A cross-sectional study of the causes of morbidity and mortality in farmed white-tailed deer

Abstract

Two questionnaires were designed and administered. The first was to a random sample of 340 farmers of white-tailed deer (Odocoileus virginianus) in Canada and the United States. The second was a 10-year retrospective survey of deer submissions to veterinary diagnostic pathology laboratories in Canada and the United States. One-year rates of mortality and common causes of morbidity and mortality for the deer are reported. The primary diagnosis for each record was used to classify diseases into categories, such as parasitic, infectious, toxicological, and neoplastic. Submissions were further classified according to the anatomical location, the pathological change, and the etiology associated with each lesion. Trauma was the most important reported cause of farmed white-tailed deer mortality; necrobacillosis was a major cause of morbidity and mortality, especially in fawns.

Introduction

Most of the existing information on diseases of farmed deer has been generated in publications about red deer and wapiti (1–5). Some publications about fallow deer are also available (6–10). The most comprehensive source of information about diseases of white-tailed deer (WTD) deals primarily with free-ranging animals (11). There is little material available about diseases of farmed WTD. In his 1877 book, Caton (12) refers to diseases of captive deer and gives a graphic description of what was probably necrobacillosis. Diseases of captive WTD are also discussed in a wider paper that covers cervids in Michigan (13).

The purpose of this project, which involved a survey of producers and a survey of submissions to laboratories, was to report the causes of sickness and mortality in farmed WTD in Canada and the USA and to estimate the age and sex-specific mortality rates, reproductive performance, productivity, and effect of mortality on productivity of farmed WTD for the 1999 to 2000 production year. The precise number of farmers of WTD is not known, but Jacobson (14) has reported that there were 4011 such farms in 15 American states and 6 Canadian provinces. The Canadian figures (537 farms) reported by Jacobson (14) tally closely with those reported by Haigh (15) (536 farms) whose data were derived from information provided by government staff who used compulsory registration figures.

In the producer survey, farmers were asked to name diseases that caused sickness or mortality on their farms, both during and before the study period. For various reasons, including farmers failing to have sick animals or mortalities examined by veterinarians, it was considered that farmers might be unable to report accurate diagnoses of sickness and mortality. In order to more accurately report specific causes of mortality in farmed deer, a 10-year survey of submissions to veterinary diagnostic pathology laboratories in Canada and the western USA was also conducted. Although it is recognized that surveys of submissions to diagnostic pathology laboratories are not representative of all causes of mortality, it was felt that the combination of a farmer and a laboratory survey would provide a more complete report of causes of mortality, than either survey alone.

Materials and methods

Producer survey

For the producer survey, 340 WTD producers were randomly selected, as described by Haigh et al (16). The format of the questionnaire followed Dillman’s recommendations for maximum response to postal questionnaires, which involves the sending out of 4 carefully timed mailings (17). Each section of the study was designed to gather different types of observational data. Productivity and reproductive performance statistics on WTD farms have been reported elsewhere (16).

A sub-set of 35 questions from the questionaire were designed to gather data about morbidity and mortality from all causes, by asking farmers to name diseases that caused sickness and death among their WTD during the period of the study. From these data, the relative importance of various causes of disease or disease syndromes were estimated for the study period. Farmers were also asked to provide the names of any diseases or disease syndromes that their WTD had experienced during previous years. Data from these questions were used to estimate the commonality of disease causes and disease syndromes. Since the period of the study was 1 production year, morbidity and mortality rates were calculated for adults and yearlings as percentages over a 1-year risk period.

When questions seeking the same information were asked in different ways and gave inconsistent results, the data were discarded.

The at-risk study population was stratified according to age and sex, and mortality rates were calculated for each age and sex class over the 1-year period of the study. Morbidity and mortality rates were calculated for adults and yearlings over the 1999 to 2000 production year and expressed as percentages. Cohort specific morbidity and mortality rates for calves born in 2000 were not based on 1-year periods; rather, they were based on specific time periods of risk. For example, neonatal mortality rates were calculated as a proportion of those fawns born during the 2000 fawning season that died within 1 mo after birth.

Pathology laboratory survey

Forty-two veterinary diagnostic pathology laboratories in the USA and Canada were contacted and asked to participate in the study. Of those laboratories willing to participate, 8 were selected for inclusion, based on the number of deer laboratory submissions that they had received over the period from 1990 to 2000 and on their ability to extract useful information from their records. These included laboratories from Alberta, British Columbia, Colorado, Minnesota, North Dakota, Saskatchewan, and South Dakota. Visits were made to 3 laboratories and detailed information was obtained from the others.

Submissions that included more than 1 dead animal were entered as a single record and each submission was considered as 1 disease outbreak, without regard to size. Data was extracted from each record and coded into a database (Microsoft Access; Microsoft Canada, Mississauga, Ontario). To maintain client confidentiality, individual pathology submissions were identified only by using an identification number. Keystone information stored for each record included submission date, age, sex, species, and primary cause of mortality. The primary diagnosis for each record was used to classify records into disease categories, such as parasitic, infectious, toxicological, neoplastic, etc. In addition, 3 lesions were recorded for each submission, based on the anatomical location, the pathological change, and the etiology associated with each lesion. Database entries were checked twice against the original records for the accuracy of each entry.

