Transmission thresholds for dengue in terms of Aedes aegypti pupae per person with discussion of their utility in source reduction efforts

The expense and ineffectiveness of drift-based insecticide aerosols to control dengue epidemics has led to suppression strategies based on eliminating larval breeding sites. The present work attempts to estimate transmission thresholds for dengue based on an easily-derived statistic, the standing crop of Aedes aegypti pupae per person in the environment. We have developed these thresholds for use in the assessment of risk of transmission and to provide targets for the actual degree of suppression required to prevent or eliminate transmission in source reduction programs. The notion of thresholds is based on 2 concepts: the mass action principal- the course of an epidemic is dependent on the rate of contact between susceptible hosts and infectious vectors, and threshold theory - the introduction of a few infectious individuals into a community of susceptible individuals will not give rise to an outbreak unless the density of the vectors exceeds a certain critical level. We use validated transmission models to estimate thresholds as a function of levels of pre-existing antibody levels in human populations, ambient air temperatures, and the size and frequency of viral introduction. Threshold levels were estimated to range between about 0.5 and 1.5 Ae. aegypti pupae per person for ambient air temperatures of 28 degrees C and initial seroprevalences ranging between 0 percent to 67 percent. Suprisingly, the size of the viral introduction used in these studies, ranging between 1 and 12 infectious individuals per year was not seen to significantly influence the magnitude of the threshold. From a control perspective, these results are not particularly encouraging. The ratio of Ae. aegypti pupae to human density has been observed in limited field studies to range between 0.3 and >60 in 25 sites in dengue-epidemic of dengue-susceptible areas in the Caribbean, Central America, and South East Asia. If, for purposes of illustration, we assume an initial seroprevalence of 33 percent, the degree of suppression required to essentially eliminate the possibility of summertime transmission in Puerto Rico, Honduras, and Bangkok, Thailand was estimated to range between 10 percent and 83 percent; however in Mexico and Trinidad, reductions of >90 percent would be required.(AU)

Transmission thresholds for dengue in terms of Aedes aegypti pupae per person with discussion of their utility in source reduction efforts

The expense and ineffectiveness of drift-based insecticide aerosols to control dengue epidemics has led to suppression strategies based on eliminating larval breeding sites. With the notable but short-lived exceptions of Cuba and Singapore, these source reduction efforts have met with little documented success; failure has chiefly been attributed to inadequate participation of the communities involved. The present work attempts to estimate transmission thresholds for dengue based on an easily-derived statistic, the standing crop of Aedes aegypti pupae per person in the environment. We have developed these thresholds for use in the assessment of risk of transmission and to provide targets for the actual degree of suppression required to prevent or eliminate transmission in source reduction programs. The notion of thresholds is based on 2 concepts: the mass action principal-the course of an epidemic is dependent on the rate of contact between susceptible hosts and infectious vectors, and threshold theory-the introduction of a few infectious individuals into a community of susceptible individuals will not give rise to an outbreak unless the density of vectors exceeds a certain critical level. We use validated transmission models to estimate thresholds as a function of levels of pre-existing antibody levels in human populations, ambient air temperatures, and size and frequency of viral introduction. Threshold levels were estimated to range between about 0.5 and 1.5 Ae. aegypti pupae per person for ambient air temperatures of 28 degrees C and initial seroprevalences ranging between 0% to 67%. Surprisingly, the size of the viral introduction used in these studies, ranging between 1 and 12 infectious individuals per year, was not seen to significantly influence the magnitude of the threshold. From a control perspective, these results are not particularly encouraging. The ratio of Ae. aegypti pupae to human density has been observed in limited field studies to range between 0.3 and >60 in 25 sites in dengue-endemic or dengue-susceptible areas in the Caribbean, Central America, and Southeast Asia. If, for purposes of illustration, we assume an initial seroprevalence of 33%, the degree of suppression required to essentially eliminate the possibility of summertime transmission in Puerto Rico, Honduras, and Bangkok, Thailand was estimated to range between 10% and 83%; however in Mexico and Trinidad, reductions of >90% would be required. A clearer picture of the actual magnitude of the reductions required to eliminate the threat of transmission is provided by the ratio of the observed standing crop of Ae. aegypti pupae per person and the threshold. For example, in a site in Mayaguez, Puerto Rico, the ratio of observed and threshold was 1.7, meaning roughly that about 7 of every 17 breeding containers would have to be eliminated. For Reynosa, Mexico, with a ratio of approximately 10, 9 of every 10 containers would have to be eliminated. For sites in Trinidad with ratios averaging approximately 25, the elimination of 24 of every 25 would be required. With the exceptions of Cuba and Singapore, no published reports of sustained source reduction efforts have achieved anything near these levels of reductions in breeding containers. Practical advice on the use of thresholds is provided for operational control projects.

