Swine Flu (Influenza A H1N1v)

Synonyms: swine influenza.

Swine flu is an influenza A virus most frequently of subtype H1N1, usually found in pigs but can be transferred to humans. Until recently, human-to-human spread was rare.

Influenza viruses often mutate and this new strain has a unique gene segment combination not previously seen. Because this influenza A virus is sufficiently different from previous viruses, the human community initially may have little immunity to it. It appears the young are at most risk and those aged >60 years at least risk.¹

The 2009 outbreak began in Mexico but has spread across the world with the first case confirmed in the UK on 27 April 2009. It has reached pandemic status in the UK - with human-to-human spread causing “community-level outbreaks”. There have so far been 621,000 cases of swine flu in the UK, with 151 deaths (up to November 2009). Epidemics are occurring worldwide with considerable morbidity and mortality.^2,3

Ongoing up-to-date incidence data are available from the Health Protection Agency (HPA).⁴

Symptoms are similar to those of the usual human seasonal influenza infection, with most cases in adults and children being mild. The incubation period is thought to be between two and five days, but may be up to seven days.

Clinicians are encouraged to diagnose swine flu based on symptoms if there is a pyrexia (≥ 38°C), fever or history of fever and flu-like illness (two or more of the following symptoms: cough, sore throat, rhinorrhoea, widespread muscle and joint aches, headache). There may also be any of the following: fatigue, loss of appetite and sometimes diarrhoea, nausea, vomiting, otitis media and, rarely, cerebral irritability ± seizures.¹

Atypical symptoms in children

These include haematemesis, photophobia, chest pain, epistaxis, croup, apnoea, and rigors. Very young children and babies can present with apnoea, reduced tone and poor feeding (without classical influenza features) and may exhibit a sudden severe collapse (apparent life-threatening episode).¹

It is no longer appropriate to test patients in primary care or emergency departments where admission is not required.²

Testing may be appropriate if patients are hospitalised (for control of symptoms or placing patients with swine flu together or if there are atypical symptoms where the diagnosis is particularly in doubt).

Flu-like symptoms may be a feature of many other diseases, such as bacterial pneumonia, meningitis, etc.
In pregnant women, always consider pulmonary embolus (chest pain, tachypnoea and tachycardia) and preeclampsia (epigastric pain, headaches and elevated BP)

The Department of Health has produced comprehensive guidelines - one for adults and children,¹ the other for pregnant women.⁵
Follow the appropriate guidance algorithm:

