Multiple vector species may be responsible for transmission of Saint Louis Encephalatis Virus in Argentina

Anna Cohuet

A recommendation of:
Beranek MD, Quaglia AI, Peralta GC, Flores FS, Stein M, Diaz LA, Almirón WR and Contigiani MS. Culex saltanensis and Culex interfor (Diptera: Culicidae) are susceptible and competent to transmit St. Louis encephalitis virus (Flavivirus: Flaviviridae) in central Argentina (2020), bioRxiv, 722579, ver. 6 peer-reviewed by Peer Community in Entomology. 10.1101/722579
Submitted: 03 August 2019, Recommended: 09 January 2020
Cite this recommendation as:
Anna Cohuet (2020) Multiple vector species may be responsible for transmission of Saint Louis Encephalatis Virus in Argentina. Peer Community in Entomology, 100002. 10.24072/pci.zool.100002

Medical and veterinary entomology is a discipline that deals with the role of insects on human and animal health. A primary objective is the identification of vectors that transmit pathogens. This is the aim of Beranek and co-authors in their study [1]. They focus on mosquito vector species responsible for transmission of St. Louis encephalitis virus (SLEV), an arbovirus that circulates in avian species but can incidentally occur in dead end mammal hosts such as humans, inducing symptoms and sometimes fatalities. Culex pipiens quinquefasciatus is known as the most common vector, but other species are suspected to also participate in transmission. Among them Culex saltanensis and Culex interfor have been found to be infected by the virus in the context of outbreaks. The fact that field collected mosquitoes carry virus particles is not evidence for their vector competence: indeed to be a competent vector, the mosquito must not only carry the virus, but also the virus must be able to replicate within the vector, overcome multiple barriers (until the salivary glands) and be present at sufficient titre within the saliva. This paper describes the experiments implemented to evaluate the vector competence of Cx. saltanensis and Cx. interfor from ingestion of SLEV to release within the saliva. Females emerged from field-collected eggs of Cx. pipiens quinquefasciatus, Cx. saltanensis and Cx. interfor were allowed to feed on SLEV infected chicks and viral development was measured by using (i) the infection rate (presence/absence of virus in the mosquito abdomen), (ii) the dissemination rate (presence/absence of virus in mosquito legs), and (iii) the transmission rate (presence/absence of virus in mosquito saliva). The sample size for each species is limited because of difficulties for collecting, feeding and maintaining large numbers of individuals from field populations, however the results are sufficient to show that this strain of SLEV is able to disseminate and be expelled in the saliva of mosquitoes of the three species at similar viral loads. This work therefore provides evidence that Cx saltanensis and Cx interfor are competent species for SLEV to complete its life-cycle. Vector competence does not directly correlate with the ability to transmit in real life as the actual vectorial capacity also depends on the contact between the infectious vertebrate hosts, the mosquito life expectancy and the extrinsic incubation period of the viruses. The present study does not deal with these characteristics, which remain to be investigated to complete the picture of the role of Cx saltanensis and Cx interfor in SLEV transmission. However, this study provides proof of principle that that SLEV can complete it’s life-cycle in Cx saltanensis and Cx interfor. Combined with previous knowledge on their feeding preference, this highlights their potential role as bridge vectors between birds and mammals. These results have important implications for epidemiological forecasting and disease management. Public health strategies should consider the diversity of vectors in surveillance and control of SLEV.

References
[1] Beranek, M. D., Quaglia, A. I., Peralta, G. C., Flores, F. S., Stein, M., Diaz, L. A., Almirón, W. R. and Montigiani, M. S. (2020). Culex saltanensis and Culex interfor (Diptera: Culicidae) are susceptible and competent to transmit St. Louis encephalitis virus (Flavivirus: Flaviviridae) in central Argentina. bioRxiv 722579, ver. 6 peer-reviewed and recommended by PCI Entomology. doi: 10.1101/722579


Revision round #3

2019-12-18

Dear authors
I ask for only minor corrections before I'll write the recommendation

Line 31 : change « objetive » to « objective »
Line 32 : remove “a” before Cx
Line 38 : change “acquire” to “acquired”
Line 39 : change “Culex saltanensis » to « Cx. Saltanensis » because already cited above
Lines 41-42 : change “Culex saltanensis and Culex interfor” to “Cx. saltanensis and Cx. interfor”
Line 51 : rephrase “Due to the hematophagous habit of females, many mosquito species are vectors of infectious…” (add “many” because not all species are vectors, remove “competent” because trophic behaviour is a parameter of vectorial capacity not of vector competence”
Line 54 : replace “one” by “few” because the cited virus can be transmitted not only by one species
Line 56 : add “are” in “arboviruses are generalist”
Line 86 : replace “competence” by “capacity”
Line 90 : add “ infected “ Culex p. quinquefasciatus if this actually what is meant
Line 210 : Table 2 instead of Table 3
Line 470 : rephrase the legend and explain better what is presented : p values , test and what is compared. Reverse the order of the columns so that they are ranked in the same order than the lines.


Revision round #2

2019-11-14

Dear Dr Beranek and co-authors I acknowledge the improvement of the revised manuscript according to the suggestions of the reviewers. However, I notice that the main comment from the first reviewer was not fully considered. Indeed, the first reviewer highlighted weakness in the statistical analyses. Each of the mosquito species was fed on a different chick individual for exposure to virus. This means that mosquito species and viremia are confounded and interpreting the levels of infection/dissemination/transmission between species as differences of vector competences does not make sense. Only the ratio between the 3 steps (infection/dissemination/transmission) could be compared. However, the sample size being very limited I do not recommend it. The manuscript reports for the first time experimental observations of the susceptibility of Culex interfor and Culex saltanensis for SLEV, from ingestion to release of viral particles in the saliva. This deserves attention by itself. Comparing the vectorial competences between species is not feasible with the current data, which is fine. I therefore strongly recommend to present the data without extrapolation on competence levels. The GLM analysis should be removed and the text and figure changed accordingly. Also, I strongly recommend the revised version to be corrected by an native English speaker. The new version of the discussion includes very long sentences; some of them are difficult to understand. Please make sure that the row data are available to readers through an open data repository


Revision round #1

2019-09-17

The reviewers highlighted the interest of the manuscript but also pointed out some limits that need to be considered/corrected before recommendation can be considered. I invite the authors to revise the manucript accordingly and to re-submit.