Infection with influenza A viruses is common in pigs. These viruses are among the pathogens responsible for porcine respiratory disease complex (PRDC) and, in addition to respiratory disease and an associated impairment of general condition, also cause fever and fertility disorders. All types of depression in performance are reflected in economic losses for the farmer and therefore in adverse effects on farm income. In addition, on account of their zoonotic potential, a major concern for public health. This is evidenced not least by the example of "swine flu", which first emerged in Mexico in 2009.
The classical clinical picture of swine influenza develops after a 1-2 day incubation period and includes high fever, lethargy and inappetence. Other symptoms include nasal discharge, conjunctivitis and laboured breathing, the latter especially when startled.
The characteristic barking cough develops on day 3-4 post infection (p.i.). All age groups are equally affected, although the course of the disease can be milder in weaners.
In pregnant sows, it can lead to abortion or to the birth of sickly piglets. If influenza infection is not complicated by any other pathogens, animals recover around 6-7 days p.i.
Morbidity is high and can reach 100% in a susceptible herd.
However, in classical cases, the infection is benign and short-lived. Mortality is generally low.
Figure: Down with influenza – virologically confirmed.
A much milder form of the disease is now being observed with increasing frequency. In this form, the virus circulates within the herd and may cause slowly spreading infections. Here, the clinical picture is characterised by non-specific respiratory problems, a reduced vitality and fertility disorders. Typically, especially in newly weaned piglets or weaners, a persistent, sporadic mild cough is observed in a few animals in a pen without raising suspicions of influenza infection. If these animals are then subjected to stress, e.g. rehousing, the clinical picture may be aggravated. In this case, establishing a diagnosis is much more difficult and only pathogen or antibody detection can provide a reliable diagnosis.
On pathological examination, the lung is not collapsed, which would be a specific finding. To establish a reliable diagnosis, histopathological examination is recommended.