Increased dietary protein-to-lipid ratio improves survival during naturally occurring pancreas disease in Atlantic salmon, Salmo salar L

This study demonstrated that increased dietary protein-to-lipid ratio (P/L-ratio) improved survival of farmed Atlantic salmon naturally affected by pancreas disease (PD). In addition to diet, body weight (BW) and delousing mortality prior to the PD outbreak also contributed significantly (p < 0.05) to explain the observed variation in PD-associated mortality. Subsequent to the PD outbreak, large amount of fish failed to grow and caused thin fish with poor condition (runts). At the end of the trial, significantly (p < 0.05) lower amounts of runt fish and increased amount of superior graded fish where detected among fish fed increased P/L-ratio and within the fish with the largest BWs prior to PD. Diet, BW and delousing mortality contributed significantly (p < 0.05) to explain the variation in the amount of superior graded fish, whereas BW and diet explained the variation in the amount of runt fish. A significant (p < 0.01) negative linear relationship was observed between the amount of superior graded fish and the total mortality, whereas a positive linear relationship was detected between percentage of fillets with melanin and the total mortality. Thus, increased dietary P/L-ratio seem to reduce the mortality and impaired slaughter quality associated with PD.

Age has a role in driving host immunopathological response to alphavirus infection

Alphaviruses are a group of arthropod-borne pathogens capable of causing a wide spectrum of clinical symptoms, ranging from milder symptoms like rashes, fever and polyarthralgia, to life-threatening encephalitis. This genus of viruses is prevalent globally, and can infect patients across a wide age range. Interestingly, disease severity of virus-infected patients is wide-ranging. Definitions of the pathogenesis of alphaviruses, as well as the host factors influencing disease severity, remain limited. The innate and adaptive immune systems are important host defences against alphavirus infections. Several reports have highlighted the roles of specific immune subsets in contributing to the immune pathogenesis of these viruses. However, immunosenescence, a gradual deterioration of the immune system brought about by the natural advancement of age, affects the functional roles of these immune subsets. This phenomenon compromises the host's ability to defend against alphavirus infection and pathogenesis. In addition, the lack of maturity in the immune system in newborns and infants also results in more severe disease outcomes. In this review, we will summarize the subtle yet diverse physiological changes in the immune system during aging, and how these changes underlie the differences in disease severity for common alphaviruses.

Mortality and weight loss of Atlantic salmon, Salmon salar L., experimentally infected with salmonid alphavirus subtype 2 and subtype 3 isolates from Norway

Pancreas disease (PD) caused by salmonid alphavirus (SAV) has a significant negative economic impact in the salmonid fish farming industry in northern Europe. Until recently, only SAV subtype 3 was present in Norwegian fish farms. However, in 2011, a marine SAV 2 subtype was detected in a fish farm outside the PD-endemic zone. This subtype has spread rapidly among fish farms in mid-Norway. The PD mortality in several farms has been lower than expected, although high mortality has also been reported. In this situation, the industry and the authorities needed scientific-based information about the virulence of the marine SAV 2 strain in Norway to decide how to handle this new situation. Atlantic salmon post-smolts were experimentally infected with SAV 2 and SAV 3 strains from six different PD cases in Norway. SAV 3-infected fish showed higher mortality than SAV 2-infected fish. Among the SAV 3 isolates, two isolates gave higher mortality than the third one. At the end of the experiment, fish in all SAV-infected groups had significantly lower weight than the uninfected control fish. This is the first published paper on PD to document that waterborne infection produced significantly higher mortality than intraperitoneal injection.

Clinical manifestations of pancreas disease outbreaks in Norwegian marine salmon farming - variations due to salmonid alphavirus subtype

Pancreas disease (PD) in Norwegian salmonid aquaculture has traditionally been caused by salmonid alphavirus (SAV) subtype 3. Following the isolation of a novel SAV subtype in 2010, marine SAV2, two separate endemic areas have developed. It has been debated whether disease outbreaks due to marine SAV2 result in milder clinical manifestations compared to outbreaks caused by SAV3. The aim of this study was to descriptively investigate site-level differences in the clinical manifestations of marine SAV2 and SAV3 at Norwegian seawater sites diagnosed with PD in 2012. The findings suggest that Norwegian PD outbreaks caused by marine SAV2 result in lower mortality and milder clinical signs compared to outbreaks caused by SAV3. For sites without reported PD-related mortality, there was no difference in the mortality levels between sites infected by marine SAV2 and SAV3. The results also indicate that there are no differences in grading quality at slaughter between the SAV subtypes.

An experimental means of transmitting pancreas disease in Atlantic salmon Salmo salar L. fry in freshwater

A challenge model for pancreas disease in Atlantic salmon, Salmo salar L. fry, was developed comparing two salmonid alphavirus (SAV) subtypes: SAV1 and SAV5. Viral doses of 3 × 10(5) TCID50  mL(-1) for SAV1 and 3 × 10(4) for SAV5 were tested in triplicate tanks, each containing 450 salmon fry. Cumulative mortalities of 1.2% were recorded. Titres of virus recovered from the mortalities ranged from 10(2) to 10(7) TCID50  mL(-1) . Fry were sampled at 3, 5 and 7.5 weeks post-challenge. Sampling after 3 weeks revealed a high prevalence of infection in the absence of clinical signs, and infectious virus was recovered from 80% and 43% of sampled fry infected with SAV1 and SAV5, respectively. After 5 weeks pancreas, heart and red skeletal muscle lesions were generally observed, whilst degeneration in white skeletal muscle was observed only in fish infected with SAV1. In situ hybridisation confirmed the presence of viral genome in infected pancreas, heart and muscle. After 7.5 weeks, infectious virus (both isolates) was recovered from 13.3% of the fish sampled, with a viral titre of 10(2) TCID50  mL(-1) . Clearly, salmon fry are susceptible to SAV infection and pancreas disease.

