Microsporidian infections in Lymantria dispar larvae: interactions and effects of multiple species infections on pathogen horizontal transmission

High temperatures and low humidity promote the occurrence of microsporidians (Microsporidia) in mosquitoes (Culicidae)

BACKGROUND

In the context of climate change, a growing concern is that vector-pathogen or host-parasite interactions may be correlated with climatic factors, especially increasing temperatures. In the present study, we used a mosquito-microsporidian model to determine the impact of environmental factors such as temperature, humidity, wind and rainfall on the occurrence rates of opportunistic obligate microparasites (Microsporidia) in hosts from a family that includes important disease vectors (Culicidae).

METHODS

In our study, 3000 adult mosquitoes collected from the field over 3 years were analysed. Mosquitoes and microsporidia were identified using PCR and sequencing of the hypervariable V5 region of the small subunit ribosomal RNA gene and a shortened fragment of the cytochrome c oxidase subunit I gene, respectively.

RESULTS

DNA metabarcoding was used to identify nine mosquito species, all of which were hosts of 12 microsporidian species. The prevalence of microsporidian DNA across all mosquito samples was 34.6%. Microsporidian prevalence in mosquitoes was more frequent during warm months (> 19 °C; humidity < 65%), as was the co-occurrence of two or three microsporidian species in a single host individual. During warm months, microsporidian occurrence was noted 1.6-fold more often than during the cold periods. Among the microsporidians found in the mosquitoes, five (representing the genera Enterocytospora, Vairimorpha and Microsporidium) were positively correlated with an increase in temperature, whereas one (Hazardia sp.) was significantly correlated with a decrease in temperature. Threefold more microsporidian co-occurrences were recorded in the warm months than in the cold months.

CONCLUSIONS

These results suggest that the susceptibility of mosquitoes to parasite occurrence is primarily determined by environmental conditions, such as, for example, temperatures > 19 °C and humidity not exceeding 62%. Collectively, our data provide a better understanding of the effects of the environment on microsporidian-mosquito interactions.

Interactions between microsporidia and other members of the microbiome

The microbiome is the collection of microbes that are associated with a host. Microsporidia are intracellular eukaryotic parasites that can infect most types of animals. In the last decade, there has been much progress to define the relationship between microsporidia and the microbiome. In this review, we cover an increasing number of reports suggesting that microsporidia are common components of the microbiome in both invertebrates and vertebrates. These microsporidia infections can range from mutualistic to pathogenic, causing several physiological phenotypes, including death. Infection with microsporidia often causes a disruption in the normal microbiome, with both increases and decreases of bacterial, fungal, viral, and protozoan species being observed. This impact on the microbiome can occur through upregulation and downregulation of innate immunity as well as morphological changes to tissues that impact interactions with these microbes. Other microbes, particularly bacteria, can inhibit microsporidia and have been exploited to control microsporidia infections. These bacteria can function through regulating immunity, secreting anti-microsporidia compounds, and, in engineered versions, expressing double-stranded RNA targeting microsporidia genes. We end this review by discussing potential future directions to further understand the complex interactions between microsporidia and the other members of the microbiome.

Metabarcoding reveals low prevalence of microsporidian infections in castor bean tick (Ixodes ricinus)

BACKGROUND

Microsporidia is a large group of eukaryotic obligate intracellular spore-forming parasites, of which 17 species can cause microsporidiosis in humans. Most human-infecting microsporidians belong to the genera Enterocytozoon and Encephalitozoon. To date, only five microsporidian species, including Encephalitozoon-like, have been found in hard ticks (Ixodidae) using microscopic methods, but no sequence data are available for them. Furthermore, no widespread screening for microsporidian-infected ticks based on DNA analysis has been carried out to date. Thus, in this study, we applied a recently developed DNA metabarcoding method for efficient microsporidian DNA identification to assess the role of ticks as potential vectors of microsporidian species causing diseases in humans.