Results

Farmer responses

One hundred and two farmers responded to the survey. The response rate for the deer questionnaire was 30.0% overall, with 25.4% of US deer farmers and 33.3% of Canadian deer farmers responding. The total number of deer included in responses about morbidity and mortality was 5035, and the average herd size was 49 deer.

Mortality and morbidity rates among various age and sex cohorts of deer are shown in Table 1. The specific causes of death and disease are listed in Tables 2 and 3. Table 2 shows combined data for all deer; Table 3 is specific to fawns. In the combined data set (Table 2), trauma was the greatest cause of problems. In Table 4, the breakdown of handling related injuries according to age cohort is shown. All classes of deer are represented in this table, with yearling bucks having the highest percentage of injuries (7.6% of handled yearlings were injured).

Table 1.

One-year mortality data for various age and and sex cohorts of farmed white-tailed deer

	Fawns	Yearling male	Yearling female	Adult male	Adult female
Number of farms	69	77	72	86	84
Deer at risk	1603	586	505	998	1343
Number of deer deada	478	37	18	53	56
% mortality	29.8%	6.5%	3.6%	5.3%	4.2%
Range (%)	0% to 100%	0% to 100%	0% to 100%	0% to 100%	0% to 50%

^a 74.5% of all deaths occurred in fawns

Table 2.

One-year farmer-reported causes of morbidity and mortality for white-tailed deer adults, yearlings, and fawns combined. The number and percentages (%) of dead and sick deer are shown. The number of outbreaks of each disease and the number of farms that consulted a veterinarian to diagnose or manage each disease (Vet) are listed

Disease name	Number dead (%)	Number sick (%)	Number of reports	Vet
Trauma	118 (23.7)	20 (7.1)	58	19
Necrobacillosis/lumpy jaw	107 (21.5)	112 (39.9)	10	7
Pneumonia	38 (38.0)	22 (7.8)	17	13
Salmonellosis	31 (6.2)	28 (10.0)	3	3
Exposure	19 (3.8)	6 (2.1)	4	1
Dystocia	11 (2.2)	1 (0.4)	6	3
Stillborn	8 (1.6)	3 (1.1)	5	1
Calf scours	8 (1.6)	8 (2.8)	3	1
Grain overload	7 (1.4)	5 (1.8)	4	3
Coccidiosis	6 (1.2)	4 (1.4)	3	2
Predation	7 (1.4)	0(	3	0
Abscess	6 (1.2)	10 (3.6)	1	1
Othera	18 (3.6)	8 (2.8)	23	18
Unknown	114 (22.9)	54 (19.2)	56	19
Total	498	281	196	91

^a Others include: Abandoned fawn, abomasal ulcer, abortion, bloat, capture myopathy, choke, congenital anomaly, cryptosporidiosis, diarrhea, Escherichia coli, fescue poisoning, foot rot, drowning, dam no milk, drug reaction, intestinal accident, lightning strike, listeriosis, neoplasia, old age, parasites, predation, and unspecified infection

Table 3.

One-year farmer-reported causes of morbidity and mortality for white-tailed deer fawns. Included are the number of dead deer, sick deer, and the number of reports of outbreaks of each disease. The number of outbreaks of each disease and the number of farms that consulted a veterinarian to diagnose or manage each disease (Vet) are listed

Condition	Dead deer (%)	Sick deer (%)	Reports	Vet
Necrobacillosis	103 (29.7)	102 (47.0)	6	4
Trauma	56 (16.1)	4 (1.8)	14	4
Pneumonia	30 (8.6)	19 (8.8)	10	7
Salmonellosis	28 (8.1)	28 (12.9)	1	1
Exposure	19 (5.5)	5 (2.3)	3	0
Stillborn	8 (2.3)	3 (1.4)	5	1
Calf scours	8 (2.3)	8 (3.7)	3	1
Grain overload	6 (1.7)	4 (1.8)	3	2
Othera	13 (3.7)	10 (4.6)	14	8
Unknown	76 (21.9)	34 (15.7)	27	7
Total	347	217	86	35

^a Others include: Abandoned fawn, abomasal ulcer, abortion, bloat, congenital anomaly, dam no milk, listeriosis, and predation

Table 4.

