Dangers in Health through Mosquito

Malaria is a mosquito-borne infectious disease of humans and other animals caused by protists (a type of microorganism) of the genus Plasmodium.

It begins with a bite from an infected female Anopheles mosquito, which introduces the protists through saliva into the circulatory system.

In the blood, the protists travel to the liver to mature and reproduce. Malaria causes symptoms that typically include fever and headache, which in severe cases can progress to coma or death.

The disease is widespread in tropical and subtropical regions in a broad band around the equator, including much of Sub-Saharan Africa, Asia, and the Americas.

Five species of Plasmodium can infect and be transmitted by humans. The vast majority of deaths are caused by P. falciparum and P. vivax, while P. ovale, and P. malariae cause a generally milder form of malaria that is rarely fatal.

The zoonotic species P. knowlesi, prevalent in Southeast Asia, causes malaria in macaques but can also cause severe infections in humans.

Malaria is prevalent in tropical and subtropical regions because rainfall, warm temperatures, and stagnant waters provide habitats ideal for mosquito larvae.

Disease transmission can be reduced by preventing mosquito bites by distribution of mosquito nets and insect repellents, or with mosquito-control measures such as spraying insecticides and draining standing water.

Malaria is typically diagnosed by the microscopic examination of blood using blood films, or with antigen-based rapid diagnostic tests.

Modern techniques that use the polymerase chain reaction to detect the parasite's DNA have also been developed, but these are not widely used in malaria-endemic areas due to their cost and complexity.

The World Health Organization has estimated that in 2010, there were 219 million documented cases of malaria. That year, between 660,000 and 1.2 million people died from the disease, many of whom were children in Africa.

The actual number of deaths is not known with certainty, as accurate data is unavailable in many rural areas, and many cases are undocumented. Malaria is commonly associated with poverty and may also be a major hindrance to economic development
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Despite a need, no effective vaccine currently exists, although efforts to develop one are ongoing. Several medications are available to prevent malaria in travellers to malaria-endemic countries (prophylaxis).

A variety of antimalarial medications are available. Severe malaria is treated with intravenous or intramuscular quinine or, since the mid-2000s, the artemisinin derivative artesunate, which is superior to quinine in both children and adults and is given in combination with a second anti-malarial such as mefloquine. Resistance has developed to several antimalarial drugs; for example, chloroquine-resistant P. falciparum has spread to most malarial areas, and emerging resistance to artemisinin has become a problem in some parts of Southeast Asia.

Signs and symptoms

Main symptoms of Malaria

The typical fever patterns of the different types of malaria

The signs and symptoms of malaria typically begin 8–25 days following infection: however, symptoms may occur later in those who have taken antimalarial medications as prevention.

Initial manifestations of the disease—common to all malaria species—are similar to flu-like symptoms, and can resemble other conditions such as septicemia, gastroenteritis, and viral diseases.

The presentation may include headache, fever, shivering, joint pain, vomiting, hemolytic anemia, jaundice, hemoglobin in the urine, retinal damage, and convulsions.

The classic symptom of malaria is paroxysm—a cyclical occurrence of sudden coldness followed by rigor and then fever and sweating, occurring every two days (tertian fever) in P. vivax and P. ovale infections, and every three days (quartan fever) for P. malariae. P. falciparum infection can cause recurrent fever every 36–48 hours or a less pronounced and almost continuous fever.

Severe malaria is usually caused by P. falciparum (often referred to as falciparum malaria). Symptoms of falciparium malaria arise 9–30 days after infection.

Individuals with cerebral malaria frequently exhibit neurological symptoms, including abnormal posturing, nystagmus, conjugate gaze palsy (failure of the eyes to turn together in the same direction), opisthotonus, seizures, or coma.

Complications

There are several serious complications of malaria. Among these is the development of respiratory distress, which occurs in up to 25% of adults and 40% of children with severe P. falciparum malaria. Possible causes include respiratory compensation of metabolic acidosis, noncardiogenic pulmonary oedema, concomitant pneumonia, and severe anaemia.

Acute respiratory distress syndrome (ARDS) may develop in 5–25% in adults and up to 29% of pregnant women but it is rare in young children. Coinfection of HIV with malaria increases mortality.

