Merel Langelaar

National Institute for Public Health and the Environment (RIVM), Utrecht, Utrecht, Netherlands

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Publications (19)26.48 Total impact


  • No preview · Article · Jun 2011 · Tijdschrift voor diergeneeskunde
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    ABSTRACT: The role of beef in human infections with Toxoplasma gondii is not clear. To get a better understanding of the value of seroprevalence as an indication of the role of beef in human infections with T. gondii we studied the seroprevalence of T. gondii in Dutch cattle and analysed the correlation between detection of antibodies and parasitic DNA. An indirect ELISA was developed and used to test a sample of the Dutch cattle population. Since validation of the ELISA was hampered by a lack of sufficient bovine reference sera, the results were analysed in two different ways: using a cut-off value that was based on the course of the OD in 27 calves followed from birth until 16 months of age, and by fitting a mixture of two normal distributions (binormal mixture model) to the log-transformed ODs observed for the different groups of cattle in the study population. Using the cut-off value, the seroprevalence was estimated at 0.5% for white veal, 6.4% for rosé veal and 25.0% for cattle. However, using the frequency distributions the prevalences were higher: 1.9% for white veal, 15.6% for rosé veal and 54.5% for cattle. Next, for 100 cattle the results with two different serological assays (ELISA and Toxo-Screen DA) were compared with detection of parasites by our recently developed sensitive magnetic capture PCR. Toxoplasma gondii DNA was detected in only two seronegative cattle. This discordance demonstrates that seroprevalence cannot be used as an indicator of the number of cattle carrying infectious parasites. Demonstrating parasitic DNA in seronegative cattle and not in seropositive cattle suggests that only recent infections are detectable. Whether beef from these PCR-positive cattle is infectious to humans remains to be studied.
    Full-text · Article · Dec 2010 · International journal for parasitology
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    Paul Overgaauw · Merel Langelaar

    Full-text · Article · Dec 2010 · Tijdschrift voor diergeneeskunde
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    ABSTRACT: To support the development of early warning and surveillance systems of emerging zoonoses, we present a general method to prioritize pathogens using a quantitative, stochastic multi-criteria model, parameterized for the Netherlands. A risk score was based on seven criteria, reflecting assessments of the epidemiology and impact of these pathogens on society. Criteria were weighed, based on the preferences of a panel of judges with a background in infectious disease control. Pathogens with the highest risk for the Netherlands included pathogens in the livestock reservoir with a high actual human disease burden (e.g. Campylobacter spp., Toxoplasma gondii, Coxiella burnetii) or a low current but higher historic burden (e.g. Mycobacterium bovis), rare zoonotic pathogens in domestic animals with severe disease manifestations in humans (e.g. BSE prion, Capnocytophaga canimorsus) as well as arthropod-borne and wildlife associated pathogens which may pose a severe risk in future (e.g. Japanese encephalitis virus and West-Nile virus). These agents are key targets for development of early warning and surveillance.
    Full-text · Article · Nov 2010 · PLoS ONE
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    ABSTRACT: Lamb and mutton are considered important sources of human Toxoplasma gondii infections, but actual data on the prevalence of T. gondii in sheep in The Netherlands is lacking. The aim of this study was to investigate the prevalence of T. gondii in slaughtered sheep to get more insight in the importance of sheep as a source of human infection. In addition, regional variation in prevalence was studied, as this may indicate differences in environmental contamination. An in-house ELISA that detects antibodies against T. gondii was developed and used to test 1179 sera collected from sheep presented at 11 Dutch slaughterhouses between October and December 2007. Since validation of the serological assay was hampered by a lack of appropriate reference sera, the diagnostic performance and seroprevalence were estimated by fitting a binormal mixture model. ROC-curve analysis on the fitted distributions showed high discriminatory power (AUC=0.995), and high sensitivity and specificity of the ELISA. The overall prevalence was estimated at 27.8% (25.6-29.9%), but was significantly higher in sheep over 1 year old, and in sheep from the central provinces. The high sensitivity and specificity of the in-house ELISA were confirmed by Bayesian analysis together with three commercially available assays: Toxo-Screen DA (bioMérieux), Chekit Toxotest Antibody ELISA (IDEXX), and Toxoplasmosis serum screening ELISA (Institut Pourquier). In conclusion, the binormal mixture model proved a useful method to obtain estimates of diagnostic performance and seroprevalence without use of reference sera. The seroprevalence in sheep was high, and as sheep with antibodies usually carry tissue cysts, this indicates that undercooked lamb and mutton may indeed be important sources of human toxoplasmosis in The Netherlands.
    No preview · Article · Sep 2010 · Preventive Veterinary Medicine

