Population dynamics of Trichostrongylus colubriformis and Ostertagia circumcincta in single and concurrent infections

Influence of Massive and Long Distance Migration on Parasite Epidemiology: Lessons from the Great Wildebeest Migration

Very little is known about the influence of massive and long distance migration on parasite epidemiology. Migration can simultaneously minimize exposure to common parasites in their habitats and increase exposure to novel pathogens from new environments and habitats encountered during migration, while physiological stress during long distance movement can lead to immune suppression, which makes migrants vulnerable to parasites. In this paper, we investigated the diversity, prevalence, parasite load, co-infection patterns and predilection sites of adult gastrointestinal helminths in 130 migrating wildebeests and tested for their relation with animal age, sex and migration time (which also could indicate different migration routes), and compared them with the non-migratory wildebeest. Surprisingly, only four parasite species were found, Oesophagostomum columbianum, Haemonchus placei, Calicophoron raja and Moniezia expansa, which were lower than in non-migratory wildebeest reported in the literature. These parasites were generalists, infecting livestock, and suggests that wildebeest and livestock, because of their interaction during migration, have a cross-infection risk. There was a negative relation between parasites diversity, prevalence and intensity of infection, and host age, which suggests that wildebeests acquire protective immunity against these parasites as they get older. Prevalence and intensity of infection were higher among wildebeest crossing the Mara Bridge (early migrants) compared to those crossing the Serena (late migrants), which suggests that early migrants (or migrants originating from different areas) have varying infection intensities. The prevalence and intensity of infection were higher in males compared to females and may be due to ecological, behavioural, or physiological differences between males and females. Our findings compared to those of previous studies suggest that migration may provide a mechanism to minimize exposure of hosts to common parasites through migratory escape, but this result awaits examination of helminths epidemiology of non-migratory wildebeests from areas of migrant origins. The potential parasitic cross-infection between wildebeests and livestock is a real risk to be taken into account in the management of wildebeest migration corridors.

A multi-species model to assess the effect of refugia on worm control and anthelmintic resistance in sheep grazing systems

OBJECTIVE

Develop a computer simulation model that uses daily meteorological data and farm management practices to predict populations of Trichostrongylus colubriformis, Haemonchus contortus and Teladorsagia (Ostertagia) circumcincta and the evolution of anthelmintic resistance within a sheep flock. Use the model to explore if increased refugia, provided by leaving some adult sheep untreated, would delay development of anthelmintic resistance without compromising nematode control.

PROCEDURES

Compare model predictions with field observations from a breeding flock in Armidale, NSW. Simulate the impact of leaving 1-10% of adult sheep untreated in diverse sheep-grazing systems.

RESULTS

Predicted populations of Tr. colubriformis and T. circumcincta were less than those observed in the field, attributed to nutritional stress experienced by the sheep during drought and not accounted for by the model. Observed variation in faecal egg counts explained by the model (R(2) ) for these species was 40-50%. The H. contortus populations and R(2) were both low. Leaving some sheep untreated worked best in situations where animals were already grazing or were moved onto pastures with low populations of infective larvae. In those cases, anthelmintic resistance was delayed and nematode control was maintained when 1-4% of adult stock remained untreated.

CONCLUSIONS

In general, the model predicted that leaving more than 4% of adults untreated did not sufficiently delay the development of anthelmintic resistance to justify the increased production risk from such a strategy. The choice of a drug rotation strategy had an equal or larger effect on nematode control, and selection for resistance, than leaving 1-10% of adults untreated.

A model to account for the consequences of host nutrition on the outcome of gastrointestinal parasitism in sheep: model evaluation

This paper describes sensitivity analyses and expectations obtained from a mathematical model developed to account for the effects of host nutrition on the consequences of gastrointestinal parasitism in sheep. The scenarios explored included different levels of parasitic challenge at different planes of nutrition, for hosts differing only in their characteristics for growth. The model was able to predict the consequences of host nutrition on the outcome of parasitism, in terms of worm burden, number of eggs excreted per gram faeces and animal performance. The model outputs predict that conclusions on the ability of hosts of different characteristics for growth to cope with parasitism (i.e. resistance) depend on the plane of nutrition. Furthermore, differences in the growth rate of sheep, on their own, are not sufficient to account for differences in the observed resistance of animals. The model forms the basis for evaluating the consequences of differing management strategies and environments, such as breeding for certain traits associated with resistance and nutritional strategies, on the consequences of gastrointestinal parasitism on sheep.

