JOHNS HOPKINS MALARIA RESEARCH INSTITUTE RESEARCHERS GENETICALLY ENGINEER MOSQUITO IMMUNE SYSTEM TO BLOCK MALARIA

The three mosquitoes with glowing eyes are from three strains genetically engineered to better kill the malaria parasites in their guts. The mosquito at the far right, with yellow eyes, is a hybrid of the two other modified strains and was the most effective of the three at killing the parasite. (Credit: George Dimopoulos, JHU)

Researchers at the Johns Hopkins Malaria Research Institute demonstrated for the first time that the Anopheles mosquito’s innate immune system could be genetically engineered to block the transmission of the malaria-causing parasite to humans. In addition, they showed that the genetic modification had little impact on the mosquito’s fitness under laboratory conditions. The researchers’ findings were published December 22 in the online journal PLoS Pathogens.

George Dimopoulos, PhD, senior author of the study and associate professor in the W. Harry Feinstone Department of Molecular Microbiology and Immunology at the Johns Hopkins Bloomberg School of Public Health said.

“The immune system of the Anopheles mosquito is capable of killing a large proportion—but not all—of the disease-causing parasites that are ingested when the mosquito feeds on an infected human. We’ve genetically engineered this immune system to create mosquitoes that are better at blocking the transmission of the human malaria parasite Plasmodium falciparum. “

For the study, Dimopoulos and his team genetically engineered Anopheles mosquitoes to produce higher than normal levels of an immune system protein Rel2 when they feed on blood. Rel2 acts against the malaria parasite in the mosquito by launching an immune attack involving a variety of anti-parasitic molecules. Through this approach, instead of introducing a new gene into the mosquito DNA, the researchers used one of the insect’s own genes to strengthen its parasite-fighting capabilities.According to the researchers, this type of genetically modified mosquito could be further developed and used to convert malaria-transmitting to Plasmodium-resistant mosquito populations. One possible obstacle for this approach is the fitness of the genetically modified malaria resistant mosquitoes, since they would have to compete with the natural malaria-transmitting mosquitoes. The researchers showed with their study that the Rel2 genetically modified mosquito strain lived as long, and laid as many eggs, as the non-modified wild type mosquitoes, thereby suggesting that their fitness had not become significantly impaired.

Dimopoulos said.

“Malaria is one of world’s most serious public health problems. Mosquitoes and the malaria parasite are becoming more resistant to insecticides and drugs, and new control methods are urgently needed. We’ve taken a giant step towards the development of new mosquito strains that could be released to limit malaria transmission, but further studies are needed to render this approach safe and fail-proof.”

Worldwide, malaria afflicts more than 225 million people. Each year, the disease kills approximately 800,000, many of whom are children living in Africa.

Authors of “Engineered Anopheles immunity to Plasmodium infection” are Yuemei Dong, Suchismita Das, Chris Cirimotich, Jayme A. Souza-Neto, Kyle J. McLean and George Dimopoulos.

The Johns Hopkins Malaria Research Institute is a state-of-the-art research facility at the Johns Hopkins Bloomberg School of Public Health. It focuses on a broad program of basic science research to treat and control malaria, develop a vaccine and find new drug targets to prevent and cure this deadly disease.

Funding was provided by the National Institutes of Health and the Johns Hopkins Malaria Research Institute.

COMMENTARY:   

Quick Facts About Malaria

• More than 40 percent of the world’s population lives in areas where there is a risk of contracting malaria.
• A child dies of malaria every 30 seconds.
• Nearly one million people die of malaria every year, mostly infants, young children, and pregnant women and most of them in Africa.
• Approximately 300 to 500 million cases of clinical malaria occur each year.
• Malaria accounts for at least $12 billion in economic losses each year in Africa, and a reduction in annual economic growth estimated at 1.3 percent.

Source: World Health Organization

What Causes Malaria

Malaria is caused by a single-celled parasite from the genus Plasmodium. More than 100 different species of Plasmodium exist. They produce malaria in many types of animals and birds, as well as in humans.

Four species of Plasmodium commonly infect humans. Each one has a distinctive appearance under the microscope, and each one produces a somewhat different pattern of symptoms. Two or more species can live in the same area and infect a single person at the same time.

