THE CANADIAN VETERINARY JOURNALLA REVUE VETERINAIRE CANADIENNE

Volume 11 May-mai 1970 No. 5

BOVINE COCCIDIOSIS IN CANADA

L. Niilo*

INTRODUCTIONIN A REPORT to the American Veterinary Medi-cal Association in 1948, a standing committeeon parasitology listed coccidiosis as the thirdmost important parasitic disease of cattle inNorth America (14). In 1949 it was estimatedthat this disease was responsible for an annualloss of ten million dollars in the U.S. cattleindustry (7) and in 1962, bovine coccidiosiswas said to have caused an annual loss of $3.5million in the eleven western states and $7.5million in the seven west north-central states ofthe U.S.A. (8). Thus coccidiosis accounted formore than one-fourth of the economic loss incattle in the U.S.A. attributed to internal para-sitism (6).A particular form of coccidiosis in cattle

known as "winter coccidiosis" has been seen infeedlots and on ranches of the beef cattle rais-ing areas of the north-western United Statesand western Canada for many years. The firstreports of this appeared in the 1920s (2, 13).These infections occur during periods of coldwinter weather and may reach epizootic pro-portions. Investigations to find factors respon-sible for the outbreaks of winter coccidiosisrevealed a number of problems which couldnot be easily explained (6, 10, 11).

So far no study has been conducted on the in-cidence, nature, distribution and economic sig-nificance of bovine coccidiosis in Canada. Thepurpose of this paper is to review reports ofbovine coccidiosis in Canada and draw atten-tion to some of the features of this disease.

METHODSData on the national incidence of bovine

coccidiosis in Canada were obtained from tabu-lated summaries of "Minor Diseases" as pub-lished in the periodical Newsletter of the Healthof Animals Branch, Canada Department of Ag-riculture (3). These tables contain data on the

*Animal Pathology Division, Health of AnimalsBranch, Canada Department of Agriculture, AnimalDiseases Research Institute (Western), P.O. Box640, Lethbridge, Alberta.

incidence of clinical bovine coccidiosis for sevengeographical areas: British Columbia, Alberta,Saskatchewan, Manitoba, Ontario, Quebec, andthe Maritimes. Compiled on a bi-monthly basissince October 1962, the tables list the numberof animals infected, premises involved and thenumber of deaths for each province or geogra-phical area. These figures are collected by fieldveterinarians of the Health of Animals Branchand based on reported observations of veter-inarians in general practice. Approximately 120field veterinarians, responsible for subdistrictswhich cover practically all livestock producingareas, are required to collect monthly farmlivestock disease figures from all veterinarypractitioners located in their respective sub-districts. This is done by asking the practition-ers to complete a uniform tabular list of themore common livestock diseases, including coc-cidiosis, for each animal species (in additionto diseases reportable under the Animal Con-tagious Diseases Act). Therefore, this methodof survey has a potential of reaching almostevery large animal practitioner in Canada.The tabulated summaries, pertaining to a

period of five years prior to October 1968, wereanalyzed and mean values calculated on geo-graphical area and seasonal basis. Since it wasassumed that some indeterminate number ofdisease reports probably were missed at origin,the available figures were considered as mini-mums. Any errors made in diagnosis and re-porting were assumed to be of equal magnitudeamong the major geographical areas and fromseason to season. The mean values were usedfor comparing major differences and trends.Detection of subtle differences was not an ob-jective. In search of possible explanations fordiffering disease patterns in different areas orseasons, the calculated incidence and the mor-tality and morbidity rates were compared withcertain environmental variables. Sources of col-lateral data were official statistical and meteoro-logical publications (5, 9, 12, 15, 16).To obtain information about clinical obser-

vations made by practitioners a questionnaire

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was prepared by the author and mailed to 175veterinary practitioners engaged in farm ani-mal practice in Alberta and parts of easternBritish Columbia and western Saskatchewan.This questionnaire asked for specific informa-tion on incidence, morbidity, mortality, path-ology, age, husbandry and other pertinent as-pects of clinical bovine coccidiosis as seen inpractice throughout the year, but especiallyduring the winter of 1968-69. A total of 73completed questionnaires was received.

