The migratory behavior of wildlife has fascinated the researchers a lot. With the onset of increasingly advanced tagging and tracking technologies, we now know that there are large numbers of migratory animals, who migrate across the globe every year in search of safety, food, and reproductive opportunities for their survival.
So the animal migration can be defined as consistent, directional movements from one place to another place. However, it is deliberately recognized that animal migrations have the probability to make changes in the ecosystem dynamics, structure and function in communities they visit. Cross border transfer of pathological agents can occur not only be through the mobility of human, but, also through the wildlife movement.
Among these migratory wildlifes, wild birds are the leading suspects which are needed to be monitored, whom play major role in transport of parasites over long distances, even across the continents, parasites are transferred by the flocks of birds in breeding season of these wild birds or during wintering when weather is unbearable for birds, like in Siberia or when the availability of food is reduced to overcome the needs of these wild birds.
Thousands of species of birds leave their tropical or subtropical habitats in winter, every year to spend the summer in high range area where they breed, returning to low ranges at the end of the mating season. These species are exposed to different parasites in their frosty or temperate breeding areas and subtropical or tropical wintering areas which play major role in dispersal/ transmission of parasites between these areas. So due to these reasons huge populations of parasites are distributed over enormous geographical sites, and it is essential to understand the epidemiology of these parasites, the patterns of virulence of parasite, and the evolution in resistance of host.
Migratory Wild Bird Species
Here we have some examples of wild birds which are the house, fox and the song sparrows (Passer domesticus, Passerella iliaca and Melospiza melodia); the cocoa, wood, clay colored, forest, bare eyed and red legged thrushes (Turdus fumigatus, Hylocichla mustelina, T. grayi, T. lherminieri, T. nudigenis and T. plumbeus); the American robin (T. migratorius); the Baltimore oriole (Icterus galbula); the reddish winged bare eye (Phlegopsis erythroptera); the rufous-tailed ant-wren (Epinecrophylla erythrura); the common yellowthroat (Geothlypis trichas); the bananaquit (Coereba flaveola); the black throated blue warbler (Setophaga caerulescens); the dark-eyed junco (Junco hyemalis); the red eyed vireo (Vireo olivaceus); the northern mockingbird (Mimus polyglottos); and the worm eating black & white warblers (Helmitheros vermivorus and Mniotilta varia).
These all wild bird species play role in transmission of malaria parasites including avian plasmodium spp. and haemoproteus which are common wild bird parasites. Plasmodium which is a protozoan parasite whose vector is mosquito which transmit plasmodium to other wildlife except in Antarctica. In some avian species the migration system involves shorter distances.
Avian infecting plasmodium spp are not transmissible to humans, but migratory wild birds play role in transmission of many other protozoan parasites, many of them are responsible for transmission of emerging diseases to wild animals and human. Waterfowl such as geese, ducks, and swans, are infected by Eimeria which are responsible for transmission of gastro-enteric protozoa which causes coccidiosis in geese, turkeys and chickens. In commercial setup there are huge losses due to coccidiosis if not properly handled.
Feeding Migratory Wildlife
There are a lot of wild animals whom migrate in search of food in rural, urban and peri-urban areas. So they have zoonotic and sometimes anthropogenic impact (transmission of pathogen from human to animals). There are various examples including butterflies, raccoons, wild rodents, wild birds and many other species, which helps in transmissibility of numerous parasites, which have negative impact on human and local animal species.
We have an example of American robins, due to widespread ornamental plantings of berry bushes in city parks and backyards means there’s food for them in the winter, or in rural areas where people have plantation of fruit plants, which attract various wild birds and insects like butterflies for food source, so this is another way of transmission.
Either way, access to new food resources, especially if they’re available all year long, so these can cause some migratory animals to stay put. When human provided food resources allow some animals to forego migration and form resident populations, the consequences can be serious. For one thing, by the time migrants’ return, resident individuals may already be taking up breeding territories and resources, putting the migrants at a competitive disadvantage.
And many of those resident animals the migrants encounter may be infected by parasites. Another concern is that resident animal populations with supplemented diets might support more virulent parasites. These resident populations might support the transmission of parasites that are more lethal to migrants. So there are possibilities that either residents can transmit parasites to migrants, migrants to residents or by interaction of both can give birth to more virulent strains of parasites, which can have negative impact on both of these or can infect human beings and lead to serious consequences.
Many animals migrate from one place to another to avoid harsh winter conditions or to follow available food as the seasons change. But migration can have another important benefit. By allowing animals to escape from habitats, where parasites have accumulated and by weeding out infected individuals that are not able to survive the journey, migration can reduce parasite infection in animal populations, but it’s very rare.
When you get resident populations forming, you might extend the parasite transmission season, and additionally if those residents are supported by food subsidies through the winter, the infected individuals might be more likely to make it through. So now these migrants are returning to areas where there are already a larger number of infected individuals, so those mechanisms for escaping parasitism become less effective. If you think about it from the parasite’s perspective, killing too many of their hosts minimizes the chances of onward transmission and could cause them to die out.
So parasites that rely on migratory animals may have evolved to be more benign to their hosts during their difficult migratory journeys. Migration is considered to be a threatened phenomenon by some, because of the many barriers resulting from human land use and activities.
Providing food and shelter for wildlife in our backyards provides us with an important connection with the natural world, with associated benefits for our well being and engagement with conservation issues. It doesn’t mean feeding to wildlife should be stopped but it should be carried out by adoption of some practices which are helpful and beneficial for both people and wildlife. But, there are practices adopted by some people, so it can help in minimization of the unintended consequences.
Hammad Ur Rehman Bajwa, Muhammad Kasib Khan, Zaheer Abbas, Abdullah Khalid Chatha
Department of Parasitology, University of Agriculture, Faisalabad