Epidemiological Importance, Threats, and Biosecurity Measures at Stopover Sites

Bird migration is a regular seasonal large-scale, long-distance flyway movement. Where birds migrate between breeding “homes” and nonbreeding grounds, for food and nesting location. Migratory birds comprise more than 80% of the global avian diversity. They have essential role in sustaining a healthy ecosystem, and contribute to shaping the global biodiversity through long-distance dispersal of various microorganisms and seeds.

Recent pandemics have brought to fore the interconnectedness among humans and exotic animals in one ecosystem. One of main key elements for predicting the emergence of infectious disease is thorough understanding of transmission dynamics that link wild and domestic animals and humans. Annually, more than 2,000 species migrate all over the world with great variation in migration time and direction among and within species, as well as travelled distance. An increasing interest is directed toward migratory birds because during their migratory journey they may encounter a broad range of pathogens and vectors, which could be easily transmitted to resident communities en-route. These birds “Migratory birds” have a unique ability to fly for long distances between continents in a short lapse of time, and crossing majestic geographical barriers with little respect for country or continental borders. For instance, many international borders were closed due to COVID-19, while numerous wild bird species continued to cross country and continental boundaries through their migrations.

During their flights, migratory birds can play a role as reservoirs “vehicles” and a transport shuttle for geographic spread and dissemination of many infectious pathogens and their arthropod hosts “vectors”. Mostly, migratory trip is divided into several migration legs, interspersed with stopover periods for feed and preparation for next migration leg. These stopover sites represent good opportunity for attachment of new vectors and detachment of already fed vectors to birds, as well as acquisition, dissemination and spread of pathogens.

Of the most commonly transmitted pathogens are; West Nile Virus, High Pathogenic Avian Influenza (HPAI), Crimean-Congo hemorrhagic fever virus, Newcastle disease virus, tick-borne encephalitis virus, Japanese encephalitis virus, and St. Louis encephalitis virus. In addition to some zoonotic disease-causing bacteria such as Salmonella enterica, Campylobacter jejuni, Mycobacterium avium, and Chlamydia pscittci. Besides, migratory birds are important reservoirs for spreading antimicrobial-resistant (AMR) bacteria.

Some of these pathogens were transferred as novel infections to new areas along wild bird flyways across Europe, Asia, and Africa. For instance, highly pathogenic avian influenza virus (HPAIV) was most likely introduced to the EU via migratory wild birds, a key driver of HPAIV emergence, through contact between poultry and wild birds. Also, seasonal fluctuations in the overall prevalence of AIV in aquatic wild bird populations were observed and found to be associated with population turnover rates and migration.

One of the most important factors affecting migratory birds is climate changes. These birds are very sensitive to climate change, which drastically affects natural habitats, living conditions, and plant population and biodiversity. Climate can affect the distribution of wintering passerines in two main ways. First, food availability which is mainly plant in origin, thus precipitation, temperature or humidity affects bird abundance. Second, low temperature will increase energy demands of birds leading increase food intake. On the other hand, warm weather enables more birds to occur at a given level of productivity.

Concerning status of migratory birds in Europe, there are four main geographical routes for the entry of wild birds to Europe: North-eastern route (through border with Russia and Belarus), Eastern route (through border with Ukraine, Moldova, Black Sea, Turkey until the southern border of Turkey), Southern route (through border from the south of Turkey to the north of Portugal) and North-western route (through border from north Portugal to north Russia).

Southern route is confined with Mediterranean region, and entry via this route is mostly occurring during spring as birds migrate back from their African wintering grounds. Mediterranean Sea is a semi-closed sea, that makes the Mediterranean region very sensitive for climate changes. Mediterranean region is considered among the world’s most vulnerable to the dire impacts of climate change. This region already suffers from extreme fluctuations in temperatures and precipitation, as well as scarcity of natural water and agricultural land. These patterns are predicted  to be more worsen in coming years, and consequently climate change will influence the extent and suitability of this wintering ground for birds. It has been predicted that many trans-Saharan species will reduce and shift their wintering range due to the effect of climate change. These changes have potential conservation implications because can affect the survival or the subsequent reproductive success of migratory birds and thus their populations trends.

Along Mediterranean Basin, there are various fundamental resting places for the millions of migratory birds through their migration flight between Eurasia and Africa. These areas represent favorable places both for the study of bird species and for the monitoring of some transboundary and zoonootic diseases transmitted by vectors.

In addition to impact of climate effect, migratory birds face a suite of threats. For instance, breeding and wintering habitats of exotic or wild migratory birds are becoming increasingly prone to encroachment due to human development. There is also a straightforward approach to decimate number of these birds to prevent/mitigate spread of infectious diseases, which is at variance with the UN sustainable goal of reducing biodiversity loss significantly whilst maintaining good health and wellbeing. Othere threats are represented by agricultural intensification on the breeding grounds, energy infrastructure development along migratory routes, and illegal taking at stopover sites.

Monitoring the main natural reservoir of Avian Influenza Virus species of migratory birds, based on migratory flyways map, is well/documented as one of the earliest ways to detect spread of AI. Annual monitoring and surveillance of these birds is necessary and should be pursued more seriously to protect the environment and poultry industry in the region/even in whole country. Stop over sites are avenues for birds to spread diseases between populations and species when overcrowded as well as into water bodies. Unfortunately, the ability to forecast stop over sites as well as understand their spatiotemporal characteristics are still lacking and need to be improved.

Finally, implementing specific biosecurity measures is valuable in reducing the probability of emerging transboundary diseases and their vectors by migratory birds. Of these measures are: 1) In advance alerting to inhabitants of the migratory path of these birds to use personal protective equipments; 2) Special vaccination of local poultry birds and animals in advance at stop over sites; 3) Disinfection stop over sites and migratory paths by fumigation; 4) Temporary lockdown of the inhabitants around the stop over sites to minimize interaction and contact with the wild birds.

In Conclusion; Much attention and interest are directed toward migratory birds because they encounter a broad range of pathogens and vectors, some of them are deadly pathogens. During their flights, these migratory birds can play a role of reservoirs or a transport shuttle for geographic spread and dissemination of these pathogens and their arthropod hosts “vectors” to resident communities. Migratory birds face a suite of threats as climate changes, human development, agricultural intensification, and energy infrastructure development along migratory routes. These threats have potential conservation implications on migratory birds and their populations trends.

Mediterranean region, Southern route of migratory birds to Europe in spring, has various fundamental resting places for the millions of migratory birds during their migration flight between Eurasia and Africa. Due to the extreme climate changes in this region, many trans-Saharan migratory birds species are predicted to reduce and shift their wintering range.

Stop over sites are avenues for birds to spread diseases between populations and species. Continuous collection of relevant weather and environmental data is an essential cornerstone key as these birds move in response to changes in these variables. Annual monitoring and surveillance of these birds is necessary to protect environment and resident humans and animals. Implementation of specific biosecurity measures is valuable in reducing the probability of emerging transboundary diseases and their vectors by migratory birds.

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