Helminth-host-environment interactions: Looking down from the tip of the iceberg.

In 1978, the theory behind helminth parasites having the potential to regulate the abundance of their host populations was formalized based on the understanding that those helminth macroparasites that reduce survival or fecundity of the infected host population would be among the forces limiting unregulated host population growth. Now, 45 years later, a phenomenal breadth of factors that directly or indirectly affect the host-helminth interaction has emerged. Based largely on publications from the past 5 years, this review explores the host-helminth interaction from three lenses: the perspective of the helminth, the host, and the environment. What biotic and abiotic as well as social and intrinsic host factors affect helminths? What are the negative, and positive, implications for host populations and communities? What are the larger-scale implications of the host-helminth dynamic on the environment, and what evidence do we have that human-induced environmental change will modify this dynamic? The overwhelming message is that context is everything. Our understanding of second-, third-, and fourth-level interactions is extremely limited, and we are far from drawing generalizations about the myriad of microbe-helminth-host interactions.Yet the intricate, co-evolved balance and complexity of these interactions may provide a level of resilience in the face of global environmental change. Hopefully, this albeit limited compilation of recent research will spark new interdisciplinary studies, and application of the One Health approach to all helminth systems will generate new and testable conceptual frameworks that encompass our understanding of the host-helminth-environment triad.

Impact of host sex and group composition on parasite dynamics in experimental populations.

To better understand the spread of disease in nature, it is fundamentally important to have broadly applicable model systems with readily available species which can be replicated and controlled in the laboratory. Here we used an experimental model system of fish hosts and monogenean parasites to determine whether host sex, group size and group composition (single-sex or mixed-sex) influenced host-parasite dynamics at an individual and group level. Parasite populations reached higher densities and persisted longer in groups of fish compared with isolated hosts and reached higher densities on isolated females than on isolated males. However, individual fish within groups had similar burdens to isolated males regardless of sex, indicating that females may benefit more than males by being in a group. Relative condition was positively associated with high parasite loads for isolated males, but not for isolated females or grouped fish. No difference in parasite dynamics between mixed-sex groups and single-sex groups was detected. Overall, these findings suggest that while host sex influences dynamics on isolated fish, individual fish in groups have similar parasite burdens, regardless of sex. We believe our experimental results contribute to a mechanistic understanding of host-parasite dynamics, although we are cautious about directly extrapolating these results to other systems.

Parting ways: parasite release in nature leads to sex-specific evolution of defence.

We evaluated the extent to which males and females evolve along similar or different trajectories in response to the same environmental shift. Specifically, we used replicate experimental introductions in nature to consider how release from a key parasite (Gyrodactylus) generates similar or different defence evolution in male vs. female guppies (Poecilia reticulata). After 4-8 generations of evolution, guppies were collected from the ancestral (parasite still present) and derived (parasite now absent) populations and bred for two generations in the laboratory to control for nongenetic effects. These F2 guppies were then individually infected with Gyrodactylus, and infection dynamics were monitored on each fish. We found that parasite release in nature led to sex-specific evolutionary responses: males did not show much evolution of resistance, whereas females showed the evolution of increased resistance. Given that male guppies in the ancestral population had greater resistance to Gyrodactylus than did females, evolution in the derived populations led to reduction of sexual dimorphism in resistance. We argue that previous selection for high resistance in males constrained (relative to females) further evolution of the trait. We advocate more experiments considering sex-specific evolutionary responses to environmental change.

Typhoid fever acquired in the United States, 1999-2010: epidemiology, microbiology, and use of a space-time scan statistic for outbreak detection.

