Gastrointestinal helminths of ponies in Louisiana: a comparison of species currently prevalent with those present 20 years ago

A survey in Louisiana of gastrointestinal helminths recovered at necropsy from 117 ponies with minimal exposure to anthelmintics between 1989 and 2000 is compared with a survey conducted 20 yr earlier in the same region. An objective of this study was to determine whether species diversity has been affected by the advent and use of the macrocyclic lactone (ML) parasiticides and by the increased anthelmintic pressure on the helminth species infecting the general equine population. Twenty-six cyathostome species and 8 strongyle species were recovered. Two cyathostome species that were not found before, Cylicostephanus asymetricus and C. bidentatus, and 1 species of large strongyle, Oesophagodontus robustus, were added to the list of species found in Louisiana. All cyathostome and large strongyle species found previously were still present. But prevalences and intensities were significantly reduced for almost all large and small strongyle species. Prevalences and intensities of Oxyuris equi adults and larvae were reduced, whereas the prevalence of Parascaris equorum remained constant. The tapeworm Paranoplocephala mamillana was added to the list of parasite species found in Louisiana. Anoplocephala perfoliata remained the most common cestode. This species was found at the same level of intensity but increased slightly in prevalence. Anoplocephala magna was found less frequently than previously. The overall diversity of species remained reatively unchanged. The reasons for the differences in intensity and prevalence of strongyles between these 2 periods are unknown but might be related to the development and use of the broad-spectrum ML anthelmintics in the intervening period, a difference in the population of equids surveyed, different techniques used to identify the parasites, or differences in numbers of parasites identified (or to all).     

Occurrence of Gasterophilus intestinalis and some Parasitic Nematodes of Horses in Sweden

A survey was performed on the occurrence of some internal parasites in 461 horses (1–30 years old) slaughtered from October 1992 to September 1993 at the Linköping abattoir in central Sweden. Macroscopical examination was carried out specifically for parasites of the tear ducts and conjunctival sacs of the eyes, and of selected parts of the alimentary tract and car-dio-vascular system. The following parasites were found in selected parts of the large intestine: encapsulated cyathostome larvae (in 35.6% of the horses), and mature strongyle worms (17.4%); in the stomach: Gasterophilus intestinalis (12.3% during October-June) and Habronema muscae (1.1%); and in the conjunctival sac: Thelazia lacrymalis (3.1%). Significantly more encapsulated cyathostome larvae were found during January to June than during other times of the year, and horses aged 1–5 years harboured significantly more larvae than older horses. Severe damage to the cranial mesenteric artery and its main branches was noticed in 16 (6.1%) out of 263 horses specifically examined. However, the 4th stage larva of Strongylus vulgaris was only recovered in 6 (2.3%) of the horses. Quantitative and qualitative faecal egg counts were done on 412 and 384 of the horses, respectively. Eggs of strongyles, Parascaris equorum and Strongyloides westeri were found in 78.1%, 1.9% and 0.2% of the faecal samples, respectively. Highest prevalence of strongyle eggs was found during July to September. Third stage larvae of the following nematodes were recovered from faecal cultures: subfamily Cyathostomum sensu lato (78.1%) Triodontophorus spp. (6.5%), Strongylus vulgaris (3.6%), Trichostrongylus axei (1.3%), while Gyalocephalus sp., Oesophagodontus sp., Poteriostomum sp. and Strongylus edentatus each comprised less than 0.5%.     

Prevalence and abundance of equine strongyles (Nematoda: Strongyloidea) in tropical Australia

A postmortem survey of 57 horses in tropical northern Queensland revealed 41 (89%) infected with intestinal strongyles. Thirty-five strongyle species (8 large strongyles and 27 small strongyles [Cyathostominae]) were recorded of which 9 species are reported from Australia for the first time. The 14 most prevalent small strongyles were Cyathostomum catinatum (in 76% of horses), Cyathostomum coronatum (65%), Cyathostomum pateratum (33%), Cyathostomum labiatum (30%), Cylicostephanus calicatus (70%), Cylicostephanus longibursatus (67%), Cylicostephanus goldi (43%), Cylicostephanus minutus (26%), Cylicocylus nassatus (67%), Cylicocyclus leptostomus (41%), Cylicocylus insigne (41%), Cylicocyclus radiatus (33%), Cylicocyclus brevicapsulates (22%), and Poteriostomum imperidentum (24%). The remaining cyathostomes were each found in less than 15% of horses. The 4 most common large strongyles were Triodontophorus serratus (30%), Strongylus vulgaris (28%), Strongylus equinus, and Strongylus edentatus (both 22%). The number of species of small strongyles per horse showed a marked variation (mean 10.3, range 2-21) but bore no relationship to either the total number of strongyles per horse, age, sex, and breed of horse, or season. Total number of strongyles per horse (mean 15,890, range 20-165,000) was less than in recent surveys in Europe and the U.S.A. Most horses had low worm burdens, whereas a very small number were heavily infected. Ninety-seven per cent of the total strongyle counts were small strongyles. Strongylus species contributed just over 1%. Small numbers of large strongyles per horse were usual with T. serratus (mean 570), S. vulgaris (mean 330), and S. equinus (mean 330) the most numerous.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal transmission of gastrointestinal parasites of equids in southern Louisiana.

