Health authorities across the globe are taking steps to try to stem the spread of swine flu after outbreaks in Mexico and the United States, and suspected cases elsewhere. RN Breakfast's science correspondent Dr Chris Smith and the World Health Organisation (WHO) explain how swine flu is spread from animals to humans.

Q: What do we know about the swine flu virus in general and what do we know about this new strain?

Dr Chris Smith: The flu is a very ancestrally old illness. It's a virus that has been around for thousands of years. It started off as an infection in birds and slowly over those thousands of years diversified to infect every single warm-blooded animal on Earth and a few cold-blooded ones, including snakes.

All these different organisms have their own forms of flu and periodically they can exchange them with other related animals. Birds can exchange their forms of flu with us, that gives us bird flu and periodically spawns one type of pandemic. Also, pigs can exchange their form of flu with birds and perhaps other large mammals, including horses. Because pigs, as very big mammals, are so similar to humans, they can also exchange viruses with humans and this is why we think the present outbreak could have occurred.

A pig could have had a form of flu and if a pig worker infects that pig with a form of human flu, the pig can then be co-infected with two very similar viruses at the same time. The way that flu organises its genetic material makes it very easy for the virus to do what is called reassortment. You can get a hybrid where the worst bits of both viruses combine, producing a super-virus.

You end up with a virus which has an outer coat resembling the outer coat of the pig form of the virus, but has the inner workings and the ability to evade the human immune system of the human form, which means it can spread very readily amongst humans. Because it looks like nothing we've seen before, there is no "herd immunity" in the population, so it spreads very effectively and very easily.

Q: When and where do swine flu outbreaks occur?

WHO: Outbreaks in pigs occur year round, usually in the autumn and winter in temperate zones. Since international health regulations were implemented in 2007, WHO has been notified of swine influenza cases from the United States and Spain.

Governments do not have to notify international animal health authorities about swine flu outbreaks, therefore its international distribution in animals is not well known. The disease is considered endemic in the United States. Outbreaks in pigs are also known to have occurred in North America, South America, Europe (including the UK, Sweden, and Italy), Africa (Kenya) and in parts of eastern Asia including China and Japan.

Q: This super-virus we're looking at now is said to be a cross between a pig flu, a human flu and also the bird flu, which has been deadly too. Does this make it even more dangerous?
 

Dr Smith: Flu is broken down into a number of different types: there's flu A, flu B and flu C. Flu A is the bird form and, of course, humans also have a flu A. Then we further categorise it according to the surface molecules, the H (for Haemagglutinin) and the N on the surface.

This new strain is a H1N1 virus, which we know circulates in humans, pigs and birds. It's possible that the pig initially got some of its flu from a bird and then co-mixed that with the human form to produce this hybrid, which is why you can find elements of all three.

It is very much early days and the big focus now, through infection control organisations like the Centres for Disease and Prevention (CDP) and the WHO, will be to interrogate this virus at a molecular level.

In other words, look at the genetic make-up, sequence the virus, and once they've got the genetic sequence you can begin to unpick where it's come from, what its origins are, and therefore begin to build the story as to what its likely outcome will be.

WHO: Swine flu viruses are most commonly of the H1N1 subtype but other subtypes g (H1N2, H3N1 and H3N) are also circulating. The H3N2 swine virus was thought to have been originally introduced into pigs by humans.

An influenza virus containing genes from a number of sources is called a "reassortant" virus. Swine flu viruses are normally species specific and only infect pigs, but sometimes they cross the species barrier to cause disease in humans.

Q: The World Health Organisation has activated stage three of its six stage global influenza plan and that puts nations on pandemic alert. Stage three is activated when there is "no or limited human-to-human transmission". But how could the virus have spread as it has without human-to-human transmission?

Dr Smith: The key thing here is going to be the epidemiology. In other words, looking at the numbers and how they relate to populations. What Mexico and then the subsequent cases in America tell us is that there are no confirmed examples of all of these other cases all having links to pigs.

Normally when we see these "zoonotic jumps" (a virus which is normally found in one animal getting into people, like H5N1 bird flu), when you see human cases there is some association with an animal. You cannot account for the cases that are happening in other countries like America on those grounds.

So there must be some other factor, some other ingredient. We think that's probably human-to-human transmission and so that's why people are worried because we think we're seeing onward transmission now, and not just to a few cases but to lots of cases.

