Swine Influenza A (H1N1) Virus
Author: Rick Kulkarni, MD, Medical Director, Assistant Professor of Surgery, Section of Emergency Medicine, Yale-New Haven Hospital
Contributor Information and Disclosures
Updated: Apr 28, 2009
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
Treatment Recommendations
Treatment is largely supportive and consists of bedrest, increased fluid consumption, cough suppressants, and antipyretics and analgesics (eg, acetaminophen, nonsteroidal anti-inflammatory drugs) for fever and myalgias. Severe cases may require intravenous hydration and other supportive measures. Antiviral agents may also be considered for treatment or prophylaxis (see Medications).
Patients should be encouraged to stay home if they become ill, to avoid close contact with people who are sick, to wash their hands often, and to avoid touching their eyes, nose, and mouth. The CDC recommend the following actions when human infection with swine flu is confirmed in a community:6
Home isolation
Patients who develop flulike illness (ie, fever with either cough or sore throat) should be strongly encouraged to self-isolate in their home for 7 days after the onset of illness or at least 24 hours after symptoms have resolved, whichever is longer.
To seek medical care, patient should contact their health care providers to report illness (by telephone or other remote means) before seeking care at a clinic, physician’s office, or hospital.
Patients who have difficulty breathing or shortness of breath or who are believed to be severely ill should seek immediate medical attention.
If the patient must go into the community (eg, to seek medical care), he or she should wear a face mask to reduce the risk of spreading the virus in the community when coughing, sneezing, talking, or breathing. If a face mask is unavailable, ill persons who need to go into the community should use tissues to cover their mouth and nose while coughing.
While in home isolation, patients and other household members should be given infection control instructions, including frequent hand washing with soap and water. Use alcohol-based hand gels (containing at least 60% alcohol) when soap and water are not available and hands are not visibly dirty. Patients with swine influenza should wear a face mask when within 6 feet of others at home.
Household contacts who are not ill
Remain home at the earliest sign of illness.
Minimize contact in the community to the extent possible.
Designate a single household family member as caregiver for the patient to minimize interactions with asymptomatic persons.
School dismissal and childcare facility closure
Strong consideration should be given to close schools upon a confirmed case of swine flu or a suspected case epidemiologically linked to a confirmed case.
Decisions regarding broader school dismissal within these communities should be left to local authorities, taking into account the extent of influenzalike illness within the community.
Cancelation of all school or childcare related gatherings should also be announced.
Encourage parents and students to avoid congregating outside of the school if school is canceled.
Duration of schools and childcare facilities closings should be evaluated on an ongoing basis depending on epidemiological findings.
Consultation with local or state health departments is essential for guidance concerning when to reopen schools. If no additional confirmed or suspected cases are identified among students (or school-based personnel) for a period of 7 days, schools may consider reopening.
Schools and childcare facilities in unaffected areas should begin preparation for possible school closure.
Social distancing
Large gatherings linked to settings or institutions with laboratory-confirmed cases should be canceled (eg, sporting events or concerts linked to a school with cases); other large gatherings in the community may not need to be canceled at this time.
Additional social distancing measures are currently not recommended.
Persons with underlying medical conditions who are at high risk for complications of influenza should consider avoiding large gatherings.
Patient education
Patients should be referred to the eMedicine Health article Swine Flu.
Medications
Laboratory testing has found the swine influenza A (H1N1) virus susceptible to the prescription antiviral drugs oseltamivir and zanamivir, and the CDC has issued interim guidance for the use of these drugs to treat and prevent infection with swine influenza viruses.7 As part of its preparation for the emergency, the US Department of Homeland Security is releasing 25% of stockpiled antiviral agents (ie, oseltamivir [Tamiflu], zanamivir [Relenza]).
The usual vaccine for influenza administered at the beginning of the flu season is not effective for this viral strain. Also, other antiviral agents (eg, amantadine, rimantadine) are not recommended because of recent resistance to other influenza strains documented over the past several years.
Basic supportive care (ie, hydration, analgesics, cough suppressants) should be prescribed. Empiric antiviral treatment should be considered for confirmed, probable, or suspected cases of swine influenza. Treatment of hospitalized patients and patients at higher risk for influenza complications should be prioritized.
