Dan W. Meyrowitsch

Is the current decline in malaria burden in sub-Saharan Africa due to a decrease in vector population?

BackgroundIn sub-Saharan Africa (SSA), malaria caused by Plasmodium falciparum has historically been a major contributor to morbidity and mortality. Recent reports indicate a pronounced decline in infection and disease rates which are commonly ascribed to large-scale bed net programmes and improved case management. However, the decline has also occurred in areas with limited or no intervention. The present study assessed temporal changes in Anopheline populations in two highly malaria-endemic communities of NE Tanzania during the period 1998-2009.MethodsBetween 1998 and 2001 (1st period) and between 2003 and 2009 (2nd period), mosquitoes were collected weekly in 50 households using CDC light traps. Data on rainfall were obtained from the nearby climate station and were used to analyze the association between monthly rainfall and malaria mosquito populations.ResultsThe average number of Anopheles gambiae and Anopheles funestus per trap decreased by 76.8% and 55.3%, respectively over the 1st period, and by 99.7% and 99.8% over the 2nd period. During the last year of sampling (2009), the use of 2368 traps produced a total of only 14 Anopheline mosquitoes. With the exception of the decline in An. gambiae during the 1st period, the results did not reveal any statistical association between mean trend in monthly rainfall and declining malaria vector populations.ConclusionA longitudinal decline in the density of malaria mosquito vectors was seen during both study periods despite the absence of organized vector control. Part of the decline could be associated with changes in the pattern of monthly rainfall, but other factors may also contribute to the dramatic downward trend. A similar decline in malaria vector densities could contribute to the decrease in levels of malaria infection reported from many parts of SSA.

Change in composition of the Anopheles gambiae complex and its possible implications for the transmission of malaria and lymphatic filariasis in north-eastern Tanzania

BackgroundA dramatic decline in the incidence of malaria due to Plasmodium falciparum infection in coastal East Africa has recently been reported to be paralleled (or even preceded) by an equally dramatic decline in malaria vector density, despite absence of organized vector control. As part of investigations into possible causes for the change in vector population density, the present study analysed the Anopheles gambiae s.l. sibling species composition in north-eastern Tanzania.MethodsThe study was in two parts. The first compared current species complex composition in freshly caught An. gambiae s.l. complex from three villages to the composition reported from previous studies carried out 2–4 decades ago in the same villages. The second took advantage of a sample of archived dried An. gambiae s.l. complex specimens collected regularly from a fourth study village since 2005. Both fresh and archived dried specimens were identified to sibling species of the An. gambiae s.l. complex by PCR. The same specimens were moreover examined for Plasmodium falciparum and Wuchereria bancrofti infection by PCR.ResultsAs in earlier studies, An. gambiae s.s., Anopheles merus and Anopheles arabiensis were identified as sibling species found in the area. However, both study parts indicated a marked change in sibling species composition over time. From being by far the most abundant in the past An. gambiae s.s. was now the most rare, whereas An. arabiensis had changed from being the most rare to the most common. P. falciparum infection was rarely detected in the examined specimens (and only in An. arabiensis) whereas W. bancrofti infection was prevalent and detected in all three sibling species.ConclusionThe study indicates that a major shift in An. gambiae s.l. sibling species composition has taken place in the study area in recent years. Combined with the earlier reported decline in overall malaria vector density, the study suggests that this decline has been most marked for An. gambiae s.s., and least for An. arabiensis, leading to current predominance of the latter. Due to differences in biology and vectorial capacity of the An. gambiae s.l. complex the change in sibling species composition will have important implications for the epidemiology and control of malaria and lymphatic filariasis in the study area.

Bancroftian filariasis: analysis of infection and disease in five endemic communities of north-eastern Tanzania

Clinical and parasitological surveys for bancroftia filariasis were carried out in five endemic communities in north-eastern Tanzania, covering a population of 3086 individuals. High microfilarial (mf) prevalences (17.7%-34.7%) and mf geometric mean intensities (251-1122 microfilariae/ml) were observed in the communities. The mf prevalence generally increased with age, but often levelled out in the older age groups. Larger variability was observed in individual mf intensities and no clear association between mf geometric mean intensity and age or sex was seen. Hydrocele was the most common clinical manifestation (with a prevalence of 30.2%-40.0% in male subjects aged > or = 20 years) followed by leg elephantiasis (with a prevalence of 2.0%-6.8% in all subjects aged > or = 20 years). In four of the five communities, there was no significant difference in mf prevalence in males aged > or = 20 years between those with and without hydrocele. In all the communities, the mf geomtric mean intensities in microfilaraemic males with and without hydrocele were not significantly different. The present study therefore did not indicate any association between hydrocele in males (the most common type of chronic clinical manifestation seen) and presence or absence of microfilaraemia. In contrast, only two (4.4%) of the 45 subjects with leg elephantiasis were microfilaraemic. In children aged 1-15 years, mf prevalence was significantly higher among those with microfilaraemic mothers (18.0%) than among those with amicrofilaraemic mothers (7.9%). The children of microfilaraemic mothers were therefore at 2.3-fold higher risk of becoming microfilaraemic than the children of amicrofilaraemic mothers. No relationship between the mf prevalence of the children and the mf status of their fathers was observed.

