Paratuberculosis

Paratuberculosis

C D BUERGELT, S S BASTIANELLO AND A L MICHEL

Introduction

Paratuberculosis or Johne’s disease is a chronic infectious disease of cattle, sheep, goats and certain captive or free-living wild ruminant species. The disease is caused by Mycobacterium avium subsp. paratuberculosis, a facultative intracellular acid-fast bacillus. In infected herds or flocks the majority of infected animals are asymptomatic. 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. Paratuberculosis has a virtual worldwide distribution.

Johne and Frothingham were the first to describe paratuberculosis in Germany in 1895 as an unusual case of tuberculosis in a cow.46 The true causative agent, M. avium subsp. paratuberculosis, was isolated and grown in pure culture in 1902.22 The disease was officially recognized as an infection distinct from tuberculosis in 1906, and it was suggested that it be named paratuberculosis.7

Paratuberculosis is widely distributed throughout the world and is endemic in some regions. In some countries, including the USA, it is now recognized as one of the most economically important infectious diseases that affect cattle and sheep. In Africa, it has been reported from Mauritania, Sudan, Ethiopia, Kenya, Uganda,Tanzania, Zaire, Ghana, Nigeria, Gabon, Zimbabwe, Swaziland, Zambia and South Africa.68

The first case of paratuberculosis in South Africa was diagnosed in a cow in 1923. During the period between 1976 and 1990 the disease was confirmed in 42 individual cattle in South Africa. After 1990 its prevalence in South African cattle is estimated at 0,0002 per cent43 but the actual prevalence is believed to be higher. In sheep, paratuberculosis was first recorded in South Africa in 1967 in a single Merino ram which had been imported from Germany and the second case to be diagnosed was in 1981 in a local ram. In 1988 the disease was diagnosed in a flock of sheep on a state-owned farm in the Mpumalanga Province and in a sheep from the commercial sheep sector in the Western Cape Province.

Since 1996, 52 infected sheep farms have been identified in the Western and Eastern Cape provinces.60 The overall estimate of Johne’s disease prevalence in South Africa for sheep outside the much more severely affected Western Cape Province is 0,16 per cent.43 Paratuberculosis has been classified as a notifiable disease in South Africa since 1997.

Although precise data for the USA are not available, the prevalence of paratuberculosis has greatly increased over the last few decades.113 In the USA it is estimated that 3 to 10 per cent of cattle are infected. In a recent national survey 22 per cent of dairy herds were found to be infected.19 The disease is responsible for severe economic losses in the cattle industry in the USA where direct and indirect losses greater than US$ 200 per infected cow per year can occur in affected herds, and overall, the economic impact may be as much as US$ 1,7 billion annually.22, 25

In Australia, it is estimated that 2 to 40 per cent of cattle herds107 and 9 to 23 per cent of sheep flocks are infected,20 and that the annual loss to the sheep and cattle industries is Aus. $ 92 million20 and Aus. $ 7,5 respectively.107 In Spain paratuberculosis appears to be widely distributed in sheep and goat flocks, between 24 and 60 per cent of sheep flocks being infected in certain regions2 although vaccination of sheep has apparently significantly reduced the infection rate.73 In southern England 1 per cent of cattle farms has been reported to be infected.61 In the Netherlands and Belgium the seroprevalence in cattle has been calculated to be 3,3 per cent, the percentage differing between regions, being as high as 5 per cent in some regions.6,16 The prevalence in beef cattle in Argentina has been reported to be between four and 51 per cent.64

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 which aids in its identification. A network of intercellular filaments is considered to be responsible for the clumping of the bacilli.13

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

Primary 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 in Norway.86 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 M. avium subsp. paratuberculosis are its slow growth pattern and its dependence on exogenous mycobactin which is used by the bacterium as an iron chelator for respiratory activity.52 Mycobacterium avium subsp. paratuberculosis requires higher levels of iron to sustain multiplication in vitro than do all the other Mycobacterium spp.70, 95 To be considered as M. avium subsp. paratuberculosis, the organism must primarily be acid-fast and...

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