanied by symptoms such as runny nose, nasal congestion, cough, sore throat, headache, muscle ache, and general discomfort. Some patients may have gastrointestinal symptoms such as nausea, abdominal pain, diarrhea, and loose stools, and most mild cases have a good prognosis.

　　The condition of severe patients develops rapidly, and may have signs of lung consolidation, and may also have complications such as pneumonia, acute respiratory distress syndrome, lung hemorrhage, pleural effusion, decreased blood cell count, renal failure, sepsis, shock, and Reye syndrome. Severe cases can lead to death. If the body temperature remains above 39℃ during treatment, be vigilant of the tendency towards severe illness. H7N7 infected individuals have relatively mild symptoms, most patients can have conjunctivitis, and a few patients may have mild influenza-like symptoms. H9N2 infected individuals only cause transient influenza-like symptoms, and there have been no reported deaths.

　　The measures for preventing infection with highly pathogenic avian influenza virus are similar to those for preventing general infectious diseases, and should start with controlling the source of infection, cutting off the route of transmission, and protecting susceptible populations. The following are 4 methods summarized based on these three aspects:

　　1. Monitoring and controlling the source of infection

　　The health department and the agriculture department should cooperate to carry out simultaneous monitoring of human and avian H5N1 influenza outbreaks, exchange intelligence, strengthen quarantine, and prevent the entry of avian influenza virus, especially highly pathogenic avian influenza virus, into China. Special attention should be paid to the epidemic prevention and disinfection of transportation vehicles from countries or regions with animal epidemic outbreaks, and the prohibition of passengers from carrying or mailing related animals and their products into the country. Adhere to the 'all-in, all-out' breeding method for poultry, strengthen disinfection in daily life, and implement general immunization against epidemic diseases to improve the resistance of poultry. In the event of an avian influenza outbreak, it is necessary to handle it in accordance with the relevant provisions of the 'Animal Quarantine Law'. Early rapid diagnosis should be carried out, and if highly pathogenic strains such as H5 and H7 are found and diagnosed, strict isolation, blockade, culling, and destruction of the infected chicken flock should be carried out. The chicken farm should be thoroughly cleaned, washed, and disinfected. The current measure is to cull all chicken flocks within 3km of the epidemic source and to implement mandatory immunization for chicken flocks within 5km. The breeding personnel and all relevant personnel should take protective measures and strengthen monitoring. When influenza-like symptoms appear in these personnel, they should be isolated immediately and the epidemic reported. At the same time, conduct an epidemiological investigation to prevent the deterioration of the disease and the expansion of the epidemic. While isolating and treating patients, collect nasal and oropharyngeal secretions, gargle fluid, sputum, or tracheal aspirates, and serum and send them to designated laboratories for virus isolation and antibody detection to clarify the diagnosis as soon as possible.

　　2. Cut off the route of transmission

　　In the event of an avian influenza outbreak, thorough disinfection should be carried out in poultry breeding farms, market stalls selling poultry, slaughterhouses, and the units and homes of patients. Dead poultry and poultry waste should be destroyed or buried deeply; the outpatient and inpatient departments of hospitals treating patients should implement isolation and disinfection measures to prevent the contamination of the hospital environment and medical supplies by patient excrement and blood; medical staff should take personal protective measures, wear masks, gloves, and isolation gowns when in contact with avian influenza patients, and wash their hands after contact. Strengthen the management of detection samples and laboratory avian influenza virus strains, and ensure that laboratories conducting avian influenza virus isolation meet P3 level standards. Strictly implement operational specifications to prevent hospital and laboratory infections and transmission.

　　3. Promote a healthy lifestyle

　　Strengthen physical exercise, avoid overexertion, pay attention to nutrition, do not smoke, wash hands frequently, pay attention to food hygiene, do not drink unboiled water, try to avoid contact with poultry when an epidemic occurs, thoroughly cook chicken and other foods, do not eat raw or undercooked animal products, keep the indoor air fresh and circulating, and for close contacts, oral antiviral drugs such as rimantadine and oseltamivir can be used for prevention.