• Plasmodium falciparum is responsible for most malaria deaths, especially in Africa. The infection can develop suddenly and produce several life-threatening complications. With prompt, effective treatment, however, it is almost always curable.
• Plasmodium vivax, the most geographically widespread of the species, produces less severe symptoms. Relapses, however, can occur for up to 3 years, and chronic disease is debilitating. Once common in temperate climates, P. vivax is now found mostly in the tropics, especially throughout Asia.
• Plasmodium malariae infections not only produce typical malaria symptoms but also can persist in the blood for very long periods, possibly decades, without ever producing symptoms. A person with asymptomatic (no symptoms) P. malariae, however, can infect others, either through blood donation or mosquito bites. P. malariae has been wiped out from temperate climates, but it persists in Africa.
• Plasmodium ovale is rare, can cause relapses, and generally occurs in West Africa.

Four Species of Plasmodium malaria parasite (Click Image To Enlarge)

Transmission

The malaria parasite typically is transmitted to people by mosquitoes belonging to the genus Anopheles. In rare cases, a person may contract malaria through contaminated blood. Malaria also may be transmitted from a mother to her fetus before or during delivery ("congenital" malaria).

Because the malaria parasite is found in red blood cells, malaria can also be transmitted through blood transfusion, organ transplant, or the shared use of needles or syringes contaminated with blood. 

The Life Cycle of the Malaria Parasite

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1. A female Anopheles mosquito carrying malaria-causing parasites feeds on a human and injects the parasites in the form of sporozoites into the bloodstream. The sporozoites travel to the liver and invade liver cells.
2. Over 5-16 days*, the sporozoites grow, divide, and produce tens of thousands of haploid forms, called merozoites, per liver cell. Some malaria parasite species remain dormant for extended periods in the liver, causing relapses weeks or months later.
3. The merozoites exit the liver cells and re-enter the bloodstream, beginning a cycle of invasion of red blood cells, asexual replication, and release of newly formed merozoites from the red blood cells repeatedly over 1-3 days*. This multiplication can result in thousands of parasite-infected cells in the host bloodstream, leading to illness and complications of malaria that can last for months if not treated.
4. Some of the merozoite-infected blood cells leave the cycle of asexual multiplication. Instead of replicating, the merozoites in these cells develop into sexual forms of the parasite, called male and female gametocytes, that circulate in the bloodstream. 
5. When a mosquito bites an infected human, it ingests the gametocytes. In the mosquito gut, the infected human blood cells burst, releasing the gametocytes, which develop further into mature sex cells called gametes. Male and female gametes fuse to form diploid zygotes, which develop into actively moving ookinetes that burrow into the mosquito midgut wall and form oocysts. 
6. Growth and division of each oocyst produces thousands of active haploid forms called sporozoites. After 8-15 days*, the oocyst bursts, releasing sporozoites into the body cavity of the mosquito, from which they travel to and invade the mosquito salivary glands. The cycle of human infection re-starts when the mosquito takes a blood meal, injecting the sporozoites from its salivary glands into the human bloodstream .

Glossary

Diploid: Cells containing a full set of chromosomes. 
Gametes: Reproductive elements, male and female. 
Gametocytes: Precursors of the sexual forms of the malaria parasite, which release either male or female gametes within the stomach of the mosquito. 
Haploid: Cells containing a half set of chromosomes.
Merozoite: The form of the malaria parasite that invades red blood cells.
Oocyst: A stage of the malaria parasite within the mosquito which is produced when male and female gametes combine. 
Ookinete: The actively moving zygote of the malarial organism that penetrates the mosquito stomach to form an oocyst under the outer gut lining. 
Sporozoite: The infectious form of the malaria parasite, which is injected into people by mosquitoes. 
Zygote: The diploid cell resulting from union of a male and a female gamete. 

Symptoms

Malaria typically produces a string of recurrent attacks, or paroxysms, each of which has three stages—chills, followed by fever, and then sweating. Along with chills, the person is likely to have headache, malaise, fatigue, muscular pains, occasional nausea, vomiting, and diarrhea. Within an hour or two, the body temperature rises, and the skin feels hot and dry. Then, as the body temperature falls, a drenching sweat begins. The person, feeling tired and weak, is likely to fall asleep.

The symptoms first appear some 10 to 16 days after the infectious mosquito bite and coincide with the bursting of infected red blood cells (RBCs). When many RBCs are infected and break at the same time, malaria attacks can recur at regular time periods—every 2 days for Plasmodium vivaxmalaria and P. ovale, and every 3 days for P. malariae.

