Density Of Microfilariae Research Articles

Onchocerciasis in Ecuador, 1980?1986: Epidemiological evaluation of the disease in the Esmeraldas Province

An increase in both the prevalence (68.3%) and intensity of infection (106.2%) occurred in the Ecuadorian onchocercal focus during the 6 year period, 1980 to 1986. In 1986, a higher prevalence of infection was noted in males (53.6%) than females (33.1%), but a greater rate of increase of infection over the 6 years was seen in females than males (75.7% vs 61.9% respectively). In all endemic areas, the Chachi race had a higher prevalence of infection as well as higher increase rate of infection than the Black. In the hyperendemic and hypoendemic areas the highest increase of infection was seen in the Chachi female and Chachi male respectively. The prevalence of infection in 1986 was greater than that in 1980 for all ages, with the greatest increase of infection in the 1-4 year age groups. There was an average increase of 106.2% in the microfilarial skin density with an increase seen in all age groups. A higher microfilariae density increase was seen in the hypoendemic (100.0%) than the hyperendemic (66.5%) areas. The Blacks showed a greater microfilarial density increase than the Chachi (155.1% vs 70.7% respectively) with the Black female showing the highest increase (117.6%) in the hyperendemic area and the Black male showing the highest (155.6%) in the hypoendemic area. Along the Rio Cayapas, high microfilarial densities were found in 3 major areas on the river, a reflection of man-vector exposure.

Evaluation of a specific enzyme immunoassay for onchocerciasis using a low molecular weight antigen fraction of Onchocerca volvulus.

A micro-ELISA technique has been developed for the diagnosis of onchocerciasis. With the use of a low molecular weight (LMW) fraction of a soluble, adult Onchocerca volvulus extract as an antigen, the problem of nonspecificity observed with the crude worm extract is negligible. Seventy-three sera from proven cases of other filarial infections were weakly reactive or nonreactive. From cross-sectional and longitudinal surveys in a West African village hyperendemic for onchocerciasis, we concluded that this assay detects antibodies elicited when female worms start to produce microfilariae. Thirty-two of 33 children aged less than or equal to 15 years with positive skin snips were seropositive. High antibody levels could also be found in children who only became parasitologically positive 1-4 years after seroconversion. This leads to the conclusion that serology is more sensitive than the 2-snip method. In individuals with chronic infection (exposed for greater than 15 years), antibody titers were consistently lower (independent of the density of skin microfilariae) than in the early phase of a patent infection. A comparative serological study in a mesoendemic village revealed that the assay can serve to assess onchocerciasis prevalence by testing individuals aged 5-15 years.

Extracellular proteases of Onchocerca

Two important events in infection by Onchocerca parasites involve cutaneous tissue migration by larval stages. L3 larvae migrate from the blackfly bite site to subcutaneous locations for adult development, and microfilariae from subcutaneous nodules to distant regions of the skin and sometimes the eye. By analogy to other tissueinvasive helminth larvae, it has been proposed that migration of Onchocerca larvae through cutaneous tissue is facilitated by secretion of proteolytic enzymes. To test this hypothesis, neutral protease activity capable of degrading a model of cutaneous extracellular matrix was assayed using live L3 larvae of O. lienalis and microfilariae of O. cervicalis and O. cervipedis. Five hundred L3 larvae degraded most of the matrix within 24 hr of incubation. Substrate gel electrophoresis and other protease assays showed a 43-kDa serine elastase was secreted by O. lienalis L3 larvae. Larvae and adults of the free-living nematode, Caenorhobditis elegans, by contrast, did not secrete neutral proteases and large numbers of motile C. elegans juveniles and adults produced no degradation of the extracellular matrix. Expression of Onchocerca neutral protease activity was stage specific. No protease activity corresponding to that seen in L3 larvae was found in adult worms. Microfilariae of O. cervicalis and O. cervipedis produced both a serine and a metalloprotease, but the level of protease activity of these microfilariae was substantially lower than that of L3 larvae, and no significant protease activity was detected in extracts of O. linealis microfilariae. Uterine microfilariae of O. cervicalis had different protease species than skin microfilariae, suggesting that changes in protease expression parallel other morphologic and biochemical changes in the development of skin microfilariae. The serine protease of L3 larvae probably plays an important parasitic function, facilitating L3 migration from the blackfly bite site to distant regions of the body where adults will develop and form nodules. The protease activity of microfilariae, while individually considerably less than that of L3 larvae, may still contribute to the tissue destruction seen with heavy skin densities of microfilariae.

