The effect of Mycobacterium avium subspecies paratuberculosis exposure on animal health

ABSTRACT

Background: Johne's disease is an incurable wasting condition that affects ruminant and non-ruminant animals. Each year, Johne's is responsible for losses in the billions of dollars in the United States cattle industry alone. Mycobacterium avium subspecies paratuberculosis (MAP) is the microorganism responsible for Johne's disease. MAP can spread very fast among animals, and this patogen has been isolated across the world and in several different animal species including humans. Therefore, MAP is classified as having a major impact on both animal and human health, and therefore the economy. MAP has also been associated with Crohn's disease in humans, which necessitates great concerns regarding public health. The objective of this literature review is to identify problems and challenges associated with this illness and highlight possible approaches to minimize the economic losses and the incidence of Johne's, two avenues to reduce human exposure with this pathogen. Review Following ingestion and exposure to MAP, the bacterium will infect the host through the ileum and than it will proliferate inside of host-cells; MAP can therefore be considered an intracellular parasite. After infection, this pathogen goes to a latency period that can be from months to several years without causing the presence of clinical signs in the host. This bacteria can cause an inflammatory response in the intestine, decreasing the ability of the animal to absorb nutrients. Depending on the level of infection bacteria strain and the genetic composition of the animal, individuals can become or not infected, if infected they can shed variable levels of MAP into the environment, increasing the exposure to other animals. Thus, it is of importance to eliminate MAP infected animals from herds ; aiming to reduce the environment contamination with this bacteria. Several chromosomal regions have been associated and linked with MAP infection in cattle. It is proposed that Johne's disease has a polygenic effect with multiple genes involved in the process of susceptibility and tolerance to the disease. Selection for animals that are tolerant to Johne's disease has also been proposed, whereby tolerance was defined as the ratio of MAP tissue infection and MAP fecal shedding. Animals that are shedding low or no levels of MAP in the environment were considered tolerant, thus are preferred in comparison with the ones that are eliminating high levels of MAP. Some positional and functional candidate genes have being identified and explored. The major problem with genetic studies with Johne's disease is the correct classification of the phenotype. ELISA, PCR and fecal culture are methods of testing for Johne's disease but variation exists regarding the degree of accuracy and effectiveness for each test, as discussed further within this review. Discussion: In this study, we presented the importance of preventive control of MAP transmission amongst animals and humans, respectively. Several approaches to reduce the incidence of this illness among animals were evaluated; however the number of infected animals is still increasing annually, especially within dairy herds. Genetic selection for animals that are less susceptible might be one solution to reduce the spread and contamination of other animals with these bacteria. This necessitates the better understanding of the genes involved with host-immune-defense mechanisms for development of an accurate selection method. Questions related to the zoonotic potential of MAP, the causative agent of Johne's disease, and Crohn's disease in humans is still of great concern to the population, therefore efforts to control and eradicate this disease are needed.