METHODS

In total, 1070 (493 juvenile and 577 adult) unfed host-seeking Ixodes ricinus ticks collected at urban parks in the city of Poznan, Poland, and 94 engorged tick females fed on dogs and cats were screened for microsporidian DNA. Microsporidians were detected by PCR amplification and sequencing of the hypervariable V5 region of 18S rRNA gene (18S profiling) using the microsporidian-specific primer set. Tick species were identified morphologically and confirmed by amplification and sequencing of the shortened fragment of cytochrome c oxidase subunit I gene (mini-COI).

RESULTS

All collected ticks were unambiguously assigned to I. ricinus. Potentially zoonotic Encephalitozoon intestinalis was identified in three fed ticks (3.2%) collected from three different dogs. In eight unfed host-seeking ticks (0.8%), including three males (1.1%), two females (0.7%) and three nymphs (0.7%), the new microsporidian sequence representing a species belonging to the genus Endoreticulatus was identified.

CONCLUSIONS

The lack of zoonotic microsporidians in host-seeking ticks suggests that I. ricinus is not involved in transmission of human-infecting microsporidians. Moreover, a very low occurrence of the other microsporidian species in both fed and host-seeking ticks implies that mechanisms exist to defend ticks against infection with these parasites.

Factors That Determine Microsporidia Infection and Host Specificity

Microsporidia are a large phylum of obligate intracellular parasites that infect an extremely diverse range of animals and protists. In this chapter, we review what is currently known about microsporidia host specificity and what factors influence microsporidia infection. Extensive sampling in nature from related hosts has provided insight into the host range of many microsporidia species. These field studies have been supported by experiments conducted in controlled laboratory environments which have helped to demonstrate host specificity. Together, these approaches have revealed that, while examples of generalist species exist, microsporidia specificity is often narrow, and species typically infect one or several closely related hosts. For microsporidia to successfully infect and complete their life cycle within a compatible host, several steps must occur, including spore germination, host cell invasion, and proliferation of the parasite within the host tissue. Many factors influence infection, including temperature, seasonality, nutrient availability, and the presence or absence of microbes, as well as the developmental stage, sex, and genetics of the host. Several studies have identified host genomic regions that influence resistance to microsporidia, and future work is likely to uncover molecular mechanisms of microsporidia host specificity in more detail.

Increased mortality of the European pepper moth Duponchelia fovealis (Lepidoptera:Crambidae) using entomopathogenic fungal consortia

The European pepper moth (Duponchelia fovealis) is an invasive pest affecting crops in many countries. The use of chemicals to control D. fovealis is not only ineffective but is hazardous to the environment. The most effective way to reduce this invasive species is biological control using entomopathogenic fungi. Furthermore, the use of combining entomopathogenic fungi is a novel and underexplored approach in the field of biocontrol research. The compatibility of different strains of Beauveria bassiana, Purpureocillium lilacinum, and Isaria javanica was evaluated by forming two-fungi consortia. The pathogenicity of these consortia against D. fovealis, as well as the related enzymatic activities, were investigated. Seven consortia increased D. fovealis mortality, showing synergistic activity. One consortium formed by two strains of B. bassiana produced highest control. Moreover, these consortia also demonstrated increased chitinase and lipase activities. Higher mortality of D. fovealis by these consortia was mainly associated with enzyme production. One consortium, also formed by two strains of B. bassiana, was unique in producing lower D. fovealis mortality than the two strains alone. The potential use of entomopathogenic fungal consortia is a promising alternative approach for biological control. Most of the consortia used in this study improved control of D. fovealis, showed synergistic activity and could be a suitable strategy to control this pest.