Mortality related to pancreas disease in Norwegian farmed salmonid fish, Salmo salar L. and Oncorhynchus mykiss (Walbaum)

High mortality during the salmonid seawater phase is a continuous problem for the salmonid aquaculture industry, although the mortality levels show a large variation both in farms with a disease history and those without. We wanted to examine the mortality patterns in farms with pancreas disease (PD) and compare it to farms without this diagnosis. Further, we wished to investigate the factors influencing the maximum mortality in both groups. We examined data from all salmonid farms in Norway stocked after January 2003 and slaughtered before December 2007. In total, 1884 cohorts were included, and 150 of these were diagnosed with PD. We found that season accounted for more of the variation in mortality than water temperature in PD-positive cohorts and that infection pressure influenced the mortality in non-PD cohorts, suggesting outbreaks of disease that are not diagnosed. We also found that the mortality in PD cohorts decreased significantly from 2003 to 2007, suggesting that increased knowledge about PD and targeted actions have been effective. Our study further suggests that crude mortality figures may be of limited use when wanting to examine a particular disease and risk factors for increased mortality. We suggest farmers and legislation should turn to a more modern approach with cause-specific mortality records.

[Severe forms of chikungunya virus infection in a pediatric intensive care unit on Reunion Island]

In 2005-2006, an unexpected, massive outbreak of chikungunya occurred on Reunion Island, a French overseas territory in the Indian Ocean. This arboviral infection transmitted by a mosquito of the Aedes genus is usually benign. A surprising feature of the Reunion Island epidemic was the occurrence of rare severe forms involving adults as well as children.

OBJECTIVES

The purpose of this report is to describe severe forms of chikungunya observed in children hospitalized in a pediatric intensive care unit.

PATIENTS AND METHODS

This retrospective single-center study was conducted from January 1st to April 30th, 2006. Children between 1 month and 15 years admitted to the pediatric intensive care unit with proven chikungunya infection were included.

RESULTS

A total of 9 children were included. The main manifestations were extensive skin blisters in 5 cases, neurological symptoms (encephalopathy) in 4, cardiac complications (myocarditis, hemodynamic disorders) in 5 and bleeding in 1. Two children died. The causes of death were circulatory failure associated with coma and massive hemorrhage in one case and post-infectious encephalitis in the other. Three survivors present long-term neurologic or dermatologic sequels.

DISCUSSION

Severe cases of chikungunya in children provide a stark reminder of the cardiac and neurological tropism of the virus and its hemorrhagic forms with high potential mortality and morbidity. These cases underline the need for personal protection measures and for research to develop specific antiviral therapy and vaccines to prevent potentially lethal forms of the disease.

[Chikungunya outbreak on Reunion Island in 2005/2006: role of hospital physicians in raising alert]

The epidemic of chikungunya (CHIK) that swept through Reunion Island from late 2005 to mid 2006 affected 38.2% of the population, i.e., 300000 people. Although this outbreak took place in a French overseas department with high public health standards, failure to anticipate a large-scale epidemic associated with unprecedented severity and unexpectedly high mortality led to a major public health crisis. The purpose of this report is to provide a complete account of the experience of hospital intensive care physicians in addressing problems ranging from discovery of severe forms to management of a major health crisis. This report underlines the role of the head hospital physician and the necessity of mutual trust and collaboration with supervisory authorities.

Evaluation of an inactivated Ross River virus vaccine in active and passive mouse immunization models and establishment of a correlate of protection

Ross River Virus has caused reported outbreaks of epidemic polyarthritis, a chronic debilitating disease associated with significant long-term morbidity in Australia and the Pacific region since the 1920s. To address this public health concern, a formalin- and UV-inactivated whole virus vaccine grown in animal protein-free cell culture was developed and tested in preclinical studies to evaluate immunogenicity and efficacy in animal models. After active immunizations, the vaccine dose-dependently induced antibodies and protected adult mice from viremia and interferon α/β receptor knock-out (IFN-α/βR(-/-)) mice from death and disease. In passive transfer studies, administration of human vaccinee sera followed by RRV challenge protected adult mice from viremia and young mice from development of arthritic signs similar to human RRV-induced disease. Based on the good correlation between antibody titers in human sera and protection of animals, a correlate of protection was defined. This is of particular importance for the evaluation of the vaccine because of the comparatively low annual incidence of RRV disease, which renders a classical efficacy trial impractical. Antibody-dependent enhancement of infection, did not occur in mice even at low to undetectable concentrations of vaccine-induced antibodies. Also, RRV vaccine-induced antibodies were partially cross-protective against infection with a related alphavirus, Chikungunya virus, and did not enhance infection. Based on these findings, the inactivated RRV vaccine is expected to be efficacious and protect humans from RRV disease.

