CLINICAL MATTERS
Living in the tropics, most Filipinos are familiar with the two big mosquito borne illnesses—dengue and malaria. But there are many other infectious diseases borne by mosquitoes that infect a significant number of Filipinos every year. These diseases include Chinkungunya, Zika, Japanese encephalitis, and lymphatic filariasis. Being aware of these diseases is worthwhile since they can happen to anyone especially when living or traveling in known hotspots for these illnesses.
Dengue is hyperendemic in the Philippines. That means it is transmitted the whole year round, although there are some seasonal increases especially during the rainy season. There are four serotypes of dengue which means that a person can get infected up to four times in their lifetime. Getting infected with a specific serotype confers lifelong immunity to that serotype. The mosquito involved in transmitting dengue is known as Aedes aegypti. A second mosquito species Aedes albopictus is becoming an increasingly important vector.
The most dangerous complication of dengue is severe dengue where the platelet count can precipitously drop and result in hemorrhage. Severe dengue can also cause an unsafe drop in blood pressure. Infection with a second serotype has the highest risk for severe dengue, although severe dengue can occur with the first, third, and fourth infection as well. About 87 percent of Filipinos by ten years of age have already had at least one episode of dengue and this is why children of this age group are at highest risk for severe disease during an outbreak.
With early diagnosis and supportive care, most bouts of dengue are self-limited and can be managed at home. Danger signs of dengue include abdominal pain, bleeding, and low blood pressure. If you have a fever lasting more than three days without any respiratory symptoms, with or without a rash, it may be time to get tested for dengue. Your physician will also check a complete blood count and if the platelets are below 100,000 or there are other warning signs, your doctor may recommend that you be admitted to the hospital for supportive care and closer monitoring. The fever usually lasts three to seven days on average, and the platelets begin to drop during the febrile phase. These continue to drop for another two to three days after the fever goes away. It is during this time where bleeding and hypotension are more likely to occur. Prevention of dengue is still the best and most cost-effective way to combat dengue and this is done through different modalities including vector control, use of mosquito repellants and education and awareness campaigns. A new dengue vaccine which does not seem to preempt the risk of severe dengue to the level of a second infection in seronegative individuals has already completed clinical trials and may be another useful tool once it becomes available.
The incidence of malaria in the Philippines has precipitously dropped in the last two decades thanks to an effective malaria control program funded by the Global Fund and the Philippine government. This program has been so successful that it has been cited as a global model for malaria. Only a few places (Palawan, Sultan Kudarat, Mindoro) in the Philippines are still considered endemic for malaria, and most of the malaria infections seen in nonendemic places like Metro Manila are from people infected abroad, particularly returning travelers and overseas workers.
Malaria is transmitted by mosquitoes of the genus Anopheles. Malaria is caused by protozoans of the genus Plasmodium. There are five different malaria species that infect humans: Plasmodium falciparum, vivax, ovale, malariae, and knowlesi, and all are found in the Philippines. P. falciparum is the deadliest form and can kill people when the infection reaches the brain or when the parasite causes the kidneys to shut down. Treatment of malaria has changed over the years due to emerging resistance to different drugs. Two malaria species, P. ovale and P. vivax, need additional treatment with a drug known as primaquine to prevent relapse. Most recently, an effective malaria vaccine has been rolled out in some countries and is expected to have a tremendous impact especially on childhood malaria. Travelers going to endemic areas can take prophylactic medications to decrease the risk of infection. Malaria control in the Philippines has been quite successful but this can easily relapse if current gains are not sustained and protected.
Chikungunya is a mosquito-borne viral disease that is similar to dengue. It is also transmitted by Aedes aegypti and albopictus. It is not as common as dengue and was first discovered in the Philippines in 1968 with sporadic outbreaks in the late 1990s and 2010s. Recently, case numbers of Chikungunya have been growing, with more and more cases showing up in 2022 and 2023 after the pandemic. Symptoms of Chikungunya are very similar to dengue and include fever and rash. Fortunately, platelet counts do not drop as much as seen in dengue. The hallmark of Chikungunya is arthritis or joint pain and swelling. As much as 10 percent of patients with Chikungunya can have joint pain lasting more than two weeks, with a few unfortunate people having persistent pain for months up to years. Chikungunya is rarely fatal but can easily be confused with dengue and so most people with suspected of Chikungunya should also be checked for dengue. Prevention of Chikungunya is similar to dengue, and there are ongoing efforts to produce an effective vaccine.
Zika is a recent arrival to the Philippines with the first described case in 2012. It is also transmitted by Aedes aegypti and albopictus. It causes a similar illness to dengue but there is just a mild drop in platelets. Some patients present with conjunctivitis (swelling of the eyes) like that seen in sore eyes. This disease is generally mild. Rarely, some patients develop a neurologic condition called Guillain-Barre syndrome which causes profound muscle weakness that can be life-threatening. A Zika outbreak in Brazil among pregnant women caused birth defects in children and so women of child-bearing age so take extra precautions when there is a Zika outbreak. Zika can also be easily confused with dengue and so dengue should be checked in Zika suspects. Several vaccine candidates are being studied.
Japanese encephalitis is a viral infection of the brain that is carried by Culex mosquitoes. Approximately one in 250 patients infected with the virus develop brain swelling and neurologic problems that can be debilitating or even be fatal. Early symptoms are nonspecific, including fever and headaches that can progress to seizures or coma. There is an effective vaccine that can be given to children, especially in areas that have historically been hotspots for this disease. Other prevention measures include control of mosquitoes and use of mosquito repellants.
Finally, lymphatic filariasis is caused by two kinds of worms known as Wuchereria bancrofti and Brugia malayi. The juvenile form of the worm can be found in the blood and are known as microfilariae. These microfilariae are ingested when a mosquito bites an infected person or animal. Several types of mosquitoes including those from Aedes, Culex, Anopheles and Mansonia species are able to transmit the disease. The adult worm causes severe and chronic swelling of the legs (elephantiasis) by blocking ¬lymphatic channels. Some infected men also develop an enlarged scrotum known as a hydrocoele. The worms can be treated with medication, preferably before the legs or scrotum enlarge and the condition becomes irreversible. Whole populations in endemic areas are sometimes preemptively treated with medication once a year, and this mass drug administration has significantly decreased the burden of disease.
Mosquitoes in the Philippines can be deadly and they cause a lot of pain and suffering through the diseases they transmit. Be aware of your risk, get vaccinated for those diseases that can be prevented, avoid mosquitoes especially in endemic areas, and talk to your doctor if you think you might be infected.