Rigors - Causes

Authored by Dr Mary Lowth, 30 May 2017

Patient is a certified member of
The Information Standard

Reviewed by:
Prof Cathy Jackson, 30 May 2017

Our body temperature is controlled by a part of the brain called the hypothalamus. This acts as a thermostat and 'sets' the normal body temperature to around 37°C (98.6°F). The body then does all it can to obey the hypothalamus and hold its temperature at this setting. It generates heat through the processes of its metabolism. If it starts to get cold it takes steps to warm up; if it starts to get too hot it takes steps to cool down.

Rigors are triggered by the presence of chemicals called pyrogens in the blood which 'turn up' the body's thermostat setting, telling the body to aim for a higher target temperature. Pyrogens are produced by our own immune systems in response to certain triggers, of which infection is the most common.

The symptoms of a rigor are caused by the body responding to its internal thermostat having been turned up.

When the body's thermostat has been set higher than the body's actual temperature, the mechanisms that kick in are those that the body uses to warm itself up. These include:

  • Feeling a sensation of cold and so behaving in a way that increases temperature (wrapping up, curling up, seeking warmth).
  • Shivering.
  • Hairs standing on end to hold in heat, causing 'goosebumps'.
  • Reduced sweating.
  • Reducing heat loss from the skin by shrinking down (constricting) the tiny blood vessels in the skin, leading to a pale face and fingers that feel cold to the touch.

All of these tend to raise body temperature to match the new thermostat setting (shivering raises our temperature because it is a muscular activity - running around would have a similar effect). Once the body reaches that new set temperature, shivering stops and you stop feeling cold.

As your body fights the infection, the immune system stops releasing pyrogens, and the hypothalamus drops the thermostat setting back towards normal. This tends to happen when the numbers of bacteria or viruses circulating in the blood fall. Your body then uses various mechanisms to lower your temperature again. These include:

  • Feeling hot and behaving in a way to lower temperature (fanning, taking off clothes).
  • Sweating.
  • Expanding or opening up (dilation) of the blood vessels in the skin to increase heat loss, making you look flushed and feel warm and clammy to the touch.

If the process is repeated because a new rush of pyrogens is released into the blood, there will be another rigor. Some infections tend to cause repeated rigors, generally because there are repeated releases of infective agents. Malaria typically causes repeated rigors with intervals of several days between them, as the malaria parasite is released into the blood in sudden rushes every few days. Parasites are living things (organisms) that live within, or on, another organism.

Any infection that pushes the temperature up quickly can cause a rigor. Some people are more likely than others to have rigors, and the same infection will not cause a rigor in everyone. Children with a temperature and rigors are slightly more likely to have a bacterial infection than children with a temperature without rigors, but the difference is small.

A germ (bacterium) called Streptococcus pneumoniae, which typically causes respiratory tract and ear infections, is the most common infectious bacterium to cause rigors in children. There are many other possible causes, although some are unusual (or not seen in the UK) and some occur only in adults.

Common infectious causes of rigors

This is not a complete list but it includes the most likely causes of rigors in children and adults, together with some of the more uncommon ones:

Uncommon infectious causes of rigors

Non-infectious causes of rigors

  • Widespread skin inflammation (including sunburn, burns, psoriasis).
  • Reactions to medicines (including some street drugs).
  • Inflammatory conditions, such as Crohn's disease, ulcerative colitis.
  • Some cancers.
  • Blood transfusion (rarely).
  • Rigors which can occur in pregnant women during labour.
  • Dialysis.
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