Evaluating host-parasite co-adaptation relationships involving Angiostrongylus costaricensis.

Angiostrongylus costaricensis is a parasite that infects rodents, including the wild cotton rat Sigmodon hispidus and pygmy rice rats Oligoryzomys spp., among others. However, urban Rattus norvegicus and Mus musculus have not been identified as important hosts of A. costaricensis. In this study, Swiss mice (SW), Wistar R. norvegicus (RN), wild Oligoryzomys nigripes (ON) and a local strain of M. musculus (RGS) were experimentally infected with A. costaricensis. Survival, elimination of L1 (total sum per group, A0), and the number of adult worms recovered divided by the dose of each L3 inoculum (yield ratio, YR) were examined for each group after a 40-day post-infection period. The survival rates, A0 and YR values were: 27%, 207,589 and 0.42 for the SW group; 81%, 8691 and 0.01 for the RN group; and 63.6%, 26,560 and 0.16 for the RGS group, respectively, in each case. The survival rate for the ON group was 100% and the A0 value was 847,050. A YR was not calculated for the ON group since the ON group was maintained up to 565 days post-infection (pi) to examine long-term mortality. At 500 days pi (16 months), 50% of the ON group had died, while one animal (10%) survived 595 days pi (20 months). Taken together, these data indicate that A. costaricensis has undergone a greater degree of adaptation to the wild rodent, O. nigripes, than to R. norvegicus or a local M. musculus strain. In addition, titre curve (A0) modelling of adaptation status proved to be useful in evaluating A. costaricensis-rodent interactions.

Short- and long-term association between individual levels of milk antibody against Ostertagia ostertagi and first-lactation heifer's production performances.

It is agreed that exposure of adult dairy cattle to helminths on pasture can negatively affect production performances as milking herd. Young animals, especially replacement heifers, represent the future of a dairy farm and are among the most vulnerable to helminth infections in a dairy herd. For this reason, dairy farmers tend to frequently treat heifers against helminths, although the impact of helminths on heifers' production performances is still poorly understood. Using different epidemiological and serological tools, this study examines the relationship between heifer exposure to helminths on pasture and production performances over time. During a one-year period, 1,454 individual milk samples were collected from first-lactation heifers in England and tested for Ostertagia ostertagi (O. ostertagi) antibodies. After controlling for other confounders, increasing milk antibody levels against O. ostertagi were significantly associated with decreased milk yield at sampling but not at day 305 of heifer lactation. We did not observe any relationship between milk antibody levels against O. ostertagi in heifers and yields in fat and protein. However, heifers with a high level of milk antibodies against O. ostertagi were more likely to produce dead calf at first calving and present a delay in second calving. Moreover, these heifers had significantly higher levels of milk antibodies against Mycobacterium paratuberculosis (M. paratuberculosis) during their first lactation and were more likely to die before the end of the study. We argue that epidemiological approaches can be useful but must be complemented by other methodologies to better understand the impact of helminth infections in dairy heifers. In order to address the complex dynamics of helminth infections in dairy cattle production we require more comprehensive approaches that include triangulation between data sources and interdisciplinary studies.

Mass Mortality Attributed to Acanthocephaliasis at a Gull-billed Tern (Gelochelidon nilotica) Colony in Coastal California, USA.

From 12 May 2013 to 29 May 2013, the Gull-billed Tern (Gelochelidon nilotica) colony at the San Diego Bay National Wildlife Refuge, California, US, experienced a mass die-off of at least 92 adults, representing 71-92% of the breeding population on the US west coast. Cause of death was determined to be peritonitis due to perforations of the intestine by a large quantity of acanthocephala (Profilicollis [=Polymorphus] altmani). This is a unique report of P. altmani infecting G. nilotica, and a report of a great impact to a tern population in southern California. Mole crabs (Emerita analoga), the intermediate host for P. altmani and a major component of the Gull-billed Tern diet in San Diego, were found in the stomachs of necropsied terns along with cystacanths, and are the presumed source of the parasite infection. The tern's dietary reliance upon mole crabs likely amplified parasite transmission and infection. We suggest additional research to determine factors that influence parasite infection of intermediate and definitive hosts, particularly mole crabs, given that they are a vital resource for migrating birds within the coastal zone.

