By Dr. Saif Ur Rehman
WILD BIRDS and especially migratory species transmit a wide range of parasitic infections to human and other living organisms. Certain types of pathogens are closely associated with migratory birds in comparison to other animal species. There is no truism indirect transmission of parasitic infection from wild bird to human being. There exist many human infections that can theoretically be transmitted from the aforementioned bird species although the base of science for most of the associations remains theoretical. Such results are expected for zoonosis, pre-requisite for which is the amplification in an animal species cycle before the spill over to humans. The wild birds as well as the migrant ones are the most significant regarding the introduction of pathogens into these amplification systems. This elaborates why most of the association with transmission from bird to human may only occur indirectly.
On the contrary, there is ample evidence for the dispersal of pathogens to various locations by migrating birds but how this phenomenon will affect in humans still seems to unclear. The potential mechanisms and factors involved in the transfer of such infectious agents from birds to humans require further clarification. Furthermore, socio-cultural factors like captivation, importation and sale of wild birds as pets should also be considered as salient factors, which can enhance the spread of pathogens from wild birds to humans. Many vectors with the capacity to serve as pathogen carriers have also been isolated from wild birds. For example, ticks and mosquitoes are the main vectors of pathogens like West Nile Virus and Borrelia burgdorferi, respectively. Borrelia burgdorferi is a bacterium that has been reported in robins, songbirds, cardinals, thrushes, sparrows and other ground foraging birds, same assign some cases waterfowl causing Lyme disease in human which could transmit through the bite of Black-Legged Tick (Ixodes scapularis) and the Western Black-Legged Tick (Ixodes pacificus) that play a role in zoonosis.
A diverse number of protozoa such as Babesia and other blood parasites can be scattered by the wild and migratory birds. The matter of the transmission of different infections from wild birds and the migratory ones for humans is complicated. Numerous factors determine the possibility of such a spread, some factors relate to the affected species including the birds themselves (e.g. the avian species involved, susceptible local vertebrate recipients or invertebrate vectors), others to the parasite itself (e.g. stability of the parasite in the environment), and lastly some factors relate to the environmental conditions of a particular area (e.g. air, temperature, humidity).
There are various possible transmissions of bacteria, viruses and other pathogens from wild birds to human through a mechanical vector, which are arthropods e.g Coxiella burneti a bacteria, possible transmission through pigeons ticks in Asia. On the other hand in viruses Tick-borne Encephalitis Virus (TBE) present in blackbirds, sandpipers, wild mallards, wild grouse, and other wild birds. Same as possible transmission of Sindbis virus to humans, as migratory birds serve as hosts of mosquitoes which are vectors or transmitters for these viruses. In some reports mosquitoes act as potent vectors of Eastern Encephalitis Virus, Venezuelan Equine Encephalitis Virus and Western Equine Encephalitis Virus to humans through wild birds. Moreover, Cryptosporidium, coccidia, are more prevalent in Asia and their possible zoonosis; from some non-migratory birds present in captivity and dynasties, through contamination with fecal material.
Wild and migratory birds helminthes are also a source of infection to humans, possibly food-borne through the consumption of small water fish which grow on the fecal material of migratory birds. Mainly water fowls of Asia are responsible for this infection including: gulls, ducks, passerines, and waterfowl species. Meanwhile helminthes are also a source of cercarial dermatitis (swimmers itch) due to exposure to marine schistosomes present in the water pound contaminated with the wild birds fecal material. Same as sarcocystis is possible transmitted to human beings through the drinking of contaminated water with fecal material of cowbirds, exotic mallards, birds, wading birds passerines, wild anseriforms, swans, geese. Thus, the vulnerability for pathogen exchange is enhanced among avian species, which make use of the same stopover sites.
In the aforementioned examples duration and concentration of the agent in the blood or the gastrointestinal tract of migrating birds are important for the subsequent infection of another competent vector that feeds or gets exposed in crowding situations e.g. several studies have recorded infections e.g. B. burgdorferi and Human Granulocytic ehrlichiosis (HGE) in ticks removed from birds. Ticks commonly infest and plague a wide range of avian species, especially thrushes, sparrows and other ground foraging birds. Although many wild birds are reported that has been parasitized by a wide range of tick species as Ixodes species are most likely to carry infections (e.g. B. burgdorferi). Ixodid ticks often stick to hosts for 24 to 48 hours while acquiring some blood feed.
In tick-borne bacterial, viral and protozoal diseases, the infectious larvae or nymph (a stage during the development of tick) may remain attached to the host i.e. a migratory bird for several days and then deposited in a new geographic location during migration. In the same time span during migration some birds travel hundreds or even a few thousand miles before ticks complete feeding and drop off. Each bird carrying small tick burdens, integrates to the local tick populations in coastal areas. There is also indubitable evidence of trans-hemispheric exchange or shuffling of spirochete infected ticks by seabirds indicating the capacity for wild birds to carry infected ticks for long distances.
Vulnerabilities to tick-borne diseases are mainly peri-domestic, so the contribution to tick related human infection of avian ticks relative to mammalian ticks around dwellings is critical. Birds that are implicated in peri-domestic transmission of tick related infections to humans, especially in Asia, include cardinals, robins and song sparrows that frequently seen in backyard environments and some are mostly found at bird feeders.
Migrants of most bird species in the new world seldom use the same stopover sites on northward, spring migration as they do on southward, fall migration. Seasonality is a major factor influencing both, wild birds (wild resident and migratory species) and other vectors e.g. ticks, mosquitoes leading to variations in transmission dynamics. Mean season is important for vector borne pathogens.
Investigating connections between infectious disease risk and environmental factors are essential to understanding how human induced environmental changes will affect the dynamics of human and wild bird diseases. In depth study of large wetland areas, and intact wetland bird communities, may portray a valuable ecosystem based approach for controlling infections caused by migratory birds including parasites associated risk factors of human population.
To efficacy of the diminution of the risk of infections associated with wild birds, the public health and animal health sectors must collaborate to strategies seldom human exposure to pathogens carried by wild birds. An effective public educational campaign could also put in perspective and clarify realities about the risk of acquiring infections associated with wild birds.
The writer is faculty member of Veterinary Science, University of Agriculture, Faisalabad, Pakistan. He has written this article under the guidance of
Dr. Saif ur Rehman