Ivermectin in loiasis and concomitant O. volvulus and M. perstans infections.

Two clinical studies were carried out in Gabon, Africa to evaluate the efficacy, safety, and tolerability of ivermectin in the treatment of patients with Loa loa infection. In the first study, 35 patients received single oral doses of ivermectin, 5-200 mcg/kg body weight. Blood microfilariae levels did not decrease after a single oral 5, 10, 30, or 50 mcg/kg dose of ivermectin, but levels did decrease after doses of 100, 150, and 200 mcg/kg. The most efficacious dose was 200 mcg/kg; mean blood microfilariae levels decreased to 12% of mean pretreatment values by day 15 and remained decreased for 28 days. A second study evaluated the safety and efficacy of ivermectin in patients with multifilarial infections. All 17 patients had concomitant Loa loa and O. volvulus infection. M. perstans affected 5 of the patients. Sixteen patients also had infections due to intestinal nematodes. The patients each received single oral doses of 200 mcg/kg ivermectin. Ten days later, the mean Loa loa blood microfilariae level had decreased to 20% of the mean pretreatment level. O. volvulus dermal microfilariae densities were reduced to 2% of the pretreatment levels. A minimal increase in blood microfilaria levels was observed on day 28. In contrast, dermal microfilariae levels remained near zero for the duration of the study. Intestinal infection due to Ascaris was eradicated in all of the affected patients by day 23; efficacy against Trichuris and hookworm infections, however, was poor. All patients tolerated ivermectin well including those with multiple infections.

Epidemiology of Wuchereria bancrofti in Leogane, Haiti

A survey for Wuchereria bancrofti in Leogane, Haiti, revealed that 140 of 421 individuals (33%) had a patent infection, of which 40% lived in the suburban outskirts of the city. The median microfilaria density was 19·1 per 20 mm 3 of blood for suburban dwellers compared with only 8·8 for those living in the city. The vector, Culex quinquefasciatus (Say), breeds mosdy in and around numerous rum distilleries, located exclusively around the periphery of the city, and this undoubtedly accounts for the higher prevalence and intensity of infection among suburban dwellers.

Microfilaria density distribution in the human population and its infectivity index for the mosquito population.

A new method of computing the infectivity index of microfilariae (mf) for the mosquito population is proposed using the estimated mf density distribution in the human population. the observed density distribution is considered a compound of the Poisson and the gamma distributions. The former distribution describes the probability of a specimen containing a specified number of mf and the latter describes the density distribution of mf in the host population. The mf infectivity index is the probability that a blood meal will include at least 1 mf, conditional on the population-density distribution of mf as specified by the gamma distribution. Actual data indicate that this population-density-based infectivity index can be considerably different from the conventional index based on the survey-density distribution. The level of the carrier rate of mf in a survey is greatly influenced, apart from the sample variation, by the average volume of blood taken from each person. The rate computed on the estimated population-density distribution of mf is convertible to any base amount of blood.

Bancroftian filariasis in coastal north Trinidad: the effects of mass chemotherapy using spaced doses of diethylcarbamazine citrate on human microfilaraemias and vector infection rates

The effects of a mass chemotherapy programme using spaced doses of diethylcarbamazine citrate on Wuchereria bancrofti microfilaraemias and vector infection rates were studied in an isolated rural community of 650 people in north Trinidad where a microfilaria rate of 15% had previously been recorded. A single oral dose of 6mg/kg body weight was given at monthly intervals for one year. After 6 months, 79% (52/66) of persons previously microfilaraemic had become negative. After 12 months the negative proportion had increased to 90% (63/70). The geometric mean microfilaria density of those still microfilaraemic was reduced from 19.7 to 4.6 per 100mm 3 of blood. Of 147 initially negative persons, none had become positive following treatment. Filarial infection rates in Culex quinquefasciatus from randomly sampled houses fell from 6.4% to zero; in houses where occupants with microfilaraemia had been identified before treatment, the rate was reduced from 15.8% to zero. In a supplementary study, however, weekly collections from one initially positive house showed a rapid decline in the vector infection rate from 43% to 2% after three months but an additional 3 months elapsed before the index reached zero. There was negligible microfilarial uptake by the vectors from residual low-density microfilaraemias (<10 microfilariae per 100mm 3 of blood) after chemotherapy.