• Initially manage the patient by telephone where possible - avoid patient coming to surgery.
  Children under 1 year who are unwell with fever and flu should be seen to exclude serious infection.
• Antivirals - use as early as possible (ideally within first 48 hours) - but benefit has been shown up to seven days:¹
  • Give priority to early treatment with antivirals of people in higher-risk groups:⁶
    • Young children under 5 years old. Children below 1 year of age may be treated with low-dose oseltamivir (seek paediatric advice if any concerns)
    • Pregnant women - patients with uncomplicated influenza should be offered antivirals. Zanamivir is preferred (inhaled, hence less systemic exposure), although either can be used and oral may be better if there is respiratory disease or difficulty with inhalers.⁵
    • Immunosuppression (whether caused by disease or treatment)
    • Chronic lung, heart, kidney, liver or neurological disease
    • Diabetes mellitus
    • Patients who have had drug treatment for their asthma within the past three years
    • People aged 65 years and older
  • In considering treatment of people aged 5-64 years with no underlying illness, doctors are advised to take a precautionary approach and offer antivirals to patients with symptoms strongly suggestive of swine flu. It is acknowledged that doctors must use their clinical judgement and will not usually choose to offer antivirals in circumstances where there are minor or unrelated respiratory symptoms.
  • Testing has shown that the human swine influenza A/H1N1v can be treated with the antiviral oseltamavir (Tamiflu®) and zanamivir (Relenza®).⁷
    • Standard treatment dose of oseltamivir in adults (aged >13 years old) is 75 mg bd for 5 days. Paediatric suspension of oseltamavir is available (see SPC for dosages),⁸ and the hard capsules can be opened and contents sprinkled on food when suspension is not available (see SPC).⁹
    • Standard treatment dose of zanamivir is 10 mg bd for 5 days (inhaled),¹⁰ and is more suitable for pregnant patients and those with renal failure.¹¹
  • Prophylaxis should not usually be offered to contacts of cases unless, for example, a household member has serious underlying health problems or there are other special circumstances.
• Antibiotics¹
  • Consider empiric 5-day course of antibiotics where patient appears to have influenza, complicated by lower respiratory tract infection, pustular tonsillitis/sore throat, severe painful cervical lymphadenopathy or acute suppurative otitis media.
  • Consider prophylactic antibiotics in patients with co-morbidities (e.g. chronic lung, heart, renal, liver or neuromuscular disease and immunosuppression) or chronic obstructive pulmonary disease.
  • Children and pregnant women should be given co-amoxiclav or clarithromycin if penicillin-allergic. Other adults should receive doxycycline or co-amoxiclav.¹
• If severe or life-threatening illness, refer to hospital for further management.
  Apply algorithm to see whether admission criteria are met - and refer those patients to hospital:¹²
  1. Severe respiratory distress:
     • Adult: severe breathlessness, e.g. unable to complete sentences in one breath. Use of accessory muscles, supraclavicular recession, tracheal tug or feeling of suffocation.
     • Child: lower chest wall indrawing, sternal recession, grunting, or noisy breathing when calm.
  2. Increased respiratory rate measured over at least 30 seconds:
     • Adult: over 30 breaths per minute.
     • Child: ≥50 breaths per minute if under 1 year, or ≥40 breaths per minute if ≥1 year.
  3. Oxygen saturation ≤94% on pulse oximetry (breathing air). Absence of cyanosis is a poor discriminator for severe illness.
  4. Respiratory exhaustion:
     • Adult: new abnormal breathing pattern, e.g. alternating fast and slow rate or long pauses between breaths.
     • Child: respiratory exhaustion or apnoeic episode - apnoea defined as a ≥20-second pause in breathing.
  5. Evidence of severe clinical dehydration or clinical shock:
     • Adult: systolic blood pressure <90 mmHg and/or diastolic blood pressure <60 mmHg. Sternal capillary refill time >2 seconds, reduced skin turgor.
     • Child: evidence of severe clinical dehydration or clinical shock. Sternal capillary refill time >2 seconds, reduced skin turgor, sunken eyes or fontanelle.
  6. Altered conscious level or seizures:
     • Adult: new confusion, striking agitation or seizures.
     • Child: strikingly agitated or irritable, seizures, or floppy infant.
  7. Any sign of sepsis.
  8. Causing other clinical concern to their own GP or clinical team: for example, a rapidly progressive or an unusually prolonged illness.

Antiviral prophylaxis should not ordinarily be given to contacts. However, clinical judgement should be used where risk is identified to particularly vulnerable individuals.

Swine flu vaccination began in the UK in November 2009 (see separate record Influenza Vaccination).

As described in the separate Influenza record. Influenza-related viral pneumonia, acute respiratory distress syndrome and bacterial co-infection are common. Myocarditis with associated tachycardia has been observed.¹

1. Pandemic H1N1 2009 influenza: clinical management guidelines for adults and children. Department of Health (2009)
2. Swine Influenza - Health Protection Agency UK
3. Swine Influenza - World Health Organization
4. Influenza Weekly Epidemiological Update - Health Protection Agency.
5. Pandemic H1N1 2009 influenza: clinical management guidelines for pregnancy, Department of Health (2009)
6. New H1N1v Influenza: Current situation and next steps. Chief Medical Officer, Department of Health CEM/CMO/2009/16 (2 July 2009)
7. Summary of Product Characteristics - Relenza® 5 mg/dose inhalation powder (zanamivir) GlaxoSmithKline UK; Electronic medicines compendium (updated May 2009).
8. Summary of Product Characteristics - Tamiflu® 12 mg/ml powder for oral suspension (oseltamivir); Roche Products Limited, Electronic Medicines Compendium (updated Jan 2009)
9. Summary of Product Characteristics - Tamiflu® 75 mg hard capsule (oseltamivir); Roche Products Limited, Electronic Medicines Compendium (updated May 2009)
10. British National Formulary; 57th Edition (March 2009) British Medical Association and Royal Pharmaceutical Society of Great Britain, London.
11. NHS Choices website - Pandemic Flu
12. Swine Flu Clinical Package, Department of Health (July 2009)

• Swine Influenza - Health Protection Agency UK
• Swine Flu Clinical Package, Department of Health (July 2009)
• Pandemic H1N1 Swine Flu and Hajj pilgrims, RCGP November 2009
• Swine influenza cases: Questions and Answers, Health Protection Agency
• Information about Swine Flu - UK Government Website
• Swine Influenza - Frequently Asked Questions, World Health Organization.