Haemonchus contortus egg excretion and female length reduction in sheep previously infected with Oestrus ovis (Diptera: Oestridae) larvae

Mixed parasitic infection of animals is a common phenomenon in nature. The existence of one species often positively or negatively influences the survival of the other. Our experimental study was started with the objectives to demonstrate the interaction of Haemonchus contortus and Oestrus ovis in relation to cellular and humoral immune responses in sheep. Twenty-two sheep of Tarasconnais breed (France) were divided into four groups (O, OH, H and C) of five or six animals. Group O and OH received 5 weekly consecutive inoculations with O. ovis L1 larvae (total = 82 L1) in the first phase of the experiment between days 0 and 28. On the second phase, groups OH and H received 5000 L3 of H. contortus on day 48 while group C served as our control throughout the experimental period. Parasitological, haematological, serological and histopathological examinations were made according to standard procedures and all animals were slaughtered at day 95. There was no significant variation in the number and degree of development of O. ovis larvae between the two infected groups. Furthermore, in tissues examined in the upper respiratory tract (nasal septum, turbinate, ethmoide and sinus), group O and OH has responded similarly on the basis of cellular inflammatory responses (blood and tissue eosinophils, mast cells and globule leucocytes (GL)) and serum antibody responses against the nasal bots. This may indicate that the presence of H. contortus in the abomasa of group OH had no marked influence over the development of O. ovis larvae in the upper respiratory tract. On the other hand, we have observed a significantly lower H. contortus female worm length, fecal egg count (FEC) and in utero egg count in animals harbouring the nasal bot (OH) than in the mono-infected group (H). This was significantly associated with higher blood eosinophilia, higher packed cell volume (PCV) and increased number of tissue eosinophils and globule leucocytes. We conclude that, the establishment of O. ovis larvae in the upper respiratory tract has initiated higher inflammatory cellular activity in group OH there by influencing the development and fecundity of H. contortus in the abomasum.

Local immune responses in sensitized sheep following challenge infection with Teladorsagia circumcincta

Sheep were sensitized by weekly infections with Teladorsagia circumcincta over a 9-week period. After a 12-week rest, sheep were divided into four groups and killed without challenge or 3, 5 and 10 days post challenge (DPC) with 50000 L3. Recovery of challenge larvae from abomasal scrapings was highest at 3 DPC while no parasites were recovered by 10 DPC. Abomasal lymph nodes (ALN) of challenged sheep were significantly larger at 5 DPC, coinciding with an increase in the proportion of CD4 T cells and a decrease in CD21+ cells, probably reflecting the loss of CD21 from terminally differentiated antibody secreting cells. A significant increase was observed in gammadelta-TCR+ cells at 3 DPC in the ALN, while their number slightly decreased in the abomasal tissues throughout the challenge period. The number of tissue eosinophils was dramatically increased after challenge compared with the unchallenged controls, with a peak at 3 DPC, coinciding with the peak in larval recovery. CD4+ cells significantly increased in the abomasal tissues at 5 DPC, while no changes in globule leucocytes were observed until 10 DPC. Antibody-secreting cell probes (ASC-probes) generated from the ALN showed highest reactivity against larval antigens at 5 DPC. This reactivity was predominantly directed against regions between 90 and 100 kDa and 30-35 kDa in the L3 preparation and lower molecular weight antigens in the L4. No reactivity was observed against the adult extract. The 30-35 kDa antigen seemed to exist as a high molecular weight complex in L3 homogenate and was not susceptible to protease K treatment, suggesting it may be non-protein in nature.

The immune response of sheep to larval challenge with Ostertagia circumcincta and O. ostertagi

One of three groups of sheep was challenged twice-weekly with infective-stage larvae (L3) of the sheep parasite O. circumcincta, another with the cattle parasite O. ostertagi while the third received no larval challenge. Positive faecal egg counts (FEC) and a rise in plasma pepsinogen levels were observed only in those animals given O. circumcincta. Anti-O. circumcincta L3 IgG titres were rapidly elevated during parasite challenge with either O. circumcincta or O. ostertagi. Throughout the experiment, no rise in anti-adult IgG titres or eosinophil numbers was observed in peripheral blood in any group. On evidence of self-cure of the trickle-infection, determined by a reduction in FEC, all groups were drenched and challenged with 15,000 O. circumcincta L3. No effect of previous challenge on parasite establishment or FEC was observed, although egg viability was significantly reduced in both groups given prior challenge. Significant differences in adult female worm length were observed between groups. Those recovered from animals previously challenged with O. circumcincta were shorter than from those given O. ostertagi which were in turn shorter than those from previously unchallenged animals. In utero egg counts were significantly lower in worms from animals previously challenged with O. circumcincta than in those from unchallenged control animals. The results indicate that a level of immunity to O. circumcincta can be conferred by exposure to O. ostertagi.