Parasite-naive pony foals were used as sentinels to monitor transmission of gastrointestinal parasites of equids in Louisiana during 4 seasons of the year. Two annual periods were studied, 1988-1989 and 1992-1993. Two or 3 foals each season were turned out to graze a contaminated pasture along with resident parasitized mares and their foals. After a grazing period of 8 wk, sentinel ponies were held in a parasite-free box stall for a period of 6 wk to allow parasites to develop, thus enhancing the evaluation of hypobiotic stages. Following this holding period, necropsies were performed for complete parasite recoveries. Data show that transmission of large and small strongyles occurs during all seasons in southern Louisiana, with highest levels of transmission occurring in the winter and only minimal transmission taking place in the summer. Numbers of mucosal cyathostomes, as well as total cyathostome numbers, were highest in the winter, and luminal cyathostome numbers were highest in the spring. Transmission of Anoplocephala perfoliata and Parascaris equorum occurred during all seasons of the year, although numbers of P. equorum were reduced in spring 1989 and 1993. Gasterophilus intestinalis instars were recovered from fall and winter sentinels only. Oxyuris equi L4 were found all seasons 1 yr, but only during the fall and winter of the final year.     

Strongylids (Nematoda: Strongylidae) of domestic horses in Ukraine: modern state of Fauna and structure of the parasite community

Species composition and structure of the strongylid community was studies on helminthological material collected from 162 domestic horses from 11 regions of Ukraine by the in vivo method. Animals were treated with anthelmintic drug "Univerm" (0.2% aversectin C, Russia). Faecal samples (200 g each) were collected from every horse at 24, 36, 48 and 60 hours after treatment; all nematodes expelled (90.851 specimens) were collected and identified. Thirty-three strongylid species from 12 genera (8 species of subfamily Strongylinae and 25--of Cyathostominae) were found in domestic horses in Ukraine. Cyathostominae dominated in the strongylid community; they were found in 100 % horses and composed 98.21% of community. "Core" of the strongylid community was composed by 7 cyathostome species. Decreasing of proportion of Strongylinae in the community for last 40 years was registered; strongylines were found in 37.6% of horses and composed 1.25% of community. Maximal prevalence was 20.98% (Strongylus vulgaris). Bray-Curtis cluster analysis revealed high similarity of strongylid communities in horses from various regions of Ukraine. Difference in general structure of strongylid communities of horses from different horse-keeping conditions was established. Horses from farms with stable-paddock keeping conditions had bimodal strongylid community structure; while horses from stable-pasture keeping conditions possessed multimodal community structure.     

Control of gastrointestinal strongyles in goats

Gastrointestinal (GI) parasitism are the most serious problem affecting sheep and goats worldwide. Economic losses caused by GI strongyles are related to decreased production, costs for treatment and prophylaxis and animal death. Effective control of internal parasites in small ruminants is one of the most difficult challenges encountered by veterinary in practice. For control of helminth parasites in grazing animals it is important knowledge of epidemiology of the parasite as it interacts with the host in a specific climatic, management and production environment. Sheep and goats are infected by many of the same GI nematodes, but feeding and management practices alter levels of parasitism between the two ruminants. The most commonly anthelmintics used for helminth control in small ruminants include three chemical groups: Imidazothiazoles/Pyrimidines (Im/Pm), Benzimidazoles/Probenzimidazoles (Bz/Pbz) and Macrocyclic Lactones (ML). The control of gastrointestinal strongyles infections in goats shows specific patterns. The use of anthelmintics in goats is based upon anthelmintics goats-suited dose rates taking into account the specific pharmacokinetics features of some anthelmintics in this species. Several types of control strategies: deworming programs, grazing management, biological control, supplementary feeding and breeding approaches are discussed.     