Laboratory colonization and maintenance of Toxorhynchites moctezuma

A colony of Toxorhynchites moctezuma was established at the Caribbean Epidemiology Centre in Trinidad in 1984. Toxorhynchites moctezuma was maintained in cages with high humidities. Eggs were deposited most frequently in a cut bicycle tire containing water. A minimum of 42 h was required for hatching, but 94 percent hatched between 43 and 51 h. Aedes aegypti larvae were supplied as prey. Larval development times varied with the quantity of prey offered, but when fed ad lib, peak developmental time was 18 days. Mean pupal developmental time was 5.5 days. Although only 12 percent of larvae survived to pupation in 3 years of production, our experience indicates this species would be a likely candidate for mass production and release.(AU)

Incidence and diel pattern of oviposition outdoors of the mosquito Aedes aegypti (L.) (Diptera: Culicidae) in Trinidad W. I. in relation to solar aspect

The incidence and diel patterns of oviposition of domestic Aedes aegypti in Trinidad, West Indies during the middle of the wet season (July and August) was compared, using modified ovitraps, in unshaded peridomestic outdoor sites facing east or west. Sites close to each other along a transect differed greatly in the total eggs they received, 18.2 percent of sites receiving 50.6 percent of the eggs. Sites receiving most eggs were close to other breeding sites. The incidence of oviposition in sites facing west, as measured by the number of eggs laid (>59 percent) and by the number of occurrences of eggs (>57 percent), was greater than in sites facing east. The diel pattern of oviposition was closely similar in sites facing east and west, indicating that the sun's azimuth at the moment of oviposition does not determine whether a female lays in sites facing east or west. The diel pattern of oviposition resembled an earlier study in being bimodal but differed from it in featuring a much larger morning peak (comprising more than 34 percent of eggs laid v. about 5 percent in the earlier study). This bimodal pattern closely resembles, the diel periodicity of landing on humans described previously by several authors and is probably close to the typical oviposition pattern for Ae. aegypti; an artefact may have modified the oviposition periodicity characterized earlier. In the evening significantly more eggs were laid facing east. These findings have implications for effective placement of ovitraps for surveillance or suppression of Ae. aegypti, and give rise to an hypothesis which invokes responses to microclimate to account for the form of the bimodal oviposition pattern (AU)

Observations on the seasonal incidence and diel oviposition periodicity of haemagogus mosquitoes (Diptera: Culicidae) in Trinidad, W.I.: part I

The diel oviposition periodicity of sylvan Haemagogus janthinomys Dyar in the Pt. Gourde Forest, Trinidad, West Indies was monitored weekly for 53 weeks using conventional ovitraps. During the wet season (September to November 1981 and May to September 1982) a large well-defined unimodal peak occurred six to eight hours after sunrise (comprising 94 percent of eggs laid and 91 percent occurrences). During the dry season (December 1981 to May 1982), a small less-well-defined peak occurred two to four hours before sunset (comprising 6 percent of eggs laid). The combined data showed a diurnal oviposition pattern with eggs laid from noon to sunset. The numbers of eggs and of used ovitraps found during the wet season were significantly (P<0.001) higher than those found during the dry season.(AU)

Bacteriological aspects of chicken processed in Jamaica

Bacteriological aspects of chicken processed in Jamaica were studied. The effect of processing on the number of bacteria on chicken from the largest processing plant (Jamaican Broilers) was investigated with reference to international standards. Microbial numbers, temperature and total organic matter content of selected washing tanks were examined. The number of bacteria on the hands of the workers and the final temperature of the carcasses after packaging and before freezing were also determined. Bacterial counts on the Jamaica Broilers chickens were compared with those of chickens from a smaller processing plant. Bacterial numbers on chickens sampled at retail outlets were also investigated and found to differ significantly from those on chicken sampled at the processing plant. Microbial examination of the delivery trucks, and observation of the hygienic condition of the outlets were conducted. The question of whether the transport system or condition of the retail outlets was the major factor responsible for the increase, was discussed. A correlation was found between the level of hygiene of the outlets and the number of bacteria on retail chickens. The shelf life and dynamics of the microbial population of chicken during storage at 4§C was examined. Chicken purchased at a retail outlet was stored at 4§C and rinsed in 0.1 percent peptone every 48 hours. The number of bacteria per ml of rinse water was determined. A total of 50 to 100 organisms were isolated and purified from total count plates of each sample day. Preliminary identification was by the method of Shewan et al. (1960) with confirmation by further biochemical tests. Pseudomonas species were found to be the major group of spoilage bacteria. Pseudomonas isolates were classified into different biochemical groups which were shown to undergo changes in distribution during storage. They were also characterised by their intrinsic antibiotic resistance patterns. While all the isolates were found to be resistant to penicillin and ampicillin, only 28 percent displayed high resistance to one or more of six other antibiotics tested. This investigation of bacteriological conditions of chicken processed in Jamaica is a pioneer effort. Nevertheless, it has provided sufficient information for the compilation of recommendations which may help to improve the microbial quality of chickens, with consequent increase in the shelf life. This study may also be useful to other Caribbean islands in which the conditions of processing, transportation and retail handling of chickens are similar (AU)

