What Veterinarians Need to Know About Avian Influenza

Avian influenza (AI), also known as bird flu, is a highly contagious respiratory illness affecting both wild and domestic birds. It belongs to the type A influenza virus family, the same type that causes influenza in humans. Genetic and antigenic differences exist between the influenza A virus subtypes that typically infect only birds and those that can infect birds and people.  

Related: Avian Influenza and Highly Pathogenic Avian Influenza 

Avian influenza etiology and pathology 

Type A influenza viruses that primarily infect birds (avian influenza viruses) are divided into subtypes based on a combination of two proteins surface proteins: hemagglutinin (H), which has 16 recognized types (H1–H16), and neuraminidase (N), which has nine (N1–N9). 

Avian influenza A viruses are classified into the following two categories based on their ability to cause disease in poultry: 

  • High Pathogenicity Avian Influenza (HPAI): These strains cause severe illness in birds, often leading to high mortality rates (up to 100%) within 48 hours. HPAI viruses can cause internal bleeding, swelling, and extensive organ damage. 
  • Low Pathogenicity Avian Influenza (LPAI): These strains cause little (such as ruffled feathers and a drop in egg production) to no clinical signs in infected birds. However, LPAI viruses can mutate into HPAI strains, posing a significant threat to poultry production. 

Both HPAI and LPAI viruses can spread rapidly through poultry flocks.  

In the United States, most outbreaks of avian influenza have been associated with LPAI strains. These strains may cause mild disease in poultry. However, outbreaks also have occurred involving HPAI strains. These strains, many of which are harmless in wild birds, can cause severe disease and death in poultry. HPAI can rapidly devastate entire flocks, result in major economic losses, and potentially infect people and other animal species (AVMA.org). 

Current landscape of avian flu 

Avian influenza continues to be a significant threat to poultry health and economic stability worldwide. There are several predominant circulating strains. 

  • H5N1: This highly pathogenic avian influenza (HPAI) subtype remains the primary concern. Since late 2021, there have been numerous outbreaks of H5N1 affecting poultry farms and wild bird populations across various regions, including Europe (France, Germany, and the Netherlands), the United States, Asia (China, India, and Japan), and Africa (Egypt and Nigeria).  
  • Other strains: While H5N1 dominates the current landscape, other strains, such as H7N9, have also caused sporadic outbreaks in certain regions.  

Changes in epidemiology 

There are several concerns regarding the new developments and changes in the epidemiology of H5N1. These include: 

  • The prolonged persistence and wider geographical distribution of H5N1 compared to previous outbreaks. This trend raises concerns about potential mutations and adaptations of the virus.  
  • The wild bird populations, particularly waterfowl and shorebirds, which are the major reservoirs for avian influenza, continue to increase and spread in many regions of the world. The continuous increase in wild bird populations makes control efforts difficult and emphasizes the need for biosecurity protocols for chicken operations. 
  • The potential for zoonotic transmission of H5N1. Veterinarians should be fully aware of the zoonotic potential of the current H5N1 strain. While it primarily affects birds, there have been recorded cases of spillover infection to humans working closely with infected poultry. Appropriate precautions to minimize occupational exposure should be undertaken.  
  • The ability of the influenza virus to mutate calls for continued surveillance to monitor for the emergence of new strains with altered virulence or zoonotic potential.  

What are the clinical symptoms of avian influenza? 

Early detection of avian influenza is a critical step in preventing its transmission. Various body systems (respiratory, digestive, and nervous systems) can be affected, resulting in a wide array of clinical signs.  

Table 1. Clinical Symptoms of Avian Influenza 

Poultry with LPAI Poultry with HPAI 
Mild respiratory symptoms (nasal discharge, sneezing) Decreased feed consumption  Ruffled feathers Decreased egg production Asymptomatic Sudden mortality without prior symptoms Lack energy Inappetence Purple discoloration or swelling of various parts of the body Petechiae or ecchymoses, on the non-feathered portions of the feet and legs Decreased egg production, or soft-shelled/misshaped eggs Respiratory symptoms (nasal discharge, coughing, or sneezing) Incoordination Diarrhea 

HPAI in companion animals 

While HPAI infections in cats and dogs have not been extensively described, laboratory confirmation is necessary to arrive at a definitive diagnosis. Most of the symptoms, such as fever, lethargy, inappetence, conjunctivitis, dyspnea, neurologic signs, and death, are commonly encountered in other respiratory diseases affecting cats and dogs.   