Anthelmintic resistance: the state of play revisited

Helminthosis is one of the major constraints in the successful wool and mutton industry throughout the world. Anthelmintic Resistance (AR) is said to have been established when previously effective drug ceases to kill exposed parasitic population at the therapeutically recommended dosages. Anthelmintic resistance is almost cosmopolitan in distribution and it has been reported in almost all species of domestic animals and even in some parasites of human beings. Some of the most important species of parasites of small ruminants in which AR has been reported include: Haemonchus spp., Trichostrongylus spp. Teladorsagia spp., Cooperia spp. Nematodirus spp., and Oesophagostomum spp. All the major groups of anthelmintics have been reported for development of variable degrees of resistance in different species of gastrointestinal nematodes. This paper describes the global scenario of prevalence and methods used for detection of AR in small ruminants. Different mechanisms and contributory factors for the development of AR are discussed. Various options and alternate strategies for the control and/or delay in the onset of AR are suggested in the light of available information.     

Anthelmintic resistance in human helminths: Learning from the problems with worm control in livestock

During the past decade, the prevalence of anthelmintic resistance in some economically important helminths of sheep, goats and horses has increased dramatically. In some regions of Australia, South America and South Africa, anthelmintic resistance has become a serious threat to the survival of the sheep industry. Mass treatment programmes and exclusive reliance on anthelmintics for worm control in livestock are amongst the most important reasons for the development of anthelmintic resistance. In this article, Stanny Geerts, Gerald Coles and Bruno Gryseels draw the attention to a number of errors that have occurred in the control of helminths in livestock and that should be avoided in the control of worms in humans.     

Nematode parasites from Burchell's zebras in South Africa

Twenty-five Burchell's zebras (Equus burchelli antiquorum) which were culled at monthly intervals in the Kruger National Park were examined for helminths. Twenty-nine species of nematodes belonging to the families Atractidae, Habronematidae, Onchocercidae, Oxyuridae, Strongylidae, Strongyloididae and Trichostrongylidae were recovered. The cyathostomes (small strongyles) most abundant were Cyathostomum tetracanthum, Cylicostephanus calicatus, Cylindropharynx sp. (? C. intermedia Theiler, 1923) and Cylicocyclus auriculatus. Cyathostomum alveatum, Cyathostomum montgomeryi, Cylicostephanus calicatus and Cylindropharynx sp. (? C. intermedia Theiler, 1923) were the most prevalent cyathostomes (small strongyles) while Craterostomum acuticaudatum was the most prevalent of the large strongyles. Of all the species recovered those most abundant were Crossocephalus viviparus and Probstmayria vivipara with intensities of 100 to 3,857,772 and 18,400 to 104,120,467, respectively. Four new species, two Triodontophorus spp. (Strongylidae) and two Habronema spp. (Habronematidae) were identified. Furthermore, this study furnishes a first report of Triodontophorus minor in zebras. The fourth stage cyathostomes as well as the adults of 11 of the 14 species were present in significantly greater intensities in autumn and winter.     

A survey of gastrointestinal parasites of olive baboons (Papio anubis) in human settlement areas of Mole National Park, Ghana

Fecal samples from 55 free-ranging olive baboons (Papio anubis) in Mole National Park, Ghana, were collected 22 June-7 July 2008 and analyzed for gastrointestinal parasites. This is the first survey of baboon gastrointestinal parasites in Ghana and provides baseline data for this area. Ninety-three percent of samples were infected, leaving 7% with no parasites observed. Of those infected, there was a 76% prevalence of strongyles, 53% Strongyloides spp., 11% Abbreviata caucasica , 62% prevalence of Balantidium coli (trophozoites and cysts identified), 4% Entomeba hystolytica/dispar, and 47% unidentified protozoan parasites. Of the strongyle infections, 9% were identified as Oesophagostamum sp. One sample contained an unidentified spirurid nematode that resembled Gongylonema sp. Mole has a mixed forest-savanna habitat, and baboons frequently range into human areas, which makes them subject to parasites from each habitat and multiple sources of exposure. We found a high prevalence of nematode parasites, consistent with a wet or cooler forest environment, or high rates of fecal contamination. The presence of Strongyloides sp., E. hystolitica/dispar, and B. coli suggest potential public health risk from baboons, but molecular identification of these parasites, and documentation of their presence in local human populations, would be necessary to confirm zoonotic transmission.     

