Abstract
In this study, we examined the in vitro invasion and proliferation capacities of the Nc-Liv and ten Spanish Neospora caninum isolates (Nc-Spain 1 H - Nc-Spain 10). The invasion rate was determined as the number of tachyzoites that completed their internalisation into MARC-145 cells at 2, 4, and 6 h post-inoculation (pi). The proliferation rate was evaluated by determining the doubling time during the exponential proliferation period. Significant differences in the invasion rates of these isolates were detected at 2 and 4 h pi (P < 0.0001, Kruskal-Wallis test). At 4 h pi, the Nc-Spain 4 H and Nc-Liv isolates displayed the highest, while the Nc-Spain 3 H and Nc-Spain 1 H isolates had the lowest invasion rates (by Dunn's test). Variations in the proliferation kinetics of these isolates were also observed. Between different isolates, the lag phase, which occurs before the exponential growth phase, ranged from 8 to 44 h, and the doubling time ranged from 9.8 to 14.1 h (P = 0.0016, ANOVA test). Tachyzoite yield, which combines invasion and proliferation data, was also assessed and confirmed marked differences between the highly and less prolific isolates. Interestingly, a direct correlation between the invasion rates and tachyzoite yields, and the severity of the disease that was exhibited by infected pregnant mice in previous works could be established for the isolates in this study (Spearman's coefficient > 0.62, P < 0.05). The results of this study may help us to explain the differences in the pathogenicity that are displayed by different isolates.
Highlights
Neospora caninum is an obligate intracellular parasite that is phylogenetically related to Toxoplasma gondii and causes neuromuscular disease in dogs and abortion in cattle, it can infect other host species [1,2]
Very little information is known about the inherent factors of this parasite that contribute to its intra-specific pathogenicity, but the capacity to produce pathology has been associated with the behaviour of different N. caninum isolates in the host
Previous in vitro studies have reported that the growth [18,19] and bradyzoite conversion rates [14,20,21] are variable among different N. caninum isolates
Summary
Neospora caninum is an obligate intracellular parasite that is phylogenetically related to Toxoplasma gondii and causes neuromuscular disease in dogs and abortion in cattle, it can infect other host species [1,2]. The dissemination capacity, the parasite burdens that are reached in target tissues, the ability to avoid the immune response produced against the infection by the host and the rate of tachyzoite-bradyzoite conversion in the host may all contribute to the different levels of pathogenicity that are caused by different isolates [11,12,13,16,17]. Similar to T. gondii, the inherent pathogenicity of different N. caninum isolates may be directly related to specific virulence traits, which include the migration capacity, the ability to cross barriers and the cell invasion and intracellular proliferation efficiencies [22,23,24,25]
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