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We detected infection with highly pathogenic avian influenza A(H5N1) virus clade 2.3.4.4b in 2 red fox (Vulpes vulpes) cubs found in the wild with neurologic signs in the Netherlands. The virus is related to avian influenza viruses found in wild birds in the same area.

We collected data on mass mortality in Sandwich terns (Thalasseus sandvicensis) during the 2022 breeding season in the Netherlands. Mortality was associated with at least 2 variants of highly pathogenic avian influenza A(H5N1) virus clade 2.3.4.4b. We report on carcass removal efforts relative to survival in colonies. Mitigation strategies urgently require structured research.

Since 1998, notifiable bluetongue virus (BTV) serotypes 1-4, 6, 8, 9, 11, and 16 have been reported in Europe. In August 2006, a bluetongue (BT) outbreak caused by BTV serotype 8 began in northwestern Europe. The Netherlands was declared BT-free in February 2012, and annual monitoring continued. On September 3, 2023, typical BT clinical manifestations in sheep were notified to the Netherlands Food and Product Safety Consumer Authority. On September 6, we confirmed BTV infection through laboratory diagnosis; notifications of clinical signs in cattle were also reported. We determined the virus was serotype 3 by whole-genome sequencing. Retrospective analysis did not reveal BTV circulation earlier than September. The virus source and introduction route into the Netherlands remains unknown. Continuous monitoring and molecular diagnostic testing of livestock will be needed to determine virus spread, and new prevention strategies will be required to prevent BTV circulation within the Netherlands and Europe.

Two mass mortality events (MMEs) of marine mussels that took place in the Oosterschelde, the Netherlands—the first in 2015/2016 and the second in 2019—both severely affected mussel production. The current study presents our observations on the onset and course of both MMEs and discusses probable putative causes. The two MMEs displayed a distinct course of events. The first event started in November 2015 with high mortality rates on culture plots, which remained elevated until the autumn of 2016. Approximately 40–50% of mussels from all age classes were lost on culture plots and 100% were lost from wild seed beds. The second event started in April–May 2019 and continued until the end of July, with mortality ranging from 20 to 100%, again from all age classes. Culture areas other than the Oosterschelde and other shellfish species were not affected. Histological and bacteriological screening produced no evidence for common pathogens or pollution as a primary mortality factor and there is no indication of abnormal environmental conditions preceding or during the events. We hypothesize that a cumulation of stressors results in weakening of the mussels and in elevated mortality rates. In 2019, this cumulation of stressors could be high spawning activities (an unusual high concentration of mussel larvae was found in April) that resulted in very low condition from April to June, a Phaeocystis bloom in April to May that prevented a quick recovery, and the development of granulocytomas that were found in up to 60 to 70% of live mussels as a consequence of cumulative stress. Although no (single) putative causes could be identified, this study contributes to the knowledge on MMEs in mussels and fits in a wider and disturbing trend on mortality events in shellfish.

Wild carnivore species infected with highly pathogenic avian influenza (HPAI) virus subtype H5N1 during the 2021–2022 outbreak in the Netherlands included red fox (Vulpes vulpes), polecat (Mustela putorius), otter (Lutra lutra), and badger (Meles meles). Most of the animals were submitted for testing because they showed neurological signs. In this study, the HPAI H5N1 virus was detected by PCR and/or immunohistochemistry in 11 animals and was primarily present in brain tissue, often associated with a (meningo) encephalitis in the cerebrum. In contrast, the virus was rarely detected in the respiratory tract and intestinal tract and associated lesions were minimal. Full genome sequencing followed by phylogenetic analysis demonstrated that these carnivore viruses were related to viruses detected in wild birds in the Netherlands. The carnivore viruses themselves were not closely related, and the infected carnivores did not cluster geographically, suggesting that they were infected separately. The mutation PB2-E627K was identified in most carnivore virus genomes, providing evidence for mammalian adaptation. This study showed that brain samples should be included in wild life surveillance programs for the reliable detection of the HPAI H5N1 virus in mammals. Surveillance of the wild carnivore population and notification to the Veterinary Authority are important from a one-heath perspective, and instrumental to pandemic preparedness.

Brucella canis had not been isolated in the Netherlands until November 2016, when it was isolated from a dog imported from Romania. Including this case, 16 suspected cases were notified to the authorities during the following 25 months. Of these 16 dogs, 10 were seropositive; tracking investigations found another 8 seropositive littermates. All seropositive animals were rescue dogs imported from Eastern Europe. B. canis was cultured from urine, blood, and other specimens collected from the dogs. Genotyping of isolates revealed clustering by litter and country. Isolating B. canis in urine indicates that shedding should be considered when assessing the risk for zoonotic transmission. This case series proves introduction of B. canis into a country to which it is not endemic through import of infected dogs from B. canis–endemic areas, posing a threat to the naive autochthonous dog population and humans.

