Swine influenza epidemiology and demographics

Swine influenza Microchapters
Home
Patient Information
Overview
Historical Perspective
Classification
Pathophysiology
Causes
Differentiating Swine influenza from other Diseases
Epidemiology and Demographics
Risk Factors
Natural History, Complications and Prognosis
Diagnosis
Diagnostic Criteria
History and Symptoms
Physical Examination
Laboratory Findings
Other Imaging Findings
Other Diagnostic Studies
Treatment
Medical Therapy
Surgery
Primary Prevention
Secondary Prevention
Future or Investigational Therapies
Case Studies
Case #1
Swine influenza epidemiology and demographics On the Web
Most recent articles
Most cited articles
Review articles
CME Programs
Powerpoint slides
Images
American Roentgen Ray Society Images of Swine influenza epidemiology and demographics All Images X-rays Echo & Ultrasound CT Images MRI
Ongoing Trials at Clinical Trials.gov
US National Guidelines Clearinghouse
NICE Guidance
FDA on Swine influenza epidemiology and demographics
CDC on Swine influenza epidemiology and demographics
Swine influenza epidemiology and demographics in the news
Blogs on Swine influenza epidemiology and demographics
Directions to Hospitals Treating Swine influenza
Risk calculators and risk factors for Swine influenza epidemiology and demographics

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Epidemiology

Outbreaks in swine

2007 Philippine outbreak

On August 20, 2007 Department of Agriculture officers investigated the outbreak of swine flu in Nueva Ecija and Central Luzon, Philippines. The mortality rate is less than 10% for swine flu, unless there are complications like hog cholera. On July 27, 2007, the Philippine National Meat Inspection Service (NMIS) raised a hog cholera "red alert" warning over Metro Manila and 5 regions of Luzon after the disease spread to backyard pig farms in Bulacan and Pampanga, even if these tested negative for the swine flu virus.

Outbreaks in humans

Swine flu has been reported numerous times as a zoonosis in humans, usually with limited distribution, rarely with a widespread distribution. The 1918 flu pandemic in humans was associated with H1N1, thus may reflect a zoonosis either from swine to humans or from humans to swine. Evidence available from that time is not sufficient to resolve this question.

1976 U.S. outbreak

On February 5, 1976, an army recruit at Fort Dix said he felt tired and weak. He died the next day and four of his fellow soldiers were later hospitalized. Two weeks after his death, health officials announced that swine flu was the cause of death and that this strain of flu appeared to be closely related to the strain involved in the 1918 flu pandemic. Alarmed public-health officials decided that action must be taken to head off another major pandemic, and they urged President Gerald Ford that every person in the U.S. be vaccinated for the disease.

However, the vaccination program was plagued by delays and public relations problems. But on Oct. 1, 1976, the immunization program began and by Oct. 11, approximately 40 million people, or about 24% of the population, had received swine flu immunizations. That same day, three senior citizens died soon after receiving their swine flu shots and there was a media outcry linking the deaths to the immunizations, despite not having any positive proof. According to science writer Patrick Di Justo, however, by the time the truth was known - that the deaths were not proven to be related to the vaccine - it was too late. "The government had long feared mass panic about swine flu -- now they feared mass panic about the swine flu vaccinations." This became a strong setback to the program.

There were reports of Guillain-Barré syndrome, a paralyzing neuromuscular disorder, affecting some people who had received swine flu immunizations. As a result, Di Justo writes that "the public refused to trust a government-operated health program that killed old people and crippled young people." In total, less than 33 percent of the population had been immunized by the end of 1976. The National Influenza Immunization Program was effectively halted on Dec. 16.

Overall, about 500 cases of Guillain-Barré syndrome (GBS), resulting in death from severe pulmonary complications for 25 people, which, according to Dr. P. Haber, were probably caused by an immunopathological reaction to the 1976 vaccine. Other influenza vaccines have not been linked to GBS, though caution is advised for certain individuals, particularly those with a history of GBS.

2009 swine flu outbreak

See also: Live map of swineflu, H1N1 live map The new strain of influenza involved in the 2009 swine flu outbreak strain is a reassortment of several strains of influenza A virus subtype H1N1 that are, separately, endemic in humans and in swine. Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in United States pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions present in European swine flu isolates. Viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, but there is no formal national surveillance system to determine what viruses are circulating in pigs in the United States.

The origins of this new strain remain unknown. One theory is that Asian and European strains traveled to Mexico in migratory birds or in people, then combined with North American strains in Mexican pig factory farms before jumping over to farm workers.

  Confirmed cases followed by death

  Confirmed cases

  Unconfirmed or suspected cases

The earliest known human influenza A virus subtype H1N1 case was at a Mexican pig farm whose nearby neighbors had been complaining about the manure smell and flies.

Edgar Hernandez, 4, was suffering from ordinary influenza but laboratory testing has since shown that he had contracted human influenza A virus subtype H1N1. The boy went on to make a full recovery.

The Mexican health agency acknowledged that the original disease vector of the virus may have been flies multiplying in manure lagoons of pig farms near Perote, Veracruz, owned by Granjas Carroll, a subsidiary of Smithfield Foods.

Dr. Anne Schuchat, interim Deputy Director for CDC Science and Public Health, said that the American cases were found to be made up of genetic elements from four different flu viruses – North American swine influenza, North American avian influenza, human influenza, and swine influenza virus typically found in Asia and Europe – "an unusually mongrelised mix of genetic sequences." Pigs have been shown to act as a potential "mixing vessel" in which reassortment can occur between flu viruses of several species. This new strain appears to be a result of reassortment of human influenza and swine influenza viruses, presumably due to superinfection in an individual human. Influenza viruses readily undergo reassortment because their genome is split between eight pieces of RNA (see Orthomyxoviridae).

The influenza A virus subtype H1N1 can adapt and spread more efficiently than previously known H1N1 strains. Moreover, co-infection of H1N1 swine flu and Oseltamivir resistant H1N1 season flu can lead to acquisition of Influenza A virus subtype H274Y (H274Y) by the swine flu via recombination or reassortment. Swine H1N1 with human H1 and N1 have been reported. Moreover, the swine flu can also infect swine and acquire more polymorphisms that could lead to increased virulence.

The 1918 pandemic strain has polymorphism from swine and human H1N1 in all eight pieces of RNA gene segments. Similar swapping of polymorphism in human co-infected with season and swine H1N1 can lead to rapid evolution.