Dynamics of parasitic equilibrium in cotton rat filariasis

POPULATIONS AND SITE SELECTION OF DIROFILARIA URSI (NEMATODA: ONCHOCERCIDAE) IN AMERICAN BLACK BEARS (URSUS AMERICANUS)

A total of 4,846 (84.5±87.15, 3-429: mean±SD, range) subadult or adult Dirofilaria ursi were recovered from 56 American black bears (Ursus americanus) in Ontario, 1975-77. Yearling bears had fewer worms than older bears; age classes greater than 2.5 yr had similar numbers of worms. Dead worms comprised 3.7% of all worms. There was one dead worm in total in 10 yearling bears and consistently more dead worms in older bears. The occurrence of worms within local sites in bears differed over time. Increasing numbers of worms appeared first in peritracheal sites (Site 1) following 1-3 seasons of transmission, in abundance in perirenal sites (Site 2) following the second season of transmission, and in lateral and ventral trunk and abdomen plus medio-proximal legs (Site 3) following three and four seasons of transmission. Few worms occupied other sites (Site 4). The proportion of worms recovered from the four sites was 46, 30, 19.8, and 3.9%, respectively. Dead worms comprised only 3.2% of worms in the first three definitive sites but 14% of worms in other sites. These data are consistent with primary, secondary, and tertiary site preferences for mature D. ursi in this species. Circulating microfilaremia in peripheral blood was periodic in 7/10 experiments and revealed a pattern of variable numbers during the day, highest in the evening and lowest during the night. The time of high density of microfilariae in peripheral blood was synchronous with the feeding cycle of Simulium venustum, the vector of D. ursi. Immediately postmortem, microfilariae were at highest densities in blood of lungs regardless of their relative density in peripheral blood. These data can direct diagnostic efforts to maximize detection of adult D. ursi in tissues and larvae in blood samples of naturally infected bears.

A comprehensive, model-based review of vaccine and repeat infection trials for filariasis

SUMMARY Filarial worms cause highly morbid diseases such as elephantiasis and river blindness. Since the 1940s, researchers have conducted vaccine trials in 27 different animal models of filariasis. Although no vaccine trial in a permissive model of filariasis has provided sterilizing immunity, great strides have been made toward developing vaccines that could block transmission, decrease pathological sequelae, or decrease susceptibility to infection. In this review, we have organized, to the best of our ability, all published filaria vaccine trials and reviewed them in the context of the animal models used. Additionally, we provide information on the life cycle, disease phenotype, concomitant immunity, and natural immunity during primary and secondary infections for 24 different filaria models.

A model for the dynamics of human lymphatic filariasis

In this paper, Bryan Gren fell, Edwin Michael and David Denham review the appropriateness of feline filariasis as a model of the population dynamics of human lymphatic filarial infection and disease. Because of the longevity of infection and our inability to measure the adult parasite population in humans, research in filariasis is particularly dependent on the use of laboratory animal models. We demonstrate that Brugia pahangi infection patterns in the cat closely parallel those of Brugia and Wuchereria in humans. Although primary infections in 'susceptible' cats are long-lived, repeatedly infected animals show evidence of concomitant immunity which prevents the establishment of later cohorts of infective larvae. Furthermore, there is some evidence from macro filarial length distributions of 'stunting' of adult worms during long-term repeat infections. Cats can also show an 'acute' response that spontaneously eliminates infections, and this appears to be due to a combination of intrinsic and dynamic mechanisms. As in humans, pathology in cat filariasis develops as a sequel to the asymptomatic microfilaremic state, largely as a result of re-expression of immunity. The relationship between macro filarial burdens and microfilariae in blood is positive but portrays a high degree of variability. The cat model provides an important tool for elucidating the relationships between infection, immunity and disease dynamics in lymphatic filariasis, and we conclude by suggesting directions for further work in this area.

