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Biomedical subjects

J B Lok

Publications and source records attributed to J B Lok.

36 records · Page 2Linked to original sources
PubMed1987

Studies of the growth-regulating effects of ivermectin on larval Onchocerca lienalis in vitro.

At concentrations of 0.1-100 ng/ml ivermectin inhibited L3-L4 molting by Onchocerca lienalis in vitro. The degree of inhibition was dose-dependent with a significant effect apparent at 0.1 ng/ml and complete inhibition occurring at 100 ng/ml. The ED50 for molt inhibition was 0.19 ng/ml. Molt-inhibiting levels of the drug were not acutely toxic to the worms. In the presence of 10 ng/ml, a concentration giving 95% molt inhibition, motility at day 7 postinoculation was 71% of that seen in nontreated controls. A more pronounced effect on motility was apparent in larvae under long-term cultivation in the presence of ivermectin. Kinetic studies indicated that the majority of the larvae respond irreversibly to the drug within the first 2 hr of exposure. Twenty-four hours of exposure were required for a maximal response. The inhibitory effects of ivermectin were less pronounced if larvae were allowed to develop under normal culture conditions for 24 or more hours prior to the initiation of drug treatment.

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PubMed1986

Punctate keratitis induced by subconjunctivally injected microfilariae of Onchocerca lienalis.

An experimental model of human onchocercal keratitis was produced by injecting guinea pigs subconjunctivally with microfilariae (Mf) of Onchocerca lienalis. Actively motile O lienalis Mf spontaneously penetrated the central cornea and produced gray-white midstromal opacities, 0.2 to 0.4 mm in diameter, resembling those of human onchocercal punctate keratitis. Histologically, small foci of eosinophil and mononuclear cell infiltration associated with small pockets of interstitial edema were present in the central corneal stroma, with eosinophil infiltrates in the conjunctiva, episclera, limbus, and ciliary body. The severity of the punctate keratitis was increased by repeated subconjunctival inoculations of Mf. Punctate lesions were not seen following subconjunctival injection of Mf in animals previously hyperimmunized by three subcutaneous injections of Mf. Hyperimmunization may produce an immune response capable of destroying the subconjunctivally injected Mf and preventing their migration into the central cornea.

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PubMed1986

Entomological studies at an enzootic Venezuelan equine encephalitis virus focus in Guatemala, 1977-1980.

The ecology of several potential mosquito vectors of Venezuelan equine encephalitis (VEE) alphavirus was studied in an enzootic focus of that virus on the Pacific coast of Guatemala over a four-year period. Four species-Culex taeniopus, Mansonia titillans, Culex nigripalpus and Aedes taeniorhynchus-were most prevalent during the wet season when transmission normally occurs. However, only Cx. taeniopus yielded VEE virus. The bloodfeeding patterns of these species revealed that Ae. taeniorhynchus and Ma. titillans fed almost exclusively on bovine and equine hosts. Conversely, Cx. nigripalpus was highly ornithophilic but occasionally fed on mammals. Cx. taeniopus exhibited a wide host range, utilizing both large and small mammals as well as birds and, rarely, reptiles. The versatility in feeding pattern displayed by this mosquito coupled with its ability to become infected with relatively low levels of enzootic VEE virus suggests that vertebrates other than rodents may serve as amplifying hosts in this habitat. Nepuyo virus was also isolated from Cx. taeniopus, suggesting that this mosquito might be an endemic vector of this rodent-associated bunyavirus. A single isolate of St. Louis encephalitis virus was made from Cx. nigripalpus.

Aedes↗
PubMed1986

Experimental ocular onchocerciasis in cynomolgus monkeys.

