A Review of Morbidity andMortality Rates andDisease Occurrence inNorth American Feedlot CattleAndrew P. Kelly and Eugene D. Janzen

Department ofHerd Medicine andTheriogenology, Western College ofVeterinary Medicine, University ofSaskatchewan, Saskatoon,Saskatchewan S7N OWO

IntroductionMorbidity and mortality rates and

their patterns are extremelyimportant epidemiological statistics. Asquantitative descriptions of diseaselevels and time of occurrence they givemajor insights into etiological and

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pathological processes. Many descriptiveepidemiological studies have reportedmorbidity and mortality rates in NorthAmerican feedlots. However, they arenot well standardized and considerablevariation occurs in the definition of rates.

In this review we have examinedmorbidity and mortality rates and otherepidemiological parameters reportedfrom recent literature on feedlot cattlein North America. Papers which con-

tained data that allowed the calculationofmorbidity or mortality incidence ratesfor feedlot cattle were examined. Theyspanned the period 1955-1984.

This review gives an outline ofdiseaseoccurrence in the feedlot industry. Theproblems of morbidity and mortalityrate definitions are discussed, andselected papers are used to examine therange of disease incidence rates incalves. Epidemic curves and other epi-demiological descriptions are reviewed,and the common clinical and necropsydiagnoses are noted.

Disease Occurrence inthe IndustryFeedlots are an integral part of NorthAmerican beef production. Beef calvestypically start their lives on breedingranches and remain with their mothersfor several months untiL after weaning,they are transported to a feedlot and ini-tiated into the more intensively managedhusbandry system. Occasionally cattledo not enter feedlots until their secondyear (yearlings). In either case, a periodofconsiderably increased disease occur-rence is recognized soon after the arrivalof cattle at the feedlots (1-7). The post-arrival disease peak consists largely ofrespiratory infections (2, 4, 5, 7-9). Thepeak occurs so reliably that feedlotmanagers usually observe calves closelyduring this time to allow early detectionand treatment of clinical cases. Simi-larly, the studies examined in this reviewconcentrated on the postarrival period.

Definitlon of Morbidity andMortality RatesAll morbidity rates encountered in thereview were incidence (or attack) rates.The simple method of calculating theincidence of morbidity in a group ofanimals is to count the animals whichdevelop an illness (cases) over a periodof time, and divide this number by thetotal number of animals in the group atthe start of the time period. If thepopulation-at-risk changes, an averagemay be used for the denominator(10-13). The result is the proportion of

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the group which fell ill, and its com-plement is the proportion which stayedhealthy. The mortality incidence rate issimilar, but only deaths are counted ascases.

Important variations may arise in themeaning of these terms when differentcase-definitions, time periods, and evendenominator definitions are used. Toenable meaningful summarization oftheliterature it was necessary to firstexamine these variations.

Variaton in DefinitionsThe papers reviewed were of a varietyof types. Many were experimental trialsexamining the effects of various treat-ments on health. Some were retro-spective surveys, others were prospectiveobservational studies, and some werestudies ofoutbreaks or high-risk diseasesituations. All used morbidity or mor-tality statistics as a measure of animalhealth, and in many it was the majordependent variable. Table I summarizes,in- chronological order, the location(state or province), the type ofstudy (asdefined above), and the type of cattleinvolved (calves or yearlings) in the30 reports examined.

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Case-definitions used for morbidityrates in these studies were often crudelydefined, and most depended on ananimal receiving chemotherapy as acriterion. In 15 of the reports (2,5,8,9,16, 18,21-24,31,32,34,35,37) "treat-ment" was used as a case-definition.For these, the morbidity rates calculatedcould be called "treatment rates". Eightpapers (1, 3, 14, 26, 27, 29, 30, 36)simply stated "sickness" or "morbidity"as a case-definition. Five others (17, 19,25, 28, 33) listed "respiratory disease"(some including a list ofsigns), and onestudy (15) used a high rectal temperatureas the sole definition. The remainingpaper (20) was concerned only withmortalities.

The time periods over which countswere made also varied. Twenty-seven ofthe studies commenced their obser-vations with the arrival of a knowngroup (cohort) of cattle in a feedlot,however, eight of these papers (2, 14,19,21,23,25,31,35) gave no indicationof the duration for which they thencounted cases! The time period of the19 others varied from 11 days to "thewhole feeding period", which is usuallya span of several months. The denomi-

nator used in calculating rates for allthese studies was the size of the groupunder observation.

