Genetic control of liability to infection with Nematospiroides dubius in mice: selection of refractory and liable populations of mice

The Use and Underuse of Model Systems in Infectious Disease Ecology and Evolutionary Biology

AbstractEver since biologists began studying the ecology and evolution of infectious diseases (EEID), laboratory-based model systems have been important for developing and testing theory. Yet what EEID researchers mean by the term "model systems" and what they want from them is unclear. This uncertainty hinders our ability to maximally exploit these systems, identify knowledge gaps, and establish effective new model systems. Here, we borrow a definition of model systems from the biomolecular sciences to assess how EEID researchers are (and are not) using 10 key model systems. According to this definition, model systems in EEID are not being used to their fullest and, in fact, cannot even be considered model systems. Research using these systems consistently addresses only two of the three fundamental processes that underlie disease dynamics-transmission and disease, but not recovery. Furthermore, studies tend to focus on only a few scales of biological organization that matter for disease ecology and evolution. Moreover, the field lacks an infrastructure to perform comparative analyses. We aim to begin a discussion of what we want from model systems, which would further progress toward a thorough, holistic understanding of EEID.

Variability in the susceptibility of the fish host, Poecilia reticulata, to infection with Gyrodactylus bullatarudis (Monogenea)

Heterogeneity in susceptibility of guppies (Poecilia reticulata) to infection with Gyrodactylus bullatarudis was examined by comparing the establishment, reproduction and survival of the parasite on 4 inbred strains of the fish host following low and high levels of exposure to infection. The 4 strains fell into 2 broad categories of innately ‘resistant’ and ‘susceptible’ fish on the basis of strain differences in (a) the rates of growth and decay of the parasite populations, (b) the proportional occurrence of susceptible and resistant fish within each strain, (c) the average duration of primary infection, and (d) the duration of resistance to reinfection following recovery from a primary exposure. The results suggest genetic control of resistance/susceptibility traits and experiments with hybrids tentatively suggest dominance of resistance over susceptibility. The potential of the guppy – Gyrodactylus laboratory system for the investigation of the role of genetic factors in the study of host–parasite interactions is discussed.

A QUANTITATIVE APPROACH TO THE EXPERIMENTAL TRANSMISSION SUCCESS OF ECHINOSTOMA FRIEDI (TREMATODA: ECHINOSTOMATIDAE) IN RATS

Using a range of parameters, the ability of rats (Rattus norvegicus) to successfully transmit Echinostoma friedi to the next host was examined under experimental conditions. The concept of Experimental Transmission Success (TM), defined as the number of hosts that become successfully infected after exposure to a number of infective stages produced by a previous host per unit of inoculation at which this latter host was exposed, was introduced. Using data for the egg output and miracidium hatching and infectivity, the TM permits us to estimate the ability of a particular definitive host species to successfully transmit a parasite species. This concept may be also useful to compare the transmission fitness of a parasite in different definitive host species. Moreover, variations of the Experimental Transmission Success over the course of the infection were calculated by the use of the Weekly Experimental Transmission Success (TMW). Overall, considering the complete duration of the experiment, the TM of E. friedi using rats as definitive hosts was 0.68 infected snails/metacercaria. However, positive values of the TMW were only obtained from 2 to 4 wk post-infection, with a maximum during the third wk post-infection. When comparing the TM values of E. friedi in rats with those calculated in hamsters on the basis of previously published data, E. friedi appears to be more appropriate to move through this portion of its life cycle when using hamsters (Mesocricetus auratus) as the final host than rats.

H-2 genes and resistance to infection with Heligmosomoides polygyrus in selectively bred mice

Two lines of mice bred selectively for high resistance (RH) and susceptibility (SL) to reinfection with Heligmosomoides polygyrus demonstrated disparate levels of resistance to infection but did not differ in the frequency of H-2 antigens when assayed with antisera against antigens of 5 inbred H-2 haplotypes. The selected RH and SL mice were crossbred with, and backcrossed to, the inbred CBA mice. F1 mice from crosses between RH and CBA were as resistant to reinfection with H. polygyrus as their RH parents. F1 mice from crosses between SL and CBA were more resistant than either of their parents. BC1 mice were either positive or negative for H-2 antigens from RH and SL mice. BC1 mice that were positive for RH H-2 antigens were more resistant to infection than their negative littermates, but they were significantly more susceptible to infection with H. polygyrus than their F1 parents. These results demonstrated that genes within and mapped outside H-2 complex control the level of resistance to H. polygyrus in the selected mice and suggested that selective breeding of mice for resistance fixed the relevant genes in and outside the H-2 complex.

