Back-transmission of HPAI viruses from poultry to wild birds has occurred and is implicated in mass die-offs and intercontinental spread.AIVs have occasionally infected humans through aerosols, fomites, and contaminated water. Direction transmission from wild birds is rare. Instead, most transmission involves domestic poultry, mainly chickens, ducks, and geese but also a variety of other birds such as guinea fowl, partridge, pheasants, and quails. The primary risk factor for infection with AIVs is exposure to birds in farms and live poultry markets. Typically, infection with an AIV has an incubation period of 3–5 days but can be up to 9 days. H5N1 and H7N9 cause severe lower respiratory tract illness, whereas other AIVs such as H9N2 cause a more mild upper respiratory tract illness, commonly with conjunctivitis. Limited transmission of avian H2, H5-7, H9, and H10 subtypes from one person to another through respiratory droplets, aerosols, and fomites has occurred, but sustained human-to-human transmission of AIVs has not occurred. Before 2013, H5N1 was the most common AIV to infect humans. Since then, H7N9 has been responsible for most human cases. Pigs
Influenza in pigs is a respiratory disease similar to influenza in humans and is found worldwide. Asymptomatic infections are common. Symptoms typically appear 1–3 days after infection and include fever, lethargy, anorexia, weight loss, labored breathing, coughing, sneezing, and nasal discharge. In sows, pregnancy may be aborted. Complications include secondary infections and potentially fatal bronchopneumonia.
Both lineages are in circulation in humans, disproportionately affecting children. IBVs contribute to seasonal epidemics alongside IAVs but have never been associated with a pandemic.ICV, like IBV, is primarily found in humans, though it also has been detected in pigs, feral dogs, dromedary camels, cattle, and dogs. ICV infection primarily affects children and is usually asymptomatic or has mild cold-like symptoms, though more severe symptoms such as gastroenteritis and pneumonia can occur. Unlike IAV and IBV, ICV has not been a major focus of research pertaining to antiviral drugs, vaccines, and other measures against influenza. ICV is subclassified into six genetic/antigenic lineages.IDV has been isolated from pigs and cattle, the latter being the natural reservoir. Infection has also been observed in humans, horses, dromedary camels, and small ruminants such as goats and sheep. IDV is distantly related to ICV. While cattle workers have occasionally tested positive to prior IDV infection, it is not known to cause disease in humans. ICV and IDV experience a slower rate of antigenic evolution than IAV and IBV. Because of this antigenic stability, relatively few novel lineages emerge. Genome and structure
Influenza viruses have a negative-sense, single-stranded RNA genome that is segmented. The negative sense of the genome means it can be used as a template to synthesize messenger RNA (mRNA). IAV and IBV have eight genome segments that encode 10 major proteins. ICV and IDV have seven genome segments that encode nine major proteins.