West Nile Virus has relatively recently gained a foothold in the consciousnesses of Americans. It has a long history in Northern Africa and the Middle East and was first identified in a Northern Uganda in 1937 (Sejvar, 2003). It was first recognized in New York City in 1999 and the first major outbreak in the United States was seen in 2002. Early on in its introduction to the US it generally caused mild fever-related symptoms and more recently has been causing outbreaks with more severe symptoms and higher mortality (Sejvar, 2003). Most people infected with WNV, 70-80%, do not develop symptoms. About 20% have symptoms including fever, headache, joint and body aches, nausea/vomiting, diarrhea, and possibly a rash (CDC, 2013). While the symptoms will abate and the infected person will likely recover, fatigue from the infection can remain for months. While this type of WNV infection is serious and concerning, it is not likely to be life threatening. The remaining type of West Nile infection is different. Less than 1% of people infected with WNV have much more serious symptoms, leading to encephalitis or meningitis. According to the CDC, the symptoms of these conditions include “headache, high fever, neck stiffness, disorientation, coma, tremors, seizures, or paralysis” (CDC, 2013). In general, recovery from WNV symptoms can take weeks to months but neurological effects of the disease may be permanent. West Nile Virus has a 10% mortality rate; the patients most at risk are those who develop the severe neurological symptoms. Medical conditions that compromise the immune system such as cancer, diabetes, hypertension, and kidney disease also increase the risk with West Nile Virus (CDC, 2013; Sejvar, 2003).
There are two types of dengue fever – a mild type that causes fever, rash, and joint pain and a more serious type called dengue hemorrhagic fever which causes severe bleeding and can result in death (Mayo Clinic, 2011). These are separate diseases caused by the same viruses (Vyas, 2013). Both types of dengue fever are seen in tropical and subtropical areas such as Southeast Asia, Latin America, and Caribbean and millions of people per year contract the mosquito-borne disease (Mayo, 2011). Dengue fever is caused by four different but related dengue viruses spread by mosquitoes (particularly the Aedes aegypti) (Vyas, 2013). Much like malaria, the virus enters a mosquito when the mosquito bites an infected person and when the infected mosquito bites another human the virus is transmitted. The mild form of dengue fever causes symptoms for about a week before recovery. With dengue hemorrhagic fever, the symptoms do not abate and instead get worse – bleeding from the mouth and nose, vomiting, subdural bleeding, and problems with almost all major organs (Vyas, 2012). These symptoms can lead to a sudden drop in blood pressure (shock) and death (Mayo Clinic, 2011). There is no vaccine and no specific treatment. Treatment is generally supportive: fluids to replace those lost in vomiting, blood transusions to keep blood pressure stable, electrolyte replacement, and fever reducers (Vyas, 2012). Contracting dengue fever leads to immunity from that particular type of dengue fever but also increases the likelihood that person will contract dengue hemorrhagic fever if they are infected by one of the other three dengue viruses (Mayo Clinic, 2011). Yellow Fever is found in South America and sub-Sarahan Africa, similar to many of the other Aedes aegypti mosquito-vector diseases. Like malaria, anyone is capable of contracting the disease but there are specific high-risk groups. Unlike malaria where children are at greatest risk for malaria, the elderly are at most risk for yellow fever (Vyas, November 2013). Yellow fever occurs in three stages, known as stage 1/2/3, acute/toxic, or infection/remission/intoxication (Vyas, November 2013; Mayo Clinic, 2011). People with stage 1 yellow fever have relatively mild symptoms such as headache, aches, fever, vomiting (Vyas, 2013). From stage 1 yellow fever, patients enter stage 2, also known as remission. This is a period of symptom improvement that may signal the end of the disease course or just a slight pause before the patient enters stage 3 (also known as toxic or intoxication stage) of yellow fever (Mayo Clinic, 2011). This final stage can be life threatening; its symptoms include jaundice and liver failure, decreased urination and kidney failure, bleeding from mucous membranes, vomiting, as well as arrhythmia and heart problems (Mayo Clinic, 2011). Organ failure, internal bleeding, seizures, coma, and death are potential symptoms of stage 3 yellow fever (Vyas, 2013). While there’s no treatment for yellow fever beyond supportive treatment to relieve symptoms, a vaccine against yellow fever that is generally recommended for tourists traveling to high risk areas (Mayo Clinic, 2011). Humans are not the only species to be at risk from mosquito-transmitted diseases; dogs can contract heartworm from infected mosquitoes. When an infected mosquito carrying the heartworm larvae (Dirofilaria immitis) bites a dog the larvae are transferred into the bloodstream and grow and mature in the dog’s cardiovascular system. While dogs are the main species known to vulnerable to heartworm, several species including wild canids and wild feline species, ferrets, sea lions, and domestic cats are also at risk. Humans can also contract heartworms from mosquitoes (American Heartworm Society, 2014). West Nile Virus is also present in horses and some species of birds, transmitted to them by mosquitoes. These species act as reservoirs but it is important to understand that direct transmission from birds or horses to humans is impossible – the disease requires the mosquito vector in order to transmit (CDC, 2013). Yellow fever is present in monkeys and is essentially passed back and forth between monkeys and humans through the mosquito vector (Mayo Clinic, 2011). There are many, many other viral diseases that can be transmitted by mosquito vectors. Among them are Rift Valley fever and several arboviral encephalitides, including Eastern equine encephalitis, Japanese encephalitis, La Crosse encephalitis, St. Louis encephalitis, West Nile virus (part of the Japanese encephalitis antigenic complex and discussed in greater detail earlier), and Western equine encephalitis (NINDS, 2004). |