Host factors associated with the detection of Aeromonas salmonicida and Yersinia ruckeri in Ontario, Canada government fish hatcheries

This paper presents an epidemiological investigation of Ontario Ministry of Natural Resources Fish Health Laboratory data from 1981 to 1997, to determine whether fish species and age were associated with lot-level detection of Aeromonas salmonicida and Yersinia ruckeri in hatchery fish. In stepwise logistic regression, the species brook trout and back-cross (lake trout crossed with the hybrid “splake”) were more likely to test A. salmonicida-positive compared to all other species reared in the hatcheries. Similarly, the species brook trout was significantly more likely to test Y. ruckeri-positive compared to all other species. For both pathogens, the 1–5-month age group was associated significantly with detection. These findings suggest that purposive sampling of higher-risk fish lots could increase the likelihood of detecting both study pathogens.     

Factors associated with the incidence of bacterial gill disease in salmonid lots reared in Ontario, Canada government hatcheries

Bacterial gill disease (BGD) (causative agent: Flavobacterium branchiophilum) has been a persistent problem in early-rearing salmonids in the Ontario Ministry of Natural Resources (OMNR) fish hatchery system. Retrospective epidemiological investigations of BGD diagnoses and treatments in OMNR fish hatcheries during the period 1991–2001 were conducted using University of Guelph Fish Health Laboratory and OMNR central office data. All investigations were conducted at the lot-level, which is the major within-hatchery-level of population aggregation. Survivorship of BGD diagnosis in early-rearing lots within seven individual hatcheries ranged from 84.2 to 100%; within individual species groups, survivorship was lowest (84.6%) in brook trout (Salvelinus fontinalis) lots. Annual risk percentages (cumulative incidence) for BGD diagnosis within hatchery and species groups varied considerably among years. Multivariable proportional hazards survival analysis indicated that the species brook trout, and the Spring (March–May) season, were significantly associated with treatment for BGD. Combined, these results emphasize the importance of hatchery, species, and time on the development of BGD. Future observational research on this disease must consider these factors in their design and analysis.     

Rearing unit-level factors associated with bacterial gill disease treatment in two Ontario, Canada government salmonid hatcheries

Early-rearing salmonids in Ontario Ministry of Natural Resources (OMNR) fish hatcheries have been consistently affected by bacterial gill disease (BGD) (causative agent: Flavobacterium branchiophilum) for many years. Separate retrospective epidemiological investigations of BGD treatments at two OMNR fish hatcheries (Hatcheries A and B) for the 1999 production year were conducted using on-site hatchery records. Both investigations were carried out at the rearing unit-level, with early-rearing (<9 months of age) “tank-lot” as the unit of analysis to identify unique fish populations over time. Multivariable repeated measures logistic regression models were created for both hatchery datasets, controlling for lot-level and species effects. For Hatchery A, the species brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) were significantly associated with BGD treatment, as well as lower water exchange rate, and higher feeding and mortality percentages during the 2 weeks previous to BGD treatment. At Hatchery B, the species brook trout (S. fontinalis) and splake (Salvelinus namaycush × S. fontinalis) were significantly associated with BGD treatment, as well as lower individual fish weights and treatment for BGD during the previous week. These results emphasize the importance of water quality, feeding rate, fish size and prior mortality on the development of BGD. Significant hatchery and species effects were evident, and future observational research on BGD must account for these factors in their design and analysis.     

Immune modulation by probiotic strains: quantification of phagocytosis of Aeromonas salmonicida by leukocytes isolated from gut of rainbow trout (Oncorhynchus mykiss) using a radiolabelling assay

A method is described for the rapid and sensitive assay of phagocytosis that utilizes radioactively labelled bacteria. With this method, we observed that phagocytosis of heat-inactivated Aeromonas salmonicida by leukocytes isolated from gut of rainbow trout fed with different viable probiotics (Lactococcus lactis subsp. lactis, Lactobacillus sakei, and Leuconostoc mesenteroides) was significantly higher (P<0.05) after 2 weeks of probiotic-feeding than the control group. However, only phagocytosis of live A. salmonicida by the leukocytes isolated from gut of rainbow trout fed with L. lactis subsp. lactis was significantly higher (P<0.05) than the control group.     

