Virus Surveillance and Discovery

Background

  The WHO Reference Centre, located in the Centre for Virology Research at KEMRI in Nairobi, is a collaborative effort with the CDC and USAMRU.  Its primary responsibility is to operate the national yellow fever surveillance network, which monitors suspected yellow fever cases at 22 hospitals and clinics around Kenya.  Because other hemorraghic viruses often present with symptoms of inhibited coagulation similar to those of classic yellow fever (and about 40% of yellow fever cases initially present without those symptoms), the network is designed to screen for a wide variety of other viruses.  Case definition therefore for sample collection is 2 of the following: temperature >38^oC, Jaundice, Hemorrhage, encephalitis, and renal dysfunction.  Isolations are done from mosquito C6-36 and Vero cell cultures in which cytopathic effects (CPE) are noted. Viruses, as determined by presence of CPE or IFA, are inoculated into mice.   Routine IFA uses the NIH Arbovirus Pooled Grouping Serum for alpha viruses, flaviviruses, and bunya viruses.  In addition, the Reference Centre, is often called upon to support the response to suspected viral epidemics, such as the outbreak of Ebola in Uganda during 2000.

Recent Work

In October 2000, the Reference Centre diagnosed the first confirmed case of human Crimean-Congo Haemorrhagic Fever (CCHF) in Kenya. Using RT-PCR a diagnosis was made within 24 hours after receipt of the serum sample from one of the Kenya Yellow Fever surveillance sites. The tick-borne CCHF virus is notorious for secondary transmission in hospital settings but fortunately no secondary cases amongst the hospital staff or close family members were recorded. Follow-up studies suggested that exposure of the single fatal case to CCHF virus was most probably through the bite of an infected tick.

Ticks have the ability to transmit a wide range of viral pathogens, including CCHF virus mentioned above. The Reference Centre is currently investigating 16,250 ticks collected at two Nairobi slaughterhouses for viral pathogens. The ticks have been taxonomically identified to species level, pooled and inoculated in to newborn mice and cell culture systems for virus isolation. To date 39 viruses have been isolated and these are being characterised using indirect immunofluorescence, in conjunction with pooled immune grouping serum, and RT-PCR. Common East African tick-borne viruses including Dugbe virus (Bunyavirus) and Thogoto virus (Orthomyxovirus) have been characterised, but the majority of the virus isolates may require additional expertise to fully identify them. It is expected that some of the isolates may prove to be ‘new’ viruses. As a follow-up to this study, the Department of Veterinary Sciences, Kenya has given the Reference Centre the permission to conduct a sero-survey of the slaughterhouse staff from where the ticks were collected to determine exposure levels to the viruses isolated during the study and job associated risk factors for exposure (the slaughtering process in Kenya is completely manual from start to finish with large numbers of staff involved).

Dengue

Background

Dengue a flavivirus transmitted by aedes mosquitoes is the most cosmopolitan and important arbovirus in the world.The virus occurs in four types (1,2,3 and 4). Infection with one type confurs life long immunity to that type but not to the others. For reasons not yet fully understaood, but apparently related to the type or sequence in which one is infected by different types, some people develop dengue Haemorraghic Fever Syndrome (DHF), which is often fatal.Although usually associated with Asia and he Americas, it has been recorded spoardically from Africa including form coastal Kenya in 1980.Because cases of dengue have been found so infrequently in Kenya, epidemics have generally been thought to originate from introduced cases in 2000 we began to test the hypothesis that dengue transmissiom was endemic to Kenya and if true to what extent transmission occured during inter epidemic periods.

      In collaboration with the Oxford University / Wellcome Foundation research unit operating at Kilifi, about 50 Km north of Mombasa, we obtained specimens from pediatric admissions with undiagnosed fever.  From these, three isolations were made of dengue, serotype 2; one of which was sequenced at the CDC, Ft. Collins and found to be nearly identical to that isolated from a Canadian tourist infected at the coast in 1980.  Subsequently we found nearly 8% of more than 160 other sera from ill children presenting to Kilifi District Hospital had elevated antibodies to dengue, including from several neonates whose mothers presumeably were recently infected.  None of the positive cases manifested DHF.

Objectives

• Determine the prevalence of dengue at several sites along the Indian Ocean coast.

• Identify vectors and risk factors

• Determine serotypes and search for cases of DHF syndrome

Recent Work

   The current 12 month study at Kilifi focuses on detailing risk to the putative vector, Aedes aegypti.  This common mosquito, which is also the vector of yellow fever, breeds in containers.  But unlike in Southeast Asia, where Ae. aegypti breeds in domestic water containers, rests in houses and attacks in shady areas in and around houses, we have found virtually no domestic habitation by the mosquito in Kilifi.  This suggests that dengue epidemiology is significantly different in Kenya and may possibly include other vectors.  We have also begun to test a dengue control system based on lethal domestic ovitraps devised by the Army, which has shown promising efficacy in Southeast Asia; but the difference in transmission dynamics at Kilifi suggest it may not be effective in Kenya.  In 2001 we will begin a 12 month cohort study that will measure both seroconversion in the human population and vector bionomics at a site near Malindi, 50 Km north of Kilifi.

Tick Typhus

Background

    Typhus is caused by Rickettsia, small, bacteria-like parasites that can invade the cells of humans and other mammals, but usually infect insects, mites and ticks, which pass them vertically in their eggs.  Tick typhus has only recently been recognized as a serious endemic disease of humans in Africa.  The species of parasite responsible, Rickettsia africae, had been recorded only once from Kenya, in a tourist diagnosed in UK.  The disease caused by R. africae includes fever and rash but has never been recorded as fatal.

    In 2000 we became aware of  the death of an American teacher with symptoms indicative of typhus at Kijabe A. I. C Hospital, a yellow fever sentinel site in the Rift Valley, 1 hour west of Nairobi.  Susequent amplification and sequencing of DNA from autopsy specimens by collaborators at the University of Maryland Medical School incriminated R. africae.  This is the first known fatality attributed to R. africae, possibly signifying a more virulent strain.

Recent Work

    With a team from the University of Maryland we did a dry season collection of ticks from vegetation and domestic and wild animals at Masai Mara refuge, where it had been reported that a group of school children, also from Kijabe, had contracted a typhus like disease on a camping trip.  One pool of 7 ticks was positive for R. africae.  A more extensive wet season collection that netted thousands of specimens from the same locality is now being processed in Baltimore. 

    Our future objectives will be to include typhus surveillance in routine surveillance at the yellow fever sites, as well as as active case detection and risk assessment, including serosurveys and vector incrimination, at foci of transmission.  This will be augmented by DNA comparison of the Kenyan strains with those from elsewhere in Africa and a search in caught ticks for other Rickettsia and viruses.