Number Of Infective Larvae Research Articles

Activity of commercial Bacillus thuringiensis preparations against Trichostrongylus colubriformis and Nippostrongylus brasiliensis

Control of nematode parasites of livestock is focused almost entirely on the parasitic stages within the host. Current methods rely on anthelmintic drugs, but these are under increasing threat with the development of resistance covering the whole spectrum of anthelmintics amongst the important nematode species of a range of livestock. However, invariably the greatest proportion of the parasite biomass resides not within the animal hosts, but in the external environment - commonly on pasture. It is in this environment that the free-living stages are vulnerable to a range of abiotic factors (extremes in temperature and desiccation) and biotic factors (macro- and micro-organisms) that may decimate their numbers. Of the latter, there are organisms, which exert their effects either indirectly by rendering faecal deposits inimical for the development of nematode eggs through to infective larvae, or directly by acting as pathogens or by exploiting the free-living stages as a food source. Within this vast assemblage of organisms, which include microarthropods, protozoa, viruses, bacteria and fungi, could well emerge a variety of biological control agents of nematode parasites. At present, greatest interest lies with the nematode-destroying fungi. Work has progressed from Petri dishes, to plots, to paddocks with several species of the genus Arthrobotrys and Duddingtonia flagrans. These studies indicate that the voracious nematophagous capabilities of these fungi, clearly demonstrated in vitro, translate to reductions in the number of infective larvae on pasture and indicate that levels of control, comparable to conventional schemes using anthelmintics, can be achieved. The challenge now lies in developing methods of administration of fungi to animals which can be applied under practical farm conditions.However, the pursuit of candidates for biological control of nematode parasites of livestock should not involve just a few species of nematophagous fungi. More than 100 species of fungi have been identified as possessing nematode destroying capabilities. These need to be more specifically investigated for their effects on free-living stages of nematode parasites of livestock, together with other classes of organisms, particularly bacteria, which have proved successful as biological control agents of arthropods.

Factors affecting the survival and migration of the free-living stages of gastrointestinal nematode parasites of cattle in central queensland

Faecal pats containing parasitic nematode eggs were deposited monthly on worm-free pasture, from mid-1975 to early in 1979, near Rockhampton in central Queensland. Pasture samples were collected monthly from beside these pats and the number of infective larvae on the samples was counted. Cooperia spp. were the most numerous larvae on pasture all year round and Haemonchus placei were commonly present in low numbers. Small numbers of Oesophagostonum radiatum larvae were found, mostly during summer. Dung beetle activity and rainfall influenced larval populations on pasture, but temperature did not. Beetles were not active in winter, and pats deposited in spring, summer and autumn when beetles were active yielded only 42, 44 and 26%, respectively, as many larvae per 1000 eggs deposited as winter pats. Pats in which beetle activity was minimal (feeding only), moderate and intense (complete destruction), yielded 43, 10 and 6%, respectively, as many larvae per 1000 eggs as intact pats. Larval densities on pasture were highest after the first saturating rains during the spring-summer period and most of these larvae migrated from unattacked pats deposited in winter. Beetle numbers and activity increased with the summer rains and so few larvae were available to migrate onto pasture during late summer and autumn when the highest falls of rain were recorded. The regression of larval recovery on rainfall was positive and statistically significant when data collected soon after these very heavy rainfall periods were omitted from the analysis. In 1977, drought-breaking rains increased the normal larval density on pasture 10-fold because larvae in pats deposited in the last 4 months of the drought migrated onto pasture immediately after the rains. This work suggests that in summer rainfall areas where dung beetles are active, helminth control may be achieved by reducing the worm egg output from cattle during the winter.

Ethyl-methanesulfonate-induced changes in filarial susceptibility in Aedes aegypti (Diptera: Culicidae).

The susceptibility to Brugia pahangi infections of F1 progeny derived from female Aedes aegypti treated with ethyl methanesulfonate (EMS) was tested. Three-day-old females were treated with 0.025, 0.050, and 0.075% EMS by allowing them to feed freely for 5 days on sugar cubes containing the chemical. Control and treated females were then mated with normal, untreated males to recover F1 progeny. Percent susceptibility and mean number of infective larvae (Ls) in F1 females were analyzed to test for changes in the fm gene for filarial susceptibility. Sterility was induced in populations exposed to 0.075% EMS. Susceptibility in both the 0.025 and 0.050% EMS F1 populations was reduced by 9 and 20%, respectively. The 0.025% EMS F1 females showed a 35% decrease in mean Ls numbers. At 0.050% EMS, mean Ls numbers decreased by 64%.

