Effects of the seasonal variations on within-host parasitaemia are still not well understood and potentially due to numerous factors, e.g. host and parasite species, host sex or age, or geographical regions. In this study, over three years in Switzerland, Pigeault et al. (2024) collected data on great tits reproductive outputs – laying date, clutch size, fledging success – to determine whether they were associated with avian Plasmodium parasitaemia before (winter), during (spring) and after (autumn) the breeding season. They focused on two lineages from two species: a highly generalist lineage Plasmodium relictum (lineage SGS1; Bensch et al. 2009) and a more specialized lineage Plasmodium homonucleophilum (lineage SW2). As previously found, they showed that parasitaemia level is low during the winter and then increase in spring (Applegate, 1970; Applegate 1971). Spring recurrences have been intensively studied but are still not well understood since many non-exclusive factors can provoke them, i.e environmental stressors, reproductive hormones, co-infections or bites of mosquitoes (Cornet et al. 2014).

Interestingly, the parasitaemia level during the winter before and during the breeding season were not associated to the reproductive success, meaning that birds in their populations with low parasitaemia during the winter had not more fledglings than the ones with a higher parasitaemia. However, the individuals who invested the most in the reproduction with a higher number of fledglings had also a higher parasitaemia in the following autumn. The number of laid eggs was not associated with the parasitaemia during the following autumn, showing that the initial investment in the reproduction is less important than the parental care (e.g. chicks feeding) in terms of mid/long term cost. The originality here is that authors followed populations during three periods of the year, which is not an easy task and rarely done in natural populations. Their results highlight the mid/long-term effect of higher resource allocation into reproduction on individuals’ immune system and ability to control parasite replication. Further analyses on various lineages and bird populations from other geographical regions (i.e. different latitudes) would be the next relevant step.

References

Applegate JE (1971) Spring relapse of Plasmodium relictum infections in an experimental field population of English sparrows (Passer domesticus). Journal of Wildlife Diseases, 7, 37–42. https://doi.org/10.7589/0090-3558-7.1.37

Applegate JE, Beaudoin RL (1970) Mechanism of spring relapse in avian malaria: Effect of gonadotropin and corticosterone. Journal of Wildlife Diseases, 6, 443–447. https://doi.org/10.7589/0090-3558-6.4.443

Bensch S, Hellgren O, Pérez‐Tris J (2009) MalAvi: a public database of malaria parasites and related haemosporidians in avian hosts based on mitochondrial cytochrome b lineages. Molecular Ecology Resources, 9, 1353-1358. https://doi.org/10.1111/j.1755-0998.2009.02692.x

Cornet S, Nicot A, Rivero A, Gandon S (2014) Evolution of plastic transmission strategies in avian malaria. PLoS Pathogens, 10, e1004308. https://doi.org/10.1371/journal.ppat.1004308

Pigeault R, Cozzarolo CS, Wassef J, Gremion J, Bastardot M, Glaizot O, Christe P (2024) Spring reproductive success influences autumnal malarial load in a passerine bird. bioRxiv ver 3. Peer reviewed and recommended by Peer Community In Infections. https://doi.org/10.1101/2023.07.28.550923

Most plant-infecting viruses rely on an animal vector to be transmitted from one sessile host plant to another. A fascinating aspect of virus-vector interactions is the fact that viruses from different clades produce different proteins to bind vector receptors (1). Two major processes are described. In the “capsid strategy”, a motif of the capsid protein is directly binding to the vector receptor. In the “helper strategy”, a non-structural component, the helper component (HC), establishes a bridge between the virus particle and the vector’s receptor.   

In this exhaustive review focusing on hemipteran insect vectors, Di Mattia et al. (2) are revisiting the helper strategy in light of recent results. The authors first place the discoveries of the HC strategy in a historical context, suggesting that HC are exclusively found in non-circulative viruses (viruses that only attach to the vector). They present an overview of the nature and modes of action of helper components in the major virus clades of non-circulative viruses (Potyviruses and Caulimoviruses). Authors then detail recent advances, to which they have significantly contributed, showing that the helper strategy also appears widespread in circulative transmission categories (Tenuiviruses, Nanoviruses). 

In an extensive perspective section, they raise the question of the evolutionary significance of the existence of HC in numerous unrelated viruses, transmitted by unrelated vectors through different mechanisms. They explore the hypothesis that the helper strategy evolved several times independently in distinct viral clades and for different reasons. In particular, they present several potential benefits of plant virus HC related to virus cooperation, collective transmission and effector-driven infectivity.

