Disease transmission – an introduction

Transmission is a process in which a pathogen leaves its primary host, travels through suitable media, and finally enters and infects a new, susceptible host. It is a cyclic mechanism by which a pathogen ensures the survival of its progeny.

A generic transmission cycle consists of the following elements:

1. The causative organism (pathogen)
2. Host organism
3. Transmission media – these may include air, water, soil, direct physical contact, sexual contact, transmission through body fluids (such as blood), droplets and faecal-oral transmission
4. Susceptible organism (potential future host)

What is bird flu?

Avian influenza, more commonly known as bird flu, is an infectious viral illness that primarily affects birds. It is caused by the influenza virus, which are member of the family Orthomyxoviridae, and is classified as a negative-sense single-stranded RNA virus.^1,2 The influenza virus species consists of four main subspecies – A, B, C, and D. Among these, type A influenza is known to infect birds and some mammals.² Certain strains of the influenza A virus are also known to cause the seasonal flu in humans.³

A brief history of bird flu

The first documented case of bird flu (avian influenza) was first identified by Edoardo Perroncito in northern Italy in 1878.⁴ It was initially described as a contagious, poultry-related disease of high mortality – initially being referred to as “fowl plague”.⁵

Transmission

Virus source and avian transmission

Waterfowl, namely orders Anseriformes (eg. ducks, geese, etc.) and Charadriformes (eg. gulls, etc.) are known to be natural reservoirs of influenza A virus.^6,7 These birds subsequently carry and spread pathogens over large distances during annual migrations.

Infected birds shed viruses through body fluids, secretions and waste such as saliva, mucous and droppings – transmission occurs when other birds come into contact with these biological secretions. The disease spreads to poultry when birds such as chickens come in contact with infected waterfowl, and infection subsequently spreads among other poultry birds from there on.⁴ 

Transmission to humans

The H5N1 and H7N9 influenza A variants are the most common cause of bird flu in humans.⁸ Transmission to humans is a relatively rare phenomenon – the Centre for Disease Control describes it as a “low” public health risk. In the US, 14 human cases of H5 bird flu have been reported since March 24th, 2024.⁹ Strict surveillance of this disease is done to keep track of any mutations which might pose a greater risk to human health.¹⁰

Human transmission of avian influenza A virus may occur through two main routes – from direct exposure with secretions from an infected bird, or through intermediate hosts (other animals). Transmission may occur when the virus enters the human body through oral, respiratory routes, or through the eyes.¹¹ High-risk individuals for bird flu include people who are in close contact with birds, for example, people who work with poultry in farms or coops. Bird flu rarely shows human-to-human transmission.^8,10

Human bird flu cases have not been reported from the consumption of properly handled poultry or eggs.^12,13 The United States Department of Agriculture however, recommends four important basic precautionary measures when food is handled – clean, separate, cook, and chill.¹³

Symptoms in affected humans

Although rare, bird flu has a relatively high fatality rate. Among a total of 258 confirmed cases of H5N1 bird flu cases between January 1st 2003 and July 19th 2024, 141 cases were fatal – a fatality rate of 55%.

The following symptoms are observed in infected humans.¹⁰

• Sore throat, cough, fever, stuffy/runny nose
• Nausea
• Fatigue, muscle aches
• Conjunctivitis (pink eye)
• Dyspnea (shortness of breath)

Diagnosis

Bird flu is usually diagnosed by testing nose/throat swab cultures.¹⁰

Management

Vaccination for birds is possible against certain strains of influenza A.¹⁴ Management of avian influenza outbreaks in birds primarily includes infection monitoring, biosecurity, and mass culling of suspected birds.

In humans, confirmed cases of bird flu are treated using antiviral medication such as oseltamivir or zanamivir.¹⁵

Summary

Although rare, bird flu is a potentially life-threatening condition should a human get infected from another species. Chances of infection may be minimised by precautionary measures and maintaining basic hygiene – regularly wash hands with soap and water (with proper hand-washing technique), use separate utensils for raw and cooked meat, ensure poultry and eggs are properly cooked, avoid contact with birds, or their secretions.¹⁵ Proper protection and care must be taken if poultry/farmwork is to be undertaken – this involves the use of gloves, masks, and protective eyewear.¹⁰