Malaria: Hope on the horizon?

Vaccine reduces burden of disease but fails to achieve criteria for minimum efficacy and protection duration

By Henrylito D. Tacio

THE WORLD Health Organization (WHO) had previously set a target of 2015 for having a malaria vaccine that was at least 50 percent effective with protection lasting longer than a year. Those goals, however, were missed with a new vaccine evaluated for its efficacy and safety in diagnosed cases of malaria. The study was published recently in the journal The Lancet. “Everyone accepts that this is not the perfect or the last malaria vaccine,” observed Brian Greenwood of the London School of Hygiene and Tropical Medicine, who is the lead author of the published study. “It’s not good enough to stop transmission but it will cut the huge burden of disease.”

Although the world’s leading malaria vaccine candidate is “a letdown,” it is still being hailed as “it could still reduce malaria cases,” according to a report released by Associated Press. Manufacturer GlaxoSmithKline has spent hundreds of millions of dollars on the vaccine.

“While the levels of protection the vaccine offers against clinical malaria may seem relatively low, they are better than any other potential vaccine we currently have,” Prof. Mike Turner, head of infection at the Wellcome Trust, was quoted as saying by BBC News.

“The findings are not only important in their own right but also in signposting a road to developing better vaccines in the future,” Prof Turner said adding that it had taken two decades to get to this point.

James Whiting, from the nongovernmental movement, Malaria No More UK, considered it “a huge achievement” to get the vaccine this far. “There are still a number of considerations and approval processes to be undertaken, but it has the potential to be an important additional tool to fight malaria and save lives from a disease that kills a child every minute,” he pointed out.

Impact of climate change

According to the latest WHO estimates, released in December 2014, there were about 198 million cases of malaria in 2013 (with an uncertainty range of 124 million to 283 million) and an estimated 584,000 deaths (with an uncertainty range of 367,000 to 755,000).

The figure above is expected to increase as world temperature continues to rise. Models suggest, although conservatively, that risk of malaria will increase 5-15 percent by 2100 due to climate change.

Mosquitoes, which can carry the virus that caused malaria, are very sensitive to temperature changes. “Warming of their environment — within their viable range — boosts their rates of reproduction and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse,” wrote Dr. Paul R. Epstein in The New England Journal of Medicine.

“In highland regions, as permafrost thaws and glaciers retreat, mosquitoes and plant communities are migrating to higher ground,” adds Dr. Epstein, who is the associate director of the Center for Health and the Global Environment of Harvard Medical School in Boston.

Malaria carriers

Malaria is caused by a bite of a mosquito, which has about 2,000 species. The species that transmit malaria are classified in the genus Anopheles. “About 20 different Anopheles species are locally important around the world,” the UN health agency says. “All of the important vector species bite at night.”

Anopheles mosquitoes breed in water and each species has its own breeding preference. For example, some species prefer shallow collections of fresh water, such as puddles, rice fields, and hoof prints. Actually, malaria is caused by four types of plasmodium, a single-cell parasite transmitted by the bite of the female Anopheles. There are four parasite species that cause malaria in humans: Plasmodium falciparum, P. vivax, P. alariaeand P. ovale.

Plasmodium falciparum and P. vivax are the most common. Among the two, Plasmodium falciparum is the most deadly.

In recent years, some human cases of malaria have also occurred with Plasmodium knowlesi — a species that causes malaria among monkeys and occurs in certain forested areas of South-East Asia.

“Transmission is more intense in places where the mosquito lifespan is longer (so that the parasite has time to complete its development inside the mosquito) and where it prefers to bite humans rather than other animals,” the WHO points out.

Transmission also depends on climatic conditions that may affect the number and survival of mosquitoes, such as rainfall patterns, temperature and humidity. According to the WHO, in many places, transmission is seasonal, with the peak during and just after the rainy season.

“Malaria epidemics can occur when climate and other conditions suddenly favor transmission in areas where people have little or no immunity to malaria,” the WHO says. “They can also occur when people with low immunity move into areas with intense malaria transmission, for instance to find work, or as refugees.”

Symptoms

Malaria is an acute febrile illness. According to the WHO, in a non-immune individual, symptoms appear seven days or more (usually 10–15 days) after the infective mosquito bite. The first symptoms – fever, headache, chills and vomiting – may be mild and difficult to recognize as malaria. If not treated within 24 hours, P. falciparum malaria can progress to severe illness often leading to death.

On the other hand, children with severe malaria frequently develop one or more of the following symptoms: severe anemia, respiratory distress in relation to metabolic acidosis, or cerebral malaria. In adults, multi-organ involvement is also frequent. In malaria endemic areas, persons may develop partial immunity, allowing asymptomatic infections to occur.

For both P. vivax and P. ovale, clinical relapses may occur weeks to months after the first infection, even if the patient has left the malarious area. These new episodes arise from dormant liver forms known as hypnozoites (absent in P. falciparum and P. malariae); special treatment – targeted at these liver stages – is required for a complete cure.

According to a WHO publication, Rural Health, the malaria parasite is not simply transferred from one person to another but must live part of its life in the mosquito. It is for this reason that malaria is such a threat to health in the tropics but not in cooler countries or at high altitudes (where the temperature is lower).

“The cycle of malarial infection begins when a female mosquito bites a person with malaria,” explains The Merck Manual of Medical Information. “The mosquito ingests blood that contains malarial parasites. Once inside the mosquito, the parasite multiplies and migrates to the mosquito’s salivary gland. When the mosquito bites another person, the parasites are injected along with the mosquito’s saliva. Inside the person, the parasites move to the liver and multiply again. They typically mature over an average of one to three weeks, then leave the liver and invade the person’s red blood cells.”

It is at this point that the bitten person begins to feel the symptoms. Many people who have had malaria feel that they can tell it from other illnesses, but this is a dangerous belief in reality since at this stage malaria is easily confused with many other illnesses such as an attack of flu.

If treatment is not started quickly, the patient gets more ill. “Malaria can kill an individual in just an hour, if the one being infected is not aware that he’s been bitten,” the WHO points out.

Malaria-free

The WHO lists the Philippines among the 10 malaria endemic countries in the Western Pacific region. One of the goals of the Department of Health is to eliminate malaria from the country by 2020.

Last year, during the 2014 Asia Pacific Malaria Elimination Network (APMEN) VI in Makati, former health secretary Enrique Ona reported that the Philippines is on the verge of attaining malaria-free status.

Dr. Ona said malaria cases in the country declined since the mid-2000s, and has resulted in an 83 percent reduction from 2005 to 2013, while there was a 92 percent reduction in the number of deaths within the same period. The number of cases went down from 46,342 cases in 2005 to 7,720 in 2013. Deaths were 150 in 2005 to 12 in 2014.

Of 53-known provinces that are endemic for the disease, 27 have already been declared malaria-free. These were: Cavite, Batangas, Marinduque, Catanduanes, Albay, Masbate, Sorsogon, Camarines Sur, Iloilo, Aklan, Capiz, Guimaras, Bohol, Cebu, Siquijor, Western Samar, Eastern Samar, Northern Samar, Northern Leyte, Southern Leyte, Biliran, Camiguin, Surigao Del Norte, Benguet, Romblon, Batanes, and Dinagat Islands.

Vital Signs Issue 75 Vol. 4, May 1-31 2015

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