H1N1 Influenza

Q&A with CDC Neuroepidemiologist James J. Sejvar, MD

May 4, 2009

1. What is H1N1 influenza (swine flu)?

Swine flu refers to strains of the influenza virus that are endemic (e.g., passed back and forth naturally) to pigs.

2. How do you define "epidemic?"

An epidemic is new cases of a disease which occur over a period of time at an occurrence of greater than what would be "expected". Put simply, it is the occurrence of more cases of an illness than would be expected to occur in the population.

3. Is this H1N1 flu virus different than previous ones? If so, how?

This strain of swine flu (H1N1 flu) appears to be different than strains that have come before it, as it appears to have components of the genetic material of human, avian (bird), and swine influenza viruses. This combination has not been previously observed. We do not know how or if it will behave differently in humans, with respect to illness.

4. Are there any clinically useful tests to establish diagnosis?

H1N1 flu can be suspected on the basis of a clinically compatible respiratory illness. However, it can only be confirmed by laboratory testing. The preferred specimen for testing is a nasopharyngeal swab or nasal aspirate. Instructions on specimen collection are available on the CDC website.

As for laboratory testing, real-time RT-PCR for influenza A, B, H1, H3 at a State Health Department Laboratory is recommended. Currently, swine-origin influenza A (H1N1) virus will test positive for influenza A and negative for H1 and H3 by real-time RT-PCR. If reactivity of real-time RT-PCR for influenza A is strong (e.g., Ct <30) it is more suggestive of a novel influenza A virus. Confirmation as swine-origin influenza A (H1N1) virus is performed at CDC currently, but will be available at state public health laboratories in the very near future.

5. How should physicians handle patients with possible H1N1 flu?

Physicians should practice good infection control and hygiene when interacting with patients with suspected H1N1 flu, particularly in a health care setting. Stringent hand washing between patients is very important. Physicians should avoid touching their nose, eyes, or mouth following touching a patient with suspected influenza until hand washing has been accomplished. Physicians who feel that they are ill with influenza should remain at home. Physicians should emphasize these same principles (careful hand washing, covering mouth with coughing, remaining at home) if feeling ill with "the flu" to patients and family members as well.

6. Are there immediate or delayed neurologic complications of H1N1 flu?

To date, no neurologic illnesses or events have been seen in the H1N1 cases. Ongoing surveillance is being conducted to assess these potential complications, however.

In the setting of seasonal influenza, rare cases of influenza-associated encephalopathy (IAE) have been reported, particularly among children. The underlying mechanism of the encephalopathy is not well understood. IAE is characterized by the occurrence of neurologic symptoms within a few days to a week following initial influenza-like illness; fever and altered mental status are common, and seizures are frequently reported. Focal neurologic signs, including paresis, movement disorders, cranial nerve palsies, and aphasia, may develop. CSF is generally unremarkable, with absence of pleocytosis and normal or mildly elevated protein levels.

IAE has been more frequently reported with influenza A, especially with A (H3N2) virus infection, but has been reported with influenza B. We do not know if the H1N1 strain might be associated with similar cases. Acute necrotizing encephalopathy (ANE), a particularly severe manifestation of para-infectious encephalopathy, has been described with seasonal influenza. ANE is characterized by a fulminant and monophasic course; multifocal necrotizing brain lesions predominant in the thalami and brainstem are seen on neuroimaging.

Case reports of other neurologic features, including acute demyelinating inflammatory neuropathy (AIDP), acute disseminated encephalomyelitis (ADEM) and transverse myelitis (TM), as well as brachial neuritis have also been reported in the setting of seasonal influenza, although such reports are few.

Whether these neurologic features might be observed in the setting of H1N1 influenza infection is not known.

7. What are the treatments for H1N1 flu?

Recommendations for use of antivirals may change as data on antiviral effectiveness, clinical spectrum of illness, adverse events from antiviral use, and antiviral susceptibility data become available. Antiviral treatment should be considered for confirmed, probable or suspected cases of swine-origin influenza A (H1N1) virus infection. Treatment of hospitalized patients and patients at higher risk for influenza complications should be prioritized.

Antiviral treatment with zanamivir (Relenza) or oseltamivir (Tamiflu) should be initiated as soon as possible after the onset of symptoms. Evidence for benefits from treatment in studies of seasonal influenza is strongest when treatment is started within 48 hours of illness onset, with treatment continued for five days. Areas that continue to have seasonal influenza activity, especially those with circulation of oseltamivir-resistant human A (H1N1) viruses, might prefer to use either zanamivir or a combination of oseltamivir and rimantadine or amantadine to provide adequate empiric treatment or chemoprophylaxis for patients who might have human influenza A (H1N1) infection.

8. Are there immediate or delayed neurologic complications of treatments for H1N1 flu?

The neurologic complications of zanamivir are generally benign and include headache, dizziness, or vertigo; reported in less than 5% of patients.

Side effects from oseltamivir are similar. In addition, various neuropsychiatric side effects, including delirium or self-injury, have been seen rarely following oseltamivir use. The FDA recommends that people be monitored closely for abnormal behavior following oseltamivir use.

 email to a friend  |   print