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Avian Influenza

Its science and management

May 18, 2010
By Dr. Rob Porter DVM PhD Minnesota Veterinary Diagnostic Laboratory University of Minnesota


What is it? Avian influenza (AI) is an infectious vir-al disease, most
often associated with disease in poultry, and exhibiting a range of
clinical signs varying from none (subclinical), respiratory (cough,
head shake), depression, and lowered feed and water consumption, to
high mortality, depending on the virulence of the virus. In breeding
flocks there can be drops in egg production and hatchability that also
vary with the virulence of the virus.

What is it? Avian influenza (AI) is an infectious vir-al disease, most often associated with disease in poultry, and exhibiting a range of clinical signs varying from none (subclinical), respiratory (cough, head shake), depression, and lowered feed and water consumption, to high mortality, depending on the virulence of the virus. In breeding flocks there can be drops in egg production and hatchability that also vary with the virulence of the virus.

Diagnosis of LPAI is based on serology and virus isolation, to differentiate the infection from other diseases such as colibacillosis, Newcastle disease and infectious bronchitis.



AI virus is generally quite labile and is rapidly destroyed by most commercial disinfectants.

In nature, the virus is found most often in wild waterfowl and shorebirds, which serve as natural reservoirs by carrying and transmitting the virus, usually without showing clinical signs.

The virus exists in high pathogenicity (HPAI, ability to cause severe disease) and low pathogenicity (LPAI, mild or no disease) forms identified by both live bird inoculation and characterization of amino acid sequences at the hinge region of the hemagglutinin molecule.

HPAI is defined as a virus that is lethal for 6 of 8, 4- to 6-week-old chickens when injected intravenously with a 0.2 ml of a 1:10 dilution of allantoic fluid. Additionally, the pathogenicity of the virus can be assessed by the molecular sequence of basic amino acids at the hinge region of the hemagglutinin.

Why are there so many AI viruses?
Avian influenza (AI) viruses are characterized by the glycoproteins attached to the surface of the viral envelope. One glycoprotein is hemagglutinin (HA), of which 16 types (HA1-HA16) can be encoded by the viral genome. Hemagglutinin allows the virus to attach to the membrane of the host cell.

A second glycoprotein, neuraminidase (NA), of which there are nine types (NA1-NA9), facilitates the escape of newly formed viruses through the plasma membrane and out of the host cell. AI virus is primarily characterized by the H and N types on the virus surface. A single virus will have one H and one N on the surface. There are potentially 16 (HA) x 9 (NA) = 144 combinations of AI virus that occur in nature.

Hemmaglutinin plays a key role in the virulence of an AI virus. H must be cleaved to HA1 and HA2 by host proteases in order for the virus to be infectious. High pathogenicity AI (HPAI) viruses are cleaved by a wide variety of cells in the infected host because HPAI viruses have multiple basic amino acids at the carboxy terminus of HA-1 (hinge region). These multiple basic amino acids of HPAI virus are degraded by a wider array of host proteases and hence, more easily infect birds, spread through many cells of the body, replicate, and cause disease. HPAI virus can replicate in a large number of cells or organs in the host (systemic infection) while low pathogenic AI (LPAI) virus generally replicate only in the respiratory and digestive tract of poultry.

Antigenic shift and drift
Two mechanisms that AI virus uses to change form are by mutations in the viral genes (antigenic drift) and the ability to re-assort gene segments with other AI viruses (antigenic shift). These changes can be particularly important when hemagglutinin or neuraminidase gene expression are affected. Because antibody responses to the surface hemagglutinin can promote resistance to AI infection, changes in the hemagglutinin can permit the virus to evade the current humoral immunity of the host. In theory, point mutations in the hemagglutinin could convert a virus from low to high pathogenicity; this change in virulence is a rare event, but has been shown to occur. In addition, antigenic shift is characterized by sharing of gene segments between influenza viruses when a host is exposed to two or more antigenically unique viruses. Because the population has no protective immunity to the new virus, the virus can temporarily evade the immune response and spread quickly in the population. Antigenic shift is not a major issue in poultry because of the short-lived production cycles, but this phenomenon may be more significant in congregating wild waterfowl.

Why the concern about H5 and H7?
For nearly fifty years, all outbreaks of high pathogenicity AI have been H5 or H7 hemagglutinin subtypes; however, most H5 or H7 AI viruses are of low pathogenicity. Outbreaks of HPAI in the U.S. have occurred in 1920, 1983 and 2004. Antigenic drift (point mutations) of LPAI of H5 or H7 subtype virus that is circulating in a poultry flock could, in rare instances, convert the virus from low path to high path form, as was documented in field outbreaks in Pennsylvania.

The 1983 outbreak in Pennsylvania resulted in 17 million birds dying or being euthanized at a cost of $65 million dollars. The H5N2 isolate from the outbreak was traced to virus cycling in live bird markets in the north-eastern U.S.