HPAI should be on the list of differential diagnoses in any dog or cat presented with respiratory signs and a recent history of bird contact, especially dead birds or those from confirmed HPAI outbreaks. 

Spread and transmission of avian influenza 

Highly contagious avian influenza spreads primarily through two main routes: 

  • Direct contact: Infected birds shed the virus through their respiratory secretions (sneezing, coughing) and droppings. Healthy birds become infected when they come into direct contact with these contaminated materials or with the infected birds themselves. 
  • Indirect contact: The virus can survive for extended periods in the environment, particularly in cool, moist conditions. Contaminated equipment, cages, clothing, and vehicles can harbor the virus and introduce it into new flocks. Additionally, contaminated water sources and feed can also be sources of infection for birds. The virus can be carried on the feet and feathers of wild birds, potentially spreading it over long distances. 

Bird-to-bird transmission 

The primary culprit for bird-to-bird spread is contact with infected bird excrement. Airborne secretions, however, play a significant role within enclosed environments like poultry houses. This highlights the importance of proper ventilation and minimizing overcrowding to reduce airborne transmission risk. For flocks with outdoor access, wild waterfowl droppings pose a particular threat, potentially introducing the virus into domestic flocks. 

Farm-to-farm transmission of avian flu 

While long-distance airborne transmission of avian influenza between poultry farms is uncommon, the primary cause of spread remains the movement of infected birds and contaminated materials.  

  • Introducing infected birds into a new flock is the most direct route for the virus to spread. 
  • People working with infected birds can unknowingly carry the virus on their clothing, boots, or vehicles, transmitting it between farms. Strict biosecurity measures are essential to prevent this. 

While long-distance airborne spread between farms is considered unlikely, it’s not entirely impossible. Here’s why: 

  • Short-range airborne transmission: The virus can potentially travel short distances through the air, particularly within densely populated poultry regions. 
  • Environmental factors: Sunlight and drying do deactivate the virus, but their effectiveness depends on weather conditions. In cool, moist environments, the virus can survive longer in the air, posing a higher risk for short-range airborne spread. 

Therefore, biosecurity measures should not solely rely on distance as a barrier. Implementing strict protocols for bird movement, equipment disinfection, and personnel hygiene remains essential to prevent the spread of avian influenza between farms. 

Are indoor birds at risk of HPAI? 

While the presence of HPAI in a region may raise concern, indoor birds generally face a lower risk of contracting the virus compared to their outdoor counterparts. This is because strict biosecurity measures in well-maintained housing facilities can significantly reduce exposure to infected birds or contaminated materials. 

However, for birds housed outdoors, the risk of encountering the virus increases. Numerous HPAI cases have been documented in backyard flocks in the United States, highlighting the vulnerability of these birds. Similarly, outdoor pet birds, zoo birds, and birds in captive wildlife facilities require additional protection. Measures to minimize contact with wild migratory birds, especially waterfowl and shorebirds, are of optimum significance. This includes preventing access to areas frequented by these birds and their droppings, and any water sources they may use. 

Zoonotic potential of HPAI 

Highly pathogenic avian influenza viruses (HPAI) can, in rare cases, be transmitted from birds to humans. This typically occurs through extensive and unprotected contact with infected birds, such as during slaughter, de-feathering, or close handling in live bird markets. 

Public health officials are particularly concerned about the possibility of viral mutation or combination with other influenza viruses to form a new strain. If an HPAI virus acquires the ability to efficiently transmit from person to person, it could pose a significant pandemic threat. This is why the subtypes most commonly associated with human infections, H5N1 and H7N9, are closely monitored. 

In April 2024, the World Health Organization (WHO) was notified about a laboratory-confirmed case of human infection with an influenza A (H5N1) virus by the United States of America IHR National Focal Point (NFP). The patient had a history of exposure to dairy cows presumed to be infected with the influenza A (H5N1) virus.  

