Pathogenic Elizabethkingia miricola Infection in Cultured Black-Spotted Frogs, China, 2016.
The black-spotted frog, Pelophylax nigromaculatus, is a typical amphibian species, largely endemic to east Asia. Owing to the success of rearing it on an artificial diet, this frog has been widely farmed under special government approval as an edible animal in south-central China in recent years. In 2016, epidemic meningitis-like disease outbreaks in cultured black-spotted frogs occurred in separate farms. We identified E miricola as the predominant pathogen and used whole-genome sequencing (WGS) to further characterize this Asian epizootic isolate and phylogenetically compare it with the available typical Elizabethkingia genomes.
Since May 2016, many black-spotted frogs in farms in Hunan Province in south-central China have experienced an emerging, contagious disease characterized mainly by severe neurologic dysfunction. The first clinical sign is intermittent swimming in circles. Thereafter, the frogs develop signs of torticollis (Figure 1, panel A), disorientation (Video, https://wwwnc.cdc.gov/EID/article/23/12/17-0942-V1. htm), and anepithymia or meteorism (Figure 1, panel E). These signs are followed by cataracts (Figure 1, panel C); proptosis or hyperemia (Figure 1, panels B, D); agitation or lethargy; and, ultimately, death. The frogs are farmed in artificial ecologic wetlands or ponds with running water and shelter (online Technical Appendix Figure 1, https:// wwwnc.cdc.gov/EID/article/23/12/17-0942-Techapp1. pdf). Most ponds in 1 farm, which share a common water supply, were infected sequentially within a short time. More than 60% of the frogs in the infected farms had signs of varying appearance, and 60%-90% of the diseased frogs died in the next few days or weeks. The disease continued until hibernation and returned the following spring.
During July-October 2016, we collected 213 abnormal frogs from 7 separate farms in Hunan Province, China (online Technical Appendix Figure 2). Histopathologic examination showed severe meningitis with denatured, incrassate meninges. We observed inflammatory infiltrates, moderate multifocal gliosis, and perivascular cuffing in the cerebellum (online Technical Appendix Figure 3). Results of the diagnostic tests for Batrachochytrium dendrobatidis and ranaviruses were negative (Table 1). Although we observed Myxosporidia protozoa in the gallbladder and some protists in the intestine, they were not identified as the etiologic agents, considering the proportion of infection (online Technical Appendix Figure 4).
We confirmed bacterial infections in 190 (89.2%) of the 213 frogs; 90% were E. miricola according to the 16S rRNA gene sequence, which shared 99.36%-99.86% similarity with E. miricola DSM14571 (online Technical Appendix). We selected bacterial strain FL160902, isolated from frog no. 160, as the representative isolate and conducted experimental pathogenicity testing by various infection routes, including intramuscular injection, immersion infection, and cohabitation with infected frogs. All animal handling was done in compliance with the National Institutes of Health protocols (online Technical Appendix). After 2 weeks of observations (Table 2), we found that the cumulative mortality (10%70%) increased with dose in the injection trial and that 100% of frogs exposed to E. miricola by immersion died. In the cohabitation studies, 30% mortality was recorded, indicating cross-infection. Koch's postulates were satisfied by identification of isolates from dead frogs as E. miricola, identical to FL160902.
To characterize E. miricola FL160902, we conducted WGS with the Illumina HiSeq 2500 platform (Illumina Inc., San Diego, CA, USA), producing 2 x 150-bp paired-end reads. We assembled the trimmed reads using SOAPdenovo (http://soap.genomics.org.cn/soapdenovo.html). We constructed a phylogenetic tree (Figure 2) of orthologous genes using RAxML (9) with 100 bootstrap replicates to examine the evolutionary relatedness between E. miricola FL160902 (GenBank accession no. NHPR00000000) and other Elizabethkingia genomes. The results showed that FL160902 was most closely related to CSID_3000517120, a clinical isolate of E. miricola from the United States sequenced by the Centers for Disease Control and Prevention (CDC) (10), revealing the potential of E. miricola FL160902 for pathogenicity in humans.
