History of epidemiological aspects of yellow fever.

This review attempts to follow the trail of the development of epidemiological aspects and concepts of yellow fever and yellow fever transmission (vectors, vertebrate hosts, spacing of epidemic outbreaks) with less emphasis on well-documented early history and more emphasis on epidemiological problems still remaining, plus discussion of possible means of resolving certain of these problems.

The known and the unknown in yellow fever ecology and epidemiology.

Earlier epidemiological concepts are discussed briefly. Attention is focussed on several of the puzzling problems in the epidemiology of the disease as seen today. The recurring outbreaks of yellow fever in regions where it has been absent, as well as the absence of yellow fever in regions where the vectors exist are discussed. Features of vaccination are discussed, including the long persistence of humoral antibodies and long duration of effective protection. The possibility of changes in virulence of yellow fever strains is mentioned. In some of the modern outbreaks, virulence for human beings appears to be lower than virulence noted some centuries ago, and a hypothesis is advanced. The possible influence of virus-protecting antibodies acquired by infection by other flaviviruses is mentioned. With respect to the vectors of yellow fever, the possibility of differing susceptibility and of different behaviour of various strains is discussed. Transovarial transmission of the virus in the vector mosquito is mentioned as a possible mechanism for long persistence of the virus in nature. Infection of ticks and transovarial transmission of virus in ticks are also mentioned. The need for a diagnostic test to differentiate immunity produced by a naturally acquired infection from immunity induced by vaccination is stressed.

Persistence of neutralizing antibody 30-35 years after immunization with 17D yellow fever vaccine.

Previous studies on the duration of antibody following vaccination with 17D yellow fever (17D YF) virus vaccine have indicated that immunity persists for at least 17 years and suggest that the vaccine may provide lifelong immunity. We studied sera obtained from 149 veterans of the Second World War, 30 - 35 years after military service during which YF vaccination was required for defined groups. A significantly high proportion of "vaccinated" subjects was found to be seropositive to 17D YF virus. The highest proportion of seropositive "vaccinated" veterans (97%) was among navy and air corps personnel, while only 60% of "vaccinated" army personnel and 19% of "unvaccinated" personnel were seropositive. This study suggests that (i) antibody to 17D YF virus, as measured by the plaque-reduction neutralization test (PRNT), persists for 30 years or more following administration of a potent vaccine; (ii) army personnel often had not received potent vaccine, even though their service history indicated that they should have been vaccinated; (iii) some personnel were vaccinated, although their service did not include vaccination-designated areas; and (iv) 88% of veterans with persistent PRNT antibody to 17D YF virus also had mouse-protective antibody against French neurotropic YF virus.

Powassan virus in Ixodes cookei and Mustelidae in New England.

Powassan virus was recovered from a pool of 3 nymphal and 1 adult female Ixodes cookei removed from a striped skunk (Mephitis mephitis) trapped in Massachusetts during 1967 and from a pool of 9 nymphal I. cookei from a long-tailed weasel (Mustela frenata) captured in Connecticut during 1978. Virus was detected in the blood of both mammals. Hemagglutination-inhibiting (HI) antibody to Powassan virus was demonstrated in 16.0% of the skunks sampled in Connecticut, and neutralizing antibody was detected in 83.3% of the skunks tested from Massachusetts. HI antibody was found in 1 of 6 long-tailed weasels from Connecticut and 1 of 6 short-tailed weasels (Mustela erminea) from Maine.

Aedes aegypti strain fitness for yellow fever virus transmission.

Three geographical strains of Aedes aegypti from Thailand (Amphur), East Africa (Kampala), and the West Indies (Santo Domingo) were compared for susceptibility to infection with low-passage yellow fever virus (French viscerotropic) as well as for ability to transmit virus by bite at varying extrinsic incubation periods. Santo Domingo strain appeared the most competent and Kampala the least when mosquitoes were exposed to a low level virus-infecting blood meal; at higher virus levels, a similar trend was noted but differences were less evident and in no case were the differences statistically significant. All three strains were infected with and transmitted yellow fever virus.

Avian arboviruses of the Witless Bay seabird Sanctuary, Newfoundland, Canada.

A virologic and serologic survey of arbovirus infections among seabirds and seabird ticks, Ixodes uriae, on Great Island. Witless Bay, Newfoundland, Canada, was conducted during 1971 and 1972. Kermerovo (Great Island, Bauline) and Sakhalin (Avalon) group viruses previously reported from birds and/or ticks on Great Island were prevalent among avian populations, while conclusive evidence of known nonindigenous serotypes was lacking. Circumstantial evidence-hemagglutination inhibiting antibody-of an unidentified member of the group B tick-borne encephalitis complex transmitted among marine birds of North America by I. uriae is reported. No evidence of human infections with any of these viruses was detected in a small number of biologists doing research on Great Island.

Lassa fever in Onitsha, East Central State, Nigeria in 1974.

Three cases of Lassa fever occurred in Onitsha, East Central State, Nigeria, in January and February 1974. The first case was a 19-year-old Nigerian; the other 2 cases were German missionary physicians at St Charles Borromeo Hospital, Onitsha, one of whom cared for the patient who was the first case. Thus, 2 of the 3 cases were hospital acquired. Investigations failed to discover a village outbreak or the source of virus for the first case. A serosurvey of 258 hospital staff members and contacts of the 3 cases showed no other persons with antibody to Lassa virus. The absence of Lassa virus antibody in a high-risk group indicates a low or nonexistent level of past Lassa virus activity in southeastern Nigeria.