Trichostrongyle larval populations on herbage from grazed and ungrazed cattle pastures in south west Scotland

In the course of studies on the control of bovine parasitic gastroenteritis in south west Scotland using a morantel sustained release bolus the populations of trichostrongyle infective larvae on herbage were monitored over a two-year period. From the results of the herbage analysis and the worm burdens in parasite naive tracer calves introduced at the end of 18 months, it was clear that considerable numbers of infective larvae of Ostertagia ostertagi and Cooperia oncophora survived for at least 18 months on pastures not grazed at all. The results suggest that in south west Scotland after one, or even two, seasons with no further contamination of grazing, permanent cattle pastures cannot be assumed to be helminthologically 'safe' and that this should be taken into consideration when determining prophylactic strategies.

Bancroftian filariasis in Kenya. V. Mass treatment given by members of the local community.

Diethylcarbamazine citrate was given for a total of 13 days to all people above one year of age in the following areas: (1) Mambrui, a town with a population of about 1300; (2) Jaribuni, a rural area with a population of about 1500; (3) The towns and villages on the islands off the north coast of Kenya with a population of over 20 000. The people were not weighed, but the daily dose was calculated according to age, being approximately 6 mg kg-1; drug taking was not supervised. The drug was mainly distributed by local people who also treated the side-effects; a doctor remained available in the background for consultation and advice. In Mambrui and Jaribuni, where baseline data had been collected, the number of infective larvae per 1000 vector-mosquitoes had decreased by more than 90% in the six to 12 months period after the campaign. Two years after the campaign, the microfilaria rate in Mambrui had dropped by 75%; in Jaribuni, a reduction of 75% was recorded in men, but in women it was only 48%. The overall rates after the mass treatment were 5.3 and 9.3% in Mambrui and Jaribuni, respectively. On the islands to the north, particularly good results were obtained in the village of Siyu, where the highest elephantiasis rate had been recorded; three years after the campaign, the microfilaria rate there had fallen to 3.6%, while none of 48 children under ten were found infected. The spectacular results in Lamu town may have been partly due to excellent mosquito control in preceding years. However, no control was achieved on the remote island of Ndau, probably because the treatment team left before the side-effects had disappeared, causing many people to stop the course of tablets. Thus, on the East African coast it is possible and cheap to control Bancroftian filariasis by mass treatment campaigns carried out by local, non-medical personnel.

Grazing study in Ireland using the morantel sustained release bolus for controlling nematodiasis in calves.

The morantel sustained release bolus was administered at turnout to first-season grazing calves in order to assess its efficacy in the seasonal control of infection by nematode parasites in Ireland. The pastures grazed by control calves showed a marked increase in gastrointestinal trichostrongylid infective larvae by September, while numbers of infective larvae on pasture grazed by bolus-treated calves remained at a low level throughout the grazing season. In consequence, the controls showed significantly higher worm egg counts in late season and significantly higher worm burdens (mainly Ostertagia spp) at necropsy carried out in November on representative number of principal animals selected from each group. These reduced worm burdens were attributed to the suppression of egg output during the early part of the season as a result of treatment with the morantel sustained release bolus at turnout in the spring. Pasture contamination with Dictyocaulus viviparus larvae was present on all treatment pastures. The bolus-treated calves however were subjected to an increase in D. viviparus infection which occurred on their pasture in late season after the active life of the bolus had expired. It was concluded that bolus treatment delayed (rather than prevented) the buildup of D. viviparus infection on the pasture by 60-90 days.

Effects of gregarine parasites on the development of Dirofilaria immitis in Aedes triseriatus (Diptera: Culicidae).

