Mathematical models of Neospora caninum infection in dairy cattle: transmission and options for control

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 publishe d literature. Three routes of transmission were considered: maternal vertic al transmission with a high probability (0.95), horizontal transmission fro m infected cattle within the herd, and horizontal transmission from an inde pendent external Source. Putative infection via. pooled colostrum was used as an example of within-herd horizontal transmission, and the recent findin g that the dog is a definitive host of N. caninum supported the inclusion o f an external independent source of infection. The predicted amount of hori zontal transmission required to maintain infection at levels commonly obser ved in field studies in the UK and elsewhere, was consistent with that obse rved in studies of post-natal seroconversion (0.85-9.0 per too cow-years). A stochastic version of the model was used to simulate the spread of infect ion in herds of 100 cattle, with a mean infection prevalence similar to tha t observed in UK studies (around 20%). The distributions df infected and un infected cattle corresponded closely to Normal distributions, with S.D.s of 6.3 and 7.0, respectively. Control measures were considered by altering bi rth death and horizontal transmission parameters. A policy of annual cullin g 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 sourc e of horizontal infection. If the level of within-herd transmission was abo ve a critical threshold, then a combination of reducing within-herd, and bl ocking external sources of transmission was required to permanently elimina te infection. (C) 1999 Australian Society for Parasitology Inc. Published b y Elsevier Science Ltd. All rights reserved.