Immunization with Usutu virus and with a chimeric West Nile virus (WNV) harboring Usutu-E protein protects immunocompetent adult mice against lethal challenges with different WNV lineage 1 and 2 strains

West Nile virus (WNV) and Usutu virus (USUV), two antigenically related flaviviruses co-circulating in Europe, can cause severe neurological disease in animals and humans. The immune response against USUV and WNV and their immunopathogenesis are still poorly investigated. Here we present results upon sequential infections of adult immunocompetent CD-1 and BALB/c mice primed with two different doses (high dose, HD or low dose, LD) of an USUV isolate and challenged with HD or LD of three different WNV isolates. CD-1 and BALB/c LD USUV-primed mice, regardless of the dose, are largely protected from lethal WNV challenges despite showing no detectable neutralizing antibodies. Furthermore, mice immunized with a chimeric virus harboring the E protein of USUV within the WNV backbone (WNVE-USUV) are protected against a lethal challenge with WNV. We believe these findings could contribute to understanding the dynamics of the interaction during sequential infection of these two flaviviruses.

Synergistic pathogenicity by coinfection and sequential infection with JXA1-like HP-PRRSV and PCV2d in PCV2 antibody-positive post-weaned pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) and Porcine circovirus (PCV) are two important pathogens, which caused respiratory disease in pigs. PRRSV and PCV2 had caused great economic losses to the pig industry. Pigs coinfection with PCV2 and PRRSV were common in the clinic, PCV2 antibodies can be detected in most of the pigs. PCV2d and HP-PRRSV(JXA1-like) were two major viruses circulating in the pigs in China. In this study, HP-PRRSV (JXA1-like) and PCV2d were used to coinfect and (or) sequential infect 5-week-old weaned PCV2-antibody positive pigs and the clinical indications, pathological, virus load, and specific antibodies of the challenged post-weaned piglets were evaluated. Thirty 5-week-old post-weaned pigs were divided into six groups infected with PBS, PCV2, PRRSV, PCV2-PRRSV, PRRSV-PCV2, and Co-PRRSV-PCV2 according to the PCV2 specific antibodies. Pigs infected with PRRSV can experience diarrhea, increased body temperature, weight loss, and even death. The pigs in PRRSV and PRRSV-PCV2 infected groups showed severe clinical symptoms, high mortality, and low average daily gain. The main pathological changes were widening of the lung interstitium, lung adhesion, and so on. The PRRSV-PCV2-infected group showed high levels of TNF-α and IL-2. In conclusion, PRRSV and PRRSV-PCV2 sequential infected pigs showed most pathogenic signs, and PCV2-PRRSV sequential infected pigs showed less pathogenicity than pigs of PCV2 and PRRSV coinfection and PRRSV monoinfection from day 10-14, partially suppressing the cytokine storm produced by PRRSV.

Vector competence of Malaysian Aedes aegypti to Zika virus and impact of sequential arbovirus infections

Zika virus (ZIKV) is a mosquito-borne flavivirus with global impact since 2015. Although ZIKV was first isolated from Aedes aegypti in Malaysia in 1965, not much is known about the competency of Malaysian Ae. aegypti to ZIKV. To date only 9 cases of ZIKV have been reported in Malaysia despite the abundance of mosquito vectors. This study aimed to determine the susceptibility of Ae. aegypti to ZIKV, and the impact of sequential infections in Ae. aegypti mosquitoes with DENV serotype 2 (DENV-2) followed by ZIKV. Field-caught urban Ae. aegypti were orally challenged with a Martinique strain of ZIKV, and midgut, head/thorax and saliva were collected at 3, 7 and 14 days post-infection (dpi). At 14 dpi, ZIKV-exposed mosquitoes had infection and dissemination rates of 59% (n=10/17) and 90% (n=9/10), respectively. Average titres of 3.9 and 4.4 log pfu infectious ZIKV were recovered in midgut and head/thorax, respectively. In sequential infection, prior exposure of Ae. aegypti to DENV did not affect the subsequent ZIKV infection in head/thorax albeit with a low sample size. In conclusion, Malaysian urban Ae. aegypti is susceptible to the contemporary Asian lineage of ZIKV. The established and continuous DENV circulation in Ae. aegypti did not suppress ZIKV emergence in Malaysia. Other factors contributing to low level of ZIKV circulation in Malaysia remain to be explored.

Dynamics of gamete production and mating in the parasitic protist Trypanosoma brucei

BACKGROUND

Sexual reproduction in Plasmodium falciparum and Trypanosoma brucei occurs in the insect vector and is important in generating hybrid strains with different combinations of parental characteristics. Production of hybrid parasite genotypes depends on the likelihood of co-infection of the vector with multiple strains. In mosquitoes, existing infection with Plasmodium facilitates the establishment of a second infection, although the asynchronicity of gamete production subsequently prevents mating. In the trypanosome/tsetse system, flies become increasingly refractory to infection as they age, so the likelihood of a fly acquiring a second infection also decreases. This effectively restricts opportunities for trypanosome mating to co-infections picked up by the fly on its first feed, unless an existing infection increases the chance of successful second infection as in the Plasmodium/mosquito system.

RESULTS

Using green and red fluorescent trypanosomes, we compared the rates of trypanosome infection and hybrid production in flies co-infected on the first feed, co-infected on a subsequent feed 18 days after emergence, or fed sequentially with each trypanosome clone 18 days apart. Infection rates were highest in the midguts and salivary glands (SG) of flies that received both trypanosome clones in their first feed, and were halved when the infected feed was delayed to day 18. In flies fed the two trypanosome clones sequentially, the second clone often failed to establish a midgut infection and consequently was not present in the SG. Nevertheless, hybrids were recovered from all three groups of infected flies. Meiotic stages and gametes were produced continuously from day 11 to 42 after the infective feed, and in sequentially infected flies, the co-occurrence of gametes led to hybrid formation.

CONCLUSIONS

We found that a second trypanosome strain can establish infection in the tsetse SG 18 days after the first infected feed, with co-mingling of gametes and production of trypanosome hybrids. Establishment of the second strain was severely compromised by the strong immune response of the fly to the existing infection. Although sequential infection provides an opportunity for trypanosome mating, the easiest way for a tsetse fly to acquire a mixed infection is by feeding on a co-infected host.