Certain species of mosquitoes, including Aedes aegypti and the Asian tiger mosquito, are vectors for several arboviruses. In the United States alone, West Nile Virus has been detected in 65 species of mosquito (CDC & ArboNET, 2013). After biting an infected organism, the vector mosquito can transmit viruses obtained from its blood meal when it bites another organism in the future. Mosquitoes are the deadliest organism in terms of human mortality, with a death toll of an estimated two million human deaths each year (CDC, 2013; IDPH, 2007).
Malaria is the most well-known and most universally feared of the mosquito-related illnesses. As we learned in class “malaria is the most important global arbor disease” and an average of 5% of the world’s population is infected at any given time (Miller & Walton, 2014). However, at least in the United States, malaria is something that happens to other countries (IDPH, 2007). Malaria is transmitted through bites of infected female mosquitoes belonging to the Anopheles genus (WHO, Jan 2014). Even respected sources vary in which mosquitoes are malaria vectors: the WHO (Mar 2014)reports that there are about 20 different species Anopheles genus that transmit the disease while the CDC (2012) says 30-40 Anopheles mosquitoes are vectors for malaria. The actual parasite that causes malaria (remember, mosquitoes only transmit the disease) belongs to protozoan genus Plasmoium; there are four species that cause human malaria, including P. falciparum, P. ovale, P. vivax, and (somewhat obviously) P. malariae (WHO, “Malaria,” n.d.). The blood meals that female mosquitoes require to produce eggs provides the opportunity for the parasite to transfer from its mosquito host to its human target (CDC, 2012). The mosquito acquires the malaria parasite from biting humans infected with malaria (WHO, Jan 2014). The mosquito provides an environment for the malaria parasite to develop from its gametocyte state to the sporozoite state (CDC, 2012). Transmission rates depend on intersection of the parasite’s developmental requirements and the mosquito’s lifespan and habits. If the lifespan of the mosquito is too short, the parasite may not complete its development in time to infect any human hosts. If the mosquito’s blood source of choice is non-human, the parasite is out of luck. The long lifespan and preference for human blood of African malaria-carrying mosquitoes is a contributing factor as to why almost 90% of malaria deaths occur in Africa (WHO, Mar 2014). Human immunity and climate conditions are other contributing factors. Malaria initially causes symptoms of fever, chills, headache and vomiting one to two weeks after infection (WHO, March 2014). Severe malaria proceeds from those untreated early (“uncomplicated”) symptoms (CDC, 2012). Symptoms of severe malaria can include anemia, respiratory distress, cerebral malaria, kidney failure, and metabolic acidosis (WHO, March 2014) When the plasmodium parasite enters the body it heads for the liver, where it invades liver cells and reproduces. The reproducing cells eventually rupture the liver cells they are inhabiting and spread through the body, reproducing in red blood cells and rupturing those as well, resulting in anemia (CDC, 2012). Toxic waste products of the parasite’s reproduction stimulate the body’s reaction and cause many of the symptoms seen in malaria (CDC, 2012). Prevention of malaria, as well as any other disease with a mosquito vector, consists of various ways of preventing mosquito bites. In developing countries in many cases this means DDT. Release of sterile male mosquitoes in attempts to wipe out mosquito populations have been effective in small, isolated areas but are not a global strategy (CDC, 2012). Insecticide-treated bed nets are useful because while there is considerable variety between the species, they do share one important characteristic useful for prevention: they bite at night (WHO, Mar 2014). Nets in general are useful to prevent mosquito bites but insecticide treated nets have been shown to be much more effective at killing mosquitoes and reducing malaria rates (CDC, 2012). Indoor residual spraying one tactic used in malaria-prone regions. Just as it sounds, it involves spraying the insides of homes with a long-lasting insecticide to kill mosquitoes if they land on the treated surfaces. However, over 80% of structures in any given area have to be treated in order to be an effective prevention (WHO, Mar 2014). Because of this high percentage needed, unless there is a concerted and cooperative effort to spray each and every structure, this method will not have a high success rate. This was the method primarily used in the 1950s and 1960s during the Global Malaria Eradication Campaign – using DDT (CDC, 2012). When the health and environmental effects of DDT became better understood it was no longer a viable option for global prevention. However, developing countries retain the right to spray a certain amount per year, something that may still be too high for human health (Cone, 2009). Weighing the need to prevent malaria and the deaths it causes with the disastrous health and environmental consequences from DDT is an ethical debate that continues even today. Risk of malaria rises when a woman is pregnant, as any previous partial immunity is disturbed because of changes in her immune system during pregnancy (CDC, 2012). The CDC considers treatment of pregnant women to be a cornerstone of fighting malaria transmission in developing countries. Treatment generally includes iron to prevent anemia, use of bed nets, and treatment with anti-malarial drugs such as sulfadoxine-pyrimethamine (WHO, Mar 2014). Larvacide is a popular approach and seems like it would be quite effective. However, because the many types of mosquitoes that transmit malaria prefer different types of habitats in which to breed, it can be next to impossible to even make a dent in the larval survival rates (CDC, 2012). In addition larvae hatch within days, some species need such small spaces a hoofprint filled with water is enough, and the landscape is filled with possible sites and nearly impossible to effectively search (Plymouth, 2014). In addition to chemical methods of control, preventing many of the mosquito-borne diseases is composed of various ways of personally avoiding mosquito bites. Insect repellents such as DEET when outside and wearing clothing that covers as much of the body as possible to protect yourself outside. To protect yourself from mosquito bites while indoors, the CDC recommends window and door screens, keeping the home closed, and using air conditioners whenever possible. In general, denying mosquitoes inviting environments in which to lay eggs by emptying areas of standing water in your area is essential (CDC, 2013). |