Helminth infection is associated with hen mortality in Danish organic egg production.

The aim of this study was to investigate whether two highly prevalent helminth infections (Ascaridia galli and Heterakis species) are associated with an increased mortality rate for hens at the peak of lay. An observational event study with 11 farms was conducted between 2012 and 2013, with weekly mortality recordings and grouping of the farms into low-infected with A galli and Heterakis species (0-200 epg of faeces) or high-infected (over 200 epg of faeces). Survival analysis was performed using a discrete time proportional hazards model. The difference between the hazard functions for low-infected farms and high-infected farms in either summer (August to September) or winter (January to March) were analysed. No statistically significant associations were found between the mortality rate in winter and summer in low-infected farms. However, the mortality rate was doubled for hens from high-infected farms observed in the summer season compared with hens from low-infected farms (winter and summer), whereas high-infected farms observed in the winter did not have a significant association between mortality rate and season compared with low-infected farms (summer and winter). The results suggest that the mortality in organic egg production may be reduced by measures to control A galli and Heterakis species infections.

Apparent mortality as a result of an elevated parasite infection in Presbytis rubicunda.

We provide the first parasite assessment of wild red langurs (Presbytis rubicunda). Seven helminth taxa and 4 protozoa were recovered from faecal samples. We report the details of the death of an adult female red langur with potential association of an elevated parasite infection. There is a paucity of published literature on red langurs and nothing is known of the factors affecting interspecific parasite transmission and disease spread between other primate species. Parasites may be substantial determinants of host health and may present a significant influence on the survival and reproduction of individuals and the subsequent fecundity of populations. Further research into the connection of parasite burden with individual deaths and potential population declines is recommended for monitoring population health.

Risk factors for Merino ewe mortality on the Northern Tablelands of New South Wales, Australia.

OBJECTIVE: To measure the association of worm egg count (WEC), body weight (BW) and fat score (FS) with mortality in Merino ewes. METHODS: On six farms, Merino ewes (n = 1440) were yarded at approximately 3-month intervals for 2 years for a range of experimental measurements. Mortality was defined by failure to present at two consecutive measurement days and at next shearing. Survival analysis was conducted with phenotypic traits measured both as continuous variables and categorised appropriately for industry guidelines. RESULTS: Overall mortality was 10%, with most deaths occurring in autumn followed by winter. Age class (2006 drop vs mature-age ewes) did not affect mortality. High WEC, low BW and low FS significantly increased the risk of mortality. WEC > 1200 epg significantly increased risk of mortality in summer (risk ratio 3.76) and autumn (risk ratio 3.61) compared with the reference group. Risk of mortality increased 1.1-fold for each kilogram unit decrease in BW and 7-fold for each unit decrease in FS. CONCLUSIONS: Mortality of Merino ewes can be significantly reduced by management methods that reduce WEC, maintain BW and increase the FS. For Merino ewes on the Northern Tablelands of NSW, improved animal welfare and economic benefits may be achieved by maintaining sheep with WEC < 1200 epg, BW > 35 kg and FS > 2.5.

Anthelmintic and relaxant activities of Verbascum Thapsus Mullein.

