Veterinary Preventive Medicine-II

CLINICAL MANIFESTATIONS

The clinical and pathologic manifestation of infection in individual cattle varies with age and pregnancy status. Three situations are considered,

• Postnatal infection in non pregnant cattle
• Infection in pregnant cattle
• Persistent infection in calves and mucosal disease.

Postnatal infection of susceptible non pregnant cattle

• Acute infection
  • Infection is most common in animals 8-24 months of age. Although calves may receive antibodies in colostrum, antibody levels decline by 3-8 months of age and animals can then become infected.
  • Incubation period 5-7 days followed by pyrexia (40-41°C), leucopenia, viraemia. In majority it is sub clinical infection.
  • There may be diarrhoea of explosive nature with very high morbidity but no mortality.
  • Drop in milk yield in dairy cows, oculo-nasal discharge and mouth ulcers is referred to as BVD. Animals develop serum neutralizing antibodies which persist life long.
• Immuno suppression
  • Virus induces transient but profound immuno suppression.
  • The virus suppresses the interferon production, affects lymphocyte function, humoral antibody production and phagocytosis which paves way for many respiratory and other diseases in calves.
• Venereal infection
  • Semen from persistently infected bulls by AI or natural service infect cows which results in early embryonic mortality and repeat breeding.

Infection of susceptible pregnant cattle

• Transplacental infection
  • The infection to the fetus causes abortion, weak calves, under sized calves and congenitally deformed calves.
  • The virus replicates in almost all the fetal tissues and the extent of damage is more in actively dividing cells.
  • Regarding the nervous system it causes cerebellar hypoplasia, dysplasia, cavitation of the cerebrum and retinal displacement.
• Immunological competence of the fetus
  • Infection in early fetal life becomes persistent viral infection in many tissues and organs. After birth, the calf remains infected for life.
  • These calves excrete the virus in large quantity and transmit to all susceptible healthy animals. Therefore, high probability of development of MD is possible.

Persistently infected cattle

• In susceptible healthy herds in which virus was recently introduced, high proportion of calves in the next calving season become persistently infected.
• Mortality rate exceeds 50% in the preslaughter age i.e., 6-18 months age and the most classical clinical manifestations are that of MD. This disease is characterized by pyrexia, anorexia, profuse watery diarrhoea, nasal discharge, buccal ulceration and sometimes lameness. Death is within a few days to 3 weeks.