Identification and characterization of three microsporidian genera concurrently infecting a silkworm, Bombyx mori, in Brazil

Microsporidia are important entomopathogens known for infecting insects such as the silkworm (Bombyx mori) thus impairing global silk production. This study aimed to identify and characterize the microsporidia isolated from a diseased larva of silkworm, collected from a sericulture farm in southern Brazil. Identification was performed by phylogenetic analysis of the nucleotide sequences of the SSU rRNA genes. Characterization was performed by analyzing spore sizes, tissue tropism, internal and external symptoms, and pathogenicity against B. mori. Microsporidia belonging to three different genera were identified, namely, Endoreticulatus, Nosema and Tubulinosema. After inoculation of the mixed spores of the microsporidian isolates into B. mori larvae, a high prevalence of Tubulinosema spp. was observed. This isolate showed high prevalence on the silk glands and a late mortality, initially of around 10% until the 20th day post-inoculation but reaching 91.5% upon pupation. Therefore, we demonstrated that Tubulinosema spp. causes chronic infection with slow pathogenicity. We identified for the first time three different microsporidians concurrently infecting B. mori in Brazil. Tubulinosema is of particular interest because of its potential threat to silk production; it affects the formation of silk glands in B. mori while not presenting distinguishable external symptoms or causing the immediate death of these insects. Further studies focusing on this species, mainly regarding its life cycle within the host and the sublethal effects of surviving individuals, demonstrate the importance of describing it as a new species and improving the characterization of the disease in order to prevent its spread.

Twenty-five-year study of Nosema spp. in honey bees (Apis mellifera) in Serbia

A total of 7386 samples of adult honey bees from different areas of Serbia (fifteen regions and 79 municipalities) were selected for light microscopy analysis for Nosema species during 1992–2017. A selection of honey bee samples from colonies positive for microsporidian spores during 2009–2011, 2015 and 2017 were then subjected to molecular diagnosis by multiplex PCR using specific primers for a region of the 16S rRNA gene of Nosema species. The prevalence of microsporidian spore-positive bee colonies ranged between 14.4% in 2013 and 65.4% in 1992. PCR results show that Nosema ceranae is not the only Nosema species to infect honey bees in Serbia. Mixed N. apis/N. ceranae infections were detected in the two honey bee samples examined by mPCR during 2017. The beekeeping management of disease prevention, such as replacement of combs and queens and hygienic handling of colonies are useful in the prevention of Nosema infection.

Interspecific competition in honeybee intracellular gut parasites is asymmetric and favours the spread of an emerging infectious disease

There is increasing appreciation that hosts in natural populations are subject to infection by multiple parasite species. Yet the epidemiological and ecological processes determining the outcome of mixed infections are poorly understood. Here, we use two intracellular gut parasites (Microsporidia), one exotic and one co-evolved in the western honeybee (Apis mellifera), in an experiment in which either one or both parasites were administered either simultaneously or sequentially. We provide clear evidence of within-host competition; order of infection was an important determinant of the competitive outcome between parasites, with the first parasite significantly inhibiting the growth of the second, regardless of species. However, the strength of this 'priority effect' was highly asymmetric, with the exotic Nosema ceranae exhibiting stronger inhibition of Nosema apis than vice versa. Our results reveal an unusual asymmetry in parasite competition that is dependent on order of infection. When incorporated into a mathematical model of disease prevalence, we find asymmetric competition to be an important predictor of the patterns of parasite prevalence found in nature. Our findings demonstrate the wider significance of complex multi-host-multi-parasite interactions as drivers of host-pathogen community structure.

