Articles and other publications by RIVM employees
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Differential susceptibility of geographically distinct Ixodes ricinus populations to tick-borne encephalitis virus and louping ill virusTick-borne encephalitis virus (TBEV) is an emerging pathogen in the Netherlands. Multiple divergent viral strains are circulating and the focal distribution of TBEV remains poorly understood. This may, however, be explained by differences in the susceptibility of tick populations for specific viruses and viral strains, and by viral strains having higher infection success in their local tick population. We investigated this hypothesis by exposing Dutch Ixodes ricinus ticks to two different TBEV strains: TBEV-NL from the Netherlands and TBEV-Neudoerfl from Austria. In addition, we exposed ticks to louping Ill virus (LIV), which is endemic to large parts of the United Kingdom and Ireland, but has not been reported in the Netherlands. Ticks were collected from two locations in the Netherlands: one location without evidence of TBEV circulation and one location endemic for the TBEV-NL strain. Ticks were infected in a biosafety level 3 laboratory using an artificial membrane feeding system. Ticks collected from the region without evidence of TBEV circulation had lower infection rates for TBEV-NL as compared to TBEV-Neudoerfl. Vice versa, ticks collected from the TBEV-NL endemic region had higher infection rates for TBEV-NL compared to TBEV-Neudoerfl. In addition, LIV infection rates were much lower in Dutch ticks compared to TBEV, which may explain why LIV is not present in the Netherlands. Our findings show that ticks from two distinct geographical populations differ in their susceptibility to TBEV strains, which could be the result of differences in the genetic background of the tick populations.
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In vitro assay to determine inactivation of Toxoplasma gondii in meat samples.Consumption of raw and undercooked meat is considered as an important source of Toxoplasma gondii infections. However, most non-heated meat products contain salt and additives, which affect T. gondii viability. It was our aim to develop an in vitro method to substitute the mouse bioassay for determining the effect of salting on T. gondii viability. Two sheep were experimentally infected by oral inoculation with 6.5 × 104 oocysts. Grinded meat samples of 50 g were prepared from heart, diaphragm, and four meat cuts. Also, pooled meat samples were either kept untreated (positive control), frozen (negative control) or supplemented with 0.6 %, 0.9 %, 1.2 % or 2.7 % NaCl. All samples were digested in pepsin-HCl solution, and digests were inoculated in duplicate onto monolayers of RK13 (a rabbit kidney cell line). Cells were maintained for up to four weeks and parasite growth was monitored by assessing Cq-values using the T. gondii qPCR on cell culture supernatant in intervals of one week and ΔCq-values determined. Additionally, 500 μL of each digest from the individual meat cuts, heart and diaphragm were inoculated in duplicate in IFNγ KO mice. Both sheep developed an antibody response and tissue samples contained similar concentrations of T. gondii DNA. From all untreated meat samples positive ΔCq-values were obtained in the in vitro assay, indicating presence and multiplication of viable parasites. This was in line with the mouse bioassay, with the exception of a negative mouse bioassay on one heart sample. Samples supplemented with 0.6 %-1.2 % NaCl showed positive ΔCq-values over time. The frozen sample and the sample supplemented with 2.7 % NaCl remained qPCR positive but with high Cq-values, which indicated no growth. In conclusion, the in vitro method has successfully been used to detect viable T. gondii in tissues of experimentally infected sheep, and a clear difference in T. gondii viability was observed between the samples supplemented with 2.7 % NaCl and those with 1.2 % NaCl or less.
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Chronic toxicity of core-shell SiC/TiO2 (nano)-particles to Daphnia magna under environmentally relevant food rations in the presence of humic acidTo date, research on the toxicity and potential environmental impacts of nanomaterials has predominantly focused on relatively simple and single-component materials, whilst more complex nanomaterials are currently entering commercial stages. The current study aimed to assess the long-term and size-dependent (60 and 500 nm) toxicity of a novel core-shell nanostructure consisting of a SiC core and TiO2 shell (SiC/TiO2, 5, 25, and 50 mg L-1) to the common model organism Daphnia magna. These novel core-shell nanostructures can be categorized as advanced materials. Experiments were conducted under environmentally realistic feeding rations and in the presence of a range of concentrations of humic acid (0.5, 2, 5, and 10 mg L-1 TOC). The findings show that although effect concentrations of SiC/TiO2 were several orders of magnitude lower than the current reported environmental concentrations of more abundantly used nanomaterials, humic acid can exacerbate the toxicity of SiC/TiO2 by reducing aggregation and sedimentation rates. The EC50 values (mean ± standard error) based on nominal SiC/TiO2 concentrations for the 60 nm particles were 28.0 ± 11.5 mg L-1 (TOC 0.5 mg L-1), 21.1 ± 3.7 mg L-1 (TOC 2 mg L-1), 18.3 ± 5.4 mg L-1 (TOC 5 mg L-1), and 17.8 ± 2.4 mg L-1 (TOC 10 mg L-1). For the 500 nm particles, the EC50 values were 34.9 ± 16.5 mg L-1 (TOC 0.5 mg L-1), 24.8 ± 5.6 mg L-1 (TOC 2 mg L-1), 28.0 ± 10.0 mg L-1 (TOC 5 mg L-1), and 23.2 ± 4.1 mg L-1 (TOC 10 mg L-1). We argue that fate-driven phenomena are often neglected in effect assessments, whilst environmental factors such as the presence of humic acid may significantly influence the toxicity of nanomaterials.