Additionally, since 1997 attention has been directed to international outbreaks of H5N1 virus (bird flu as described below) associated with mortality in humans, poultry and wild birds. The World Organization for Animal Health or Office Internationale des Epizooties (OIE) has designated any LPAI virus of H5 or H7 subtype as low pathogenicity notifiable avian influenza (LPNAI). The OIE promotes science-based methods to maintain safety in international trade of animals and animal products, and requires reporting of any H5 or H7 virus isolated from poultry. Unfortunately, OIE reports of H5 or H7 LPAI in U.S. s poultry usually result in international embargos on U.S. poultry meat even though there is minimal risk of the product harbouring virus.

The OIE program, although well-intentioned, might discourage a poultry company from seeking diagnostic work (passive surveillance) or at least virus isolation on a flock showing respiratory signs or diminished egg production. Additionally, the public health sector must be reminded that most AI viruses, including those of H5 and H7 subtypes, pose no health risk to humans or to food safety; i.e., influenza in poultry and humans are usually unrelated. It has been reported that controlled vaccination against LPAI, including H5 and H7, can reduce the spread of virus and should be considered as part of a science-based AI control program for at-risk flocks, but government regulatory agencies have been slow to accept this approach for day-to-day prevention of AI in poultry, reserving vaccine use for control during an outbreak. Vaccination is considered of least importance for meat-type poultry; however, could play a role in protection of longer lived breeder birds as long as the other facets of a management/control program (see below) are prioritized.

The OIE website indicates that since 1997 the H5N1 infection has been described in domestic poultry and wildlife in sixty-two countries. It is likely that the extremely high concentration of poultry in Asian countries, poor biosecurity practices, flight patterns of migratory birds and covert movement of poultry from an infected region (in order to avoid testing and destruction of birds) has contributed to rapid spread of HPAI.

The World Health Organization website indicates that human cases of H5N1 have been described in Azerbaijan, Bangladesh, Cambodia, China, Djibouti, Egypt, Indonesia, Iraq, Mynamar, Nigeria, Pakistan, Thailand, Turkey and Vietnam.

There is currently no high pathogenicity H5N1 in the United States (North America). The U.S.D.A. and poultry industry remain vigilant to prevent this infection in commercial poultry.

Potential routes for high pathogenicity avian influenza to enter the United States (North America) are:

  1. On contaminated live poultry or poultry products
  2. Via migration of infected wild birds
  3. 3Via infected human beings
  4. Agroterrorism: unscrupulous and malicious intent of one or more individuals to introduce a virus, bacterium or toxin into the United States food supply.
  5. Measures taken to prevent the entry of HPAI into the country include:
  • Restrict imports of poultry and poultry products from all Asian countries
  • Birds entering country must be quarantined for thirty days and tested for AI and exotic Newcastle disease
  • Poultry meat is not imported from Asia because USDA-FSIS (equivalent to the CFIA in Canada) has not approved Asian poultry processing plants
  • Products containing feathers in finished form for sale (e.g., jackets, pillows, mattresses) do not require veterinary import permit, but must be inspected at port of entry to be free of manure, blood and skin.
  • U.S.D.A. has established regulations for AI surveillance of live bird markets
  • AI surveillance of wild migratory birds in Alaska flyway
  • Stock piling of AI vaccine for use in poultry during AI outbreak
  • Education of public on safe handling and cooking of poultry products
  • Promotion of biosecurity programs
  • National Poultry Improvement Plan and state AI surveillance programs

As of March 2010 there is no evidence of Asian H5N1 avian influenza (bird flu) in the United States. There is no evidence that avian influenza virus can be transmitted to humans through the consumption of properly cooked poultry products. U.S. poultry products remain safe and the reports regarding avian influenza outbreaks in birds and humans should not be used to promote fear and deprive the public of nutritious and affordable food products.

Poultry infected with LPAI virus may not show clinical signs, but still are capable or harbouring and transmitting virus to other birds. The LPAI virus can replicate in the respiratory and intestinal tracts of poultry, resulting in virus being shed in oculonasal secretions and manure. Free-flying aquatic birds, such as ducks, geese, gulls and shorebirds, are major reservoirs for a genetically diverse population of LPAI viruses.There is also evidence that the LPAI viruses from wild birds can infect domestic poultry; for example, range-rearing of turkeys and subsequent contact with wild birds was considered to be a significant source of LPAI infection in Minnesota. In most instances the source of AI virus in a poultry flock is rarely confirmed.