Seasonal Changes in Density and Tissue Distribution of Onchocerca cervicalis Microfilariae in Ponies and Related Changes in Culicoides variipennis Populations in Louisiana

Seasonal changes in density and spatial distribution of Onchocerca cervicalis microfilariae were studied in ventral-midline skin of 15 infected pony mares in southern Louisiana. Triple running mean analysis of data over a 13-mo period indicated that a distinct pattern exists in total microfilariae population density and in microfilariae occurrence in different levels of the dermis. Microfilariae density reaches peak levels in the spring followed by a 58% decrease in the summer, a 19% increase in the fall, and a decrease to the lowest numbers in the winter. Microfilariae were found in all levels of the skin during the spring, summer, and fall but were not found in the superficial layers of the dermis during the winter months. The population density of Culicoides variipennis, a demonstrated vector of O. cervicalis, appeared to have seasonal fluctuations similar to the changes in microfilarial density. Harmonic wave analysis of microfilariae density data in individual ponies showed that all individuals did not follow the population trend.

Studies on the dynamics of transmission of onchocerciasis in a Sudan-savanna area of North Cameroon V. What is a tolerable level of Annual Transmission Potential?

The prevalence and intensity of infection with Onchocerca volvulus were assessed in population surveys in nine villages, situated at different distances from Simulium damnosum s.l. breeding sites. The prevalence varied from 48 to 89%, the arithmetic mean densities of microfilariae per skin snip were between 16 and 109, and severe ocular lesions were found in from 1 to 22% of patients. Annual Transmission Potentials (ATP) were measured for up to three years in the near vicinity of nine villages at several fly-catching sites. Weighted means of the ATP over the three years, and of the sojourn times of the human population, were calculated at three of the villages, where the prevalence of onchocerciasis was 51, 61 and 89%. An average ATP of 100 larvae or less in the head, thorax and abdomen of the flies was associated with an onchocerciasis prevalence of 50 to 60%, a mean microfilarial density below 40 microfilariae per skin-snip, less than 5% of ocular lesions, and no onchocercal blindness. This value might therefore be considered to be an indication of the level to which the transmission must be reduced in the savanna in order to prevent the occurrence of severe ocular lesions or blindness. It is lower than the present level accepted by the Onchocerciasis Control Programme in the Volta River Basin.

Efrect of diethylcarbamazine against Brugia malayi infection on Cheju Island, evaluated in 1965

A study on the effect of diethylcarbamazine (DEC) (Supatonin) against Brugia malayi infection was conducted on Cheju Island in September 1965. A total of 182 persons living in a village of Aiwol Myun, Bukcheju-Gun was examined for microfilaraemia. Microscopic examination of smears of 20 microliter of blood revealed a microfilaria positivity rate of 28.5 per cent. At the end of September 1965, 34 confirmed microfilaria positive cases were treated with DEC at a daily dosage of 5 mg/kg body weight. A full course of 12 days of drug administration divided of two rounds for 6 days each was used. The first round of treatment was given under a strict supervision of the author in order to observe carefully side-effects of the drug. The second round of treatment was given in January 1966. The microfilaria density in 20 microliter of blood of those who received the drug was checked four times; before the treatment, during the first round of the treatment, 2 weeks and 4 months after the completion of the first round. The pre-treatment mean microfilaria density of 104.6 diminished to nearly zero (only two cases with one microfilaria respectively) 2 weeks after the first round and again slightly rose up to 0.5 four months after the first round. These results indicate that DEC (Supatonin) is highly effective to eliminate the microfilaria of B. malayi. However, severe side-effects, e.g. fever (average 38.6C, maximum 39.7C), headache, backache and seldom abdominal discomfort ect. were observed. There were two cases of withdrawal from the scheme due to refusal.