ELISA for Schistosoma mansoni infection: durability of blood spots on filter paper

We investigated the persistence of seropositivity for Schistosoma mansoni infection by ELISA in blood spots on filter paper strips stored for up to 16 weeks under conditions ranging from "tropical" temperatures and humidity to storage at -20 degrees Centigrade. High humidity (60 to 85 percent RH) appeared to cause greater deterioration of samples than high temperatures. Storage in a dessicator at 30 degrees Centigrade was as effective as storage in a freezer at -20 degrees Centigrade (Summary)

Tumours of the testis in Jamaica

Malignant tumours of the testis are reported to be rare in negroes. All cases reported in the records of the University Hospital of the West Indies and the Jamaica Cancer Registry over the period June 1958 to December 1976 were studied. There were 26 cases, giving a world standardised incidence of 0.40 per 100,000 per year. This incidence is higher than that seen in African negroes but lower than that of negroes in North America. The peak age incidence is a decade later that commonly seen in high incidence countries (AU).

Direct determination of luminal diameter changes in intrapulmonary airways

A method is decribed for the direct measurement of changes in luminal diameter at all levels of the airway. Using this method it was found that (i) abrupt bronchiolar collapse occurred in the preterminal and terminal bronchioles once the luminal diameter was reduced to a critical level: (ii) decreased temperatures resulted in airway narrowing which was reversed by increasing the temperature to above 25 degrees C; as a rule, airway narrowing followed a cranial to caudal direction, and higher concentration of a drug being required to close the peripheral airways; (iii) bronchodilators except Carbuterol had no effect on resting bronchial tone or on acetylcholine-induced constriction in the absence of O-adrenoreceptor blockade; (iv) at 35 degrees C rhythmic waves (frequency 6--20/min) were observed; these waves travelled from the periphery in a cranial direction. (AU)

Climate and painful crisis of sickle-cell disease in Jamaica

In a 10-year retrospective study a close correlation was found between low temperatures and hospital admissions for severe painful crises of sickle-cell disease.(Summary)

Predation on peridomestic mosquitoes by hylid tadpoles on Grand Bahamas Island

In order to determine whether tadpoles of Hyla septentrionalis are effective predators of larval peridomestic mosquitoes, larvae were exposed to tadpoles and the association between larvae and tadpoles in nature was recorded. Tadpoles were cannibalistic, eating egg masses of their own species, as well as a variety of material of both plant and animal origin. Tadpoles captured and ate more than 13 larval Culex pipiens per day. In natural breeding sites on Grand Bahamas Island, larval C. papiens were never abundant in sites that contained tadpoles and when tadpoles were introduced into vessels that contained mosquitoes, these containers soon became free of mosquitoes. The habitat of H. septentrionlis tadpoles in the study area seemed identical to that of larval C. papiens and more than half of apparently suitable sites contained tadpoles. Thus, the presence of these frogs appeared to limit the abundance of C. papiens.(AU)

The influence of temperature on the biology of Biomphalaria glabrata (Say), intermediate host of Schistosoma mansoni on St. Lucia, West Indies

Life tables were obtained over 16 fortnights for three cohorts of Biomphalaria glabrata at 20§C, 25§C and 30§C respectively. At 35§C no eggs hatched. The intrinsic rate of natural increase r was greater at 25§C(0.8834) but the mean generation time T was similar to that at 30§C(8-9 fortnights). Egg production and survival were reduced at 30§C but r(0.6866) was higher than at 20§C(0.4725). T was greatest at 20§C(12 fortnights) due to the delay in the onset of egg laying. The growth rate at 30§C was only a little faster than that at 25§C, but both were greater than that at 20§C. Comparison of these results with those of similar studies elsewhere suggests that the St. Lucians strain of B. glabrata is adapted to living in unstable, temporary habitats. B. glabrata, unlike its African relatives, is capable of surviving prolonged exposure to temperatures of 30§C so that, despite reduced fecundity, it can colonize the hot, low lying coastal plains of northeastern South America. (AU)

Environmental temperature and response to premedicant drugs

A total of 105 subjects have been studied, 70 in the hot climate of Jamaica and 35 in the temperate climate of England. Following the same premedication a very significant alteration from resting levels in the arterial pH and PaCO3 occurred in the subjects in Jamaica, but there was no change in the Jamaican and English groups in England. This sensitivity was shown to be related to a lowereing of the metabolic rate in the hot climate. There was no relationship to race, liver disease, or immediate ambient room temperature (AU)

Arbovirus studies in Bush Bush Forest, Trinidad, W.I., September 1959-December 1964. I. Description of the study area

Bush Bush Island and the surrounding Nariva Swamp, eastern Trinidad, are described in terms of their topography, flora, and fauna as an introduction to a series of papers reporting the results of 5 years of arbovirus studies in this area. (AU)