Swine influenza

Swine flu viruses cause high levels of illness and low death rates in pigs. Swine influenza viruses may circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to outbreaks in humans.

The classical swine flu virus (an influenza type A H1N1 virus) was first isolated from a pig in 1930.

How many swine flu viruses are there?

Like all influenza viruses, swine flu viruses change constantly. Pigs can be infected by avian influenza and human influenza viruses, as well as swine influenza viruses.

When influenza viruses from different species infect pigs, the viruses can reassort (in other words, swop genes), and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge. Over the years, different variations of swine flu viruses have emerged.

At this time, there are four main influenza type A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2 and H3N1. However, most of the recently isolated influenza viruses from pigs have been H1N1 viruses.

Swine flu in humans
Can humans catch swine flu?

Swine flu viruses do not normally infect humans. However, sporadic human infections with swine flu have occurred.

Most commonly, these cases occur in persons with direct exposure to pigs (for example, children near pigs at a fair or workers in the swine industry). In addition, there have been documented cases of one person spreading swine flu to others.

For example, an outbreak of apparent swine flu infection in pigs in Wisconsin, USA in 1988 resulted in multiple human infections, and, although no community outbreak resulted, there was antibody evidence of virus transmission from the patient to health-care workers who had close contact with the patient.

How common is swine flu infection in humans?

In the past, there have been reports of approximately one human swine influenza virus infection every one to two years in the US, but from December 2005 through February 2009, 12 cases of human infection with swine influenza have been reported.

What are the symptoms of swine flu in humans?

The symptoms of swine flu in people are expected to be similar to the symptoms of regular human seasonal influenza and include fever, lethargy, lack of appetite and coughing. Some people with swine flu also have reported runny nose, sore throat, nausea, vomiting and diarrhoea.

Can people catch swine flu from eating pork?

No. Swine influenza viruses are not transmitted by food. You can not get swine influenza from eating pork or pork products. Eating properly handled and cooked pork and pork products is safe. Cooking pork to an internal temperature of 71 degrees Celsius kills the swine flu virus as it does other bacteria and viruses.

How does swine flu spread?

Influenza viruses can be directly transmitted from pigs to people and from people to pigs. Human infection with flu viruses from pigs are most likely to occur when people are in close proximity to infected pigs, such as in pig barns and livestock exhibits housing pigs at fairs.

Human-to-human transmission of swine flu can also occur. This is thought to occur in the same way as seasonal flu occurs in people, which is mainly person-to-person transmission through coughing or sneezing of people infected with the influenza virus. People may become infected by touching something with flu viruses on it and then touching their mouth or nose.

What do we know about human-to-human spread of swine flu?

In September 1988, a previously healthy 32-year-old pregnant woman was hospitalised for pneumonia and died eight days later. A swine H1N1 flu virus was detected. Four days before getting sick, the patient visited a county fair swine exhibition where there was widespread influenza-like illness among the swine.

In follow-up studies, 76% of swine exhibitors tested had antibody evidence of swine flu infection, but no serious illnesses were detected among this group. Additional studies suggest that one to three health-care personnel who had contact with the patient developed mild influenza-like illnesses with antibody evidence of swine flu infection.

How can human infections with swine influenza be diagnosed?

To diagnose swine influenza A infection, a respiratory specimen would generally need to be collected within the first four to five days of illness (when an infected person is most likely to be shedding virus). However, some persons, especially children, may shed virus for 10 days or longer.

Identification as a swine flu influenza A virus requires sending the specimen for laboratory testing.

What medications are available to treat swine flu infections in humans?

At this time, authorities recommend the use of oseltamivir (available in South Africa as Tamiflu) or zanamivir (not available in South Africa) for the treatment and/or prevention of infection with swine influenza viruses.

What other examples of swine flu outbreaks are there?

Probably the most well known is an outbreak of swine flu among soldiers in Fort Dix, New Jersey in 1976. The virus caused disease with x-ray evidence of pneumonia in at least four soldiers and one death. All of these patients had previously been healthy.

The virus was transmitted to close contacts in a basic training environment, with limited transmission outside the basic training group. The virus is thought to have circulated for a month and disappeared.

The source of the virus, the exact time of its introduction into Fort Dix, and factors limiting its spread and duration are unknown. The Fort Dix outbreak may have been caused by introduction of an animal virus into a stressed human population in close contact in crowded facilities during the winter. The swine influenza A virus collected from a Fort Dix soldier was named A/New Jersey/76 (Hsw1N1).

Is the H1N1 swine flu virus the same as human H1N1 viruses?

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.

What to look out for:

• A recent onset of fever (38°C) with sore throat, runny nose/nasal congestion, cough, myalgia and/or gastrointestinal symptoms; and
• Recent travel or close contact with someone who is a suspected/confirmed case of swine flu in the seven days prior to the onset of symptoms.

The WHO provides the following precautionary advice for individuals who are well:

Maintain a distance of at least 1m from any individual with influenza-like symptoms, and:

• Refrain from touching your mouth and nose;
• Perform hand hygiene frequently, by washing with soap and water or using an alcohol-based handrub, especially if touching your mouth and nose and surfaces that are potentially contaminated;
• Reduce as much as possible the time spent in close contact with people who might be ill;
• Reduce as much as possible the time spent in crowded settings;
• Improve airflow in your living space by opening your windows as much as possible.

For individuals with influenza-like symptoms:

• Stay at home if you feel unwell and follow the local public health recommendations;
• Keep distance from well individuals as much as possible (at least 1m);
• Cover your mouth and nose when you're coughing or sneezing, with tissues or other suitable materials, to contain respiratory secretions;
• Dispose of the material immediately after use, or wash it;
• Clean your hands immediately after contact with respiratory secretions; and
• Improve airflow in your living space by opening your windows as much as possible.

Source: US Centers for Disease Control and Prevention (CDC), http://www.cdc.gov/swineflu/key_facts.htm
World Health Organisation