Why we shouldn’t be too quick to blame migratory animals for global disease


Alice Risely, Deakin University; Bethany J Hoye, University of Wollongong, and Marcel Klaassen, Deakin University

Have you ever got on a flight and the person next to you started sneezing? With 37 million scheduled flights transporting people around the world each year, you might think that the viruses and other germs carried by travellers would be getting a free ride to new pastures, infecting people as they go.

Yet pathogenic microbes are surprisingly bad at expanding their range by hitching rides on planes. Microbes find it difficult to thrive when taken out of their ecological comfort zone; Bali might just be a tad too hot for a Tasmanian parasite to handle.

But humans aren’t the only species to go global with their parasites. Billions of animals have been flying, swimming and running around the globe every year on their seasonal migrations, long before the age of the aeroplane. The question is, are they picking up new pathogens on their journeys? And if they are, are they transporting them across the world?


Read more: A tale of three mosquitoes: how a warming world could spread disease


Migratory animals are the usual suspects for disease spread

With the rate of zoonotic diseases (pathogens that jump from animals to humans) on the rise, migratory animals have been under increasing suspicion of aiding the spread of devastating diseases such as bird flu, Lyme disease, and even Ebola.

These suspicions are bad for migrating animals, because they are often killed in large numbers when considered a disease threat. They are also bad for humans, because blaming animals may obscure other important factors in disease spread, such as animal trade. So what’s going on?

Despite the logical link between animal migration and the spread of their pathogens, there is in fact surprisingly little direct evidence that migrants frequently spread pathogens long distances.

This is because migratory animals are notoriously hard for scientists to track. Their movements make them difficult to test for infections over the vast areas that they occupy.

But other theories exist that explain the lack of direct evidence for migrants spreading pathogens. One is that, unlike humans who just have to jump on a plane, migratory animals must work exceptionally hard to travel. Flying from Australia to Siberia is no easy feat for a tiny migratory bird, nor is swimming between the poles for giant whales. Human athletes are less likely to finish a race if battling infections, and likewise, migrant animals may have to be at the peak of health if they are to survive such gruelling journeys. Sick travellers may succumb to infection before they, or their parasitic hitchhikers, reach their final destination.

Put simply, if a sick animal can’t migrate, then neither can its parasites.

On the other hand, migrants have been doing this for millennia. It is possible they have adapted to such challenges, keeping pace in the evolutionary arms race against pathogens and able to migrate even while infected. In this case, pathogens may be more successful at spreading around the world on the backs of their hosts. But which theory does the evidence support?

Sick animals can still spread disease

To try and get to the bottom of this question, we identified as many studies testing this hypothesis as we could, extracted their data, and combined them to look for any overarching patterns.

We found that infected migrants across species definitely felt the cost of being sick: they tended to be in poorer condition, didn’t travel as far, migrated later, and had lower chances of survival. However, infection affected these traits differently. Movement was hit hardest by infection, but survival was only weakly impacted. Infected migrants may not die as they migrate, but perhaps they restrict long-distance movements to save energy.

So pathogens seem to pose some costs on their migratory hosts, which would reduce the chances of migrants spreading pathogens, but perhaps not enough of a cost to eliminate the risk completely.


Read more: Giant marsupials once migrated across an Australian Ice Age landscape


But an important piece of the puzzle is still missing. In humans, travelling increases our risk of getting ill because we come into contact with new germs that our immune system has never encountered before. Are migrants also more susceptible to unfamiliar microbes as they travel to new locations, or have they adapted to this as well?

Guts of migrants resistant to microbial invasion

To investigate the susceptibility of migrants, we went in a different direction and decided to look at the gut bacteria of migratory shorebirds – grey, unassuming birds that forage on beaches or near water, and that undergo some of the longest and fastest migrations in the animal kingdom.

Most animals have hundreds of bacterial species living in their guts, which help break down nutrients and fight off potential pathogens. Every new microbe you ingest can only colonise your gut if the environmental conditions are to its liking, and competition with current residents isn’t too high. In some cases, it may thrive so much it becomes an infection.

The Red-necked stint is highly exposed to sediment microbes as it forages for the microscopic invertebrates that fuel its vast migrations.
Author provided

We found the migratory shorebirds we studied were exceptionally good at resisting invasion from ingested microbes, even after flying thousands of kilometres and putting their gut under extreme physiological strain. Birds that had just returned from migration (during which they stopped in many places in China, Japan, and South East Asia), didn’t carry any more species of bacteria than those that had stayed around the same location for a year.

The ConversationAlthough these results need to be tested in other migratory species, our research suggests that, like human air traffic, pathogens might not get such an easy ride on their migratory hosts as we might assume. There is no doubt that migrants are involved in pathogen dispersal to some degree, but there is increasing evidence that we shouldn’t jump the gun when it comes to blaming migrants.

Alice Risely, PhD candidate in Ecology, Deakin University; Bethany J Hoye, Lecturer in Animal Ecology, University of Wollongong, and Marcel Klaassen, Alfred Deakin Professor and Chair in Ecology, Deakin University

This article was originally published on The Conversation. Read the original article.

Fences are an increasing threat to Africa’s migratory wildlife


File 20170808 10926 wuf5ra
In the Serengeti wildebeest will move more than 2000km during their annual migration.
Sarah Durant

Sarah Durant, Zoological Society of London

Wildebeest rarely stay still for long. With sloping hindquarters, and an easy loping gait, their bodies are designed to move. In the Serengeti ecosystem, for instance, a wildebeest will move over more than 2,000 kilometres during their annual migration.

