Prolonged fever among children is hard to diagnose ,some time only specialists can diagnose the problem.Some common reasons for prolonged fever are given below which may help its diagnosis.

Common causes of the prolonged fever.
Relatively common causes
1.   Infections.
2.   Diseases of hypersensitization e.g., rheumatoid arthritis and systemic lupus erythematosus
3.   Neoplastic disorders especially , Hodgkin disease,lymphoma and leukemia.
Not that common cases
1.   Immune deficiency disorders.
2.   Hematologic disorders like agranulocytosis.
3.   Neurologic  disorders   e.g.,  familial  dysautonomia, hypothalamic and third ventricle lesions.
4.   Genetic disorders  like anhidrotic ectodermal dysplasia.

Infections

In all cases of prolonged fever, infections especially those which are epidemiologically more relevant must be considered in differential diagnosis. Pulmonary or extra pulmonary tuberculosis, typhoid and paratyphoid fevers, malaria and Kala azar; amoebic hepatitis and amebic liver abscess; and subacute bacterial endocarditis should always be excluded by “appropriate clinical, laboratory and radiological evaluation.
Liver, urinary tract, pelvic viscera, ovaries, retroperitoneum, lungs, pleural cavity, mechastinum, subdiaphragmatic region, bones, and brain should always be considered as possibiliites of chronic pyogenic infections. Rickettsial infections (e.g., typhus, psittacosis, Q fever); brucellosis; leptolpirosis and relapsing fever are rare but should be considered in differential diagnosis if epidemiologically relevant. Infectious mononucleosis is common in the subtropical and temperate zones. Nosocomial infections with unusual organisms e.g., anaerobic bacilli should be considered in differential diagnosis of patient who had received various antibiotics especially aminoglycosides for several days in the hospital.
Appropriate laboratory investigations such as total leucocyte count, differential leucocyte count; blood smear examination for malaria and filaria (night blood); serological test for typhoid, brucellosis, leishmaniasis, toxoplasmosis, amebiasis; bacteriological culture of blood for salmonella and brucella should be undertaken. Erythrocyte sedimentation rate is non-specific and is elevated in several inflammatory disorders.
Diseases of hypersensitization (Collagen vascular disorders)
(i) Rheumatic fever in childhood may occur without arthritis or arthralgia. The diagnosis of rheumatic fever should never be considered unless there is evidence of recent streptococcal infection prior to illness with  elevated titer of ASLO and acute phase reactants.
(ii) Rheumatoid disease. History of prolonged hectic fever without localizing signs, continuing for weeks, without the evidence of arthritis is not unusual in the early phase of rheumatoid arthritis. Erythrocyte sedimentation rate is elevated and there may be leucocytosis with high neutrophil count. A careful history and physical examination may reveal history of evanescent maculopapular rashes, which should arouse suspicion of a collagen vascular disorder.
(iii) Systemic lupus erythematous.
Leukemia and other malignancies
(Refer chapter on malignancies)
THE MANAGEMENT OF FEVERS
1. Evaluate the medical history, physical examination and laboratory investigations to develop a working diagnosis for the etiology of fever.

2.   Specific treatment of the infections, if present, should be undertaken.
3.   The general treatment of pyrexia is symptomatic and supportive.
(a)   Most children appear quite comfortable until their temperature reaches 38.5°C, and therefore no antipyretic measures are indicated in children with temperature below 38.5°C.
(b)   If the temperature exceeds 38.5°C, drugs such as acetaminophen (paracetamol) may be used in, a small dose at more frequent intervals, just to lower the temperature to around 38°C.
(c)   Environment. The child’s environment should be cool and airy.
(d)   The clothing should be loose and absorbent.
(e)   The body should be massaged gently so that the cutaneous vessels dilate and body heat is dissipated.
(f) Hydrotherapy. An absorbent towel should be soaked in cold water, rinsed and placed on the legs, trunk and forehead in order to reduce the body temperature. Hydrotherapy should be continued till the body temperature comes down to 38°C.
(g)   A close watch should be kept on the vital signs.
4.   Hyperpyrexia. When the temperature exceeds 41°C the body of the child below the neck should be immersed in the cold water without further delay to prevent irreversible brain damage. The parents should be reassured that this seemingly drastic measures will not induce shock. Ice cold bath does not cause significant vaso constriction. The rectal temperature should be recorded continuously and the hydrotherapy should be discontinued as the temperature falls below 38°C.
While hydrotherapy is being given, a lytic cocktail consisting of very small doses of chlorpromazine, promethazine and pethidine may be given intravenously at intervals of 30 minutes to reduce shivering and rebound rise of temperature after hydrotherapy.
5.   Hydration. Dehydration should be treated by intravenous administration of fluids.
6.   Other supportive measures for associated complications should be adopted. Patient with unexplained bleeding should be investigated for disseminated intravascular coagulation. In these cases fibrinogen degradation products in the serum are elevated, prothrombin time is increased and platelets are reduced. Such patients are treated with fresh blood transfusion if the bleeding is significant.