　　4. Vaccine

　　The current influenza vaccines for H1N1, H3N2, and B influenza cannot prevent infections with H5N1, H7N7, and H9N2 viruses. The H9N2 vaccine has undergone phase I clinical trials in humans and is considered to have certain safety and tolerance. Recently, the WHO is organizing safety and immunogenicity tests for the H5N1 vaccine.

　　Diagnosis refers to the People's Republic of China's diagnosis and treatment program for human avian influenza (trial), and a diagnosis of human avian influenza can be made after excluding other diseases based on the epidemiological history, clinical manifestations, and laboratory examination results.

　　1. Peripheral blood and bone marrow

　　The total white blood cell count is generally not high or low, platelets are normal, and severe patients often have a decrease in total white blood cell count and lymphocytes. Bone marrow cytomorphology examination shows active cell proliferation, reactive tissue cell hyperplasia with hemorrhagic phagocytosis.

　　2. Viral antigen and gene detection

　　Collect respiratory specimens from patients, detect the nucleoprotein antigen (NP) of influenza A virus and H subtypes of avian influenza virus using immunofluorescence (or enzyme-linked immunosorbent) methods, and detect the subtyping-specific H antigen gene of avian influenza virus using RT-PCR. Recently, Lau et al. used a nucleic acid sequence amplification technology (NASBA) to rapidly detect H5 and H7, which has high sensitivity and specificity and can distinguish to some extent between pathogenic and non-pathogenic avian influenza viruses.

　　3. Viral isolation

　　Isolate avian influenza virus from respiratory specimens of patients (such as nasopharyngeal secretions, oral rinse solutions, tracheal aspirates, or respiratory epithelial cells).

　　4. Serological examination

　　Collect two serum samples from the early and convalescent stages of the disease, and detect antibodies against avian influenza virus by hemagglutination inhibition test, complement fixation test, or enzyme-linked immunosorbent assay. If the titer increases by 4 times or more, it can be used as a reference index for retrospective diagnosis.

　　5. X-ray examination

　　X-ray examination of the chest in severe patients can show unilateral or bilateral pneumonia, and a few patients may have pleural effusion.

　　To prevent infection with highly pathogenic avian influenza virus, it is necessary to strengthen physical exercise, avoid overexertion, pay attention to nutrition, and rely on the body's resistance. Do not smoke or drink alcohol, wash hands frequently, pay attention to food hygiene, and do not drink unboiled water. When an epidemic occurs, people should try to avoid contact with poultry, thoroughly cook chicken and other foods, and do not eat raw or undercooked animal products, keeping the indoor air fresh and circulating. For close contacts, oral antiviral drugs such as rimantadine and oseltamivir can be used for prevention.

　　The treatment principles for highly pathogenic avian influenza virus infection are the same as for influenza, both requiring antiviral treatment and symptomatic treatment.

　　1. Isolation

　　Isolation and treatment should be provided for suspected and confirmed cases to prevent the progression of the disease and the spread of the disease.

　　2. Symptomatic Supportive Treatment

　　Heat-reducing and analgesic drugs, decongestants for nasal mucosal congestion, expectorants, and mucus-thinning drugs can be used. Children should avoid using salicylate drugs such as aspirin for fever reduction to prevent Reye syndrome. Pay attention to rest, drink plenty of water, eat light meals, and appropriately supplement nutrition and intravenous fluid replacement.

　　3. Antiviral Treatment for Influenza

　　Antiviral drugs against influenza should be tried within 48 hours of onset of the disease.

　　(1) M2 Ion Channel Blockers: Amantadine and rimantadine. These drugs mainly inhibit the replication of influenza virus strains by interfering with the activity of the viral M2 ion channels. Drug sensitivity tests show that they are effective against avian influenza. Early application can prevent the progression of the disease, alleviate the symptoms, accelerate the recovery of the disease, and improve the prognosis. The use of these drugs is prone to induce drug resistance in influenza viruses. There are also reports suggesting that the efficacy of these antiviral drugs against avian influenza viruses in humans may not be conclusive.

