2016 Mosquito-Borne Disease Year in Review

West Nile Virus (WNV): WNV is the most common virus transmitted by mosquitoes to humans in the United States. WNV is typically transmitted to humans by mosquitoes that have previously fed upon an infected bird. While over 150 species of mosquitoes have been known to carry WNV, the main vector species in the U.S. are Culex pipiens, Culex tarsalis, and Culex quinquefasciatus. These mosquitoes are all active at night, and most cases of infection occur during the summer months. Approximately 20% of people affected by WNV will experience flu-like symptoms including fever, headache, nausea, muscle pain, and swollen lymph glands. Other symptoms may include a stiff neck, rash, sleepiness or disorientation. Less than 1% of those infected will develop West Nile Encephalitis or Meningitis, which can lead to coma, tremors, convulsions, paralysis, and even death.

As of January 17, 2017, 47 out of 50 states and the District of Columbia have reported West Nile virus infections in people, birds, or mosquitoes in 2016. Overall, 2,038 cases of WNV were reported in humans, and there were 94 confirmed deaths (4.61%) in 2016. The total is a slight improvement from 2015, where there were 2,060 human cases and 119 confirmed deaths (5.8%). To learn more about the symptoms, treatment, and mosquito species that vector this virus, visit our educational page on West Nile virus.

Eastern equine encephalitis (EEE): EEE, also referred to as Triple E, is a rare but deadly illness for humans. Thankfully, only a few human cases of EEE are reported in the United States each year. From 2004 to 2013, an average of eight cases of EEE were reported annually in the U.S. The reason EEE is less common in humans is that the primary mosquito vector (Culiseta melanura), does not typically feed on humans. It is believed that EEE virus is mainly transmitted to humans and horses by bridge vectors that have contracted the virus by feeding on infected birds. Symptoms typically occur four to ten days after a bite from an infected mosquito and include fever, headache, vomiting, muscle aches, joint pain, and fatigue. In rare cases, infection occurs in the brain and spinal cord leading to sudden high fever, stiff neck, disorientation, seizures, and coma. The mortality rate of those that develop EEE is about 33%, the highest among human arboviruses (a virus transmitted by arthropod vectors) cases reported in the U.S. Currently, there is no human vaccine for EEE. The disease is also a concern for horses; however, a vaccine is available for horse owners.

Although preliminary, five human cases of EEE were reported nationally in 2016. In 2015, there were six reported human cases of EEE and 50% of those cases were fatal. To learn more about the symptoms, treatment, and mosquito species that vector this virus, visit our educational page on EEE virus.

Zika Virus (ZIKV): The Zika virus has been prevalent in the news in 2016, but has actually been around since 1947. In the Americas, it has only been linked to transmission by Aedes aegypti. Ae. aegypti is also responsible for the transmission of dengue virus, yellow fever virus, and chikungunya virus. Recently in Africa, the virus was detected in Aedes albopictus or the Asian tiger mosquito; hence, it is possible that Ae. albopictus could vector the virus in the Americas. If a pregnant woman is infected with Zika virus, it may result in microcephaly, a birth defect causing underdevelopment of the head and brain in newborn children. When symptoms occur, they typically begin with a mild headache and fever. Within a day or two, a maculopapular rash may appear and can cover many parts of the body. Following the rash, people generally report continued fever, malaise, and body aches. Other symptoms can include diarrhea, constipation, abdominal pain, and dizziness. Treatment for symptoms includes hydration, rest, and the use of acetaminophen to relieve fever. Currently, there is no vaccine or cure for ZIKV.