A double-blind comparison of the efficacy and safety of ivermectin and diethylcarbamazine in a placebo controlled study of Senegalese patients with onchocerciasis

Ivermectin (MK-933) has been compared with diethylcarbamazine (DEC) and placebo in a double-blind study in 30 adult male Senegalese patients with Onchocerca volvulus infection. 10 patients were randomly assigned to each treatment group. Ivermectin was administered as a single oral dose of 12 mg and DEC as 50 mg daily for two days and 100 mg twice daily for the following six days, total 1·3 g in eight days. Skin O. volvulus microfilaria densities remained near pre-study values in the placebo patients, but decreased rapidly with both active drugs to mean values about 2% of pretreatment (Day 8) and then increased slowly, reaching in 12 months about 4% of pre-treatment (ivermectin) and 18% (DEC). This difference is statistically significant. Clinical adverse reactions were recorded in four ivermectin, ten DEC and three placebo patients. One ivermectin and six DEC patients received steroid treatment for relief of these reactions. Serious adverse ocular changes were not seen in any patients, possibly because of the steroid therapy in the DEC patients. Adult O. volvulus from onchocercal nodules one and six months after treatment showed no effect of either drug on viability. Intra-uterine developing forms of the microfilariae appeared normal in all three treatment groups at the one month examination but deformed and degenerated forms were evident at six months in the ivermectin group but not in the DEC and placebo patients. Ivermectin as a single oral dose appears to be a safer and more effective microfilaricidal drug in human onchocerciasis than DEC in the standard multi-dose regimen.

Mansonella ozzardi in the Federal Territory of Roraima, Brazil. Distribution and finding of a new vector in the area of Surumu River

A survey conducted among the Makuxi Indians from 15 settlements in the northeastern part of the Territory of Roraima, Brazil, revealed the occurrence of Mansonella ozzardi in 3,2% (21/652) of the persons examined. The absence of demonstrable infection--with one exception--in persons under 15 years of age, and the low microfilaria density in adults suggest that mansonelliasis has been acquired by the Makuxi Indians outside their villages. As many Indians from the region pan gold on the Upper Maú (Ireng) river--where black flies occur in great quantity--the mining camps are probably the sites of transmission. Experimental infection with M. ozzardi of Simulium oyapockense s.l. (or Simulium roraimense) showed that this species, at the least in the Surumu river area, is capable of supporting the full development of the microfilariae. Although S. oyapockense has a wide distribution in the extreme north of Brazil, it does not appear to be an efficient vector, since only 20,6% (19/92) of the specimens collected after a blood meal on a naturally infected Indian contained larval stages of M. ozzardi (with an average of 1-2 larvae per fly). The high prevalence rate of infection found, in a previous survey, among the Sanumá and Mayongong, two Indian groups living at the Auaris river area, on the other side of the Territory of Roraima, indicates that a more competent intermediate host should exist in that region.

Filariasis transmission in Samoa

Transmission experiments of laboratory-bred and wild-caught Aedes polynesiensis and wild-caught Ae. samoanus on carriers with different levels of microfilaraemia showed that the percentage of mosquitoes infected, the average number and range of larvae found in each infected mosquito were directly proportional to the microfilarial densities in the carrier at the time of feeding. There was no difference between the results for laboratory-bred and wild-caught mosquitoes. Aedes polynesiensis fed on low carriers (around 20 Mf ml-1 and less) gave an average infection rate of 4.9%. Each infected mosquito harboured only one larva. A plot of the probit of percentage Ae. polynesiensis positive against the logarithm of microfilarial density showed a linear relationship. Using the regression line, at 95% confidence interval, a microfilarial density of 1 ml-1 would give an infection rate of 0.22-2.51%, a density of 0.1 Mf ml-1 an infection rate of 0.01-1.0%. The concentrating capacity of Ae. polynesiensis ranged from 0.70 to 4.74. As microfilaria densities decreased, concentration increased. The microfilarial intake, the subsequent worm burden and concentrating capacity were less in Ae. samoanus than Ae. polynesiensis. There was no evidence of any association between microfilarial density and concentration in Ae. samoanus.