Single and mixed-species trypanosome and microsporidia infections elicit distinct, ephemeral cellular and humoral immune responses in honey bees

Frequently encountered parasite species impart strong selective pressures on host immune system evolution and are more apt to concurrently infect the same host, yet molecular impacts in light of this are often overlooked. We have contrasted immune responses in honey bees to two common eukaryotic endoparasites by establishing single and mixed-species infections using the long-associated parasite Crithidia mellificae and the emergent parasite Nosema ceranae. Quantitative polymerase chain reaction was used to screen host immune gene expression at 9 time points post inoculation. Systemic responses in abdomens during early stages of parasite establishment revealed conserved receptor (Down syndrome cell adhesion molecule, Dscam and nimrod C1, nimC1), signaling (MyD88 and Imd) and antimicrobial peptide (AMP) effector (Defensin 2) responses. Late, established infections were distinct with a refined 2 AMP response to C. mellificae that contrasted starkly with a 5 AMP response to N. ceranae. Mixed species infections induced a moderate 3 AMPs. Transcription in gut tissues highlighted important local roles for Dscam toward both parasites and Imd signaling toward N. ceranae. At both systemic and local levels Dscam, MyD88 and Imd transcription was consistently correlated based on clustering analysis. Significant gene suppression occurred in two cases from midgut to ileum tissue: Dscam was lowered during mixed infections compared to N. ceranae infections and both C. mellificae and mixed infections had reduced nimC1 transcription compared to uninfected controls. We show that honey bees rapidly mount complex immune responses to both Nosema and Crithidia that are dynamic over time and that mixed-species infections significantly alter local and systemic immune gene transcription.

Specific detection and localization of microsporidian parasites in invertebrate hosts by using in situ hybridization

We designed fluorescence in situ hybridization probes for two distinct microsporidian clades and demonstrated their application in detecting, respectively, Nosema/Vairimorpha and Dictyoceola species. We used them to study the vertical transmission of two microsporidia infecting the amphipod Gammarus duebeni.

Comparative virulence of Nosema ceranae and Nosema apis in individual European honey bees

Nosema apis and Nosema ceranae are intracellular microsporidian parasites infecting the midgut epithelial cells of adult honey bees. N. ceranae was considered to be restricted to the Asian honey bee, Apis cerana, but is nowadays a parasite found also in the European honey bee (Apis mellifera) across most of the world. Recent surveys and experimental work suggest that N. ceranae is a serious threat to the global beekeeping industry. It has been suggested that N. ceranae induces significantly higher mortality in honey bees than N. apis, but little is known about their comparative virulence. In this study, we used in vivo infection experiments to study the two parasites' different virulence (i.e. multiplication rate and infectivity). A qPCR was developed to elucidate within host competition between the two parasites using mixed infections. The outcome of the experiments indicates minor differences in infectious dose and multiplication rate between the two species. Moreover, the mortality caused by N. ceranae was not significantly higher than for N. apis and N. ceranae appeared to have no competitive advantage within host.

Interaction between Paranosema locustae and Metarhizium anisopliae var. acridum, two pathogens of the desert locust, Schistocerca gregaria under laboratory conditions

The interaction between two pathogens, the microsporidian Paranosema locustae Canning and the fungus Metarhizium anisopliae var. acridum Driver and Milner was studied under laboratory conditions in an attempt to develop an improved method of microbial control for the desert locust, Schistocerca gregaria Forskål. Fifth-instar locust nymphs, reared in the laboratory, were treated with various concentrations of one of the two pathogens or with both pathogens. The numbers of locusts killed were recorded each day and the production of pathogen spores within the dead locusts was assessed at the end (day 21) of each experiment. Locust nymphs treated with both P. locustae and M. anisopliae died sooner than nymphs infected with only one of the pathogens. At the lower concentrations of pathogen tested, the effects of the two pathogens were additive. At the higher concentrations the combined effects were synergistic. In terms of locust mortality, there was no evidence of any antagonistic effects between the two pathogens. However, the production of spores by P. locustae was reduced considerably when the host insects were infected also with M. anisopliae. For example, nymphs treated initially with P. locustae and then treated 3 and 10 days later with M. anisopliae produced 3-20 times and 2.5-8 times fewer spores, respectively, than nymphs treated only with P. locustae. Hence, in areas where M. anisopliae is applied, the natural persistence of P. locustae in the local grasshopper and locust populations may be diminished.