Behaviour of filariae: morphological and anatomical signatures of their life style within the arthropod and vertebrate hosts

This paper attempts to pinpoint the most original morphological anatomical features of the biology of filariae per se and those which are or could be important for triggering regulatory processes in the arthropod vector and uncontrolled pathogenic processes in the vertebrate hosts. The following stages are considered: the motile egg or newly-hatched larva, the microfilaria, in the lymphatic or blood vessels of its vertebrate host; the larva, its migrations and its intrasyncitial development in the hematophagous arthropod subverted as vector; its transfer to the vertebrate host, migratory properties through the lymphatic system, maturation, mating and, finally, egg laying in the tissues they reach. This synthesis is based on parasite morphological features and their functional interpretation, histological features in the different niches the filariae reach, and on quantitative analyses of filarial development at its different phases, as well as on the rare and valuable observations of living parasites in situ. Data have been drawn from various species of Onchocercidae from amphibians, reptiles, birds and mammals. These comparative analyses have revealed the major constraints to which the filariae, including those parasitizing humans, have been subjected during their evolution from their ancestors, the oviparous and heteroxenic spirurids. Emphasis is placed on mechanical events: resistance of the microfilariae to the currents in the blood or lymph vessels, regulatory processes induced in the vector mesenteron by the movements of the ingested microfilariae, transient disruption by the microfilarial cephalic hook of the vectors' tissues and cell membranes during microfilarial translocation, attachment of males to females during mating by means of 'non-slip' systems, etc. Like other nematodes, filariae are equipped with sensory organs and a locomotor system, composed of the muscles and of the original osmoregulatory-excretory cell. Any change in one of these elements will result in the destruction of the filaria, at some stage of its development. In the vertebrate host, the intravascular stages will no longer be able to resist being carried passively towards the organs of destruction such as the lymph nodes or the lungs.

Cellular immune responses of filaria (Litomosoides sigmodontis) infected BALB/c mice detected on the level of cytokine transcription

Cellular immune responses of BALB/c mice infected with 80 or 160 L3 of Litomosoides sigmodontis were studied over a period of 200 days postinfection (p.i.) by stimulating spleen cells with specific microfilariae and adult antigens and Concanavalin A (Con A). Effects were determined as the level of transcription of cytokine genes [interleukin (IL)-2, interferon (IFN)-gamma, IL-4, IL-5, IL-10, IL-13] employing a semiquantitative reverse transcriptase-polymerase chain reaction technique. Con A stimulation resulted in generally enhanced transcription levels in infected animals. Exposure to filarial antigens stimulated T cells of infected animals dependent on time p.i. There was a general strong response in the early prepatency (24 days p.i.), a temporary almost complete downregulation of cytokine gene transcription except IL-10 towards the end of prepatency (45 days p.i.), and subsequently strong reactions particularly concerning IFN-gamma and IL-13 during patency and postpatency. The dose of infection as well as the mode of antigenic stimulation had generally only small effects on the cytokine gene transcription: following the same type of kinetics, infection with 160 L3 as well as the use of microfilarial antigen generally induced lower levels of cytokine gene transcription compared with infection with 80 L3 and stimulation with female antigen, respectively.

The host-parasite relationships in pyridoxine (vitamin B6) deficient cotton rats infected with Litomosoides carinii (Nematoda: Filaroidea)

