Avian Influenza

The family Orthomyxoviridae contains the influenza viruses, which are divided into three types (A, B and C). Influenza A virus is the only type that is considered of veterinary importance, and this virus has been recovered from a wide range of hosts, including humans, other mammals and birds. It has been associated with respiratory disease in a number of mammals and avian species. Influenza B and C have primarily been recovered from humans, so it is influenza A that we are concerned with when it comes to disease involving birds and humans.

To understand the technical information associated with an outbreak, we must have a basic understanding of the identification schemes used to identify the influenza A virus isolates. One common identification scheme is based on the variances in the hemagglutinin (H) and neuraminidase (N) proteins found on the surface of the virus. Currently, 14 different hemagglutinin proteins and 9 different neuraminidase proteins have been detected. Individual sub-types of the influenza A viruses found in birds can be composed of any mixture of one of the hemagglutinin and one of the neuraminidase proteins. For example, the current outbreak of influenza A in Thailand, Cambodia, China, Indonesia, Japan, South Korea, Laos, Vietnam and Hong Kong is being classified as H5N1. The H5N1 strain is considered to be a very virulent, highly pathogenic strain of this virus. This strain has caused serious symptoms and fatalities. The influenza A virus outbreak currently identified in British Columbia is H7N3.

Another identification scheme used has five parameters: the type of virus, host from which the virus was originally recovered, geographic location from which the original isolate was made, reference number and year of isolation. So, A/duck/Ireland/113/84 (H5N8) is a type A influenza that was recovered initially from ducks in Ireland in 1984, and the isolate contains the number 5 hemagglutinin and the number 8 neuraminidase protein.

The hundreds of subtypes of influenza A viruses that have been isolated from free-ranging birds, domestic poultry, humans, swine and horses are related. This group of viruses undergoes constant change, resulting in the frequent appearance of new serotypes to which a population of hosts do not have immunity. This is why "flu" epidemics are common year after year. Large congregations of migratory birds, particularly waterfowl, are thought to serve as reservoirs for the virus.

In 2003, there was an outbreak in the Netherlands with H7N7 avian influenza A, causing symptoms of upper respiratory disease and conjunctivitis. This outbreak of avian influenza A H7 occurred mostly among poultry workers. H7 viruses are not as dangerous as the H5N1 viruses that are currently causing serious problems in Asia at this time. The H7 avian influenza A viruses are not nearly as dangerous as some of the other strains that are not H7. The H7 viruses seem to cause less serious disease symptoms, and it has primarily caused illness among poultry workers.

It should be noted that the use of personal protective equipment is mandatory for all persons involved in poultry culling activities, which should minimize infection in poultry workers. Human infection with the H7 avian influenza viruses is rare, however persons who have had close contact with infected birds may become infected and exhibit symptoms, most commonly conjunctivitis and/or upper respiratory symptoms. The risk of infection to poultry workers is low, especially when persons wear appropriate personal protective equipment and follow standard depopulation procedures when in contact with infected birds.

The reported symptoms of avian influenza in humans have ranged from typical influenza-like symptoms (cough, sore throat, fever and muscle aches) to eye infections, pneumonia, acute respiratory distress, viral pneumonia and other severe and life-threatening complications.

Diagnosis is made by using certain tests, such as reverse transcription-polymerase chain reaction (PCR) and cell culture of material obtained from nasal swabs. An antiviral medication, oseltamivir, TamifluTM, Roche Laboratories, is used as a treatment for influenza A and B, and as prophylaxis within 48 hours of exposure, to help treat a person exposed to or infected with this virus. Studies to date suggest that these prescription medications approved for human influenza strains would be effective in preventing avian influenza infections in humans. However, sometimes flu strains can become resistant to these drugs and so they may not always be effective.

