Yellow fever and Zika virus epizootics and enzootics in Uganda

Data of monkey serology are presented which, together with past evidence, support the view that yellow fever (YF) virus circulates in its primary sylvan host populations, i.e., forest monkeys, in an enzootic state in Bwamba County in western Uganda but as series of epizootics in the forest-savanna mosaic zone of central Uganda. Evidence of an epizootic of Zika virus at the Zika Forest near Entebbe is described which occurred in two episodes, the first (in 1969) apparently following the build-up of non-immune monkey populations since a previous epizootic of 1962-63 and the second (in 1970) when Aedes africanus biting densities rose. This was followed only 18 months later by an intensive epizootic of YF virus, contradictory to the hypothesis that Zika virus alone would suppress subsequent epizootics of YF virus in nature, at least when redtail monkeys are involved. Conclusions are finally reviewed in the light of more recent evidence of transovarial flavivirus transmission in mosquitoes, pointing out that phlebotomine sandflies also require fresh attention.     

A yellow fever epizootic in Zika forest, Uganda, during 1972: Part 1: Virus isolation and sentinel monkeys.

The results of the yellow fever immunity survey of Central and East Africa reported by SAWYER & WHITMAN in 1936 prompted scientists to undertake well-planned epidemiological studies on yellow fever in eastern Africa. A Yellow Fever Research Institute (the present East African Virus Research Institute) was established at Entebbe in 1936 for this purpose. One of the areas where much work has been carried out is a strip of typical tropical forest, the Zika Forest, 12 kilometres from the Institute. Routine surveillance work, particularly on the biting activity of the yellow fever vector mosquitoes, has been going on since 1946. It was during one of these studies in 1972 that the first yellow fever virus strain was isolated from Aedes africanus collected from the Zika and Sisa forests and one strain was isolated from Coquillettidia fuscopennata, also from the Zika Forest. Three sentinel rhesus monkeys, nomimmune to YF, which were kept in the Zika Forest during the time of the epizootic died of YF disease. The present observations indicate that YF is still present in Africa, and as such it still remains a potential menace to the human population. The epidemiological implications are discussed.     

Isolations of yellow fever virus from Haemagogus leucocelaenus in Rio Grande do Sul State,Brazil

Following howling monkey (Alouatta caraya) deaths and yellow fever (YF) antigen detection by immunohistochemistry in the liver sample of a dead monkey in April and May 2001 in the municipalities of Garruchos and Santo Ant?nio das Miss?es, Rio Grande do Sul State, Brazil, epidemiological field investigations were initiated. Two strains of YF virus were isolated in suckling mice from 23 Haemagogus (Conopostegus) leucocelaenus Dyar & Shannon mosquitoes collected from the study sites. The YF virus was isolated from this species in the 1930s in Brazil and in the 1940s in Colombia. No human cases were reported during the current epizootic outbreak. The YF virus isolation and the absence of Hg. (Haemagogus) janthinomys Dyar from the area suggest that Hg. leucocelaenus may be a secondary YF vector and play an important role in the epidemiology of this disease in the Southern Cone.     

Zika virus infections in Nigeria: virological and seroepidemiological investigations in Oyo State.

A study of Zika virus infections was carried out in four communities in Oyo State, Nigeria. Virus isolation studies between 1971 and 1975 yielded two virus isolations from human cases of mild febrile illness. Haemagglutination-inhibition tests revealed a high prevalence of antibodies to Zika and three other flaviviruses used. The percentages of positive sera were as follows: Zika (31%), Yellow fever (50%), West Nile (46%), and Wesselsbron (59%). Neutralization tests showed that 40% of Nigerians had Zika virus neutralizing antibody. Fifty per cent of zika virus immune persons had neutralizing antibody to Zika alone or to Zika and one other flavivirus. A total of 121 sera had antibody to Zika virus; of these 48 (40%) also showed antibody to two other flaviviruses, and 12 (10%) had antibodies to three or more other viruses. The percentage of neutralizing antibodies to other flaviviruses in Zika virus immune sera was 81% to Dengue type 1, 58% to Yellow fever, 7% to Wesselsbron, 6% to West Nile and 3% to Uganda S.     

The 1970 yellow fever epidemic in Okwoga District, Benue Plateau State, Nigeria. 3. Serological responses in persons with and without pre-existing heterologous group B immunity

Serological studies of persons infected with yellow fever (YF) during the 1970 epidemic in Okwoga District, Nigeria, indicated that epidemic YF occurred despite a high prevalence of pre-existing group B arbovirus immunity, which increased with age. The viruses involved were primarily dengue, Zika, and Wesselsbron. Patterns of responses of haemagglutination-inhibiting, complement-fixing, and neutralizing antibodies in primary YF and in superinfections are defined in this paper.     

