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?
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.
Trypanosomes are single-celled protozoan organisms, one species of which causes sleeping sickness in people and several of which cause a similar disease in animals. In its “classic” form, the animal disease is spread from wildlife to cattle in much of sub-Saharan Africa through the bite of a tsetse fly, resulting in a slow wasting away of the affected livestock (but with typically no signs of illness in the wildlife hosts).
I arrived in northeast South Sudan in 2013 to work on a livestock project for the German branch of Veterinarians Without Borders. The animal form of sleeping sickness (which I will call AAT, short for African animal trypanosomosis) was at the time a major problem in the herds of the 120,000 refugees from neighboring Sudan living in four camps in the area. But the situation was far from “classic.”
Nearly a quarter-century of civil war had led to the almost complete elimination of large wildlife species that tend to act as reservoir hosts for trypanosomes. In addition, tsetse fly vectors, the poster child for sleeping sickness in people and animals, were nowhere to be found. Our subsequent joint effort with the community to control this disease taught me valuable lessons in how good intentions can go awry in animal (and human) health planning through failure to consider every aspect.
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.
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.
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.
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.
With 70% of emerging infectious diseases estimated by the World Health Organisation to be zoonotic in nature, livestock and wildlife often fall ill from these pathogens before they spread to people. As a result, veterinarians and other animal health workers (AHWs) can play a crucial role in early detection of emerging zoonotic diseases, especially in remote areas of poorer countries where human health care infrastructure is sparse or absent.
Yet, despite significant numbers of livestock in rural areas of developing countries, AHWs are few and far between, unable to generate sufficient income to make a living. Well-intentioned policies from national and international organizations in some cases end up driving AHWs away, or into more lucrative pursuits. As a result, animal disease outbreaks, many with public health impacts, may run their course for weeks before being detected and addressed by the authorities.
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.
The ancestors of Florida’s rhesus macaques arrived in the late 1930s on the supposed whim of a local tour boat guide. What started as a handful of individuals is now believed to number in the hundreds, worrying public health officials that the macaques could transmit a deadly herpesvirus to human admirers imprudent enough to venture too close to the animals.