Due to the possibility that bird flu viruses could mutate and gain the ability to spread easily between people, avian influenza poses a significant pandemic threat to birds and humans alike. The U.S. Centers for Disease Control and Prevention (CDC) has been monitoring for illness among humans exposed to infected birds since outbreaks were detected in poultry and wild bird populations in late 2021-2022. As of July 5, 2022, the U.S. Department of Agriculture (USDA) reported 40.09 M birds in 36 U.S. states to be infected with highly pathogenic avian influenza (HPAI). Out of the 382 flocks to test positive for the virus, 186 were commercial and 196 were backyard flocks. Yet, only 2 human cases associated with this particular group of avian influenza have been detected globally—1 in the U.S. and the other in the U.K. Understanding the history of this virus, as well as current guidelines and recommendations for investigations and response (i.e., monitoring for human infection and person-to-person spread) are extremely important for public health.
History of Highly Pathogenic Avian Influenza
It was 1878 in northern Italy, and veterinarian Edoardo Perroncito observed a problem. A contagious respiratory disease was spreading among poultry with a high fatality rate. At the time, “fowl cholera,” a deadly respiratory disease, caused by the bacterium Pasteurella multocida, was common and believed to be the culprit of the disease. As other investigators studied Perroncito’s findings throughout the 1900s, they determined that the disease was caused by a virus small enough to pass through a filter, which was later determined to be a highly pathogenic avian influenza (HPAI) and was officially renamed as such in 1981.
Advertisement offering treatment for birds with Fowl Cholera, 1890.
In 1996, things changed. A new strain of HPAI subtype H5N1 was introduced to poultry via migratory waterfowl in the Guangdong province of China, resulting in a mortality of greater than 40% in aquatic birds. Subsequent reassortment events between HPAI and low pathogenic avian influenza strains (LPAI) led to an HPAI strain that was disastrous for Hong Kong, resulting in the culling of the entire poultry population, 18 human infections and 6 human deaths. This was the first time a lethal case of HPAI in humans had been documented; human infection with HPAI was previously believed to be improbable.
After the outbreak of HPAI in Hong Kong in 1997, things remained quiet until 2003 when 2 humans were diagnosed with HPAI H5N1 after returning from China. From 2003-2018, human cases were reported from 16 countries resulting in hundreds of deaths.
Clinical Picture in Animals and Humans
Symptoms and Transmission of Avian Influenza in Birds
Fortunately, many avian influenza A viruses exhibit low pathogenicity, causing few signs of disease in infected wild birds. Low pathogenic avian influenza (LPAI) viruses are either asymptomatic or cause mild/subclinical disease (such as ruffled feathers and a drop in egg production) in chickens and poultry.
However, these viruses are primed for continual emergence and pandemic potential, and some low-pathogenic viruses can mutate in poultry into highly pathogenic avian influenza viruses. HPAI viruses often cause severe disease and high mortality in infected poultry. HPAI A(H5) or A(H7) virus infections can cause disease that impacts multi-organ systems with mortality as high as 90-100% in chickens, often within 48 hours. HPAI A(H5N1) is one of the most contagious viruses occurring among birds, and the clinical outcome is typically deadly, especially in domestic poultry. However, ducks can be infected without any signs of illness.
HPAI A(H5) and A(H7) virus infections in poultry can also spill back into wild birds, resulting in rapid geographic dissemination of the virus with bird migration. While some wild bird species can be infected with some HPAI A(H5) or A(H7) virus subtypes without appearing sick, other HPAI A(H5) and A(H7) virus subtypes can cause severe disease and mortality in some infected wild birds, as well as in infected poultry.
Transmission of Avian Influenza from Birds to Humans
When birds are infected with this virus, they shed bird flu virus through their saliva, mucous and feces. Human infections are likely caused by accidental inoculation of virus into a person’s eyes, nose or mouth or via respiratory routes. Inhalation of virus occurs when the virus is in the air (in droplets or possibly dust) and a person breathes it in, or possibly when a person touches a fomite (contaminated object) that has virus on it then touches their mouth, eyes or nose. No human bird flu infections have been reported from proper handling of poultry meat or from eating properly cooked poultry or poultry products.
Bird flu transmission from human–to-human is very rare, and when it has happened it has only spread to a few people. Human infections with bird flu viruses have ranged in severity from asymptomatic/mild illness to severe disease resulting in mortality. Asian lineage H7N9 and Asian lineage H5N1 viruses have been responsible for most human illness from bird flu viruses globally, including the most serious illnesses and illness with the highest mortality.
Research has also reported that companion animals, especially cats, may be at risk of disease and/or death from HPAI. Dogs may also be susceptible, but only in rare instances. Some strains of the virus can affect mammals, such as pigs, cats, horses, dogs and ferrets.
Symptoms of Avian Influenza in Humans
What does a bird flu infection in people look like? The reported signs and symptoms of this type of infection in humans range from asymptomatic to mild (e.g., conjunctivitis/red eye) or mild, flu-like (e.g., upper respiratory symptoms), to more severe (e.g., pneumonia requiring hospitalization). Fever (temperature of 100ºF [37.8ºC] or greater), cough, sore throat, congestion, achiness, headaches, fatigue and shortness of breath or difficulty breathing are common. Diarrhea, nausea, vomiting or seizures may be observed more rarely.
One Health Epidemiology for Tracking and Prevention
The One Health initiative is a collaborative, multisectoral and transdisciplinary approach—working at the local, regional, national and global levels—with the goal of achieving optimal health outcomes, recognizing the interconnection between people, animals, plants and their shared environment.
HPAI is a 1 of many examples that illustrates the ongoing and critical importance of a One Health initiative for zoonotic and/or vector borne diseases that intersect animal and plant health, environmental space and human health. A recent Health Alert Network (HAN) CDC Advisory for Highly Pathogenic Avian Influenza A(H5N1) Virus: Recommendations for Human Health Investigations and Response was released for multiple entities and personnel.
For full guidelines and descriptions of the recommendations, please see the CDC HAN Advisory.
Current State of Highly Pathogenic Avian Influenza (HPAI) and Prevention
The greatest risk of infection is for individuals who directly handle or work around poultry. This is an exceptionally important risk to consider, given that the COVID-19 pandemic and fears of food shortages contributed to an exponential increase in backyard poultry ownership. The CDC has developed individual guidance documents for various audiences, which include hunters, the general public, poultry producers (including those who have backyard poultry), poultry outbreak responders and healthcare providers.
Early in the summer of 1999, county health departments across New York began to report an alarming and widespread uptick in the number of dead birds found on the ground. Between May and November, there would be more than 15,000 dead birds reported. What caused this die-off? Read the article, featured in the Winter 2021 issue of Microcosm, our flagship member magazine. Now available for open access viewing.
Andrea Prinzi, Ph.D., MPH, SM(ASCP) is an infectious disease medical science liaison and works to bridge the gap between clinical diagnostics and clinical practice.