Renal failure is a feature of blackwater fever, where hemoglobin from lysed red blood cells leaks into the urine.

Infection with P. falciparum may result in cerebral malaria, a form of severe malaria that involves encephalopathy. It is associated with retinal whitening, which may be a useful clinical sign in distinguishing malaria from other causes of fever.

Splenomegaly, severe headache, hepatomegaly (enlarged liver), hypoglycemia, and hemoglobinuria with renal failure may occur.

Malaria in pregnant women is an important cause of stillbirths, infant mortality and low birth weight,^[10] particularly in P. falciparum infection, but also with P. vivax.^[11]

Cause

Malaria parasites belong to the genus Plasmodium (phylum Apicomplexa). In humans, malaria is caused by P. falciparum, P. malariae, P. ovale, P. vivax and P. knowlesi.

Among those infected, P. falciparum is the most common species identified (~75%) followed by P. vivax (~20%).

Although P. falciparum traditionally accounts for the majority of deaths, recent evidence suggests that P. vivax malaria is associated with potentially life-threatening conditions about as often as with a diagnosis of P. falciparum infection.  

P. vivax proportionally is more common outside of Africa.^[16] There have been documented human infections with several species of Plasmodium from higher apes; however, with the exception of P. knowlesi—a zoonotic species that causes malaria in macaques^[13]—these are mostly of limited public health importance.^[17]

Life cycle

The life cycle of malaria parasites: A mosquito causes infection by taking a blood meal. First, sporozoites enter the bloodstream, and migrate to the liver. They infect liver cells, where they multiply into merozoites, rupture the liver cells, and return to the bloodstream. Then, the merozoites infect red blood cells, where they develop into ring forms, trophozoites and schizonts that in turn produce further merozoites. Sexual forms are also produced, which, if taken up by a mosquito, will infect the insect and continue the life cycle.

In the life cycle of Plasmodium, a female Anopheles mosquito (the definitive host) transmits a motile infective form (called the sporozoite) to a vertebrate host such as a human (the secondary host), thus acting as a transmission vector. A sporozoite travels through the blood vessels to liver cells (hepatocytes), where it reproduces asexually (tissue schizogony), producing thousands of merozoites. These infect new red blood cells and initiate a series of asexual multiplication cycles (blood schizogony) that produce 8 to 24 new infective merozoites, at which point the cells burst and the infective cycle begins anew.^[18] Other merozoites develop into immature gametes, or gametocytes. When a fertilised mosquito bites an infected person, gametocytes are taken up with the blood and mature in the mosquito gut. The male and female gametocytes fuse and form zygotes (ookinetes), which develop into new sporozoites. The sporozoites migrate to the insect's salivary glands, ready to infect a new vertebrate host. The sporozoites are injected into the skin, alongside saliva, when the mosquito takes a subsequent blood meal.^[19]
Only female mosquitoes feed on blood; male mosquitoes feed on plant nectar, and thus do not transmit the disease. The females of the Anopheles genus of mosquito prefer to feed at night. They usually start searching for a meal at dusk, and will continue throughout the night until taking a meal.^[20] Malaria parasites can also be transmitted by blood transfusions, although this is rare.^[21]

Recurrent malaria

Symptoms of malaria can reappear (recur) after varying symptom-free periods. Depending upon the cause, recurrence can be classified as either recrudescence, relapse, or reinfection. Recrudescence is when symptoms return after a symptom-free period. It is caused by parasites surviving in the blood as a result of inadequate or ineffective treatment.

Relapse is when symptoms reappear after the parasites have been eliminated from blood but persist as dormant hypnozoites in liver cells. Relapse commonly occurs between 8–24 weeks and is commonly seen with P. vivax and P. ovale infections.

P. vivax malaria cases in temperate areas often involve overwintering by hypnozoites, with relapses beginning the year after the mosquito bite. Reinfection means the parasite that caused the past infection was eliminated from the body but a new parasite was introduced. Reinfection cannot readily be distinguished from recrudescence, although recurrence of infection within two weeks of treatment for the initial infection is typically attributed to treatment failure.