Changes

====Mucosal Disease====

====Mucosal Disease====

Mucosal disease is an invariably fatal condition of 6-18 month-old cattle⁴⁴. Disease follows a course of several days to weeks, and intially presents as pyrexia, depression and weakness. Anorexia leads to emaciation, and animals suffer profuse, watery, foul-smelling and sometimes bloody diarrhoea. Dehydration ensues. As suggested by the name, lesions are localised to mucosal surfaces. These include the oral mucosa, tongue, external nares,nasal cavities and conjunctiva³⁴, where large lesions  cause excessive salivation, lacrimation, and oculo-nasal discharge. The coronet and interdigital surface are also affected, causing the animal to become disinclined to walk and eventually recumbent.  Figure 1.5 shows examples of tongue and small intestine lesions.

Mucosal disease is an invariably fatal condition of 6-18 month-old cattle⁴⁴. Disease follows a course of several days to weeks, and intially presents as pyrexia, depression and weakness. Anorexia leads to emaciation, and animals suffer profuse, watery, foul-smelling and sometimes bloody diarrhoea. Dehydration ensues. As suggested by the name, lesions are localised to mucosal surfaces. These include the oral mucosa, tongue, external nares, nasal cavities and conjunctiva³⁴, where large lesions  cause excessive salivation, lacrimation, and oculo-nasal discharge. The coronet and interdigital surface are also affected, causing the animal to become disinclined to walk and eventually recumbent.

MD occurs when animals persistently infected with noncytopathic BVDV are superinfected with an antigenically similar cytopathic strain. Cytopathic virus arises from the persistent noncytopathic virus by mutation (see 1.1.4), and may then be transmitted to cause MD in animals PI with the same noncytopathic strain. Immunotolerance induced by the noncytopathic strain prevents superinfecting virus being recognised by the immune system; the biotypes are “homologous” to the immunotolerance (Brownlie, 1990). “Heterologous” superinfection with a non-related cytopathic biotype causes an antibody response and mucosal disease does not usually occur.

Mucosal disease arises from superinfection of persistently infected animals with a cytopathic virus antigenically similar to the original, non-cytopathic strain persisting in the animal. In one animal, a cytopathic virus is produced by mutation of the persistent non-cytopathic virus. The new cytopathic isolate can then be transmitted to other animals, where it will cause mucosal disease if they are persistently infected with the same non-cytopathic strain. Immune tolerance induced by the pesistent virus prevents the immune system recognising the superinfecting cytopathic strain: the two biotypes are said to be "homologous" to the immunotolerace.⁴⁵. "Heterologous" superinfection with a non-related cytopathic biotype does not result in mucosal disease because a normal immune response is mounted.

===Laboratory Tests===

===Laboratory Tests===