NSG-Mice Reveal the Importance of a Functional Innate and Adaptive Immune Response to Overcome RVFV Infection

Rift Valley fever (RVF) is a zoonotic disease caused by RVF Phlebovirus (RVFV). The RVFV MP-12 vaccine strain is known to exhibit residual virulence in the case of a deficient interferon type 1 response. The hypothesis of this study is that virus replication and severity of lesions induced by the MP-12 strain in immunocompromised mice depend on the specific function of the disturbed pathway. Therefore, 10 strains of mice with deficient innate immunity (B6-IFNARtmAgt, C.129S7(B6)-Ifngtm1Ts/J, B6-TLR3tm1Flv, B6-TLR7tm1Aki, NOD/ShiLtJ), helper T-cell- (CD4tm1Mak), cytotoxic T-cell- (CD8atm1Mak), B-cell- (Igh-Jtm1DhuN?+N2), combined T- and B-cell- (NU/J) and combined T-, B-, natural killer (NK) cell- and macrophage-mediated immunity (NOD.Cg-PrkdcscidIl2rgtm1WjI/SzJ (NSG) mice) were subcutaneously infected with RVFV MP-12. B6-IFNARtmAgt mice were the only strain to develop fatal disease due to RVFV-induced severe hepatocellular necrosis and apoptosis. Notably, no clinical disease and only mild multifocal hepatocellular necrosis and apoptosis were observed in NSG mice, while immunohistochemistry detected the RVFV antigen in the liver and the brain. No or low virus expression and no lesions were observed in the other mouse strains. Conclusively, the interferon type 1 response is essential for early control of RVFV replication and disease, whereas functional NK cells, macrophages and lymphocytes are essential for virus clearance.

Insights into the Pathogenesis of Viral Haemorrhagic Fever Based on Virus Tropism and Tissue Lesions of Natural Rift Valley Fever

Rift Valley fever phlebovirus (RVFV) infects humans and a wide range of ungulates and historically has caused devastating epidemics in Africa and the Arabian Peninsula. Lesions of naturally infected cases of Rift Valley fever (RVF) have only been described in detail in sheep with a few reports concerning cattle and humans. The most frequently observed lesion in both ruminants and humans is randomly distributed necrosis, particularly in the liver. Lesions supportive of vascular endothelial injury are also present and include mild hydropericardium, hydrothorax and ascites; marked pulmonary congestion and oedema; lymph node congestion and oedema; and haemorrhages in many tissues. Although a complete understanding of RVF pathogenesis is still lacking, antigen-presenting cells in the skin are likely the early targets of the virus. Following suppression of type I IFN production and necrosis of dermal cells, RVFV spreads systemically, resulting in infection and necrosis of other cells in a variety of organs. Failure of both the innate and adaptive immune responses to control infection is exacerbated by apoptosis of lymphocytes. An excessive pro-inflammatory cytokine and chemokine response leads to microcirculatory dysfunction. Additionally, impairment of the coagulation system results in widespread haemorrhages. Fatal outcomes result from multiorgan failure, oedema in many organs (including the lungs and brain), hypotension, and circulatory shock. Here, we summarize current understanding of RVF cellular tropism as informed by lesions caused by natural infections. We specifically examine how extant knowledge informs current understanding regarding pathogenesis of the haemorrhagic fever form of RVF, identifying opportunities for future research.

Rift Valley Fever: A survey of knowledge, attitudes, and practice of slaughterhouse workers and community members in Kabale District, Uganda

Background

Rift Valley Fever virus (RVF) is a zoonotic virus in the Phenuiviridae family. RVF outbreaks can cause significant morbidity and mortality in humans and animals. Following the diagnosis of two RVF cases in March 2016 in southern Kabale district, Uganda, we conducted a knowledge, attitudes and practice (KAP) survey to identify knowledge gaps and at-risk behaviors related to RVF.