Infections of Aedes triseriatus with the gregarine parasite Ascogregarina barretti were examined to determine the influence of this parasite on host development and survival, and to quantify the effects of this parasite on Dirofilaria immitis development. Infections with A. barretti caused a reduction in the weight of female pupae and a slight increase in the time to pupation of males. Male pupal weight, female time to pupation, larval mortality, adult emergence, and adult male survival were not affected by A. barretti. Over 95% of the gregarine-infected and uninfected adult females that ingested comparable levels of microfilariae of D. immitis became infected, and in over 90% of the mosquitoes D. immitis completed development to the infective L3 stage. No significant differences in the number of infective larvae were found between gregarine-infected and uninfected mosquitoes. However, in the Malpighian tubules, significantly more L3 and L2 larvae were found in gregarine-infected mosquitoes. The survival of adult female Ae. triseriatus was not influenced by either gregarine infections, D. immitis infections, or concomitant infections with both parasites. Overall, infections of A. barretti do not appear to affect the vector competence of Ae. triseriatus for D. immitis.

Life Cycle of Brugia pahangi (Nematoda) in Nude Mice, C3H/HeN (nu/nu)

The development of Brugia pahangi (Nematoda: Filarioidea) was studied in nude (congenitally athymic) mice C3H/HeN (nu/nu) and in their phenotypically normal littermates (nu/+). Nude mice were highly susceptible to this parasite. As in the natural host (the cat), the nematodes' third molt in nude mice occurred at 7 to 10 days. The final molt occurred at about 24 days for male worms and 33 days for female worms. Adult worms were smaller than those from other hosts, such as the cat. After inoculation of various numbers of infective larvae, recoveries of adult worms averaged about 15% of the inoculum. In long-term infections initiated with 100 larvae, about 75% of the worms localized in the heart or lungs. Patent infections were seen as early as day 50 PI. Microfilaremia developed in most nude mice given 100, 50, or 25 infective larvae, but was less frequent in those given only 10. Mean filaremias generally rose during the first 6 mo, but in individuals usually did not exceed 500-600/20 mm3 of blood. As in the Mongolian jird, intraperitoneal inoculations yielded large quantities of worms and microfilariae. Few worms could be recovered from normal mice after day 40, even when large (1,000 larvae) inocula were used. Microfilaremia was not detected in normal mice. Although recoveries of adult worms from some nude females were not as high as those from nude males, neither nude nor normal mice showed consistent evidence of a differential susceptibility based on sex. Given the strong, consistent dichotomy of response to B. pahangi between nude and normal mice, this system may be useful in studies of protective immune responses in filariasis.

Epidemiological Aspects of the Infection With Oswaldocruzia Filiformis (Goeze, 1782), Travassos, 1917 (Nematoda: Trichostrongylidae) in the Common Toad (Bufo Bufo L., 1785) in the Netherlands

Twenty-eight males and fifteen females of B. bufo were collected before and after spawning and in November 1979 at a fieldstation for examination for the presence of O. filiformis. From this fieldstation another sample of the adult toad population was collected in the same spring and placed in an outdoor enclosure (experimental plot). The fluctuation of the parasitic stages in these hosts (twenty-three males, thirty-one females) was observed monthly from May till September/October. The availability of infective larvae of O. filiformis was investigated each month in the experimental plot by using wormfree Rana temporaria as tracers from May till November. Tracers were also used to registrate survival of preparasitic stages following the winters of 1979 and 1980. All parasitic stages overwintered in the toads and resumed their development after spawning of the host. In the experimental plot this development resulted in a first wave of adult parasites in B. bufo during May/June. They caused a midsummer-increase of infective larvae on the ground resulting in significant values in tracers in June and in the toads in June and July. It is suggested that these larvae result in a second wave of adults from the end of August on. From that moment increasing numbers of fifth stage larvae and adults were observed in toads, the increase being significant for adult female parasites. During the summer a significant decrease in available numbers of infective larvae on the ground was seen in the results from the tracers, but the chances for infection considerably increased in late summer and autumn. After both winters tracers became infected, but it is suggested that survival capacity of preparasitic stages is probably low. Under natural circumstances no infection of B. bufo up to the end of May was observed. No significant differences in the intensity of the infection between males and females of both host species, or first and second year of life tracers were found. Instead of 0-20 parasites per animal, as seen in toads in the fieldstation, intensities of 100-200 parasites in both host species were seen most frequently in the experimental plot. Traces became infected earlier than toads, in which new infections were not observed until June. Under outdoor conditions the prepatent period of O. filiformis in R. temporaria apparently takes more than one month.

