December 9, 1997
From the drug sensitivity study of the isolates from the 2 H5N1 cases at Centres for Disease Control and Prevention (CDC), it has been shown that the H5N1 virus is sensitive to amantadine. This drug is an effective agent for the treatment and prophylaxis of influenza A (but not B). However, it is prudent to note that the influenza viruses can rapidly develop resistance to this drug. Hence, doctors are advised to use the drug appropriately for treatment or prophylaxis of influenza A. The following guidelines which have incorporated the advice from the CDC experts are recommended for doctors' reference.
Confirmed case of H5N1 infection
Amantadine 100mg twice a day for 5 days can be used to treat cases of H5N1 infection. If started within 48 hours of the start of illness, amantadine can reduce the severity and shorten the duration of illness. Doses should be reduced for children and elderly, and those with underlying renal diseases. For children aged 1 to 9, the dosage is 5mg/kg/day in 2 divided doses up to 150 mg. For children aged greater than 9, adult dosage can be used but if the body weight of the child is less than 45kg, use the regime of 5mg/kg/day in 2 divided doses up to 150 mg.
Symptomatic or Positive Contacts of H5N1 cases
Close contacts, i.e. home contacts and medical staff providing direct care to patients with H5N1 infection, should be put on medical surveillance. If they develop symptoms compatible with influenza (fever of 38 o C or higher, together with cough or sore throat), they should have a throat swab taken for viral cultures. Treatment with amantadine (100mg twice for 5 days) can be started pending viral culture results.
Chemoprophylaxis for contacts
Amantadine should only be prescribed as chemoprophylaxis for close contacts upon the advice of the health officer in charge of the investigation of the institutional outbreaks.
Side effects
Amantadine can cause neurological and gastrointestinal side effects. In one study of healthy adults, approximately 14% of those treated with amantadine developed side effects. Neurological side effects include nervousness, anxiety, difficulty in concentrating and dizziness. More serious neurological side effects like marked behavioural changes, delirium, hallucinations, agitation and seizures have been observed. Gastrointestinal side effects include nausea, vomiting, abdominal pain and constipation. These side effects will stop once the drug has been withdrawn. Cautions must be exercised for people with renal insufficiency and in the elderly age group. The drugs are contraindicated for persons with seizure disorders.
It should be noted that winter season is not the high season for influenza in Hong Kong. Many common cold or flu-like illnesses are caused by other viruses like adenoviruses, parainfluenza viruses etc.