Antigens in phenotypes of Heligmosomoides polygyrus raised selectively from different strains of mice

Antigens from Heligmosomoides polygyrus, which had been passaged selectively for over 40 generations through naive (Hpn) and immune (Hpa) mice, were extracted as whole worm (WWA) and membrane bound antigens (MBA), soluble adult worm homogenates (AWH), and excretory/secretory (ES) products. The antigen complexes were separated by SDS-PAGE, and 2-dimensional electrophoresis and assayed by western blot. Qualitative and quantitative differences were observed between profiles and antigenic reactivity of the constituents from selected parasites, which reflected their genetic heterogeneity. The survival of Hpa parasite phenotypes was improved compared with that of their Hpn counterparts in homologous strains of immunized mice but this did not correlate strongly with the antigenic differences observed. Three small molecules at 18, 21 and 23 kDa, respectively, dominated the somatic and ES components of all worms but they were of low immunogenicity during natural infection in mice and after vaccination in rabbits; and their role in immunomodulation is discussed.

Studies on the reproductive capacity of Echinostoma caproni (Trematoda) in hamsters and jirds

Total egg output per infective stage inoculated was used as a measurement of the reproductive capacity in Echinostoma caproni infections in hamsters and jirds. Egg production per adult E. caproni recovered was comparable in the hamster and in the jird, but the reproductive capacity of E. caproni in infections with 6 metacercariae in the hamster markedly exceeded that in the jird. This was due mainly to the expulsion of worms from the jird. Eggs per gram faeces produced was shown to be an invalid parameter in comparing the reproductive capacity in E. caproni infections in jirds and hamsters due to differences in amounts of faeces produced, and evidence was obtained that uterine egg counts may not be a useful parameter in assessment of overall reproductive potentials. In the hamster, the reproductive capacity of infections with 25 or 6 metacercariae of E. caproni was comparable. This reflected a density independent recovery and egg production of the adult worms. The reproductive capacity, being an expression of the importance of a particular host species for the reproduction of a parasite population, is suggested as a convenient and useful quantitative parameter in the establishment of standardized criteria for evaluating host-parasite relationships in definitive host models.

Infectivity, growth, distribution and fecundity of a six versus twenty-five metacercarial cyst inoculum of Echinostoma caproni in ICR mice

Female ICR mice, 6-8 weeks old, were exposed to 6 (group A) or 25 (group B) metacercarial cysts/host of Echinostoma caproni to determine the effects of these cyst dosages on infectivity, growth, distribution and fecundity of worms in the small intestines. All 30 mice exposed (15 in group A and 15 in group B) were infected and there was no significant difference in the percentage of worm recovery between group A and B at 2, 4 and 8 weeks postinfection (PI). Growth was rapid in both groups with worm body areas increasing from about 3mm2 at 2 weeks PI to about 7 mm2 at 8 weeks PI. More worms from group B were in the anterior sections of the small intestines at 2 and 4 weeks PI than those from group A. However, at 8 weeks PI more worms from Group A were in the anterior sections of the small intestines than those from group B. There were marked differences in our findings on infectivity, growth and distribution of E. caproni in ICR mice using 6 and 25 cyst inocula compared to a previous study using the same echinostome and cyst inocula in NMRI mice, presumably related to the mouse strain. Fecundity studies were somewhat comparable in both studies showing an approximate three to four times increase in the average number of eggs/gram of faeces in mice receiving 25 versus 6 cyst inocula.