Methodology/Principal findings

A multidisciplinary team interviewed 657 community members, including abattoir workers, in and around Kabale District, Uganda. Most participants (90%) had knowledge of RVF and most (77%) cited radio as their primary information source. Greater proportions of farmers (68%), herdsmen (79%) and butchers (88%) thought they were at risk of contracting RVF compared to persons in other occupations (60%, p<0.01). Participants most frequently identified bleeding as a symptom of RVF. Less than half of all participants reported fever, vomiting, and diarrhea as common RVF symptoms in either humans or animals. The level of knowledge about human RVF symptoms did not vary by occupation; however more farmers and butchers (36% and 51%, respectively) had knowledge of RVF symptoms in animals compared to those in other occupations (30%, p<0.01). The use of personal protective equipment (PPE) when handling animals varied by occupation, with 77% of butchers using some PPE and 12% of farmers using PPE. Although most butchers said that they used PPE, most used gumboots (73%) and aprons (60%) and less than 20% of butchers used gloves or eye protection when slaughtering.

Conclusions

Overall, knowledge, attitudes and practice regarding RVF in Kabale District Uganda could be improved through educational efforts targeting specific populations.

Rift Valley Fever (RVF) virus is transmitted to humans from contact with infected livestock and through mosquito bites. Several human cases of RVF were diagnosed in Kabale District, Uganda in March 2016, over 40 years after the last RVF case was identified in Uganda. We administered a knowledge, attitudes, and practice survey to people living in Kabale District, near where the cases occurred. Survey results demonstrated that knowledge, attitudes and practice surrounding RVF could be improved within the community.

Risk factors for severe Rift Valley fever infection in Kenya, 2007

A large Rift Valley fever (RVF) outbreak occurred in Kenya from December 2006 to March 2007. We conducted a study to define risk factors associated with infection and severe disease. A total of 861 individuals from 424 households were enrolled. Two hundred and two participants (23%) had serologic evidence of acute RVF infection. Of these, 52 (26%) had severe RVF disease characterized by hemorrhagic manifestations or death. Independent risk factors for acute RVF infection were consuming or handling products from sick animals (odds ratio [OR] = 2.53, 95% confidence interval [CI] = 1.78-3.61, population attributable risk percentage [PAR%] = 19%) and being a herds person (OR 1.77, 95% CI = 1.20-2.63, PAR% = 11%). Touching an aborted animal fetus was associated with severe RVF disease (OR = 3.83, 95% CI = 1.68-9.07, PAR% = 14%). Consuming or handling products from sick animals was associated with death (OR = 3.67, 95% CI = 1.07-12.64, PAR% = 47%). Exposures related to animal contact were associated with acute RVF infection, whereas exposures to mosquitoes were not independent risk factors.

Rift Valley fever--a threat for Europe?

Rift Valley fever (RVF) is a severe mosquito-borne disease affecting humans and domestic ruminants, caused by a Phlebovirus (Bunyaviridae). It is widespread in Africa and has recently spread to Yemen and Saudi Arabia. RVF epidemics are more and more frequent in Africa and the Middle East, probably in relation with climatic changes (episodes of heavy rainfall in eastern and southern Africa), as well as intensified livestock trade. The probability of introduction and large-scale spread of RVF in Europe is very low, but localized RVF outbreaks may occur in humid areas with a large population of ruminants. Should this happen, human cases would probably occur in exposed individuals: farmers, veterinarians, slaughterhouse employees etc. Surveillance and diagnostic methods are available, but control tools are limited: vector control is difficult to implement, and vaccines are only available for ruminants, with either a limited efficacy (inactivated vaccines) or a residual pathogenic effect. The best strategy to protect Europe and the rest of the world against RVF is to develop more efficient surveillance and control tools and to implement coordinated regional monitoring and control programmes.