Arbovirus infections and viral haemorrhagic fevers in Uganda: a serological survey in Karamoja district, 1984

Sera collected in May 1984 from 132 adult residents of Karamoja district, Uganda, were examined by haemagglutination inhibition tests for antibodies against selected arboviruses, namely Chikungunya and Semliki Forest alphaviruses (Togaviridae); dengue type 2, Wesselsbron, West Nile, yellow fever and Zika flaviviruses (Flaviviridae); Bunyamwera, Ilesha and Tahyna bunyaviruses (Bunyaviridae); and Sicilian sandfly fever phlebovirus (Bunyaviridae); and by immunofluorescence tests against certain haemorrhagic fever viruses, Lassa fever arenavirus (Arenaviridae), Ebola-Sudan, Ebola-Za?re and Marburg filoviruses (Filoviridae), Crimean-Congo haemorrhagic fever nairovirus and Rift Valley fever phlebovirus (Bunyaviridae). Antibodies against Chikungunya virus were the most prevalent (47%), followed by flavivirus antibodies (16%), which were probably due mainly to West Nile virus. No evidence of yellow fever or dengue virus circulation was observed. A few individuals had antibodies against Crimean-Congo haemorrhagic fever, Lassa, Ebola and Marburg viruses, suggesting that these viruses all circulate in the area.     

Production of yellow fever 17DD vaccine virus in primary culture of chicken embryo fibroblasts: yields, thermo and genetic stability, attenuation and immunogenicity

While a good vaccine against yellow fever (YF) virus has been available for decades, the basic technology for the production of YF vaccine in chicken embryos has remained substantially unchanged since the 1940s. Here we describe the highly efficient and economic production of the 17DD strain of YF virus in chicken embryo fibroblast (CEF) cell cultures with viral titers ranging from 6.3 to 6.7 log10PFU/mL. Thermostability of two different formulations (5 and 50-dose vials) of the CEF vaccine virus was found to be as high as the current vaccines retaining the minimal titer required for YF 17D vaccines. The production passage in CEF did not lead to the selection of genetic variants as shown by nucleotide sequence analyses of the CEF-derived vaccine lots or the sequence of viruses recovered from monkeys experimentally inoculated with the CEF virus. YF 17DD virus produced in CEF was also indistinguishable from its seed lot virus parent in terms of plaque size and immunogenicity in mice and monkeys. Comparison of the CEF virus and the seed lot virus made in chicken embryo in the internationally accepted monkey neurovirulence test (MNVT) revealed a higher clinical score for the former. The differences in central nervous system (CNS) histological scores for monkeys inoculated with the chicken embryo and experimental CEF vaccines were at the borderline level of statistical significance. These data warrant further studies on the monkey attenuation of other batches of CEF-derived vaccines.     

Evidence of recent jungle Yellow-Fever activity in Eastern Panama

Outbreaks of jungle yellow fever in man have been recorded twice from eastern Panama of recent years, first in 1948 and again in 1956. Since then, a close surveillance has been maintained on virus activity in eastern Panama. Recent field observations and serological tests on 402 monkey sera indicate that there was an outbreak of yellow fever among monkeys of southern Darién Province some time between 1963 and 1965. It does not appear that the outbreak has spread as yet to other areas. Virus transmission may have been permanently disrupted during the drought which affected the region in 1965. However, if the virus had managed to survive this unfavourable period, an epizootic wave might have evolved, invading forested areas immediately east of the Panama Canal, now inhabited by a dense non-immune human population.     

Immune correlates of protection against yellow fever determined by passive immunization and challenge in the hamster model

Live, attenuated yellow fever (YF) 17D vaccine is highly efficacious but causes rare, serious adverse events resulting from active replication in the host and direct viral injury to vital organs. We recently reported development of a potentially safer β-propiolactone-inactivated whole virion YF vaccine (XRX-001), which was highly immunogenic in mice, hamsters, monkeys, and humans [10] and [11]. To characterize the protective efficacy of neutralizing antibodies stimulated by the inactivated vaccine, graded doses of serum from hamsters immunized with inactivated XRX-001 or live 17D vaccine were transferred to hamsters by the intraperitoneal (IP) route 24 h prior to virulent, viscerotropic YF virus challenge. Neutralizing antibody (PRNT₅₀) titers were determined in the sera of treated animals 4 h before challenge and 4 and 21 days after challenge. Neutralizing antibodies were shown to mediate protection. Animals having 50% plaque reduction neutralization test (PRNT₅₀) titers of ≥40 4 h before challenge were completely protected from disease as evidenced by viremia, liver enzyme elevation, and protection against illness (weight change) and death. Passive titers of 10-20 were partially protective. Immunization with the XRX-001 vaccine stimulated YF neutralizing antibodies that were equally effective (based on dose response) as antibodies stimulated by live 17D vaccine. The results will be useful in defining the level of seroprotection in clinical studies of new yellow fever vaccines.     

