[Preliminary results of an entomological survey of the potential arbovirus vectors in the French Territory of Afars and Issas]

The preliminary results of an entomological survey of the potential arbovirus vectors in the French Territory of Afars and Issas are exposed. 25 culicid species were recorded, specially Anopheles gambiae, Culex tritaeniorhynchus, C. univittatus, C. pipiens fatigans, Aedes caspius, Ae. vittatus, Ae. arabiensis. Ae. aegypti larvae were collected on a dhow arriving in Djibouti. 5 species of ticks and 4 sandfly speices were also recorded. The epidemiological modalities of certain arboviroses in this territory are discussed, depending on these entomological data and the uniterrupted travelling of local populations.

[Arthropodan vectors of human parasites: their pathology and defence reactions (author's transl)]

Many infectious diseases of man are transmitted by arthropods. It is not known whether the carriers were primarily susceptible or insusceptible as the cycle first came into existence. Similarly so little is understood, why some arthropods are suitable vectors and other close relatives are not suitable. Only the hypothesis that the agents are original parasites and not at all commensals or symbionts can reasonably explain the evolution of the cycles. It is shown by means of numerous examples how the vector can be damaged by the parasite. Some organs of the vector might become unfunctional, the reproduction rate might be lowered and the vector itself may die. On the other hand the vector has at its disposal defence mechanisms which are according to our present point of view limited to the midgut barrier and the hemolymph reactions. Both components of the system, the vector and the parasite together are capable of change under the influence of the other. For example through the high mortality of the susceptible part of a vector population the resistant and the tolerant individuals will be selected. On the contrary harmful parasites cannot transmit their genetic information when their virulence is so high that the vector will suffer death. Due to this the parasites succumb to a selection pressure and only the careful treatment of the vector is rewarding. Besides this rough pattern of actions and reactions exists also the possibility of developing finer adjustments, with the "molecular mimikry" as a well known example.

The differential transmissibility of Myxoma virus strains of differing virulence grades by the rabbit flea Spilopsyllus cuniculi (Dale)

Laboratory studies showed that few rabbit fleas (Spilopsyllus cuniculi (Dale)) transmitted myxomatosis after removal from wild rabbits (Oryctolagus cuniculus (L) that had been infected for fever than 10-12 days, irrespective of the virulence of the myxoma virus strain involved. Rabbits infected with fully virulent (Grade I) strains died within 10-15 days and few fleas from these hosts became infective; averaging all the samples takem. 12% of the fleas were infective. Also, few fleas acquired infectivity on individual rabbits which covered from infection with attenuated strains; the mean was 8% infective. Rabbits which died between 17 and 44 days after infection had higher proportions of infective fleas at all sampling times; the mean was 42% infective. Male and female fleas transmitted virus with equal efficiency. For rabbits infected with any of the attenuated virus strains the mean percentage of infective fleas was inversely related to the survival time of the host. Rabbits infected with moderately attenuated strains (Grades IIIA and IIIB) had, on average, the highest proportion of infective fleas; hence such strains have a selective advantage and have become predominant under natural conditions in Britain. The changes that might occur if there is an increase in host resistance to myxomatosis are discussed.

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.

Distribution and ecological consideration of Breinlia booliati infecting wild rodents in Malaysia

Seven of the 18 species of lowland forest terrestrial and semi-arboreal murids were found naturally infected with Breinlia booliati. Of these, two species, Rattus sabanus and R. cremoriventer, were found to be the most preferred hosts. None of the murids from the highland, field or human-inhabited areas was infected. This could have been due more to the greater scarcity of the vectors in these habitats than to the susceptibility of the hosts. The absence of this parasite in the squirrels examined may be attributed either to host specificity or to the normal activity cycles or vertical stratification of the vectors, separating them in space and/or time from the squirrels. The pattern of dispersion of the parasite is influenced by the wide distribution of suitable hosts, and the hypothesis that the parasite is of forest origin is discussed.

Arthropod-borne viral infections of man in Nigeria, 1964-1970

During the years 1964 to 1970, 171 arboviruses of 15 different types were isolated from humans in Nigeria. Isolation rates were highest in 1969, and lowest in 1965 and 1967. Monthly arbovirus activity was highest in the rainy season months of June, July and August and lowest in the dry months of January and February. Viruses were isolated from all age groups, with the majority from children one to four years old. The viruses isolated in largest numbers were chikungunya and yellow fever, which caused epidemics in 1969, and dengue types 1 and 2 and Tataguine, which are endemic in Ibadan. Bwamba virus was isolated in 1964 and 1969, and Bunyamwera group viruses were encountered for the first time in 1969. Other viruses recovered less frequently were Zika, Igbo-Ora (an agent related to o'nyong-nyong), two viruses related to the Uganda mosquito virus Ug MP 359, Dugbe, Thogoto, Lebombo and Shuni. Several of these are new agents and have not previously been isolated from man. Clinical details are presented where available.