Seroprevalence and Potential Risk Factors Associated with Neospora spp. Infection among Asymptomatic Horses in Jordan

This study aimed to determine the seroprevalence and to identify risk factors associated with Neospora spp. infection in horses in Jordan. Management related data were collected from each farm and individual horses. Sera from 227 horses from 5 of 6 climatic regions in Jordan were analyzed for the presence of antibodies to Neospora spp. by ELISA kit. The study was performed during spring of 2010. The association between seropositivity and risk factors was analyzed. A total of 7 (3%) of 227 sera had antibodies for Neospora spp. There was a significant regional difference (P=0.018) between the 5 climatic regions. Positive cases were located in Amman and Irbid, while the other regions (Zarqa, Jordan Valley, and Wadi Mousa) had zero prevalence. The use of anthelmintics at least once a year resulted in a significant reduction of the seroprevalence to Neospora spp. (1.6% vs 9.8%). However, this might be a phenomenon by chance and a better hygiene since owners can invest in anthelmintics. Other risk factors such as age, gender, breed, usage, body condition score, grazing, presence of other animals mixed with the horses in the same property, and a history of previous diseases were not significantly associated with the seroprevalence to Neospora spp. infection. This is the first study to report on the presence of Neospora seropositive horses in Jordan. Further studies are warranted to better understand the role of certain risk factors in the transmission of Neospora spp. among horse population and to determine which Neospora spp. are responsible for the infection.     

Concurrent infections with the ruminant nematodes Haemonchus contortus and Trichostrongylus colubriformis in jirds, Meriones unguiculatus, and use of this model for anthelmintic studies

Haemonchus contortus- and Trichostrongylus colubriformis-infected jirds (Meriones unguiculatus) are useful for anthelmintic studies. With concurrent infections of these parasites established in the jird, questions of not only anthelmintic activity, but to some extent spectrum, could be assessed in a single model system. This report outlines a model using immunosuppressed (0.02% hydrocortisone in feed) jirds concurrently infected with H. contortus and T. colubriformis. Immunosuppressed jirds were inoculated with approximately 1,000 exsheathed infective larvae of each species, treated per os on day 10 postinoculation (PI), and killed on day 13 PI. Stomachs and small intestines were removed, opened longitudinally, incubated in distilled water at 37 C for 5 hr, fixed in formaldehyde solution, and stored for subsequent examination. Contents of both organs were examined using a stereomicroscope (15-45 x). Various standard anthelmintics were evaluated in the model; modern broad-spectrum ruminant anthelmintics (benzimidazoles, febantel, ivermectin, levamisole hydrochloride, and milbemycin D) are active uniformly and in most cases at doses comparable to those required for efficacy against these parasites in ruminants. This model, using worms of 2 genera living in distinct sites, allows preliminary evaluation of anthelmintic activity and spectrum for experimental compounds in a single cost- and resource-efficient experiment.     

First detection of anti-Besnoitia spp. specific antibodies in horses and donkeys in Italy

Among Apicomplexa protozoa infecting equids, Besnoitia spp., Toxoplasma gondii and Neospora spp. represent important issues from a sanitary and zootechnical viewpoint. However, only scarce epidemiological data are available on the spread of the infections in horses and donkeys in Europe. Therefore, a serosurvey was planned to estimate the prevalence of these Sarcocystidae species in Italian equids. Serum samples from 268 horses and 18 donkeys raised in Italy were collected and serologically analyzed to detect anti-Besnoitia spp., anti-T. gondii and anti-Neospora spp. antibodies: an approach based on an initial screening by in-house ELISA followed by a confirmatory WB was used. Two horses (0.7%) and four donkeys (22.2%), showed antibodies anti-Besnoitia spp. Ten horses (3.7%) resulted positive to T. gondii and one of these (0.4%) was seropositive also to Neospora spp. This is the first detection of anti-Besnoitia spp. specific antibodies in Italian horses and donkeys. The study confirmed the circulation of Besnoitia spp. among equids in Europe. Low prevalence of T. gondii and Neospora spp. in horses raised in Italy was reported. Nevertheless, it is noteworthy to consider that consumption of horse meat could represent a source for human toxoplasmosis.     