Although rare, typhoid fever cases acquired in the United States continue to be reported. Detection and investigation of outbreaks in these domestically acquired cases offer opportunities to identify chronic carriers. We searched surveillance and laboratory databases for domestically acquired typhoid fever cases, used a space-time scan statistic to identify clusters, and classified clusters as outbreaks or non-outbreaks. From 1999 to 2010, domestically acquired cases accounted for 18% of 3373 reported typhoid fever cases; their isolates were less often multidrug-resistant (2% vs. 15%) compared to isolates from travel-associated cases. We identified 28 outbreaks and two possible outbreaks within 45 space-time clusters of ⩾2 domestically acquired cases, including three outbreaks involving ⩾2 molecular subtypes. The approach detected seven of the ten outbreaks published in the literature or reported to CDC. Although this approach did not definitively identify any previously unrecognized outbreaks, it showed the potential to detect outbreaks of typhoid fever that may escape detection by routine analysis of surveillance data. Sixteen outbreaks had been linked to a carrier. Every case of typhoid fever acquired in a non-endemic country warrants thorough investigation. Space-time scan statistics, together with shoe-leather epidemiology and molecular subtyping, may improve outbreak detection.

Review of genetic diversity in malaria vectors (Culicidae: Anophelinae).

We review previous studies on the genetic diversity of malaria vectors to highlight the major trends in population structure and demographic history. In doing so, we outline key information about molecular markers, sampling strategies and approaches to investigate the causes of genetic structure in Anopheles mosquitoes. Restricted gene flow due to isolation by distance and physical barriers to dispersal may explain the spatial pattern of current genetic diversity in some Anopheles species. Nonetheless, there is noteworthy disagreement among studies, perhaps due to variation in sampling methodologies, choice of molecular markers, and/or analytical approaches. More refined genealogical methods of population analysis allowing for the inclusion of the temporal component of genetic diversity facilitated the evaluation of the contribution of historical demographic processes to genetic structure. A common pattern of past unstable demography (i.e., historical fluctuation in the effective population size) by several Anopheles species, regardless of methodology (DNA markers), mosquito ecology (anthropophilic vs zoophilic), vector status (primary vs secondary) and geographical distribution, suggests that Pleistocene environmental changes were major drivers of divergence at population and species levels worldwide.

Concurrent nematode infection and pregnancy induce physiological responses that impair linear growth in the murine foetus.

This study examined concurrent stresses of nematode infection and pregnancy using pregnant and non-pregnant CD1 mice infected 3 times with 0, 50 or 100 Heligmosomoides bakeri larvae. Physiological, energetic, immunological and skeletal responses were measured in maternal and foetal compartments. Resting metabolic rate (RMR) was elevated by pregnancy, but not by the trickle infection. Energy demands during pregnancy were met through increased food intake and fat utilization whereas mice lowered their body temperature during infection. Both infection and pregnancy increased visceral organ mass and both altered regional bone area and mineralization. During pregnancy, lumbar mineralization was lower but femur area and mineralization were higher. On the other hand, infection lowered maternal femur bone area and this was associated with higher IFN-gamma in maternal serum of heavily infected pregnant mice. Infection also reduced foetal crown-rump length which was associated with higher amniotic fluid IL-1 beta.

Tissue mineral distributions are differentially modified by dietary protein deficiency and a murine nematode infection.

This study was designed to investigate whether mineral concentrations in the spleen, serum, and liver were modified by challenge infection with a gastrointestinal nematode, by infection dose, or by protein deficiency despite adequate dietary intakes of minerals. BALB/c mice fed protein-sufficient (PS, 24%) or protein-deficient (PD, 3%) diets were infected with 100 L3 of Heligmosomoides bakeri, drug-treated, and then re-infected with either 0, 100, or 200 L3. Protein deficiency and infection, but not dose, independently modified tissue mineral distributions. H. bakeri infection lowered serum iron concentrations in both diet groups. Despite this, PD mice had elevated iron and calcium concentrations and Ca/Zn ratio in the spleen as well as Fe/Zn ratio in liver, but they had reduced calcium, zinc, copper, and sulfur concentrations, and Cu/Zn ratio in the liver. Infection reduced calcium and iron concentrations and the Ca/Zn ratio in the spleen. We suggest that tissue mineral distribution is a consequence of Th2 immune and inflammatory responses induced by infection in PS mice and the switch to predominant Th1 inflammation in PD, nematode-infected mice.

Temporal dynamics of epidermal responses of guppies Poecilia reticulata to a sublethal range of waterborne zinc concentrations.