Salmonid alphavirus (SAV) and pancreas disease (PD) in Atlantic salmon, Salmo salar L., in freshwater and seawater sites in Norway from 2006 to 2008

A cohort study was initiated in the spring of 2006 to investigate epidemiological aspects and pathogenesis of salmonid alphavirus (SAV) subtype 3 infections and pancreas disease (PD). The aims were to assess involvement of the freshwater production phase, the extent and frequency of subclinical infections and to follow PD-affected populations throughout the entire seawater production cycle, as well as investigate possible risk factors for PD outbreaks. Fish groups from 46 different Atlantic salmon freshwater sites in six counties were sampled once prior to seawater transfer and followed onto their seawater sites. A total of 51 Atlantic salmon seawater sites were included, and fish groups were sampled three times during the seawater production phase. SAV subtype 3 was not identified by real-time RT-PCR from samples collected in the freshwater phase, nor were any SAV-neutralizing antibodies or histopathological changes consistent with PD. In the seawater phase, SAV was detected in samples from 23 of 36 (63.9%) studied sites located within the endemic region. No SAV subtype 3 was detected in samples from seawater sites located outside the endemic region. The cumulative incidence of PD during the production cycle amongst sites with SAV detected was 87% (20 of 23 sites). Average fish weight at time of PD diagnosis ranged from 461 to 5978 g, because of a wide variation in the timing of disease occurrence throughout the production cycle. Mortality levels following a PD diagnosis varied greatly between populations. The mean percentage mortality was 6.9% (+/-7.06) (range 0.7-26.9), while the mean duration of increased mortality following PD diagnosis was 2.8 months (+/-1.11) (range 1-6).

Prospective longitudinal studies of salmonid alphavirus infections on two Atlantic salmon farms in Ireland; evidence for viral persistence

Prospective longitudinal studies of two outbreaks of pancreas disease in Atlantic salmon (AS), Salmo salar L., in Ireland were conducted. Both outbreaks occurred during the marine phase of production, with one caused by salmonid alphavirus subtype 1 (SAV1) and the other by SAV4. In addition to screening a range of tissues by real-time reverse transcriptase polymerase chain reaction (RRT-PCR), virological, serological and histopathological examinations were performed along with partial genome sequencing and results were related to environmental and production data and farm history. On Farm 1 (marine sampling only), infection was detected within 3 weeks of smolts being placed on the farm, while on Farm 2 (freshwater and marine sampling), infection was first detected 315 days after transfer to sea. In both outbreaks, RRT-PCR signals were detected in a range of tissues including gill, heart, kidney, pancreas/pyloric caeca, brain and serum. Persistence of signal was longest in gill and heart (> or =265 days on both farms) and shortest in serum. Mortalities on the two farms varied from 10.9% to 30%. In both cases, partial genome sequence of the causative viruses were identical to SAV strains detected in previous populations of AS on each of the study farms, including populations with which the study populations overlapped in time and space.

Clinical profile of chikungunya fever in patients in a tertiary care centre in Maharashtra, India

BACKGROUND & OBJECTIVE

In India a chikungunya fever outbreak started in December 2005 when the country experienced more than 13 lakhs of chikungunya infected cases. We undertook this study to study detailed clinical profile of chikungunya fever in both indoor and outdoor patients in a tertiary care hospital in Nagpur, Maharashtra in 2006.

METHODS

Suspected cases of chikungunya fever (n=405) during the period of July to September 2006, having clinical triad of fever, arthralgia and/or rashes were included in the study. Clinical profile was studied in all the cases. Of the 405 samples collected, 166 were tested for serum CHIK IgM antibodies.

RESULTS

Of the 166 samples tested for CHIKV IgM antibodies, 87 (52.4%) were positive (confirmed cases). Male: female ratio was 2.3:1. Fever and arthralgia were present in all cases. Rash was present in 27 (31%) confirmed and 38 (12%) suspected cases. Lymphadenopathy was present in 12 (13.8 %) confirmed and 4 of suspected cases. Chronic polyarthritis was seen in 22 (25.3%) confirmed and 75 (23.6%) suspected cases. Neurological manifestations were observed in 08 (9%) confirmed and 10 (3.14%) suspected cases. Mortality was 7 (2.2%) in 318 suspected cases and 3 (3.4%) in 87 confirmed cases.

INTERPRETATION & CONCLUSION

Our findings showed that about half of the serum samples for CHIKV IgM antibody tested positive from cases suspected to have chikungunya fever. Fever, joint pain and headache were major symptoms. Certain rare manifestations like lymphodenopathy, oral ulcers and encephalitis were also seen. Mortality in confirmed cases was about 3.4 per cent.

A major epidemic of chikungunya virus infection on Reunion Island, France, 2005-2006

In January 2005, an epidemic of chikungunya fever broke out in the Comoro Islands and lasted until May 2005. In April, cases were also reported in Mayotte and Mauritius. On Réunion Island, the first cases were reported at the end of April. Surveillance of this epidemic required an adaptive system, which at first was based on active and retrospective case detection around the cases reported, then relied on a sentinel network when the incidence increased. Emerging and severe forms of infection were investigated. Death certificates were monitored. By April 2006, the surveillance estimate was 244,000 cases of chikungunya virus infection, including 123 severe cases and 41 of maternoneonatal transmission, with an overall attack rate of 35%. Chikungunya infection was mentioned on 203 death certificates and significant mortality was observed. This epidemic highlighted the need for a mutual strategy of providing information on arboviral diseases and their prevention and control between countries in the southwestern Indian Ocean.

