The United States has identified five cases of malaria in people without any history of international travel in Florida and Texas over the last two months. This has led to the US Centres for Disease Control and Prevention (CDC) issuing an alert asking clinicians to consider malaria as a diagnosis in persons with fever of unknown origin, laboratories and public health experts to aid clinicians in identifying and diagnosing malaria cases, and people to take precautions to prevent mosquito bite. Why have just five cases of malaria in the US prompted such an alert? The five cases have raised alert because this is the first time in 20 years that there has been local transmission of malaria in the United States. The last time the infection was transmitted by a mosquito locally in the country was in 2003 when eight people in Florida were infected. While there hasn’t been local transmission in years, United States has been reporting around 2,000 cases of malaria every year mostly among international travellers from regions where the disease is still in transmission. Around 300 of these persons end up developing severe disease and between five and ten persons die of the infection each year in the United States, according to the CDC. The alert raised by the organisation also said that there could be an uptick in the number of imported malaria cases with summer travel increasing to the pre-pandemic levels in 2023. “Despite these cases, the risk of locally acquired malaria remains extremely low in the United States,” the CDC alert said, adding that the risk is higher in areas where the Anopheles mosquitoes that transmit the infection can thrive for most of the year due to climactic conditions and where travellers from malaria-endemic regions live. Malaria is a parasitic infection transmitted by the bite of an infected mosquito – while the country continues to have the anopheles mosquito that transmits the infection, the parasite has been missing. What do we know about the five infections? The four cases in Florida have been reported from a single county. In fact, it has been reported in people living in close proximity, with the government now carrying out active surveillance to find any more cases in the area. As for the case in Texas, CDC says that there is no evidence to show that it is linked to the cases from Florida. “All patients have received treatment and are improving,” the CDC said. The CDC has also said all the cases have been caused by plasmodium vivax. There are five plasmodiums that can cause malaria in humans – p. falciparum, p. vivax, p. malariae, p. ovale, and p. knowlesi. P. falciparum is the deadliest form of the disease, resulting in several complications including cerebral malaria if left untreated. Cases of p. falciparum are mostly seen on the African continent. Outside the African continent, p vivax causes most of the malaria cases. Though not as deadly as p. falciparum, it poses a unique challenge as the parasite is capable of laying dormant in the liver for extended periods. “P. vivax is always the last parasite surviving in countries that try to or have eliminated the infection. This is because of the challenges it comes with. First, p. vivax can remain dormant in the liver and cause repeated bouts of malaria. Second, the parasite count is usually lower than p. falciparum making it harder for rapid diagnostic tests as well as microscopy to pick it up,” said Dr Anup Anvikar, director, National Institute of Malaria Research. He added: “Third, patients have to take 14-day course of medicine to clear the parasite from the liver but the compliance is poor. Fourth, the medicine may also lead to haemolysis or destruction of red blood cells in people who have a deficiency of enzyme called G6PD. This means in the absence of a test for G6PD deficiency many clinicians to not prescribe the medicine that can clear the parasite completely.” What is the status of malaria in the world? Nearly half of the world’s population continued to be at risk for the infection in 2021, according to the latest available World Malaria Report. The report said that there were an estimated 247 million cases across the world and 619,000 deaths due to malaria during the year. Around 95 per cent of these cases were from Africa. The report says that malaria cases across the world went up from 2020, but at a slower rate than the year before despite disruptions in prevention, testing, and treatment due to the pandemic. Several strides have been made to help eliminate the infection – more effective artemisinin-based combination therapies have been introduced, shorter course treatments for p. vivax have been developed, and a vaccine has been deployed for children living in regions with high transmission of p. falciparum. The sustainable development goals have set the target of reducing malaria incidence by 90 per cent, reducing malaria mortality by 90 per cent, eliminating malaria in at least 35 countries, and preventing resurgence in all countries that are malaria-free by 2030. Towards this end, there are already 35 countries that have reported fewer than 1,000 indigenous malaria cases in 2021, up from 2020 as per the World Malaria Report. Countries that achieve zero indigenous transmission for three consecutive years can apply for WHO certification of malaria elimination. Nine countries have been certified since 2019, with the latest being China and El Salvador in 2021. How does India fare? Although malaria cases have been on the decline in India – there was an 85.1 per cent decline in malaria cases and an 83.36 per cent decline in deaths between 2015 and 2022 according to Union health minister Dr Mansukh Mandaviya – it is still one of the countries with a high burden of the infection. In 2021, 1.7 per cent of the malaria cases in the world and 1.2 per cent of all the deaths were reported in India. The country accounted for 79 per cent of the malaria cases and 83 per cent of the deaths from the WHO Southeast Asia region, according to the latest World Malaria Report. Another challenge that India faces is that in reducing cases of p. falciparum, it is faced with a high burden of the difficult-to-treat p. vivax infections. A 2021 study published in PLOS Medicine found that the highest the number of p. vivax cases is seen in India, Pakistan, and Ethiopia. Could there be a resurgence of malaria due to climate change? Dr Anvikar says that climate has a very important role to play in the transmission of the vector-borne disease. He said, “The mosquitoes that transmit malaria are very sensitive to temperatures, they can thrive only in certain not-too-hot and not-too-cold temperatures. If temperatures increase in a high-burden state like Odisha, the number of months when malaria can be transmitted may go down. But, in foothill states like Himachal if the temperatures go up, malaria will be transmitted for more months.” A 2021 study published in the Lancet says that the increasing temperatures is likely to increase the climate suitability for malaria for an additional 1.6 months in tropical highlands in the African region, the Eastern Mediterranean region, and the Americas. As Dr Anvikar said the study predicts a move of the disease towards higher altitudes and more rural areas. This would mean, “Outbreaks can occur in areas where people might be immunologically naive and public health systems unprepared,” the study said.
