Swine flu: not common, but there

In 1988 a 32 year old woman, 36 weeks pregnant, checked into a community hospital in Wisconsin. She'd had flu-like symptoms with a moderately high (spiking to 102 degrees F.) fever for the previous week. Three days before admission she started a cough that brought up sputum and a day before started to get short of breath. On x-ray both lungs showed a consolidated pneumonia in the lower lobes and she was started on broad spectrum antibiotics, transferred to a tertiary care hospital and started on assisted ventilation. Labor was induced and she delived a healthy baby, just over 6 lbs. Four days later she was dead, the victim of what appeared to be a primary viral pneumonia, as all cultures for bacteria and fungi were negative.

Post mortem, influenza virus was cultured from a clinical specimen. It turned out to be the same general subtype as a seasonal influenza (A/H1N1) but unlike any strain then circulating but the same as a strain that was normally found in pigs: it was a swine flu. Four days before her first symptoms, this unfortunate young mother-to-be attended a county fair and visited a pig barn where veterinarians said some pigs had had pig influenza. No other illnesses were reported, but 76% of the exhibitors and several health care workers eventually seroconverted.

Influenza strains of the same subtype as human seasonal influenza (H1N1 and H3N2) are endemic in the US swine population but rarely infect humans and even more rarely are transmitted further, human to human. But it has happened, as in this case. The most famous incident was an outbreak among Army recruits at Fort Dix, NJ in 1976, involving hundreds of cases and several fatalities. Most swine flu viruses are so-called triple reassortants, i.e., the include among their 8 segmented genetic material segments originally from human, pig and bird influenza viruses.

Now infections with novel influenza A subtypes are now reportable in the US and we are learning that perhaps infections with swine flu viruses are not as rare as we previously thought:

On December 7, 2005, a previously healthy 17-year-old boy with no history of recent travel became ill; symptoms were headache, rhinorrhea, low back pain, and cough without fever. He had received inactivated influenza vaccine administered intramuscularly on November 11, 2005. During an outpatient clinic visit on December 8, 2005, a nasal wash specimen was obtained and tested positive for influenza A by rapid influenza diagnostic test (BinaxNow A&B, Binax, Inc., Scarborough, ME, USA). Results of a chest radiograph were normal. The patient's symptoms resolved on December 10, 2005. The specimen was sent to the Wisconsin State Laboratory of Hygiene (WSLH), and an influenza A virus was isolated by shell vial tissue cell culture (MDCK cells, WSLH, Madison, WI, USA). Real-time reverse transcription-PCR (rRT-PCR) was positive for influenza A virus but negative for human subtypes H1, H3, and Asian avian H5. At the Centers for Disease Control and Prevention (CDC), rRT-PCR testing of the shell vial viral culture material was positive for influenza A virus, but negative for human subtypes H1 and H3, as well as avian subtypes H5, H7, and H9. Complete genomic sequencing of the virus at CDC identified it as a swine influenza A (H1N1) triple reassortant virus, A/Wisconsin/87/2005 H1N1. (CDC, Newman et al., Emerging Infectious Diseases), CDC)

This is a case that would almost certainly have gone unreported without systematic surveillance and new diagnostic tools. Whether his relatively mild experience is related to his having just been vaccinated we don't know.

Now, via CIDRAP Reports, we are alerted to still another swine flu case via the CDC surveillance system:

The CDC said the patient was infected with a swine influenza A/H1N1 virus. Although human infections with swine flu viruses are uncommon, many years bring reports of isolated cases, the report said.

The Texas Department of State Health Services, in a flu surveillance activity report for the week ending Nov 15, said the patient got sick in mid October. His or her specimen was collected and the virus identified during routine influenza surveillance. Texas officials, who gave no details about the patient's illness, said their investigation found no illnesses in his or her household or close contacts. (Lisa Schnirring, CIDRAP News)

The more we learn about flu the more we learn that there's a lot more to learn about flu.