BACKGROUND: Verbascum thapsus is used in tribal medicine as an antispasmodic, anti-tubercular agent and wormicide. In this study, we investigated the antispasmodic and anthelmintic activities of crude aqueous methanolic extract of the plant. METHODS: V. thapsus extracts were tested against roundworms (Ascaridia galli) and tapeworms (Raillietina spiralis). Each species of worm was placed into a negative control group, an albendazole treatment group, or a V. thapsus treatment group, and the time taken for paralysis and death was determined. In addition, relaxation activity tests were performed on sections of rabbit's jejunum. Plant extracts were tested on KCl-induced contractions and the relaxation activities were quantified against atropine. V. thapsus calcium chloride curves were constructed to investigate the mode of action of the plant extracts. RESULTS: We detected flavonoids, saponins, tannins, terpenoids, glycosides, carbohydrates, proteins, fats and fixed oils in V. thapsus. For both species of worm, paralysis occurred fastest at the highest concentration of extract. The relative index values for paralysis in A. galli were 4.58, 3.41 and 2.08, at concentrations of 10, 20 and 40 mg/ml of plant extract, respectively. The relative index for death in A. galli suggested that V. thapsus extract is wormicidal at high concentration. Similarly, the relative indexes for paralysis and death in R. spiralis suggested that the extract is a more potent wormicidal agent than albendazole. The mean EC(50) relaxation activity values for spontaneous and KCl induced contractions were 7.5 ± 1.4 mg/ml (6.57-8.01, n = 6) and 7.9 ± 0.41 mg/ml (7.44-8.46, n = 6), respectively. The relaxation activity of the extract was 11.42 ± 2, 17.0 ± 3, 28.5 ± 4, and 128.0 ± 7% of the maximum observed for atropine at corresponding concentrations. The calcium chloride curves showed that V. thapsus extracts (3 mg/ml), had a mean EC(50) (log molar [calcium]) value of -1.9 ± 0.06 (-1.87 - -1.98, n = 6) vs. control EC(50) = -2.5 ± 0.12 (-2.37 - -2.56, n = 6), whereas the verapamil (0.1 µM) EC(50) was -1.7 ± 0.1 (-1.6 - -1.8, n = 6) vs. control EC(50) = -2.4 ± 0.09 (-2.3 - -2.47, n = 5). CONCLUSIONS: Our results suggest that V. thapsus, which is currently used by some tribes in the Malakand region of Pakistan, has anthelmintic and antispasmodic value.

The effect of helminth co-infection on malaria in mice: a meta-analysis.

The question of how helminths may alter the course of concurrent malaria infection has attracted much interest in recent years. In particular, it has been suggested that by creating an anti-inflammatory immune environment, helminth co-infection may dampen both protective and immunopathological responses to malaria parasites, thus altering malaria infection dynamics and disease severity. Both synergistic and antagonistic interactions are reported in the literature, and the causes of variation among studies are not well understood. Here, meta-analysis of 42 mouse co-infection experiments was used to address how helminths influence malaria parasite replication and host mortality, and explore the factors explaining variation in findings. Most notably, this analysis revealed contrasting effects of helminth co-infection in lethal and resolving malaria models. Whilst co-infection exacerbated mortality and increased peak parasitaemia in ordinarily resolving malaria infections (Plasmodium chabaudi and Plasmodium yoelii), effects among lethal malaria infections (Plasmodium berghei) tended to be in the opposite direction with no change in parasitaemia. In the subset of experiments on cerebral malaria models (P. berghei ANKA strain in a susceptible host), helminth co-infection significantly delayed death. These findings are consistent with the hypothesis that depending on the existing balance of pro- and anti-inflammatory responses mounted against malaria parasites in a given host, immune responses elicited by helminth co-infection may either promote or inhibit malarial disease. However, despite such broad patterns, a prominent feature of this dataset was great heterogeneity in effects across studies. A key future challenge therefore lies in explaining the biological causes of this variation, including a more thorough exploration of non-immunological mechanisms of helminth-malaria interaction.

Causes of calf mortality in peri-urban area of Nairobi, Kenya.