Biological parameters of Trichostrongylus colubriformis in Meriones unguiculatus

In a series of three experiments, 64 jirds, Meriones unguiculatus, were infected with 700 infective larvae of Trichostrongylus colubriformis for the study of several biological parameters of this laboratory host-parasite model. In jirds the third stage larvae of T. colubriformis were shown to reach the fourth larval stage by the Day 6 post-infection (PI). By Day 10 PI, all the worms harvested had reached the immature adult stage. All immature adult stages of T. colubriformis developed to sexually mature adult stage by Day 12 PI when the female worms showed developing eggs in their uteri. Developed eggs were observed in the uteri of females on Days 13 and 14 PI. The first eggs of T. colubriformis appeared in the faeces of jirds on Day 13 PI. The peaks of egg production were recorded between Day 21 and Day 31 PI. Immunosuppression of jirds infected with 700 L3, by administration of dexamethasone from Day 57 to Day 94 PI led to increased faecal egg count when compared to untreated controls. All the worms were located in the small intestine. The jirds of dexamethasone treated group harboured higher number of adult worms than those of the untreated group. The number of adult worms was significantly higher in the first part of the small intestine than in the three other similar parts of the small intestine. The coefficient of correlation between the faecal egg count and worm number on the day of necropsy of jirds ranged between r = 0.58 and r = 0.89. Patent infections in jirds were maintained till the end of experiment on Day 100 PI, indicating that in this host and unlike other laboratory hosts, T. colubriformis is responsible for long lasting infections similar to what happens in domestic ruminants. The results of the present study suggest that the jird is a suitable laboratory model to study various aspects of this host-parasite relationship.

The influence of a Cooperia oncophora priming on a concurrent challenge with Ostertagia ostertagi and C. oncophora in calves

The development of immunity to Ostertagia ostertagi and Cooperia oncophora and interactions between these species were investigated in experimentally infected calves. Parasitological, serological and histological parameters were used for assessing immune responses. No conclusive evidence of an effect of C. oncophora on the course of an O. ostertagi infection in calves could be shown. Following a challenge with C. oncophora and O. ostertagi of C. oncophora primed calves, no significant reductions in establishment rate, faecal egg counts, worm length or the percentage of early fourth stage larvae could be demonstrated. Results also confirmed earlier work showing the very different degrees of immunity conferred following immunisation with either C. oncophora or O. ostertagi. While a protective immunity was generated in the case of C. oncophora, continuous infection of calves with 420000 L3 of O. ostertagi during almost 5 months induced immune reactions which affected growth and fecundity of the worms but not the establishment rate.

The dynamics of nematode infections of farmed ruminants

In this paper the dynamics and control of nematode parasites of farmed ruminants are discussed via a qualitative analysis of a differential equation model. To achieve this a quantity, 'the basic reproduction quotient' (Q0), whose definition coincides with previous definitions of R0 for macroparasites, but extends to models with periodic time-varying transition rates between parasite stages or management interventions, is introduced. This quantity has the usual threshold property: if Q0 is less than one the parasite population cannot maintain itself in the host population, and in the long term becomes extinct; but if Q0 is greater than one the parasite can invade the host population. An alternative quantity, R(E), that is often easier to calculate is also introduced, and shown to have the same threshold property. The use of these two quantities in analysing models for the dynamics of nematodes in complex situations is then demonstrated, with reference to the dynamics of mixed parasite species in one host; the effects of breeding host animals for resistance to parasitism; and the development of parasite strains that are resistant to chemotherapy. Five examples are discussed using parameters for the dynamics of nematode infections in sheep, and some statements on control policies are derived.

Interaction between Ostertagia circumcincta and Haemonchus contortus infection in young lambs

Twenty-one-week-old, worm-free, pen-reared lambs were infected with either 6000 O. circumcincta L3 per week, or 3000 H. contortus L3 per week, or both (9000 L3 per week). Egg counts were monitored throughout the experiment, and worm burdens and larval establishment rates of both worm species were estimated after 4, 7, 10 and 13 weeks of infection. After 10-13 weeks of infection with H. contortus only, establishment of O. circumcincta was lower than in previously uninfected controls, demonstrating that a high level of immunity to H. contortus affords some cross-protection against O. circumcincta. Total H. contortus worm burdens and egg counts (about 2000 worms and 3000 e.p.g., respectively) in sheep infected with both worm species were less than half those observed in sheep infected with H. contortus alone (about 5000 worms and 10,000 e.p.g., respectively). Cross-protection between the two species was observed, but was probably less important than the reduction in H. contortus establishment that was caused by O. circumcincta disrupting abomasal physiology.

