Influenza virus Type A causes avian influenza and can affect several species of birds, such as chickens, turkey, guinea fowl, and quails. However, it can also affect pet birds and wild birds. Some strains of avian influenza have high mortality rates. The virus has also been found in mammals such as pigs, weasels, ferrets, cats, dogs, and tigers. Avian influenza viruses are not new viruses. Throughout the documented literature, numerous outbreaks of the virus have been found.
There are many strains of avian influenza viruses and they can be separated into two categories: low pathogenic avian influenza (LPAI), which causes virtually no clinical symptoms in birds, and highly pathogenic avian influenza (HPAI), which causes severe clinical symptoms and/or mortality in birds.
The Asian H5N1 strain of the avian influenza virus has attracted much attention over the past few years because of its pathogenicity and because of several outbreaks in the domestic world. Concerns were raised because of the degree of virulence that affects not only poultry but wild birds as well. It was also documented to spread to mammalian species. Although avian influenza viruses are usually highly specific, the H5N1 virus also infected humans.
The Asian lineage H5N1 HPAI viruses are of special importance because of their ability to infect humans. They have caused fatal diseases. In addition, milder diseases have been caused as well, but there are cases of asymptomatic infections. The disease caused by the H5N1 virus is characterized by severe pneumonia that rapidly progresses. The disease’s common symptoms are not specific to it. Instead, they are typical of acute respiratory infections, such as shortness of breath, fever, and cough. Complications include septic shock, acute respiratory distress syndrome (ARDS) and multi-organ failure requiring mechanical ventilation and intensive care. On-going influenza-like surveillance systems have detected a small number of patients with the mild clinical illness. Severe illness, however, is more likely to be found in older persons with severe comorbid conditions.
Other symptoms of the disease are a dry cough, fatigue, muscle pain, limb and joint pain, headache, chills and shivering, sore throat, runny nose, blocked nose, conjunctivitis, difficulty sleeping, loss of appetite, diarrhea, and upset stomach.
The cause of the avian influenza H5N1 dates back to 1997 when a series of poultry breakouts occurred in Hong Kong. An H5N1 strain was isolated from chickens, as well as humans. This outbreak included the first human-human transmission and the first infection of a health worker by the virus. The outbreak was rapidly contained by culling infected birds and biosecurity measures. However, this particular strain of virus is thought to have been circulating in Mainland China before 1997.
These H5N1 viruses are part of a group of rapidly evolving viruses that have gained genetic stability. They have gained the ability to infect a wide range of bird and animal species. The ability to transmit between mammals has raised the concern of public health officials. These viruses, furthermore, were not detected until their reappearance in Hong Kong in 2003 in humans. The infection was contained through culling of poultry, poultry vaccination, and tight biosecurity measures. However, the virus spread to Cambodia, Vietnam, Thailand, and other parts of the world.
The routes of entry of the H5N1 virus into humans is poorly understood, however, it is posited that they are transmitted through air droplets suspended after coughing or sneezing. Therefore, risk factors would include contact with persons who are already infected with the virus, especially with high doses of the virus. Humans can also become infected from contact with wild birds or poultry, either dead or alive, that are infected. The probability of transmission is thought to be linked to both virus and host factors. Eating uncooked or undercooked meat is a risk factor since the virus is able to survive in these conditions. In some cases, handling raw or uncooked food products infected with H5N1 causes the virus to be transmitted.
Direct exposure to poultry, especially for people who work in the poultry industry, is a significant risk factor. In addition, poultry that is raised outdoors tends to have higher viral loads, making the probability of human transmission more significant.
Individuals who work with wild birds are also at risk. For instance, people who attempt to catch or de-feather wild birds that are infected with the virus may be at risk.
In some cases, children may be at an increased risk compared to adults because of their closer contact with pet birds. They may also play with poultry at farms and other places.
Healthcare workers caring for those infected with H5N1 are at risk for contracting the virus themselves. Veterinarians, in addition, are another susceptible group since they work closely with animals and poultry. People who work in sewage lines may also be at risk because sewers may be reservoirs of viruses.
The complications of H5N1 are ARDS, multi-organ system failure, and septic shock. The endpoint of these is death. However, other complications may arise at the population level. Pandemics have been reported in Asia when the Asian lineage H5N1 first became recognized in 2003. These pandemics caused widespread panic in the region and many individuals were prohibited from traveling to other countries by their governments. Other complications from the disease include the amount of money lost in poultry investments, since many poultry are culled when they are infected, or as preventive measures.
The diagnosis of H5N1 should be included as a differential diagnosis of individuals presenting with febrile illnesses in countries or territories where H5N1 viruses have been reported. Commonly, the signs and presenting symptoms of H5N1 are non-specific. A detailed exposure history needs to be taken from the patient, including exposure to sick/dead birds, other ill persons, work in laboratories handling samples of H5N1, or travel to an area with H5N1 activity.
Specimens should be collected from people who are suspected of having H5N1. These specimens should be collected from the nose, trachea, or throat. These specimens should be tested at laboratories that are capable of detecting H5N1 in samples, such as H5 Reference Laboratory or a WHO Collaborating Center. Current tests for diagnosing H5N1 are not highly sensitive and are not recommended.
The primary antiviral of choice for the treatment of H5N1 is oseltamivir, which is only available in oral formulations. This is the most effective treatment against H5N1, although no clinical trials have yet been conducted. Once a person is suspected of having H5N1, then it is recommended that he undergo a 5-day course of oseltamivir. Higher doses of oseltamivir may be considered on a case by case basis in patients since the drug has not been tested that much in humans. In children, the possible risks of the drug should be considered against the benefits before commencing treatment.
Neuraminidase inhibitors are another type of drug that can be used to treat H5N1. Zanamivir is one of these drugs and it is orally inhaled. As with oseltamivir, clinical trials for the drug are lacking, thus, their full effectiveness and safety is unknown.
Adamantine and rimantadine have also been successfully used. Early treatment of patients with these drugs has been shown to improve outcomes. Combination therapy is often times recommended in order to prevent complications associated with the infection.
Immunotherapy may also be tried in patients in whom the previously mentioned therapies have not been successful. The administration of anti-H5N1 antibodies in the form of monoclonal antibodies have been proven effective in animal models. Early administration of convalescent blood products may have therapeutic effects on people who are infected with the virus.
The prevention of H5N1 rests on the premise that reduced contact with infected persons, poultry, and wild birds will reduce transmission. Reducing exposure to wild birds, such as staying indoors, is a preventive measure. In addition, preventive measures such as hand washing have also been proven to reduce the risk of acquiring the infection from other people. In terms of infections from animals, refraining from eating raw or undercooked meat may reduce the risk of transmission. Refraining from handling these food products will also reduce the risks.
The prognosis of patients who have been diagnosed with H5N1 depends on their age, comorbid conditions, and viral RNA loads in the upper respiratory tracts. Persistent replication is associated with fatal outcomes. Thus, it is important to monitor virological response by reverse transcriptase polymerase chain reaction (RT-PCR). If left untreated, hypotension and septic shock have been reported in patients who were infected with H5N1. The prognosis for these patients is poor. H5N1 is fatal if left untreated and especially if the person has other co-morbid conditions. In addition to this, the complications arising from the disease, such as ARDS and multi-organ system failure, cause fatalities. The prognosis for those infected with the virus is poor if they are not promptly treated by experienced healthcare workers. However, the prognosis is better for those whose illnesses are treated early on. These individuals have a chance to recover from the illness completely.