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Animal-human diseases that may spread in a warming world

Posted by Laura Grant on October 13, 2008
Posted in Green News

In the past week the effects of climate change on human health have come to the fore. The World Health Organisation made it a priority research area at its conference in Barcelona last week after acknowledging that the issue had up to now received little research attention, ENS reports.

Also in Barcelona, the World Conservation Society (WCS) released a report in which it listed 12 animal-human diseases, the so-called “Deadly Dozen”, that might spread to new regions as a result of projected temperature and rainfall changes.

The Intergovernmental Panel on Climate Change has warned us that the range of vector-borne diseases like malaria is likely to change thanks to climate change. Kenya has already reported cases of the disease in previously malaria-free areas. But the WCS report includes other diseases such as avian flu, cholera, tuberculosis and Ebola that can spread between animals and humans.

The report just happens to coincide with reports of the deaths of three people in Johannesburg in the past month of an as yet unidentified viral haemorrhagic fever – Ebola is one of the haemorrhagic fever viruses. More than 100 people are under clinical observation for signs of the fever.

WCS health experts have been looking into the effects of climate change on the health of wild animals and, the subsequent effects on human populations. Diseases such as Ebola and avian influenza, for example, originate from or move through wildlife populations. By monitoring wildlife we could have an early warning system for where these diseases are likely to occur, the expert say. Measures could be put in place to contain their impact.

A programme (Global Avian Influenza Network for Surveillance) already monitors wild bird populations for avian flu around the world. Another in the Republic of Congo involved recruiting a network of hunters and other locals to report on sightings of gorillas and chimpanzees that have died from outbreaks of Ebola. This project has prevented any human outbreaks of the deadly disease in northern parts of the country for three years because the hunters no longer bring dead animals to their villages, hastening the spread of Ebola among people, the Times reports.

“The health of wild animals is tightly linked to the ecosystems in which they live and influenced by the environment surrounding them, and even minor disturbances can have far reaching consequences on what diseases they might encounter and transmit as climate changes. Monitoring wildlife health will help us predict where those trouble spots will occur and plan how to prepare,” said Dr Steven E Sanderson, president and CEO of the Wildlife Conservation Society.

THE DEADLY DOZEN

  • Avian influenza: A strain of the disease – H5N1 – has proven deadly to domestic and wild birds, as well as humans, and has the potential to evolve into a strain that can spread from human to human. The movement of H5N1 from region to region appears to be largely driven by the trade in poultry, but changes in climate such as severe winter storms and droughts can disrupt the movements of wild birds bringing them into greater contact with domestic bird populations.
  • Babesiosis: These tick-borne diseases affect domestic animals and wildlife, and are emerging disease in humans. When infections are severe because of large numbers of ticks, the host becomes more susceptible to other infectious diseases. This has been seen in large die-offs of lions in East Africa from canine distemper. Climate factors fostered heavy infestations of ticks.
  • Cholera: Cholera is a water-borne diarrhoeal disease that affects people mainly in the developing world. It is caused by a bacterium, Vibrio cholerae, which survives in small organisms in contaminated water sources and may also be present in raw shellfish such as oysters. Cholera is highly temperature dependent, and increases in water temperature are directly correlated with occurrence of the disease.
  • Ebola: Ebola haemorrhagic fever virus and the closely related Marburg fever virus kill humans, gorillas, and chimpanzees, and there is currently no known cure. There is significant evidence that outbreaks of both diseases are related to unusual variations in rainfall/dry season patterns.
  • Intestinal and external parasites: Parasites are widespread on land and in water. As temperatures and precipitation levels shift, parasites in the environment will increase in many places, infecting an increasing number of humans and animals. Many species of parasites spread between wildlife and humans. Monitoring parasites in wildlife and livestock will help us identify transmission of these infections between domestic and wild animals and humans.
  • Lyme disease: This disease is caused by a bacterium and is transmitted to humans through tick bites. Tick distributions will shift as a result of climate change. The monitoring of tick distributions will be necessary to assess the impacts of climate change on this disease.
  • Plague: This is one of the oldest infectious diseases known and still causes significant death rates in wildlife, domestic animals, and humans in certain locations. Plague is spread by rodents and their fleas. Alterations in temperatures and rainfall are expected to change the distribution of rodent populations around the globe, which would impact the range of rodent-born diseases such as plague.
  • Red tides: Harmful algal blooms off global coasts create toxins that are deadly to both humans and wildlife. These occurrences – commonly called “red tides” – cause mass fish kills, marine mammal strandings, penguin and seabird mortality, and human illness and death. Similar events in freshwater have resulted in animal die-offs in Africa. Altered temperatures or food-web dynamics resulting from climate change will have unpredictable impacts on the occurrences of this worldwide phenomenon. Effects of harmful algal blooms on sea life are often the first indicators that such an event is taking place.
  • Rift Valley Fever: Rift Valley fever virus affects livestock and people Africa and the Middle East particularly. In infected cattle, sheep, goats and camels, abortions and high death rates are common. People can get the virus from butchering infected animals. The virus is transmitted by mosquitoes and changes in climate raise concerns about the spread of the virus.
  • Sleeping sickness: Also known as trypanosomiasis, this disease affects people and animals. It is caused by the protozoan, Trypanosoma brucei, and transmitted by the tsetse fly. The disease is endemic in certain regions of Sub-Saharan Africa, affecting 36 countries, with estimates of 300,000 new cases every year and more than 40,000 human deaths each year in eastern Africa. Domestic cattle are a major source of the disease, but wildlife can be infected and maintain the disease in an area. Direct and indirect effects (such as human land-use patterns) of climate change on tsetse fly distributions could play a role in the distribution of the disease.
  • Tuberculosis: As humans have moved cattle around the world, bovine tuberculosis has also spread. It now has a global distribution and is especially problematic in Africa, where it was introduced by European livestock in the 1800s. The disease infects vital wildlife populations, such as buffalo and lions in the Kruger National Park. The disease also infects humans in Southern Africa through the consumption of unpasteurised milk. Human forms of tuberculosis can also infect wild animals. Climate change has an effect on the availability of water and is likely to increase the contact of wildlife and livestock at limited water sources, resulting in increased transmission of the disease between livestock and wildlife and livestock and humans.
  • Yellow fever: Found in the tropical regions of Africa and parts of Central and South America, this virus is carried by mosquitoes, which will spread into new areas as changes in temperatures and precipitation levels permit. One type of the virus – jungle yellow fever – can be spread from primates to humans and vice-versa via mosquitoes that feed on both hosts. In some countries in South America, monitoring of wild primates has resulted in early detection of disease activity and allowed vaccination programs to be rapidly implemented to protect humans.

Update (14.10.2008): The disease that killed three people in Johannesburg has been classified as a rodent-borne member of the arenavirus family, related to Lassa fever. [IOL]

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2 Responses to “Animal-human diseases that may spread in a warming world”

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