In three studies (8, 20, 26) obser-vations were made over a set period ofcalendar time (for example, January toDecember 1974). These works used afeedlot population turnover figure forthe period of observation as a denomi-nator. This is a valid technique, pro-viding the observation period is longrelative to the disease occurrence periodunder investigation. Since all threestudies involved periods ofat least eightmonths, this denominator probablyapproximates the approach used in theother reports.

Morbidity and Mortalityin CalvesIn order to examine more closely themorbidity and mortality incidence ratesexperienced by calves in the immediatepostarrival period, selected papers wereused. All studies of calves, in nonout-break situations, were examined. Thosewhich used a case-definition of "treat-ment", "sickness", or "respiratorydisease", and which covered the firsttwo to ten weeks postarrival, wereselected.To summarize each study, an overall

morbidity or mortality incidence ratewas calculated for all cattle, regardlessofsubdivisions into experimental groupsif such were used. If the data allowed,the range ofmorbidity or mortality ratesof any sub-groups involved was alsocalculated. Table II summarizes the14 selected studies, in morbidity rateorder, including summary morbidityand mortality incidence rates andranges, the number of cattle and thenumber of sub-groups, the case defini-tion, and the time period involved.

In the selected studies, summarymorbidity incidence rates ranged from8% to 56%, with sub-group rates rangingfrom 0% to 69%. Most reports were inthe range 15% to 45%. Summary mor-tality incidence rates ranged from 0% to5.6%, with sub-groups ranging from 0%to 15%. Most reports were between 1%and 5%.

Epidemic CurvesA graph showing the changes inmorbidity or mortality rate with time iscalled an epidemic curve. It displays thedynamics of disease activity in a popu-lation and provides a basic epidemio-logical description (10-12, 38).

Literature on epidemic curves infeedlot cattle populations is rare. Martin

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(7) published summary treatment-ratecurves from the first five weeks post-arrival for a large number of groups ofcattle, and Woods etal (2) and Andrews(39) showed epidemic curves for res-piratory disease occurring over the firstfour weeks in small experimentalgroups. Other reports contained datawhich allowed extrapolation of roughepidemic curves for small groups (3, 15,16,34,40). The conclusion drawn fromall studies is that morbidity rates peak infeedlots in the first three weeks aftercattle arrive. This is followed by areduction to a lower rate, by four to fiveweeks, which is maintained through therest of the feeding period.

Studies using mortality instead ofmorbidity for epidemic curves offeedlotgroups, although sparse, show a similarpattern. Rothwell et al (6) published amortality rate curve for the first12 weeks postarrival showing a peakaround two weeks after arrival and adrop to a lower level by week 4. Jensenet al, in the Colorado study (4, 41-46),described in outline terms the epidemiccurves, throughout the period offeedlotresidence, for seven different diseaseconditions (shipping fever, atypicalinterstitial pneumonia, bronchiectasis,brisket disease, embolic pulmonaryaneurysms, fatal abomasal ulcers, andsudden deaths). Many of these condi-tions showed a fairly even distribution,but shipping fever, which accounted forthe largest number of fatalities, wasconcentrated in the early postarrivalperiod. Niilo et al (20) confirmed thispattern.

The common rise-and-fall pattern ofboth morbidity and mortality ratessupports one simple conclusion. Theregular occurrence of a disease peaksoon after cattle arrive at a feedlotdemonstrates the association betweenmanagement and disease. Investigationsinto this link are widespread and haveaddressed most aspects ofthe process oftransition of a calf from ranch-life tofeedlot, including weaning, transport,mixing and processing (2, 9, 22, 30,47-55).

Other EpidemiologicalDescriptonsApart from the basic disease descriptionparameters already discussed, there area host of less commonly reported fea-tures which may be used to describedisease patterns. These include othertemporal distributions of disease (sea-son, day of the week), the geographicaldistribution, and the distribution by typeof animal (sex, age, breed).

Seasonal patterns have been com-mented on by Jensen et al (26) whostated that morbidity and mortality rateswere highest in fall, less in winter, andleast in spring and summer. Patternsreported for respiratory conditions alsopoint to peak occurrence in fall (4, 39,56). However, it may be that theincreased proportion of newly-arrivedyoung calves at that time of yearexplains these findings, rather than atrue seasonal effect since none of thestudies controlled for this complication.The distribution of disease by day of

the week has not been described for

feedlot cattle. Geographic patterns havenot been reported either, although itwould appear from the papers reviewedthat, within North America, diseaseoccurs wherever there are feedlots.The types of animals affected by

disease within feedlots are also poorlydescribed. Breed and sex differenceshave not been well reported. Age isconsidered an important factor influenc-ing disease levels (57, 58), with youngercattle more disease-prone than older.Age effects appear in the literaturemainly by the distinction between"calves" and "yearlings" although theseterms are not always mutually exclusive.Jensen and Mackay (59) commentedthat shipping fever morbidity for calveswas about 25% compared to 3% to 4%for yearlings. Mortality rates weresimilarly related. Church (58) describeda similar pattern in a review of diseaselevels reported for calves and yearlingsbut none of the studies in his summarycompared the two age groups directlyand no conclusion was drawn about thedifferences involved.