Neutrophil, glass-adherent, nitroblue tetrazolium assay gives early indication of immunization effectiveness in rainbow trout

Neutrophil activity in rainbow trout (Oncorhynchus mykiss) is increased upon antigenic stimulation with the Yersinia ruckeri O-antigen bacterin. The characteristics of neutrophil attachment to glass and nitroblue tetrazolium (NBT) staining were used to determine the effectiveness of immunization programs with fingerling rainbow trout. Fish immunized by intraperitoneal injection with doses of 100, 10, or 1 μg of the bacterin showed the highest responses in that order in numbers of glass adherent, NBT-positive neutrophils. Studies on the kinetics of the occurrence of numbers of glass-adherent, NBT-positive staining cells from the fish injected with the 10 μg dose showed the numbers of positive cells were largest on Day 2 after injection. The specific immune response was confirmed by demonstrating the presence of plaque-forming cells by the passive hemolytic plaque assay and the rise in humoral antibody titers by passive hemagglutination 12 days after injection. The effects of immunization in trout could be detected earlier by using the neutrophil glass adherence and NBT reduction assays than by using assays based on observations of the specific immune response.     

Herd-level factors associated with isolation of Salmonella in a multi-state study of conventional and organic dairy farms I. Salmonella shedding in cows

The objective of this study was to evaluate associations between herd characteristics and the isolation of Salmonella from dairy cows in Minnesota, Wisconsin, Michigan, and New York. Study farms were 129 conventional and organic farms enrolled without regard to previous history of Salmonella infection. Herds were sampled at 2-month intervals over a 1-year period. This is the largest study to date on Salmonella shedding in dairy cows and the only study evaluating herd-level risk factors using longitudinal sampling to characterize Salmonella shedding on dairy farms. Salmonella was isolated in fecal samples from 1026 (4.9%) of 20,089 cows. Over the course of the study, 113 (87.6%) of 129 farms had at least one positive cow sample. Multi-variable logistic regression using the generalized estimating equations approach was used to test the association between herd-level risk factors and the dependent variable of within-herd prevalence by visit (number of Salmonella-positive cows/number of cows sampled) after adjustment for effects of herd size, season, state of origin, and the multiple sampling occasions per herd. Factors retained in the final model included lack of use of tiestall or stanchion facilities to house lactating cows (OR = 1.9; 95% CI: 1.1–3.3), not storing all purchased concentrate or protein feeds in an enclosed building (OR = 2.5; 95% CI: 1.3–4.9), not using monensin in weaned calf or bred heifer diets (OR = 3.2; 95% CI: 2.0–5.4), access of lactating or dry cows to surface water (e.g., lake, pond, river, or stream) (OR = 2.3; 95% CI: 1.3–3.9), disposal of manure in liquid form (slurry or irrigation, as opposed to disposal of manure by broadcast/solid spreader only) on owned or rented land (OR = 1.8; 95% CI: 1.3–3.9), and cows eating or grazing of roughage from fields where manure was applied in solid or liquid form and not plowed under during the same growing season (OR = 1.8; 95% CI: 1.0–3.0). A seasonal association was also present as cows were more likely to be Salmonella-positive in summer, spring, and fall compared to winter. Herd size was not associated with Salmonella shedding in the final multi-variable model. The herd-level risk factors identified in this study could potentially be implemented in Salmonella control programs on dairy farms.     

Accuracy of two commercial enzyme-linked immunosorbent assays for the detection of antibodies to Salmonella spp. in slaughter pigs from Canada

Large discrepancies are usually found when different ELISAs for the diagnosis of pig salmonellosis are compared. Thus, our main goal was to estimate the diagnostic accuracy through Bayesian approaches of two commercial assays (Svanovir™ “test A” and HerdCheck™ “test B”) for the detection of antibodies to Salmonella spp. in slaughter pigs. Previously, we estimated the agreement between both tests and their relative sensitivity (Se) and specificity (Sp) with respect to bacteriology on caecal content and ileocaecal lymph nodes. Test A, at a cut-off OD% ≥20%, indicated higher prevalence than test B (OD% ≥10%) (14.6% vs. 8.6%). Relative Se with respect to overall bacteriology was low (≈30%) and similar for both tests, but the relative Sp was significantly lower for test A compared to B (88% vs. 95%). Both tests failed to detect some pigs infected with Salmonella serogroups B and C₁, which they were supposed to identify. In general, tests showed only fair-to-moderate agreement when they were compared (kappa: 0.41). In the Bayesian models, Se of test A varied between 63% and 77%, while Se of test B was 73%. Sp of A was always lower than that of test B (89% vs. 95%). The implications derived from the use of these imperfect serological tests will have to be accounted for in large-scale Salmonella-control programs.     