This study assessed the histological changes in the epidermis of guppies Poecilia reticulata induced by waterborne zinc (Zn). Laboratory-reared P. reticulata fry were maintained individually in separate vessels containing artificial water (8 microg l(-1) Zn) to which 0, 15, 30, 60 or 120 microg l(-1) Zn was added. Their epidermal response to Zn was monitored regularly over 4 weeks. Compared with controls, mucus was rapidly released and mucous cell numbers decreased at all concentrations. Thereafter mucous release, epidermal thickness, numbers and size of mucous cells fluctuated at a rate that varied with Zn concentration, but fluctuations declined after day 18. Results clearly highlight the dynamic nature of the epidermal response to sublethal concentrations of waterborne Zn. In general, low concentrations of Zn induced a rapid response with reduced numbers and size of mucous cells and shift in mucin composition, and a subsequent thickening of the epidermis. Epidermal thickness and mucous cell area fluctuated over time but were normal after a month of exposure to low Zn concentrations. The number of mucous cells, however, remained low. Virtually all mucous cells from fish maintained in 15 and 60 microg l(-1) Zn contained acidic mucins throughout the month, whereas fish maintained at 30 microg l(-1) Zn responded by production of neutral mucins during the first 12 days followed by a mixture of neutral and acidic mucins. At 120 microg l(-1) Zn, the most dramatic effects were the gradual but sustained decrease in numbers and area of mucous cells, and the shift to acidic mucins in these cells. Thus, as concentration of Zn increased, the epidermal responses indicated a disturbed host response (dramatic decline in mucous cell numbers, with mixed composition of mucins), which may have been less effective in preventing Zn uptake across the epithelium.

Species composition and distribution of adult Anopheles (Diptera: Culicidae) in Panama.

Anopheles (Diptera: Culicidae) species composition and distribution were studied using human landing catch data over a 35-yr period in Panama. Mosquitoes were collected from 77 sites during 228 field trips carried out by members of the National Malaria Eradication Service. Fourteen Anopheles species were identified. The highest average human biting rates were recorded from Anopheles (Nyssorhynchus) albimanus (Wiedemann) (9.8 bites/person/night) and Anopheles (Anopheles) punctimacula (Dyar and Knab) (6.2 bites/person/night). These two species were also the most common, present in 99.1 and 74.9%, respectively, of the sites. Anopheles (Nyssorhynchus) aquasalis (Curry) was encountered mostly in the indigenous Kuna Yala Comarca along the eastern Atlantic coast, where malaria case history and average human biting rate (9.3 bites/person/night) suggest a local role in malaria transmission. An. albimanus, An. punctimacula, and Anopheles (Anopheles) vestitipennis (Dyar and Knab) were more abundant during the rainy season (May-December), whereas An. aquasalis was more abundant in the dry season (January-April). Other vector species collected in this study were Anopheles (Kerteszia) neivai (Howard, Dyar, and Knab) and Anopheles (Anopheles) pseudopunctipennis s.l. (Theobald). High diversity of Anopheles species and six confirmed malaria vectors in endemic areas of Panama emphasize the need for more detailed studies to better understand malaria transmission dynamics.

Redescription of Pseudanthobothrium hanseni Baer, 1956 and description of P. purtoni n. sp. (Cestoda: Tetraphyllidea) from different pairs of rajid skate hosts, with comments on the host-specificity of the genus in the Northwest Atlantic.

During a parasitological survey of Leucoraja erinacea, L. ocellata, Malacoraja senta and Amblyraja radiata from Passamaquoddy Bay and waters surrounding the West Isles of the Bay of Fundy, NB, Canada, seven species of cestodes were recovered. Examination of these skates revealed the presence of two distinct species of Pseudanthobothrium Baer, 1956: one was retrieved from M. senta and A. radiata, identified as P. hanseni Baer, 1956 and redescribed herein; the other was retrieved from L. erinacea and L. ocellata and differs from previously described species. The new species is described herein as P. purtoni n. sp. on the basis of the degree of apolysis, the maximum width of the strobila, the length of the cirrus-sac and the number of testes. Additionally, the distinctiveness of both species of Pseudanthobothrium is supported by the characterisation of a 643 base-pair nuclear marker, which includes most of the D2 variable region of the large subunit ribosomal DNA (LSU). The recovery of two different tetraphyllidean species, each from two different host species, challenges the oioxeny (strict host-specificity) of echeneibothriine cestodes and can be explained, at least in part, by the similarities in diet and substrate preference within each host pair.

