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Tuberculosis in bovine animals
Tuberculosis in bovine animals

 Last update 2021/12/28

Epidemiology of tuberculosis in bovine animals

M. bovis is the main causative agent of bovine TB and it has received special consideration in livestock owing to the economic impact of infections in this context. Moreover, M. bovis can infect a wide variety of hosts, including wild animals, captive species, primates, and even humans. Consequently, the zoonotic potential of M. bovis has raised public health concerns [1].

M. bovis as zoonotic agent

Tuberculosis due to M. bovis is a rare infection in humans in the EU, with 125 confirmed human cases reported in 2012. The case numbers in the EU have decreased in the last two years. There was no clear association between a country’s status as officially TB-free (OTF) from bovine TB and notification rates in humans. This could be due to many of the cases in both OTF and non-OTF countries being persons who have immigrated to the country; thus, the infection might have been acquired in their country of origin. Cases native to the country could have been infected before the disease was eradicated from the animal population as it may take years before disease symptoms develop [2]. The routes of transmission of TB from animals to humans are well known and include direct exposure to infected animals or consumption of contaminated animal products. Application of fingerprinting tools facilitates analysis of the molecular epidemiology of M. bovis in animal-to-human and human-to-human transmission. Nevertheless, other animal species and members within the M. tuberculosis complex can contribute to the zoonosis. Improvements in diagnostic techniques, application of more advanced discriminatory genotyping tools, and collaboration between veterinary and human health care researchers are key to our understanding of this zoonosis. Humans are generally assumed a dead-end host for infection by M. bovis. Therefore, human-to-human transmission is considered rare and restricted mainly to exposed immunocompromised persons. The development of genotyping techniques has allowed the demonstration of person-to-person transmission events involving M. bovis. The few reports that have been able to identify such events investigate clusters comprising a small number of cases with close epidemiological relationships [1].

Tuberculosis in cattle

Cattle to cattle TB transmission occurs mainly by the respiratory route thorough the inhalation of aerosols containing the bacteria when the animals are in close contact. Oral route of infection due to the ingestion of mycobacteria from the environment cannot be discarded in some cases since it has been also described. Nowadays, M. bovis infection rarely presents as clinical disease and it normally appears as apparently healthy animals reacting to the diagnostic tests [3]. Infectious diseases in general arise from an interaction between the infectious agent, the host, and a range of covariables, which may include other infectious diseases and the environment. Risk factors (biological, behavioural, environmental, or genetic) are known to influence both transmission and susceptibility. They may operate at different scales; regional-level, herd-level, and animal-level and may vary across regions due to factors, such as, differing farm structures, farm management practices, bovine TB control and eradication programs, regional TB incidences, wildlife densities, and the relative importance of specific risk factors by area. The risk of a bovine TB episode is accepted to vary between herds with some herds experiencing multiple breakdowns over time, whilst others appear to remain free of infection [3]. Many epidemiological studies about bovine TB has been published in the last decade and the modelling raises as the main tool in the recent years. Different variables are taken into account in these studies as type of herd, location movements or the presence of other domestic or wildlife species that can act as reservoirs.

Tuberculosis in wild animals

Reservoir host Bovine Tuberculosis

Wild boar and badger, important resevoirs of tuberculosis in bovine animals in the European Union.

Wild animals are susceptible to TB by many of the same pathogens that afflict domestic animals, and transmission between domestic animals and wildlife can occur in both directions. Nevertheless, the original event was often the transmission of a domestic animal disease to wildlife. In most cases, eradication efforts have successfully decreased the incidence of bovine TB as well as M. bovis infection in humans. Notwithstanding, in some areas, eradication has not been possible due to the presence of M. bovis-infected wildlife and wildlife-to-cattle transmission. As the prevalence of bovine TB in cattle decreases, the relative importance of M. bovis-infected wildlife increases and disease control measures are required for both livestock and wildlife. An understanding of the role wildlife play in the epidemiology of M. bovis infection is required. It is generally accepted that the main wildlife reservoir hosts of M. bovis in Europe include the European badger in Great Britain and Ireland and the wild boar in the Iberian Peninsula. Moreover, TB has been also observed in other wildlife species in Europe as red deer, roe deer, red fox, buffalo, bison, mink, ferret and others [4][5].

Presence of M. bovis in wildlife–domestic animal interface represents an obstacle to bovine TB eradication in several countries [6]. In spite of long-standing, expensive and somewhat successful efforts over many decades, animal health officials have found that traditional test and slaughter methods, the centerpiece of most bovine TB eradication programs, are of limited success when affected cattle herds have contact with infected wildlife. Two methods described for dealing with tuberculosis and other diseases in wildlife include: (i) limiting the number of receptive and infected individuals by culling (lethal control) or vaccination, and (ii) decreasing the number of infected animals through treatment or selective test and cull practices [7].

The evaluation of the TB hazard posed to cattle from wildlife across Europe demonstrated that cattle pose the greatest current and potential TB hazard for the majority of countries [4]. The most common host communities for TB hazard in Europe are cattle-deer-wild boar systems, which were found in 15 countries across Europe. Wild boar pose the greatest hazard of all the wildlife species, indicating that wild boar have the greatest ability to transmit disease to cattle. This is of particular concern given current population and distribution increases of some susceptible wildlife species, especially wild boar and deer which can help to spread pathogens to new areas and the paucity of wildlife TB surveillance programs. In countries and regions with high wild boar populations in particular, there are likely to be significant advantages to TB control from developing integrated TB surveillance schemes incorporating both cattle and wild boar [4].