Infections of Edwardsiella tarda among Brook Trout in Quebec

Abstract

Acute bacterial septicemia is commonly diagnosed in brook trout Salvelinus fontinalis of Quebec, Canada. The agents most commonly isolated include Aeromonas salmonicida (furunculosis), Aeromonas hydrophila (motile aeromonad septicemia), and Pseudomonas species. Septicemia in brook trout caused by the gram-negative bacterium Edwardsiella tarda was diagnosed for the first time in the province of Quebec from two different fish farms producing stock for fee fishing establishments. Affected fish displayed nonspecific lesions associated with bacterial septicemia including hemorrhages on the gills and viscera and exophthalmia. Stress-associated immunosuppression due to an increase in summer water temperatures and lack of precipitation were considered as primary causes of these disease outbreaks.     

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 μg/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 LD₅₀. Although differences in response to IVM in the in vivo determinations were not significant, “strain A” H. contortus had a significantly higher LD₅₀ 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.     

Prevalence and risk factors for Salmonella spp. and Campylobacter spp. caecal colonization in broiler chicken and turkey flocks slaughtered in Quebec, Canada

We conducted an observational study to estimate prevalence and risk factors for Salmonella spp. and Campylobacter spp. caecal colonization in poultry. Eighty-one broiler chicken and 59 turkey flocks selected among flocks slaughtered in the province of Quebec, Canada, were included in the study. Flock status was evaluated by culturing pooled caecal contents from about 30 birds per flock. Exposure to potential risk factors was evaluated with a questionnaire. Odds ratios were computed using multivariable logistic regression.

The prevalence of Salmonella-positive flocks was 50% (95% CI: 37, 64) for chickens and 54% (95% CI: 39, 70) for turkeys, respectively. Odds of Salmonella colonization were 2.6 times greater for chicken flocks which failed to lock the chicken house permanently. In turkeys, odds of Salmonella colonization were 4.8–7.7 times greater for flocks which failed to be raised by ≤2 producers with no other visitors allowed onto the premises, or origin from a hatchery.

The prevalence of Campylobacter-positive flocks was 35% (95% CI: 22, 49) for chickens and 46% (95% CI: 30, 62) for turkeys. Odds of colonization were 4.1 times higher for chicken flocks raised on farms with professional rodent control and 5.2 times higher for flocks with manure heap >200 m from the poultry house, and also increased with the number of birds raised per year on the farm and with the age at slaughter. For turkeys, odds of Campylobacter flock colonization were 3.2 times greater in flocks having a manure heap at ≤200 m from poultry house and 4.2 times greater in flocks drinking unchlorinated water.     

Growth of white sturgeon (Acipenser transmontanus) following recovery from the stunted stage in a commercial farm in California, USA

Runt white sturgeon (Acipenser transmontanus) develop during grow-out and are characterized by atrophied muscles and decreased growth. Our first objective was to compare the growth (and body condition) of previously-runt white sturgeon after they recovered from the runt state and sturgeon that had never been runts. On 12 occasions, recovered runts and age- and size-matched controls that had never been runts were tagged and put in a tank that already contained fish of similar age and size. Tagged groups were followed for 119–134 days. Median relative growth rates (RGRs) of the recovered runts were significantly (p ≤ 0.05) higher than those of the controls in three tanks. Multiple linear regression was used to model final weight as a function of initial weight and status (recovered runt or control). Status was not significantly related (p = 0.71) to final weight, after adjusting for initial weight, “tank” and time of follow-up. Our second objective was to determine factors that influenced the loss of tags by white sturgeon during the follow-up period. Logistic regression analysis indicated that higher initial weight and being a control fish might have been associated with losing both tags. We concluded that once white sturgeon runts recovered and started growing, they grew at least as well as fish that had never been runts.     

Grass steer production system to improve carcass and meat quality

Three different production systems for autumn born dairy steers, managed more or less intensively, have been investigated. The aim of the production systems were to use a basic diet of grass, either grazed or conserved as silage or hay, combined in various proportions. The animals were slaughtered, aiming for the same degree of fattening, but at three ages. The intensive management aimed at slaughter in January–February at 26–28 months of age after indoor finishing with silage or hay. An “intensive out-of-season” management wished improve at slaughter at 28–30 months of age. Finally an extensive management with finishing at grazing resulted in slaughter in May–June at 30–33 months of age.