Additional information on field cases utilizedin this paper was drawn from 41 case reportsof outbreaks referred to diagnostic laboratories,and from consultation with farmers in southernAlberta. The period covered for some of thisinformation was from the autumn of 1966 tothe spring of 1969, a span of two and a halfyears which included three winters.

RESULTS AND DIscussION

National DistributionIncidence. Figure 1 depicts the average an-

nual incidence of clinical cases of coccidiosison a province or area basis (3) in proportionto the cattle population in each area (5). Theincidence per 10,000 head (a morbidity rate)is based on the reported figures and is assumednot to be the true incidence of the disease. Themorbidity rate of clinical bovine coccidiosis inall the areas, except Alberta, was 2.3 cases per10,000; in contrast, this figure for Alberta was19.6. As seen in Figure 1, there was no direct

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diosis by province or area in proportion to cattlepopulation. The incidence is the mean annual num-ber of reported cases per 10,000 head of cattlepopulation over a five-year period. The cattle popu-lation is based on 1966 census figures.

relationship between the morbidity rate and thenumber of cattle in a particular area. Two areashave been selected for comparison: Ontario andAlberta. These two areas have the largest cattlepopulations in Canada but present striking dif-ferences in the incidence of bovine coccidiosis.This comparison will be discussed later.

Figure 2 depicts the seasonal incidence ofbovine coccidiosis for each area or province asdescribed below. The area comparisons on aseasonal basis, calculated for each bi-monthlyperiod, are expressed as a percentage of thetotal cases. It is important to note that in On-tario, and to some extent in the Maritimes, coc-cidiosis occurred throughout the year with littleapparent seasonal fluctuation, whereas in thewestern provinces there was a marked tendencytoward winter coccidiosis. A somewhat transi-tional pattern was seen in Quebec and fromManitoba westward, the peak of the diseasewas in December and January and the lowestpoint was during the summer months. Here

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FIGURE 2. Reported seasonal incidence of bovinecoccidiosis by province or area. The incidence isexpressed as a percentage of the mean annual re-ported cases on a bimonthly basis over a five-yearperiod.

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COCCIDIOSIS

again, the contrast between Ontario ais evident.Herd outbreaks. During the peak

in December and January the averaiof cases per infected herd in Alberta20 (Figure 3). In Ontario, this ratiithan five at all times without shar]differences. (The average herd inabout 40% larger than that in Ontspeak ratio appeared later in the seaskatchewan (a value of nearly tenAlberta. These trends indicate thaiinfections in Alberta and Saskatchevwto be sporadic in summer and epizooter. The infections in Ontario appessporadic throughout the year.

Mortality. The percentage mortalno. dead/no. affected) among the in:mals varied seasonally as shown inThe mortality rate was higher in alling the summer months; however, titario was the highest. The mortalitycoccidiosis was relatively low (3.1% iiThis could be explained on the basister coccidiosis affects the calves wheolder and able to survive the infecticdial infections in summer are assurvolve primarily young, highly susceptwhich succumb readily to this diseas

Coccidiosis in the WestIn the preceding section, the av;

tional data indicated a higher inc

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FiGuRE 3. Seasonal appearance and elof bovine coccidiosis in Alberta, SaskatcOntario.

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:incidencege numberwas nearlyo was lessp seasonalAlberta isario.) Theson in Sas-) than int coccidialran tendedtic in win-ared to be

ity (100 xfected ani-Figure 4.areas dur-lat for On-r in winteri Alberta).s that win-in they are

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FiGURE 4. Seasonal mortality in bovine cocci-diosis in Alberta, Saskatchewan and Ontario.