Interactions between Ostertagia ostertagi and Cooperia oncophora in calves

Development of immunity to Ostertagia ostertagi and Cooperia oncophora and interactions between both species in primed calves were investigated after homologous, heterologous and concurrent challenge infections. Worm counts, faecal egg output, IgG1 and IgG2 antibodies and the presence of globule leucocytes were used to evaluate the possible interactions. Results show that immunity build-up against O. ostertagi is slow in comparison with C. oncophora. The presence of early fourth-stage larvae and globule leukocytes in the O. ostertagi primed groups was significantly different to that of a previously uninfected control group. Ostertagia ostertagi and C. oncophora IgG1 antibodies were high in the previously exposed groups compared with uninfected controls and C. oncophora antibodies cross-reacted strongly with O. ostertagi antigens. There was no conclusive evidence for an interaction between C. oncophora and O. ostertagi. Globule leukocytes, IgG1 antibodies and early fourth-stage larvae seem to be related to development of immunity to O. ostertagi.

Persistence of acquired immunity to Trichostrongylus colubriformis in sheep after termination of infection

Lambs were infected with 6000 Trichostrongylus colubriformis L3 per week for 18, 12 or 6 weeks, beginning at ages 14, 20 and 26 weeks, respectively. At the end of the primary infection subgroups had geometric mean adult worm burdens of 5000, 15,000 and 18,000, respectively. In the remaining sheep the worm population was removed with anthelmintic, and sheep had no further larval intake until challenged with T. colubriformis 1, 6, 12, 18 or 24 weeks later. In the groups given 18 or 12 weeks primary infection, establishment of challenge doses was low (less than 25% of establishment in helminth naive controls) in most animals at all challenge times. However, for the groups given 6 weeks primary infection, establishment was low only at the first two challenge times. Thereafter it had similar mean to control groups, but much greater variance. Other subgroups were challenged with T. colubriformis, Ostertagia circumcincta and Haemonchus contortus 1 week after worm removal. In these animals T. colubriformis establishment was not different to animals challenged at the same time with T. colubriformis alone, however immunity to T. colubriformis afforded little protection against the other species. The results of this experiment were incorporated into a simulation model of the population dynamics of T. colubriformis.

Rejection of heterologous nematodes by sheep immunized with larval or adult Trichostrongylus colubriformis

Sheep immunized by truncated larval infections or by the adoptive transfer of adult Trichostrongylus colubriformis were subsequently challenged with single infections of T. colubriformis, Nematodirus spathiger, Haemonchus contortus or Ostertagia circumcincta or combinations of the parasites. Sheep vaccinated with larval infections were > 90% protected by 4 days after challenge (DAC) against T. colubriformis L3 given in a single or combined infection, whereas no significant protection was exhibited against a single-species infection with the unrelated nematodes N. spathiger or O. circumcincta. Sheep challenged with T. colubriformis together with N. spathiger or O. circumcincta were equally protected against both intestinal nematodes, but O. circumcincta was not affected. Sheep immunized with adult T. colubriformis and challenged with a combined infection of T. colubriformis, N. spathiger and H. contortus expelled around 40 and 80% of the intestinal parasites by 4 and 11 DAC, respectively, but showed no protection against the abomasal parasite, H. contortus. The results confirm the previous findings on 'self-cure' and the non-specific rejection of unrelated parasites living in the same or downstream niches in the gut when the nematode used to induce immunity is included in the challenge infection. The results also indicate that even though L3 antigens effectively elicited the non-specific rejection, antigens produced by L4 and later stages could also induce rejection of unrelated worms in the second week after infection.

The survival of Ostertagia circumcincta and Trichostrongylus colubriformis in faecal culture as a source of bias in apportioning egg counts to worm species

When cultured alone or concurrently with Trichostrongylus colubriformis in sheep faeces, Ostertagia circumcincta produced fewer infective larvae per 100 eggs than did T. colubriformis. Averaged over five trials 60% of T. colubriformis eggs were recovered as infective larvae while for O. circumcincta the figure was only 39%. This result was observed for two strains of O. circumcincta and was independent of when larvae were harvested from culture (days 6-10 at 25 degrees C). The mortalities of both species occurred at the first and second larval stages. These observations are of concern when using larval differentiation from faecal culture to make quantitative estimates of worm egg numbers for each species present. Species such as T. colubriformis which have a low mortality during culture are likely to have their egg numbers overestimated when cultured with a species, like O. circumcincta, that suffers high mortality in culture.