The distribution of the microfilariae of Onchocerca volvulus in the different body regions in relation to the attacking behaviour of Simulium damnosums.l. in the sudan savanna of northern Cameroon

Densities of Onchocerca volvulus microfilariae in four volunteers with low to moderate infections were estimated at five body sites by paired skin snips. The landing of Simulium damnosum s.l. females on the body of these volunteers was recorded during 12 hours for six days. Most flies fed at the ankles (53% and 51%) and calves (28% and 27% respectively) in both the standing and sitting positions. The density of microfilariae in the skin was highest in the pelvic region (24·1 mf/mg) and relatively low in the calf (14·8 mf/mg) and ankle (1·0 mf/mg) regions. From the biting rate (♀♀/body part) and the microfilarial density (mf/mg) a transmission index was calculated for the different body regions. This was highest for the calves showing that this part of the leg, if unclothed, accounts for the highest rate of contact (50 to 60% of total) between vector and parasite.

Circulating immune complexes and their possible relevance to other immunological parameters in Guatemalan onchocerciasis.

Circulating immune complexes (CIC) were demonstrated in sera of Guatemalan patients with onchocerciasis by Raji cell radioimmunoassay. 44% of patients but none of controls had abnormally high concentrations of CIC in their sera. The increased concentrations of CIC were found more frequently in patients with lower density of microfilariae in their skin biopsies. Patients with higher concentrations of CIC appeared to have increased titers of serum antibodies to Onchocerca volvulus. A depression of both humoral immune response to tetanus toxoid and delayed hypersensitivity reaction to PPD were found in patients with onchocerciasis. CIC may be involved in modulation of the immune response in onchocerciasis.

The concentration of Mansonella ozzardi microfilariae in skin capillaries

The densities of microfilariae of Mansonella ozzardi in different regions of the body of infected individuals were studied. The density in venous blood closely approximated to that found in finger capillaries, but was significantly lower than the densities in capillaries of the scapular region and buttocks. A possible explanation for this concentration of microfilariae is the lack of glomi in these two sites, which consequently have a lower flow of blood than areas where glomi occur and where microfilarial densities are lower. No relationship was found between blood and skin densities in the buttocks and scapular region, probably due to the inadequacy of the skin snip method used for the detection of this species of filaria. It was concluded that the most acceptable detection method for M. ozzardi remains the sampling of finger capillary blood. It could not be decided, due to the paucity of data, whether any relationship exists between skin microfilarial densities and the biting sites of the vectors.

Time courses of antibody levels in Mastomys natalensis after infections with Litomosoides carinii, Dipetalonema viteae, Brugia malayi or B. pahangi, Determined by ELISA.

Using a Litomosoides carinii adult antigen, time courses of antibody levels were followed by an ELISA in L. carinii, Dipetalonema viteae, Brugia malayi and B. pahangi infected Mastomys natalensis. Using various groups of infected animals, periods up to 400 days after infection were covered. In L. carinii infected Mastomys, antibodies were first detected 11 days p.i. and levels increased rapidly until day 40. Temporarily reduced levels about the beginning of patency were followed by increasing values until about 100 days p.i. Then the antibody content of the sera remained more or less constant until about 250 days p.i. although maximum levels were found at day 170. Thereafter, the antibody concentration in the sera declined slowly but high levels were still observed 390 days p.i. The antibody content was usually higher in animals with high microfilariae densities than in those with low microfilariae counts but relations could not be proven statistically. In D. viteae infected Mastomys, maximum antibody values were reached within the beginning of patency. Levels were not altered markedly until about 110 days p.i. Thereafter they decreased slightly but then remained constant until the end of the investigation period 350 days p.i. B. malayi infected animals showed a rapid increase of the antibody content in the sera; a maximum was reached by 20 days after the infection. Thereafter, somewhat constant levels were found for 4--5 months. After 300 days p.i. the antibody levels declined progressively, accompanied with increasing parasitaemia densities; after 380 days the levels reached about 2/3 of the maximum. However, despite this, no relation was found between the levels of parasitaemia and antibody in individual animals. In B. pahangi infections the main prepatent antibody increase occurred during week 5 p.i., when maximum values were observed. The beginning of patency and the early patency were accompanied with slightly declining antibody levels. From 150 days p.i. until the end of the investigation 400 days p.i., the antibody content of the sera was fairly constant.