Non-pathogenic Sindbis virus causes hemorrhagic fever in the absence of alpha/beta and gamma interferons

The role of interferon-gamma (IFNgamma) in antiviral innate immune responses during acute alphavirus infection is not well defined. We examined the contribution of IFNgamma to the protection of adult mice from Sindbis virus (SB)-induced disease by comparing subcutaneous infection of mice lacking receptors for either IFNalpha/beta (A129), IFNgamma (G129) or both (AG129) to normal mice (WT129). While neither G129 nor WT129 mice exhibited clinical signs of disease, infection of A129 or AG129 mice was fatal with AG129 mice succumbing more rapidly. Furthermore, AG129 mice developed signs of viral hemorrhagic fever (VHF), including extensive hepatocellular damage, inflammatory infiltrates in multiple organs and vascular leakage, which were significantly delayed and/or partially ameliorated during fatal A129 infections. We conclude that: (i) IFNalpha/beta is the primary mediator of innate immunity to SB infection, however; (ii) IFNgamma is directly antiviral in vivo, acting before the adaptive immune response appears and; (iii) development of VHF may involve viral suppression of both IFNalpha/beta and IFNgamma responses.

Early restriction of alphavirus replication and dissemination contributes to age-dependent attenuation of systemic hyperinflammatory disease

Severity of alphavirus infection in humans tends to be strongly age-dependent and several studies using laboratory-adapted Sindbis virus (SB) AR339 strains have indicated that SB-induced disease in mice is similarly contingent upon host developmental status. In the current studies, the consensus wild-type SB, TR339, and in vivo imaging technology have been utilized to examine virus replication and disease manifestations in mice infected subcutaneously at 5 days of age (5D) vs 11D. Initial virulence studies with TR339 indicated that this age range is coincident with rapid transition from fatal to non-fatal outcome. Fatal infection of 5D mice is characterized by high-titre serum viraemia, extensive virus replication in skin, fibroblast connective tissue, muscle and brain, and hyperinflammatory cytokine induction. In contrast, 11D-infected mice experience more limited virus replication and tissue damage and develop mild, immune-mediated pathologies including encephalitis. These results further establish the linkage between hyperinflammatory cytokine induction and fatal outcome of infection. In vivo imaging using luciferase-expressing viruses and non-propagative replicons revealed that host development results in a restriction of virus replication within individual infected cells that is manifested as a delay in reduction of virus replication in the younger mice. Thus, an important contributing factor in age-dependent resistance to alphavirus infection is restriction of replication within first infected cells in peripheral tissues, which may augment other developmentally regulated attenuating effects, such as increasing neuronal resistance to virus infection and apoptotic death.

Epidemiological observations of pancreas disease of farmed Atlantic salmon, Salmo salar L., in Ireland

Epidemiological investigations into the pancreas disease (PD) of farmed salmon were conducted on populations of Atlantic salmon reared in Ireland during 2003 and 2004. The investigations surveyed all marine salmon farms operating in Ireland through a detailed questionnaire with follow-up farm visits. Information was gathered on 21 populations of fish in 2003 and 14 populations in 2004. Thirteen of the 21 populations suffered PD in 2003 and 12 of the 14 in 2004. The mean mortality due to PD on affected farms was 18.8% in 2003 and 14.8% in 2004 and the loss of growth due to PD was estimated at 11.4% over the 2-year period. The highest risk periods for outbreaks of PD were early summer and early autumn and the farms most seriously affected by PD mortality were in the western counties of Ireland. Factors which showed an indication of association with a PD outbreak or high mortality during a PD outbreak were: livestock movement to another sea site, high feeding rate prior to any PD outbreak, the presence of another PD positive farm in the same water body, greater than 250000 fish on a site, a previous history of PD on a site, a high sea lice burden, and sites located in the western regions of Ireland which reared a specific strain of salmon.

Acid sphingomyelinase deficiency increases susceptibility to fatal alphavirus encephalomyelitis

Sindbis virus (SV), an enveloped virus with a single-stranded, plus-sense RNA genome, is the prototype alphavirus in the Togaviridae family. In mice, SV infects neurons and can cause apoptosis of immature neurons. Sphingomyelin (SM) is the most prevalent cellular sphingolipid, is particularly abundant in the nervous systems of mammals, and is required for alphavirus fusion and entry. The level of SM is tightly regulated by sphingomyelinases. A defect in acid sphingomyelinase (ASMase) results in SM storage and subsequent intracellular accumulation of SM. To better understand the role of the SM pathway in SV pathogenesis, we have characterized SV infection of transgenic mice deficient in the ASMase gene. ASMase knockout (ASM-KO) mice were more susceptible to SV infection than wild-type (WT) or heterozygous (Het) animals. Titers of SV were higher in the brains of ASM-KO mice than in the brains of WT mice. More SV RNA was detected by in situ hybridization, more SV protein was detected by immunohistochemistry, and more terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling-positive cells were present in the cortex and hippocampus of ASM-KO mice than in those of WT or Het mice. Interleukin-6 (IL-6), but not IL-1beta or tumor necrosis factor alpha, was elevated in infected ASM-KO mice compared to levels in WT or Het mice, but studies with IL-6-KO mice and recombinant SV expressing IL-6 showed no role for IL-6 in fatal disease. Together these data indicate that the increase in susceptibility of ASM-KO mice to SV infection was the result of more-rapid replication and spread of SV in the nervous system and increased neuronal death.