The study reported data from 507 post-mortem records in the Department of Pathology, Microbiology and Parasitology, Faculty of Veterinary Medicine, University of Nairobi, Kenya. The records were from carcasses obtained from the peri-urban area of Nairobi during a 20-year period between 1990 and 2009. Approximately 80% (393/507) of the calf carcasses had their diagnosis made through post-mortem examination, while the rest (114/507) were inconclusive. Just less than half (48.3%) of the calf carcasses presented had their age specified by the owners compared to 51.7% whose age was not specified. For calf carcasses whose age was specified by the owners, those indicated as more than 3 months were one-and-a-half times as many as those below 3 months old. The proportion of female carcasses (53.8%, 273/507) presented for post-mortem were slightly higher than the male carcasses (46.2%, 234/507). Diseases or conditions of the respiratory system were the most common 17.7% (97/507) while gastrointestinal tract (GIT) was second and affected 16.1% (88/507) of the cases. Another small number, 3.3% (18/507), died from bloat giving the total cases associated with GIT as 19.4% (106/507). Severe calf malnutrition and septicaemia were the third most reported causes of calf mortality in similar proportions at 14.3% (78/507) and 14.4% (79/507), respectively. Other minor causes of calf mortality were tick-borne diseases 8.6% (47/507), helminthiasis and poisoning, 2.9% (16/507) and 1.8% (10/507), respectively.

All hosts are not equal: explaining differential patterns of malformations in an amphibian community.

1. Within a community, different host species often exhibit broad variation in sensitivity to infection and disease. Because such differences can influence the strength and outcome of community interactions, it is essential to understand differential disease patterns and identify the mechanisms responsible. 2. In North American wetlands, amphibian species often exhibit extraordinary differences in the frequency of limb malformations induced by the digenetic trematode, Ribeiroia ondatrae. By coupling field studies with parasite exposure experiments, we evaluated whether such patterns were due to differences in (i) parasite encounter rate, (ii) infection establishment, or (iii) parasite persistence within hosts. 3. Field results underscored the broad variation in malformations and infection between host species; while nearly 60% (n = 618) of emerging American toads exhibited severe limb deformities such as bony triangles, skin webbings and missing limbs, fewer than 4% (n = 251) of Eastern gray treefrogs from the same pond were abnormal. Despite similarities in the phenology and larval development period of these species, they differed sharply in Ribeiroia infection. On average, toads supported 75x more metacercariae than did metamorphic treefrogs. 4. Experimental exposures of larval toads and treefrogs to a realistic range of Ribeiroia cercariae revealed strong differences in the sensitivity of these species to infection; exposed toads suffered elevated mortality (up to 95%), delayed metamorphosis, and severe limb malformations consistent with field observations. Treefrogs, in contrast, exhibited limited mortality and no malformations, regardless of exposure level. Ribeiroia cercariae were substantially less successful in locating and infecting Hyla versicolor larvae. 5. Our results indicate that the observed differences in infection and malformations owe to a lower ability of Ribeiroia cercariae to both find and establish within larval treefrogs, possibly stemming from a heightened immune response to infection. Because Ribeiroia is a highly pathogenic parasite with negative effects on larval and metamorphic amphibian survival, variation in infection resistance among species could have important implications for understanding patterns of species co-occurrence, competition, and community diversity.

Intestinal Acanthocephaladiosis in Olrog's gulls (Larus atlanticus): Profilicollis chasmagnathi as possible cause of death.

In 2003 and 2005, mortality events were observed among hatch-year Olrog's gulls (Larus atlanticus) in a breeding colony in the Bahía Blanca estuary, Argentina. Freshly dead chicks were collected for parasitologic and parasite-associated pathologic studies. Profilicollis chasmagnathi was found at various intensities in all of the birds examined (n=28). On gross and histopathologic examinations, severe lesions ranging from intestinal obstruction to complete perforation were present in small and large intestines of the birds. Larval forms of P. chasmagnathi were being found in prey items of L. atlanticus in the study area, which suggests that diet may play a central role in the epidemiology of disease and mortality events in this species.

Parasite community and mortality of overwintering young-of-the-year roach (Rutilus rutilus).