  • No preview · Article · May 2010 · Tijdschrift voor diergeneeskunde

  • No preview · Article · May 2010 · Tijdschrift voor diergeneeskunde
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    ABSTRACT: The burden of diseases caused by food-borne pathogens remains largely unknown. Importantly data indicating trends in food-borne infectious intestinal disease is limited to a few industrialised countries, and even fewer pathogens. It has been predicted that the importance of diarrhoeal disease, mainly due to contaminated food and water, as a cause of death will decline worldwide. Evidence for such a downward trend is limited. This prediction presumes that improvements in the production and retail of microbiologically safe food will be sustained in the developed world and, moreover, will be rolled out to those countries of the developing world increasingly producing food for a global market. In this review evidence is presented to indicate that the microbiological safety of food remains a dynamic situation heavily influenced by multiple factors along the food chain from farm to fork. Sustaining food safety standards will depend on constant vigilance maintained by monitoring and surveillance but, with the rising importance of other food-related issues, such as food security, obesity and climate change, competition for resources in the future to enable this may be fierce. In addition the pathogen populations relevant to food safety are not static. Food is an excellent vehicle by which many pathogens (bacteria, viruses/prions and parasites) can reach an appropriate colonisation site in a new host. Although food production practices change, the well-recognised food-borne pathogens, such as Salmonella spp. and Escherichia coli, seem able to evolve to exploit novel opportunities, for example fresh produce, and even generate new public health challenges, for example antimicrobial resistance. In addition, previously unknown food-borne pathogens, many of which are zoonotic, are constantly emerging. Current understanding of the trends in food-borne diseases for bacterial, viral and parasitic pathogens has been reviewed. The bacterial pathogens are exemplified by those well-recognized by policy makers; i.e. Salmonella, Campylobacter, E. coli and Listeria monocytogenes. Antimicrobial resistance in several bacterial food-borne pathogens (Salmonella, Campylobacter, Shigella and Vibrio spp., methicillin resistant Staphylcoccus aureas, E. coli and Enterococci) has been discussed as a separate topic because of its relative importance to policy issues. Awareness and surveillance of viral food-borne pathogens is generally poor but emphasis is placed on Norovirus, Hepatitis A, rotaviruses and newly emerging viruses such as SARS. Many food-borne parasitic pathogens are known (for example Ascaris, Cryptosporidia and Trichinella) but few of these are effectively monitored in foods, livestock and wildlife and their epidemiology through the food-chain is poorly understood. The lessons learned and future challenges in each topic are debated. It is clear that one overall challenge is the generation and maintenance of constructive dialogue and collaboration between public health, veterinary and food safety experts, bringing together multidisciplinary skills and multi-pathogen expertise. Such collaboration is essential to monitor changing trends in the well-recognised diseases and detect emerging pathogens. It will also be necessary understand the multiple interactions these pathogens have with their environments during transmission along the food chain in order to develop effective prevention and control strategies.
    Full-text · Article · May 2010 · International journal of food microbiology
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    ABSTRACT: Different transmission routes, including the ingestion of undercooked meat, can result in Toxoplasma gondii infection in humans. The development of effective prevention strategies is hampered by a lack of quantitative information on the contamination level of different types of meat. Therefore, we developed a method for detection and quantification of T. gondii. The method involved preparation of crude DNA extract from hundred gram samples of meat, magnetic capture of T. gondii DNA and, quantitative real-time PCR targeting the T. gondii 529-bp repeat element. The detection limit of this assay was approximately 230 tachyzoites per 100 g of meat sample. There was a linear relation between the number of parasites added to the samples and Cp-values. Results obtained with the PCR method were comparable to bioassay results for experimentally infected pigs, and to serological findings for sheep. In addition, the T. gondii in 50% of the positive sheep samples could be genotyped by sequencing of the GRA6 gene, after isolation of the gene by magnetic capture. Two subtypes of GRA6 type II were identified in the 16 samples from sheep. For seven samples, the identification of T. gondii as type II was confirmed by microsatellite typing. The PCR method can be used as an alternative to bioassay for detection and genotyping of T. gondii, and to quantify the organism in meat samples of various sources.
    Full-text · Article · Mar 2010 · International journal of food microbiology
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    Paul Overgaauw · Merel Langelaar

