Scientists studying the clinical features of 11 sporadic human infections by triple-reassortant swine influenza A (H1) viruses that occurred in the three or so years leading up to the current swine flu epidemic, found that most of the patients had been exposed to pigs directly or indirectly, and that although they all recovered, some of them had severe symptoms, including some who were previously healthy.

The study is the work of Dr Lyn Finelli, head of influenza surveillance at the US Centers for Disease Control and Prevention in Atlanta, and colleagues from state health departments throughout the US, and was published online on 7 May in the New England Journal of Medicine.

Triple-reassortant swine influenza A (H1) viruses have scraps of genetic material from avian, human, and swine influenza viruses. They started to appear in and become native to ("enzootic") pig populations in North America in the late 1990s, said the authors in their background information.

The viruses also infected humans, and for this study Finelli and colleagues examined the clinical characteristics of the first 11 sporadic cases in humans that occurred between December 2005 and February 2009, or up to just before the current epidemic of A/H1N1 in humans.

To study the cases, the researchers looked at reports from national flu surveillance and investigations by public and animal health authorities.

The results showed that:

* The ages of the 11 patients ranged from 16 months to 48 years, with a midpoint (median) at age 10.

* 4 of the 11 patients had underlying health problems.

* 9 of the 11 patients had been exposed to pigs: 5 had been in direct contact and 4 had visited pig facilities but had had no direct contact.

* 1 of the 11 patients was thought to have become infected through human to human contact.

* The incubation period (from last known exposure to start of symptoms) was between 3 and 9 days.

* 10 of the 11 patients had known clinical symptoms that included fever (9 of the 10), cough (all 10), headache (6 of the 10) and diarrhea (3 of the 10).

* Complete blood tests were done for 4 of the 11 patients. These showed that 2 patients had low white blood cell counts (leukopenia), one had a low lymphocyte count (lymphopenia) and one had a low platelet count (thrombocytopenia).

* 4 of the 11 patients were hospitalized, with 2 requiring invasive mechanical ventilation.

* 4 of the 11 patients were treated with the antiviral drug oseltamivir (Tamiflu), and all 11 patients recovered from the influenza infection.

The authors concluded that:

Before the current epidemic of swine-originating flu, there were sporadic infections of triple-reassortant swine influenza A (H1) viruses among humans in the US.

"Although all the patients recovered, severe illness of the lower respiratory tract and unusual influenza signs such as diarrhea were observed in some patients, including those who had been previously healthy," they added.

The CDC said in an announcement on their website this weekend that they expect more cases, more hospitalizations and more deaths as the current outbreak of the novel influenza A (H1N1) continues to expand in the United States in the coming weeks.

The current response strategy is to reduce spread and illness severity, and continue to give up to date information to health care workers, officials and the public.

On Sunday, the CDC reported that the latest number of confirmed lab cases of novel H1N1 virus infections in the US (including the District of Columbia) was 2,532 in 44 states, including 3 deaths.

The CDC has developed a PCR diagnostic test kit that identifies the new H1N1 virus genetically. The kit is being distributed throughout the US, Puerto Rico and other countries.

The authorities anticipate that the number of reported cases will go up as diagnostic capacity increases, which should give a more accurate picture of the spread and extent of the new virus.