Before WGS was commonly used, E. meningoseptica (previously Flavobacterium meningosepticum) was found to be separated into 2 main hybridization groups, UBI and UBII, that were [approximately equal to]40%-55% interrelated; UBII could be further divided into 4 subgroups (11,12). However, because the isolates from different groups are phenotypically very similar, these genomic groups remain assigned at this time to E. meningoseptica (13). In our phylogenetic tree, UBI group E. meningoseptica isolates did not group with the other Elizabethkingia spp. and were distantly related to UBII. Considering the low DNA-DNA relatedness (<70%) between the 2 groups and phylogenomic analysis based on WGS (3,11,12), we propose that UBII are not E. meningoseptica. The UBII subgroups branching separately supports the view that they ate different Elizabethkingia species (3). The UBII:1 group species E. anophelis and E. endophytica formed a clade with strong support of 100%, favoring the suggestion that E. endophytica is a later subjective synonym of E. anophelis (14). Our FL[60902 isolate grouped with E. miricola, which is thought to be closely related to UBII:2 (3,10). The taxonomic status o fE. miricola ATCC 33958 and BM I 0 should be reconsidered be cause they clustered with UBII:3 and not with UBII:2 E miricola species. Our resuhs agree with Eriksen's conclusion about the genetic diversity in Elizabethkingia; a more comprehensive taxonomic system is needed to clarify the Elizabethkingia genus (3).
In this natural outbreak of meningitis-like disease in cultured frogs in Hunan Province, China, in 2016, E miricola was the most predominant pathogen. The neurologic signs and pathologic brain lesions suggested that E. miricola could break through the blood-brain barrier and damage the nervous system. The etiologic analyses combined with the results of experimental challenge support the conclusion that the E. miricola strain represented by isolate FL160902 is highly contagious for frogs, especially by immersion infection. We suspect that contaminated water is the primary vehicle of transmission, considering the infection assay and the epidemiology in 1 farm with different ponds. However, diverse transmission routes might be involved because there is no obvious interconnectivity among independent farms, which needs to be investigated further. Close attention should be paid to whether this disease affects the wild population of amphibians. Our results indicated the gradual expansion of its host and suggest that amphibians may serve as a reservoir for infection in humans. Black-spotted frog farming is a major aquacuhure industry in south-central China; thus. animals and humans that have close contact with infected frogs should be continually monitored for emerging E. miricola infections, even though no human E. miricola infection cases were reported related to frog consumption of farming in Hunan in 2016. Our results demonstrate a contagious disease in frogs caused by E. miricola that poses a potential zoonotic threat to humans, generating a need for consideration of the role of Elizabethkingia bacteria in public health.
We thank Junguang Ruan and the frog farmers for their cooperation with the sample collection. We also thank Dandan Zhao for parasite identification and Yang Liu for the excellent bioinformatics technical assistance.
This study was supported by China Agriculture Research System CARS-46. Hubei Agricultural Science and Technology Innovation Center (2016-620-007-001), Featuring Talents Cultivation Project (4006-4611300108), and Pre-research Foundation of Huazhong Agricultuml U niversiiy (52209-814058).
Ms. Hu is a PhD candidate at Huazhong Agricultucal University, Wuhan. China. Her primary research interest is pathogenic microbiology and infectious diseases of amphibians.