Infection of cynomolgus monkeys with microfilariae (Mf) of Onchocerca lienalis was studied as a model for human ocular onchocerciasis. Normal monkeys and immunized monkeys were given intracorneal/subconjunctival, intracameral, or intravitreal injections of Mf or bovine serum albumin (control). Selected animals were given diethylcarbamazine citrate (DEC) orally (15 mg/kg) daily after the ocular infection. Following intravitreal challenge, living Mf and fibrinous exudates were visible by slit-lamp in both the anterior chamber and vitreous. Eosinophils and macrophages surrounded the Mf in the vitreous, with degranulated eosinophils adherent to the Mf. Eosinophils infiltrated the uvea and surrounded the retinal vessels. After intracorneal injection of Mf, living Mf were visible by slit-lamp biomicroscopy in the cornea for 3 days, with minimal inflammation of the corneas occurring over the 7 days after injection. Intracamerally injected Mf induced anterior uveitis. The extent of the inflammatory reactions was not substantially altered by DEC treatment following intraocular injection of Mf. In vitro proliferative responses of peripheral blood leukocytes to a crude Mf antigen were not observed in infected monkeys and proliferative responses to mitogen declined in these animals. Responses to mitogen were inhibited by addition of Mf antigen in vitro in normal monkeys. Circulating IgG antibodies were present in the sensitized, intracorneally, and intravitreally challenged animals. No obvious correlations were present between IgG antibody level and ocular inflammation.

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PubMed1984

In vitro development of third- and fourth-stage larvae of Dirofilaria immitis: comparison of basal culture media, serum levels and possible serum substitutes.

In vitro development and survival of third-stage larvae of Dirofilaria immitis were compared in four different culture media and in the presence of varying concentrations of four different medium supplements. Motility and the incidence of third- to fourth-stage molting were used as criteria for evaluating different culture conditions. No significant differences in either motility or molting response were detected between larvae cultured in NCTC-135, F12(K), CMRL 1066 or Dulbecco's Modified Eagle's Medium. Fetal calf serum enhanced development and survival of the cultured larvae in dose-dependent fashion. Its effects were maximal at a concentration of 20 percent of the total medium volume. Addition of a commercial medium supplement, NuSerum, also gave a dose-related increase in larval development and viability. The activity of NuSerum in this respect was comparable to that of fetal calf serum. The tripeptide glycylhistidyllysine and bovine serum albumin, fraction V both failed to stimulate development of third-stage D. immitis larvae in vitro.

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PubMed1984

Recovery and viability of Dirofilaria immitis microfilariae.

The viability of Dirofilaria immitis microfilariae recovered from canine blood by different methods was determined. Microfilaria recovery techniques included saponin lysis, saponin lysis with a trypsin treatment, dextran sedimentation and phytohemagglutinin treatment. Criteria for evaluating viability were microfilarial motility in vitro at 37 degrees C, microfilarial development in mosquitoes and the ability of microfilariae to circulate in mice. Although each method produced motile microfilariae, differences among groups of microfilariae recovered by different techniques were apparent by each of the criteria for viability. Saponin lysis gave superior yields of viable microfilariae.

Aedes↗
PubMed1984

Development of onchocerca lienalis and O. volvulus from the third to fourth larval stage in vitro.

Third-stage larvae of Onchocerca volvulus and O. lienalis were observed to molt to the fourth stage in various cell-free in vitro systems. The percentage of O. lienalis completing the molt was similar in the three culture media and two gas phases tested ranging from 44.8% (1:1 IMDM:NCTC + 5% CO2: 95% N2) to 56.7% (L-15 + 5% CO2: 95% air). Percent molting in O. volvulus ranged from 0% (F12(K) + 5% CO2: 95% N2) to 33.3% (L-15 + 5% CO2: 95% N2). All media were supplemented with either 20% FCS or 20% horse serum. Molting by O. lienalis occurred on days 2-5 in culture. Molting by O. volvulus was observed as early as day 5 and as late as day 10. Incomplete casting of the third-stage cuticle was frequently observed in O. volvulus. Larvae of both species entered a lethargus 24-48 hours prior to the onset of molting. Maximum survival in culture was 42 days for O. lienalis and 25 days for O. volvulus. Significant growth of larval O. lienalis was noted early in the culture period, but neither species continued development to the fifth stage.

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PubMed1983

Cryopreservation of Dirofilaria immitis microfilariae and third-stage larvae.

