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Abstract
The transmission and control of Neospora caninum infection in dairy cattle was examined
using deterministic and stochastic models. Parameter estimates were derived from recent
studies conducted in the UK and from the published literature. Three routes of transmission
were considered: maternal vertical transmission with a high probability (0.95), horizontal
transmission from infected cattle within the herd, and horizontal transmission from
an independent external source. Putative infection via pooled colostrum was used as
an example of within-herd horizontal transmission, and the recent finding that the
dog is a definitive host of N. caninum supported the inclusion of an external independent
source of infection. The predicted amount of horizontal transmission required to maintain
infection at levels commonly observed in field studies in the UK and elsewhere, was
consistent with that observed in studies of post-natal seroconversion (0.85-9.0 per
100 cow-years). A stochastic version of the model was used to simulate the spread
of infection in herds of 100 cattle, with a mean infection prevalence similar to that
observed in UK studies (around 20%). The distributions of infected and uninfected
cattle corresponded closely to Normal distributions, with S.D.s of 6.3 and 7.0, respectively.
Control measures were considered by altering birth, death and horizontal transmission
parameters. A policy of annual culling of infected cattle very rapidly reduced the
prevalence of infection, and was shown to be the most effective method of control
in the short term. Not breeding replacements from infected cattle was also effective
in the short term, particularly in herds with a higher turnover of cattle. However,
the long-term effectiveness of these measures depended on the amount and source of
horizontal infection. If the level of within-herd transmission was above a critical
threshold, then a combination of reducing within-herd, and blocking external sources
of transmission was required to permanently eliminate infection.