Mechanisms underlying reduced expulsion of a murine nematode infection during protein deficiency.

Balb/c mice infected with the gastrointestinal nematode Heligmosomoides bakeri were fed protein sufficient (PS, 24%) or deficient (PD, 3%) diets to investigate whether diet, infection or dose of larval challenge (0, 100 or 200 larvae) influenced gut pathophysiology and inflammation. Among the PS mice, worms were more posteriorad in the intestine of mice infected with 200 compared with 100 larvae, suggesting active expulsion in the more heavily infected mice. This was consistent with the positive correlation between worm numbers and fluid leakage in PS mice; similar patterns were not detected in the PD mice. Infection also induced villus atrophy, which was more pronounced in PS than in PD mice. Our cytokine screening array indicated that infection in PD mice elevated a wide range of pro-inflammatory cytokines and chemokines. Whereas serum leptin concentrations were higher in PD mice, monocyte chemotactic protein-5 (MCP-5) in serum increased with increasing larval dose and concentrations were lower in PD than PS mice. We suggest that elevated MCP-5 together with villus atrophy may contribute to the apparent dose-dependent expulsion of H. bakeri from PS mice but that delayed expulsion in PD mice appeared related to a predominant Th1 cytokine profile that may be driven by leptin.

Re-feeding rapidly restores protection against Heligmosomoides bakeri (Nematoda) in protein-deficient mice.

This study determined whether the timing of re-feeding of protein-deficient mice restored functional protection against the gastrointestinal nematode, Heligmosomoides bakeri. Balb/c mice were fed a 3% protein-deficient (PD) diet and then transferred to 24% protein-sufficient (PS) diet either on the day of primary infection, 10 days after the primary infection, on the day of challenge infection, or 7 days after the challenge infection. Control mice were fed either the PD or PS diet. Onset of challenge, but not primary, infection caused short-term body weight loss, anorexia and reduced feed efficiency. Weight gain was delayed in mice when re-feeding commenced on the day of challenge infection; alkaline phosphatase (ALP) was also elevated in these mice on day 28 post-challenge. In contrast, other re-feeding groups attained similar body weights to PS mice within 4 days and had similar ALP at day 28. Serum leptin was higher in PD than PS mice and positively associated with food intake. As expected, worm survival was prolonged in mice fed the PD diet. However, egg production and worm burdens were similar in all re-feeding groups to the PS mice, indicating that protein re-feeding during either the primary or challenge infection rapidly restored normal parasite clearance.

Performance of beef heifers grazing stockpiled fescue as influenced by supplemental whole cottonseed.