Anthelmintic resistance in nematodes: extent, recent understanding and future directions for control and research

Resistance has now been reported to all of the broad spectrum anthelmintic types currently available, namely to the benzimidazoles, levamisole/morantel and to ivermectin. The problem causes most concern for parasite control in sheep, but anthelmintic resistance has also been reported in nematodes of horses, goats, pigs and more recently cattle. Our understanding of the factors which select rapidly for resistance has increased and programmes of worm control which minimize selection for anthelmintic resistance are being developed and tested. One of the greatest problems encountered in attempting to reduce the selection for overt drug resistance is the need for more sensitive tests for developing resistance. In the long term, new approaches to chemotherapy and to overcoming anthelmintic resistance problems will arise from improving our understanding of the modes of action of, and mechanisms of resistance to, anthelmintics at the level of the receptor proteins and their genes.     

Horses grazing with cattle have reduced strongyle egg count due to the dilution effect and increased reliance on macrocyclic lactones in mixed farms

Strongyle infection is an important issue in horse breeding. It impairs horse health and performance, with young horses being the most sensitive. Strongyle control has long relied on the systematic use of chemical treatments. However, expanding anthelmintic resistance among strongyles calls for alternative options. Mixed grazing is assumed to reduce strongyle load on the pasture as the result of a dilution effect. This has been shown in small ruminants grazing with cattle, but the putative benefits of co-grazing between horses and cattle have not yet been evaluated. Here, we conducted field surveys and face-to-face interviews on 44 farms from two contrasted saddle-horse production areas, Normandy and northern Massif Central, to compare equine strongyle management practices between specialized systems and mixed horse-cattle systems. Our goals were (i) to quantify breeders’ awareness of the putative benefits associated with the co-grazing of horses and cattle, (ii) to establish whether mixed farming was associated with different strongyle management strategies and (iii) to test whether strongyle egg excretion was reduced in horses grazed with beef cattle. Every breeder relied on systematic calendar treatments, and only 8 out of the 23 mixed breeders were aware that co-grazing of horses with cattle could be used as part of their strongyle control strategy. Management practices were similar across both systems in Normandy. In Massif Central, mixed breeders formed a distinct cluster from their specialized counterparts: deworming was less frequent and stocking density was higher in mixed farms, while specialized breeders seemed more willing to integrate herd and plot management into control strategies. Faecal egg counts measured in horses from Massif Central were significantly reduced when horses were grazed with cattle. This was the result of an increased reliance on macrocyclic lactones in mixed farms (P < 0.01) and a significant dilution effect (P < 0.01). When considering a subsample of horses treated with macrocyclic lactones only, young horses grazed with cattle had 50% fewer strongyle eggs excreted in their faeces than horses grazed in equine-only pastures (P < 0.01). This is the first evidence of the benefits of mixed grazing with cattle as an alternative to control strongyle infection in horses, although this promising alternative remains largely unknown by horse breeders.     

A repeatable and quantitative DNA metabarcoding assay to characterize mixed strongyle infections in horses

Horses are ubiquitously infected by a diversity of gastro-intestinal parasitic helminths. Of particular importance are nematodes of the family Strongylidae, which can significantly impact horse health and performance. However, knowledge about equine strongyles remains limited due to our inability to identify most species non-invasively using traditional morphological techniques. We developed a new internal transcribed spacer 2 (ITS2) DNA metabarcoding ‘nemabiome’ assay to characterise mixed strongyle infections in horses and assessed its performance by applying it to pools of infective larvae from fecal samples from an experimental herd in Kentucky, USA and two feral horse populations from Sable Island and Alberta, Canada. In addition to reporting the detection of 33 different species with high confidence, we illustrate the assay’s repeatability by comparing results generated from aliquots from the same fecal samples and from individual horses sampled repeatedly over multiple days or months. We also validate the quantitative potential of the assay by demonstrating that the proportion of amplicon reads assigned to different species scales linearly with the number of larvae present. This new tool significantly improves equine strongyle diagnostics, presenting opportunities for research on species-specific anthelmintic resistance and the causes and consequences of variation in mixed infections.     

Parasite control practices on Swedish horse farms

Background  

Virtually all horses are infected with helminth parasites. For some decades, the control of parasites of Swedish horses has been based on routine treatments with anthelmintics, often several times per year. Since anthelmintic resistance is becoming an increasing problem it is essential to develop more sustainable control strategies, which are adapted to different types of horse management. The aim of this study was to obtain information on practices used by Swedish horse owners for the control of endoparasites.     

