Category Archives: Public health

Are Biting Flies Important Disease Spreaders?

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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.

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Venezuelan Equine Encephalitis: How Small Effects Lead to Big Consequences

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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.

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Central Asian Wetlands as Predictors of Avian Influenza Spread

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Lake Uvsu-Nur area on the Russian Federation-Mongolian border. Detection here of highly pathogenic H5N8 avian influenza in wild birds this past spring led to (correct) warnings that the virus would likely spread to the Middle East, Europe, and Africa.  © Alexey Butorin/Greenpeace

Another highly pathogenic avian influenza (HPAI) virus is marching across Western Asia, Europe, and North Africa, killing domestic flocks and a number of wild birds, from India in the east to the Atlantic Ocean in the west.

This is the 4th wave of HPAI to sweep across large swathes of the globe in the past 11 years. The culprit this time around, an H5N8 virus, appeared in India in October and the Mediterranean basin in November 2016, leading so far to the deaths of hundreds of thousands of domestic birds and dozens of wild birds from over 30 species.

A potential silver lining to this unfolding story is that this particular H5N8 virus was first detected 4 months earlier, from a lake on the Mongolian-Russian Federation border. Prompt reporting of the find led to warnings by experts of a high likelihood of spread to exactly those regions affected so far.

Was the early warning a lucky break, or have we learned enough about HPAI epidemiology to make such predictions routine? The answer is a bit of both.

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Pandora’s Box? The Risks of Pathogen Escape from Laboratories

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Laboratory procedures for working on dangerous pathogens has changed significantly over the past 40 years. Randal J. Schoepp

Laboratory procedures for working on dangerous pathogens has changed significantly over the past 40 years. Randal J. Schoepp, James Gathany

Pathogens are maintained in laboratories around the world for many reasons. They can be used to develop vaccines, to provide materials for diagnostic tests, or to study genomes, offering clues as to how pathogens may evolve so that we are better prepared to deal with them.

There is debate within the scientific community as to exactly what kinds of research should be done on especially nasty organisms commonly called Potential Pandemic Pathogens, such as the deadly SARS respiratory virus or highly pathogenic avian influenza viruses. Some believe the risks of escape, though small, are not worth taking as an accidental release could sicken or even kill millions of people, animals, or both.

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New World Screwworm Outbreak in the United States: Sterile Flies to the Rescue

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New World screwworm maggots on an endangered Key deer. The injuries on rutting males are particularly vulnerable to the fly. Early infestations can be treated effectively in pets, livestock, and people. But identifying and treating infested wildlife is difficult. Charles Hoots

New World screwworm maggots on an endangered Key deer. The injuries on rutting males are particularly vulnerable to the fly. Early infestations can be treated effectively in pets, livestock, and people. But identifying and treating infested wildlife is difficult.             Charles Hoots

When the presence of New World Screwworm (Cochliomyia hominivorax) was confirmed in the Florida Keys in September 2016, it was the first non-isolated appearance of the parasite in the United States in over 30 years. While the devastation to endangered wildlife on this archipelago has been significant, if the fly spreads unchecked to the mainland it could result in losses approaching $1 billion annually.

The release of large numbers of sterile male flies is the only known method to eliminate established New World Screwworm (NWS) populations. Identifying the geographical extent of the invaders is the critical first step. The flies typically are not great wanderers and refuse to cross open water. But when those in Florida began turning up on nearby islands with no land links to neighboring islands, the sterile fly release campaign became that much more complicated.

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An Imperfect Storm: Why Lyme Disease Is Uncommon in the Southern United States

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Ixodes tick. Primary bridging vectors for Lyme disease. Jerzy Gorecki

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 southern 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?

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Migratory Birds as Spreaders of Emerging Diseases: Fact and Fiction

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Storks on migration over Haifa, Israel. Some of these carried a particularly virulent form of West Nile Virus from Europe in 1998. David King

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.

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Zika Virus and its Animal Hosts: Why We Need to Know More

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Space-fill drawing of the outside of one Zika virus particle, and a cross-section through another as it interacts with a cell. The outer shell of viral capsid proteins are in pink, the membrane layer with purple proteins, and the RNA genome inside the virus in yellow. The cell-surface receptor proteins are in green, the cytoskeleton in blue, and blood plasma proteins in gold. Drawn by David Goodsell.

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.

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Plague Epidemics in Madagascar May Offer Answers for Rest of World

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Cats occasionally get plague from infected prey. Eddy Van 3000

Cats occasionally get plague from infected prey.            Eddy Van 3000

The mention of bubonic plague still sends shivers down the spines of people in much of the world. The disease ravaged Asia and Europe for at least 1,500 years, until the advent of antibiotics in the mid-20th century. Many people today believe that plague has been eradicated, and are surprised to learn that the disease continues to thrive in much of the world, though in a rather different form from in its heyday.

Plague is but a shadow of its former self, but it refuses to go away completely. The United States and Madagascar, two reservoirs of the Yersinia pestis bacteria that cause plague, continue to suffer regular outbreaks of the disease. While this scourge may well continue to decline to very low levels, its eradication will be all but impossible unless we understand better where these bacteria like to hide in between outbreaks.

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The Mysteries of Emerging Anthrax Outbreaks in Bangladesh

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Water-logged soils in the monsoon season make carcass burial problematic in Bangladesh

Water-logged soils in the monsoon season make carcass burial problematic in Bangladesh Wikimedia

The month of May 2016 has seen over 100 human anthrax cases in Bangladesh, the seventh such outbreak in the past eight years. The Bacillus anthracis bacteria that cause anthrax began wreaking havoc in Bangladesh beginning in 2009, reportedly a quarter century after the last known human case of the disease in the country.

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