Smallpox

Epidemiology of Infectious Diseases: Smallpox

The last indigenous case of smallpox occurred in Somalia in 1977 and in 1980 The World Health Assembly officially certified the global eradication of smallpox. However, the public health importance of smallpox remains due to the potential deliberate release of the virus in a bioterrorist attack.

Causal agent
Variola virus

Common clinical features
Two clinical forms of smallpox have been described. 

1. Variola major is the severe form of smallpox and is characterised by a more extensive rash, higher fever and a greater degree of prostration. The case fatality rate among unvaccinated individuals is 30%.

There are four types of variola major smallpox:

• Ordinary -accounting for over 90% of cases among unvaccinated individuals.
• Modified - mild and occurring in previously vaccinated individuals, the prodrome may be less severe. This form is rarely fatal and may be confused with chickenpox.
• Flat and haemorrhagic smallpox are both severe and uncommon forms smallpox which are usually fatal.

2. Variola minor is a less severe form with a case fatality of <1%.

Clinical Features
The prodrome phase is characterized by the abrupt onset of fever (≥101⁰F), malaise, headache, severe back pain, prostration, nausea and less often abdominal pain and vomiting.

After 2-4 days, the fever declines and the development of a deep-seated rash in which individual lesions containing infectious virus progress through successive stages of macules, papules, vesicles, pustules, then crusted scabs that fall off 3-4 weeks after the appearance of rash^1,3.

Epidemiology

• Epidemics of smallpox have occurred globally for thousands of years causing widespread mortality.
• In 1798 Edward Jenner demonstrated that inoculation with cowpox could protect against the smallpox virus².
• By the early 1950s an estimated 50 million cases of smallpox occurred worldwide.
• Following the World Health Organisations global smallpox vaccination programme (1967-1977) The World Health Assembly officially certified the global eradication of smallpox in 1980.
• The last naturally acquired case of smallpox occurred in Somalia in 1977.
• The last known stocks of the smallpox virus are maintained at two WHO collaborating laboratories in the United States and in Russia.

Reservoir
Humans

Mode of transmission
Person to person spread via infected aerosols and air droplets.
Transmission may also occur via contaminated clothing and bedding (less common).

Incubation period
7-17 days.
Commonly 12-16 days to onset of illness and 2-4 days more to the onset of rash.

Period of Communicability
From the development of the earliest lesions to the disappearance of all scabs (about 3 weeks).

Treatment
There is no specific treatment for smallpox other than treatment of symptoms.

Prevention and control
In the UK routine vaccination against smallpox has not been carried out since the 1970s and globally since 1982. Vaccination is generally believed to confer a high level of protection for up to 10 years. However, studies of the level of the protection beyond 10 years has been conflicting.

Vaccination given up to 4 days following exposure can modify the course of the disease and reduce mortality by up to 50%.

In the event of a deliberate release - activate national and local plans.

In response to the threat of a bioterrorist release of smallpox, the Department of Health published Guidelines for smallpox response and management in the post-eradication era (smallpox plan) available at: http://www.dh.gov.uk/PublicationsAndStatistics/

References

1. Heymann D L, editor, Control of Communicable Disease Manual. 18^th ed. American Public Health Association; 2004.
    
2. National Immunizatin Program, Centers for Disease Control and Prevention, Epidemiology and Prevention of Vaccine-Preventable Disease 'The Pink Book', 9^th edition, U.S. Department of Health and Human Services, 2006.
    
3. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.

© CM Kirwan 2006