Normally when these viruses jump out of their host species and get into a new species, it's somewhat constrained in its ability to multiply and spread, because the virus has found itself in foreign territory - us, rather than an animal - to which its ill-adapted.

This virus doesn't seem to be ill-adapted in that way, suggesting it's well-adapted to life in a human, enabling it to spread efficiently from one person to the next.

Given how infectious the flu is, and given how mobile the human population is around the planet, the UK government in a working party report in 2000 suggested there is something like 500,000 people airborne around planet Earth at any instant in time - this means the potential for this to spread far and wide, very quickly, is high.

WHO: People usually get swine influenza from infected pigs, however, some human cases lack contact history with pigs or environments where pigs have been located. Human-to-human transmission has occurred in some instances but was limited to close contacts and closed groups of people.

It is likely that most people, especially those who do not have regular contact with pigs, do not have immunity to swine influenza viruses that can prevent the virus infection.

If a swine virus establishes efficient human-to human transmission, it can cause an influenza pandemic. The impact of a pandemic caused by such a virus is difficult to predict: it depends on virulence of the virus, existing immunity among people, cross protection by antibodies acquired from seasonal influenza infection and host factors.

Q: What are the symptoms of this flu and how does it kill you?

Dr Smith: The health protection agency in the UK has drawn up an algorithm as to how they intend to assess people. The first point on the algorithm is geography. Have people come in from an area where we're seeing disease activity? That includes obviously parts of the US now but chiefly Mexico.

Then there is a symptom algorithm. It's a temperature of more than 38 degrees, or a history of a temperature of more than 38 degrees and not only respiratory symptoms (for example, a runny nose) or a headache but also diarrhoea and vomiting.

Some of the US cases have presented with diarrhoea, though it's not clear actually if those symptoms were incidental. But because this is a foreign virus getting into people it could present in an atypical way.

They're using that initial screening and then activating various molecular tests, using tests that can detect the genetic material of the virus to see if we're onto this form of flu.

WHO: Swine flu tends to be associated with high morbidity (that is, it infects a large proportion of an area's population) but low mortality (less than 5 per cent of infected patients die of the disease).

Generally clinical symptoms are similar to seasonal influenza but reported clinical presentation ranges broadly from asymptomatic infection to severe pneumonia resulting in death.

Since typical clinical presentation of swine influenza infection in humans resembles seasonal influenza and other acute upper respiratory tract infections, most of the cases have been detected by chance through seasonal influenza surveillance. Mild or asymptomatic cases may have escaped from recognition; therefore the true extent of this disease among humans is unknown.

Q: Is it safe to eat pork meat and pork products?

WHO: Yes. Swine influenza has not been shown to be transmissible to people through eating properly handled and prepared pork (pig meat) or other products derived from pigs. The swine influenza virus is killed by cooking temperatures of 70 degrees Celsius, corresponding to the general guidance for the preparation of pork and other meat.

Q: Is there a human vaccine to protect from swine influenza?

WHO: There are no vaccines that contain the current swine influenza virus causing illness in humans. It is not known whether current human seasonal influenza vaccines can provide any protection.

Influenza viruses change very quickly. It is important to develop a vaccine against the currently circulating virus strain for it to provide maximum protection to the vaccinated people. This is why WHO needs access to as many viruses as possible in order to select the most appropriate candidate vaccine virus.

Q: What drugs are available for treatment?

WHO: Antiviral drugs for seasonal influenza are available in some countries and effectively prevent and treat the illness. There are two classes of such medicines, adamantanes (amantadine and remantadine) and inhibitors of influenza neuraminidase (oseltamivir and zanamivir).

Most of the previously reported swine flu cases recovered fully from the disease without requiring medical attention and without antiviral medicines.

Some flu viruses develop resistance to the antiviral medicines, limiting the effectiveness of chemoprophylaxis and treatment. The viruses obtained from the recent human cases with swine flu in the United States were sensitive to oselatmivir and zanamivir but resistant to amantadine and remantadine.

There is not enough information for the WHO to make recommendations on the use of the antivirals in prevention and treatment of swine flu virus infection. Clinicians have to make decisions based on the clinical and epidemiological assessment and harms and benefit of the prophylaxis/treatment of the patient.

For the ongoing outbreak of the swine flu infection in the United States and Mexico, the national and the local authorities are recommending to use oseltamivir or zanamivir for treatment and prevention of the disease based on the virus's susceptibility profile.

resource: extracted from ABC News - Dated April 27 2009