Two poultry workers in the Fraser Valley region of British Columbia were confirmed by testing. Clinical signs were unilateral (one sided) conjunctivitis and upper respiratory symptoms in one patient, and unilateral conjunctivitis and headache in the second. There is currently no evidence of person-to-person transmission of avian influenza in this outbreak.

In the United States in 2002, Virginia experienced an outbreak of avian influenza A (H7N2) in which 4.7 million turkeys and chickens were destroyed. One culler developed upper respiratory signs and was subsequently tested and found to have antibodies to avian influenza A (H7N2). Nasal swabs and other specimens suitable for viral isolation were not collected, and therefore virus isolation could not be performed.

The current outbreak of avian influenza A H5N1in Asia has resulted in human cases in Vietnam and Thailand. It has killed 30 of the 42 Southeast Asians it has infected in the past year, as well as millions of chickens and wild birds across Asia. It has also infected pigs, housecats, and even zoo tigers. At this time, it is believed that these cases resulted from contact with infected birds or surfaces contaminated with excretions from infected birds. An investigation is ongoing to determine the source of human infections in Asia.

The latest reported death in Vietnam from avian flu was a 14 month old who got sick August 28 and died September 5.

A handful of cases of human-to-human transmission may have occurred during bird flu outbreaks in Hong Kong in 1997 and in Europe last year, but neither has resulted in a pandemic.

Influenza A H5N1 is a subtype of the Type A influenza virus. Wild birds are the natural hosts of the virus, hence the name avian influenza or bird flu. The virus was first isolated from birds (terns) in South Africa in 1961. The virus circulates among birds worldwide. It is very contagious among birds and can be deadly to birds, particularly domesticated birds like chickens. Influenza A does not typically infect humans. However, in 1997, the first instance of direct bird-to-human transmission of H5N1 was documented during an outbreak of avian influenza among poultry in Hong Kong; the virus caused severe respiratory illness in 18 people, resulting in six deaths. Since that time, there have been other instances of H5N1 infection among humans. But so far, H5N1 viruses have not been capable of efficient human-to-human transmission. This is something that is being watched carefully and is being investigated during this outbreak.

The key to containing an outbreak is by culling sick and exposed birds. This has been thought to have been crucial to averting many more human cases. For the current outbreak in Asia, the governments are culling poultry to try to contain the virus. Patients are being treated and isolated (although human to human transmission has not been documented to have occurred). Investigations are underway to determine the source of the infection.

The virus can mutate (change) and it could change to infect humans and could spread easily from person to person if that happens. Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. IF an avian influenza virus were able to infect people and gain the ability to spread easily from person to person, an "influenza pandemic" could begin.

An influenza pandemic is a global outbreak of influenza and occurs when a new influenza virus emerges, spreads and causes worldwide disease. Past influenza pandemics have led to high levels of illness, death, social disruption and economic loss. There were three pandemics in the 20th century. All of them spread worldwide within one year of being detected. They are:

Once a pandemic influenza virus emerges and spreads, it typically becomes established among people and circulates for many years. The CDC in the United States, and the World Health Organization (WHO) conduct extensive surveillance programs to monitor the occurrence of influenza activity worldwide, and follow the emergence of potential pandemic strains of influenza virus. Pandemic influenza virus infections ARE spread person to person, and not bird to person.

The CDC (Centers for Disease Control) advises travelers to countries in Asia with documented H5N1 outbreaks should avoid poultry farms, contact with animals in live food markets, and any surfaces that appear to be contaminated with feces from poultry or other animals.

Certain water birds act as hosts of influenza viruses by carrying the virus in their intestines and shedding it. Infected birds shed virus in saliva, nasal secretions and feces. Avian influenza viruses spread among susceptible birds when they have contact with contaminated nasal, respiratory and fecal material from infected birds; however fecal-to-oral transmission is the most common mode of transmission.

Most influenza viruses cause no symptoms, or only mild ones in wild birds. However, the range of symptoms in birds can vary greatly, depending on the strain of the virus and the type of bird. Infection with certain avian influenza A viruses (for example some H5 and H7 strains) can cause widespread disease and death among some species of wild and especially domesticated birds such as chickens and turkeys.

