The avian influenza virus is orthomyxovirus, a member of the genus Influenza Virus. Influenza viruses are distinguished by specific non-cross-reactive ribonucleoprotein antigens into three different antigen types, namely A, B, and C. Human influenza viruses can cause seasonal epidemics, usually in October and May. Wild waterfowl are natural hosts for all known influenza A viruses, especially certain mallards, geese, geese, swans, gulls, waterfowl, and terns. Influenza A viruses can infect humans, birds, pigs, horses, dogs, marine mammals, and other animals. Influenza A viruses are classified into different genotypes based on two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). For example, "H7N2 virus" refers to a subtype of influenza A virus that has HA7 protein and NA2 protein. Similarly, the "H5N1 virus" contains the HA5 protein and the NA2 protein. At present, we know that influenza A viruses have 17 HA subtypes and 10 NA subtypes. Currently, only the H1N1 and H3N2 subtypes of influenza viruses can be transmitted from person to person. Studies have also shown that some subtypes of influenza viruses can infect other species of animals, such as H7N7 and H3N8 influenza viruses can cause disease in horses, and H3N8 can also cause disease in dogs.

Recently, there have been cases of new avian influenza A (H7N9) infecting humans in humans. Although avian influenza A viruses do not usually infect humans, since 1997, when it was discovered that humans can also be infected with avian influenza A, the disease has attracted great concern from health organizations around the world, and there have been sporadic outbreaks. Most cases of human infection with avian influenza A virus have been linked to direct or close contact with infected poultry, and the patients will have mild to severe clinical symptoms.

Since November 2003, more than 600 people infected with the H5N1 highly pathogenic avian influenza virus (HPAIV) have been reported in 15 countries in Asia, Africa, Asia and the Pacific, Europe and the Near East, with high mortality rates. The H5N1 highly pathogenic avian influenza virus circulating in birds has been evolving into different subgroups of viruses, known as "clades". As H5N1 highly pathogenic avian influenza is evolving in unpredictable ways, it is crucial to closely monitor the spread and prevalence of these viruses among poultry and other birds, mainly to grasp the risk of transmission to humans. In the case of human infection with the H5N1 influenza virus, the cause of the illness is thought to be the result of direct or close contact with infected sick or dead poultry.

Other subtypes of avian influenza viruses may also be transmitted to humans, including the low-pathogenic avian influenza A virus (LPAI) and the highly pathogenic avian influenza A virus (HPAI).

1. H7 subtype avian influenza virus

There are 9 possible genotypes of H7 subtype avian influenza virus, including H7N1, H7N2, H7N3, H7N4, H7N5, H7N6, H7N7, H7N8 and H7N9. Cases of human infection with the H7 subtype virus are rare, and the cases that have been shown to have been developed through direct contact with infected birds, particularly during outbreaks of the H7 subtype avian influenza virus in poultry. The onset of symptoms in patients mainly consists of conjunctivitis and/or upper respiratory tract infections.

In humans, infection with the low-pathogenic avian influenza virus (H7N2, H7N3 and H7N7) causes mild to moderate cases.In 2002, during an outbreak of the H7N2 low-pathogenic avian influenza virus in a poultry and turkey farm in Virginia, USA, the H7N2 influenza virus was isolated from a slaughtering man who developed flu symptoms. In November 2003, an adult woman in New York developed respiratory illness and the H7N2 influenza virus was isolated from a respiratory sample. In February 2004, the Canadian Food Inspection Agency (CFIA) announced an outbreak of the H7N3 avian influenza virus in poultry in the Fraser Valley region. Initially a case of low pathogenic avian influenza, it later developed into an outbreak of highly pathogenic avian influenza. This was followed in March by two staff members who had close contact with poultry with conjunctivitis and mild flu symptoms. Laboratory analysis confirmed that they were all infected with the H7 subtype avian influenza virus. However, the patients recovered well after treatment with the anti-disease essay model essay.

Highly pathogenic avian influenza viruses (H7N3, H7N7) can cause infection of varying severity and even death.On April 1, 2013, the first case of human infection with the H7N9 avian influenza virus was reported in Shanghai, China, and patients experienced severe respiratory symptoms and death. So far (April 6), a total of 16 confirmed cases of human infection with H7N9 avian influenza have been reported nationwide, of which 6 have died. No epidemiological link was found between the 16 confirmed cases reported.

2. H9 subtype avian influenza virus

There are also 9 possible genotypes of H9 subtype avian influenza virus, including H9N1, H9N2, H9N3, H9N4, H9N5, H9N6, H9N7, H9N8 and H9N9. The H9N2 virus has been detected in flocks of birds in Asia, Europe, the Middle East, and Africa. Studies have confirmed that there have been sporadic cases of H9N2 avian influenza virus infection in humans, which generally cause mild symptoms of upper respiratory tract infection.