Bancroftian filariasis: the pattern of microfilaraemia and clinical manifestations in three endemic communities of Northeastern Tanzania

Individuals from three villages in northeastern Tanzania, located 40 km inland from the Indian Ocean coast, were surveyed for parasitological and clinical evidence of bancroftian filariasis. Microfilarial (mf) prevalences ranged from 22.2 to 37.6%, and mf geometric mean intensities (GMI) ranged from 546 to 735 mf/ml blood, in the three villages. Microfilaraemia was rare in children below five years. The mf prevalences increased with age, reaching from 35.9 to 49.2% in individuals aged 45 years and above. No association between mf GMI and age was observed in any of the villages. Hydrocele was the most common chronic clinical manifestation, with prevalences ranging from 14.5 to 21.3% for all males, and from 52.9 to 62.1% for males aged 45 years and above. From 0.6 to 3.3% of the inhabitants in the three villages had elephantiasis, which most often affected the legs. Microfilaraemia was common in males with hydrocele, and in males of 45 years and above there was no significant difference in mf prevalence between males with (42.5%) and without (55.2%) hydrocele. In contrast, microfilariae were only detected in the blood from one of 18 individuals with elephantiasis. With respect to hydrocele, the present results do not support recently forwarded hypotheses assuming a general negative relationship between microfilaraemia and chronic clinical manifestations in bancroftian filariasis.

Wuchereria bancrofti in Tanzania: microfilarial periodicity and effect of blood sampling time on microfilarial intensities

The circadian periodicity of Wuchereria bancrofti microfilarial (mf) intensities in peripheral blood was analysed in a group of infected individuals from an endemic community in north‐eastern Tanzania. The mf density was quantified at two‐hourly intervals for 24 hours. A clear nocturnal periodic pattern was observed. Mathematical analysis of the data indicated a peak at 0152 h and a periodicity index of 117.5. A periodicity equation was developed describing the average relation between mf intensity and hour of the day for the study area. Based on the observed periodicity pattern, the effect of blood sampling before peak time is discussed, and the importance of taking sampling time into consideration when analysing data from epidemiological studies is emphasized. A simple method is devised which can be used to adjust for the influence of time on mf intensities, in studies where accurate information on mf intensities is necessary, and where it is impossible to obtain all samples at peak time.

Cost and cost effectiveness of mass diethylcarbamazine chemotherapy for the control of bancroftian filariasis: comparison of four strategies in Tanzania

This study examines the costs and cost effectiveness of four different mass diethylcarbamazine (DEC) chemotherapy regimens—standard dose, semi‐annual single dose, low monthly dose and DEC‐medicated salt—in reducing microfilarial (mf) prevalence at the community level. Costs were estimated for each intervention in relation to both ingredient and activity, by the derivation and use of detailed itemized cost menus. The most expensive and most effective strategy in reducing community mf prevalence over 2 years was DEC salt intervention, followed in order of costs by the standard, low monthly and semi‐annual DEC strategies. The most cost effective strategy was the low monthly DEC treatment. Cost and sensitivity analyses, however, suggest that the optimal choice of mass DEC strategy for reducing mf is very sensitive to programme design parameters. In particular, the results demonstrate that if the salt delivery structure is simplified, DEC salt has the potential to be the dominant intervention for filariasis control. The results suggest that economies of scale considerations might militate against the adoption of this intervention for large‐scale applications, unless perhaps offset by its potential for cost recovery by direct patient purchase. Further analyses require a more realistic evaluation of filariasis intervention effectiveness by addressing changes in infection intensity and by accounting for the population dynamics of parasite transmission and control.

Bancroftian filariasis: patterns of vector abundance and transmission in two East African communities with different levels of endemicity.

Abstract Intensive monitoring of Wuchereria bancrofti vector abundance and transmission intensity was carried out in two communities, one with high-level endemicity for bancroftian filariasis (Masaika, Tanzania) and the other with low-level (Kingwede, Kenya), on the East African coast. Mosquitoes were collected in light traps, from 50 randomly selected households in each community, once weekly for 1 year. They were identified, dissected and checked for parity and filarial larvae. Anopheles gambiae s. l., An. funestus and Culex quinquefasciatus transmitted W. bancrofti in the two communities but the importance of each of these taxa differed between the communities and by season. The overall vector densities and transmission intensities were significantly higher in Masaika than in Kingwede (the annual biting rate by 3.7 times and the annual transmission potential by 14.6 times), primarily because of differences in the available breeding sites for the vectors and in the vectorial capacity of the predominant vector species. A marked seasonal variation in vector abundance and transmission potential contributed to the complex transmission pattern in the communities. Generally, these indices were higher during and shortly after the rainy seasons than at other times of the year. Considerable differences in W. bancrofti transmission were thus observed between communities within a relatively small geographical area (mainly because of environmentally-determined differences in vector habitats), and these were reflected in the marked differences in infection level in the human populations. The variation in vector abundance, vector composition and transmission intensity in the two communities is discussed in respect to its cause, its effects, and its significance to those attempting to control bancroftian filariasis.