Initiation of antiviral agents within 48 hours of symptom onset is imperative for providing treatment efficacy against influenza virus. In studies of seasonal influenza, evidence for benefits of treatment is strongest when treatment is started within 48 hours of illness onset. However, some studies of treatment of seasonal influenza have indicated benefit, including reductions in mortality or duration of hospitalization, even in patients in whom treatment was started more than 48 hours after illness onset. The recommended duration of treatment is 5 days.7
Prophylaxis with antiviral agents should also be considered in the following individuals (pre-exposure or postexposure):
Close household contacts of a confirmed or suspected case who are at high risk for complications (eg, chronic medical conditions, persons >65 y or <5>65 y or <5>65 y, pregnant women)
Individuals not considered to be at high risk but who are nonetheless traveling to Mexico, first responders, or border workers who are working in areas with confirmed cases
Antiviral Agents
Drugs indicated for treatment of swine influenza A (H1N1) virus include neuraminidase inhibitors (ie, oseltamivir and zanamivir).
Oseltamivir (Tamiflu)
Oseltamivir inhibits neuraminidase, which is a glycoprotein on the surface of influenza virus that destroys an infected cell's receptor for viral hemagglutinin. By inhibiting viral neuraminidase, this agent decreases the release of viruses from infected cells and, thus, viral spread. Oseltamivir is effective in the treatment of influenza A or B and must be administered within 48 hours of symptom onset. The sooner the drug is administered after symptom onset, the better the likelihood of a good outcome. Oseltamivir reduces the length of illness by an average of 1.5 days. (In a subgroup of high-risk patients, illness was reduced by 2.5 d.) In addition, the severity of symptoms is also reduced.
Oseltamivir is available as 30-mg, 45-mg, and 75-mg oral capsules and as a powder for suspension that contains 12 mg/mL after reconstitution.
Adult dose
Treatment for acute illness: 75 mg PO bid for 5 d
Prophylaxis: 75 mg PO qd
Pediatric dose
Treatment for acute illness and age <1 year
<3>1 year
<15>40 kg: Administer as in adults
Prophylaxis and age <1 year
<3>1 year
<15>40 kg: Administer as in adults
Zanamivir (Relenza)
Zanamivir inhibits neuraminidase, which is a glycoprotein on the surface of the influenza virus that destroys the infected cell's receptor for viral hemagglutinin. By inhibiting viral neuraminidase, release of viruses from infected cells and viral spread are decreased. Zanamivir is effective against both influenza A and B. The preparation of zanamivir is in powder form for inhalation via the Diskhaler oral inhalation device. Circular foil discs that contain 5-mg blisters of drug are inserted into the supplied inhalation device. Individuals with asthma or other respiratory conditions that may decrease ability to inhale drug should be given oseltamivir.
Adult dose
Treatment for acute illness: 10 mg inhaled orally bid for 5 d
Prophylaxis of household contact: 10 mg inhaled orally qd for 10 d (initiate within 36 h)
Prophylaxis for community outbreak: 10 mg inhaled orally qd for 28 d (initiate within 5 d of outbreak)
Pediatric dose
Treatment for acute illness
<7>7 years: Administer as in adults
Prophylaxis in household contact
<5>5 years: Administer as in adults
Prophylaxis in community outbreak
Adolescents 12-16 years: Administer as in adults
Additional pediatric considerations
Aspirin or aspirin-containing products (eg, bismuth subsalicylate [Pepto Bismol]) should not be included in the treatment of confirmed or suspected viral infection in persons aged 18 years or younger because of the risk of Reye syndrome. For relief of fever, other antipyretic medications (eg, acetaminophen, nonsteroidal anti-inflammatory drugs) are recommended.
Pregnant women
Oseltamivir and zanamivir are "Pregnancy Category C" medications, indicating that no clinical studies have been conducted to assess the safety of these medications in pregnant women. Because of the unknown effects of influenza antiviral drugs on pregnant women and their fetuses, oseltamivir or zanamivir should be used during pregnancy only if the potential benefit justifies the potential risk to the embryo or fetus; the manufacturers' package inserts should be consulted. However, no adverse effects have been reported among women who received oseltamivir or zanamivir during pregnancy or among infants born to women who have received oseltamivir or zanamivir. Pregnancy should not be considered a contraindication to oseltamivir or zanamivir use. Because zanamivir is an inhaled medication and has less systemic absorption, some experts prefer zanamivir over oseltamivir for use in pregnant women, when feasible.
A big read, but hey, your first scientific paper. Congrats.