This is the second confirmed human case of influenza A (H5N1) detected in the country. It also appears to be the first human infection with A (H5N1) acquired from contact with an infected mammal. The patient works on a dairy farm where the H5N1 virus has been identified in cows. As of May 29, 2024, the U.S. Department of Agriculture (USDA) has confirmed highly pathogenic avian influenza (HPAI) in dairy herds in nine states. 

However, despite these cases, WHO assesses the risk of infection in occupationally-exposed persons to be low-to-moderate, and low for the general population. 

Avian influenza from dairy cows 

The emergence of H5N1 in cattle raises a new concern for animal health officials. Genetic and epidemiological data suggest the virus spilled over from wild birds to dairy cows, with some instances of secondary spread between dairy farms and even to nearby poultry facilities. 

Laboratory analysis reveals a significant finding: the complete genome sequence of the virus found in infected cattle herds across eight states is identical to the strain affecting commercial poultry flocks in two separate states. This genetic similarity suggests a potential link between the outbreaks in cattle and poultry. 

While the exact transmission pathways remain under investigation, one key factor is the high concentration of the virus shed in infected cows’ milk. This raises concerns about the potential spread through contact with unpasteurized milk, spilled milk, or contaminated equipment and surfaces. Animals, vehicles, and other objects exposed to these materials could become fomites and contribute to the spread of the virus. 

Symptoms of avian influenza in cattle

Based on data from APHIS-USDA, as of late March 2024, H5N1 was detected in milk and other bovine-origin samples associated with mild illness in dairy cattle. The virus has been characterized as highly pathogenic. Pathways of disease transmission are still being investigated, and lateral transmission has been documented. The estimated herd level incubation period in dairy cattle appears variable, ranging from approximately 12 to 21 days.  Several factors in incubation are to be considered, including the route of exposure, viral dose, the production phase of the animal, and likely other factors that are still unknown at this time. 

Infected cattle may be asymptomatic or clinical. Symptoms may include: 

  • A decrease in feed consumption with a simultaneous decrease in rumination and rumen motility 
  • Respiratory signs, including clear nasal discharge 
  • Subsequent acute drop in milk production  
  • Abnormal tacky or loose feces 
  • Lethargy 
  • Dehydration 
  • Fever 
  • May have thicker, concentrated, colostrum-like milk or produce no milk at all

Diagnostic tests for avian flu 

Several tests are used for avian influenza screening and confirmation in birds. These include  

  1. Antigen capture immunoassay (ACIA)  
  1. Direct RNA detection that involves real-time reverse transcriptase polymerase chain reaction (rRT-PCR) 
  1. Isolation of the virus  

How to prevent the spread of avian influenza 

Strong biosecurity measures constitute the primary line of defense against the introduction and spread of avian influenza in all bird populations– companion birds, commercial poultry flocks, and backyard flocks. 

Precautions for veterinarians 

The American Veterinary Medical Association (AVMA) outlines specific protocols for veterinarians when presented with a bird suspected of having HPAI: 

  • Minimize exposure: Ideally, isolate the bird in an examination room or examine it outside the clinic altogether to limit potential contamination. 
  • Restricted access: Only essential staff should have contact with the bird, and they must wear appropriate Personal Protective Equipment (PPE). This includes a gown, gloves, face shield or safety goggles, and an N95 respirator (or a surgical mask as a last resort). Disposable head/hair covers are also essential. 
  • Biosecurity measures: Thorough hand hygiene after glove removal and disinfection of all contaminated surfaces are very important. Disinfect items you cannot throw away using appropriate protocols. 
  • Reporting and disposal: Immediate reporting to the USDA and state authorities is mandatory. Depending on the situation, they may direct sample collection for testing. Veterinarians should consult with local health officials regarding the safe disposal of contaminated waste, ensuring it’s inaccessible to wild animals. 
  • Client education: It’s vital to advise the owner on quarantining the bird and any potential contacts upon returning home. 

The World Organization for Animal Health (OIE) classifies avian influenza as a notifiable disease. Immediately report suspected cases of HPAI in any bird, or of H5 or H7 LPAI in poultry, to the Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture (USDA) and the applicable state animal health official(s). (AVMA.org)