Swine Influenza A (H1N1) Virus part 1

[size=12][left]Introduction

Swine influenza is a highly contagious respiratory disease in pigs caused by one of several swine influenza A viruses.

Human cases of swine influenza A (H1N1) have been reported worldwide. In 2009, cases of influenzalike illness were first reported in Mexico on March 18; the outbreak was subsequently confirmed as swine influenza A.1 Investigation is continuing to clarify the spread and severity of swine influenza in Mexico. Suspected clinical cases have been reported in 19 of the country's 32 states. Although only 18 of the Mexican cases have been laboratory-confirmed as Swine Influenza A/H1N1 (12 of them genetically identical to Swine Influenza A/H1N1 viruses from California),1 approximately 1,600 cases and 103 deaths have been attributed to swine influenza in Mexico.2 Cases of swine influenza were subsequently confirmed in the United States, Canada, and the United Kingdom (Scotland), with suspected cases in France, Israel, and Brazil.

On April 26, 2009, the US Department of Health and Human Services declared a national public health emergency involving swine influenza A, citing its significant potential to affect national security.3 In the United States, 64 confirmed cases of swine flu have been reported as of April 28, 2009, in California (10 cases), Kansas (2 cases), New York (45 cases), Ohio (1 case), and Texas (6 cases).4 All affected patients have had mild influenzalike illness, with only two requiring brief hospitalization. No deaths have been reported in the United States.

Unlike typical influenza, most cases of swine influenza have occurred in previously healthy young adults.1

Government and public health officials are monitoring this situation worldwide to assess the threat from swine influenza and to provide guidance to health care professionals and the public. Because the situation is changing rapidly, it is important to check regularly for changes in recommendations as new information becomes available. Online resources for daily guidance include the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO).



This preliminary negative stained transmission electron micrograph depicts some of the ultrastructural morphology of the A/CA/4/09 swine flu virus. Courtesy of CDC/ C. S. Goldsmith and A. Balish.




This preliminary negative stained transmission electron micrograph depicts some of the ultrastructural morphology of the A/CA/4/09 swine flu virus. Courtesy of CDC/ C. S. Goldsmith and A. Balish.


Morbidity and Mortality

Swine influenza tends to cause high morbidity but low mortality rates (1%-4%). At the time of this writing during the 2009 outbreak (April 28, 2009), 149 deaths in Mexico have been attributed to swine influenza, with 22 of those deaths confirmed as resulting from the infection. In the United States, all cases of swine influenza in humans have been mild, with only two patients requiring brief hospitalization, and no deaths have been reported.

Symptoms

Manifestations of swine influenza are similar to those of seasonal influenza. Patients present with symptoms of acute respiratory illness, including at least 2 of the following:


Fever
Cough
Sore throat
Body aches
Headache
Chills and fatigue
Diarrhea and vomiting (possible)
Persons with these symptoms should call their health care provider promptly. If an antiviral agent is warranted, it should ideally be initiated with 48 hours from the onset of symptoms (see Medications). The duration of illness is typically 4-6 days.

Viral Strain and Testing

Outbreaks of swine influenza are common in pigs year-round. Historically, when humans have become infected, it is a result of close contact with infected pigs. However, the current virus is a novel influenza A (H1N1) virus not previously identified in humans, and it appears to be spread by human-to-human transmission. The WHO has raised its pandemic alert level for swine influenza to phase 4, which signifies sustained community-wide outbreaks of human-to-human transmission.



Phase 4 is characterized by verified human-to-human transmission of an animal or human-animal influenza reassortant virus able to cause "community-level outbreaks." The ability to cause sustained disease outbreaks in a community marks a significant upwards shift in the risk for a pandemic. Any country that suspects or has verified such an event should urgently consult with WHO so that the situation can be jointly assessed and a decision made by the affected country if implementation of a rapid pandemic containment operation is warranted. Phase 4 indicates a significant increase in risk of a pandemic but does not necessarily mean that a pandemic is a forgone conclusion. Courtesy of the WHO.


In the current 2009 outbreak in the United States, preliminary testing has shown that, in all cases, the viruses have the same genetic pattern. The virus is being described as a new subtype of influenza A/H1N1 not previously detected in swine or humans.

Clinicians should consider the possibility of swine influenza virus infections in patients who present with febrile respiratory illness. The CDC criteria for suspected swine influenza are as follows:5

Acute febrile respiratory illness in a person who resides in a community with onset (1) within 7 days of close contact with a person who has a confirmed case of swine influenza A (H1N1) virus infection or (2) within 7 days of travel to community (within the United States or internationally) where one or more swine influenza A (H1N1) cases have been confirmed.
Acute febrile respiratory illness in a person who resides in a community where at least one swine influenza case has been confirmed.
If swine flu is suspected, the clinician should obtain a respiratory swab for swine influenza testing and place it in a refrigerator (not a freezer). Once collected, the clinician should contact his or her state or local health department to facilitate transport and timely diagnosis at a state public health laboratory.

Laboratories should send all influenza A specimens that they are unable to subtype to the Viral Surveillance and Diagnostic Branch of the CDC's Influenza Division as soon as possible for further diagnostic testing.6