Horse fly (Tabanus spp.) mouthparts. The dark, middle projection is the labium, used by the fly to lap up fluids. The much-feared scissor-like cutting parts are enclosed within the labium and not visible here. The top protrusion, though dangerous looking, is one of two antennae. Alan R Walker
Biting flies are feared around the world for their painful bites, inflicted on both animals and humans. Like houseflies, which have no piercing mouthparts, biting flies can spread disease-causing pathogens that hitchhike on the flies’ legs, abdomen, or mouthparts, transmitting disease from ill to susceptible animals. This effect is reinforced by their feeding habit of severing capillaries near the skin surface to drink the blood that pools there.
Given these characteristics, it is surprising that biting flies are not more important pathogen spreaders than they actually are. Many of the dozens of diseases they are suspected of spreading are based on having identified those pathogens somewhere on a fly in the past. But documented transmission by biting flies between animals has been rare, for several reasons.
The northeast coast of Colombia, focus of several major Venezuelan equine encephalitis outbreaks in people and animals. Charles Hoots
After a trip to Colombia last November, I wanted to do a post on Venezuelan equine encephalitis (VEE), yet another zoonotic, mosquito-borne virus of the tropics. The northeast coast of Colombia, along with neighboring Venezuela, has been the focus of several outbreaks of VEE in the past. But the last major one occurred in the 1990s and I decided it wasn’t current enough to write a post on.
Right on cue, in December 2016 Colombian authorities announced a mass equine vaccination campaign and restrictions on horse movements into and out of Colombia’s Cesar Department in response to an as yet limited VEE outbreak there. Colombia’s caution is warranted, given the unpredictable nature of this disease that in the past has vanished for decades at a time, only to reappear with devastating effects just when it was about to be written off as gone forever.
Ixodes tick, primary vectors for Lyme disease. Jerzy Gorecki
Lyme disease is caused by Borrelia burgdorferi, a bacterium transmitted by ticks to a wide range of animal species (including people) in much of the world. The great majority of human Lyme disease cases in the United States occur in the Northeast and upper Midwest states. Yet, the impact of Lyme disease in the south US remains minimal despite the abundant presence of the primary Ixodes tick vectors, numerous competent animal hosts, widespread suburban sprawl that brings people into frequent contact with ticks, and the documented presence of B. burgdorferi bacteria in the region. Why hasn’t the disease taken a stronger hold there?
Storks on migration over Haifa, Israel. Several individuals of this species were found in this area carrying a particularly virulent form of West Nile Virus from Europe in 1998. David King
Migratory birds move hundreds to thousands of kilometers twice a year, often spanning continents. As they share certain diseases with people, it is not surprising that birds are frequently blamed for transporting these diseases around the world. But while birds are undoubtedly implicated in the geographic expansion of some emerging diseases, the more interesting question is why it doesn’t happen more often, given the hundreds of millions of birds on the move.
Central European wild boar (Sus scrofa). A reservoir host for African swine fever? Richard Bartz
African swine fever (ASF) is a deadly, contagious viral disease of pigs for which there is no vaccine or treatment. While primarily a scourge of sub-Saharan Africa, the disease’s recent spread into Russia and the European Union reminds us that ASF is not just a tropical disease.
Alarmingly for large swine-producing regions, from China to the EU and the United States, ASF’s behavior in the current Eastern European outbreak differs significantly from what was expected based on previous ASF outbreaks in Western Europe. These differences make expansion of the disease much more difficult to control, threatening huge economic losses to affected countries. Through October 2015, over 750,000 domestic pigs in Europe have died from or were culled to prevent the spread of the current ASF outbreak.
Space-fill drawing of a whole Zika virus particle, and a cross-section as it interacts with a cell. The outer capsid is pink, the membrane purple, and RNA genome in yellow. Cell-surface receptors are green, cytoskeleton blue, and blood plasma gold. David Goodsell
Zika virus is one of a large number of viruses transmitted between animals (including humans) by arthropod insects. These are called arthropod-borne viruses, or arboviruses for short. The arthropod vectors in the case of Zika virus are certain mosquito species that transmit the virus from one host to another. But arboviruses also require a reservoir host: one or more species of animal within whose population the virus is maintained for long periods in relative stability. In other words, the virus circulates at low levels in the population, avoiding the infection of so many individuals that the general population becomes immune to it and the virus has nowhere to go but extinct.
Researchers are getting a pretty good handle on the various mosquito vectors of Zika virus. But we know very little about what animal species act or may act as reservoir hosts for the virus. This information is crucial for understanding the virus’s transmission dynamics and geographical distribution. Without understanding Zika’s reservoir(s) or other hosts, control and prevention will be difficult and inefficient at best, counterproductive at worst.
Construction of the Panama Canal spurred research to develop a yellow fever vaccine Library of Congress
With yellow fever vaccine in short supply in the face of an ongoing outbreak in Angola, we might be reminded of the fortuitous, and by no means inevitable, circumstances that led to the development of the yellow fever vaccine in the first place. Vaccines for mosquito-borne diseases are hard to come by, and no less so for the flaviviruses that include yellow fever, dengue, West Nile, and Zika viruses. A serendipitous event in a yellow fever virus isolated from a single person 90 years ago has given rise to all yellow fever vaccines in use today, though at a heavy cost to many of those who played a role in its discovery.
Rift Valley Fever outbreaks have long been predicted in Uganda. But when the nation declared its first ever outbreak of this zoonotic virus last week, it could hardly have occurred in a more unlikely part of the country. This is just the latest in several surprises this mosquito-borne virus has thrown at us over the past few decades.
Lake Victoria from Entebbe, Uganda, near the forest for which the Zika virus is named. Charles Hoots
With concern of a possible connection between Zika virus and microcephaly in Brazilian babies, researchers are racing to fill in the many blanks about the virus in order to better control it. But predicting the future of Zika virus in the Americas is fraught with difficulties. The behavior of closely related, recently introduced viruses offers clues. But despite similarities, the differences are many and prevent sweeping generalities. Though not reassuring to couples contemplating starting a family right now, it is this uncertainty that makes these viruses so fascinating.