Abstract
Experimental infections in pregnant sheep have been focused on studying the effect of the time of challenge on the outcome of N. caninum infection, whereas the impact of the dose and route of challenge has not been studied in depth. Therefore, clinical outcome, immune responses, parasite detection and burden, and lesion severity in placental tissues and foetal brains were investigated in 90-day-pregnant sheep inoculated intravenously with 105 (G1), 104 (G2), 103 (G3), or 102 (G4) tachyzoites or subcutaneously with 104 (G5) tachyzoites of the virulent Nc-Spain7 isolate and an uninfected group (G6). Comparing challenge doses, G1 was the only group that had 100% abortion. Likewise, IFNγ levels in G1 increased earlier than those in other intravenously infected groups, and IgG levels on day 21 post-infection (pi) were higher in G1 than those in other intravenously infected groups. Concerning vertical transmission, G1 shows a higher parasite burden in the foetal brain than did G2 and G3. Comparing routes of administration, no differences in foetal survival rate or parasite load in the foetal brain were found. Although G2 had higher IFNγ levels than G5 on day 10 pi, no differences were found in humoral immune responses. Because the outcome after intravenous infection with 105 tachyzoites was similar to that observed after intravenous infection with 106 tachyzoites used in a previous work (100% abortion and vertical transmission), we conclude that it may be reasonable to use 105 tachyzoites administered by the intravenous route in further experiments when assessing drugs or vaccine candidates.
Highlights
Neospora caninum is an obligate intracellular apicomplexan parasite considered one of the leading infectious causes of abortion in cattle worldwide [1,2,3]
There are no studies comparing the outcome of N. caninum experimental infection using different routes of inoculation in pregnant sheep, in cattle, this is crucial because intravenous inoculation is associated with a more severe clinical presentation
All dams from the uninfected group (G6) gave birth, and a significant difference was found in the foetal survival rate compared to that in G1 (P < 0.05)
Summary
Neospora caninum is an obligate intracellular apicomplexan parasite considered one of the leading infectious causes of abortion in cattle worldwide [1,2,3]. Recent studies suggest that N. caninum could be a relevant abortifacient in some small ruminant management systems [4] or even the main cause of reproductive losses in some flocks [5, 6]. In a study comparing different infective doses, a strong relationship between the challenge dose of Nc-NZ1, Nc-NZ2 and Nc-NZ3 N. caninum tachyzoites and the clinical outcome was found in pregnant sheep at midgestation [9]. There are no studies comparing the outcome of N. caninum experimental infection using different routes of inoculation in pregnant sheep, in cattle, this is crucial because intravenous inoculation is associated with a more severe clinical presentation. The Nc-Spain isolate has been evaluated at different times during gestation in pregnant sheep, suggesting that the time of infection plays a key role in the pathogenesis of the disease [8]
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