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by Amesh Adalja, MD
Fellow at the Center for Biosecurity
Chief Fellow in the Division of Infectious Diseases
University of Pittsburgh Medical Center
{last update: 11:59 am EST}
- US confirms second death (33 year old pregnant woman from Texas)
- 41 states have reported a total of 642 cases
- At least 35 hospitalizations have occurred in the US
- WHO reports 22 countries have reported cases (including Southern Hemisphere countries)
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Swine Flu Update: May 4
- 226 confirmed cases in the US
- WHO considering moving to "Stage 6"
Swine Flu Update: May 1
- 109 cases in the US in 10 states
- South Carolina newest state with confirmed cases
- 11 countries now have cases, up from 9 yesterday
- New cases in Switzerland, Netherlands
Swine Flu Update: April 30
- WHO upgrades to stage 5
- 9 countries with confirmed cases
- New cases in Germany, England, and Austria
Q: If 36,000 Americans died during a recent year from seasonal flu, is that not about 100 per day? During the flu season, maybe twice that many, with half as many during the off season? How is swine flu special? I sort of understand that this could be different because it is a unique strain, but I think we need some perspective.
Keep up the good work.
Rich Wilson DO
A: It is too early to tell how this novel virus will spread throughout the world and what level of mortality to expect. While numerous deaths have been reported in Mexico, it is unclear what the total burden of cases (including mild cases) is in Mexico. It is true that seasonal flu kills 36,000 Americans yearly, but what increment the novel virus will produce remains to be seen. However, the fact that a novel influenza virus--to which people have no immunity and no vaccine is available--is efficiently spreading from person-to-person, is an extremely serious situation.
Swine Flu Update: April 29
- There are 91 cases reported in the US
- New cases have cropped up in Arizona, Indiana, Michigan, Massachusetts and Nevada
- First US swine flu death confirmed in Texas
- 5 hospitalizations (3 in CA, 2 in TX) have occurred.
- Worldwide seven countries (USA, Canada, Mexico, Spain, New Zealand, and Israel) have confirmed cases according to the WHO.
- The CDC's antiviral guidance was updated and now does not include recommendations for dual therapy with an adamantane to cover seasonal influenza infection if oseltamivir is used.
- An Emergency Use Authorization was also issued allowing for the use of oseltamivir in those aged under one year.
Swine Flu Update: April 28
What every emergency physician needs to know about swine flu today*
On April 21, 2009, the Centers for Disease Control and Prevention (CDC) issued an MMWR Dispatch describing 2 cases of swine influenza A (H1N1) infection that occurred in Southern California in April. While both patients recovered uneventfully, the isolated viruses harbor novel genetic characteristics not seen in swine flu isolates in the U.S. prior to this event. The other striking feature of these cases is that there was no known contact with swine, raising the question of efficient human-to-human transmission of this virus [1] Subsequent investigation has uncovered 40 additional cases in the United States—all of whom have recovered uneventfully—and reports of severe morbidity and mortality in Mexico. Several other countries including Canada, Scotland, and Spain are reporting confirmed cases.
Why the Emergency Physician Should Know about Swine Flu
As with all emerging infectious diseases, emergency physicians are likely to find themselves on the front line contronting swine flu. It is vital that they be informed with the best information when confronting this potentially deadly virus.
Swine Flu 101
Influenza viruses circulate among waterfowl, swine, and humans, but other mammals may also be infected. Until 1997, avian viruses were thought to be unable to infect humans directly, as they were thought to require a “mixing-vessel” (e.g., swine) as an intermediary to allow the virus to adapt to humans. The experience with the H5N1 virus and other wholly avian viruses has proven that this step is unnecessary. However, swine do potentially play a major role in influenza epidemics given their ability to be infected efficiently with both avian and human strains, thus creating a potential platform for the recombination of viruses from different lineages. Swine are receptive to infection from avian and human viruses because they possess receptors for both. While avian influenza preferentially binds to receptors containing alpha-2,3 sialic acid-galactose linkages, human viruses bind to alpha-2,6 linkages. Humans contain only alpha 2,6 residues in the upper respiratory tract, while pigs have both types. Once an avian virus is in swine, it can develop tropism for human receptors.[2] A recent report from Indonesia indicates that H5N1 viruses have attenuated their virulence in swine, suggesting a degree of adaptation to mammalian hosts.[3] However, prior to the current situation there was no evidence to date that a pandemic strain has originated from swine, and this hypothesis was being questioned.[2]
Swine Flu in Humans
The first isolation of a swine flu virus from a human occurred in 1974, confirming a long held suspicion that swine flu viruses could infect humans.[4] A 2007 review article on this topic presented data from 50 cases reported in the medical literature and offered several salient points regarding human cases:
• Case fatality rates were 14% (likely reflecting case ascertainment bias).
• 61% of civilian cases reported swine exposure.