Migratory or nomadic animals, like wildebeest, that live in drylands need to move over vast distances to find sufficient water and nutrients. They follow localised and variable rainfall and food resources.

The Serengeti wildebeest spends the wet season, November to April, on the short grass plains of the southern Serengeti National Park and adjoining Ngorongoro Conservation area in Tanzania. Here they feed on nutritious grass shoots that grow in response to the abundant rain. But even here, they do not stay still. They constantly move across the short grass plains in search of the fresh grass that grows after each new rainfall. This allows mothers to maximise milk production for their calves, born during a simultaneous calving of more than a quarter a million, peaking in February.

When the rains cease at the end of April, the wildebeest start their long journey to their dry season grazing areas. They first move west, and then head north, following the remaining water in the rivers before moving on as they dry out. Eventually they reach the only permanent water found in the Mara River on the Kenyan border. The dry season is hard, and many wildebeest die of starvation during this period.

When the rains start in November, the wildebeest lope down south once again. They make the journey to the short grasslands nearly 200km away in just a few days. Here they graze, recover their strength and the cycle begins again.

If these Serengeti wildebeest were to face a barrier at any point in their journey, they would die, either of starvation or thirst. Sadly, this has happened to migratory animals elsewhere in Africa. For example, over 30 years ago, after a fence was erected as a veterinary cordon to separate wildlife from cattle in the Kalahari, 80,000 wildebeest and 10,000 hartebeest died when they were no longer able to access permanent water during a drought. The fence was built to satisfy European Union livestock disease regulations, and allow southern African countries to export meat into the European Union.

Unfortunately, the ability of wildlife in Africa to continue to move across landscapes is still being threatened by linear barriers, and this is particularly a problem in Africa’s drylands.

African drylands

African drylands are home to most of its large mammal species. These include semi-arid and arid savannahs, found across much of eastern and southern Africa, which support spectacular wildlife migrations, such as those found in the Serengeti. But drylands also include hyperarid deserts, such as the vast Sahara, home to distinctive nomadic species such as the critically endangered Addax and dama gazelle.

Because mobility is key for large mammals in these systems, subdividing land reduces the numbers of animals areas can support. To the extent that 300km2 of land in Laikipia will support 19% fewer cattle if subdivided into 10km2 parcels.

Large carnivores, which depend on wide-ranging herbivore prey, also need to range widely, and live at even lower densities than their prey. The Saharan cheetah, for example, occurs at one of the lowest densities ever documented for a big cat, with only one individual per 4,000km2.


Sarah Durant

The recent human migration crisis and growing insecurity in many dryland areas across the Sahara-Sahel has led to calls for large-scale border fencing in Africa, some of which stretch over several hundreds of kilometres.

There are also growing calls for large scale boundary fencing of protected areas as well as infrastructure developments, such as oil pipelines and railways, that cut across wildlife movement pathways. Kenya’s new Standard Gauge Railway line is a recent example.

On top of this is the problem of boundary fences erected around smaller plots of land. In southern Kenya fences put up around private farms have meshed together to form a large-scale barrier to wildlife movement.

International action

In the face of these pressures, migratory, nomadic and wide ranging species depend on trans-boundary action for their long term survival.

The UN Convention on the Conservation of Migratory Species of Wild Animals , also known as the Bonn Convention, lays the legal foundation to safeguard species that need to move across international boundaries. It also provides for internationally coordinated conservation measures throughout their migratory range.

Africa is not alone in facing barrier threats. In central Asia, linear barriers also threaten this region’s migratory wildlife. For example, the border fence and railroad between Kazakhstan and Uzbekistan bisects the Saiga antelope migration between these countries. It has helped to put this population on the brink of extinction.

In response to barrier threats, the Convention on the Conservation of Migratory Species established the Central Asian Mammals Initiative This produced an important set of guidelines to inform fencing interventions and to help sustain migration corridors for migratory ungulates in Asia.

These guidelines are now being followed up with action. A project has been initiated to partially remove and modify the fences along the Trans-Mongolian Railway. This had formed a major barrier to movement for kulan (wild ass) and Mongolian gazelles. Furthermore, border fence modifications recommended by the Bonn Convention on the Conservation of Migratory Species are being implemented to enable Saiga to move, once again, between Kazakhstan and Uzbekistan.

African issues on the table

The Bonn Convention on the Conservation of Migratory Species has just held the Second Meeting of the Sessional Committee of its Scientific Council. This is in the run up to the Conference of the Parties in October where countries will come together to agree on new actions to save migratory species. Under discussion was a new African Carnivore Initiative, which seeks to develop a framework for the trans-boundary conservation of existing Bonn Convention listed large carnivore species, cheetah and African wild dog, and to add two as yet unlisted species, lion and leopard, to the initiative.

Also on the table was an important new initiative to maintain connectivity for terrestrial species, including an additional decision requested by the Zoological Society of London to address the problem of linear barriers in Africa, building on the experiences under the Central Asian Mammals Initiative.

If Africa to avoid catastrophic impacts of large scale fencing on its wildlife in the future, we must avoid repeating past mistakes. This will require further scientific research to better understand potential negative impacts of fencing and other linear barriers, and how best to mitigate such impacts, not just for wildlife, but also for ecosystem services and local communities.

The ConversationAt the Bonn Convention’s next Conference of Parties, nations will need to decide whether to implement important decisions to safeguard migratory species, including maintaining terrestrial connectivity. The fate of many wide ranging species hangs in the balance, and depends on governments supporting and, importantly, implementing, these decisions.

Sarah Durant, Senior Research Fellow, Zoological Society of London

This article was originally published on The Conversation. Read the original article.