The composition and development of the parasite community of overwintering young-of-the-year (YOY) roach (Rutilus rutilus) was studied in outdoor artificial ponds. Cumulative mortality of the roach from November until May was about 20%. The prevalences of the epizoic ciliates Trichodina sp. and Apiosoma sp., and of the monogeneans Dactylogyrus suecicus, Dactylogyrus nanus, and Gyrodactylus sp. in general tended to peak in the beginning of April at a water temperature of about 8 C. However, mean parasite intensity remained constant over this interval. Prevalence of metacercariae of the digeneans Diplostomum cf. spathaceum and Tylodelphys clavata remained constant with values around 60% and 10%, respectively. Larvae of the nematode Philometra obturans and the cestode Ligula intestinalis and females of the copepod Neoergasilus japonicus were found sporadically. A concept of 2 thresholds, a critical condition factor (attained by a temperature-induced energy deficiency), and a condition-dependent critical infection intensity, which determine winter mortality of YOY, is suggested.

Mortality factors, helminth burden, and contaminant residues in white-tailed sea eagles (Haliaeetus albicilla) from Finland.

Eleven white-tailed sea eagles (WSEs) (Haliaeetus albicilla) collected in Finland between 1994 and 2001 were examined for their causes of death, including analyses of ubiquitous environmental contaminants and parasites. Four WSEs died due to electrocution. Two were lead poisoned and another had fragments of a lead bullet in its gizzard. An 11-year-old female drowned entangled in fishing gear, but also had mercury levels in its liver and kidneys known for detrimental physiological effects. One WSE was evidently killed by an intraspecific conflict, which was also assumed to be with another bird. The mortality factors of two WSEs could not be clarified, but one had a lead level of 4.604 microg g(-1) in its liver, indicating high lead exposure at a clinically relevant concentration. All organ levels of chlorinated pesticides, polychlorinated biphenyls, and cadmium were moderate and not harmful for birds of prey. Seven helminth species, but no ectoparasites or coccidians, were found in 8 of 10 WSEs investigated for parasites. The highest prevalence of 40% was found for the liver fluke Metorchis billis, but no severe parasitosis was diagnosed for the eagles. The two acanthocephalan species Corynosoma semerme and Polymorphus meyeri are both new records for WSEs.

Ovine helminthosis, a major health constraint to productivity of sheep in Ethiopia.

Small ruminants represent an important component of the Ethiopian livestock production system, providing 12% of the value of livestock products consumed at the farm level and 48% of the cash income generated. Ethiopia is second in Africa and sixth in the world in terms of sheep population. The country has, however, benefited little from this enormous resource owing to a multitude of problems, disease being the most important. Disease alone accounts for mortalities of 30% in lambs and 20% in adults. Productivity losses attributable to helminth parasites are often substantial. A loss of US $ 81.8 million is reported annually due to helminth parasites. In a country confronted with challenges of an ever-rising human population and food shortage, such enormous losses caused by helminth parasites, 'the silent predators', are intolerable. Therefore, helminth control should receive special attention in poverty reduction strategies through improved productivity of livestock if the present and future challenges of food shortage are to be addressed. Productivity loss due to helminth infections can be substantially reduced through implementation of effective disease control strategies, which require an understanding of the epidemiology and ecology of parasites and parasitic infections under local conditions. This paper reviews extensive evidence demonstrating that helminthosis is a major constraint to productivity of sheep in Ethiopia. It also discusses epidemiology and ecology of major helminth parasites in the country and suggests strategies for improved control in various agro-ecological zones and production systems.

Joint modelling of repeated measurements and event time: application to performance traits and survival of lambs bred in sub-humid tropics.

We considered the analysis of a study for Dorper, Red Maasai and crossbred lambs born over a period of 6 years at the Diani Estate, Kenya. The study was designed to compare survival and performance traits of genotypes with differing susceptibilities to helminthiasis. The available data include information on time to death and repeated measurements of body weight, packed cell volume (PCV) and faecal egg count (FEC) of the animals. In the paper, we consider joint modelling of the survival time and the repeated measurements. Such an approach allows to account for the possible association between the survival and repeated measurement processes. The advantages and limitations of the joint modelling are discussed and illustrated using the Diani Estate study data.

Role of cellular response in elimination of the monogenean Neoheterobothrium hirame in Japanese flounder Paralichthys olivaceus.

