Fever or Pyrexia

Abnormal elevation of body temperature especially when the inflammation is accompanied with infection is called Fever or pyrexia. It is also defined as abnormal elevation of body temperature resulting due to circulating toxins and exogenous pyrogens.

Fever is also known as Febris, febrility and Pyrexia.

Etiology of Fever

Fever or pyrexia may be septic due to infection or aseptic due to inflammation or traumatic injury. Generally, pyrogens are substances that elevate body temperature. They are of two types: endogenous and exogenous.

Exogenous causes of fever are:

• Infectious agents: bacteria, virus, fungi and protozoa
• Non-infectious agents: neoplasms or malignancies and tissue injuries

Endogenous causes of fever are:

• Endogenous causes of fever include cytokines produced by macrophages like interleukins (IL-1) and tissue necrosis factor (TNF)

Pathogenesis

Exogenous pathogens enter into the body and cause the release of lymphokines and lymphocyte proliferation that stimulate macrophages and monocytes to release cytokines like IL-1 and TNF. The TNF causes membrane damage by releasing phospholipids (arechidonic acid precursors), thereby causing the conversion of arechidonic acid via the cyclo-oxigenase pathway, resulting in the production of PGE2, which is further enhanced by the action of IL-1, especially in the hypothalamus, and this stimulates the hypothalamus, thereby elevating the thermoregulatory set point and resulting in an increase in the body temperature. Elevation of body temperature is also due to the hypothalamus sending a signal to the vasomotor centre, causing peripheral vasoconstriction and thereby preventing heat dissipation.

Stages of fever

Pyrexia has three stages, as described below:

1. Onset of fever

This is the stage of increment or rising phase of temperature. Here the heat production is elevated because of the increase in the thermal set point and peripheral vaso-constriction. In this stage, temperature goes on increasing and in few cases the rise in temperature accompanied with chills and rigor. Though the body temperature is rising, the extremities are cold.

2. Stage of fastidium

This is the stage of maximum temperature.

3. Stage of decrement

Decline of body temperature is noticed and heat dissipation predominate heat production. Most of the time this stage is accompanied by profuse sweating.

Beneficial effects of fever

• It is a body defense mechanism
• It improves phagocytosis
• It increase circulating polymorphs
• Increases the velocity of blood (fever increase PR,HR,velocity of blood and cell distribution to tissues)
• Increase in temperature has a bacteriostatic effect
• Improves Ag-Ab complex formation and activates complement system. But sometimes micro-organisms overtake these mechanisms and take an upper hand and hence treatment is required.

Types of Fever

1. Simple fever

Simple fever (Febris simlica) is the type of fever where the temperature does not touch the normal limit in 24 hours and the variation of temperature does not exceed 2 °F in 24 hours. this type of temperature is noted in lobar pneumonia, enteric fever etc.

2. Remittent fever

Remittent fever (Febris remmittens) is the type of fever where the temperature does not touch the normal limit in 24 hours and tha variation of temperature is >2 °F in 24 hours. This type of temperature is observed in septicaemia, bronchopneumonia, and urinary tract infection.

3. Intermittent fever or Relapsing fever

Intermittent fever (Febris intermittens or Relapsing fever) is that type of fever where the rise of temperature persists only for a few hours of thye day. There is a regular brief attack of fever (pyroxysms) with a afebrile period (apyrexia). In brief, there is high temperature for several hours followed by a drop to normal temperature and again there is rise of temperature and so on.

This type of temperature is seen in human malaria, chronic surra in horse and canine distemper (diphagic fever).

Based on febrile and afebrile period, intermittent fever can be divided into:

• Quotidian: There is daily rise and daily fall of temperature. This is seen in malaria in human beings, E.coli infection or pyelitis etc.
• Tertian: Temperature appears and disappears every alternate day ie., there is a gap of one day. Ex: Malaria in humans.
• Quartan: Here the temperature comes and goes every 4th day. Ex: Malaria in African countries.
• Hectic: There is sudden rise of temperature daily with chill or rigor, persistent for few hours and falling equally suddenly with profuse sweating. Ex: Malaria, empuema and E.coli infection.

4. Irregular fever

Irregular fever (Atypical fever) has got no definite character. Daily fluctuation of temperature does not follow a definite pattern. This type of temperature is most commonly observed in veterinary practice such as Louping ill, CD, Swine erysephalus, strangle in horses etc.

5. Transient fever

Transient fever subsides within about 24 hours after its development. this type of fever seen in Epimeral fever disease.

6. Continuous fever

In Continuous fever (Febris continua) the plateau of temperature remains for a longer period than the simple fever. this type of fever seen in Tick borne fever disease.

7. Periodic fever

Periodic attack of fever alternate with afebrile period seen in periodic fever.

Clinical classification of fever

1. Acute fever

There is sudden sharp elevation of body temperature to the level of 104 °F or more. This type of fever is recorded in acute viral or bacterial diseases. Acute fever is seen in Acute pneumonitis, acute peritonitis etc.

2. Sub-acute fever

Temperature does not go too high and remains within the range of 103 °F. this type of fever is recorded in mild inflammatory conditions like nephritis, cystitis, metritis etc.

3. Chronic fever

Temperature does not exceed above 1-2 °F beyond the normal range. Course is usually prolonger. Temperature persists for a considerably prolonged period. Chronic fever seen in pulmonary TB, chronic pyometritis, surra in horse etc.