Infection with malaria parasites may result in a wide variety of symptoms, ranging from absent or very mild symptoms to severe disease and even death. Malaria disease can be categorized as uncomplicated or severe (complicated) . In general, malaria is a curable disease if diagnosed and treated promptly and correctly.
Following the infective bite by the Anopheles mosquito, a period of time (the "incubation period") goes by before the first symptoms appear. The incubation period in most cases varies from 7 to 30 days. The shorter periods are observed most frequently with P. falciparum and the longer ones with P. malariae.
Antimalarial drugs taken for prophylaxis by travelers can delay the appearance of malaria symptoms by weeks or months, long after the traveller has left the malaria-endemic area. (This can happen particularly with P. vivax and P. ovale, both of which can produce dormant liver stage parasites; the liver stages may reactivate and cause disease months after the infective mosquito bite.)
Such long delays between exposure and development of symptoms can result in misdiagnosis or delayed diagnosis because of reduced clinical suspicion by the health-care provider. Returned travelers should always remind their health-care providers of any travel in malaria-risk areas during the past 12 months.
The classical (but rarely observed) malaria attack lasts 6-10 hours. It consists of:
- a cold stage (sensation of cold, shivering)
- a hot stage (fever, headaches, vomiting; seizures in young children)
- and finally a sweating stage (sweats, return to normal temperature, tiredness)
Classically (but infrequently observed) the attacks occur every second day with the "tertian" parasites (P. falciparum, P. vivax, and P. ovale) and every third day with the "quartan" parasite (P. malariae).
More commonly, the patient presents with a combination of the following symptoms:
- Nausea and vomiting
- Body aches
- General malaise.
In countries where cases of malaria are infrequent, these symptoms may be attributed to influenza, a cold, or other common infections, especially if malaria is not suspected. Conversely, in countries where malaria is frequent, residents often recognise the symptoms as malaria and treat themselves without seeking diagnostic confirmation ("presumptive treatment").
Physical findings may include:
- Elevated temperature
- Enlarged spleen.
In P. falciparum malaria, additional findings may include:
- Mild jaundice
- Enlargement of the liver
- Increased respiratory rate.
Diagnosis of malaria depends on the demonstration of parasites on a blood smear examined under a microscope. In P. falciparum malaria, additional laboratory findings may include mild anaemia, mild decrease in blood platelets (thrombocytopaenia), elevation of bilirubin, elevation of aminotransferases, albuminuria, and the presence of abnormal bodies in the urine (urinary "casts").
Severe malaria occurs when P. falciparum infections are complicated by serious organ failures or abnormalities in the patient's blood or metabolism. The manifestations of severe malaria include:
- Cerebral malaria, with abnormal behaviour, impairment of consciousness, seizures, coma, or other neurologic abnormalities
- Severe anaemia due to haemolysis (destruction of the red blood cells)
- Haemoglobinuria (haemoglobin in the urine) due to haemolysis
- Pulmonary oedema (fluid buildup in the lungs) or acute respiratory distress syndrome (ARDS), which may occur even after the parasite counts have decreased in response to treatment
- Abnormalities in blood coagulation and thrombocytopaenia (decrease in blood platelets)
- Cardiovascular collapse and shock
Other manifestations that should raise concern are:
- Acute kidney failure
- Hyperparasitaemia, where more than 5% of the red blood cells are infected by malaria parasites
- Metabolic acidosis (excessive acidity in the blood and tissue fluids), often in association with hypoglycaemia
- Hypoglycaemia (low blood glucose). Hypoglycaemia may also occur in pregnant women with uncomplicated malaria, or after treatment with quinine.
Severe malaria occurs most often in persons who have no immunity to malaria or whose immunity has decreased. These include all residents of areas with low or no malaria transmission, and young children and pregnant women in areas with high transmission.
In all areas, severe malaria is a medical emergency and should be treated urgently and aggressively.
In P. vivax and P. ovale infections, patients having recovered from the first episode of illness may suffer several additional attacks ("relapses") after months or even years without symptoms. Relapses occur because P. vivax and have dormant liver stage parasites ("hypnozoites") that may reactivate. Treatment to reduce the chance of such relapses is available and should follow treatment of the first attack.
Other Manifestations of Malaria
- Neurologic defects may occasionally persist following cerebral malaria, especially in children. Such defects include troubles with movements (ataxia), palsies, speech difficulties, deafness, and blindness.
- Recurrent infections with P. falciparum may result in severe anaemia. This occurs especially in young children in tropical Africa with frequent infections that are inadequately treated.
- Malaria during pregnancy (especially P. falciparum) may cause severe disease in the mother, and may lead to premature delivery or delivery of a low-birth-weigh baby.
- On rare occasions, P. vivax malaria can cause rupture of the spleen or acute respiratory distress syndrome (ARDS).
- Nephrotic syndrome (a chronic, severe kidney disease) can result from chronic or repeated infections with P. malariae.
- Hyperreactive malarial splenomegaly (also called "tropical splenomegaly syndrome") occurs infrequently and is attributed to an abnormal immune response to repeated malarial infections. The disease is marked by a very enlarged spleen and liver, abnormal immunologic findings, anaemia, and a susceptibility to other infections (such as skin or respiratory infections).