)ns. tOccC- bovine coccidiosis in Alberta than in any otherned to in- area in Canada. In this section, detailed resultsible calves obtained through questionnaires and consulta-e. tions in Alberta and parts of Saskatchewan and

British Columbia will be discussed. Althoughsome of the local observations covered the pen-

ailable na- od from autumn 1966 to spring 1969, most ofcidence of the data and trends apply for the six-month

period from October 1968 to March 1969.rA - Area incidence. The reports indicated that31 K .. 6,600 cases of clinical bovine coccidiosis occur-,K red in Alberta during the winter of 1968/69.T --- The majority of cases (88%) was seen in south-

ern Alberta. Figure 5 shows the distribution ofthe "susceptible" cattle and winter coccidiosis inAlberta. For a more realistic comparison, thedensity of market cattle, rather than the totalcattle population in the province is shown,since most of the winter coccidial infections

/: have been seen among market cattle. Thus most., of the cases were seen in southern Alberta,

a region with approximately 400,000 marketcattle among a total cattle population of 1.4million head, based on 1966 statistics (16). The

-' Cmarket cattle figures include cows, bulls and

°DLLJ cattle for direct export; butcher steers and-D ( bheifers and feeders constitute 82% of this totalI I and are the principal group affected by winterZ tQ coccidiosis prior to marketing.D J The incidence in the winter of 1968/69 was) s calculated to be 19.4 per 10,000 which agreespizootiology with the five-year average of 19.6 for Alberta±hewan and obtained from the national data, the fre-

quency, however, appears to be geographic-93

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DISTRIBUTION:

MARKET CATTLE

WNTER COCCIDIOSIS

FiGuRE 5. Approximate distribution of marketcattle in Alberta and the incidence of winter coc-cidiosis. Data: 1967-1969.

ally influenced. The incidence of this disease insouthern Alberta was 42 per 10,000 of the totallocal cattle population of 1.4 million. By con-sidering only those cattle most likely to be af-fected (feeders and butcher steers and heifers)the incidence figure then would have beennearly 180 per 10,000 or 1.8% of some 330,-000 beef cattle of this type in the region (thelatter figure is about 82% of the 400,000 marketcattle mentioned above).

Figures from Saskatchewan indicated the ex-istence of winter coccidiosis in feedlot calves,mostly in the south-west part of the province,but the incidence was lower and more scatteredthan in Alberta. In British Columbia, wintercoccidiosis in the beef cattle raising areas ofthe interior and the eastern parts of the prov-ince accounted for most of the cases reported inthe entire province.

Seasonal incidence. Most of the cases ofwinter coccidiosis occurred in December andJanuary (Figure 6). The overall incidence wasgenerally higher in January than in December.First outbreaks began at about the middle ofNovember, which is a few weeks after weaningof the calves and herding from the summerpastures. There was a gradual increase in thenumber of cases to a peak in mid-January fol-lowed by rapid subsidence.

Animals. In southern Alberta, where wintercoccidiosis may be termed epizootic, 40% of theclinical cases were in feed lots. The remainderof the infections occurred in open corrals, win-

OCT I NOV I DEC I JAN 'FEBFIGURE 6. Temporal distribution of winter coc-

cidiosis in southern Alberta during the winter of1968/69. Data: approximately 2,500 cases from 20herds, plotted as the percentage of this total onsemimonthly intervals. The lower portion of thegraph shows the mean daily temperatures recordedin Lethbridge, Alberta.

ter pastures, feeding areas or barns. Most ofthe affected calves were five to ten months old,having been born on pasture during the pre-vious spring and summer. Less than 10% ofthe infections were in yearlings. Although mostof the cattle in southern Alberta in which theinfections were seen were Herefords, no obviousdifference among breeds seemed to exist.

Clinical signs. Of the clinical signs reportedin the questionnaires, diarrhea, which variedfrom watery feces to a profuse type with strain-ing, appeared first. Blood may appear in thefeces during the second or third day after theonset of diarrhea. Fecal matter then is scanty,bloody and mucoid and may contain shreds oftissue sloughed from the intestinal mucosa. Asreported in the questionnaires in the winter of1968/69, blood appeared in the feces of anaverage of 78% of the affected animals, with amaximum of 100% in some herds. Diarrhealasted from one to 20 days in individual ani-mals, with an average duration of five days,including two days with blood.