Experimental transmission of sleeping disease in one-year-old rainbow trout, Oncorhynchus mykiss (Walbaum), induced by sleeping disease virus

Sleeping disease (SD) is a serious disease of rainbow trout, Oncorhynchus mykiss, reared in fresh water caused by sleeping disease virus (SDV). In this study a detailed clinical, histological, virological and serological description of the experimental reproduction of SD in 1-year-old rainbow trout exposed to SDV was carried out. Two hundred disease-free fish were intraperitoneally inoculated with a SDV isolate and 100 fish were inoculated with an uninfected cell culture lysate as a negative control. Infected and control fish were randomly removed at days 4, 7, 14, 21, 42 and 70 post-infection. Blood and tissues were collected for virus isolation, histopathological examination and serum neutralization. SDV was detected in serum, kidney and brain of infected fish from 4 to 21 days post-infection (dpi). Characteristic pathological lesions were observed in infected fish as early as 7 dpi. Lesions were first detected in exocrine pancreas and subsequently observed in heart and skeletal muscle. Neutralizing antibodies to SDV were detected in infected fish from 14 to 70 dpi. Infected fish displayed typical signs of SD 1-month pi and the mortality reached 18.7% within 44 days. This study experimentally reproduced all the pathognomonic features of natural outbreaks of SD in 1-year-old rainbow trout.

Antiviral profile of Nyctanthes arbortristis L. against encephalitis causing viruses

The ethanolic extracts, various fractions and two pure compounds isolated from the plant N. arbortris were tested against Encephalomyocarditis Virus (EMCV) and Semliki Forest Virus (SFV). Pronounced in vitro virus inhibitory activity was observed with the ethanolic and n-butanol fractions as well as with the pure compounds arbortristoside A and arbortristoside C. In addition, ethanolic extracts and n-butanol fraction protected EMCV infected mice to the extent of 40 and 60% respectively against SFV at a daily dose of 125 mg/kg body weight.

Isolation of Japanese encephalitis and Getah viruses from mosquitoes (Diptera: Culicidae) collected near Camp Greaves, Gyonggi Province, Republic of Korea, 2000

As part of an evaluation of the ecology of arthropod-borne diseases in the Republic of Korea (ROK), we examined 8,765 mosquitoes captured in Paju County, Gyonggi Province, ROK, for the presence of viruses. Mosquitoes were captured in propane lantern/human-baited Shannon traps, Mosquito Magnet traps, or American Biophysics Corporation (East Greenwich, RI) miniature light traps with or without supplemental octenol bait and/or dry ice. Mosquitoes were identified to species, placed in pools of up to 40 mosquitoes each, and tested on Vero cells for the presence of virus. A total of 15 virus isolations were made from 293 pools of mosquitoes. Viruses were identified by reverse transcriptase-polymerase chain reaction and sequencing and consisted of 14 isolations of Japanese encephalitis (JE) virus and one isolation of Getah (GET) virus. All JE isolates were from Culex tritaeniorhynchus Giles, and the isolate of GET was from Aedes vexans (Meigen). The minimum field infection rate for JE in Cx. tritaeniorhynchus was 3.3 per 1,000, whereas the GET virus infection rate for Ae. vexans was 0.2 per 1,000. Isolation of JE and GET indicated that both viruses were actively circulating in northern Gyonggi Province, ROK. The lack of human cases of JE among the Korean population probably is because of an effective government-mandated vaccination program. The reason for no cases among >10,000 United States military and others that reside or train nearby is unknown, but may be related to personnel protection measures (permethrin-impregnated uniforms and use of deet repellent), adult mosquito control, mosquito selection of nonhuman hosts (unpublished data), and the low symptomatic to asymptomatic ratio of disease in adults.

Age-dependent resistance to lethal alphavirus encephalitis in mice: analysis of gene expression in the central nervous system and identification of a novel interferon-inducible protective gene, mouse ISG12

Several different mammalian neurotropic viruses produce an age-dependent encephalitis characterized by more severe disease in younger hosts. To elucidate potential factors that contribute to age-dependent resistance to lethal viral encephalitis, we compared central nervous system (CNS) gene expression in neonatal and weanling mice that were either mock infected or infected intracerebrally with a recombinant strain, dsTE12Q, of the prototype alphavirus Sindbis virus. In 1-day-old mice, infection with dsTE12Q resulted in rapidly fatal disease associated with high CNS viral titers and extensive CNS apoptosis, whereas in 4-week-old mice, dsTE12Q infection resulted in asymptomatic infection with lower CNS virus titers and undetectable CNS apoptosis. GeneChip expression comparisons of mock-infected neonatal and weanling mouse brains revealed developmental regulation of the mRNA expression of numerous genes, including some apoptosis regulatory genes, such as the proapoptotic molecules caspase-3 and TRAF4, which are downregulated during development, and the neuroprotective chemokine, fractalkine, which is upregulated during postnatal development. In parallel with increased neurovirulence and increased viral replication, Sindbis virus infection in 1-day-old mice resulted in both a greater number of host inflammatory genes with altered expression and greater changes in levels of host inflammatory gene expression than infection in 4-week-old mice. Only one inflammatory response gene, an expressed sequence tag similar to human ISG12, increased by a greater magnitude in infected 4-week-old mouse brains than in infected 1-day-old mouse brains. Furthermore, we found that enforced neuronal ISG12 expression results in a significant delay in Sindbis virus-induced death in neonatal mice. Together, our data identify genes that are developmentally regulated in the CNS and genes that are differentially regulated in the brains of different aged mice in response to Sindbis virus infection.