With P. vivax malaria, the person may feel fine between attacks. Even without treatment, the paroxysms subside in a few weeks. A person with P. falciparum malaria, however, is likely to feel miserable even between attacks and, without treatment, may die. One reason P. falciparum malaria is so virulent is that the parasite can infect RBCs in all stages of development, leading to very high parasite levels in the blood. In contrast, P. vivax parasites infect only young RBCs, which means the number of parasites in the blood does not reach the same high levels as seen in P. falciparuminfection.

Diagnosis

Healthcare providers should suspect malaria in anyone who has been in the tropics recently, who received a blood transfusion, and who develops a fever and other signs that resemble the flu. They examine blood smears taken from a finger prick under a microscope to confirm the diagnosis. A "thick" smear makes it possible to examine a large amount of blood. Then, the species of parasite can be identified by looking at a corresponding "thin" smear. Because mixed infections are possible, these techniques are important for deciding the best treatment. For example, a person can be infected with Plasmodium vivax as well as the more dangerous P. falciparum.

Treatment

In most cases, healthcare providers can successfully treat people with malaria. To decide which medicine to use, they should try to identify the species of parasite responsible and the geographical location where the person was infected. International travel clinics, the Centers for Disease Control and Prevention, and the World Health Organization offer up-to-date information on the geography of malaria, including

• Which species are present in which areas
• Whether chloroquine-resistant parasites are present
• Which seasons of the year carry the greatest risk

Prevention

Before leaving home, anyone traveling to an area with malaria should consult a knowledgeable healthcare provider, an international travel clinic, a local health department, the Centers for Disease Control and Prevention (CDC), or the World Health Organization (WHO) to obtain advice on what medicines to take before, during, and after the trip. Health risks for malaria vary with the destination, conditions of travel, and types of activities the traveler will undertake. A traveler who spends even a single night in a malaria-endemic area risks getting infected.

CDC and WHO have information on how to limit contact with mosquitoes, as well as current guidelines on antimalarial drugs.

Malaria Cases and Deaths

CASES; According to the World Health Organization's World Malaria Report 2011, here were an estimated 216 million cases of malaria worldwide in 2010 of which 91% were due to P. falciparum. The vast majority of cases (81%) were in the African region followed by South-East Asia (13%) and Eastern Mediterranean Regions (5%).  The number of confirmed cases reported by NMCPs was only 11% of the estimated number of cases. The gap between case reports and estimated incidence was largest in the South-East Asia Region, and the smallest in the American and European Regions.   

The estimated number of malaria cases  per 1000 cases of persons at risk of malaria, which takes into account population growth over time, shows a reduction in case incidence of 17% globally between 2000 and 2010.  Declines in cases incidence are seen in every Region but are greatest in the European (100%), American (60%) and Western Pacific Regions (38%).

DEATHS: There were an estimated 655 000 malaria deaths worldwide in 2010. It is estimated that 91% of deaths in 2010 were in the African Region, followed by South-East Asia Region (6%) and Eastern Mediterranean Regions (3%). About 86% of deaths globally were in children under 5 years of age. 

The total number of deaths is substantially lower than that presented in the World Malaria Report 2010, 781,000 deaths, partly because of a downward revision of the number of child deaths occurring globally made by the UN Interagency Group for Child Mortality Estimation (22). This revision reduced the estimates of malaria deaths in the WHO African Region by approximately 11%. Thus, of the difference  (113,000) between the total number of deaths estimated for the African Region in 2010 (709,000), and the estimated deaths in 2011 for 2010 (596,000), approximately 78,000 of the decrease is due to methodological while approximately 35,000 is due to a real decrease in the real number of malaria deaths.   This decrease can be attributed, at least in part, to improved malaria control.

The estimated number of malaria deaths per 100 000 persons at risk of malaria shows a reduction in malaria mortality rates of 26% globally between 2000 and 2010.  The largest percentage decreases in malaria mortality rates were seen in the European (99%), American (55%), Western Pacific Regions (42%) and African Regions (33%).  

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World Malaria Map

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WHO World Malaria Report 2011

The World Health Organization (WHO) produces the WHO World Malaria Report each year.  The report is one of the most highly detailed and comprehensive reports on the incidence and deaths from malaria, latest research on the treatment of malaria, and country profiles for malaria. You can download a copy of their 2011 report by clicking on the following image. 

 Click Image To Download

Courtesy of a press release issued on December 22, 2011 by Johns Hopkins Malaria Research Institute, The National Institute of Allergy and Infectious Diseases and