Rift Valley fever virus lacking NSm proteins retains high virulence in vivo and may provide a model of human delayed onset neurologic disease

Rift Valley fever virus is a significant human and veterinary pathogen responsible for explosive outbreaks throughout Africa and the Arabian Peninsula. Severe acute disease in humans includes rapid onset hepatic disease and hemorrhagic fever or delayed onset encephalitis. A highly efficient reverse genetics system was developed which allowed generation of recombinant RVF viruses to assess the role of NSm protein in virulence in a rat model in which wild-type RVF virus strain ZH501 (wt-ZH501) results in 100% lethal hepatic disease 2-3 days post infection. While extensive genomic analysis indicates conservation of the NSm coding capability of diverse RVF viruses, and viruses deficient in NSs proteins are completely attenuated in vivo, comparison of wt-ZH501, a reverse genetics generated wt-ZH501 virus (R-ZH501), and R-ZH501 virus lacking the NSm proteins (R-DeltaNSm-ZH501) demonstrated that the NSm proteins were nonessential for in vivo virulence and lethality. Surprisingly, while 44% of R-DeltaNSm-ZH501 infected animals quickly developed lethal hepatic disease similar to wt- and R-ZH501, 17% developed delayed onset neurologic disease (lethargy, head tremors, and ataxia) at 13 days post infection. Such infections may provide the basis for study of both RVF acute hepatic disease and delayed onset encephalitic disease in humans.

Rift Valley fever epidemic in Saudi Arabia: epidemiological, clinical, and laboratory characteristics

This cohort descriptive study summarizes the epidemiological, clinical, and laboratory characteristics of the Rift Valley fever (RVF) epidemic that occurred in Saudi Arabia from 26 August 2000 through 22 September 2001. A total of 886 cases were reported. Of 834 reported cases for which laboratory results were available, 81.9% were laboratory confirmed, of which 51.1% were positive for only RVF immunoglobulin M, 35.7% were positive for only RVF antigen, and 13.2% were positive for both. The mean age (+/- standard deviation) was 46.9+/-19.4 years, and the ratio of male to female patients was 4:1. Clinical and laboratory features included fever (92.6% of patients), nausea (59.4%), vomiting (52.6%), abdominal pain (38.0%), diarrhea (22.1%), jaundice (18.1%), neurological manifestations (17.1%), hemorrhagic manifestations (7.1%), vision loss or scotomas (1.5%), elevated liver enzyme levels (98%), elevated lactate dehydrogenase level (60.2%), thrombocytopenia (38.4%), leukopenia (39.7%), renal impairment or failure (27.8%), elevated creatine kinase level (27.3%), and severe anemia (15.1%). The mortality rate was 13.9%. Bleeding, neurological manifestations, and jaundice were independently associated with a high mortality rate. Patients with leukopenia had significantly a lower mortality rate than did those with a normal or high leukocyte count (2.3% vs. 27.9%; odds ratio, 0.09; 95% confidence interval, 0.01-0.63).

Epidemic Rift Valley fever in Saudi Arabia: a clinical study of severe illness in humans

We describe the clinical patterns and case-fatality rate associated with severe Rift Valley fever (RVF) in patients who were admitted to the Gizan regional referral hospital during an outbreak of RVF in Saudi Arabia from September through November 2000. A total of 165 consecutive patients (136 men and 29 women) were prospectively studied; all were identified according to a strict case definition, were confirmed to have RVF by serologic testing, and were treated according to a predetermined protocol. The major clinical characteristics of RVF included a high frequency of hepatocellular failure in 124 patients (75.2%), acute renal failure in 68 patients (41.2%), and hemorrhagic manifestations in 32 patients (19.4%). Sixteen patients had retinitis and 7 patients had meningoencephalitis as late complications in the course of the disease. A total of 56 patients (33.9%) died. Hepatorenal failure, shock, and severe anemia were major factors associated with patient death.