Ingestion and development of Wuchereria bancrofti in Culex quinquefasciatus, Anopheles gambiae and Aedes aegypti after feeding on humans with varying densities of microfilariae in Tanzania

Laboratory observations were made on the transmission of Wuchereria bancrofti by Culex quinquefasciatus and Anopheles gambiae, natural vectors in East Africa, and by Aedes aegypti, an experimental vector. Mosquitoes were fed on human carriers with one to 2,667 microfilariae (mff/ml) of peripheral blood and the numbers of mff ingested were determined. The mosquitoes demonstrated a ‘concentrating effect’ and ingested 8.6 to 12-fold more mff than the expected number based on the density of mff in the carrier and the size of the blood meal. Studies on Culex showed that only 2.7 to 13% of the ingested mff developed into infective larvae (L3s). Emphasis was given to the transmission of Wuchereria from low density carriers with less than 10 mff/ml of peripheral blood—a value that approximates the threshold level of microfilaraemia detectable with conventional diagnostic techniques. By combining our laboratory data with published field data on the biting rates and longevity of mosquitoes in East Africa, we estimate that each low density carrier could serve as a source for nil to 15 L3s/year in Cx quinquefasciatus and 0.3 to 8 L3s/year in An. gambiae depending on the particular community. These estimates are relevant when formulating strategy to control bancroftian filariasis by mass chemotherapy with diethylcarbamazine. Experience with this drug has shown that a proportion of the carriers in a community continue to circulate mff at low levels after treatment. The present data demonstrate that these carriers serve as a reservoir of infection for mosquitoes.

Onchocerciasis in Guatemala

To provide quantitative information on the epidemiology of infection with Onchocerca volvulus and to define the association between indicators of infection and onchocercal eye disease, skin snips were obtained and skin and ocular examinations were performed on 892 persons living on seven Guatemalan coffee plantations. Skin-snip positivity and the density of microfilariae in the skin increased with age, reaching highest levels at 15-19 years, and both were greater in males than females. A history of nodulectomy was given by 67% of long-term residents and this percentage also increased with age. Over 90% of skin-snip positive subjects and 39% of skin-snip negative subjects had previous or present nodules. Microfilariae were detected in the cornea of 35.1% and in the anterior chamber of 18.9% of all persons examined and the frequencies increased with age, reaching peak levels at 10-19 years. Onchocercal eye lesions were found in 52 persons, causing bilateral blindness in six. Skin-snip positivity, microfilarial skin density, number of nodules, eye infection, and onchocercal eye lesions all correlated significantly with each other. Onchocercal blindness in one or both eyes was found only on fincas with a high prevalence (greater than 80%) and intensity of infection (greater than 22 microfilariae/mg skin).

Ingestion des microfilaires par le vecteur dans le cas d’une Filaire périodique Ditetalonema dessetae

The Filariid D. dessetae in its natural host, Proechimys oris, has a daily periodic microfilaraemia with a conspicuous peak during the day (blood from the ocular sinus). --If the mosquitoes engorge during the daily peak, the density of microfilariae ingested is equal or slightly superior to the density of microfilaria in the blood from the ocular sinus. However, if they engorge during the night they ingest 2,5 times the number of microfilariae than the density of microfilariae in the ocular sinus indicates they should. Thus during the night there is a phenomenon of persistence of microfilariae in the cutaneous blood circulation of the rodent. --With mosquitoes of similar size each ingesting a similar volume of blood the distribution of Aedes as a function of the number of microfilariae ingested resembles the Poisson distribution. This indicates that the distribution of microfilariae which are accessible to the vector in the blood of Proechimys is nearly homogeneous.