As pointed out by both reviewers, this is a very clear and synthetic review. Di Mattia et al. present an exhaustive overview of virus HC-vector molecular interactions and address functionally and evolutionarily important questions. This review should benefit a large audience interested in host-virus interactions and transmission processes.

REFERENCES

(1) Ng JCK, Falk BW (2006) Virus-Vector Interactions Mediating Nonpersistent and Semipersistent Transmission of Plant Viruses. Annual Review of Phytopathology, 44, 183–212. https://doi.org/10.1146/annurev.phyto.44.070505.143325

(2) Di Mattia J, Zeddam J-L, Uzest M, Blanc S (2023) The helper strategy in vector-transmission of plant viruses. Zenodo, ver. 2 peer-reviewed and recommended by Peer Community In Infections. https://doi.org/10.5281/zenodo.7709290

Most infectious diseases of humans are zoonoses, and many of these come from particularly species diverse reservoir taxa, such as bats, birds, and rodents (1). Because of our changing landscape, there is increased exposure of humans to wildlife diseases reservoirs, yet we have little basic information about prevalence, hotspots, and transmission factors of most zoonotic pathogens. Viruses are particularly worrisome as a public health risk due to their fast mutation rates and well-known cross-species transmission abilities. There is a global push to better survey wildlife for viruses (2), but these studies are difficult, and the problem is vast. Astroviruses (AstVs) comprise a diverse family of ssRNA viruses known from mammals and birds. Astroviruses can cause gastroenteritis in humans and are more common in elderly and young children, but the relationship of human to non-human Astroviridae as well as transmission routes are unclear.  AstVs have been detected at high prevalence in bats in multiple studies (3,4), but it is unclear what factors, such as co-infecting viruses and bat reproductive phenology, influence viral shedding and prevalence.
In this recommended study, Vimbiso et al. (5) study the prevalence and diversity of astroviruses in different insectivorous and frugivorous chiropteran species roosting in trees, caves and building basements across Zimbabwe, a region never investigated for astroviruses. Using both pooled population samples and individual samples from 11 different sites, the authors screened for astrovirus prevalence via RT-PCR and identified bat taxa using mitochondrial gene sequencing. An overall prevalence of 10-14% infection was recorded. No clear association of increased astrovirus and coronavirus coinfection was detected, and although astrovirus infection varied over the season, it did not do so in consistent ways across the two primary sampling sites, Magweto and Chirundu. A phylogeny generated by sequencing all of the astrovirus positive samples showed evidence that most of the viral lineages are transmitting within species but across Zibabwae such that most phylogenetic lineages grouped viruses from the same host species together. However, there was ample evidence for interspecies transmission between bats. Finally, a small percentage of the total astrovirus diversity from Zibabwae clustered with sequences from humans. The timing and direction of the transmission between humans and bats need further investigation.
 
This study provides important baseline data about viral diversity and does an excellent job of capturing the spatial, temporal, host species, and sequence level dynamics of the astroviruses. There are clear limitations on how this study can be interpreted due to different sampling regimes and, in particular, the fact that each of the two primary sites was only explored for temporal variation over a single calendar year. That said, the grand diversity of astroviruses demonstrated in insectivorous bats in Zibabwae shows that we are only seeing the very tip of the iceberg with respect to viral diversity with zoonotic potential. As suggested by the reviewers, more studies like this are needed to understand the basic ecology of viruses and to aid in predicting epidemics.

References

1. Mollentze N, Streicker DG. Viral zoonotic risk is homogenous among taxonomic orders of mammalian and avian reservoir hosts. Proceedings of the National Academy of Sciences. 2020 Apr 28;117(17):9423-30. https://doi.org/10.1073/pnas.1919176117
2. Carroll D, Daszak P, Wolfe ND, Gao GF, Morel CM, Morzaria S, et al. The Global Virome Project. Science. 2018 Feb 23;359(6378):872-4. https://doi.org/10.1126/science.aap7463
3. Lee SY, Son KD, Yong-Sik K, Wang SJ, Kim YK, Jheong WH, et al. Genetic diversity and phylogenetic analysis of newly discovered bat astroviruses in Korea. Arch Virol. 2018;163(11):3065-72. https://doi.org/10.1007/s00705-018-3992-6
4. Seltmann A, Corman VM, Rasche A, Drosten C, Czirják GÁ, Bernard H, et al. Seasonal Fluctuations of Astrovirus, But Not Coronavirus Shedding in Bats Inhabiting Human-Modified Tropical Forests. EcoHealth. 2017 Jun 1;14(2):272-84. https://doi.org/10.1007/s10393-017-1245-x
5. Vimbiso C, Hélène DN, Malika A, Getrude M, Valérie P, Ngoni C, et al. Longitudinal Survey of Astrovirus infection in different bat species in Zimbabwe: Evidence of high genetic Astrovirus diversity. bioRxiv, 2023.04.14.536987, ver. 6 peer-reviewed and recommended by Peer Community In Infections. https://doi.org/10.1101/2023.04.14.536987

