Why is a tetanus toxoid booster given with open wounds?

The questioner goes on to note, correctly, that Clostridium tetani is an obligate anaerobe - oxygen is toxic to it. Open wounds would therefore seem to be an area that would be safe from C. tetani then, wouldn't they?

Alas, the part of the wound exposed to the outside air isn't the part that we worry about. During traumatic injuries of sufficient magnitude (especially with deep wounds) the blood flow to the tissue is often disrupted. This renders local parts of the would ischaemic, and therefore (since oxygen is carried around in the blood) hypoxic as well. In some of the particularly hypoxic areas, the oxygen tension falls to a low enough level for C. tetani to replicate, raising the prospects of tetanus being the result.

The ways to prevent tetanus are:

• Vaccination - most vaccination programmes around the world include the tetanus vaccine. It's efficacious period lasts about 10 years, and so a booster vaccine should be given at decade intervals. Booster vaccines may also be given at the time of a new injury (if the patient isn't covered already).
• Passive immunisation with antitetanus immunoglobulin - This isn't usually necessary in terms of prevention unless the wound is particularly badly contaminated.
• Penicillin - The drug of choice against C. tetani, this is also used if the wound is badly contaminated.

Take care not to confuse this prophylactic regimen with the treatment regimen that one needs to embark on if the patient is unlucky enough to get tetanus. That can be the topic for another day, if requested.