Wildlife diseases that pose a risk to small ruminants and their farmers

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Abstract

Infectious pathogens from wild animals have become increasingly important throughout the world in recent years, as they have had a substantial impact in livestock and human health. A large number of pathogens (61% of the 1415 currently identified human pathogens within 313 different genera) are zoonotic and can infect multiple animal species. Multi-host pathogens are predominant among animal and human emerging diseases. Multi-host pathogens (including all zoonotic agents, pathogens that can infect more than one taxonomic order and pathogens that can infect wildlife hosts) have a higher relative risk for emergence than species-specific pathogens. Of 800 zoonotic diseases currently identified, 619 (77%) are caused by pathogens that affect wildlife; of 125 emerging zoonotic diseases, 113 (90%) affect wildlife. Of the diseases that have emerged in the last few decades around 75% are of wildlife origin. Many factors influence changes in disease incidence, including economic, climatic and microbiological effects. Increasingly, close interaction of humans and livestock with wild animals has led to increased frequency of zoonotic infections. Forest clearance and movement of animals or animal products are factors, which pose significant risks of introducing disease into a new region. Changing climate affects disease incidence by changes in land use or animal production practices, as well as by movement or changes in distribution of animal reservoirs or insect vectors. Local increases in biting midges or mosquito numbers, changes in the distribution of known vector species and/or recruitment of novel vector species, have increased the risk of spread or introduction of diseases. Pathogen evolution may occur in response to changes of which humans are not aware. The evolution may be occurring in many hosts, currently poorly monitored. Microbial evolution may affect the extent to which established methods of diagnosis can detect infectious agents. Other endemic diseases may also change in incidence for largely unknown reasons. Increased information on prevalence in a wide range of hosts will increase our understanding of these reasons. Wildlife can play an important role in the epidemiology of small ruminant and human diseases, by representing a source of disease via various transmission routes. Recent studies in infections of wildlife in Europe have highlighted the impact on small ruminant health. In Greece, blood/organ samples were collected from 60 wild deer and 140 wild boars (2006–2011). Serum samples were tested for presence of antibodies against Toxoplasma gondii, Neospora caninum, Mycobacterium avium subsp. paratuberculosis, Chlamydophila, Salmonella and Trichinella spirallis, by using appropriate immunodiagnostic techniques. Tissue samples were examined for Mycobacterium bovis by using PCR. In serum samples from deer, antibodies against T. gondii, N. caninum and Chlamydophila were detected in 15%, 5% and 5% of samples, respectively. In serum samples from wild boars, antibodies against Salmonella, T. gondii and T. spirallis were detected in 15%, 5% and 5% of samples, respectively. No M. bovis was found in tissue samples. In Spain, Bluetongue virus, Brucella spp., Coxiella burnetii and M. avium were detected in many wild cervid species. Spanish wild boars have been found to be greatly exposed to Salmonella spp., an important small ruminant intestinal pathogen. In Austria, Spain and Poland, Anaplasma phagocytophilum has been detected in various cervids. Finally, in Poland and Spain, wild deer and wild boars were found to be exposed to T. gondii and N. caninum. The results indicate that wildlife may be carriers of several pathogens, which can be transmitted to domestic small ruminants and their farmers. It is noteworthy that samples from many European countries will be collected and tested to ensure a broader evaluation of the epidemiological role of wildlife.

Introduction

Diseases of wildlife are presented with a variety of clinical manifestations, in a wide range of animal species and with significant diversity in demographic and population characteristics. Infectious pathogens of wildlife origin have gain interest and are considered to be of increasing importance worldwide, mainly because of their role on livestock health and productivity, as well as their zoonotic potential.

Apart from being zoonotic, a large number of pathogens (61% of the 1415 identified human pathogens within 313 different genera) also infect multiple animal species (Taylor et al., 2001). In fact, these agents predominate among animal and human emerging diseases. For instance, 90% of emerging diseases in cattle are caused by multi-host infectious agents. All multi-host pathogens that include by definition all zoonotic agents, pathogens that infect more than one taxonomic order and pathogens that infect wildlife hosts have a higher relative risk for emergence than species-specific pathogens (Cleaveland et al., 2001). Furthermore, diseases, when expressed in free-ranging animals, can have a significant effect on wildlife ecologies. While the disturbance caused in the wildlife ecology and the consequences in domestic animal and human health can remain of limited importance for years. They can lead to sudden outbreaks, epizooties or even pandemic infections characterized by high morbidity and mortality (Wobeser, 1994).