Estimate of the Number of Infective Larvae of Neoaplectana Carpocapsae (Nematoda: Steinernematidae) in a Soil Sample

Using Galleria traps, the number of infective larvae of Neoaplectana carpocapsae in soil samples was estimated. Sterile soil samples, in Galleria traps, were inoculated with different concentrations of infective larvae, from 10 to 5 × 105, and the concentration was related to the percentage of dead larvae of Galleria mellonella and the rate of mortality. The results suggest that using our method, an estimate can be made of the number of infective larvae in soil samples.

Populations of strongyloid nematode infective stages in sheep pastures: Comparison between direct pasture sampling and tracer lambs as estimators of larval abundance

Waller P.J., Dobson R.J., Donald A.D. and Thomas R.J. 1981. Populations of strongyloid nematode infective stages in sheep pastures : comparison between direct pasture sampling and tracer lambs as estimators of larval abundance. International Journal for Parasitology 11: 359–367. Over a 2-year period, numbers of infective larvae in samples of pasture herbage, and numbers of worms in previously worm-free “tracer” lambs allowed 4 weeks grazing, were compared as estimators of the abundance of infective larvae on pastures. Transformation of sample estimates of infective larval numbers per 100 g herbage dry matter (DM) and of worm numbers in tracer sheep, according to the expression y = log 10 ( x+25), was effective in stabilizing variances. Estimates of error variance for each technique did not differ significantly among the genera Haemonchus, Ostertagia or Trichostrongylus and the pooled estimate for the tracer sheep method was 4 times greater than that for pasture sampling. From these results, more tracer sheep than pasture samples would be required to achieve the same level of precision with the two techniques. Using conventional statistical methods, the effects of numbers of pasture samples or tracer sheep on the size of the difference between two means which can be detected as significant and on the width of the confidence interval about a single mean, are illustrated. These can be used as a guide in the choice of sample sizes. Error variances for Nematodirus spp. were significantly less than for the other genera by pasture sampling, and greater by the tracer sheep technique. Possible reasons for this are discussed, but it is concluded that pasture sampling is likely to be much the more precise method for estimating Nematodirus spp. infective larval availability. Changes with time in infective larval abundance, for Haemonchus, Trichostrongylus and Nematodirus spp. which were present in moderate to low numbers, followed similar trends by both techniques. However, for Ostertagia spp. larvae, which were much more abundant, peak levels were defined more sharply and occurred earlier by pasture sampling than by the tracer method. It is suggested that worm counts from tracer sheep, especially those grazing for 4 weeks rather than shorter periods, may systematically underestimate the infective larval population on pasture at high levels of abundance owing to density-dependent worm loss.

Effects on inhibition of development of the transfer of Ostertagia ostertagi between geographical regions of Australia.

Pasture plots in a warm coastal region were separately contaminated in the autumn, winter and spring with eggs of a coastal strain of Ostertagia ostertagi derived from dairy calves. At the same times, a parallel set of plots was contaminated by a strain transferred from within a beef cattle management system in a cooler tablelands region. A reciprocal transfer of the coastal strain was also made to the tablelands, where pasture plots were contaminated with eggs of both strains of O. ostertagi in winter. Successive groups of parasite-free calves wee grazed on the plots for 10-14 days at 4-week intervals and then killed for worm counts 14 days after their removal from pasture. In both environments, irrespective of the season of pasture contamination, inhibition of development reached a maximum in spring and the two strains differed only in the proportion of the population inhibited, which was significantly greater for the tablelands strain. There was no strain difference for either numbers of infective larvae on pasture or total O. ostertagi numbers. This suggested that the difference between coastal and tablelands populations in the extent of inhibition is genetically determined. If an environmental stimulus was implicated in the spring occurrence of inhibition, the stimulus was provided equally in coastal and tablelands environments and it cannot be exposure to low temperatures.