The objectives of this study were to determine the composition of stockpiled fescue from December through February over 2 yr and to determine the performance of heifers grazing stockpiled fescue with or without supplemental whole cottonseed. In early December, 36 heifers (initial BW 277 +/- 0.70 kg, yr 1; 266 +/- 2.2 kg, yr 2; and initial BCS of 5.0 +/- 0.04) were assigned randomly to 6 groups. Each group was assigned randomly to a 2.4-ha tall fescue pasture (98% endophyte infected), which had received 76 kg of N/ha on September 1. Group was the experimental unit for all measures. Forage DM available during grazing (to ground level) averaged 3,913 and 5,370 kg/ha in yr 1 and 2, respectively. The pasture was strip-grazed for 83 d, with daily forage allocation. Three groups were fed whole cottonseed (0.90 kg of DM/heifer; 24.4% CP, DM basis) daily at 0.33% of BW, and a small amount of a corn-based concentrate (0.19 kg of DM/heifer) to assure complete cottonseed consumption. Nutritive value of forage (dry basis) was determined each week by sampling each pasture to the 5-cm target grazing height. Forage disappearance was estimated every 2 wk from pre- and postgraze forage mass. Forage CP was 16.8% in yr 1 and 12.6% in yr 2. In vitro true organic matter digestibility (IVTOMD) was 82.0 and 71.9%, and ADF was 25.9 and 30.7% in yr 1 and 2, respectively. Most indicators of forage quality declined slightly through the winter, although they recovered in late winter in yr 1. The proportion of fescue that was green declined (P < 0.05) from December (79% in yr 1 and 64% in yr 2) to February (62% in yr 1 and 52% in yr 2). Green tissue averaged 20.4 and 15.2% CP, 91 and 87% IVTOMD, and 22.1 and 23.3% ADF in yr 1 and 2, respectively. Brown tissue averaged 10.3 and 8.5% CP, 64 and 62% IVTOMD, and 35.7 and 37.4% ADF in yr 1 and 2, respectively. Shrunk ADG (0.46 vs. 0.56 kg/d in yr 1 and 0.23 vs. 46 kg/d in yr 2) and change in BCS (- 0.03 vs. 0.33 in yr 1 and 0.13 vs. 0.5 in yr 2) was greater (P < 0.05) for supplemented heifers. Supplemented heifers had greater serum urea nitrogen in yr 1 (9.5 vs. 10.5 mg/dL; P < 0.07) and yr 2 (7.2 vs. 8.6 mg/dL; P < 0.01). Forage disappearance was similar between supplemented and unsupplemented heifers (3.19 vs. 3.39 kg.heifer(-1).d(-1) in yr 1 and 4.14 vs. 4.17 kg.heifer(-1).d(-1) in yr 2, respectively). Heifers responded to supplementation, but performance was lower than expected based on forage nutrient content.

High transmission rates restore expression of genetically determined susceptibility of mice to nematode infections.

This study investigated why the susceptible or resistance phenotype to the nematode Heligmosomoides polygyrus was lost when susceptible (C57BL/6) and resistant (Balb/c) strains of mice were housed together in indoor arenas with continuous transmission of the parasite larvae present in peat trays (Scott, 1991). First, both strains expressed their normal phenotype when given a controlled challenge while living in arenas, and when experimentally infected with only 5 parasite larvae. To test whether chronic exposure to peat altered the resistance phenotype, mice were given a challenge infection while living on peat. C57BL/6 mice living on peat had higher egg production and higher worm numbers than Balb/c mice, except at 2 months post-challenge. Finally, natural transmission rates were increased in arena experiments through either regular replacement of arena mice with naïve mice or direct introduction of additional larvae. A transient difference in infection levels between strains was detected in response to a modest increase in transmission whereas a 10-fold increase in transmission allowed C57BL/6 mice to exhibit the typical profile of high egg production and elevated worm numbers. These data indicate that C57BL/6 mice are less able to regulate parasite numbers at high transmission rates compared with lower transmission rates.

Microsatellite analysis reveals that female mice are indiscriminate when choosing infected or dominant males in an arena setting.

Considering that both infection and dominance status can be conveyed through urinary odours and both are thought to affect mate choice, the present study assessed the role of infection and male dominance status on female mate choice in arena enclosures. Three male CD-1 mice were simultaneously introduced into each of 4 spatially complex arenas (3.0 x 0.6 x 0.4 m high) for 24 h prior to introduction of 5 females into each arena. During the first mating sequence (i.e. Mating 1), all 3 males were uninfected. Prior to Mating 2, the dominant male in each arena was infected with 200 L3 of Heligmosomoides polygyrus (Nematoda). Prior to Mating 3, the dominant male was drug-treated to remove the parasite. Dominance was assessed by the absence of rump or tail wounds (Freeland, 1981). Females were removed from the arena when visibly pregnant, and returned for subsequent mating 2 weeks following parturition. Paternity was determined by microsatellite analysis of each pup. Multi-male mating (i.e. mating with 2 or all 3 males) was a common strategy among females as littermates were sired by 2 or all 3 males in 64 % of the litters. Contrary to expectation, the dominant male did not sire the majority of offspring in any of the mating sequences, and infection and subsequent drug treatment of the dominant male did not have a significant impact on female mate choice. In addition to methodological differences in paternity determination (i.e. DNA analysis versus behavioural observations and/or phenotypic traits), these findings may be further explained by the spatial complexity of the experimental arenas.