Other reported clinical signs were general de-pression, inappetence, and reluctance to move.Nervous signs were sometimes seen, usuallyamong older calves and yearlings; these con-sisted of mild muscular incoordination, twitch-ing, and occasional circling and loss of balance.The proportion of animals with nervous signswas less than 0.3% of those affected. Usually,such animals suffered from the disease for aweek or more and almost invariably died withinthree to five days after the onset of the nervoussigns.

Morbidity in herds varied from 10% to 80%,the highest occurring in January. An outbreakmay run its course through the entire herd

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within a period of three weeks if it occurs inJanuary. Earlier outbreaks involved fewer cattleand the spread in a herd was slower. In feed-lots the duration of the outbreaks varied morewidely. Recurrence of coccidiosis in the sameanimal appeared to be rare.

Mortality in winter coccidiosis in southernAlberta ranged from 1% to 10% on individualfarms with an average of 3% of those affected.This figure is similar to the mortality rate cal-culated from the national data for this geo-graphical area.

Diagnosis. In a majority of the cases reportedby the practitioners, the diagnosis had beenbased on clinical signs, history, herd morbidityand the seasonal occurrence. Microscopic ex-amination of feces was employed to a limitedextent in large animal practices, but more ex-tensive use was made of the diagnostic labora-tory for post mortem examination of fatal cases.Laboratory examination of samples of 41 intes-tinal tracts from cases of suspected winter coc-cidiosis revealed coccidial damage (mucosalinvasion by schizonts and gametocytes) in sam-ples from 30 (73%) of the animals. In six (15%)of the 41, few oocysts were observed in thefeces and on the basis of fecal examinationwould have been considered negative for cocci-diosis. On the other hand, the remaining fivecases had very little evidence of intestinal mu-cosal damage; these clinically-diagnosed casesmay have been due to some other condition.In districts other than southern Alberta, bloodydiarrhea was noticed occasionally and oftenwas considered as non-coccidial in origin dueto lack of confirmation by fecal examination.

Coccidial species. Information on the cocci-dial species involved in cases in southern Al-berta originated from the 41 cases submittedfor diagnostic purposes. In the confirmed casesof clinical coccidiosis Eimeria zurnii was alwaysthe predominant species and comprised 45% to99% of the entire coccidial fauna found in indi-vidual animals. The proportion of this specieswas greater in animals with more severe mu-cosal damage. Other coccidial species found inconsiderable numbers, in order of frequency,were E. bovis, E. ellipsoidalis and E. canadensis.Fecal samples from relatively mild cases occur-ring in October and November contained E.bovis in equal or greater number than E. zurnii.

Estimate of Economic LossAn estimate of the actual incidence of the

disease is a prerequisite for an assessment ofeconomic losses. The actual incidence of bovinecoccidiosis may be four times that which wasreported, since consultations with farmers, lab-oratory diagnosticians and veterinarians sug-

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gested that for each case dealt with by veteri-narians, three may not have been. This estimatemay include an unknown percentage of incorrectdiagnoses.

Direct economic losses fall into two maincategories: (i) deaths due to coccidiosis andto secondary complications; (ii) weight loss,treatment expenses and impaired future per-formance. Mortality is lower in winter cocci-diosis and higher in young calves in summer.Many of the young calves lost are dairy animalsand these are more valuable than beef calves;however, the weight loss is economically moreimportant in beef calves. In a few selected casesin southern Alberta, a mean loss per affectedbeef calf (10-months old) in the winter of1968/69 was estimated to be $18.00. This esti-mate included the value of the lost weight, feednecessary to gain that weight after the infec-tion, and the expenses of treatment. An estimateof the economic loss due to coccidiosis in vari-ous areas of Canada is shown graphically inFigure 7. The percentage distribution of theeconomic loss by areas takes into account thedifferent values of animals by breed, age, pur-pose, location and season; the greatest lossoccurred in Alberta, with Ontario in secondplace.