Polymyositis and polyneuritis in experimentalAngiostrongylus cantonensisinfection of rabbits

The effects of primary and secondary infections and anthelmintic therapy in angiostrongyliasis cantonensis were studied in rabbits. An increase in the number of infective larvae resulted in a more severe clinical course, but hardly changed the incubation period. Larvae fed to previously infected rabbits failed to reach the central nervous system; they died and were resorbed in the musculature, giving rise to polymyositis. Smaller inflammatory foci developed in the muscles if the larvae of a single infective dose were killed with Levamisole shortly after the infection. The finding of lesions in the peripheral nerves gives additional support to the hypothesis of intraneural larval migration from the muscles into the spinal cord and brain. It is suggested that signs of myositis could help to differentiate between the first attack and recurrence of the disease in human cases of eosinophilic meningitis.

Significance of inhibited development in the epidemiology of Chabertia ovina and Oesophagostomum venulosum

Worm counts from ewes, lambs and tracer lambs during a study on the epidemiology of gastro-intestinal nematode infections in sheep at Utrecht State University between 1974 and 1977 substantiated the following descriptions of the epidemiology of Chabertia ovina and Oesophagostomum venulosum infections. Development of inhibitated larvae and recently acquired infection from pasture leads to a spring rise in the numbers of adult worms and developing stages in lactating ewes and some barren yearling sheep. However, in barren ewes and some barren yearling sheep very few adult worms develop. The low residue of overwintered larvae on pasture results in infection of the lambs in spring and early summer. These larvae develop without inhibition. Numbers of infective larvae on pasture remain low until the autumn. This autumn increase leads to substantial worm burdens in lambs and ewes, but a high proportion of these worm burdens, especially in ewes, consists of inhibited larvae. A decrease in pasture larval contamination occurs during winter and the propensity of the larvae for inhibited development diminishes. The lambs lose most of their worms during winter. Experimental infection of worm free sheep shows that it is possible to condition infective larvae, especially of O. venulosum , for inhibition of the development, by storage of the larvae for 5 weeks at 15 or 4°C.

Observations on ostertagiasis in young cattle over two grazing seasons with special reference to plasma pepsinogen levels.

The epidemiology of ostertagiasis in south west Scotland was studied in groups of cattle grazed through two successive grazing seasons separated by a period of winter housing. Towards the end of the first grazing season (September) the numbers of infective larvae (L3) on the pasture had increased to high levels (up to 24,000 L3 per kg) which resulted in high faecal egg counts, worm burdens, plasma pepsinogen levels and the occurrence of clinical ostertagiasis in the calves. By late spring (May) at the onset of the second grazing season, there was an almost complete mortality of the overwintered L3 on the pasture followed by the appearance of moderately high numbers of a new population of L3 in September (up to 9000 L3 per kg). The latter increase in the numbers of L3 was reflected by negligible faecal egg counts, low worm burdens and a moderate elevation of plasma pepsinogens in the second year animals. It therefore seems that although young cattle acquire a good immunity to Ostertagia ostertagi after one season at grass the small infections established in the early part of the second season are capable of contaminating the pasture to levels which could be dangerous for susceptible stock. An allergic reaction in the abomasal mucosa could be the basis of the elevated pepsinogens present in the second year animals.

The epidemiology of equine strongylosis in southern Queensland. 2. The survival and migration of infective larvae on herbage.

The seasonal changes in longevity on herbage of the infective larvae of strongylid nematodes of the horse were studied. During the summer months, 1% of the larvae survived on herbage for 2-3 weeks, with 0.2% still viable for a further 2-3 weeks. Equivalent survival periods in winter were 7-11 weeks and over 11 weeks respectively. During spring and autumn, larvae survived for periods varying from 3-8 weeks. On Rhodes grass (Chloris gayana) growing vigorously in the summer of 1976, the majority of larvae remained in the lowest layers of the pasture, within 10 cm of the soil surface. Very few reached the highest fraction of grass sampled, above 40 cm from the soil. More larvae were recovered higher on the pasture in a period when less torrential rain had occurred. It was concluded that the parasitological benefits to be gained from short-term mixed grazing with horses and cattle may be minimal, in view of the tendency of cattle to eat only the upper layers of the pasture initially, with a consequent increase in the number of infective larvae per unit weight of herbage remaining.