Clinical DiagnosisA review of the clinical disease entitiesreported in feedlot cattle would resemblea textbook on cattle diseases. Recentliterature on the subject, both objectiveand subjective, suggests that a smallnumber of diseases dominate. A morecomplete review is available (59).

Respiratory diseases, particularlyshipping fever, are clearly the mostcommon. Jensen et al (4) reported thatrespiratory tract diseases accounted for75% of illnesses in a large survey. Apanel of American veterinarians (60)named the following disease entitiesmost commonly reported as the causesof disease in feedlots: bacterial pneu-monias, infectious bovine rhinotracheitis(IBR), other respiratory conditions,bovine virus diarrhea (BVD), parasitism,and clostridial diseases. In a review byMartin (7), pneumonia and IBR wereagain proposed as the most importantdiseases, and Church and Radostits inAlberta (8) found farmer diagnoses ofshipping fever, IBR, and footrot to bethe most common. Nonfatal diseasessuch as footrot and "bulling", whichtend to lose the limelight to more lethalconditions, also had a significant fre-quency (7, 8, 61).

Necropsy DiagnosesNecropsy surveys provide fairly object-ive information on fatal conditionswhich occur in feedlot cattle, although

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pathological diagnoses still suffer attimes from poor definition and fromconfusion in nomenclature.

Six main works have been reportedin this area in recent times. In 1967Niilo et al (20) published a necropsy.survey ofdead cattle from four southernAlberta feedlots. Jensen etalreported aseries of results from a similar survey inColorado in 1976 (4, 26, 41-46). Morerecently Rothwell etaldescibed a singlefeedlot survey (6), Church and Radostits(8) reported on farmer diagnoses ofcause of death from 24 Albertan feed-lots, and Martin et al (9) gave results ofa large observational study in Ontario.Finally, Hjerpe (62) described therespiratory conditions he found among2,000 necropsies.

These studies are difficult to comparebecause ofthe variety ofreporting styles,however, simple categorization intomajor body systems, as used by someauthors (6, 8), allows rough summari-zation. In all papers the system mostcommonly named as the cause ofdeathwas the respiratory system. The percen-tage ofdeaths due to respiratory lesionsvaried from 31% (20) to 71% (8) and allauthors stated that the principal lesionwas pneumonia.The second most commonly affected

system in the four papers which reportedfull data (6, 8, 20, 26) was the ali-mentary tract. From 10% (6) to 22%(20) of fatal conditions involved thissystem, but the diagnoses were diverse.The nervous, musculoskeletal, urogeni-tal, and cardiovascular systems eachaccounted for lower proportions rangingonly as high as 11%.

Summary and CondusionsThe North American feedlot industry,like any intensive animal industry,suffers from disease problems. A majorarea of concern is the peak of diseaseincidence, due mostly to respiratoryinfections, which occurs in the periodsoon after calves arrive in the feedlot.Around 15% to 45% ofincoming calvesrequire treatment, and around 1% to 5%die.

Deficiencies exist in current know-ledge of the basic epidemiologicalbehavior of disease in feedlots. Mor-bidity and mortality rate calculationsare ill-defined. The common use ofcrude case-definitions for morbiditycounts such as "treated" or "sick"leaves scope for large differencesbetween studies or even observers. Theiruse may introduce a large amount ofsubjectivity to the measurement of

morbidity. The failure to clearly state atime period when quoting a rate isreprehensible. Epidemiological descrip-tions other than morbidity and mortalityrates, such as epidemic curves, arelargely undescribed.Much clinical and pathological

groundwork has been done to establishbasic data on feedlot health problems.Laboratory investigations into thecause and prevention of the majordiseases are common. However, tomaintain a logical progression ofresearch, more epidemiological infor-mation is necessary.

AcknowledgmentsThe authors thank Drs O.M. Radostitsand C.S. Rhodes, Western Collegeof Veterinary Medicine, Saskatoon,Saskatchewan and Dr. R.G. Thomson,Atlantic Veterinary College, Charlotte-town, Prince Edward Island forguidance and support.

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