Abstract
Parameters such as pathogen dose and inoculation route are paramount in animal models when studying disease pathogenesis. Here, clinical findings, including foetal mortality, parasite transmission rates and lesion severity, and immune responses were evaluated in Asturiana pregnant heifers at day 110 of gestation challenged with a virulent (Nc-Spain7) Neospora caninum isolate. Four different doses of parasite tachyzoites were inoculated intravenously (IV1, 107 parasites, n = 6; IV2, 105, n = 6; IV3, 103, n = 6; and IV4, 102, n = 5), and the subcutaneous (SC) inoculation route was also assessed for the dose of 105 tachyzoites (SC, n = 6). In addition, a control group (n = 4 pregnant heifers) was evaluated. Foetal death was observed in all infected groups from 25 to 62 days post-infection, varying with the dose (IV1:4/6, IV2:3/6; IV4:2/5, IV3:1/6), and was three times less frequently associated with the SC route than IV inoculation (1/6 vs. 3/6). A dose-dependent effect for parasite loads in placental and foetal brain tissues was also detected. After SC challenge, a reduced number of tachyzoites were able to reach foetal brain tissues, and no lesions were observed. In calves, specific IgG responses in precolostral sera were mainly associated with high-dose groups (IV1 [100.0%] and IV2 [66.7%]), and cerebral parasite DNA detection was scarce (3/18). In dams, IFN-γ production and the dynamics of anti-N. caninum IgG antibodies varied with the dose, and the cell-mediated immune response was also found to be route-dependent. Our results confirm the influence of parasite dose and inoculation route on the outcome and dynamics of bovine neosporosis at mid-gestation.
Highlights
Infection with Neospora caninum is widely recognized as one of the major causes of bovine abortion worldwide and it is responsible for large economic losses, for the dairy industry, according to an economic study considering ten countries, including the worldwide leading cattle industries [3].In naturally infected cattle herds, the spread of this apicomplexan parasite is mainly associated with efficient transplacental infections following reactivation of a chronic infection during gestation in the dam [4, 5], horizontal transmission through oocyst ingestion is possible and more frequent than previously thought [6, 7]
In terms of inoculation routes, there were no significant differences in the foetal survival rate between IV2 and SC infected groups (P > 0.05) (Figure 1B)
Healthy calves were delivered from all dams that completed gestation from both infected and control groups
Summary
Infection with Neospora caninum is widely recognized as one of the major causes of bovine abortion worldwide (reviewed by [1, 2]) and it is responsible for large economic losses, for the dairy industry, according to an economic study considering ten countries, including the worldwide leading cattle industries [3].In naturally infected cattle herds, the spread of this apicomplexan parasite is mainly associated with efficient transplacental infections following reactivation of a chronic infection during gestation in the dam [4, 5], horizontal transmission through oocyst ingestion is possible and more frequent than previously thought [6, 7]. The clinical outcome of N. caninum infection can be abortion during months 3–9 of gestation (mostly between 5 and 6 months of gestation), a still-born calf, a new-born calf with neurological clinical signs, or a clinically healthy but persistently infected newborn calf [1, 2]. As demonstrated by experimental bovine challenge models, within the first term of gestation, foetal death is likely to occur after inoculation of NC1 [8–11] and Nc-Liverpool isolates [12–14], or (more recently) using well-characterized virulent isolates, such as Nc-Spain7 [15, 16]. Experimental infections during the second and third term of gestation have mostly led to the birth of persistently infected calves [12– 14]. Some N. caninum experimental primary infections in naïve dams at 110 days of pregnancy can result in foetal death [17, 18]
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