Sloppy Management Contributes
What can be stated with confidence is that sloppy management practices can contribute to the spread of AI between poultry flocks. Management practices that pose considerable risk include bird depopulation activities, association with live bird markets, manure disposal/spread, contact with contaminated equipment, transportation of infected birds, live bird disposal, and movement of vaccination and hauling crews. The virus is transmitted by direct contact between infected and susceptible birds and indirect contact including aerosol droplets or exposure to virus contaminated boots, clothing or equipment. Live bird markets are often associated with outbreaks.

Clinical Signs
Recent studies indicate that broiler chickens are more susceptible to infection with LPAI (chicken-adapted or wild bird adapted) than white leghorn chickens; however, clinical signs associated with LPAI are minimal or nonexistent. The largest LPAI outbreak in the U.S. involved 20 million broiler chickens from affected farms during the 2002 California outbreak of H6N2.

Poultry infected with low pathogenicity AI (LPAI) can show decreased egg production, respiratory signs (coughing, sneezing), or no clinical signs at all. Secondary infections with Escherichia coli can increase the flock mortality. HPAI can cause rapid death without clinical signs, or signs can involve the respiratory (cough, sneeze), nervous (paralysis, ataxia), digestive systems (diarrhea) and decreased egg production. Edema of the head and neck is commonly observed.

Poultry with LPAI may have no gross lesions or can have fibrinous exudate in the trachea, sinuses, air sacs and conjunctiva. The oviduct can be inactive, shrunken or contain fibrinous exudate. These respiratory tract lesions can resemble other infections that are commonly observed on poultry farms. With HPAI the gross lesions can be severe. The comb or wattle can be shrunken, ulcerated or cyanotic (purple). Edema of the face and feet along with hemorrhages on the shanks are commonly observed. Hemorrhages or fibrin can cover the pericardial sac, mesentery, air sacs, abdominal fat, trachea, intestine and oviduct. In addition, hemorrhage and necrosis can be observed in the cecal tonsils and proventricular glands.

Diagnosis of LPAI is based on serology (agar gel immunodiffusion test) and virus isolation, to differentiate the infection from other diseases such as colibacillosis, Newcastle disease and infectious bronchitis. HPAI will be diagnosed on the extreme clinical signs and must be differentiated from exotic Newcastle disease. Nine to ten-day embryonated chicken eggs are inoculated via the allantoic sac to cultivate virus. Polymerase chain reaction analysis can also be performed on cloacal (preferred) or oral swabs of ill or dead birds.

The most effective treatment for avian influenza virus infection is to prevent secondary bacterial infection. Antibiotic treatment has been used to reduce the effects of concurrent bacterial infections.

All AI management programs should at a minimum contain the following components:
1. Education: Educating poultry industry and allied industry employees on what AI virus is, how it is transmitted and the role that people, birds and contaminated equipment play in spreading the virus between should help to reduce the number of risky actions.

2. Biosecurity: Since AI virus is spread from infected bird to susceptible bird and by contaminated boots and equipment, strict biosecurity is important. In outbreaks involving high pathogenicity subtypes, eradication programs are used to control the disease. Carefully executed controlled marketing practices combined with routine flock testing can reduce or eliminate the spread of LPAI virus. Experience with several LPAI outbreaks in the U.S. show that we know how to spread AI virus between flocks; therefore, the true ability to learn from our mistakes is to practice just the opposite. Establishing barriers between live poultry markets and commercial poultry farms are vital. Risk factors for introduction of virus to flocks include service personnel,  catching crews, vaccination crews, employees (especially if they own birds), rendering facilities, feed trucks, egg pickup and processing (racks and crates going to different farms), shared equipment, and bird placements (spiking males, flock additions). On occasion, in cases of LPAI, use of a killed vaccine (autogenous) has been allowed. Routine vaccination as part of a control program is not permitted by the USDA.

3. Diagnostics and surveillance: Many state diagnostic labs offer pathology services, virus isolation, and antigen capture ELISA tests for diagnosis of AI infection, while labs that are part of the National Animal Laboratory Health Network offer real-time reverse transcriptase-polymerase chain reaction (RRT-PCR) assays for detection of H5/H7 subtypes of AI. Passive surveillance at the farm level is vital for early detection of, not only avian influenza, but other agents that can cause respiratory signs in broilers and broiler breeders as well as egg production drops in broiler breeders. Active surveillance programs for AI have been established through the National Poultry Improvement Plan. Active surveillance involves periodic blood sampling of meat-type breeders and blood sampling of meat-type birds either pre-slaughter or at the processing plant to detect antibodies to AI. Active laboratory surveillance will detect circulating LPAI H5/H7 virus in poultry in the absence of high mortality and can also detect any virus of the H5 or H7 subtype. The agar gel immunodiffusion test to detect antibodies (serology) is sensitive during the period that antibodies are circulating in the broilers and broiler breeders. The antibody titer rises to detectable levels in seven to 10 days after infection and declines after several months.

Presented at the 2010 Midwest Poultry Federation Conference.

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