Microfilariae of Dirofilaria immitis retained their infectivity for susceptible mosquitoes after cooling to -196 degrees C in the presence of 5% dimethylsulphoxide (Me2SO) using a two-step cooling sequence. Motility and in vitro development of cryopreserved microfilariae also compared favourably with unfrozen controls. Third-stage larvae frozen by the same cooling sequence in the presence of either 5% Me2SO or 16% hydroxyethyl starch were motile upon thawing. Thawed larvae completed the third- to fourth-stage moult in vitro at a frequency approximately 5 to 10% of that seen in unfrozen controls.

Aedes↗
PubMed1983

Epidemiology of canine heartworm infection.

Factors affecting transmission of D. immitis between dogs are summarized in table 4. Although this list may not be exhaustive, it illustrates the complexity of heartworm epidemiology. There are interesting and provocative similarities in the epidemiology of canine and human filariasis. Ottesen (148) has described the spectrum of disease observed in regions endemic for lymphatic filariasis. It includes patients with 1) microfilariae, but no symptoms; 2) disease, but no microfilariae; 3) tropical pulmonary eosinophilia of filarial etiology; and 4) no disease and no evidence of infection. A dog population in areas endemic for D. immitis may show a similar range of clinical manifestations. For example, dogs with microfilariae may not demonstrate clinical disease; dogs with occult infection may show signs ranging from none to severe, including signs similar to the human tropical pulmonary eosinophilia syndrome. Although some dogs live in endemic areas and, apparently, are not infected, it is difficult to determine whether those dogs are resistant to infection or if they have not had adequate exposure. One study has explored the relationship between human W. bancrofti infection and genetic factors (149). Although familial clustering of cases was apparent, the authors were unable to distinguish with certainty whether this association was due to environmental or genetic influences. Carefully designed seroepidemiologic studies of canine heartworm infection may shed some light on genetic or other factors which confer resistance or susceptibility to filarial infections. Natural strain variation has been recognized in several filariae of medical importance including O. volvulus (150), B. malayi (151), and L. loa (2). Strain variation may be manifested as differences in behavior patterns such as microfilarial periodicity, vector and host affinities, and pathogenicity. A laboratory study (88) demonstrated how such strains could arise. In this study, a strain of Brugia patei was adapted to a new, normally refractory host, by artificial selection. The possible existence of different physiologic races of D. immitis should be investigated. Experimental infections with D. immitis have been achieved under laboratory conditions in numerous mosquito species (77).(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1983

Further studies on the development of onchocerca spp. (Nematoda: Filarioidea) in Nearctic black flies (Diptera: Simuliidae).

Of three species of Nearctic black flies tested, Simulium pictipes Hagen proved the most efficient laboratory vector of the bovine parasite Onchocerca lienalis. Among flies inoculated intrathoracially with 40 microfilariae, numbers of 3rd-stage larvae per fly were 7.63 for S. pictipes, 7.54 for S. vittatum Zetterstedt, and 0.83 for S. decorum Walker. S. pictipes survived the longest under laboratory conditions, with 83.3% of the females remaining alive 10 days after inoculation with 40 microfilariae of O. lienalis. Using an artificial membrane feeding system, S. pictipes could be routinely infected with O. lienalis by mouth. This black fly was also susceptible to infection with the Guatemalan strain of O. volvulus. Among flies injected with 10 microfilariae the rate of infection with 3rd-stage larvae was 93%, with a mean of three 3rd-stage larvae per fly. Successful techniques for the large-scale recovery and cryopreservation of 3rd-stage larvae of O. lienalis were also developed. A motility rate of 92.7% was observed in larvae cryopreserved within vector black flies.

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PubMed1983

Simulium jenningsi Malloch (Diptera: Simuliidae): a vector of Onchocerca lienalis Stiles (Nematoda: Filarioidea) in New York.

