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Swine Influenza (swine flu)



Swine Flu

What is swine influenza ?

Swine influenza (also referred to as Swine influenza viruses or SIV) is caused by Orthomyxoviruses which are endemic among pig populations. Wild pigs can carry the virus intestinally, without being affected. Swine influenza can be highly contagious among pigs, but most outbreaks cause high levels of illness and low death rates in pigs.

Swine flu infects a number of human beings every year, and there have been increasing numbers of reports of person-to-person spread in recent years.

Analysis of the various virus sub-types found in swine flu in pigs suggests a pooling of DNA, incuding genes from swine, avian and human influenza.

Until recently, influenza viruses had been thought to be largely 'species specific', with little risk to other species, but it has now been established that the great epidemic of 1918 was a strain of avian flu.

Some of the cases in the 2009 outbreak in Mexico and the USA have been confirmed by the World Health Organization to be due to a new genetic strain of H1N1, including swine flu genetic content.

Note that swine influenza - swine flu - is the accepted medical description of the disease, even though porcine flu would probably be a little more accurate, and plain old fashioned pig flu would do just as well.

Where is swine influenza usually found?

The virus, a member of the Orthomyxoviridae group, is endemic among pig populations around the world. Other strains - including human and avian varieties - can thrive in pig populations without causing symptoms.

How common is swine influenza ?

Swine influenza can reach epidemic form among pigs but low pathogenicity forms are more common than high. Just like human flu in humans, it is an endemic respiratory ailment.

Outside of epidemics, it is not usually found in human beings; the US records cases in single figures in most years.

A pandemic is defined as an epidemic that is geographically widespread; occurring throughout a region or even throughout the world; the April outbreak lead to over one hundred deaths, mostly in Mexico, and affected people in a score of far-flung nations. But to call that an epidemic, let alone a pandemic, is a sick joke. 'Normal flu' does exactly the same thing, year in, year out. The fact that we can trace it more easily and identify the victims does not redefine epidemic or pandemic; the fact the world panics so easily is probably little defence. The pandemic will come; current farming methods, population density, and the fact that millions of us fly in poorly ventilated jet aircraft is testament to that. It will happen, but probably not yet.

How is swine influenza transmitted?

Infected pigs spread the 'pig flu' virus via saliva, and nasal secretions - like human flu, it is largely an airborne infection, and feces. In industrialised nations, pigs are 'farmed' using an industrial approach which keeps large numbers of pigs close together, it can easily and quickly spread. Additionally, pigs may be moved from one centre to another for breeding or other reasons, enabling the virus (which may be at a low level and symptom-free), to spread between centres.

In less developed countries, domestic pigs may move more freely, over the same territory as wild pigs, enabling the virus to be constantly on the move. Plus people and pigs live much closer together in such environments, easing the transfer of genetically separate viruses from man to pig and vice-versa, with the ever present risk of news forms developing.

Influenza viruses spread not just by inhalation of aerosolized virus, but also by eye and nose contact with droplets of respiratory secretions.

Can swine influenza be spread from person-to-person?

The disease can be spread by infected pigs to people, through direct contact with contaminated items.

There have been recorded cases of person-to-person spread.

For example, an isolated case in 1988 resulted in multiple human infections, with antibody evidence of virus transmission from the patient to health care workers who had close contact with the patient.

In the same year, a healthy woman (age 32) died of pneumonia; a swine H1N1 flu virus was detected. Four days before she became ill, the patient visited a county fair, where there was widespread influenza-like illness among the exhibited pigs. Tests showed that three out of four exhibitors tested had evidence of swine flu but no serious illness was detected. The virus was also found in some of the health care workers involved.

Pigs are susceptible to both human and pig viruses. If a pig were infected with the pig flu virus at the same time as a human flu virus, this could allow a new form of the pig virus to arise with the ability to spread directly from one human being to another.

Swine flu cannot be spread through eating pork.

Who is most at risk?

Normally, only people who have close contact with infected pigs are considered at risk; mostly in cultures where people and domestic fowls live in very close proximity.

In most disease epidemics, those at risk tend to be older people, and those weakened by immunosuppression of other ill health; however, the 2009 flu outbreak has been characterised by deaths among young people - a feature of the 1918 flu epidemic which killed over 20 million people worldwide, though that epidemic originated from a strain of bird flu.

What are the symptoms of swine influenza ?

The symptoms of swine flu in people include fever, lethargy, lack of appetite and coughing. Runny nose, sore throat, nausea, vomiting and diarrhea have also been reported.

More aggressive forms include: fever, disorientation, stiffness of the joints, vomiting, and loss of consciousness ending in death. The symptoms may depend on the virus strain involved.

How is swine influenza diagnosed?

Initial diagnosis may depend on the individual history, particularly recent travel and contact with pigs or swine wastes. Confirmation will depend on laboratory tests of nose and throat swabs, plus sputum, and possibly blood tests.

