GENERAL INTRODUCTION: MYCOBACTERIA

Introduction

Paratuberculosis or Johne’s disease is a chronic infectious disease of cattle, sheep, goats, camelids and certain captive or free-living wild ruminant species (bison, deer, elk, caribou). Sporadically, it occurs in several aberrant hosts such as equines, non-human primates, rodents, lagomorphs, canines, felines, birds, bears, and wild boars. The disease is caused by Mycobacterium avium subsp. paratuberculosis (MAP), a facultative intracellular acid-fast bacillus. In infected herds or flocks, the majority of animals are subclinically infected. Clinical signs usually develop in animals that are more than two years old and are manifested by progressive emaciation and/or chronic enteritis and diarrhoea.

Johne and Frothingham were the first to describe paratuberculosis in Germany in 1895 as an unusual case of tuberculosis in a cow.⁵³ The true causative agent, M. avium subsp. paratuberculosis, was isolated and grown in pure culture in 1902.²⁷ The disease was officially recognized as an infection distinct from tuberculosis in 1906, and it was suggested that it be named paratuberculosis.⁸

Paratuberculosis is distributed throughout the world, and has been reported from all continents. Prevalence of the disease is determined on herd and individual animal level. Prevalence rates vary for individual countries depending on the diagnostic tests used and management practices performed. Global herd level prevalence has been estimated to be between 30-50 per cent in dairy cattle.¹¹  Significant economic losses result from reduced weight salvage at slaughter and lower milk production.

Aetiology

Mycobacterium avium subsp. paratuberculosis is genotypically related to Mycobacterium avium and is grouped together with the Mycobacterium avium-intracellulare (MAI) complex. It is a non-motile short Gram-positive strongly acid-fast rod measuring approximately 1–2 μm × 0,5 μm. In both faeces and tissues the organism occurs in the form of clumps, a characteristic that aids in its identification. A network of intercellular filaments is considered to be responsible for the clumping of the bacilli.¹⁹

Phenotypically distinct lineages of MAP have been recognized.¹⁰³ Current classification of MAP strains has been achieved using different molecular techniques including genetic sequencing. Two major groups of strains are distinguished, namely “Sheep-type” or “Type S” and “Cattle-type” or “Type C”. The type C group is synonymous with the Type II strain. Type S strains comprise types I and III. The ovine pigmented sheep strain is mainly associated with subtype I. A different group of strains has been designated “Bison” or “B-type”. Analysis of B-type strains revealed that isolates from Indian bisons (Bos gaurus) are different from isolates from bisons in the USA (Bison bison). The Indian bison type is considered a sublineage of the type C strain. Isolates from goats mainly are C-type. Genetic heterogeneity and phenotypic differences exist between MAP strains, affecting growth, virulence, infection and geographic distribution.

Many techniques for the primary cultivation of MAP from faeces and tissues of infected animals have been developed (see Diagnosis).

Primary MAP colonies on slants are small, 1 to 5mm in diameter, moist, convex and usually white and unpigmented, but yellow-pigmented strains have, on occasion, been isolated from sheep.⁹⁴  Counts of colonies on primary isolation can provide a rough assessment of the number of mycobacteria being shed. The two principal distinguishing cultural characteristics of MAP are its slow growth pattern and its dependence on exogenous mycobactin, which is used by the bacterium as an iron chelator for metabolic activity.⁶¹  Mycobacterium avium subsp. paratuberculosis requires higher levels of iron to sustain multiplication in vitro than do all the other Mycobacterium spp.^{76, 100}   To be considered as MAP, the organism must primarily be acid-fast and mycobactin dependent.⁶

Mycobacterium avium subsp. paratuberculosis is relatively resistant to adverse environmental factors and can survive for at least 270 days in water, 264 days in bovine faeces, and 252 days in cattle slurry⁵⁹ but is inactivated by most disinfectants.

Epidemiology

Cattle,¹²¹  sheep,^{25, 90} goats,¹¹³ and many species of captive artiodactylids³⁶ and free-living ruminants, such as white-tailed deer (Odocoileus virginianus),⁴⁶ elk (Cervus elaphus),⁶⁴ bighorn sheep (Ovis canadensis),¹²⁵ Rocky Mountain goats (Oreamnos americanus),¹²⁵ and North American bison²³  are highly susceptible to infection with MAP. Findings of MAP in aberrant, non-ruminant hosts mentioned in the Introduction raise the possibility that these species can function as reservoir hosts or passive shedders and...