One season of pasture exposure fails to induce a protective resistance to cyathostomes but increases numbers of hypobiotic third-stage larvae

The development of acquired resistance to cyathostome challenge after 1 season's exposure to a cyathostome-contaminated pasture was investigated using 17 parasite-naive ponies, which were 2-3 yr of age. These were divided into 3 groups: 1 to graze a cyathostome-contaminated pasture for 4 mo (exposed ponies), 1 to graze a "clean" pasture not previously grazed by parasitized animals (nonexposed ponies), and 1 group to remain in the barn under helminth-free conditions (parasite-free ponies). After pasture exposure all ponies were housed in stalls in the barn dewormed with ivermectin (200 micrograms/kg) and oxibendazole (100 mg/kg), a treatment that eliminated most cyathostomes encysted in the mucosa as well as all luminal parasites, on the basis of necropsies of 5 animals, after 17 days. Remaining ponies were challenged with 100,000 cyathostome-infective third-stage larvae (L3) per os 3 wk after anthelmintic treatment. Necropsies were performed 7 wk after the challenge. Total cyathostome burdens (luminal plus encysted stages) were not significantly different among any of the groups. However, a significantly higher percentage of hypobiotic early L3 (EL3) and a lower percentage of adults were found in exposed ponies. This observation supports the hypothesis that resistance acquired through exposure promotes cyathostome hypobiosis. This increase in EL3 in exposed ponies was associated with a significant increase in weight of cecum and ventral colon biopsies.     

Detection and semi-quantification of Strongylus vulgaris DNA in equine faeces by real-time quantitative PCR

Strongylus vulgaris is an important strongyle nematode with high pathogenic potential infecting horses world-wide. Several decades of intensive anthelmintic use has virtually eliminated clinical disease caused by S. vulgaris, but has also caused high levels of anthelmintic resistance in equine small strongyle (cyathostomin) nematodes. Recommendations aimed at limiting the development of anthelmintic resistance by reducing treatment intensity raises a simultaneous demand for reliable and accurate diagnostic tools for detecting important parasitic pathogens. Presently, the only means available to differentiate among strongyle species in a faecal sample is by identifying individual L3 larvae following a two week coproculture procedure. The aim of the present study is to overcome this diagnostic obstacle by developing a fluorescence-based quantitative PCR assay capable of identifying S. vulgaris eggs in faecal samples from horses. Species-specific primers and a TaqMan probe were designed by alignment of published ribosomal DNA sequences of the second internal transcribed spacer of cyathostomin and Strongylus spp. nematodes. The assay was tested for specificity and optimized using genomic DNA extracted from identified male worms of Strongylus and cyathostomin species. In addition, eggs were collected from adult female worms and used to evaluate the quantitative potential of the assay. Statistically significant linear relationships were found between egg numbers and cycle of threshold (Ct) values. PCR results were unaffected by the presence of cyathostomin DNA in the sample and there was no indication of PCR inhibition by faecal sources. A field evaluation on faecal samples obtained from four Danish horse farms revealed a good agreement with the traditional larval culture (kappa-value=0.78), but with a significantly higher performance of the PCR assay. An association between Ct values and S. vulgaris larval counts was statistically significant. The present assay can reliably and semi-quantitatively detect minute quantities of S. vulgaris eggs in faecal samples.     

The sensitivity of microscopy and PCR-based detection methods affecting estimates of prevalence of blood parasites in birds

Inferences about the evolution of host-parasitic relationships are often made based on the prevalence of avian malaria, which is usually estimated in a large sample of birds using either microscopic or molecular screening of blood samples. However, different techniques often have variable accuracy; thus, screening methodology can raise issues about statistical bias if method sensitivity varies systematically across parasites or hosts. To examine this possibility, published information was collected on the prevalence of species in 4 genera of avian blood parasites ( Plasmodium, Haemoproteus, Leucocytozoon, and Trypanosoma) from various sources that used different tools. The data were tested to determine if the application of different methods provided different estimates for the same hosts. In these comparisons between the main methodologies, the PCR-based molecular methods were generally found to provide higher estimates for Plasmodium spp. prevalence than microscopic tools, while there was no significant tendency for such a trend in species of Haemoproteus and Leucocytozoon. When analyzing intraspecific variance of prevalence within molecular studies, some studies provided consistently higher estimates for Haemoproteus spp. prevalence than others, indicating that differences between studies can affect detected estimates. Within microscopic studies, surveys that examined more microscopic fields were more likely to report higher prevalence for Plasmodium spp. than those relying on fewer microscopic fields. Consequently, studies making comparisons across parasite genera and/or host species from different sources need to consider several types of bias originating from variation in method sensitivity.