    Full-text · Article · Dec 2009 · Tijdschrift voor diergeneeskunde
  • Merel Langelaar · Marieta Braks · Joke van der Giessen

    No preview · Article · Mar 2009 · Tijdschrift voor diergeneeskunde
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    ABSTRACT: Prevalences of parasitic infections in pigs from different housing systems may vary, due to their contact with the environment, and this might have consequences for food safety. In this study, 40 organic, 9 free-range and 24 intensive farms were selected and a total of 845 serum samples were tested for antibodies specific for Toxoplasma and Trichinella using ELISA assays. The overall seroprevalence of Toxoplasma in the total number of 845 serum samples tested is 2.6%, ranging from 0.38% in intensively raised pigs to 5.62% in free-range pigs. Of the housing systems tested, 4% (intensive farms) to 33% (free-range farms) is infected with Toxoplasma gondii. The risk of detecting Toxoplasma antibodies in a free-range farm are statistically higher (almost 16 times higher) than in an intensive farm. We observed that the risk of detecting specific antibodies is twice as high as in free-range compared with organic farms. Seropositivity of Trichinella spiralis antibodies was 0.12-0.35% (depending on the cut-off value at the 99.5% or 97.5% level). There was a tendency that Trichinella seropositivity was higher in organic pig farming (0.24%), but this was not significant. This serological study in pigs from different farming systems shows that the seroprevalence of antibodies specific for T. gondii is higher and for Trichinella equivalent in pigs raised in systems where there is contact with the environment than in pigs raised in intensive, indoor farming systems. This indicates that the prevalence of parasitic infections is higher in outdoor farming systems than in indoor farming systems. The possible consequences for food safety are discussed.
    Full-text · Article · Oct 2007 · Veterinary Parasitology
  • Paul Overgaauw · Merel Langelaar