This paper demonstrates that the establishment and growth of the filarial nematode parasite, Litomosoides carinii, is reduced in pyridoxine-deficient cotton rats. Young cotton rats were assigned to one of three dietary: vitamin B6-deficient cotton rats (B6-AL) were fed a pyridoxine-free diet ad libitum; pair-fed controls (B6 + PF) were fed the same amount of pyridoxine-free diet as animals in the deficient group and given daily oral supplements of 100 micrograms pyridoxine; and pyridoxine-sufficient controls (B6 + AL) were fed the pyridoxine-free diet ad libitum and supplemented daily with 100 micrograms pyridoxine. Half of each group was infected with 50 L3 of L. carinii by subcutaneous injection 8 weeks after the start of the experimental feeding period. B6-deficient cotton rats ate less (P < 0.001) and gained less weight (P < 0.001) than B6-supplemented controls. The levels of microfilaraemia in deficient animals, measured weekly throughout the experiment by taking blood smears, was significantly lower than in supplemented animals (P < 0.001). The deficient rats became latent for L. carinii at 20 weeks post-infection, whereas there was patent microfilaraemia in rats in the other dietary groups until the end of the experiment. Smaller (P < 0.001) and fewer (P < 0.05) adult worms were recovered from the pleural and abdominal cavities of deficient animals than from either pair-fed or sufficient controls at autopsy 28 week post-infection.

A major Litomosoides carinii microfilarial sheath glycoprotein (gp22): amino terminal sequence and immunological studies with corresponding synthetic peptides

The major glycoprotein of the sheath of Litomosoides carinii microfilariae (gp22) was analysed for its amino acid and amino sugar composition. It is rich in proline, glutamine/glutamic acid and glycine and contains (N-acetyl)galactosamine. The N-terminal amino acid sequence was determined up to position 37. It consists of a group of 6 repeats of the pentapeptide sequence methionine-glycine-proline-glutamine-proline with two minor modifications in repeats 3-6, while the first two repeats follow the general pattern more loosely. Identical N-terminal amino acid sequences were found in at least two other sheath polypeptides (33 kDa, 39 kDa). Antisera prepared against 3 overlapping synthetic peptides corresponding to the amino terminus of gp22 recognized different epitopes. They all reacted with identical patterns of sheath polypeptides. The antisera failed to recognize antigens of 4th-stage larvae of L. carinii. In contrast, cross-reacting epitopes were detected in other parasite stages. Antisera reacted with material surrounding embryos and microfilariae in the uterus of females, and caused patchy fluorescence on the sheath of blood-derived and in vitro-released microfilariae.

In-vivo encapsulation and killing of Litomosoides carinii in white rats

Ultrastructural studies revealed that in albino rats Litomosoides carinii was encapsulated and eventually killed in the pleural cavity by adherent host cells. Encapsulation was an organized cellular reaction which sequestered and eventually degraded the parasites. The process evolved in three phases: primary accumulation of host cells, especially eosinophils and macrophages, around the parasites with concentration of eosinophils on the parasite surface; secondary transformation of macrophages into epithelioid cells which replaced eosinophils on the parasite surface; finally, dead parasites became calcified and were gradually degraded within a tough, compacted fibrotic capsule.

Infections of forest rat filaria, Breinlia booliati, in neonate and juvenile laboratory white rats

The kinetics of Breinlia booliati infection in 3 inbred rat strains (Lewis, Wistar and Sprague Dawley) were investigated. One group of rats was infected as neonates (less than 24 hours of age) with third-stage larvae of B. booliati and the other group was infected as juveniles (4 weeks of age). The results showed that infection in the neonates were significantly different from the infection in the juveniles. The 60 rats infected as neonates, when necropsied between 8 to 10 months postinfection, yielded adult worms. The 2 neonatal infection groups of Lewis and Wistar strains showed highest susceptibility to the infections. The mean prepatent period was 85 days. Ninety to 95% of the infected rats were patent with microfilaraemia and a large percentage (33 to 47%) of them had high microfilaraemia counts exceeding 3000 mff/20 mm3 of blood and larger sizes (mean 157.11 mm for female adult worms and 61.88 mm for male adult worms. The adult worms were distributed equally in both the pleural (57%) and peritoneal cavity (43%). In most aspects, the neonatal infection group of the Sprague-Dawley strain was intermediate in susceptibility between the 2 neonatal infection groups of the Lewis and Wistar strains and the 3 juvenile infection groups. In contrast to neonatal infection groups, the 3 juvenile infection groups exhibited low infection rates (37%, 58% and 47% for the Lewis, Wistar and Sprague Dawley strains respectively), longer prepatent periods (mean 101 days), lower recovery rates (2 to 4%), lower adult worm loads (mean 0.4 to 0.8 female worms, and 0.2 to 0.8 male worms per rat), and smaller sizes (mean 141.24 mm for female adult worms and 53.75 mm for male adult worms). Forty-four to 57% of these infected rats harboured either single male or single female adult worms in the body cavity. Most (92%) of the adult worms recovered from the juvenile infection groups resided in the pleural cavity and the remaining 8% were recovered from the peritoneal cavity. Microfilaraemia could be detected in only 3/20 Lewis rats, 5/20 Wistar rats and 5/20 Sprague Dawley rats. The mean peak microfilaraemia of the 3 pooled juvenile infection groups was 632 mff/20 mm3 of blood, ranging from 7 mff/20 mm3 to 1856 mmf/20 mm3. Our results indicate that the susceptibility to B. booliati infection in white rats is both genetic and age-associated. The responses of the 2 distinct infection groups to B. booliati infections are discussed.