Susceptibility to IVM in a field strain of Haemonchus contortus subjected to four treatments in a closed sheep-goat flock in Kenya.

Susceptibility to IVM (IVM) of "strain A" Haemonchus contortus which had been exposed to IVM four times over a 2-year period was compared to IVM susceptibility of "strain C" H. contortus which had no prior field exposure to IVM, by in vivo and in vitro methods. In vivo, the percentage reduction in faecal egg counts (FEC) and the total worm counts (TWC) were compared between control animals (lambs and kids) and animals treated with low dose IVM (20 microg/kg). In vitro susceptibility to IVM was evaluated by larval migration inhibition (LMI) after the two strains of H. contortus were exposed to different concentrations of IVM. The dose response, measured as the proportion of larvae inhibited from migrating, was used to estimate LD(50). Although differences in response to IVM in the in vivo determinations were not significant, "strain A" H. contortus had a significantly higher LD(50) than "strain C" in the LMI assay. Coincident with the conduct of the in vivo experiment, it was observed that "strain A" H. contortus established and survived better than "strain C" in the control lambs.

Female mice mate preferentially with non-parasitized males.

If parasitic infection is a driving force in female mate choice, then females should preferentially select parasite-free males. The role of infection on female mate choice in mammals was assessed using a 3-chambered apparatus. A female CD-1 mouse was allowed to choose between 2 tethered male mice, 1 uninfected and 1 infected with 200 larvae (L3) of the intestinal nematode Heligmosomoides polygyrus. Both uninfected and infected males were equally receptive to the oestrous females, and females did not differ in the number of visits and time spent exploring the 2 males. Female time preference was not a useful predictor of ultimate mate choice, whereas first mount preference of the female was a reliable indicator. Results indicate that female mice preferentially mated with uninfected males as evidenced by first ejaculation choice, but that male infection status did not significantly affect female reproductive success. Interestingly, litters sired by infected males contained a significantly higher percentage of females suggesting that parasite-induced hormonal changes may alter the sex ratio of the offspring. This study provides the first direct evidence of the impact of parasitic infection on mammalian mate choice.

Standardised monitoring of patients on long-term medication in primary care.

A three-year pilot study was initiated in collaboration with three general practices to develop a standardised monitoting (SM) system to ensure that chronically sick patients on long-term medication, such as thyroxine, diuretics angiotensin convening enzyme (ACE) inhibitors statins and antirheumatics were reliabily monitored following fixed protocols. A high standard of care was achieved, which included identifying and following up patients with borderline or unacceptable results and persistent non-attendees. In addition, the scheme guaranteed that a current set of results was provided in time for clinical review.

Dietary pectin, but not cellulose, influences Heligmosomoides polygyrus (Nematoda) reproduction and intestinal morphology in the mouse.

Dietary texture has been reported to influence parasite establishment and survival, but to what degree this relationship is modified by either the type or quantity of dietary fibre is unknown. Using a 2 x 4 factorial design, we explored the relationship between fibre type (soluble pectin vs insoluble = cellulose) and fibre quantity (0, 5, 10 and 20% by dry weight) on parasitic outcomes in BALB/c mice infected with 100 Heligmosomoides polygyrus (Nematoda) larvae. Pectin, but not cellulose, exerted a significant effect on parasite egg production. Following in vitro culture of female worms, increasing levels of dietary pectin were associated with increasing release of eggs. Yet this pattern was not observed in vivo, where per capita egg production peaked at the 10% pectin concentration, but was very low in mice fed 20% pectin. Parasite establishment was elevated in mice fed 20% pectin, but was unaffected by cellulose concentration. Neither type nor quantity of fibre affected H. polygyrus survival or spatial distribution along the gastrointestinal tract. To what degree differences in parasite establishment and reproduction could be attributed to the marked effects of pectin on gut morphology (increased intestinal length, villus length, mucosa thickness and villus/crypt ratio) requires further exploration. Our data indicate that cellulose is preferable to pectin as the source of fibre for experimental diets as cellulose did not affect H. polygyrus establishment, reproduction or survival during a 4-week primary infection.