　　Amantadine: The adult dose is 100-200mg/d, and the pediatric dose is 5mg/(kg·d), divided into two oral doses, for a course of 5 days. Attention should be paid to central nervous system and gastrointestinal side effects during the treatment process, with about 14% of patients experiencing adverse reactions. Nervous system side effects include anxiety, inattention, dizziness, drowsiness, neurasthenia, and increased seizures; gastrointestinal side effects include nausea, vomiting, anorexia, and abdominal pain; and there is also a possibility of teratogenicity. Caution should be exercised in elderly patients and those with vascular sclerosis, and the dose should be reduced for those with impaired liver and kidney function. Pregnant women and patients with epilepsy are prohibited from using it. Rimantadine: The dosage is the same as amantadine, but it is absorbed more slowly after oral administration, with a lower plasma concentration and a half-life of 24-36h. It only needs to be taken once a day, and the nervous system adverse reactions are less common than those of amantadine.

　　(2) Neuraminidase Inhibitors: By inhibiting the neuraminidase of the influenza virus, these inhibitors suppress viral replication and reduce the pathogenicity of the virus. Oseltamivir (Oseltamivir): Trade name Tamiflu. It is an oral specific influenza virus NA inhibitor, and studies have shown that it has inhibitory effects on avian influenza virus H5N1 and H9N2 with good tolerance. It is still effective against influenza resistant to amantadine and rimantadine. The adult dose is 150mg/d, and the pediatric dose is 3mg/(kg·d), divided into two oral doses, for a course of 5 days. The dosing regimen for the prevention of influenza is 75mg/d, taken in a single dose, for a course of more than 7 days, starting within 2 days of contact with the source of infection. In addition, there are zanamivir (Zanamivir) and RWJ-270201, which are also neuraminidase inhibitors. Human trials have shown that after 5 days of continuous use, flu symptoms can be significantly improved, and the replication of influenza viruses in the body can be suppressed.

　　(3) Other: Ribavirin and other drugs have been confirmed to have antiviral effects against influenza virus in vitro trials, and further animal trials and clinical studies are needed.

　　4. Strengthen supportive treatment and prevent complications

　　In addition to the above routine treatments for severe patients, it is also necessary to strengthen supportive treatment and prevent various complications.

　　① Strengthen nutritional support treatment: According to the patient's general condition, urine output, as well as the detection of blood glucose, blood electrolytes, and plasma protein content, appropriate fluids, human serum albumin, amino acids, or intravenous hyperalimentation treatment should be given. Severe patients should record daily intake and output, monitor central venous pressure, and pay attention to protect the function of important organs such as the heart, liver, and kidneys. Patients with elevated transaminases can choose to use oxygen free radical scavengers and glycyrrhizin drugs for liver protection. For elderly patients or children with myocarditis, attention should be paid to prevent the occurrence of heart failure.

　　② Prevent and treat bacterial infections: Although there is no clear evidence of bacterial infection in avian influenza virus carriers at present, bacterial infection may occur in the later stage of influenza virus infection, so the use of some broad-spectrum antibacterial drugs for severe patients may help prevent sepsis and bacterial pneumonia. Antibacterial drugs should be used in combination and choose broad-spectrum antibacterial drugs mainly against cocci.

　　③ Strengthen blood oxygen monitoring and respiratory support treatment: Severe inpatients should strengthen the monitoring of blood oxygen saturation and partial pressure of oxygen, and oxygen therapy should be provided to those with respiratory difficulties; auxiliary respiratory ventilation treatment should be used when necessary. Half of the human avian influenza virus H5N1 cases in Hong Kong and in recent years in Vietnam and other countries have pulmonary complications, so respiratory support therapy is quite important.

　　④ Actively prevent and treat other complications: For patients with severe poisoning symptoms, acute respiratory distress syndrome, shock, cerebral edema, and other conditions, short-term corticosteroid shock therapy can be adopted. Actively prevent and treat hemophagocytic syndrome and Reye syndrome and other complications.