Expulsion of Trichostrongylus colubriformis by high and low responder guinea-pigs

Guinea-pigs with genetically determined susceptibility to infection with Trichostrongylus colubriformis (or low responders) rejected both primary and secondary infections with this parasite more slowly than resistant animals (high responders). Low responders were not protected with a vaccination procedure which was highly effective in outbred animals. The relatively poor protective immune responses that develop in low responder guinea-pigs are evocative of the responses of the natural host to infection with this parasite and suggest that low responder guinea-pigs have potential for the study of T. colubriformis protective antigens and for the development of adjuvants to enhance antiparasitic effector responses in vaccinated hosts.

The effects of age, sex and breed of chicken on susceptibility to Clinostomum complanatum (Digenea: Clinostomatidae)

Susceptibility and development of Clinostomum complanatum in the chicken (Gallus gallus domesticus) have been studied. The progenetic metacercariae of C. complanatum developed into ovigerous worms in 3 days in the buccal cavity of the chicken and survived only 6-7 days in this experimental host. The susceptibility is influenced by the age, sex and breed/type of the host. Both percent susceptibility and recovery of the worms decreased with increase in the age of the host. The egg laying chickens were found to be least susceptible to the infection. Mature male chickens were more susceptible than the females. The single comb white Leghorn (SCWL) was found to be susceptible while the local type (LT) and the cross breed (SCWLxLT) were not susceptible to C. complanatum. The possible reasons for these variations have been discussed.

The evolution of inducible defence

Defences against parasites are characterized by inducible, amplifiable responses, often with a memory component. Inducible defences with similar properties are common in a variety of other types of interactions, for example many aquatic invertebrates produce inducible structural defences against their predators and competitors. Most inducible defences have the following properties: (1) a threshold of activation; (2) an amplification of response with increasing stimulus; (3) a memory component. Specificity, amplification and memory are the basis for defining a defence as 'immune' (Klein, 1982), and these properties are present in both the vertebrate and invertebrate internal defence responses to pathogens. Invertebrates differ in the absence of immunoglobulins and therefore in reduced specificity. Although the reduced specificity of invertebrate internal defence systems is often viewed as proof of their 'primitiveness', the differences in defence systems of vertebrates and invertebrates may be more related to their respective selection regimes than to phylogeny. The syngeneic recognition system of vertebrates functions to recognize small departures from self, such as would arise from neoplasia. Are vertebrates under more intense selection from neoplasia, perhaps due to a greater incidence of hormonal imbalance or hypersensitivity reactions? The invertebrate internal defence systems are all less discriminating than the vertebrate, but there are marked differences in degree of discrimination depending on whether the group is colonial or not. Even the phyla of colonial animals with quite simple body plans, the sponges and cnidarians, have a more discriminating recognition system than the phyla of solitary animals with more complex body plans, such as the molluscs and arthropods. The primary effectors of all invertebrate responses to parasites are encapsulation and phagocytosis, although in some phyla there are specific antibacterial proteins than can also be induced.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors affecting the establishment of the caecal threadworm Trichostrongylus tenuis in red grouse (Lagopus lagopus scoticus)

The resistance of captive reared red grouse to Trichostrongylus tenuis was measured as the proportion of ingested infective 3rd-stage larvae which failed to develop to adult worms. Individual grouse showed wide, repeatable variations in resistance. Seasonal variations may also have occurred. Red grouse acquired little or no effective immunity to reinfection after challenge with a single dose of infective larvae. In trickle-dosed grouse, however, adult worms weakly inhibited the establishment of incoming larvae in an intensity-dependent fashion. Despite this, the proportion of larvae established in trickle infections was similar to that in single-dose challenges.

Nematospiroides dubius: passive transfer of protective immunity to mice with monoclonal antibodies

Nine hybridoma cell lines secreting monoclonal antibodies specific for Nematospiroides dubius were produced by fusion of the mouse myeloma cell line NS-1 to either spleen cells or mesenteric lymph node cells from mice repeatedly infected with N. dubius. Seven of the antibodies were identified as IgM and two as IgG1. Each monoclonal antibody bound to polypeptide epitopes on both infective larvae (L3) and adult worms. However, five antibodies bound preferentially to L3 and three to adult worms. All nine antibodies reacted with high molecular weight protein antigens. Passive protective immunity in Balb/c mice was demonstrated with monoclonal antibodies Nd2 and Nd3 in ascites fluid which stunted both male and female worms and reduced parasite fecundity.