As recently reported by the world organisation for animal health, 60% of infectious diseases are zoonotic with a significant part associated to ticks. Ticks can transmit various pathogens such as bacteria, viruses and parasites. Among pathogens known to be transmitted by ticks, Ehrlichia ruminantium is an obligate intracellular Gram-negative bacterium responsible for the fatal heartwater disease of domestic and wild ruminants (Allsopp, 2010). E. ruminantium is transmitted by ticks of the genus Amblyomma in the tropical and sub-Saharan areas, as well as in the Caribbean islands. It constitutes a major threat for the American livestock industries since a suitable tick vector is already present in the American mainland and potential introduction of infected A. variegatum through migratory birds or uncontrolled movement of animals from Caribbean could occur (i.e. Deem, 1998 ; Kasari et al 2010). The disease is also a major obstacle to the introduction of animals from heartwater-free to heartwater-infected areas into sub-Saharan Africa and thus restrains breeding programs aiming at upgrading local stocks (Allsopp, 2010).

In this context, it is essential to develop control strategies against heartwater, as developing effective vaccines, for instance. Such an objective requires a better understanding of the early interaction of E. ruminantium and its host cells and of the mechanisms associated with bacterial adhesion to the host-cell. In this study, the authors. studied the role of E. ruminantium membrane protein ERGA_CDS_01230 in the adhesion process to host bovine aortic endothelial cells (BAEC).

After successfully producing the recombinant version of the protein, Pinarello et al (2022) followed the in vitro culture of E. ruminantium in BAEC and observed that the expression of the protein peaked at the extracellular infectious elementary body stages. This result would suggest the likely involvement of the protein in the early interaction of E. ruminantium with its host cells. The authors then showed using flow cytometry, and scanning electron microscopy, that beads coated with the recombinant protein adhered to BAEC. In addition, they also observed that the adhesion protein of E. ruminantium interacted with proteins of the cell's lysate, membrane and organelle fractions. Additionally, enzymatic treatment, degrading dermatan and chondroitin sulfates on the surface of BAEC, was associated with a 50% reduction in the number of bacteria in the host cell after a development cycle, indicating that glycosaminoglycans might play a role in the adhesion of E. ruminantium to the host-cell. Finally, the authors observed that the adhesion protein of E. ruminantium induced a humoral response in vaccinated animals, making this protein a possible vaccine candidate.

As rightly pointed out by both reviewers, the results of this study represent a significant advance (i) in the understanding of the role of the E. ruminantium membrane protein ERGA_CDS_01230 in the adhesion process to the host-cell and (ii) in the development of new control strategies against heartwater as this protein might potentially be used as an immunogen for the development of future vaccines.

References

Allsopp, B.A. (2010). Natural history of Ehrlichia ruminantium. Vet Parasitol 167, 123-135. https://doi.org/10.1016/j.vetpar.2009.09.014

Deem, S.L. (1998). A review of heartwater and the threat of introduction of Cowdria ruminantium and Amblyomma spp. ticks to the American mainland. J Zoo Wildl Med 29, 109-113.

Kasari, T.R. et al (2010). Recognition of the threat of Ehrlichia ruminantium infection in domestic and wild ruminants in the continental United States. J Am Vet Med Assoc. 237:520-30. https://doi.org/10.2460/javma.237.5.520

Pinarello V, Bencurova E, Marcelino I, Gros O, Puech C, Bhide M, Vachiery N, Meyer DF (2022) Ehrlichia ruminantium uses its transmembrane protein Ape to adhere to host bovine aortic endothelial cells. bioRxiv, 2021.06.15.447525, ver. 3 peer-reviewed and recommended by Peer Community in Infections. https://doi.org/10.1101/2021.06.15.447525