Background European flat oysters ( Ostrea edulis ) are sequential hermaphrodites that alternate sex in response to environmental change. Epigenetics, including DNA methylation, are often involved in sex reversal through influencing gene transcription. Knowledge on the epigenetic mechanisms underlying sex reversal in hermaphrodite bivalves is limited to gonadal tissue and previous studies have only compared DNA methylomes of males and females. Therefore, the aim of this study is to assess whether sex-specific DNA methylation can be identified in somatic gill tissue of the flat oyster. Results By comparing whole-genome methylomes of 35 oysters of different sex phenotypes using nanopore sequencing, we demonstrate the presence of sex-specific DNA methylation patterns in somatic gill tissue. A total of 9,654 regions and 2,576 genes were differentially methylated between male, female, and hermaphrodite oysters. Functional analysis of differentially methylated genes indicated an association with energy homeostasis and metabolic processes, implying a remodeling of the energy balance. Conclusions This study is the first to characterize DNA methylomes of hermaphrodite oysters, providing new insights into the epigenetic mechanisms underlying sex reversal in a sequential hermaphrodite invertebrate. Additionally, this study characterizes sex-specific DNA methylation in somatic gill tissue, paving the way for non-lethal sex identification using epigenetic biomarkers.

An increase in temperature due to climate change may affect the geographic ranges of invasive parasites and alter their impact on native hosts. Our goal was to determine if the effects of infection by two species of invasive endoparasitic copepods on native blue mussel hosts ( Mytilus edulis ) change with increasing temperatures. We investigated this with a laboratory experiment using temperatures that represent annual mean and mean summer water temperatures of past observations and future predictions for the study area, the European Wadden Sea (10–26 °C). Over a period of 8–20 weeks, infection with Mytilicola intestinalis lowered mussel condition and infection with Mytilicola orientalis decreased mussel shell growth. High temperatures decreased mussel growth and condition in general, but only at low temperatures (10–14 °C) the parasite-induced loss of condition was evident compared to uninfected mussels. Mussel mortality and reproductive activity were not affected by parasite infection, although both were impacted by temperature: the highest temperature (26 °C) increased mussel mortality, and gamete ripening only occurred at lower temperatures (10–18 °C). Taken together, these results suggest that both infection and high temperatures have independent negative effects. However, an increase in temperature does not worsen the effect of infection on individual mussel hosts, and neither does infection decrease host tolerance for long-term exposure to high temperatures. These findings add to our understanding of the interplay between increasing temperature and the interaction between invasive parasites and native hosts, and help predicting host and parasite dynamics in systems affected by species invasions and climate change.

One of the most valuable aquaculture fish in Europe is the rainbow trout, Oncorhynchus mykiss, but the profitability of trout production is threatened by a highly lethal infectious disease, viral hemorrhagic septicemia (VHS), caused by the VHS virus (VHSV). For the past few decades, the subgenogroup Ia of VHSV has been the main cause of VHS outbreaks in European freshwater-farmed rainbow trout. Little is currently known, however, about the phylogenetic radiation of this Ia lineage into subordinate Ia clades and their subsequent geographical spread routes. We investigated this topic using the largest Ia-isolate dataset ever compiled, comprising 651 complete G gene sequences: 209 GenBank Ia isolates and 442 Ia isolates from this study. The sequences come from 11 European countries and cover the period 1971-2015. Based on this dataset, we documented the extensive spread of the Ia population and the strong mixing of Ia isolates, assumed to be the result of the Europe-wide trout trade. For example, the Ia lineage underwent a radiation into nine Ia clades, most of which are difficult to allocate to a specific geographic distribution. Furthermore, we found indications for two rapid, large-scale population growth events, and identified three polytomies among the Ia clades, both of which possibly indicate a rapid radiation. However, only about 4% of Ia haplotypes (out of 398) occur in more than one European country. This apparently conflicting finding regarding the Europe-wide spread and mixing of Ia isolates can be explained by the high mutation rate of VHSV. Accordingly, the mean period of occurrence of a single Ia haplotype was less than a full year, and we found a substitution rate of up to 7.813 × 10-4 nucleotides per site per year. Finally, we documented significant differences between Germany and Denmark regarding their VHS epidemiology, apparently due to those countries' individual handling of VHS.

Highly pathogenic avian influenza A(H5N8) virus was detected in mute swans in the Netherlands during October 2020. The virus shares a common ancestor with clade 2.3.4.4b viruses detected in Egypt during 2018-2019 and has similar genetic composition. The virus is not directly related to H5N8 viruses from Europe detected in the first half of 2020.