Quantification of vertical and horizontal transmission of Neospora caninum infection in Dutch dairy herds.

Ninety-six of 108 randomly selected Dutch dairy herds had one or more cows with a positive serostatus for N. caninum. In these 96 herds, we have quantified the probabilities of vertical transmission (VT) and horizontal transmission (HT) of N. caninum infection by combining serostatus and pedigree data in 4091 dam-daughter pairs. The probability of animals infected by vertical transmission during pregnancy (Prob(VT)) was calculated as the proportion of seropositive daughters among daughters of seropositive dams. The probability of animals infected by horizontal transmission (Prob(HT)) was the proportion of seropositive daughters among daughters of seronegative dams. These probabilities were calculated after the frequencies of observed dam-daughter combinations were corrected for (1) imperfect test-characteristics, (2) underestimation of horizontal transmission in situations that seronegative dams were horizontally infected after the birth of their daughters and (3) overestimation of vertical transmission in situations that seronegative daughters born from seropositive dams were horizontally infected. The incidence rate for horizontal transmission was calculated based on Prob(HT) and the average age of the animals in these herds. Based on the analysis of dam-daughter serology, Prob(VT) was 61.8% (95% CI: 57.5-66.0%) and Prob(HT) was 3.3% (95% CI: 2.7-3.9%). After adjusting the observed frequencies for imperfect test-characteristics, underestimation of horizontal transmission and overestimation of vertical transmission, Prob(VT) decreased to 44.9% (95% CI: 40.0-49.9%) while Prob(HT) increased to 4.5% (95% CI: 3.9-5.2%). Prob(HT) corresponded with an incidence rate for horizontal transmission of 1.4 (95% CI: 1.2-1.7) infections per 100 cow-years at risk. When stratifying herds for the presence of farm dogs, Prob(HT) was higher (5.5% (95% CI: 4.6-6.4%)) in herds with farm dogs than in herds without farm dogs (2.3% (95% CI: 1.5-3.4%)). When stratifying for within-herd seroprevalence, Prob(HT) was higher (10.3% (95% CI: 8.6-12.2%)) in herds with high (> or =10%) within-herd seroprevalence compared with herds with low (<10%) within-herd seroprevalence (2.0% (95% CI: 1.5-2.6%)). Although there was this relation between Prob(HT) and within-herd seroprevalence (crude OR(PREV) = 5.7 (95% CI: 4.0-7.9)), in herds without farm dogs, this relationship was no longer statistical significant (OR(PREV|DOG-) = 1.9 (95% CI: 0.7-5.5)). It indicated that the association between seroprevalence and Prob(HT) depended largely on the presence of farm dogs. In addition, when looking for the presence of specific age-groups with significantly higher seroprevalence compared with the rest of the herd, there were 7 herds in which two or more horizontally-infected animals were present in specific age-groups. This was an indication of a recent point-source exposure to N. caninum. These results reiterate the current control strategies to apply strict dog-management measures as well as to minimize within-herd seroprevalence by monitoring serostatus of animals.