Immunity to Litomosoides carinii in Mastomys natalensis. I. Effect of immunization with microfilariae and existing primary infections on the parasitaemia after microfilariae injection and challenge infection

Subcutaneous injections of intrauterine stages of Litomosoides carinii into Mastomys natalensis induced strong immunity to i.v. injected blood microfilariae. Immunity, developed after boostering with an i.p. and an i.v. injection of microfilariae, did not totally suppress the parasitaemia of a challenge infection but reduced significantly the microfilaraemia level. No effect was found on number and size of the worms of the challenge infection, the number of microfilariae or the number of leucocytes in the pleural cavity. Delayed type hypersensitivity reactions in challenged animals were similar to those in non-immunized, infected controls. Sera of immunized animals agglutinated microfilariae and mediated cell attachment to microfilariae. Challenge infections did not change this until the end of the fourth week post infection but sera taken 32 days after challenge and later failed to induce such reactions. Challenge infections performed 120 or 240 days after a primary infection did not increase the parasitaemia of recipients. Dissections carried out 130 days after the challenge showed that (a) the developmental rate of the challenge infection was reduced by about 50%; (b) the size of the challenge parasites was reduced; and (c) that these worms produced significantly less embryonic stages in comparison to worms of primary infections, of which about 90% were abnormal.

Immunological regulation of Angiostrongylus cantonensis infections in rats: modulation of population density and enhanced parasite growth following one or two superimposed infections

Rats acquired a degree of protective immunity to reinfection with Angiostrongylus cantonensis after a single infection with 50 infective larvae. Infected rats resisted the establishment of most challenging larvae and protective immunity increased with subsequent reinfections. Part of the primary infection was lost after a superimposed second and also following a superimposed third infection, but the total size of the concurrent adult worm populations remained the same as that from a primary infection. Worms surviving from the primary infection showed enhanced growth after each reinfection but their fecundity was impaired.

Macrophage-mediated damage to filarial worms (Dipetalonema setariosum) in vivo

An adult female Dipetalonema setariosum (Mönnig 1926) recovered from the pleural cavity of Meriones libycus and bearing an adherent cell mass examined by means of electron microscopy. The host reaction consisted exclusively of macrophages and was associated with disruption of the cuticular membrane and invasion of the cuticle itself. There was no evidence of prior activity by other cell types. Initiation of damage appeared to be associated with the release of lysosomal enzymes by the macrophages. Within the reaction a localized breaching of the cuticle had occurred and cells had penetrated the internal tissues of the worm. It is suggested that macrophages may play a role in the elimination of effect worms from an established population.