Adult worms of the blood-feeding monogenean parasite Neoheterobothrium hirame, which cause anemia in the Japanese flounder Paralichthys olivaceus, attach to the host fish by embedding their posterior part deeply into the host tissue. To investigate the possibility that cellular responses of the host fish can eliminate N. hirame, flounder were experimentally infected with N. hirame larvae and reared in either fed or starved conditions. Mature parasites were identified on the buccal cavity wall of the fish 33 d post-infection (Day 33). Monocytes/macrophages and granulocytes increased rapidly in the blood and infected sites after the appearance of mature parasites. These cells adhered to the tegument of the parasites. In addition, a few cells with large electron-dense granules (DGCs) were observed in the inflammatory foci. On Day 47, the tegument of some parasites collapsed partially and were phagocytosed by the infiltrated host cells. Some infiltrated cells adhered directly to the inner tissues of the parasites. On Day 54, in the fed fish group, the loss of the tegument led to damage of the parasites' inner tissue by a large number of infiltrated cells. In this group, the elimination of the parasites was noted from Day 47 to 54. These observations probably suggest that the cellular response of the host fish destructed the parasite's posterior part embedded in the tissue, thereby eliminating the parasites. On the other hand, a high mortality was observed in the starved group. The starved fish developed much more severe anemia than the fed fish, and the elimination of the parasites was not observed in this group. The results of the present study suggest that flounder can eliminate N. hirame if they are fed sufficiently.

Patterns of mortality in southern sea otters (Enhydra lutris nereis) from 1998-2001.

Detailed postmortem examination of southern sea otters (Enhydra lutris nereis) found along the California (USA) coast has provided an exceptional opportunity to understand factors influencing survival in this threatened marine mammal species. In order to evaluate recent trends in causes of mortality, the demographic and geographic distribution of causes of death in freshly deceased beachcast sea otters necropsied from 1998-2001 were evaluated. Protozoal encephalitis, acanthocephalan-related disease, shark attack, and cardiac disease were identified as common causes of death in sea otters examined. While infection with acanthocephalan parasites was more likely to cause death in juvenile otters, Toxoplasma gondii encephalitis, shark attack, and cardiac disease were more common in prime-aged adult otters. Cardiac disease is a newly recognized cause of mortality in sea otters and T. gondii encephalitis was significantly associated with this condition. Otters with fatal shark bites were over three times more likely to have pre-existing T. gondii encephalitis suggesting that shark attack, which is a long-recognized source of mortality in otters, may be coupled with a recently recognized disease in otters. Spatial clusters of cause-specific mortality were detected for T. gondii encephalitis (in Estero Bay), acanthocephalan peritonitis (in southern Monterey Bay), and shark attack (from Santa Cruz to Point Año Nuevo). Diseases caused by parasites, bacteria, or fungi and diseases without a specified etiology were the primary cause of death in 63.8% of otters examined. Parasitic disease alone caused death in 38.1% of otters examined. This pattern of mortality, observed predominantly in juvenile and prime-aged adult southern sea otters, has negative implications for the overall health and recovery of this population.

Helminth parasites of the southern sea otter Enhydra lutris nereis in central California: abundance, distribution and pathology.