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Address for correspondence: Zemao Gu, Huazhong Agricultural University-College of Fisheries, No. 1 Shizi Mountain St, Hongshan District Wuhan, Hubei 430070, China; email: email@example.com
Ruixue Hu, Junfa Yuan, Yin Meng, Zhe Wang, Zemao Gu
Author affiliations: Huazhong Agricultural University, Wuhan, China (R. Hu, J. Yuan, Y Meng, Z. Wang, Z. Gu); Hubei Engineering Research Center for Aquatic Animal Diseases Control and Prevention, Wuhan (R. Hu, J. Yuan, Z. Gu)
Caption: Figure 1. Clinical signs (arrows) in frogs with Elizabethkingia miricola infection in Hunan Province, China. A) Diseased frogs had neurologic signs of torticollis. B-D) Clinical signs with different appearances showing cataracts, proptosis, or hyperemia. E) Symptoms of abdominal distension. Scale bars indicate 1 cm.
Caption: Figure 2. Maximum-likelihood phylogenetic tree of Elizabethkingia miricola FL160902 from an infected frog in Hunan Province, China, and reference genomes. The tree was constructed by using the single-copy orthologous genes of all the 38 genomes with 100 bootstrap replicates. Species identifications strictly followed the National Center for Biotechnology Information submitted names. Isolates assigned into UB groups and subgroups are according to Holmes et al. (12) and Bruun and Ursing (13).Solid circles indicate type strains; open circle indicates a former type strain. Bold indicates strain isolated in this study. Scale bar indicates nucleotide substitutions per site.
Table 1. Results from etiologic detection in 213 frogs collected in Hunan, China, July-October 2016 * Tested organ Pathogen Skin Liver Spleen Kidney Bacteria NT + + + Parasite ([double dagger]) - - - - Fungus ([section]) - NT NT NT Ranaviruses NT NT - - Tested organ Pathogen Brain Intestine Muscle Bacteria + NT NT Parasite ([double dagger]) - - - Fungus ([section]) NT NT NT Ranaviruses NT NT - Tested organ Pathogen Gallbladder Heart Bacteria NT NT Parasite ([double dagger]) + - Fungus ([section]) NT NT Ranaviruses NT NT No. Pathogen positive Bacteria 190 ([dagger]) Parasite ([double dagger]) 9 Fungus ([section]) 0 Ranaviruses 0 * NT, not tested; +, positive; -, negative. ([dagger]) Predominant bacterial infection. The results were considered positive if any one of the tested organs was positive. ([double dagger]) Class Myxosporea. ([section]) Batrachochytrium dendrobatidis. Table 2. Results of the experimental exposure of frogs to Elizabethkingia miricola isolate FL160902, China, 2016 * Route of infection Concentration, No. frogs CFU/mL per trial Intramuscular injection 105 10 ([double dagger]) 106 10 107 10 SPSS ([section]) 10 Immersion inoculation 106 10 ([paragraph]) Cohabitation inoculation (#) NA 10 Control NA 10 Cumulative no. deaths, by days after exposure ([dagger]) Route of infection 2 4 6 8 10 12 14 Intramuscular injection 0 1 1 1 1 1 1 ([double dagger]) 0 0 1 1 5 5 5 1 3 6 7 7 7 7 0 0 0 0 0 0 0 Immersion inoculation 7 10 10 10 10 10 ([paragraph]) 3 Cohabitation inoculation (#) 0 0 1 3 3 3 3 Control 0 0 0 0 0 0 0 Route of infection Mortality, % Intramuscular injection 10 ([double dagger]) 50 70 0 Immersion inoculation 100 ([paragraph]) Cohabitation inoculation (#) 30 Control 0 * NA, not applicable. ([dagger]) Deaths after 14 d were not included. ([double dagger]) Injection volume 200 [micro]L. ([section]) An equivalent volume injection of 0.70% stroke-physiologic saline solution. ([paragraph]) Immersed for 30 min in E. miricola suspension. (#) Frogs in this trial cohabited with frogs previously infected with E. miricola.
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|Author:||Hu, Ruixue; Yuan, Junfa; Meng, Yin; Wang, Zhe; Gu, Zemao|
|Publication:||Emerging Infectious Diseases|
|Date:||Dec 1, 2017|
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