• Person-to-person transmission did occur.[4]
CDC reports the receipt of approximately 1 human swine flu case report every 1 to 2 years (swine flu has been a nationally reportable condition since 2007). Since 2005, 12 cases have been reported in the U.S., most with exposure to pigs.1
Most swine influenza infections do not have a presentation distinct from human influenza infections. Seroprevalence studies have shown 23% positivity in those with occupational exposure to pigs.[2]
The Fort Dix Incident
The most widely known incident of swine flu in humans centers around an outbreak of a lethal influenza virus at Fort Dix in New Jersey in 1976. During that outbreak, 13 soldiers had severe respiratory illness, and 1 soldier died. A novel H1N1 swine influenza virus (Hsw1N1) was isolated, and approximately 230 other soldiers displayed evidence of infection.5 The virus did not spread outside Fort Dix, no swine exposure was ever elucidated, and swine were never definitively established as the source.[2,5] The incident prompted a massive vaccination campaign that was plagued with problems.[2,8]
The Current Outbreak
The Basics
•Swine Flu is a reassortant type A influenza virus that originated from swine at some point in the past
•Unlike past infections with swine flu, the current outbreak strain efficiently spreads from person to person—secondary attack rates are approximately 20%
•The diagnosed cases in the US represent only the tip of the iceberg, it is likely to be circulating in your community
Symptoms
•All cases outside of Mexico reported to date have consisted of mild influenza-like illness with the exception of the prominence of nausea, vomiting, and diarrhea
•The US should be prepared to see severe cases with fulminant pneumonia given the experience in Mexico
•The incubation period is approximately one to seven days
•The infectiousness period begins 1 day prior to symptoms and continues for seven days after symptoms commence
Diagnosis
•Standard rapid tests may be falsely negative, but could be positive for influenza A
•RT-PCR testing is required to confirm the diagnosis of swine flu
•Viral culture can also be used to isolate the virus
•The CDC has published case definitions for possible, probable and confirmed cases:
A confirmed case of swine influenza A (H1N1) virus infection is defined as a person with an acute febrile respiratory illness with laboratory confirmed swine influenza A (H1N1) virus infection at CDC by one or more of the following tests:
•real-time RT-PCR
•viral culture
A probable case of swine influenza A (H1N1) virus infection is defined as a person with an acute febrile respiratory illness who is:
•positive for influenza A, but negative for H1 and H3 by influenza RT-PCR, or
•positive for influenza A by an influenza rapid test or an influenza immunofluorescence assay (IFA) plus meets criteria for a suspected case
A suspected case of swine influenza A (H1N1) virus infection is defined as a person with acute febrile respiratory illness with onset
•within 7 days of close contact with a person who is a confirmed case of swine influenza A (H1N1) virus infection, or
•within 7 days of travel to community either within the United States or internationally where there are one or more confirmed swine influenza A(H1N1) cases, or
•resides in a community where there are one or more confirmed swine influenza cases.
Treatment
•The virus is susceptible to treatment with the neuraminidase inhibitors, oseltamivir and zanamivir; it is resistant to amantadine and rimantidine
•Standard treatment for seasonal flu should be employed (i.e. either zanamivir alone or oseltamivir in combination with an adamantane to cover the possibility of resistant seasonal flu)
•Antiviral guidelines are available at: http://www.cdc.gov/swineflu/recommendations.htm
Infection Control •Full airborne protections are warranted (N-95 masks)
•Vaccination with seasonal influenza vaccine is not protective
Are we on the verge of a pandemic?
Prior to the current outbreak the World Health Organization (WHO) had designated the pandemic status of the world as stage 3, reflecting the limited cases of H5N1 Avian influenza infections in humans without evidence of human-to-human spread.On April 27, 2009 the WHO has raised the pandemic level to stage 4 indicating small clusters of localized human to human spread of the swine influenza.
The current reports of dozens of cases of swine flu worldwide, in individuals with no epidemiologic link to swine is clear evidence of intrusion of a novel influenza virus into the human species with potential for human-to-human spread. As the epidemic evolves it will be essential to understand patterns of illness and transmission characteristics. As this virus is making its debut at the tail end of the Northern Hemisphere’s influenza season, there is hope that the outbreak may abate. However Southern Hemisphere countries, whose flu season is about to begin, have reported suspect cases the virus may now begin to circulate in that hemisphere and reappear again in the Northern Hemisphere in the fall of 2009.
Emergency physicians will continue to be one of the first lines of defense against this virus and the care and advice they will provide will be an essential component of what may be a marathon battle against this virus.
References
3. Takano R, Nidom CA, Kiso M, et al. A comparison of the pathogenicity of avian and swine H5N1 influenza viruses in Indonesia. Arch Virol 2009; 154: 677-681. http://tiny.cc/FwqJd. Accessed April 23, 2009.
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