Increasing Evidence Points to Major Role Birds Have in Spreading Lyme Disease

Increasing Evidence Points to Major Role Birds Have in Spreading Lyme Disease

Back in 2010, Yale reported the increasing evidence that pointed to migratory birds spreading Lyme disease not just between states but around the world.

A team led by School of Public Health researcher Maria Diuk-Wasser analyzed studies on 71 bird species that host the black legged tick, the main carrier of Lyme disease.

They found that 58.6 percent of the bird species can infect the tick with the bacterium that is responsible for Lyme disease. The literature review was published online in December in the journal Frontiers in Ecology and the Environment.

“Although it has been known for some time that birds play some role in Lyme disease epidemiology, this study integrates all the available information and points at particular bird species that may have a critical role in dispersing the Lyme agent,” Diuk-Wasser said.

Traditionally, scientists thought small mammals such as white-footed mice, chipmunks and shrews were the main carriers of ticks, co-author Kimberly Tsao said. But the study shows that the expansion of the range of Lyme disease in the Northeast and Midwest over the past 20 to 30 years can also be attributed to birds, she said. For example, nearly 70 percent of brown thrashers, a ground-dwelling species, carried the tick, post-doctoral researcher and main author Robert Brinkerhoff said.

Diuk-Wasser said differences in when tick larvae are active may result in birds transporting tick larvae northward in the Midwest and southward in the Northeast.

Because the bacterium that is responsible for Lyme disease has a lot of genetic variation, the researchers said the next step for scientists is to determine if birds carry the same strains that infect humans. Some strains, Brinkerhoff said, are more infectious than others.

However, the problem has become much more widespread than originally thought by conservative professionals.

In 2013, a study published by the Journal of Veterinary Science & Medical Diagnosis (by researchers John D Scott,  John F Anderson, and Lance A Durden) showed that infected larvae were taken from a raptor (Cooper’s Hawk).  What shocked the researchers was finding Borrelia burgdorferi in the ticks taken from the hawk which were Ixodes Auritulus, a tick  first identified in South America on various song birds and now in Canada.  

This tick has previously been thought to carry Borrelia garnii only which previously has been thought to exist in Europe, Russia and northern parts of Asia.  This initially caused a mystery for the researchers who were perplexed at how the larvae became infected with Lyme disease, but now it is known that the larvae can be infected before hatching. This has also shown how countries such as New Zealand and Australia now have snowballing epidemics of Lyme disease when for years it was unheard of.

Numerous studies have been designed to investigate possible roles wild birds might have in the maintenance of Lyme disease enzootic cycles in nature and for the expansion of endemic ranges. Lyme disease is a multisystem and multi-stage infection caused by three species of tick-borne spirochetes in the B. burgdorferi sensu lato genogroup. These include B. burgdorferi sensu stricto (North America and Western Europe), Borrelia afzelii (Western Europe, Central Europe, and Russia), and Borrelia garinii (Europe, Russia, and Northern Asia).

In North America, including Canada, Lyme disease is perpetuated by a cycle involving rodents, such as white-footed mice, and is transmitted to humans and other animals primarily by black-legged ticks Ixodes scapularis in the northeast and Rocky Mountain Spotted tick in the north central U.S. and Ixodes pacificus in the West.

Two important features of the epidemiology of Lyme disease in the U.S. have been the continuous geographic spread of the tick vector and temporal increase in reported cases within endemic areas over the past two decades. 

Surveys of ectoparasites of birds reveal that ticks commonly infest a wide range of species, especially thrushes, sparrows and other ground foraging birds. Ticks are usually found attached to the thin skin around the eyes and ears, and on the head – areas that are difficult for the bird to preen. Tick infestation of some birds can be substantial.

Although a wide range of tick species have been reported to parasitize wild birds, Ixodes spp. are the most likely to carry B. burgdorferi. Other species of ticks, occasionally test positive for B. burgdorferi, but the significance of these findings continue to grow.

More than 300 species of birds breeding in the United States and Canada spend the winter in the West Indies, Central America, or South America.  

These long-distance migrants travel hundreds to thousands of miles from their breeding grounds to wintering ranges.

An extreme example of long-distance migration is the Arctic Tern. The intercontinental trip from its circumpolar nesting site to its winter destination at the Antarctic pack ice is over 10,000 miles long and takes several months to complete.

Preparation for such long trips varies depending on the size of the bird and the length of trip. Birds depend on increasing their nutritional intake to provide energy reserves as stored fat. This is particularly crucial for species like the Blackpoll Warbler, which breeds in the boreal forests of Canada and makes a non-stop flight of 2,500 miles to spend the winter in South America.

Most long-distance migrants make a series of shorter flights, traveling at night when the air is cooler and the atmosphere is generally calmer. Days are used to rest and replenish energy reserves.

Stopovers at these “staging areas” are important from the viewpoint of Lyme disease because they provide the opportunity for picking up different species of ticks at different resting stops along the way.

Certain family groups of birds such as geese, swans and cranes use well-established migration corridors through North America. These include the Atlantic, Mississippi, Central and Pacific flyways. Geese, in particular, take their time making their trip and are usually found to spend time in the Northeast, most endemic part of the country for Lyme disease. 

These routes are oriented north to south in part because wintering areas are generally located south of breeding sites, but also because the mountain ranges, coasts and major river valleys of North America run in a similar direction.

It is interesting to note that many migrating birds in Europe and Asia travel in a more east to west fashion, which correspond to major coastlines and other landforms.

So it is  actually dangerous to oversimplify the huge variable that these different migrations have on the spread of Lyme disease in particular.

Several aspects of long-distance migration can contribute to whether certain migratory birds are more or less likely to host ticks on their cross-oceanic flights. For instance, birds that over-winter in the rainforests of Central and South America inhabit (during their visit) the greatest biodiversity on earth. This may partially explain why ticks that are normally found there are showing up in unexplained parts of the world now, and infected with diseases that were never seen in those places.

However, it can also represent increased risk for exposure to the abundance of reservoir hosts and all types of pathogens that prosper and abound in the tropics.

Another consideration is the stress associated with migration, a known risk factor for immunosuppression and increased susceptibility to infectious diseases. For some birds the stress of migration can lead to reactivation of otherwise latent infections.

It is also very likely that man-made changes to the environment contribute to stress and other changes in the spread of disease by migratory birds. 

The year-to-year variation in the movement of certain bird species is highlighted by vagrant migration, an episodic “invasion” into areas that are greatly beyond the normal range.

During migration, this is sufficient time for some birds to travel hundreds or even a few thousand miles before ticks complete feeding and drop off. An example of the capacity for wild birds to carry spirochete-infected ticks long distances was part of  a molecular epidemiologic study which provided evidence of transhemispheric exchange of spirochete-infected ticks by seabirds from colonies in both the southern and northern hemispheres. Overall, dispersal of B. burgdorferi-infected ticks along migration routes is considered to be an important mechanism for the establishment of new areas of disease. 

Another important area of investigation has been attempts to determine whether or not birds might serve as reservoir hosts for B. burgdorferi. Spirochetes have been isolated from the blood of numerous bird species. 

For more information on the Australian epidemic of Lyme disease that has only just recently been acknowledged read:–reservoir-hosts-including-examination-of-iuriae-seabird–iauritulus-bird-ticks.html


Yale School of Public Health

Journal of Veterinary Science & Medical Diagnosis

Emerging Infectious diseases by the National Institute of Health


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