Susceptibility of selected strains of Australian mosquitoes (Diptera: Culicidae) to Rift Valley fever virus

We evaluated the ability of selected strains of the Australian mosquitoes, Aedes notoscriptus (Skuse), Ae. vigilax (Skuse), Culex annulirostris Skuse, and Cx. quinquefasciatus Say, to function as potential vectors of Rift Valley fever (RVF) virus, should that virus be introduced accidentally into Australia. After feeding on a hamster with a viremia of 10(7) plaque-forming units/ml of blood, Ae. notoscriptus and Cx. annulirostris were the most susceptible, with infection rates of 86 and 55%, respectively. Female Ae. vigilax and Cx. quinquefasciatus also were susceptible, with infection rates of 38 and 30%, respectively. All of these species transmitted RVF virus by bite 7-10 d after intrathoracic inoculation, and all, except Cx. quinquefastiatus (not tested), transmitted RVF virus 10-16 d after oral exposure. The presence of competent mosquito vectors for RVF virus in Australia indicates the potential for RVF virus epizootics to occur should this virus be introduced into Australia.

Recurrence of Rift Valley fever in Egypt

Rift Valley fever (RVF) has been recorded in man and in domestic animals in Egypt after a 12-year absence. Human infections were first noted in the Aswan Governorate in late May, 1993. Only cases of ocular disease, an infrequent and late manifestation, were reported. Of 41 cases, 35 were tested serologically and 27 (77%) had RVF virus-specific IgM antibodies. An estimated 600-1500 infections occurred in the region. Abortions in cattle and buffalo were seen concurrently and antibodies to RVFV were present in 39% of domestic livestock, presumably unvaccinated. RVFV was isolated from an aborted water buffalo fetus.

Pathogenesis of viral hemorrhagic fevers: Rift Valley fever and Lassa fever contrasted

Although many viral infections have on occasion been associated with hemorrhagic complications, infection with any of several RNA viruses regularly results in vascular involvement and the syndrome called viral hemorrhagic fever (VHF). In spite of clinically useful similarities among various VHFs, there are significant differences in their pathogenesis and clinical evolution; these are often related to characteristics of their viral taxon. Infection with Rift Valley fever (RVF) virus, a phlebovirus, appears to be regulated by interferon and terminated by neutralizing antibody. In contrast, Lassa fever (LF) virus, an arenavirus, is resistant to interferon, and LF is terminated by cellular immune effector mechanisms. The lytic virus-cell interaction typical of RVF virus suggests its major effects occur by direct, virus-induced cellular necrosis, particularly in the liver. In the primate RVF model, disseminated intravascular coagulation (DIC) may be important. LF virus--characteristically noncytopathic--may exert its effects through induction of mediator secretion from infected macrophages. DIC does not appear to be a central pathogenetic mechanism in LF. Pichinde virus, which is not pathogenic for humans, provides an alternate model for study of LF. Infected guinea pigs do not show histologic lesions that could explain their body wasting, cardiovascular deterioration, and pulmonary edema. In the heart, for example, loss of tissue mass, protein, and contractile function proceed without direct viral involvement or myocarditis. Sulfidopeptide leukotrienes have been implicated as one relevant soluble mediator participating in the disease state.

Experimental Rift Valley fever in rhesus macaques

Rift Valley fever (RVF) is a major cause of human morbidity and mortality in endemic areas of sub-Saharan Africa and has the potential to cause epidemic disease in receptive areas world-wide. In this study, a RVF viral isolate from the 1977 Egyptian epidemic (ZH-501) inoculated intravenously into rhesus macaques caused a benign viremic infection in most, but resulted in the hemorrhagic fever syndrome in 20 per cent (3 of 15). Serious disease of this type has not previously been observed in nonhuman primates inoculated with RVF virus and may be a consequence of the viral strain used or the route of inoculation. Severe disease was accompanied by extensive liver necrosis, disseminated intravascular coagulation, and microangiopathic hemolytic anemia. We also attempted to prevent RVF by passive transfer of serum from vaccinated rhesus monkeys (plaque-reduction neutralization test titer 1:2,560). As little as 0.025 ml/kg prevented the development of viremia in naive rhesus monkeys after subcutaneous inoculation of virus. The monkey model should be helpful in understanding the pathogenesis and prevention of human RVF.

