Swine Influenza

PATHCHAT NO 18: SWIN INFLUENZA

Introduction

A novel Influenza virus A/H1N1 has caused worldwide concern due to the lack of immunity in the general population and the risk of the development of a pandemic. This new virus has elements of swine, human and avian viruses normally found in Europe and Asia. Initial concern was raised when an influenza outbreak occurred out of season and caused severe and fatal infections in young, healthy adults. This virus was then genetically analysed and found to be a novel influenza A/H1N1 virus containing swine, human and avian elements. The name swine flu was given due to the viral elements traditionally found found in swine populations. There has been no epidemiological evidence of swine exposure.

Subsequent to the initial scare surrounding increased mortality, swine Influenza A\H1N1 has proven to be a mild influenza virus with relatively low morbidity and mortality. Patients often have mild symptoms indistinguishable from the common cold. Fever, which is usually one of the main presenting symptoms of influenza, is often absent. Recent data from Peru has indicated that a third of their patients who are infected with swine flu are asymptomatic.

Does the current seasonal influenza vaccine protect against swine influenza?

It is unknown whether the current seasonal vaccine will provide protection against swine Influenza A\H1N1. The vaccine does not include any swine influenza elements. Vaccines are under development internationally, but we are unsure whether they will be available in time for the next influenza season.

Antiviral therapy and Prophylaxis

The swine Influenza A\H1N1 virus identified in Mexico and USA is sensitive to zanamivir and oseltamivir, but resistant to amantadine and rimantadine. Isolated cases of oseltamivir resistance has been reported worlwide. Oseltamivir tablets (Tamiflu®) and zanamivir inhalation (Relenza®) are currently recommended for treatment of severe suspected (probable) cases and confirmed swine influenza A/H1N1 illness in high risk individuals, and as prophylaxis for high risk contacts of suspected/probable and confirmed cases. Oseltamivir tablets (Tamiflu®) are registered for use in children > 1 year of age and zanamivir inhalation (Relenza®) in children > 12 years. The recommended dosage for therapy in adults is oseltamivir 75mg bd for 5 – 7 days and zanamivir two 5mg inhalations twice daily for 5 – 7 days. The recommended dose in children >1 year and < 15kg is 30 mg oseltamivir bd, for children 15 – 23kg 45mg bd and for children 23 – 40 kg 60mg bd, all for 5 – 7 days. Children > 40 kg should receive the adult dose. The recommended dosage for prophylaxis in adults is oseltamivir 75mg daily for 10 days and zanamivir one 5mg inhalation daily for 10 days. The recommended dose for prophylaxis in children >1 year and < 15kg is 30 mg oseltamivir daily, for children 15 – 23kg 45mg daily and for children 23 – 40 kg 60mg daily, all for 5 – 7 days. Children > 40 kg should receive the adult dose.

No clinical studies have been conducted to assess the safety of oseltamivir and zanamivir for pregnant women. Oseltamivir or zanamivir should be used during pregnancy where the potential benefit justifies the potential risk to the embryo or fetus. The manufacturers' package inserts should be consulted. Because zanamivir is an inhaled medication and has less systemic absorption, some experts prefer zanamivir over oseltamivir for use in pregnant women when feasible (from: Communicable Disease Surveillance bulletin August 2009 Vol 7 No 3).

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Swine Influenza