From October 1997 to May 2001, the gastrointestinal tracts from 162 beach-cast southern sea otters Enhydra lutris nereis were examined for helminth parasites and associated lesions. Carcasses were collected opportunistically in central California between Pt. San Pedro and Pt. Arguello. The primary goals of this study were to examine spatial and temporal variability in mortality due to parasite infection, identify factors associated with increased risk of infection, and illustrate the process of intestinal perforation by Profilicollis spp. Two genera and 4 species of acanthocephalans (Profilicollis altmani, P. kenti, P. major, Corynosoma enhydri) were found in 46.3% (Profilicollis spp.) and 94.4% (C. enhydri) of the carcasses examined. Three species of Digenea (Microphallus pirum, M. nicolli, Plenosoma minimum) were found in 47% of carcasses, at times in massive numbers (> 3000 per cm2). This is the first report of the latter 2 species from the sea otter. Mortality resulting from infection by Profilicollis spp. occurred in 13.0% (n = 21) of sampled carcasses, either directly, due to perforation of the intestinal wall and peritonitis (9.9%, n = 16), or indirectly, due to inhibition of host nutrient uptake or depletion of host energy reserves to fight chronic infections (3.1%, n = 5). The most massive infections (< 8760 parasites), and all cases of intestinal perforation occurred in carcasses infected by P. altmani and/or P. kenti. Mortality due to infection by Profilicollis spp. occurred more frequently among juvenile and old-adult females (chi2 = 17.479, df = 9, p = 0.045) from sand and mixed habitats in Monterey and Santa Cruz in the north of the sea otter range (chi2 = 9.84, df = 4, p = 0.045). Spatial differences in sea otter mortality coincided with the relative distributions of Profilicollis altmani, P. kenti, and P. major, and may reflect differences in sea otter diet, or differences in intensity of infection in intermediate hosts. Mortality rate due to infection by Profilicollis spp. decreased between 1998 and 2001, though differences were not significant (chi2 = 3.983, df = 3, p = 0.40), and may vary on multi-year cycles due to environmental factors such as density of definitive hosts (e.g. the surf scoter Melanitta perspicillata), or El Niño. Corynosoma enhydri did not cause significant damage to the intestine of the host, even when present in great numbers.

Occurrence of concurrent trypanosomosis, theileriosis, anaplasmosis and helminthosis in Friesian, Zebu and Sahiwal cattle in Uganda.

An epidemiological investigation was conducted on farms in Tororo and Soroti districts of Uganda from January to February 2000 to determine the cause of reported persistent mortality of cattle. Blood and faecal material of 98 cattle comprising of 33 Friesians, 58 Zebu and 7 Sahiwal were examined. Results revealed that seven (7.1 %) cattle had trypanosome infection, mainly due to Trypanosoma vivax and T. brucei, 17 (17.3%) Fasciola infection, 28 (28.6%) gastrointestinal nematode infection, 33 (33.7%) Theileria sp. infection and 13 (13.3%) Anaplasma marginale infection. Mixed infections were detected in 30%, 20.6% and 43 % of the Friesian, Zebu and Sahiwal cattle respectively. Anaemia (PCV < 25) was detected in 24%, 19% and 14% of the Friesian, Zebu and Sahiwal cattle respectively. Persistent mortality of cattle on these farms could have been due to either single or mixed parasitic infections probably exacerbated by malnutrition.

Host factors limiting monogenean infections: a case study.

Comprehensive field data on polystomatid monogeneans record low prevalence and intensity of infection and suggest that worm burdens in this group are strongly regulated: thus, in the majority of Polystoma species infecting anuran amphibians mean abundance is typically less than one parasite/host. There is circumstantial evidence that the dominant control is attributable to host factors which over-ride variations in transmission success. This review provides a brief summary of information on Pseudodiplorchis americanus, a parasite of the desert toad, Scaphiopus couchii, and then focuses in detail on the spectrum of factors regulating infrapopulations of Protopolystoma xenopodis, a parasite of the aquatic Xenopus laevis. Infection levels of adult worms and their contribution to transmission are regulated by external environmental factors (especially temperature), by host factors (including behaviour and population density), and by a range of parasite factors including intra- and inter-specific competitive interactions and variations in intrinsic characters, especially survivorship and reproductive output. In addition to these factors whose primary effect is to modulate transmission rates, there is a major attrition in parasite numbers between invasion and maturity (3 months post-infection). Long-term laboratory experiments on the Xenopus laevis/Protopolystoma xenopodis interaction demonstrate a powerful acquired immune response. Primary infection is characterised by a high prevalence of established adult worms but the success of subsequent challenge infection is greatly reduced, leading to low prevalence and extended pre-patent period. In the small proportion of hosts supporting a second infection of adult parasites, surviving burdens are small (one to two worms/host) and show reduced egg production. These results provide an explanation for the low burdens encountered in field studies: a majority of adult X. laevis in natural populations are likely to exhibit strong, relatively long-term, post-infection immunity after the loss of a previous infection.