It is difficult to estimate the total economicloss due to bovine coccidiosis, but the annualloss in calves under one year of age in Canadacould amount to $3,800,000. This figure isbased on an estimated loss of $0.75 per headon all calves less than one year of age in theU.S.A. in 1962 (8) and adjusted to 1.00 perhead because of economic changes since 1962.Assuming a comparable geographical distribu-tion of cattle and of animal husbandry prac-tices, this estimate appears to be reasonablyappropriate to present Canadian conditions.

In summary, Table I compares the mainaspects of bovine coccidiosis in Ontario andAlberta. These two provinces, with the largestcattle population in Canada, had the greatestcontrast in patterns of the disease. Disease pat-terns resembling those in Alberta were alsoseen in Manitoba, Saskatchewan and interiorBritish Columbia. The disease in the Maritimesand coastal British Columbia had featuresapproximating those seen in Ontario. In Que-bec, the pattern was transitional.

Possible Contributing FactorsAs seen in Table I, fundamental differences

in the epizootiology of bovine coccidiosis ex-isted among different geographical areas. Theincidence and nature of this disease do notseem to be directly related to the density ofthe cattle population in a given area. There

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FIGURE 7. Annual economic loss due to bovine coccidiosis in various areas of Canada. The loss isexpressed as a percentage of the total for Canada. The dots indicate an approximate density of thetotal cattle population.

TABLE ICOMPARISON OF THE MAIN ASPECTS OF BOVINE

COCCIDIOSIS: ONTARIO vs. ALBERTA

Ontario Alberta

Season Summer, or all year WinterIncidence Lower HigherEpizootiology Sporadic EpizooticType of cattle Dairy BeefAge Younger calves Older calvesMortality Higher LowerEconomic loss Smaller Greater

appears to be no valid reason to presume thatpathogenicity or species distribution of bovinecoccidia along the same latitude are linked toa particular geographical area. Other possibleassociated factors may be considered:

1. Sanitation. Ingestion of sporulated oocystsdetermines transmission of the infection. Theresultant clinical disease in a susceptible ani-mal is assumed to be influenced, to some ex-tent, by the number of infective oocysts in-gested. This in turn is related to: (i) fecal con-tamination of feed, drinking water, housingarea and animal bodies; (ii) persistence ofcontamination which allows the oocysts toaccumulate; (iii) sufficient warmth to allow theoocysts to sporulate and become infective; (iv)favourable microclimate in which the oocystsremain viable for long periods. These factorsare likely to be present in unsanitary dairybarns in which severe coccidial infections inyoung calves have been observed (1, 4). Thismay account for a large proportion of clinicalcases of coccidiosis in Ontario calves, as there

are 3.7 times more dairy animals in that prov-ince than in Alberta (5).Not all the factors mentioned apply to winter

coccidiosis. Most of the outbreaks in winteroccur during periods of cold weather when thesporulation of the discharged oocysts is inhi-bited and effective fecal contamination greatlyminimized.

2. Climate. Because many winter outbreakscoincide with periods of cold weather, coldstress has been suspected as a predisposingfactor (2, 6, 11, 13). Marsh (11) speculatedthat coccidia would normally be present butdormant in the bovine intestinal tract, and thatclinical disease would result following exposureof an animal to severe cold. Figure 6, depict-ing recent outbreaks in southern Alberta, sug-gests that the peak of infections correspondedto a period of very low temperatures. However,other observations did not support this premise.For example, it is seen that the infections beganbefore the severe weather set in and they in-creased gradually to a peak. This peak wasreached in the first half of January in the lastthree winters studied, but the temperaturecurves were somewhat different from winter towinter. The temperatures in central and north-ern Alberta in the last three winters were lowerthan those of the Lethbridge area (the arearepresented on the graph), yet fewer cases ofclinical infections occurred in the north.

