Introduction 

Most people know about the flu or the influenza virus, which affects millions of people each year, however, many people have not heard of ‘bird flu’ and how in some rare instances it can cause disease in humans. Predominantly affecting birds, avian influenza has also been reported to affect a small number of people with strains of the influenza A virus including the H5N1, H7N9, H5N6 and H5N8 and the most pathogenic being the H5N1 strain that has the potential to cause pandemics. The virus was first detected in Hong Kong in 1997 and currently, in the UK there were only 4 cases of human avian influenza from the year 2023 to 2024. Even with recorded outbreaks being very low, it is important to understand that this virus has the ability to cause severe disease. In some cases, it can even result in death, especially in winter months where conditions are favourable for viruses or in individuals with pre-existing respiratory conditions and even in those with lowered immunity. The only factor that prevents pandemics from occurring from this virus is the inability to spread from person to person.

What exactly is bird flu? 

Bird flu is an infectious type of influenza A virus that usually spreads among birds and poultry. The natural reservoirs of the infection are animals such as swans, ducks, geese, and waders.² The avian influenza can be split up into two types, either the low pathogenicity or high pathogenicity. The high pathogenicity virus (H5N1) is the strain that affects humans and it is associated with higher mortality in poultry and birds but also more severe symptoms in humans whereas low pathogenicity strains cause milder symptoms.¹ As mentioned in terms of the ability to cause a human pandemic, this disease fulfils the criteria of undergoing genetic diversification, everyone is vulnerable due to the lack of antibodies in their innate immune systems but it is limited by the lack of person-person transmission. The disease should be closely observed and alert measures should be put on by governments and organisations like WHO to prevent this from happening.

How does bird flu spread to humans?

The avian influenza virus can be found in the saliva, mucus or faeces of infected birds.In other animals, such as poultry, it can additionally be secreted through respiratory fluids, their milk, blood and even their organs. 

The transmission from birds to humans is rare but it can occur when humans come into contact with this virus through inhaling respiratory droplets, indirectly through inhaling fomites, or through contact with contaminated surfaces and then they proceed to touch their eyes, mouth or nose. Typically, the humans who develop the infection are those who spend a prolonged amount of time around infected birds or poultry. Therefore, markets around the world, which distribute and sell live birds, may be a source of the virus. Other atypical methods of transmission are from mother to baby or through self-inoculation from conjunctiva in the eyes to a respiratory illness.^2,4 

An incorrect assumption in regard to the transmission of this virus is that it can be acquired through eating cooked poultry or eggs.¹ The groups of individuals at higher risk of the disease are children between the ages of 5 to 9 years old and those with increased exposure due to their occupation. 

Symptoms of bird flu in humans

Symptoms that have been seen in humans can widely range from no symptoms to a severe form of the disease and may even result in death. Usually, the incubation period, which is the time it takes for symptoms to appear, is approximately 2-4 days and can be up to 17 days. In general, the symptoms last a week and are self-limiting.⁷

The most common symptoms of avian influenza are as follows: 

Flu-like symptoms 

The initial symptoms are similar to the annual influenza common in humans. 

• Fever: In most cases, this is usually the earliest symptom and the patient can complain of feeling hot or shivery. The fever is recorded as anything higher than 38 degrees Celsius. Although it affects most patients it is not always present  
• Cough: Persistent coughing often worsens over time and can lead to the development of a sore throat 
• Runny nose: Receptors of the avian influenza virus can be found on the nasal mucosa cells ²
• Muscle aches and intense fatigue

These symptoms would be considered non-specific and therefore can be difficult to differentiate from other respiratory tract illnesses.  

Respiratory symptoms 

The virus can invade the respiratory epithelial cells and even further into the respiratory tract. 

• Shortness of breath: The patient may have difficulty breathing and the lungs can undergo diffuse alveolar damage in the inflammatory phase of the virus. If the length of the infection is extended, then fibrotic changes in the lungs can occur. The virus can also cause hyperplasia of type II pneumocytes (alveolar cells)  in the lungs and the virus can reside within these cells. Typically the main immune cells found in the alveoli are macrophages
• Chest pain: Due to the damage to the lungs, the patient may experience pain especially when inhaling deeply
• Pneumonia: Radiographically, it can be presented as crackles upon inspiration, respiratory distress, rapid breathing, and infiltrates on X-ray. This can become severe in some patients and may require hospitalisation⁵

Acute respiratory distress syndrome: A widespread inflammation within the lungs means that there is a severe lack of oxygen and the patient has trouble breathing.²

Gastrointestinal symptoms

They are considered atypical or less common symptoms but may present in strains of the virus like HN51 and H7N9. They usually present in the early stages of the disease. 

• Nausea and vomiting: The patient can complain about abdominal pain with these
• Watery diarrhoea 

Neurological symptoms 

Although less common the virus can lead to:⁵

• Headaches 
• Seizures/convulsion 
• Brain dysfunction: it can present as an altered mental state 

Other symptoms

• Bleeding from nose and gums 
• Conjunctivitis: There are receptors for the virus present in the eye which the virus can bind to.⁶ This can lead to redness, itchy/ irritated eyes and a watery discharge⁷

Complications related to bird flu symptoms

Complications of the virus can include:¹ 

• Low levels of oxygen in the blood 
• Secondary infections: bacterial and fungal 
• Multiple organ failure 

Death: As most human beings do not have antibodies against this virus it can cause fatality where the cause is respiratory failure. This is especially the case in young children

Summary 

Bird llu is an infectious type of influenza A that usually spreads among birds and poultry and in rare instances amongst humans. The virus was first detected in Hong Kong in 1997 and currently, in the UK, there were only 4 cases of human avian influenza from the year 2023 to 2024. The strains of the virus include H5N1, H7N9, H5N6 and H5N8 with the most pathogenic being the H5N1 strain that has the potential to cause pandemics. Symptom recognition is important as it can present in a similar way to influenza. Bird flu is transmitted to humans via the saliva, mucus or faeces of infected birds and even through poultry.  The groups of individuals at higher risk of the disease are children between the ages of 5 to 9 years old and those with increased exposure due to their occupation. Symptoms include flu-like symptoms like fever, cough, sore throat muscle pain, respiratory symptoms like shortness of breath, chest pain, pneumonia, acute respiratory distress syndrome, gastrointestinal symptoms including nausea, vomiting and diarrhoea, neurological symptoms like headaches, seizures/convulsions or brain dysfunction and also in some cases, there can be bleeding of nose and gums and conjunctivitis. In severe conditions, it can lead to low levels of oxygen in the blood, secondary infections, multiple organ failure and even death.  Currently, there are no vaccines against bird flu and, as antivirals are less effective due to resistance, relying on prevention strategies like proper hand hygiene, isolation from others, and sanitation procedures when handling birds is key.