Studies on zoonotic diseases revealed the importance of wildlife as hosts of zoonotic pathogens as 619 of the 800 zoonotic diseases currently identified are caused by pathogens that affect wildlife (77%). Moreover, 113 of the 125 emerging zoonotic diseases also affect wildlife (90%). Moreover, approximately 75% of all diseases, including zoonoses, which have emerged in the last few decades, are of wildlife origin (Jones et al., 2008).

In contrast to livestock where feeding, reproduction and movements depend on human activities and manipulations (Artois et al., 2006), disease in wild animals is strongly associated with environmental factors. Certain environmental and ecological parameters are constantly changing and could subsequently induce modifications in transmission of the pathogens. On the other hand, diseases can influence ecological factors, which are of major importance in the dynamics of wild populations altering their survival rate and fecundity (Wobeser, 2007). Changes in disease incidence result from economic and climate changes, as well as from microbiological alterations. Moreover, close interaction of humans and livestock with wild animals has led to an increased frequency of zoonotic and epizootic infections.

Appearance and spread of new diseases is partially attributed to modifications in ecosystems, either naturally occurring or resulting from human interventions (Williams et al., 2002). A well-documented association between introduction of a disease into a new previously unaffected region and ongoing forest clearances has been reported, while movement of animals or animal products have been identified as contributing factors, which pose significant risks. Natural climatic changes can increase host abundance and transmission of pathogens. The incidence of the disease is further affected by climatic changes, due to alterations in land use or altered livestock rearing practices and movement or changes in distribution of animal reservoirs or insect vectors. Naïve populations can be significantly affected by movement of pathogens within animal hosts. Interaction of wild and domestic species can result in serious outbreaks of disease in wildlife. Movement of pathogens has also been associated with the emergence of wildlife diseases in new areas. In the case of vector borne diseases, movement of vectors can be implicated in the appearance of diseases in wildlife reservoirs (Williams et al., 2002). Local increases in biting midges or mosquito numbers, changes in the distribution of known vector species and/or recruitment of novel vector species, have increased the risk of spread or introduction of diseases.

Apart from their hosts, carriers and the reservoirs the pathogens are also subjected to severe changes, such as mutations and re-combinations than can give rise to new pathogens or modification of existing ones. Pathogen evolution may occur in response to changes of which man is not aware and remain unnoticed, especially those currently being poorly monitored. Furthermore, this evolution may affect the extent to which established methods of diagnosis can successfully identify presence of infectious agents. The increase in the incidence of endemic diseases in different areas is of various and largely unknown reasons. Gaining information on the prevalence of the diseases in a wide range of hosts will enhance our understanding on these reasons.

The study of the diseases in wildlife, due to their role in the epidemiology of small ruminant and human health problems, as a source of diseases via various transmission routes, will contribute to better recognize epidemiology, pathogenicity and host–pathogen interactions.

Section snippets

European wildlife health surveillance

Recent studies in infections of wildlife in Europe have highlighted an impact on small ruminants, as well as on their farmers.

Mycobacterium tuberculosis complex infection has been reported in wild ungulates in Spain, France, Slovakia and the Czech Republic. M. tuberculosis complex has been reported in wild boars, in red deer (Cervus elaphus), in fallow deer (Dama dama) and, less frequently, in roe deer (Capreolus capreolus) (Vicente et al., 2006, Martin et al., 2011). Individual cases have been

Concluding comments

Wildlife may be carriers of several pathogens, which can be transmitted to domestic ruminants, as well as to their farmers. There is a need to increase sampling and testing from other countries to ensure a broader evaluation of the epidemiological role of wildlife.

Conflict of interest

The author declares that he has no conflict of interest with the contents of this paper in any respect.

Acknowledgements

The research leading to these results received partial funding from the European Union Seventh Framework Programme (2007–2013) under grant agreement no. 222633 (WildTech).

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    This paper is part of the special issue entitled “Lectures of the 1st European Conference on Small Ruminant Health Management”, held in Athens, Greece, October 2011. Guest Edited by G.C. Fthenakis.

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