Contribution of T cells to mortality in neurovirulent Sindbis virus encephalomyelitis

Intranasal inoculation of C57BL/6 mice with a neurovirulent strain of Sindbis virus (SV) results in fatal encephalomyelitis. Mice with selective immune deficiencies were studied to determine the role of the immune response in fatal outcome. Mortality was decreased in mice deficient in alphabeta, but not gammadelta, T cells demonstrating a contribution of alphabeta T cells. Mice lacking either CD4+ or CD8+ T cells also had reduced mortality and mice lacking interferon (IFN)-gamma were completely protected. Clearance of infectious virus was identical in mice without T cells or IFN-gamma, but clearance of viral RNA was delayed compared to normal mice. Mice unable to produce antibody, perforin, Fas, TNF-alpha receptor1, IL-6 or IL-12 were not protected. These data suggest that T cells contribute to fatal acute viral encephalomyelitis through the production of IFN-gamma.

Genetic control of neuroadapted sindbis virus replication in female mice maps to chromosome 2 and associates with paralysis and mortality

Neuroadapted Sindbis virus (NSV) infection of mice causes hindlimb paralysis and 100% mortality in the C57BL/6 mouse strain, while adults of the BALB/cBy mouse strain are resistant to fatal encephalomyelitis. Levels of viral RNA are higher in the brains of infected C57BL/6 mice than in BALB/cBy mice (D. C. Thach et al., J. Virol. 74:6156-6161, 2000). These phenotypic differences between the two strains allowed us to map genetic loci involved in mouse susceptibility to NSV and to find relationships between mortality, paralysis, and viral RNA levels. Analysis of percent mortality in H2-congenic and F(1) mice suggested that the H2 locus, sex linkage, and imprinting were not involved in determining susceptibility and that resistance was partially dominant over susceptibility. Segregation analysis using CXB recombinant inbred (RI) mice indicated that the percent mortality was multigenic. Interval mapping detected a suggestive quantitative trait locus (QTL) on chromosome 2 near marker D2Mit447. Analysis of paralysis in the RI mice detected the same suggestive QTL. Viral RNA level in F(1) mice was intermediate. Interval mapping using viral RNA levels in RI mice detected a significant QTL near marker D2Mit447 that explained 69% of the genetic variance. This QTL was confirmed in F2 mice and was designated as Nsv1. Viral RNA level, percent paralyzed, and percent mortality were linearly correlated (r = 0.8 to 0.9). These results indicate that mortality, paralysis, and viral RNA levels are related complex traits and that Nsv1 controls early viral load and determines the likelihood of paralysis and death.

Differences between C57BL/6 and BALB/cBy mice in mortality and virus replication after intranasal infection with neuroadapted Sindbis virus

Neuroadapted Sindbis virus (NSV), given intranasally, caused fatal encephalitis in 100% of adult C57BL/6 mice and 0% of BALB/cBy mice. Most C57BL/6 mice developed severe kyphoscoliosis followed by hind-limb paralysis, while BALB/cBy mice did not. In situ hybridization for detecting NSV RNA and immunohistochemistry for detecting NSV antigen indicated that virus delivered by this route infected neurons of the olfactory region and spread caudally without infection of ependymal cells. Virus antigen was more abundant and infectious virus increased more rapidly and reached higher levels in C57BL/6 mice than in BALB/cBy mice. Surprisingly, infectious virus was cleared faster in C57BL/6 mice, and this was associated with more rapid production of neutralizing antibody. However, viral RNA was cleared more slowly in C57BL/6 mice. In both mouse strains, more infectious virus was present in the lumbar spinal cord than in the cervical spinal cord. These data suggest that genetic susceptibility to fatal NSV encephalomyelitis is determined at least in part by the efficiency of viral replication and spread in the central nervous system. The differences identified in this study provide possible phenotypes for mapping genetic loci involved in susceptibility.

Replicase complex genes of Semliki Forest virus confer lethal neurovirulence

Semliki Forest virus (SFV) is a mosquito-transmitted pathogen of small rodents, and infection of adult mice with SFV4, a neurovirulent strain of SFV, leads to lethal encephalitis in a few days, whereas mice infected with the avirulent A7(74) strain remain asymptomatic. In adult neurons, A7(74) is unable to form virions and hence does not reach a critical threshold of neuronal damage. To elucidate the molecular mechanisms of neurovirulence, we have cloned and sequenced the entire 11,758-nucleotide genome of A7(74) and compared it to the highly neurovirulent SFV4 virus. We found several sequence differences and sought to localize determinants conferring the neuropathogenicity by using a panel of chimeras between SFV4 and a cloned recombinant, rA774. We first localized virulence determinants in the nonstructural region by showing that rA774 structural genes combined with the SFV4 nonstructural genome produced a highly virulent virus, while a reciprocal recombinant was asymptomatic. In addition to several amino acid mutations in the nonstructural region, the nsp3 gene of rA774 displayed an opal termination codon and an in-frame 21-nucleotide deletion close to the nsp4 junction. Replacement in rA774 of the entire nsp3 gene with that of SFV4 reconstituted the virulent phenotype, whereas an arginine at the opal position significantly increased virulence, leading to clinical symptoms in mice. Completion of the nsp3 deletion in rA774 did not increase virulence. We conclude that the opal codon and amino acid mutations other than the deleted residues are mainly responsible for the attenuation of A7(74) and that the attenuating determinants reside entirely in the nonstructural region.

Control of Sindbis virus infection by antibody in interferon-deficient mice

Antibodies clear Sindbis virus from infected animals through an unknown mechanism. To determine whether interferon-induced pathways are required for this clearance, we examined mice which are unable to respond to alpha/beta interferon or gamma interferon. Although extremely susceptible to infection, such mice survived and completely cleared virus if antibodies against Sindbis virus were given.