Pathogenesis of Rift Valley fever virus (RVFV) in inbred rats

The pathogenesis of Rift Valley fever in adult rats from 3 inbred strains (LEW, MAXX, WF) was investigated. WF rats all died by day 2 postinoculation with viral tissue titers reaching 9 log10 PFU/g. LEW and MAXX rats were resistant to liver disease, but fatal necrotising encephalitis developed in 16 and 44% of the rats, respectively. Detection of serum neutralising antibody on day 3 coincided with clearance of virus from serum and liver, although infectious virus was detected in spleen homogenates as late as day 19 postinfection. Viral titers in LEW and MAXX rats did not exceed 4.5 log10 PFU/g. Cyclophosphamide immunosuppression of LEW rats led to death 5-9 days postinfection; early patterns of viral replication were not affected, but continued growth in the liver resulted in fatal hepatitis. These animals could be protected by passive antibody therapy administered on days 2-5 postinfection to mimic the serum neutralising antibody pattern seen in unmanipulated infected LEW rats. Thus, RVF virus replication and spread is rapid in the WF rats tissues, whereas in LEW and MAXX rats viral growth is less due to an intrinsic mechanism which allows sufficient time for an immune response to terminate infection. A slightly diminished immune response may lead to the development of encephalitis more frequently in MAXX than LEW rats. These rat strains should be useful in elucidating those mechanisms of resistance which limit RVFV-induced hepatitis and encephalitis.

Comparative pathogenicity and antigenic cross-reactivity of Rift Valley fever and other African phleboviruses in sheep

Homologous and heterologous haemagglutination-inhibition (HAI), complement-fixation (CF), immunodiffusion (ID) and mouse neutralization tests were performed with the Lunyo (LUN) and a Zimbabwean strain of Rift Valley fever (RVF) virus, the prototype and a South African strain of Arumowot (AMT) virus and prototype strains of Gordil (GOR), Saint-Floris (SAF) and Gabek Forest (GF) viruses, using immune mouse ascitic fluids prepared against these viruses. Reactions of identity occurred in all tests between LUN and the Zimbabwean strains of RVF and between the two strains of AMT virus. Otherwise, cross-reactions occurred between all the phleboviruses in HAI tests, while reactions in CF, ID and neutralization tests were monospecific for virus serotypes, except that weak cross-reaction occurred between GOR and SAF viruses in CF and ID tests. Four sheep infected subcutaneously with the Zimbabwean strain of RVF virus developed transient fever, viraemia, leucopaenia, relative thrombocytopaenia, haemoconcentration and raised serum enzyme levels, which indicated that the sheep had developed necrotic hepatitis. Disseminated focal necrotic hepatitis was confirmed in a sheep killed for examination on day 4 post-infection. The other three sheep recovered uneventfully after only mild depression and anorexia. Groups of three sheep infected with SAF, GOR, AMT and GF viruses had no demonstrable viraemia or other sign of infection or illness, except that the sheep infected with AMT developed mild fever lasting less than 24 h. Antibody responses were monitored at intervals over a period of 24 weeks in all sheep by homologous and heterologous HAI, CF and cell culture neutralization (CPENT) tests. Homologous antibody responses were marked in the RVF-infected sheep and their sera cross-reacted strongly in HAI tests with antigens of the other viruses. The sera of the RVF-infected sheep cross-reacted less markedly in CF and CPENT tests. Homologous antibody responses were poor in all the sheep infected with phleboviruses other than RVF, and the cross-reactivity of their sera for RVF antigen or virus was negligible. All sheep were challenged with RVF virus 48 weeks after their initial infection. The sheep which had originally been infected with RVF virus were immune and developed neither fever nor viraemia. All other sheep developed fever, viraemia and antibodies to RVF virus. It was concluded that the African phleboviruses, other than RVF, are unlikely to cause disease in livestock or to induce antibodies which could cause confusion in the diagnosis of RVF.