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Line 16: − There is a long incubation period between infection and the development of clinical signs ranging from a year to several years. Animals may be affected subclinically before overt clinical signs are displayed. In the early stages, the disease is characterised by reduced milk production, reduced reproductive performance and increased susceptibility to infection or disease.+ + + Line 28:
Line 30: − − − There are many large macrophages (epithelioid macrophages) in mucosa, submucosa and lymph nodes. − The mesenteric lymph nodes are pale and enlarged (though not necrotic), and the lamina propria is infiltrated by sheets of macrophages with some lymphocytes. Acid-fast bacteria are found in the macrophages and giant cells, and this is detected by Ziehl-Neelson stain. − − − − 60% of cases have lesions in [[Colon - Anatomy & Physiology|colon]] and [[Rectum - Anatomy & Physiology|rectum]] and can be diagnosed by rectal biopsy.
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==Clinical Signs==
==Clinical Signs==
Cattle become infected by the bacteria as calves but do not display the clinical signs of disease until between two and five years of age. In addition animals may be affected subclinically before overt clinical signs are displayed.
In the early stages, the disease is characterised by reduced milk production, reduced reproductive performance and increased susceptibility to infection or disease.
In the later phases of infection, the clinical signs become more severe. The characteristic clinical signs are of watery 'pipe-stem' diarrhoea and severe wasting (despite maintaining a good appetite). The disease is progressive and advanced cases may develop submandibular or ventral oedema due to a protein losing enteropathy.
In the later phases of infection, the clinical signs become more severe. The characteristic clinical signs are of watery 'pipe-stem' diarrhoea and severe wasting (despite maintaining a good appetite). The disease is progressive and advanced cases may develop submandibular or ventral oedema due to a protein losing enteropathy.
[[Image:johnes disease proliferative ileitis.jpg|thumb|right|150px|Proliferative ileitis in Johnes disease (Courtesy of Bristol BioMed Image Archive)]]
[[Image:johnes disease proliferative ileitis.jpg|thumb|right|150px|Proliferative ileitis in Johnes disease (Courtesy of Bristol BioMed Image Archive)]]
==Diagnosis==
==Diagnosis==
Diagnosis is difficult, particularly in the case of subclinical disease as there is no single test that will detect all stages of the disease. The most commonly used diagnostic test is the ELISA which detects antibodies to ''M. paratuberculosis'' in clinically affected animals. This is typically used in combination with post mortem and identification of the classic pathological lesions of the disease. Other commercially available tests include baceterial culture, PCR and agar gel immunodiffusion. Test sensitivity may be increased by using different tests in combination.
Diagnosis is difficult, particularly in the case of subclinical disease as there is no single test that will detect all stages of the disease. The most commonly used diagnostic test is the ELISA which detects antibodies to ''M. paratuberculosis'' in clinically affected animals. This is typically used in combination with post mortem and identification of the classic pathological lesions of the disease. Other commercially available tests include baceterial culture, PCR and agar gel immunodiffusion. Test sensitivity may be increased by using different tests in combination.
==References==
==References==