Figure 8 shows the 0°F isothermal line fromcoast to coast for the mean January daily tem-perature (15). The majority of the cattle popu-lation is located south of this line in Alberta aswell as in Ontario. There seem to be no readily

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II

VARIABLE t'

FIGURE 8. Distribution of cattle in relation to air temperature in Canada. The dots indicate an ap-proximate density of the total cattle population. The solid line projects 00 F isothermal line from coastto coast for the mean January daily temperature. The broken line shows a zone of the greatest varia-bility in the mean temperature for January.

discernible differences in minimum air tem-peratures. Sudden changes in climate andlarge variations in temperature occur, however,in a zone running through Alberta. The zone,shown in Figure 8, has a standard deviation of10°F in the mean January temperature (9).Most of the region with a high incidence ofwinter coccidiosis shown in Figure 5 seems tocoincide with a zone having a standard devia-tion of 11F and is known as the chinook zone.The standard deviation of temperature forcattle raising areas of Ontario and Quebec is4°F to 5F (9). These sudden climatic changesin Alberta may contribute to outbreaks of win-ter coccidiosis.

3. Type of husbandry. Basic differences inhusbandry and behaviour of beef and dairycattle are likely to influence the epizootiologicalpattern of bovine coccidiosis, and such differ-ences in cattle industries do exist betweenAlberta and Ontario. Dairy calves are subjectto exposure to infective oocysts from contami-nated barns during the first few weeks of life.Consequently, infections at an early age withrelatively high mortality are the features ofcoccidiosis in dairy calves. Beef type calves,usually born on pasture, stay with their damson pasture or range all summer, widely dis-persed and with minimal exposure to infectivecoccidial oocysts. The exposure to coccidia onpasture may be too low in many cases to stimu-late immunity. When these calves are weanedand crowded into corrals and feedlots for win-ter feeding their bodies may become contami-

nated with fecal matter by direct bodily contact.Oocysts adhering to the hair coat may sporulateunder the influence of body heat and transmis-sion of the infection may result from licking thehair. However, this does not fully explain theoutbreaks of winter coccidiosis in calves kepton winter pastures where crowding is not aserious factor and the calves are not mixedwith new arrivals.

It appears that no single factor alone couldbe sufficient to explain the occurrence of wintercoccidiosis. The problem appears to be morecomplex than is commonly realized and furtherstudy is required.

SUMMARY

The incidence, nature, morbidity, mortalityand economic significance of bovine coccidiosisin various areas of Canada are reviewed andassessed.

Statistics indicated that in eastern Canada,particularly Ontario, bovine coccidiosis occursin summer or throughout the year, is somewhatsporadic with moderate mortality, affects mostlyyoung calves, and causes considerable economicloss.

In beef cattle areas of western Canada, par-ticularly in southern Alberta, the disease isepizootic in winter and affects older, weanedcalves. It causes lower mortality but greatereconomic loss than in other areas of Canada.

It is estimated that coccidiosis causes aneconomic loss in excess of $3,800,000 annually.

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Sanitation, climate and animal husbandry havebeen discussed as possible factors influencingthe pattern of bovine coccidiosis. No singlefactor was found adequate to fully explain thecomplex etiological mechanism of winter coc-cidliosis.

RE'sum1

]Rtude et evaluation de l'incidence, de lanature, de la morbidite, de la mortalite et del'importance economique de la coccidiose bo-vine dans les diverses regions du Canada.