TNFalpha, interferon, and stress response induction as a function of age-related susceptibility to fatal Sindbis virus infection of mice

The age-related acquisition of resistance to fatal Sindbis virus infection was examined using a molecularly cloned laboratory strain of the AR339 isolate designated TRSB. TRSB caused 100% mortality in mice up to 5 days of age. Resistance to fatal infection developed abruptly between 5 and 9 days of age. Lethal Sindbis virus infection of mice inoculated at 4 days of age was characterized by high levels of virus replication, induction of high levels of interferon-alpha/beta and TNF-alpha and severe thymic involution indicative of a systemic stress response. These changes correlated with predominantly noninflammatory lesions. In contrast, TRSB infection of older mice was characterized by survival, more limited virus replication, reduced cytokine induction, and the development of inflammatory responses leading to encephalitis, myositis, and myocarditis. Previous studies utilized infections of neonatal mice with TRSB and an attenuated mutant of TRSB to compare fatal and nonfatal Sindbis infection (Trgovcich et al., 1996. Virology 224, 73-83). The experiments reported here utilize mouse age at the time of infection to create conditions for examination of fatal and nonfatal TRSB infections. Both experiments suggest that fatal infection is associated with a shock-like syndrome and little or no inflammatory pathology, while survival is correlated with greatly reduced cytokine levels and inflammatory lesions.

SOCS1 is a critical inhibitor of interferon gamma signaling and prevents the potentially fatal neonatal actions of this cytokine

Mice lacking suppressor of cytokine signaling-1 (SOCS1) develop a complex fatal neonatal disease. In this study, SOCS1-/- mice were shown to exhibit excessive responses typical of those induced by interferon gamma (IFNgamma), were hyperresponsive to viral infection, and yielded macrophages with an enhanced IFNgamma-dependent capacity to kill L. major parasites. The complex disease in SOCS1-/- mice was prevented by administration of anti-IFNgamma antibodies and did not occur in SOCS1-/- mice also lacking the IFNgamma gene. Although IFNgamma is essential for resistance to a variety of infections, the potential toxic action of IFNgamma, particularly in neonatal mice, appears to require regulation. Our data indicate that SOCS1 is a key modulator of IFNgamma action, allowing the protective effects of this cytokine to occur without the risk of associated pathological responses.

Adaptation of Sindbis virus to BHK cells selects for use of heparan sulfate as an attachment receptor

Attachment of Sindbis virus to the cell surface glycosaminoglycan heparan sulfate (HS) and the selection of this phenotype by cell culture adaptation were investigated. Virus (TR339) was derived from a cDNA clone representing the consensus sequence of strain AR339 (K. L. McKnight, D. A. Simpson, S. C. Lin, T. A. Knott, J. M. Polo, D. F. Pence, D. B. Johannsen, H. W. Heidner, N. L. Davis, and R. E. Johnston, J. Virol. 70:1981-1989, 1996) and from mutant clones containing either one or two dominant cell culture adaptations in the E2 structural glycoprotein (Arg instead of Ser at E2 position 1 [designated TRSB]) or this mutation plus Arg for Ser at E2 114 [designated TRSB-R114]). The consensus virus, TR339, bound to baby hamster kidney (BHK) cells very poorly. The mutation in TRSB increased binding 10- to 50-fold, and the additional mutation in TRSB-R114 increased binding 3- to 5-fold over TRSB. The magnitude of binding was positively correlated with the degree of cell culture adaptation and with attenuation of these viruses in neonatal mice. HS was identified as the attachment receptor for the mutant viruses by the following experimental results. (i) Low concentrations of soluble heparin inhibited plaque formation on and binding of mutant viruses to BHK cells by >95%. In contrast, TR339 showed minimal inhibition at high concentrations. (ii) Binding and infectivity of TRSB-R114 was sensitive to digestion of cell surface HS with heparinase III, and TRSB was sensitive to both heparinase I and heparinase III. TR339 infectivity was only slightly affected by either digestion. (iii) Radiolabeled TRSB and TRSB-R114 attached efficiently to heparin-agarose beads in binding assays, while TR339 showed virtually no binding. (iv) Binding and infectivity of TRSB and TRSB-R114, but not TR339, were greatly reduced on Chinese hamster ovary cells deficient in HS specifically or all glycosaminoglycans. (v) High-multiplicity-of-infection passage of TR339 on BHK cell cultures resulted in rapid coselection of high-affinity binding to BHK cells and attachment to heparin-agarose beads. Sequencing of the passaged virus population revealed a mutation from Glu to Lys at E2 70, a mutation common to many laboratory strains of Sindbis virus. These results suggest that TR339, the most virulent virus tested, attaches to cells through a low-affinity, primarily HS-independent mechanism. Adaptive mutations, selected during cell culture growth of Sindbis virus, enhance binding and infectivity by allowing the virus to attach by an alternative mechanism that is dependent on the presence of cell surface HS.