Considering the potential for widespread infection and transmission of influenza A virus in humans, public health authorities closely monitor human disease cases associated with influenza A virus infection. Few studies have confirmed that human-to-human transmission of avian influenza A virus is rare, inefficient, and does not occur sustainably due to long-term close contact with sick people without appropriate protective measures. However, because avian influenza A viruses have the potential to mutate and can easily acquire the ability to spread from person to person, close monitoring of human avian influenza infection and human-to-human transmission is extremely important for public health.

3. Signs and symptoms of human infection with influenza A virus

The signs and symptoms of human infection depend mainly on which subtype of avian influenza A virus is causing the infection. Infection of humans with the low pathogenic avian influenza virus generally shows only mild clinical symptoms and does not cause fatal injury. In the past, the signs and symptoms of low-pathogenic avian influenza virus infection in humans were mostly conjunctivitis and flu-like symptoms, such as fever, cough, sore throat and muscle aches, and patients often need to be hospitalized for treatment in order to relieve respiratory conditions. In contrast, people infected with highly pathogenic avian influenza virus will have clinical symptoms of varying severity, ranging from conjunctivitis alone, to severe respiratory symptoms accompanied by multi-organ illness (such as shortness of breath, dyspnea, pneumonia, acute respiratory distress, viral pneumonia, respiratory failure, etc.), and sometimes nausea, abdominal pain, diarrhea, vomiting, and neurological symptoms (altered mental status, epilepsy); More severe cases can even lead to death, especially infection with the H5N1 highly pathogenic avian influenza virus. Signs and symptoms alone are not sufficient for an accurate clinical diagnosis of human influenza A virus infection, as the symptoms of these diseases are likely to be caused by other pathogenic infections, including seasonal influenza A or B viruses, which are very likely to occur together.

(1) Identification of human infection with avian influenza A virus

Human infection with influenza A virus cannot be diagnosed by clinical signs and symptoms alone, but must be diagnosed with laboratory differential diagnosis. The diagnosis of avian influenza A virus infection is generally to collect a swab sample from the nose or throat of the patient in the first few days of illness and send it to a laboratory for analysis. Laboratories generally diagnose avian influenza A viruses by molecular identification, or by attempting to culture the virus, or by using both methods. It is important to note that the culture of influenza A viruses should only be carried out in laboratories with high levels of biosafety protection measures (P3 laboratories).

In critically ill patients, collecting and testing samples from their lower respiratory tract may diagnose the H5N1 highly pathogenic avian influenza virus. And for some patients who are no longer sick or have fully recovered, it may be difficult to find the avian influenza A virus from the collected samples by using the above methods. Sometimes, we can also indirectly diagnose avian influenza A virus infection by detecting specific antibodies produced by the body in response to the immune response caused by the virus. However, this is not the best option because it requires the collection of two blood samples (one from the first week of illness; In addition, this takes a few weeks to verify the results and must be performed in a specific laboratory, such as the Centers for Disease Control and Prevention (CDC).

(ii) Treatment of human infection with avian influenza A virus

Currently, the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) recommend the use of oseltamivir (Tamiflu) or zanamivir, two prescription anti-disease essay examples, for the treatment and prevention of human infection with avian influenza A virus. Analysis of the H5N1 highly pathogenic influenza virus currently circulating globally shows that the vast majority of viruses are susceptible to oseltamivir (Tamiflu) and zanamivir. However, some studies on oseltamivir (Tamiflu) resistance have shown that the H5N1 highly pathogenic avian influenza virus isolated in individual patients develops resistance to oseltamivir (Tamiflu). Therefore, it is important to closely monitor the resistance of influenza A viruses against the disease, and it is also helpful for the CDC and WHO to make recommendations for antiviral therapy.

(3) Prevention of human infection with avian influenza A virus

The best way to prevent infection with avian influenza A virus is to avoid contact with the source of infection. Most people infected with the avian influenza A virus occur in direct or close contact with infected poultry. Seasonal influenza vaccination does not prevent infection with avian influenza A virus, but it can reduce the risk of co-infection with avian influenza A virus. Although there has been no sustained human-to-human transmission of previously reported H5N1 HPAI virus, contact with patients with suspected or confirmed HPAI virus infection should be avoided. Healthcare workers should wear personal protective equipment and follow recommended infection control measures (including droplet, contact, and air precautions) when caring for patients with suspected or confirmed H5N1 HPAI virus infection.