Antigen-specific suppressor T lymphocytes in human lymphatic filariasis

Immune responses to parasite antigens are much lower in patients with microfilaremia than in persons with other manifestations of brugian filariasis. To determine whether hyporeactivity is associated with changes in populations of lymphocytes that regulate immune responses, we quantitated helper and suppressor T cells in the blood of patients infected with Brugia malayi. Increased numbers of suppressor T cells were present in 15 of 17 patients with microfilaremia and in six of 11 patients with elephantiasis. This increase correlated with hyporeactivity to filarial antigens but not to nonparasite antigens. Removal of suppressor T cells activated in vivo or in vitro improved reactivity to filarial antigens. These results suggest that immunosuppression induced by filarial parasites is a possible mechanism of survival of these organisms in an immunocompetent host.

Studies on acquired resistance of the cotton rat against microfilariae of Litomosoides carinii. 2. Injection of microfilariae during prepatency

Cotton rats infected by infective third-stage larvae of Litomosoides carinii were treated at increasing time intervals by a threefold injection of living homologous microfilariae (mf) during the prepatent period. Starting with the first treatment 3, 4 or 5 weeks p.i. seven animals remained completely and two almost mf-negative (1 or 2 mf/mm3 each only once) until 16 weeks p.i. Starting 6, 7 or 8 weeks p.i. six animals developed a normal level of parasitaemia between 42 and 436 mf/mm3, two animals developed a continuous level of 1-2 mf/mm3. The number of fertile adult worms shedding great numbers of microfilariae in the pleural cavity was equal in all animals. However, in mf-negative animals the lung capillary blood showed, in the geometric mean, only 0.6% of the mf-concentration seen in mf-positive animals. The hypothesis is proposed that microfilariae accumulating primarily in the lung capillaries absorb all aggressive components specifically reacting with microfilarial antigens, i.e. neutralize the immune response against them to enable the development of the parasitaemia in the peripheral blood.

Immune responses in human infections with Brugia malayi: specific cellular unresponsiveness to filarial antigens

We evaluated the cellular immune competence of 101 subjects living in an area of South Kalimantan (Borneo) where Malayan filariasis is endemic. All patients with elephantiasis but none with other clinical stages of filariasis reacted with adult worm antigens. The majority of subjects without clinical or parasitological evidence of filariasis and approximately one-half of those with amicrofilaremic filariasis reacted with microfilarial antigens. In contrast, most patients with patent microfilaremia did not respond to microfilarial antigens. The in vitro reactivity of all patient categories to nonparasite antigens was similar to that of the distant control group. These results indicate that patent microfilaremia is associated with a state of specific cellular immune unresponsiveness and are consistent with the current hypothesis that the various clinical manifestations of filariasis result from different types of immune responses to distinct antigens associated with different developmental stages of filarial worms.

Studies on the Malayan forest rat filaria, Breinlia booliati (Filarioidea: Onchocercidae): course of development in rat host

The development of Breinlia booliati is described in its natural host, Rattus sabanus and in an inbred strain of laboratory albino rat. The growth of the parasite is similar in both the rat hosts. The third moult occurs between six-eight days and the final moult between 24-28 days. Larvae were recovered initially from the skin and carcass. After five weeks, developing stages were seen only in the thoracic and abdominal cavities, the site of development of the adult worms. Worms became sexually mature by 11-12 weeks and there was considerable growth in length of the female worms after this stage.

Studies on the Malayan forest rat filaria, Breinlia booliati: periodicity and microfilaraemic patterns during the course of infection

Breinlia booliati exhibited nocturnal subperiodicity in its natural host, Rattus sabanus in contrast to experimentally infected laboratory-reared albine rats which showed irregular fluctuations of microfilariae throughout the 24 hour cycle. All the infected albino rats showed a prepatent period between 11-14 weeks postinoculation. Three patterns of microfilaraemia were discerned during the course of infection 38/49 rats displayed a single peak, 4/49 displayed 2 peaks about 12-15 weeks apart and 7/49 showed a sustained high plateau-like pattern of microfilaraemia. Cortisone had no effect on microfilarial levels when administered to rats near postpatency and some at postpatency.