Les statistiques indiquent que dans l'Est duCanada, particulierement en Ontario, la cocci-diose bovine se rencontre en ete ou pendanttoute l'annee. La maladie est sporadique, samortalite est relativement faible. Elle affectesurtout les jeunes veaux et cause des pertesconsiderables.Dans l'Ouest du Canada, particulierement

dans le sud de l'Alberta, la maladie est epizoo-tique en hiver et affecte les veaux sevres plusAges. Sa mortalite est plus faible mais son im-portance economique est plus importante.On estime que la coccidiose cause des pertes

annuelles qui excedent $3,800,000. L'hygiene,le climat et la regie des troupeaux sont con-sideres comme des facteurs susceptibles d'in-fluencer l'evolution de la maladie. Aucun ele-ment par lui-meme ne semble expliquer com-pletement les mecanismes etiologiques com-plexes de la coccidiose d'hiver.

ACKNOWLEDGMENTS

The author is indebted to Dr. R. J. McClena-ghan, Director of Contagious Diseases Division,and Dr. J. F. Frank, Director of Animal PathologyDivision, Health of Animals Branch, Canada De-partment of Agriculture, Ottawa, for assistance inobtaining some of the statistical data used foranalyses, and to Dr. W. N. Harries, VeterinaryDiagnostic Laboratory, Alberta Veterinary ServicesDivision, Lethbridge, for supplying data and diag-nostic material. The author also wishes to ack-nowledge the co-operation of the veterinary prac-titioners in the western provinces who suppliedinformation on the questionnaires. The informationsupplied by farmers and ranchers on their livestockand the weather records made available by theMeteorological Observing Station, Department ofTransport, Lethbridge, are greatly appreciated.

Thanks are due to Dr. S. E. Magwood for review-ing the manuscript.

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2. BRUCE, E. A. Bovine coccidiosis in BritishColumbia, with a description of the parasite,Eimeria canadensis sp.n. J. Am. vet. med. Ass.11: 638. 1921.

3. CANADA DEPARTMENT OF AGRICULTURE. Minordiseases summaries. Newsletter. Health ofAnimals Branch, Ottawa. 1962-1968.

4. DAVIS, L. R. and G. W. BOWMAN. Coccidiosisin cattle. Proc. U.S. Livestock Sanit. Ass. 55:39. 1951.

5. DOMINION BUREAU OF STATISTICS. CanadaYear Book. Ottawa: 1968.

6. FITZGERALD, P. R. Coccidia in Hereford calveson summer and winter ranges and in feedlotsin Utah. J. Parasit. 48: 347. 1962.

7. FOSTER, A. 0. The economic losses due tococcidiosis. Ann. N.Y. Acad. Sci. 52: 434.1949.

8. HAMMOND, D. M. Coccidiosis of cattle. Someunsolved problems. Faculty Honor Lecture.Logan: Faculty Ass. Utah State Univ. 1964.

9. KENDALL, G. R. and S. R. ANDERSON. Standarddeviations of monthly and annual mean tem-peratures. Climatological Studies No. 4. To-ronto: Meteorological Branch, Canada De-partment of Transport. 1966.

10. MARQUARDT, W. C. Subclinical infections withcoccidia in cattle and their transmission tosusceptible calves. J. Parasit. 48: 270. 1962.

11. MARSH, H. Healthy cattle as carriers of coc-cidia. J. Am. vet. med. Ass. 92: 184. 1938.

12. METEOROLOGICAL BRANCH, CANADA DEPART-MENT OF TRANSPORT. Temperature and Pre-cipitation Tables. Volumes I-VI. Toronto:1967.

13. RODERICK, L. M. Epizoology of bovine coc-cidiosis. J. Am. vet. med. Ass. 73: 321. 1928.

14. SWALES, W. E., D. W. BAKER, H. E. KEMPER,R. E. REBRASSIER and R. D. TURK. Report ofthe Committee on Parasitology. J. Am. vet.med. Ass. 113: 235. 1948.

15. THOMAS, M. K. Climatological Atlas of Can-ada. Ottawa: National Research Council, andDepartment of Transport. 1953.

16. WILLIAMS, T. L. Statistics of Agriculture,Alberta, 1966 and 1967. Publ. No. 853-4.Edmonton: Statistics Branch, Alberta Depart-ment of Agriculture. 1968.

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