            Parasites represent the most diverse and adaptable ecological group of the biosphere (Timm & Clauson, 1988; De Meeûs et al., 1998; Poulin & Morand, 2000; De Meeûs & Renaud, 2002). The human species is known to considerably alter biodiversity, though it hosts, and thus sustains the maintenance of a spectacular diversity of parasites (179 species for eukaryotic species only) (De Meeûs et al., 2009). Among these, the five species of malaria agents (genus Plasmodium) remain a major public health issue around the world. Plasmodium falciparum is the most prevalent and lethal of these (Liu et al., 2010). With a pick of up to 2 million deaths due to malaria in 2004, deaths decreased to around 1 million in 2010 (Murray et al., 2012), to reach 619,000 in 2021, most of which in sub-Saharan Africa, and 79% of which were among children aged under 5 years (World Health Organization, 2022). 

            As stressed by Taconet et al. (2023), reduction in malaria deaths is attributable to control measures, in particular against its vectors (mosquitoes of the genus Anopheles). Nevertheless, the success of vector control is hampered by several factors (biological, environmental and socio-economic), and in particular by the great propensity of targeted mosquitoes to evolve physiological or behavioral avoidance of anti-vectorial measures.

            In their paper Taconet et al. (2023) aims at understanding what are the main factors that determine the evolution of insecticide resistance in several malaria vectors, in relation to the biological determinisms of behavioral resistance and how fast such evolutions take place. To tackle these objectives, authors collected an impressive amount of data in two rural areas of West Africa. With appropriate modeling, Taconet et al. discovered, among many other results, a predominant role of public health measures, as compared to agricultural practices, in the evolution of physiological resistance. They also found that mosquito foraging activities are mostly explained by host availability and climate, with a poor, if any, association with genetic markers of physiological resistance to insecticides. These findings represent an important contribution to the field and should help at designing more efficient control strategies against malaria.

References

De Meeûs T, Michalakis Y, Renaud F (1998) Santa Rosalia revisited: or why are there so many kinds of parasites in “the garden of earthly delights”? Parasitology Today, 14, 10–13. https://doi.org/10.1016/S0169-4758(97)01163-0

De Meeûs T, Prugnolle F, Agnew P (2009) Asexual reproduction in infectious diseases. In: Lost Sex: The Evolutionary Biology of Parthenogenesis (eds Schön I, Martens K, van Dijk P), pp. 517-533. Springer, NY. https://doi.org/10.1007/978-90-481-2770-2_24

De Meeûs T, Renaud F (2002) Parasites within the new phylogeny of eukaryotes. Trends in Parasitology, 18, 247–251. https://doi.org/10.1016/S1471-4922(02)02269-9

Liu W, Li Y, Learn GH, Rudicell RS, Robertson JD, Keele BF, Ndjango JB, Sanz CM, Morgan DB, Locatelli S, Gonder MK, Kranzusch PJ, Walsh PD, Delaporte E, Mpoudi-Ngole E, Georgiev AV, Muller MN, Shaw GM, Peeters M, Sharp PM, Rayner JC, Hahn BH (2010) Origin of the human malaria parasite Plasmodium falciparum in gorillas. Nature, 467, 420–425. https://doi.org/10.1038/nature09442

Murray CJ, Rosenfeld LC, Lim SS, Andrews KG, Foreman KJ, Haring D, Fullman N, Naghavi M, Lozano R, Lopez AD (2012) Global malaria mortality between 1980 and 2010: a systematic analysis. The Lancet, 379, 413–431. https://doi.org/10.1016/S0140-6736(12)60034-8

Poulin R, Morand S (2000) The diversity of parasites. Quarterly Review of Biology, 75, 277–293. https://doi.org/10.1086/393500

Taconet P, Soma DD, Zogo B, Mouline K, Simard F, Koffi AA, Dabire RK, Pennetier C, Moiroux N (2023) Physiological and behavioural resistance of malaria vectors in rural West-Africa : a data mining study to address their fine-scale spatiotemporal heterogeneity, drivers, and predictability. bioRxiv, ver. 4 peer-reviewed and recommended by Peer Community in Infections. https://doi.org/10.1101/2022.08.20.504631

Timm RM, Clauson BL (1988) Coevolution: Mammalia. In: 1988 McGraw-Hill yearbook of science & technology, pp. 212–214. McGraw-Hill Book Company, New York.

World Health Organization (2022) World malaria report 2022. Geneva: World Health Organization; 2022. Licence: CC BY-NC-SA 3.0 IGO. https://iris.who.int/bitstream/handle/10665/365169/9789240064898-eng.pdf?sequence=1.