In three successive factorial experiments, 106 steers of two genotypes, Holstein and Montbéliard were reared to test the effects of the three production systems. The animals were evaluated for productive traits, carcass quality and biochemical characteristics of m. longissimus thoracis and m. rhomboideus thoracis.

For both breeds, the “extensive” animals produced heavier carcasses (+ 20 kg between “intensive” and “extensive”; P < 0.002). The carcass conformation evaluated by the EUROP classification did not differ between the production systems. The estimated percentage of adipose tissue of the carcass was significantly lower for the “extensive” steers (P < 0.05). The shear force measured on raw meat was not significantly affected by the production system, but after cooking, the shear force was higher for the extensive management as was the total collagen content. The haeminic iron content was always lowest in the intensive management. The results of these three series of experiments show that it is possible to produce steers with a feeding regime consisting mainly of grass (from 87% to 94% of the dry matter intake) grazed or harvested as hay or silage. To make the best use of the herbage resources of the farm and to limit the purchase of concentrates, the production of autumn born steers slaughtered at 32 months seems to be an attractive production system for dairy farmers.     

Experimental study of antibiotic-induced immunosuppression in mice—II. Th, Ts and NC cell involvement

In penicillin(pen), streptomycin(str), kanamycin(kan) and tetracycline(tet)-treated CBA/J adult mice, no difference was noticed as concerned spleen T “helper” (Th) cell activity, as studied by means of response to PHA of X-irradiated/whole T fraction cell mixtures in vitro. On the contrary, in erythromycin(erm), colistin(col) and chloramphenicol(chl)-treated groups, Th cell activity was significantly decreased. On the other hand, spleen T “suppressor” (Ts) cell activity (assayed by response to PHA of mixtures containing pre-incubated with Concanavalin A and whole T cells, respectively) was augmented in samples arising from chl-treated group. These results are also supported by experiments testing Th or Ts soluble factors, induced in spleen T cells belonging to each antibiotic-treated group and purified by affinity chromatography (Concanavalin A-Sepharose 4B columns). Thus, it was confirmed both Th cell deficiency in erm, col or chl-groups, and Ts cell augmentation in chl-group.

As regards spleen “natural cytotoxic” (NC) cell activity, as tested in a xenogeneic “target” cell substrate, a diminished cytotoxic capacity manifested chl-group-derived NC cells, possibly by richness in own “non-specific suppressor” (NSS) cells. NC cell samples in vitro supplemented with NSS cells arising from suckling mouse spleens and a lower cytotoxic activity, in a larger extent in chl-group-derived NC cells, as compared to other groups. The pre-incubation of control-group-NC cells with several antibiotic preparations in vitro was followed by decrease of the cytotoxic values in erm, col and chl-samples, suggesting a drug-induced NC receptor “masking”, that prevented “target” cell recognition in mice—II. In the case of NSS addition in vitro, as strong inhibition of the cytotoximity occurred in chl-treated NC cells derived from the control group, that proves a possible chloramphenicol-induced immunodepression by potentiation of NSS inhibitory effect on NC cells.

Based upon data from the present work, as well as from the previous work, the authors suggest a classification of several mechanisms by which the antibacterial antibiotics can act as immunosuppressive.     

Modulation of juvenile brook trout (Salvelinus fontinalis) cellular immune system after Aeromonas salmonicida challenge

In fish, the first line of defence against infectious microorganisms is based on non-specific cellular immune mechanisms (innate immunity). In this study, we measured the non-specific immune parameters (natural cytotoxic cells (NCC) activity, lymphoproliferation, percentage of phagocytosis and phagocytic activity) in brook trout (Salvelinus fontinalis) infected by a virulent strain of Aeromonas salmonicida. Eight days post-infection, the mortality of infected fish reached 70%. A transient immunostimulation of the NCC activity was noticed 24h post-infection, but there was no significant difference at 48 h. Then, infection of brook trout with A. salmonicida induced a biphasic immune response. At 24h post-infection, lymphoproliferation was drastically depressed but returned to control level at 96 h. A slight increase in the percentage of phagocytosis and the phagocytic activity was noticed throughout the experiment. Conversely the cell mortality was significantly higher in infected fish compared to control. The modulation of immunological parameters might reveal important clues on how innate immunity might protect fish from bacterial infections.     