Marburg, Ebola and Rift Valley Fever virus antibodies in East African primates

Sera from 464 primates held at four institutes in Kenya were tested by indirect immunofluorescence for the presence of antibodies against Marburg, Ebola, Congo haemorrhagic fever, Rift Valley fever and Lassa viruses. Four of 136 vervet monkeys were positive for Marburg virus antibodies and three of 184 baboons had antibodies against Ebola virus. One baboon was positive for Marburg virus antibodies. Two vervet monkeys, three baboons and one grivet monkey (of 56 tested) had antibodies against Rift Valley fever virus. No Congo or Lassa virus antibodies were detected. A sample of 88 sera of more arboreal primates (Sykes, blue and colobus monkeys) were negative against all five antigens, as were sera from 58 staff members of the institutes who worked with or near the animals.     

An outbreak of yellow fever with concurrent chikungunya virus transmission in South Kordofan, Sudan, 2005

From September through December 2005, an outbreak of hemorrhagic fever occurred in South Kordofan, Sudan. Initial laboratory test results identified IgM antibodies against yellow fever (YF) virus in patient samples, and a YF outbreak was declared on 14 November. To control the outbreak, a YF mass vaccination campaign was conducted and vector control implemented in parts of South Kordofan. Surveillance data were obtained from the Sudan Federal Ministry of Health. Clinical information and serum samples were obtained from a subset of patients with illness during the outbreak. Nomads, health personnel and village chiefs were interviewed about the outbreak. Mosquitoes were collected in 11 villages and towns in North and South Kordofan. From 10 September to 9 December 2005 a total of 605 cases of outbreak-related illness were reported, of which 45% were in nomads. Twenty-nine percent of 177 patients seen at clinics in Julud and Abu Jubaiyah had illness consistent with YF. Five of 18 unvaccinated persons with recent illness and 4 of 16 unvaccinated asymptomatic persons had IgM antibodies to YF virus. IgM antibodies to chikungunya virus were detected in five (27%) ill persons and three (19%) asymptomatic persons. These results indicate that both chikungunya and YF occurred during the outbreak.     

Immunogenicity and safety of yellow fever vaccine among 115 HIV-infected patients after a preventive immunisation campaign in Mali

The immune response to yellow fever (YF) vaccine and its safety among HIV-infected individuals living in YF endemic areas is not well understood. Following a national YF preventive immunisation campaign in Mali in April 2008, we assessed the immunogenicity and safety of 17D yellow fever vaccine (17DV) among HIV-infected patients in two HIV treatment centres in Bamako, Mali, by testing for neutralising antibodies and identifying serious adverse events following immunisation (AEFI). A YF neutralisation titre (NT) of 1:≥20 was considered to be adequate and protective. A serious AEFI included hospitalisation, any life-threatening condition, or death, occurring within 30 days following 17DV administration. Of 115 HIV-infected patients who reported having received 17DV, 110 (96%) were on combination antiretroviral therapy and 83 patients were tested for neutralising antibodies. Around the time of vaccination, median CD4 cell count was 389 cells/mm(3) (IQR 227-511cells/mm(3)); HIV-RNA was undetectable in 24 of 46 patients tested. Seventy-six (92%) of 83 participants had adequate immune titres 9 months after the immunisation campaign. Previous vaccination or flavivirus exposure could contribute to this finding. No serious AEFI was found in the 115 participants. In this small series, YF vaccine appeared to be immunogenic with a favourable safety profile in HIV-infected patients on antiretroviral therapy. Higher CD4 cell counts and suppressed HIV-RNA were associated with the presence of an adequate immune titre and higher NTs.     

A multipurpose serological survey in Kenya. 2. Results of arbovirus serological tests

Arbovirus infections are of public health interest in East Africa, where a very widespread epidemic of o''nyong-nyong fever was reported in 1959-60 and where the threat of yellow fever, present in neighbouring areas such as Ethiopia, remains. Sera collected in a serological survey in Kenya were therefore tested for antibodies against 3 group-A arboviruses (chikungunya, o''nyong-nyong and Sindbis), 6 group-B arboviruses (Zika, yellow fever, West Nile, Banzi, Wesselsbron and dengue 1), and Bunyamwera virus. The sera were examined mainly by the haemagglutination-inhibition test but a small proportion were also subjected to virus neutralization tests.     

Genetic and serologic properties of Zika virus associated with an epidemic, Yap State, Micronesia, 2007

Zika virus (ZIKV) is a mosquito-borne flavivirus first isolated in Uganda from a sentinel monkey in 1947. Mosquito and sentinel animal surveillance studies have demonstrated that ZIKV is endemic to Africa and Southeast Asia, yet reported human cases are rare, with <10 cases reported in the literature. In June 2007, an epidemic of fever and rash associated with ZIKV was detected in Yap State, Federated States of Micronesia. We report the genetic and serologic properties of the ZIKV associated with this epidemic.     

