Pathogenesis:

Influenza infection is initiated by virus inoculation in the upper or lower airways. This infection can be transmitted by the hand-to-nose route, by droplets of infectious respiratory secretions, or by small droplet aerosol. It attaches to the respiratory epithelium by interaction of the HA molecule with cell-membrane receptors. In successful infection, virus begins to replicate in the respiratory epithelium; it is then shed into the respiratory secretions, and local spread ensues. The eventual result is death of respiratory epithelial cells with desquamation, loss of ciliary function and decreased mucus production [2]. The entire airway from pharynx to alveoli may be involved. Viral infection of the alveolar epithelium can result in a diffuse pneumonia that can be life-threatening. These changes permit secondary bacterial invasion either directly through the epithelium or, in the case of the middle ear space, through obstruction of the normal drainage through the eustachian tube. In humans, the influenza replicative cycle is confined to the respiratory epithelium. With primary infection, virus replication continues for 10-14 days [1].

This course is determined, in part, by the presence or absence of virus-specific serum IgG and secretory IgA antibody. Antibody to the HA of the virus is apparently most important in resistance to infection, but antibody to the NA also has a contributory role. Cellular immune responses are important in terminating established infection. Respiratory mucus contains certain glycopeptides that inhibit virus attachment to the cells of the respiratory mucosa. However, these glycopeptides can be inactivated by viral NA if this enzyme is not neutralized by specific antibody of the host. Cytokines like Tumor necrosis factor and Interferon may also be responsible for inhibiting viral replication. Heterotypic immunity is generally not seen in humans.

Clinical Manifestations:

Manifestations of swine influenza are similar to those of seasonal influenza except that most cases of swine influenza have occurred in previously healthy young adults [4]. Patients present with symptoms of acute respiratory illness, including the following: Fever, cough, sore throat, body aches, headache, chills and fatigue, diarrhea and vomiting are also possible. Persons with these symptoms should call their health care provider promptly. If an antiviral agent is warranted, it should ideally be initiated with 48 hours from the onset of symptoms. The duration of illness is typically 4-6 days. The infectious period for a confirmed case is defined as 1 day prior to the onset of symptoms to 7 days after onset. Signs of severe disease include apnea, tachypnea, dyspnea, cyanosis, dehydration, altered mental status, and extreme irritability [11].

Diagnosis:

As per CDC guidelines [12] a confirmed case of Swine origin influenza virus (S-OIV) infection is defined as a person with an acute febrile respiratory illness with laboratory confirmed S-OIV infection at CDC by one or more of the following tests:

      1.  real-time RT-PCR
      2.  viral culture

A probable case of S-OIV infection is defined as a person with an acute febrile respiratory illness who is positive for influenza A, but negative for H1 and H3 by influenza RT-PCR.