Signs of avian influenza are extremely variable. In some flocks, the only evidence of the infection is seroconversion (the birds develop a detectable antibody titer to AI). AI can also manifest as respiratory, enteric, reproductive or nervous system disease. Decreased food consumption and a drop in egg production are among the earliest and most predictable signs of disease. Signs can include coughing, sneezing, ruffled feathers, swollen heads, nervous signs such as depression, and diarrhea may occur. In some cases, with very virulent strains, birds may die rapidly without clinical signs of disease.

Investigations into the prevalence of influenza A viruses in captive birds are limited; however, there is potential for many avian species to be infected by the virus. Most species of birds of all ages are considered susceptible to influenza A viruses. The morbidity and mortality associated with an infection vary widely with the species of bird and the strain of the infecting virus. Birds infected with the less virulent strains of the virus usually remain asymptomatic, while highly susceptible birds infected with virulent strains may develop clinical signs and die. Many isolates of influenza A virus are pathogenic in one species and apathogenic in others. Because influenza A viruses are commonly recovered from asymptomatic birds and the clinical changes induced in experimentally infected birds may be less severe than those that occur in natural outbreaks, many of the clinical changes that are attributed to the virus may actually be caused by other organisms or concomitant disease processes.

Wild birds and their excrement are the major sources of avian influenza. Preventing direct contact with free-flying birds and protecting domestic poultry from contact with the feces of wild birds is an important way to prevent AI. Live bird markets historically have been an important source of AI, especially on the East coast of the United States. It is important to avoid live markets, educate employees about the dangers posed by these markets, and to prevent the spread of disease from these markets to your flock by preventing any contact.

Infected birds shed virus in the saliva, nasal secretions and feces in the first two weeks of infection. Four weeks after infection, virus can no longer be detected. It makes sense then, that prevention is best accomplished by preventing contact between newly infected birds and susceptible birds. Since AI can also be spread from infected birds by the transfer of fecal material on contaminated equipment and clothing.

The AI virus is very sensitive to most detergents and disinfectants. They are also readily inactivated by heating and drying. However, if organic material is present (feces, dander, food), the flu viruses are well-protected from inactivation. Infectious virus can be recovered from manure for up to 105 days. Complete removal of ALL organic material is necessary for effective disinfection. Contaminated houses are heated for several days to inactivate the virus. Organic material must be removed, followed by complete cleaning and disinfection of all surfaces. Contaminated litter and manure should be composted or buried to ensure that it does not spread infectious virus.

What about vaccines for protection? Vaccines can effectively prevent clinical signs of influenza infections in many species, including poultry. However, since the vaccines are not cross-protective for the 15 virus sub-types that can infect poultry, vaccines are generally not practical to prevent infections. There is really no way to predict which type will infect a flock.

Influenza A virus has been recovered from asymptomatic psittacine birds as well as from those that die suddenly or following an acute onset of depression, diarrhea and neurologic signs. In companion birds, influenza A virus should be considered a possible cause of respiratory and gastrointestinal disease, particularly when family members are ill or there is an epidemic occurring in humans.

The main thrust for the government is to protect their country's poultry industry, and to prevent the spread of AI to humans. While it may be necessary to cull infected poultry, there should be very little threat to the psittacines in areas of outbreaks, unless there has been direct or indirect exposure.

Conventional flu vaccine is not believed to provide protections against A (H5N1) avian influenza. Scientists with the federal CDC and Prevention in Atlanta are racing to complete a genetic sequence of the virus from this case to determine whether it has acquired mammalian influenza genetic material, which could make it more transmissible, and the government has ordered 2 million doses of experimental vaccine. Human trials of the new vaccine are not expected to begin until the end of this year (2006), at the earliest.

Cadeusus
Copyright 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.
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