Is there a treatment for swine influenza ?

Amantadine, rimantadine, oseltamivir and zanamivir have all been used successfully to treat swine influenza. However, the most recent swine influenza viruses isolated from humans are resistant to amantadine and rimantadine. This leaves the option of oseltamivir or zanamivir for the treatment and/or prevention of infection.

Is there a way to prevent infection?

For those dealing with pigs, respiratory precautions when pigs show symptoms - protective masks and eye protection as a minimum would be wise. There is a bigger question of how the disease in pigs is dealt with, to avoid the reservoir of infection that may be a time bomb - but that is beyond the scope of this article!

In the human community, isolation and quarantine are clearly just not practical in most places, but the time honoured approaches of exercise, good ventilation and keeping a maximum distance from those with infection would be wise.

Early in the Mexico outbreak, public buildings were closed and hundreds of public events cancelled. Schools in and around Mexico City were temporarily closed, and most bars and restaurants in the capital were closed. People were being strongly urged to avoid shaking hands, and the US embassy advised visitors to maintain a distance of 2m (6 feet) from other people.

What is the mortality rate for swine influenza ?

Very small in between epidemics; rare in pigs, rare in human beings.

Is there an swine influenza vaccine?

No. The H1N1 swine flu viruses are antigenically very different from human H1N1 viruses and, therefore, vaccines for human seasonal flu would not provide protection from H1N1 swine flu viruses - quoted from CDC advice.

There currently is no commercially available vaccine to protect humans against the virus that is being seen in the Americas. However, vaccine development efforts are taking place.

Can swine influenza be controlled environmentally?

While the swine influenza virus vaccines currently available may not completely eliminate the infection, vaccination of pigs can reduce the levelsof virus shed by infected animals, and thus reduce the potential for human exposure and zoonotic infections.

While the human influenza vaccines produced on a yearly basis contain only human, not swine, strains of viruses, these vaccines are likely to provide some level of protection against infection with swine viruses of the same hemagglutinin sub-type. Thus the vaccination of farm workers may reduce the risk of human influenza virus infection of their pigs, which in turn may reduce the risks to humans.

Sick-leave policies for farm employees should encourage them to remain away from work when they are suffering from acute respiratory infections (approximately 3-7 days).

Ventilation systems should be designed to minimize re-circulation of air within animal housing rooms. This is will reduce the exposure of pigs to viruses from other pigs, reduce their exposure to human influenza viruses, and also reduce exposure of workers to swine influenza viruses.

Farm workers should change clothes prior to leaving swine barns; hand-to-face contact should be minimized and hand-washing stations should be available throughout the animal housing areas. Boots should be specific for the pig barns, reducing the risk of spreading infected faecal content.

The known infection of pigs with waterfowl-origin influenza viruses, the risks for reassortment of avian viruses with swine and/or human influenza viruses in pigs, and the risk for transmission of influenza viruses from pigs to domestic turkeys all indicate that contact between pigs and both wild and domestic fowl should beminimized.

All doorways, windows and air-flow vents in swine housing units should be adequately sealed or screened to prevent entrance of birds. Although small birds such as sparrows, swallows, finches,wrens etc. are not thought to be important in the overall ecology of influenza viruses, they may carry influenza viruses from waterfowl faeces into barns on their bodies.

Untreated surface water should not be used for drinking water or for cleaning in swine facilities (because of waterfowl faecal contamination with influenza viruses).

Likewise, it may be prudent to attempt to minimize waterfowl use of farm lagoons.

Pigs and domestic fowl should not be farmed on the same premises.

Pig feed should be kept in closed containers to prevent contamination with faeces from over-flying waterfowl.

These recommendations clearly cannot apply when pigs are raised outdoors. Outdoor housing may put pigs at increased risk for infection from avian viruses, but the animals may well be at reduced risk of passing flu amongst the stock, and may have greater defense against infection and strionger immune systems - research is needed in this area.

Is there legal protection for workers and others?

All workers who may have contact with wild pigs, or domestic pigs that have had contact with wild pigs, are entitled to full training and education; sufficient for them to assess the risks to themselves, and for them to be able to work in a way that minimises or removes the risks to the community. Failure to provide full education and sufficient protective gear, including gloves, masks, eye shields, general protective clothing (as appropriate to the job) and handwashing facilities, would almost certainly be seen by the courts as negligence, particularly if this led to confirmed disease spread.

Bibliography and Further Information Sources


If this article hasn't answered your question, email me at the address below, and I'll try to get the information you seek. I regret I cannot assist with individual cases or essays and school projects, but if it's something I've missed, I'll be happy to try and help.

Article written by Andrew Heenan BA (Hons), RGN, RMN

First Published: 16 April 2009
Last updated: 16 April 2009
© Andrew Heenan 2009 et seq


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