Preliminary results from three researches indicate that the novel influenza A virus (H1N1) is transmittable in swine populations, earlier Swine Influenza Virus immunity does not seem to protect against the virus, but the novel virus does not seem to affect any other tissue than the respiratory tract.
A British study, released in the end of May and, coordinated by the Veterinary Laboratories Agency in Weybridge, UK, found that the virus can be transmitted among pigs, but did not create any mortality.
"Pigs are susceptible to infection with influenza A (H1N1) virus that results in the induction of detectable levels of clinical disease, virus shedding and pathology in an experimental setting. Importantly, mortality was not a feature and infected animals were able to transmit the virus to naïve contact pigs successively for at least three cycles of transmission, suggesting the virus could become established in susceptible pig populations if introduced."
Effect on tissues
By insulating four five-week old cross-bred pigs from a herd free of swine influenza virus (SIV) and PRRS, American researchers hoped to reach more knowledge about the question whether meat, blood and tissue from pigs infected with the new 2009 H1N1 Influenza A Virus is free of infectious virus.
The research, carried out by the American Research Service (ARS) and conducted at the National Animal Disease Center (NADC), yielded the result that "live 2009 A/H1N1 Influenza A Virus was only detected in the respiratory tract of infected pigs and the virus does not appear to spread and replicate in other tissues based on the day 5 post infection samples."
Susceptibility to viruses
Another US research focused on the question whether whether US commercial swine herds are susceptible to the 2009 A/H1N1 influenza viruses isolated from persons in California, New York and Mexico.
"Results of this experiment suggest that pre-existing immunity induced by swine influenza viruses circulating in the US swine herd may not protect pigs against the new 2009 A/H1N1 influenza viruses presently circulating in people. Importantly, vaccines currently used to protect pigs in US swine operations against swine influenza virus may not be effective against the new 2009 H1N1 influenza viruses."
• Clinical signs: Included nasal discharge, pyrexia (temperature >39.5ºC), respiratory signs (cough mainly, some increased respiratory rate), ocular discharge, lethargy and inappetence. Limited morbidity. Clinical scores peaked at dpi 4-6 with progressive recovery evident (n=2) after dpi 7. For in contact animals, pyrexia was evident from day post-contact (dpc) 5-9. Clinical signs appeared milder than in those pigs directly infected.
• Virus shedding: Determinations by real-time RT-PCR; all inoculated animals became infected and shed virus mainly via the naso-pharyngeal route from dpi 1-9, peak shedding occurred between dpi 3-5. Oral and ocular shedding was detected intermittently, but rectal shedding was not detected. Shedding appeared to have ceased by dpi 10. No viral RNA was detected in plasma samples collected between day -1 and dpi 7.
• Gross pathology: Necropsy at dpi 2 revealed only a mild/moderate catarrhal rhinitis with no gross pulmonary pathology. At dpi 3 and dpi 4, PME of pigs (n=2 each day) revealed no evidence of rhinitis with only minimal lung pathology (discrete, rare focal areas of lobular consolidation). At dpi 7 more extensive gross lung lesions were present, characterised as an acute lobular bronchopneumonia with associated marked lymphadenopathy in the bronchial and retropharyngeal lymph nodes.
• Transmission: The contact animals developed similar profiles to the directly infected animals, particularly after transmission cycles one and two (72 hour contact period). Evidence of successful transmission was apparent between both the direct infected and first pair of contact animals (transmission cycle 1), and between the first and subsequent pairs of contact pigs (transmission cycles 2, 3 and 4), based on RRT-PCR shedding profiles. For transmission cycle 1, shedding peaked at dpc 3, and for transmission cycle 2 at dpc 5-6, with shedding apparently ceased by dpc 6-9. Transmission cycles three and four appeared substantially delayed (third cycle >4 days).