By Kris Hickman, with Bara Vaida
The World Health Organization (WHO) defines vectors as organisms that pass diseases from animals to humans or between humans. Many vectors are insects that suck infected blood from the animal or human host. Then they pass the disease-causing microorganism to other animals or humans when they have their next blood meal. Mosquitoes are probably the most widely known vector, but vectors are not exclusively mosquitoes. Ticks, fleas, sand flies, freshwater snails, and triatomine bugs are all examples of vectors that can transmit disease.
Vector-borne diseases account for about 17 percent of infectious diseases around the world and cause as many as 1 million deaths annually. The WHO has a good primer here.
There are a number of vector-borne diseases circulating in the United States, including Zika, West Nile and Lyme disease. In the coming years, these diseases may be a growing threat to an increasing number of Americans as a result of climate change. The CDC’s Division of Vector-Borne Diseases within the National Center for Emerging and Zoonotic Infectious Diseases (NCEZID) provides more information on how researchers and health professionals combat those diseases in the United States, where mosquitoes and ticks are the most common vectors.
Globally, threats like malaria and dengue fever, impact millions.
Zika virus
Zika virus is the vector-borne disease that has garnered the most recent media attention in the U.S. The virus is transmitted by mosquitoes, and can be sexually transmitted, as well. It is carried by Aedes mosquitoes (the same genus that transmits dengue, chikungunya, and yellow fever), which are well-adapted to urban areas. See this short video from the World Health Organization on Zika transmission and mosquito control.
Zika symptoms include fever, headache, skin irritation, conjunctivitis, and body aches, and usually last for 2-7 days. The virus was identified in African monkeys in 1947 and in humans in 1952, but its symptoms were mild and no complications of the disease were identified.
Then during a 2015 outbreak of Zika in Brazil, doctors began to suspect a connection between Zika and microcephaly (a syndrome characterized by a tiny head and underdeveloped brain) in newborns. Physicians also noticed the virus seemed to trigger Guillain-Barré syndrome (which causes paralysis) in infected individuals.
In February 2016, the WHO declared Zika a public health emergency, igniting fears Zika would spread to the U.S. during the summer months. Pregnant American women were advised against traveling to many South, Central and Caribbean countries. The WHO subsequently conducted a literature review that concluded Zika infection in pregnant women can cause microcephaly, and that Zika infection can trigger Guillain-Barré syndrome. However, in 2016, there were far fewer cases of microcephaly in Brazil than originally feared and in November 2016, the WHO called an end to the public health emergency. Researchers are investigating possible links between Zika and similar viruses dengue and chikungunya as a possible explanation.
However, those links are not yet well understood. A recent study from the Arthropod-Borne and Infectious Diseases Laboratory at Colorado State University suggests an Aedes mosquito can carry two or more of those diseases at the same time, and an individual bitten by that mosquito might then experience a coinfection. Researchers concluded coinfections are becoming more common, but they were also unsure of the clinical and epidemiological effects, since it is unknown whether the viruses compete for resources within the host, or work together to increase immunosuppression. The CSU team is now studying this question in detail to determine the effect of coinfection on the viruses within the mosquito, and if that affects the severity of sickness within a coinfected person.
Zika is still considered a serious threat and there is controversy in the scientific community about whether or not the WHO should have lifted its public health emergency.
Since Jan. 1, 2015, there have been 224 cases of locally transmitted Zika infection in the United States. Another 47 cases reported in the U.S. have been determined to be sexually transmitted. Over 5,100 cases of Zika infection in the U.S. have been reported in travelers returning from an infected area. (Data current to Aug. 31, 2017.) Pregnant American women are still cautioned about travel to places where Zika has been reported.
This Zika podcast series from WHO expands on preparedness, prevention, and epidemiology of Zika, and it also addresses how to manage Zika complications like microcephaly.
West Nile
Another mosquito-borne disease is West Nile Virus (WNV). West Nile is transmitted by Culex mosquitoes, and while it grabs headlines, it is rarely fatal. About 80 percent of infected people do not show symptoms, while about 20 percent develop fever and other flu-like symptoms. A small number of those who become infected — about 1 in 150 cases — will develop a severe and potentially fatal form of the disease.
To drill down on local and statewide West Nile data, see preliminary data on West Nile for 2017 from the CDC. For information on past years, see the CDC-USGS Disease Map, which shows reported cases of West Nile and certain other vector-borne diseases at the state and county levels. This CDC fact sheet will help those who cover hurricane-prone areas understand how hurricanes affect disease transmission.
Tick-borne diseases
Ticks also transmit disease, and affect thousands of people in the U.S. each year. Dozens of tick-borne diseases are found in the United States.
Lyme disease is the most common of these. Blacklegged ticks transmit the bacterium Borrelia burgdorferi, which causes fatigue, headache, fever, and a skin rash called erythema migrans. Untreated infection can spread to the heart, joints, and nervous system. Lyme disease is the most commonly reported vector-borne illness in the United States, but it is not found nation-wide and can be difficult to diagnose and treat. It is most commonly found in the northeast, Minnesota, and Wisconsin. See this report from the Harvard Kennedy School Shorenstein Center on Media, Politics and Public Policy on climate change and Lyme disease.
Also see this AHCJ Covering Health blog post from 2014 on covering Lyme disease.
Many rickettsial bacterial diseases are also tick-borne, such as Rocky Mountain Spotted Fever (RMSF) and other spotted fevers. Providers diagnose these illnesses by the presence of a blotchy rash. The case fatality rate of spotted fevers has dropped to less than one percent in the last 60 years, but researchers are unsure how many cases are RMSF, which can be fatal if left untreated, and how many cases are caused by other, less-severe spotted fevers. For more information on RMSF and other rickettsioses, see this guide from the Morbidity and Mortality Weekly Report (May 2016).
Some tick-borne diseases in the United States are also fairly new, and are therefore considered to be emerging infectious diseases. Examples identified in recent years include Heartland virus and Bourbon virus. Since 2014, there have been only a handful of confirmed cases of Bourbon virus in Missouri, Kansas, and Oklahoma. Fatalities from Bourbon virus have occurred near Saint Louis and in the Kansas City area. It is believed to be tick-borne, but researchers know little about it at this point.
For an in-depth look at tick-borne illness in the United States, see the fourth edition of the CDC’s handbook on common tick-borne diseases, released in 2017, and watch these webinars on trends in tick-borne diseases.
Globally malaria might be the most familiar vector-borne disease. It is a potentially fatal disease that affects millions of people annually in tropical and subtropical areas worldwide. When female mosquitoes of the Anopheles genus feed on humans, they pick up and transmit parasites called Plasmodium. (See this animation for more information on Plasmodium’s life cycle.) The parasites then reproduce and develop with the female mosquitoes. Once inside humans, they can cause severe fever, headache, chills, and vomiting. In extreme cases, Plasmodium parasites can kill their human hosts by clogging the blood vessels of the brain and other vital organs.
