Infectious Diseases

Citrus Huanglongbing (HLB), also called citrus greening disease, is a highly destructive disease threatening citrus production worldwide. “ Candidatus Liberibacter asiaticus” is one of the most common putative causal agents of HLB. Phages of “ Ca . Liberibacter asiaticus”

The Asian citrus psyllid (ACP) is an important vector of the HLB pathogen, which is a major threat to citrus production around the world. Bacterial communities harbored by insects could be affected by different environmental factors.

Although the familyHalieaceae(OM60/NOR5 clade) is an abundant and cosmopolitan clade widely found in coastal seas and involved in interactions with phytoplankton, a limited number of cultured isolates are available. In this study, we isolated six pure culturedHalieaceaestrains from coastal seawaters and performed a comparative physiological and genomic analysis to give insights into the phylogeny and metabolic potential of this family.

Diaphorina citri Kuwayama (D. citri) is an insect vector of phloem-limited ‘Candidatus Liberibacter asiatus’ (CLas), the presumed pathogen of citrus Huanglongbing (HLB). Recently, our lab has preliminarily found it acquired and transmitted Citrus tristeza virus (CTV), which was previously suggested to be vectored by species of aphids. However, the influences of one of the pathogens on the acquisition and transmission efficiency of the other pathogen remain unknown. In this study, CLas and CTV acquisition and transmission by D. citri at different development stages under field and laboratory conditions were determined. CTV could be detected from the nymphs, adults, and honeydew of D. citri but not from the eggs and exuviates of them. CLas in plants might inhibit CTV acquisition by D. citri as lower CTV–positive rates and CTV titers were detected in D. citri collected from HLB-affected trees compared to those from CLas–free trees. D. citri were more likely to obtain CTV than CLas from host plants co-infected with the two pathogens. Intriguingly, CTV in D. citri facilitated the acquisition and transmission of CLas, but CLas carried by D. citri had no significant effect on the transmission of CTV by the same vector. Molecular detection and microscopy methods confirmed the enrichment of CTV in the midgut after a 72-h acquisition access period. Collectively, these results raise essential scientific questions for further research on the molecular mechanism of pathogen transmission by D. citri and provide new ideas for the comprehensive prevention and control of HLB and CTV.

Spotted fever illness caused by the tick-borne pathogen Rickettsia parkeri has emerged in the Pampa biome in southern Brazil, where the tick Amblyomma tigrinum is implicated as the main vector. Because domestic dogs are commonly parasitized by A. tigrinum, this canid is also a suitable sentinel for R. parkeri-associated spotted fever. Herein, we investigate rickettsial infection in ticks, domestic dogs and small mammals in a natural reserve of the Pampa biome in southern Brazil. The ticks A. tigrinum, Amblyomma aureolatum and Rhipicephalus sanguineus were collected from dogs. Molecular analyses of ticks did not detect R. parkeri; however, at least 34% (21/61) of the A. tigrinum ticks were infected by the non-pathogenic agent ‘Candidatus Rickettsia andeanae’. Serological analyses revealed that only 14% and 3% of 36 dogs and 34 small mammals, respectively, were exposed to rickettsial antigens. These results indicate that the study area is not endemic for R. parkeri rickettsiosis. We tabulated 10 studies that reported rickettsial infection in A. tigrinum populations from South America. There was a strong negative correlation between the infection rates by R. parkeri and ‘Candidatus R. andeanae’ in A. tigrinum populations. We propose that high infection rates by ‘Candidatus R. andeanae’ might promote the exclusion of R. parkeri from A. tigrinum populations. The mechanisms for such exclusion are yet to be elucidated.