Biting flies were collected from the umbilical areas of Onchocerca lienalis-infected cattle in New York (state) from June through September of 1980. Of the 766 flies collected, 705 were Simulium jenningsi. Microfilariae were detected in the midguts of 37 (50%) of 73 females dissected immediately after the flies had fed. The mean number of larvae per positive fly (fly with microfilariae) was 15.2. Third-stage larvae were recovered from 25 (21.9%) of 114 S jenningsi dissected 8 to 13 days after they had fed on the infected cattle; the mean number of 3rd-stage larvae per positive fly was 3.5. Dissections of flies performed on days 1 through 7 after feeding yielded various numbers of 1st and 2nd-stage larvae from the thoracic muscles. Ovarian dissections performed on 304 S jenningsi attacking cattle indicated an overall parous rate of 58%. Naturally occurring infections with filarial larvae indistinguishable from O lienalis were found in 7.3% of the parous females. Three of these flies, or 1.7% of the parous collection, harbored 3rd-stage larvae. The onset of naturally occurring filarial infections in the population of S jenningsi coincided with a peak in the parous rate in late June. Thereafter, filarial infections were generally detected when the parous rate was above 50%.

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PubMed1981

Intestinal threshold of an enzootic strain of Venezuelan encephalitis virus in Culex (Melanoconion) taeniopus mosquitoes and its implications to vector competency and vertebrate amplifying hosts.

The minimal intestinal dose of an enzootic strain of Venezuelan encephalitis (VE) virus for Culex (Melanoconion) taeniopus mosquitoes caught at a marsh habitat of VE virus in Guatemala was less than five plaque forming units (pfu) of virus. Ingestion of this dose of virus in blood of viremic hamsters resulted in transmission of virus to other hamsters. This low intestinal threshold of an enzootic strain of VE virus indicates that the natural Guatemalan population of Cu. (Mel.) taeniopus can acquire VE virus from vertebrates that have viremia levels as low as 1,000-5,000 pfu/ml of blood, provided other factors do not limit virus interchange between mosquitoes and vertebrates.

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PubMed1981

Complete generation rearing of Simulium damnosum s.l. (Diptera: Simuliidae) in the laboratory.

A closed system of water circulation previously devised for rearing nearctic black flies was used for two cohorts of Simulium damnosum s.l. (Ghana strain). Complete generation development from field-collected eggs occurred in both instances with the production of F1 adults and F1 pupae respectively. Survival of larvae form the 3rd instar to pupation was extremely high for both parental and filial generations and ranged form 63-98%. In 3 of the 4 rearings (1P, 2P, 1F1), pupal survival ranged form 98-100%. Mating attempts were frequently seen but insemination rates were low (greater than 1%). Nulliparous females were anthropophilic and exhibited bloodfeeding rates ranging form 67.5-87.5%. Parous females also readily engorged on humans. Porcine and rabbit hosts proved less attractive while two membrane systems (Baudruche, chicken skin)using equine, bovine, or chicken blood elicited practically no engorgement. Gravid females readily deposited fertile and infertile eggs in an oviposition chamber designed originally for Simulium decorum.

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PubMed1980

The development of Onchocerca spp. in Simulium decorum Walker and Simulium pictipes Hagen.

Two species of Nearctic black fly, Simulium decorum and S. pictipes, exhibit partial susceptibility to infection with the bovine parasite, Onchocerca lienalis, when inoculated intrathoracically with microfilariae. In addition, a proportion of S. decorum females will support the development of the human parasite, O. volvulus, to the third larvae stage. Infection rates with second- or third-stage larvae seven or more days after varied among geographic strains of S. decorum, ranging from 6.5% in a strain from northern New York State to 48.7% in a strain from Georgia. The average number of larvae per infected female ranged from 1.00 to 1.78 in the three strains examined. Partial susceptibility to O. lienalis was found to persist in colonized strains of s. decorum, and cryopreserved microfilariae of this parasite retained their infectivity. Seven or more days after inoculation with microfilariae of the Guatemalan strain of O. volvulus, 16.7% of the females of S. decorum harbored second- or third-stage larvae. Development of O. lienalis and O. volvulus proceeded normally in these black flies, and moderate increases in susceptibility rate and number of infective larvae were noted in response to increased microfilarial dosages.

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