What causes a rigor?
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.
Why do we shiver with a rigor?
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).
- 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).
- 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.
What type of infection can cause a rigor?
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.
What are some possible causes of rigors?
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:
- Bacterial infection including ear infection, chest infection, kidney and bladder (urinary tract) infection, gastroenteritis.
- Many common viral infections, including winter flu (influenza).
- Childhood viruses - for example, measles, mumps, chickenpox, rotavirus, roseola.
- Widespread skin infection - for example, infected eczema, cellulitis, impetigo.
- Eczema infected by the cold sore virus, which is called eczema herpeticum
- Infection after surgery, particularly to the tummy (abdomen) or to a joint.
- Cat scratch disease.
- Joint infection (septic arthritis).
- Sinusitis (adults and older children only).
- Infection associated with kidney stones or gallstones (adults).
- Malaria - an important cause of rigors if you have visited a malarial zone.
- Mastitis and breast abscess when breast-feeding.
- Womb infection (endometritis) after childbirth or miscarriage.
- Infection of the prostate gland (prostatitis).
Uncommon infectious causes of rigors
- Meningococcal infection.
- Kawasaki disease.
- Lyme disease.
- Rheumatic fever.
- Infection of the heart valves (infective endocarditis).
- Some fungal infections (for example, histoplasmosis).
- Dental abscess.
- Other uncommon (in the UK) infections, such as tuberculosis.
- Some 'tropical' diseases, such as dengue and typhoid.
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.
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Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.