Genetic control of liability to infection with Nematospiroides dubius in mice: direct and correlated responses to selection of mice for faecal parasite egg count

Mice selected as liable (L) and refractory (R) over 10 generations voided significantly more and less Nematospiroides dubius eggs compared with randomly mated (Rd) mice after primary infection with 100 larvae. Strong positive correlation was found between the numbers of N. dubius eggs in mouse faeces and the numbers of adult N. dubius recovered from mice culled from the R, Rd and L colonies. Selection limit based on a faecal e.p.g. was reached in the L mice after 9 generations whereas the faecal N. dubius e.p.g. voided by the R mice continued to decline throughout selection. In contrast, no change in worm numbers was found in L or R mice after 6 generations. The refractory state of the trait, liability to infection with N. dubius, was inherited as a dominant character with a realized heritability value of approximately 0.2. Differential correlated responses from N. dubius infectivity compared with N. dubius fecundity and growth indicated murine genetic control of this trait by 2 major genetic units. In general, there were negative phenotypic and genetic correlations between the faecal N. dubius e.p.g. of mice and murine morphological conformation, but positive correlations between the faecal N. dubius e.p.g. of selected mice and establishment, growth and fecundity of N. dubius populations in these mice. There was little correlation between the faecal N. dubius e.p.g. after primary infection and anti-N. dubius antibody titres and parasite female/male sex-ratio.

Host specificity in mice selected for innate immunity to Nematospiroides dubius: infections with Nippostrongylus brasiliensis, Mesocestoides corti and Salmonella typhimurium

The selection of mice for innate immunity to Nematospiroides dubius was not specific. Mice bred as refractory (R) to N. dubius infection were more refractory to primary infections with Nippostrongylus brasiliensis than other mice bred as liable (L) to infection with N. dubius. R and L, as well as randomly-bred (Rd) and inbred C3H mice were all immune to challenge infection with N. brasiliensis. Previous infections with N. brasiliensis failed to influence the course of N. dubius infections or the status of the selected mice as R, Rd and L to infection with N. dubius. R and L mice were equally susceptible to Mesocestoides corti infection but more resistant than Rd mice. R and L mice died sooner after infection with Salmonella typhimurium than Rd, although R survived longer than L mice.

Heritable true fitness and bright birds: a role for parasites?

Combination of seven surveys of blood parasites in North American passerines reveals weak, highly significant association over species between incidence of chronic blood infections (five genera of protozoa and one nematode) and striking display (three characters: male "brightness," female "brightness," and male song). This result conforms to a model of sexual selection in which (i) coadaptational cycles of host and parasites generate consistently positive offspring-on-parent regression of fitness, and (ii) animals choose mates for genetic disease resistance by scrutiny of characters whose full expression is dependent on health and vigor.

Genetic control of resistance to infection with Nematospiroides dubius in mice: selection of high and low immune responder populations of mice

F1, Quackenbush (Q) x Wild (W) Mus musculus were assayed and selected for their level of immunity to secondary infections with 100 Nematospiroides dubius using faecal parasite egg counts. A significant positive linear correlation was found between the number of parasite eggs voided in the faeces and the number of N. dubius recovered after secondary infections in mice. Selection for immunity was not influenced by the conformation of the mice. High (H) and low (L) responder colonies together with a randomly (Rd) bred colony of mice were established over 5 generations. Adaptive immunity was more efficacious in protecting female than male mice and appeared to be controlled by a few dominant genes; the heritability (h2) of the trait was estimated at 0.56 for the parental stock and 0.55 for the F5 Rd colony and the realized heritability was 0.49. Higher anti-N. dubius antibody titres were detected in H than L mice. H mice harboured fewer and less fecund parasites than Rd colony mice; L mice had more worms with greater fecundities than Rd mice. The levels of primary infections in H and L mice related to the levels developed after secondary infection from F4. This suggested at least partial linkage of genes controlling innate with genes controlling adaptive immunity against N. dubius.