Evidence That Sulfisoxazole, an Antibacterial Sulfonamide, Can Adversely Affect the Development of Brugia pahangi in Aedes aegypti Mosquitoes

The average number of infective larvae recovered from Brugia pahangi-infected Aedes aegypti was approximately one-half that recovered from the controls after the former group of infected mosquitoes had ingested a 1.0% solution of sulfisoxazole diolamine (SXZ) in 10% sucrose-water for 4 consecutive days, beginning 4 days after infection. Most of the filarial larvae from the SXZ-treated mosquitoes were small and sluggish compared with those from the controls. There was no increased mortality of mosquitoes that ingested 1.0% SXZ in sugar-water for 4 days. Average filarial larval burdens were not decreased in mosquitoes that ingested a solution of 10(-6) M methotrexate (MTX), a potent dihydrofolate reductase inhibitor, in sugar-water for 4 days, beginning 4 days after infection. The distributional pattern of larval burdens in mosquitoes that ingested combined 1.0% SXZ and 10(-6) M MTX in sugar-water for 4 days closely resembled that seen in mosquitoes that had imbibed 1.0% SXZ only. Average filarial larval burdens were not decreased in mosquitoes with 4-day-old B. pahangi infections that fed upon jirds which received intraperitoneal injections of SXZ (2 g/kg) and MTX (1 mh/kh), alone and in combination, 1 hr previously. Survival of the mosquitoes that fed upon the drug-treated hosts was unaffected, as was the hatchability of their eggs and subsequent growth and development of the mosquito larvae.

Dirofilaria immitis: Experimental Infections in the Ferret (Mustela putorius furo)

The ferret, Mustela putorius furo, was found to be susceptible to Dirofilaria immitis infection when exposed to low (14) or high (280-420) numbers of infective larvae harvested from Aedes aegypti. Eight ferrets (half of them cortisonized) were inoculated subcutaneously with 14 larvae each. All of them were subsequently found to harbor D. immitis in the heart, and all but one of them had worms of both sexes. Six of these ferrets were examined for microfilaremia at 31 to 35 weeks after inoculation; 3 were positive (one observed only at postmortem examination) and there was evidence that fertilization of female worms had occurred in one other. Females up to 25.5 cm and males up to 16.0 cm were recovered. There was no evidence that the cortisonization of some ferrets had affected the infections. Both male and female ferrets became infected. Four cortisonized ferrets were inoculated with 280 or 420 larvae of D. immitis (divided equally between subcutaneous and intraperitoneal routes). All of them died 16 to 18 weeks after inoculation, yielding 102 to 125 immature D. immitis. In these lethal infections, worms were recovered from the heart and adjoining vessels, and also from vascular and extravascular sites throughout the body.

Studies on the epidemiology of Dictyocaulus filaria infection in Blackface sheep on a low-ground Scottish farm

The excretion of Dictyocaulus filaria larvae in the faeces of Blackface sheep on a heavily stocked lowland farm in south east Scotland, showed considerable individual, week to week and year to year variation. Patent infections were first observed in lambs in May or June but the heaviest and most prevalent infections occurred in the autumn. Infection levels were generally low but infections tended to be prolonged and reinfection occurred following anthelmintic treatment until the late autumn or winter. Only a few very light, short-lived, patent infections were observed in yearlings or adult ewes. Pasture sampling and the use of tracer animals showed that the numbers of infective larvae on the pasture were minimal during the summer months but reached a low peak in the autumn.

Host-parasite relations in cotton rat filariasis

This paper introduces the long-term and continuing studies of the authors and their colleagues on several aspects of the host-parasite relations in cotton rat filariasis. Five methods are described for the calculation of the number of infective larvae of Litomosoides carinii transmitted to cotton rats and other laboratory animals by groups of Liponyssus bacoti mites. The relation between the calculated transmission intensities and the number of worms recovered from individual infected animals has been determined by each of the five methods. The accuracy and relevance of each method is discussed and one (method B) has been found to give a better correlation with worm recovery than the others. Some effects of infection with filariasis on the mites are described. The role of heavily infected mites in transmission is questioned and their influence on the calculation of transmission is discussed.