Experimental Infection of Brook Trout with Infectious Hematopoietic Necrosis Virus Types 1 and 2

Abstract

Fry of brook trout Salvelinus fontinalis became infected and diseased after immersion exposure to infectious hematopoietic necrosis virus (IHNV), but a long-lasting IHNV carrier state was not induced. Duplicate groups of 100 fish were immersed for 6 h in baths containing a type 1 (Round Butte, RB) or a type 2 (Rangen, RA) IHNV isolate at a high or low dose. Brook trout mortalities induced by immersion in a bath of the RB or RA IHNV isolate at 10² plaque-forming units (pfu) per milliliter were equivalent (1 and 0%), but fish were more susceptible to infection with RA IHNV. Only the single dead fish in the RB group was infected, but 24% of the RAexposed fish were infected 1 week after exposure. At a dose of 10⁶ pfu/mL, exposure to RB IHNV resulted in a higher mortality (35%) and prevalence of infection (89% of live fish sampled at 1 week postexposure), but no infectious virus was detectable by 5 weeks after exposure. In contrast, RA IHNV exposure at a dose of 10⁴ pfu/mL resulted in only 5% mortality, and live fish killed at 1 week postexposure had a 22% prevalence of infection, but infectious virus was not detectable by week 3. Although brook trout have been previously considered to be resistant to IHNV, this study has shown that brook trout become diseased and die after exposure to a high dose of one type I IHNV isolate and can be infected after immersion exposure to even a low dose of type 1 or type 2 IHNV.     

Application of the California mastitis test in intramammary Streptococcus agalactiae and Staphylococcus aureus infections of camels (Camelus dromedarius) in Kenya

A study was conducted on 207 lactating camels in six herds in Kenya to evaluate the California mastitis test (CMT) for the detection of intramammary infections (IMIs) caused by Streptococcus agalactiae and Staphylococcus aureus and to investigate the prevalence of both the pathogens in the camel udder. IMI with S. agalactiae was found in 12% of all camels sampled. IMI with S. aureus was present in 11% of all camels sampled. The herd-level prevalence of IMI varied between 0 and 50% for S. agalactiae and between 0 and 13% for S. aureus. Longitudinal observations over 10–12 months confirmed persistent infections for both pathogens. Observations in one herd suggested that camel pox was a contributing factor in spreading and exacerbating S. agalactiae udder infections.

The CMT had quarter-level sensitivities of 77 and 68% for S. agalactiae and S. aureus in camels, respectively. The CMT specificities were 91% for both the pathogens.     

Risk factors for equine acute abdominal disease (colic): Results from a multi-center case-control study

Many causal hypotheses for acute equine abdominal disease (colic) have been proposed. However, few epidemiologic studies have been undertaken to identify risk factors for the syndrome. Risk factors for equine colic were identified using a hospital-based, multi-center, case-control study. Horses admitted with abdominal pain to Cornell, Guelph, Ohio State, Pennsylvania or Tufts university veterinary hospitals between March and December 1991 were selected. Control horses were randomly selected from a list of all other non-colic admissions. The person(s) responsible for the day-to-day care of the horse completed a telephone questionnaire. Risk factors were identified using multivariable logistic regression analysis.

A total of 406 colic and 406 control horses were used in the final analysis. Age, breed, outdoor access (pasture or drylot), outdoor water supply, use of a daily worming product, amount of concentrates and whole-grain corn consumed, type of person responsible for the day-to-day care of the horse and previous colic history were all significantly associated with colic risk. Arabian horses were more than twice as likely to be colic cases (OR = 2.0, 95% CI = 1.0–3.9), whereas Standardbred horses were nearly half as likely (OR = 0.6, 95% CI = 0.4–1.0) to be colic cases compared with the comparison Thoroughbred group. Horses that had access to outdoor enclosures without a continuous supply of water were more than twice as likely (OR = 2.2, 95% CI = 1.2–4.3) to be colic cases compared with horses that had an adequate supply of water on outside enclosures. The use of a daily worming product for at least 60 days during the 12-month period prior to presentation was associated with a significantly lower colic risk (OR = 0.11, 95% CI = 0.02–0.55). Colic risk increased with greater consumption of whole-grain corn; however, when all non-roughage concentrate feeds were combined, colic risk decreased with increased intake of concentrates. Significant interactions were identified between age and drylot use and between the type of care-person responsible for the day-to-day care of the horse (owner or non-owner) and previous colic history.

