Pathology of Avian Influenza (Bird Flu)

Dr Sampurna Roy MD

Avian influenza in birds:

It is an infection caused by avian (bird) influenza (flu) viruses.

These viruses occur naturally among wild birds worldwide in their intestines, but they do not get sick.

It is very contagious among birds including domestic birds eg. chickens, ducks and turkeys and kill them.

Virus are excreted from infected birds in their saliva, nasal secretions, and feces.

Birds including domestic ones  become infected through direct contact with other infected poultry, or through contact with infected surfaces (such as dirt or cages) or materials (such as water or feed).

It causes two main forms of diseases in poultry.

The "low pathogenic" form go undetected and  causes only mild symptoms (such as ruffled feathers and a drop in egg production).

The "high pathogenic" form spreads rapidly through flocks of poultry, affecting multiple internal organs and has a mortality rate that can reach 90-100% often within 48 hours.

Cases of Human infection :

Confirmed cases of human infection from several subtypes have been reported since 1997.

In most cases human infection have resulted from contact with infected poultry (Example: domesticated chicken, ducks, and turkeys) or surfaces contaminated with secretion/excretions from infected birds.

The spread from one ill person to another is very rare.

“Human influenza virus” are those subtypes that spread widely among humans. There are only three known A subtypes of influenza viruses (H1N1, H1N2, and H3N2).Viruses are constantly changing, and they adapt over time to infect and spread among humans.

Symptoms vary from typical human influenza-like symptoms (e.g., fever, cough, sore throat, and muscle aches) to eye infections, pneumonia, severe respiratory diseases (such as acute respiratory distress).

Avian Influenza A (H5N1):

Subtype Influenza A  “H5N1 virus”  that occurs mainly in birds, is highly contagious and are deadly to them. Infections with these viruses have also occurred man.

Human health risks during the H5N1 outbreak:

In the outbreaks in Asia and Europe more than half of those infected with this virus have died. Most cases have occurred in previously healthy children and young adults. 

So far, the spread of H5N1 virus from person to person has not been recorded. Because viruses have the ability to change, one day it could be able to infect men and spread easily from one person to another. In that case an influenza pandemic (worldwide outbreak of disease) could begin.

Avian Influenza A (H7N9) virus:

Infections with a new avian influenza A (H7N9) virus were first reported in China in March 2013. The first case outside China was reported in Malaysia  in February 2014.

By December 2013 there were nearly 139 confirmed cases identified in 10 provinces and 2 municipalities, primarily in eastern China.

Most of the patients with confirmed H7N9 virus infection were elderly (median age, 61 years).

Since this H7N9 virus appears to have emerged recently to infect humans, immunity is expected to be low, and persons of any age may be susceptible to infection.

Patients were hospitalized with severe lower respiratory tract illness.

On the basis of the experience with increased circulation of H5N1 virus among poultry during periods of cooler temperatures, the spread of H7N9 virus among poultry is likely to increase during the winter and spring months, with the potential for increased transmission to exposed persons.

Treatment:

The H5N1 virus that has caused human illness and death in Asia is resistant to amantadine and rimantadine, two antiviral medications commonly used for influenza. Two other antiviral medications, oseltamavir and zanamavir, would probably work to treat influenza caused by H5N1 virus, but additional studies still need to be done to demonstrate their effectiveness.

Antiviral drugs like Tamiflu and Relenza block the flu virus from escaping an infected cell and spreading further. But the drugs have to be taken within 48 hours of getting the flu.

H5N1 mutates rapidly and  has a history of being able to acquire genes from viruses infecting other animal species.

It can cause severe disease in humans ;  

Laboratory studies have demonstrated that isolates from this virus have a high pathogenicity and can cause severe disease in humans ;  

Birds that survive infection excrete virus for at least 10 days making it easier to spread the virus at live poultry markets and by migratory birds.