Observational study on immune response to yellow fever and measles vaccines in 9 to 15-month old children. Is it necessary to wait 4 weeks between two live attenuated vaccines?

BackgroundThe use of 2 live attenuated vaccines (LAV) is recommended to be simultaneous or after an interval of at least four weeks between injections. The primary objective of this study was to compare the humoral response to yellow fever (YF) and measles vaccines among children vaccinated against these two diseases, either simultaneously or separated by an interval of 7–28 days.Subjects and methodsA prospective, multicenter observational study was conducted among children aged 9–15 months. The primary endpoint was the occurrence of positive yellow fever antibodies after YF vaccine by estimating the titers of neutralizing antibodies from venous blood samples. Children vaccinated against YF 7–28 days after receiving the vaccine against measles (test group) were compared with children vaccinated the same day against these two diseases (referent group).ResultsAnalysis was performed on 284 children. Of them, fifty-four belonged to the test group. Measles serology was positive in 91.7% of children. Neutralizing antibodies against YF were detected in 90.7% of the test group and 92.9 of the referent group (p =0.6). In addition, quantitative analysis of the immune response did not show a lower response to YF vaccination when it took place 1–28 days after measles vaccination.DiscussionIn 1965, Petralli showed a lower response to the smallpox vaccine when injected 4–20 days after measles vaccination. Since then, recommendations are to observe an interval of four weeks between LAV not injected on the same day. Other published studies failed to show a significant difference in the immune response to a LAV injected 1–28 days after another LAV. These results suggest that the usual recommendations for immunization with two LAV may not be correct.ConclusionIn low income countries, the current policy should be re-evaluated. This re-evaluation should also be applied to travelers to yellow fever endemic countries.     

Absence of YF-neutralizing antibodies in vulnerable populations of Brazil: A warning for epidemiological surveillance and the potential risks for future outbreaks

Yellow Fever (YF) is an acute febrile illness caused by yellow fever virus (YFV), a mosquito-borne flavivirus transmitted to humans and non-human primates. In Brazil, YF is a public health threat and may cause recurrent epidemics, even with the availability of a vaccine. We evaluated the sero-status for YFV in 581 individuals living in a risk area for YF in Brazil. The area presents history of cases and is located in the southeast region of country where outbreaks of YF have been reported since 2016. Through, a PRNT assay, we found 25.8% of individuals lacking YF-neutralizing antibodies. Furthermore, neutralizing antibodies were not detected in 10 individuals with proven vaccination. Our findings reinforce the importance of surveillance systems and the need of an urgent intensification of immunization programs in regions with YFV circulation. Monitoring susceptible individuals that could act as potential disseminators for YFV in risk areas should also be considered.     

Observations on the epidemiology of Rift Valley fever in Kenya.

The epizootic range of Rift Valley fever in Kenya is defined from the results of virus isolations during epizootics, and form an extensive serological survey of cattle which were exposed during an epizootic. A study of the sera from a wide range of wild bovidae sampled immediately after the epizootic, showed that they did not act as reservoir or amplifying hosts for RVF. Virus isolation attempts from a variety of rodents proved negative. Rift Valley fever did not persist between epizootics by producing symptomless abortions in cattle in areas within its epizootic range. A sentinel herd sampled annually after an epizootic in 1968 revealed not one single seroconversion from 1969 to 1974. Certain forest and forest edge situations were postulated as enzootic for Rift Valley fever, and a small percentage of seroconversions were detected in cattle in these areas, born four years after the last epizootic. This has been the only evidence for the persistence of the virus in Kenya since 1968, and may be a part of the interepizootic maintenance cycle for Rift Valley fever in Kenya, which otherwise remains unknown.     

A study of the specificities of sequential antisera to variola and monkeypox viruses by radioimmunoassay

The specificities of antisera during development of the humoral antibody response to poxvirus antigens were examined in monkeys injected with chimp-9 whitepox virus or monkeypox virus. Sera were obtained from 3 African green (vervet) monkeys inoculated with chimp-9 whitepox virus, 1 rhesus monkey inoculated with monkeypox virus, and 2 rhesus monkeys inoculated with soluble monkeypox viral antigen. The sequentially obtained sera from each animal were adsorbed with uninfected chicken chorioallantoic membranes (CAM) or vaccinia virus-infected CAM. The adsorbed sera were tested by radioimmunoassay to determine the specificity of the residual antibodies to vaccinia, variola, and monkeypox viruses. The adsorbed sera at different stages of the immune response showed increasing specificity with time after inoculation. Generally, antibodies in sera collected earlier than 21-27 days after immunization could not be identified after adsorption, but late sera could be identified unequivocally.