    No preview · Article · Jul 2006 · Tijdschrift voor diergeneeskunde
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    ABSTRACT: Paratuberculosis is a chronic granulomatous inflammation of the small intestine of cattle and other ruminants, caused by infection with Mycobacterium avium ssp. paratuberculosis (MAP). The disease can be found in ruminant herds worldwide, causing substantial economic losses at farm level due to premature culling and production losses. In previous studies, it has been shown that immune responses to recombinant MAP Hsp70 proteins were predominantly cell mediated. As protective immunity to the intracellular mycobacterial pathogens is thought to be cell-mediated in origin, we have studied the use of a recombinant MAP Hsp70 as a subunit vaccine in cattle experimentally infected with MAP. The results of the current study demonstrate that recombinant MAP Hsp70 can be successfully used as a subunit vaccine against bovine paratuberculosis, significantly reducing shedding of bacteria in feces during the first 2 years following experimental infection.
    Full-text · Article · Apr 2006 · Vaccine
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    ABSTRACT: Mycobacterium avium paratuberculosis (M.a.p.) resides and replicates in macrophages. Many of the of immune mechanisms aiding M.a.p. survival in the host's cells are known. However, little is known about interactions of M.a.p. with dendritic cells (DC). As DC are important for the induction of protective immunity against infectious diseases, we investigated the interaction of M.a.p. with these cells. Quantitative real-time PCR (RT-PCR) was used to analyse differential expression of cytokine genes after 6 h and 24 h of incubation by immature DC that phagocytosed either M.a.p. or Escherichia coli (E. coli). We hypothesized that phagocytosis of E. coli would induce pro-inflammatory cytokines due to abundant presence of lipopolysaccharide (LPS) and that the cytokine expression profile induced by phagocytosis of live M.a.p. would differ. In addition we hypothesized that incubation of immature DC with rHsp70, an immunodominant antigen of M.a.p., would induce a similar profile of cytokine gene expression as phagocytosis of intact M.a.p. However, phagocytosis of both E. coli and M.a.p. resulted in a cytokine gene expression pattern representative of a (pro-)inflammatory reaction, dominated by strong induction of IL-12 gene expression, that was higher after 24 h than after 6 h of incubation, although the response to M.a.p. was less vigorous than to E. coli. Incubation with rHsp70 resulted in a more inhibitory type of cytokine gene expression, with delayed IL-12 gene expression and downregulation of the genes for IL-1beta and IL-6 after 24 h of incubation. We conclude that bovine DC produce an immuno-stimulatory, anti-mycobacterial response to infection with M.a.p., while Hsp70 potentially contributes to pathogen virulence by allowing the bacteria to invade the host cell.
    No preview · Article · Sep 2005 · Veterinary Immunology and Immunopathology
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    ABSTRACT: Abstract Heat shock proteins (Hsp) can deliver antigen into the major histocompatibility complex class I presentation pathway of antigen-presenting cells (APC), a process called cross priming, thus stimulating antigen-specific CD8+ T-cell reactions. Hsp were shown to elicit proinflammatory responses in APC. Both processes require interaction of Hsp with APC via specific receptors. This study describes the interaction of recombinant Hsp70 (rHsp70) of Mycobacterium avium subspecies paratuberculosis with bovine peripheral blood mononuclear cells that was restricted to CD14+ cells. Characterized monocyte-derived macrophages, monocyte-derived dendritic cells (DC) and BoMac, an immortalized bovine macrophage cell line, were used to investigate the interaction of rHsp70 with different bovine APC. Saturation of immature DC with high concentrations of rHsp70 is demonstrated, and it was found that interaction of rHsp70 with DC was related to the maturation stage of the DC. Involvement of CD91 as a cellular receptor for rHsp70 was demonstrated; however, competition studies with immature DC demonstrated that other receptors exist on bovine APC. These data suggest that rHsp70-based vaccines may be useful for the successful immunization of cattle.
    Full-text · Article · Apr 2005 · Scandinavian Journal of Immunology

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  • Alfons Olde Loohuis · Frans van Knapen · Merel Langelaar

    No preview · Article · · Tijdschrift voor diergeneeskunde
  • A. Hoek · V.P.M.G. Rutten · R. Bouwstra · M.F.M. Langelaar · A.P. Koets

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Publication Stats

595 Citations
26.48 Total Impact Points

Institutions

  • 2007-2010
    • National Institute for Public Health and the Environment (RIVM)
      • Laboratory for Zoonoses and Environmental Microbiology (LZO)
      Utrecht, Utrecht, Netherlands
  • 2005-2010
    • Universiteit Utrecht
      • • Institute for Risk Assessment Sciences (IRAS)
      • • Division of Immunology
      Utrecht, Provincie Utrecht, Netherlands