Mass DEC chemotherapy for control of bancroftian filariasis: comparative efficacy of four strategies two years after start of treatment

The efficacy of 4 strategies for control of bancroftian filariasis using mass diethylcarbamazine (DEC) chemotherapy was evaluated and compared in 4 endemic communities in Tanzania 2 years after the start of treatment. The strategies used were the standard 12 d treatment (strategy I), a semi-annual single dose treatment (strategy II), a monthly low dose treatment (strategy III), and DEC medicated salt (strategy IV). Treatment took place during the first study year, and no treatment was given during the second year. Among individuals who were microfilaraemic before treatment, the microfilaria (mf) clearance rates were 41.5%, 75.0%, 84.0% and 89.1%, and the pre-treatment mf geometric mean intensities (GMIs) were reduced by 97.1%, 98.9%, 99.8% and 99.8%, for strategies I, II, III and IV, respectively, 2 years after starting treatment. Statistical analysis indicated that strategies III and IV were equally effective, and superior in clearing microfilaraemias and in reducing mf GMIs compared to strategies I and II; strategy II was significantly more effective than strategy I. The rate of occurrence of new cases of microfilaraemia among individuals who were amicrofilaraemic during the pre-treatment surveys was negligible over the study period in all communities. In all the communities combined, 66.6% of males presenting hydrocele before treatment, and 61.5% of individuals presenting elephantiasis before treatment, showed improvements in these conditions 2 years after the start of treatment, either as a reduction in size or complete disappearance.

Mass diethylcarbamazine chemotherapy for control of bancroftian filariasis through community participation : comparative efficacy of a low monthly dose and medicated salt

The efficacy of 2 strategies for the control of bancroftian filariasis using diethylcarbamazine (DEC) mass chemotherapy delivered through community participation was evaluated and compared in 2 endemic communities in Tanzania with pre-treatment microfilarial (mf) prevalences of 34.7% and 31.0%, and mf geometric mean intensities (GMI) of 1122 mf/mL and 933 mf/mL, respectively. In the first community, all individuals aged > or = 1 year were offered treatment for one year with a low monthly dose (50 mg DEC to children aged < 15 years and 100 mg DEC to adults aged > or = 15 years; given independently of body weight), and in the second community all households were offered 0.33% w/w DEC-medicated cooking salt for one year. Both treatment strategies resulted in dramatic reductions in the mf loads. Among those microfilaraemic before treatment, the low monthly dose and the DEC-medicated salt gave mf clearance rates of 55.3% and 92.1%, respectively, and the pre-treatment mf GMIs were reduced by 99.4% and 99.9%, respectively, one year after starting treatment. At community level, the mf prevalences were reduced to 15.8% and 2.4% (reductions of 54.5% and 92.3%) and the mf GMIs were reduced to 100 mf/mL and 32 mf/mL (reductions of 91.1% and 96.6%), one year after starting treatment with the low monthly dose and DEC-medicated salt respectively. Males with hydrocele before treatment improved remarkably one year after the start of treatment. Since both strategies were simple to administer and well accepted by the communities, they appear highly feasible for integration into large scale control programmes based on community participation.

Exploring knowledge, attitudes and practices related to diabetes in Mongolia: a national population-based survey

BackgroundNon-communicable diseases (NCDs) are now the leading causes of mortality in Mongolia, and diabetes, in particular, is a growing public health threat. Mongolia is a nation undergoing rapid and widespread epidemiological transition and urbanisation: a process that is expected to continue in coming decades and is likely to increase the diabetes burden. To better inform policy and public-health responses to the impact of the growth in NCDs, a national NCD Knowledge, Attitudes and Practices survey was implemented in Mongolia in 2010; a section of which focused on diabetes.MethodsThis survey was a nationally-representative, household-based questionnaire conducted by field-workers. Households were selected using a multi-stage, cluster sampling technique, with one participant (aged 15–64) selected from each of the 3540 households. Questions explored demographic and administrative parameters, as well as knowledge attitudes and practices around NCDs and their risk factors.ResultsThis research suggests low levels of diabetes-related health knowledge in Mongolia. Up to fifty percent of Mongolian sub-populations, and one in five of the total population, had never heard the term diabetes prior to surveying. This research also highlights a high level of misunderstanding around the symptomatology and natural progression of diabetes; for example, one-third of Mongolians were unaware that the disease could be prevented through lifestyle changes. Further, this study suggests that a low proportion of Mongolians have received counseling or health education about diabetes, with lowest access to such services for the urban poor and least educated sub-populations.ConclusionsThis research suggests a low prevalence of diabetes-related health-knowledge among Mongolians. In this light, health-education should be part of any national strategy on diabetes.