Sindbis virus infection of neonatal mice results in a severe stress response

Neonatal mice were infected with virus derived from a molecular clone of a laboratory strain of Sindbis virus, TRSB. The resulting acute fatal infection was typified by few if any of the classic hallmarks of encephalitis, very high levels of interferon-alpha/beta (IFNalphabeta), and lesions in the thymus and hematopoietic tissues usually associated with a severe stress response. Infection with an attenuated mutant of TRSB, which harbors a single amino acid change in the E2 surface glycoprotein (TRSBr114), was characterized by encephalitis, reduced mortality, low levels of IFNalphabeta, and no thymic pathology (J. Trgovcich, J. F. Aronson, and R. E. Johnston, 1996, Virology 224, 73-83). Here we report that infection of neonatal mice with TRSB, but not TRSBr114, resulted in induction of high levels of tumor necrosis factor-alpha as well as high and sustained levels of adrenalcorticotropin-releasing hormone and corticosterone. This syndrome of potentially toxic cytokine and stress hormone induction correlates with lethal Sindbis virus infection and constitutes a previously unrecognized aspect of Sindbis virus pathogenesis in mice.

Inhibition of nitric oxide synthesis increases mortality in Sindbis virus encephalitis

Sindbis virus (SV) is an alphavirus that causes acute encephalomyelitis in mice. The outcome is determined by the strain of virus and by the age and genetic background of the host. The mortality rates after infection with NSV, a neurovirulent strain of SV, were as follows v: 81% (17 of 21) in BALB/cJ mice; 20% (4 of 20) in BALB/cByJ mice (P < 0.001); 100% in A/J, C57BL/6J, SJL, and DBA mice; and 79% (11 of 14) in immunodeficient scid/CB17 mice. Treatment with Nomega-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthetase (NOS) inhibitor, increased mortality to 100% (P < 0.05) in NSV-infected BALB/cJ mice, to 95% (P < 0.001) in BALB/cByJ mice, and to 100% in scid/CB17 mice. BALB/cJ and BALB/cByJ mice had similar levels of inducible NOS mRNA in their brains, which were not affected by L-NAME or NSV infection. Brain NOS activity was similar in BALB/cJ and BALB/cByJ mice before and after infection and was markedly inhibited by L-NAME. NSV replication in the brains of BALB/cJ mice, BALB/cByJ mice, and mice treated with L-NAME was similar. Treatment of N18 neuroblastoma cells with NO donors S-nitroso-N-acetylpenicillamine or sodium nitroprusside in vitro before infection increased cell viability at 42 to 48 h compared with untreated NSV-infected N18 cells with little effect on virus replication. These data suggest that NO protects mice from fatal encephalitis by a mechanism that does not directly involve the immune response or inhibition of virus growth but rather may enhance survival of the infected neuron until the immune response can control virus replication.

Alphavirus-induced apoptosis in mouse brains correlates with neurovirulence

Sindbis virus induces apoptotic cell death in cultured cell lines, raising the possibility that apoptosis of infected neurons and other target cells in vivo may contribute to the resulting disease and mortality. To investigate the role of apoptosis in Sindbis virus pathogenesis, infected mouse brains were assayed by the in situ terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling technique and for DNA ladder formation. Infection with recombinant Sindbis virus strain 633 resulted in widespread apoptosis in newborn mouse brains and spinal cords, but few apoptotic cells were observed following infection of 2-week-old animals. This finding correlates with the age-dependent mortality observed in mice. The more neurovirulent virus TE, which differs from 633 by a single amino acid in the E2 glycoprotein, induced significant apoptosis in brains and spinal cords of 2-week-old animals, consistent with its ability to cause fatal disease in older animals. Double-labeling experiments demonstrated that the apoptotic cells were also infected with Sindbis virus. Thus, Sindbis virus-induced apoptosis appears to be a result of virus infection and is likely to reflect pathogenic mechanisms for other viruses.

Protective effects of melatonin in mice infected with encephalitis viruses

We examined the effect of the pineal neurohormone melatonin (MLT) on protection from viral encephalitis. The antiviral activity of MLT was evaluated in normal mice inoculated with Semliki Forest virus (SFV) and in stressed mice injected with the attenuated non-invasive West Nile virus (WN-25). Administration of MLT (s.c.) daily from 3 days before through 10 days after virus inoculation reduced viremia and significantly postponed the onset of disease and death by 7 to 10 days. Moreover, MLT injection reduced mortality of SFV (10 PFU) inoculated mice from 100% to 44%. In mice inoculated with high dose of SFV (100 PFU), MLT postponed death and reduced mortality by 20%. In all of the surviving mice anti-SFV antibodies were detected 22 days after virus inoculation. Infection of mice stressed by either isolation or dexamethasone injection with WN-25 induced mortality of 75% and 50% respectively, which was reduced by MLT administration to 31% and 25%, respectively. The efficiency of MLT in protecting from lethal viral infections warrants further investigations on its mechanisms of action.

Age-dependent susceptibility to fatal encephalitis: alphavirus infection of neurons

Sindbis virus encephalitis in mice provides a model for studying age-dependent susceptibility to acute viral encephalitis. The AR339 strain of SV causes fatal encephalitis in newborn mice, but weanling mice recover uneventfully. Increased virulence for older mice is associated with a single amino acid change from Gln to His at position 55 of the E2 glycoprotein. Weanling mice with normal immune systems clear infectious virus from neurons through an antibody-mediated mechanism. This does not happen in newborn mice because the infected neurons die soon after they are infected. Death in immature neurons, as well as most other mammalian cells infected with Sindbis virus, occurs by induction of apoptosis. This can be prevented by cellular expression of bcl-2, an inhibitor of apoptosis, which is expressed by mature neurons in culture. We conclude that mature neurons are resistant to induction of apoptosis after infection with SV through expression of cellular inhibitors of apoptosis. This provides the opportunity for antibody to clear virus by a noncytolytic mechanism.