The findings of this study illustrate the complex multi-factorial nature of equine colic and point towards the importance of the overall quality of stable management and care, as well as the type of outdoor enclosure (pasture or drylot), water access, carbohydrate intake, parasite control and intrinsic (breed-related) factors.     

A comparative study on certain enzymes of the granulocyte from different ruminant species

In the present study the level of enzyme hydrolases (alkaline phosphatase, myeloperoxidase, elastase, arginase, lysozyme and β-galactosidase) of polymorphonuclear cell (PMN) granules in different ruminant species and their release in response to activation was studied. Buffalo PMN alkaline phosphatase activity was higher (P < 0.01) than in PMNs of cattle and goats. Interestingly, myeloperoxidase was higher in cattle PMNs and least in goat PMNs (P < 0.01), a similar pattern was observed in the distribution of enzyme arginase. As far as lysozyme is concerned, its activity was significantly higher (P < 0.01) in PMNs of buffaloes than in the case of cattle and goat PMNs. On activation, these cells released MPO and elastase, in all the species studied, while lysozyme was secreted only in buffalo PMN cells. Activity of certain enzymes related to oxidant defence systems such as glutathione peroxidase and glutathione reductase were higher in cattle and goats compared to that in buffaloes. These observations are likely to have bearing on immunodefense roles played by PMNs and reflected differences among the ruminant species studied.     

Early Life Stage Survival and Susceptibility of Brook Trout,Coho Salmon,Rainbow Trout,and Their Reciprocal Hybrids to Infectious Hematopoietic Necrosis Virus

Abstract

Triploid (heat-shocked) and diploid groups of rainbow trout Oncorhynchus mykiss, brook trout Salvelinus fontinalis, coho salmon Oncorhynchus kisutch, and reciprocal hybrids were produced, monitored for early life stage survival, and evaluated for susceptibility to infectious hematopoietic necrosis virus (IHNV). The female rainbow trout × male brook trout triploid hybrids had significantly greater (P < 0.01) survival than the diploid hybrids of this cross. The heat-shocked hybrid group of the female rainbow trout × male coho salmon also exhibited significantly greater survival to the eyed egg stage of development than the untreated group of this hybrid. Studies of the susceptibility of treatment groups to a 1990 IHNV isolate from the Hagerman Valley were conducted by using a standardized immersion exposure procedure at one or two different mean body weights. The diploid brook trout and coho salmon and two triploid hybrids (female rainbow trout × male brook trout or male coho salmon) were significantly less (P < 0.05) susceptible to IHNV than the pure-species diploid and triploid rainbow trout groups.     

Use of site occupancy models to estimate prevalence of Myxobolus cerebralis infection in trout

Empirical estimates of pathogen prevalence in samples of fish may underestimate true prevalence because available detection techniques are incapable of perfect detection. Trout of several species were collected from enzootic (Myxobolus cerebralis, causative agent in whirling disease) habitats, and individual fish were examined for presence of the parasite two or six times by one of four methods: pepsin-trypsin digest (brown trout Salmo trutta), plankton centrifuge (brown trout), polymerase chain reaction (rainbow trout Oncorhynchus mykiss), or histopathology (brook trout Salvelinus fontinalis). The presence-absence data were modeled for prevalence of infection (psi) and probability of detection (p) of the parasite via occupancy models that accounted for imperfect detection of the organism. Based on estimates from the most-supported model for comparison, two myxospore concentration methods underestimated prevalence by about 12% for whole-head results and 34% for the expected value of half-head analysis. Polymerase chain reaction and histopathology gave virtually the same prevalence estimates for whole-head results as the best models but underestimated prevalence by about 6% and 12%, respectively, for the expected value of half-head analysis. The probability of detecting the parasite in a single survey of a fish head, conditional on the parasite's presence, was 0.66 for myxospore concentration methods, 0.81 for histopathology, and 1.0 (left halves) or 0.89 (right halves) for polymerase chain reaction. The occupancy models used in this study may be extended to large-scale monitoring of M. cerebralis to estimate expansion or contraction of the parasite's range over time.