Swine influenza

Influenza is an important disease for the pig industry

mainly for two reasons: the first one is the great economic impact on affected farms. And the second one, due to the fact that swine flu is a zoonosis. There is interspecies transmission of viruses between pigs and humans, in such a way that some people in close contact with pigs can acquire the infection and develop flu-like symptoms. In the same way, certain human viruses are capable of multiplying in pigs and cause symptoms of influenza in animals. The latter case is known as “reverse zoonosis” and the best example was the pandemic H1N1 virus circulating in the swine population in 2009 causing great alarm. Remember, we have seen the subtypes of viruses in human Influenza. Why does this happen?

Because pigs have in their respiratory tract, cells with receptors able to recognize swine, human and avian viruses. Thus, traditionally the pig has played a main epidemiological role in the global cycle of influenza. This is possible because this species can become infected with viruses of different origins that could give rise to the appearance of new variant subtypes of virus, as we already have seen in previous steps. The swine flu is caused by Influenza A viruses, usually of subtypes H1N1, H3N2, and H1N2. These viruses are enzootic in the pig populations and present antigenic divergence between the Eurasian and North American lineages.

Likewise, other viral subtypes have been identified in pigs: H1N7, H3N1, H4N6, and H9N2. Influenza viruses are introduced into a farm for the first time through infected animals. Airborne transmission is very fast and in less than a week the farm can be 100% infected. Therefore, the appearance of clinical signs is sudden. The infection is highly contagious among susceptible pigs, the evolution is fast, reaches high morbidity and presents as outbreaks. In this situation, the clinical signs in animals are anorexia, prostration, fever, cough, conjunctivitis, nasal discharge, weight loss, respiratory distress, etc. followed by rapid clinical recovery, if there are no secondary bacterial infections. Mortality in farms without major secondary infections can range between 1 and 4%.

This will depend on the virus subtype, the type of farm, and the age of the animals (greater mortality in fattening pigs). The most common situation in a farm is the enzootic circulation of influenza virus. Thus, the infection is always present and the disease can appear periodically. The majority of the infections are subclinical or a low percentage of animals (around 25%) may have respiratory signs, which leads to a decrease in the conversion rate, an extension of the fattening period, and a decrease in the overall profitability of the farm. Once the infection has established in the farm, it is very difficult to eliminate it.

To control swine Influenza there are inactivated vaccines that aim to increase the farm population immunity against the infection. However, these vaccines confer little protection, and their use, in general, is limited. The main prevention and control measures are designed to improve or increase biosecurity in farms and identify potential routes of introduction and spread of the infection. In the context of the prevention and control of swine influenza it is important to take protection measures during human-pig interaction, especially if the person is suffering from flu symptoms. The main objective of this is to minimize the economic impact of the disease and its incidence on the farms.