Injuries and deaths related to handling in farmed white-tailed deer as reported by farmers

	Fawns	Yearling male	Yearling female	Adult male	Adult female	Total
Number of farms	77	60	56	67	69	—
Deer at risk	1574	473	442	710	1089	4288
Number injured (%)	41 (2.6)	36 (7.6)	17 (3.8)	40 (5.6)	50 (4.6)	184 (4.3)
Range %	0 to 100	0 to 60	0 to 100	0 to 100	0 to 100	—
% died acutely	3.2	3.6	0.2	0.9	0.9	—
Range %	0 to 100	0 to 43.5	0 to 10	0 to 20.0	0 to 16.7	—
Number died following injury (%)	20 (48.8)	17 (47.2)	4 (23.5)	16 (40.0)	12 (24.0)	—
Range %	0 to 100	0 to 100	0 to 66.7	0 to 100	0 to 100	—
Number died of CM (%)	23 (8.7)	18 (5.6)	0	17 (11.8)	12 (41.7)	70
Range %	0 to 50	0 to 100	—	0 to 50.0	0 to 100	—

CM — capture myopathy

Pathology laboratory responses

A total of 505 WTD were submitted for pathologic diagnosis. Of these, 194 were female, 153 were male; in 158 cases, the sex was not recorded. There were 4 fetuses. The ages of the other deer were as follows:<24 h, 5 cases; 24 h to 7 d, 8 cases; 8 d to 30 d, 36 cases; 31 d to 1 y, 97 cases; 1 to 2 y, 53 cases; and >2 y 161 cases: In 141 instances, ages of submitted deer were not recorded. The primary pathologic diagnoses are listed in Table 5.

Table 5.

Primary diagnosis of white-tailed deer submissions to diagnostic pathology laboratories, sorted by number of cases for each primary diagnosis

Primary diagnosis	Fawns< 30 d	Fawns 30 d to 1 y	Deer > 1 y	Age not recorded	Total cases
Total cases	49	97	214	141 + 4 fetuses	505
Trauma	10	12	28	12	62
No diagnosis	5	11	31	14	61
Pneumonia	4	4	29	7	44
Enteritis/diarrhea	—	16	12	13	41
Emaciation	—	9	6	13	28
Abscess	—	3	10	8	21
Parasitism	—	3	4	9	16
Malignant catarrhal fever	—	1	9	5	15
Septicemia	5	3	2	4	14
Grain overload	—	3	5	3	11
Necrobacillosis	3	5	3	0	11
Encephalopathy	—	2	4	4	10
Neoplasia	—	—	1	7	8
Capture myopathy	—	1	2	5	8
Pulmonary edema	—	—	2	6	8
Othera	22	24	66	35	147

^a Others include, in descending order of frequency, copper deficiency (n = 7), polioencephalomalacia (n = 7), encephalitis (n = 6), meningitis (n = 6), nutritional muscular dystrophy (n = 5), epizootic hemorrhagic disease (n = 5), kidney disease (n = 5), rumenitis (n = 5), abortion (n = 5), Johne’s disease (n = 4), intestinal accident (n = 4), laryngitis (n = 4), hepatitis (n = 4), pleuritis (n = 4), hemmorhagic gasteroenteritis (n = 3), myocarditis (n = 3), starvation (n = 3), cellulitis (n = 3), necrotic enteritis (n = 3), osteomalacia (n = 3), cardiomyopathy (n = 3), intestinal hemorrhage (n = 2), abomasal ulcer (n = 2), heat stroke (n = 2), abomasitis (n = 2), diaphragmatic hernia (n = 2), infectious arthritis (n = 2), congenital anomaly (n = 2), entertoxemia (n = 2), lymphadenitis (n = 2), liver fatty/lipidosis (n = 2), peritonitis (n = 2), liver flukes (n = 2), myopathy (n = 2), metritis (n = 2), caseous lymphadenitis (n = 1), rumen impaction (n = 1), rumen ulcer (n = 1), pericarditis (n = 1), and unspecified arthritis (n = 1)

There were 44 reports where pneumonia was the primary diagnosis. No pathogen was named in 18 of these. The pathogens involved were as follows: Arcanobacterium pyogenes (n = 10), Mannheimia haemolytica (n = 4), Escherichia coli (n = 3), Pasteurella multocida (n = 2), Aspergillus spp., unspecified Pasteurella sp., and Fusobacterium necrophorum (n = 1). In addition, unspecified bacteria were listed in 2 reports, and foreign bodies were recorded twice.

There were 20 deaths due to parasitism with a recognized cause: coccidiosis (n = 8), liver fluke (Fascioloides magna) (n = 2), strongylosis (n = 4), trichostrongylosis (n = 3), specific infections with Strongyloides spp. (n = 1), Trichuris spp. (n = 1), and Parelaphostrongylus tenuis (n = 1).

Other than the gastrointestinal parasitic agents, the following bacterial pathogens were implicated in cases of enteritis and diarrhea: E. coli (n = 4), Clostridium perfringens (n = 2), Salmonella spp. (n = 2), and Yersinia sp. (n = 1). Viruses reported in cases of enteritis and diarrhea included rotavirus (n = 4), adenovirus (n = 1), and parvovirus (n = 1).

A total of 32 abscesses were diagnosed at necropsy (21 as primary diagnoses). In 17 of these, no bacteria were isolated. The remainder were classed as follows: F. necrophorum (n = 11), A. pyogenes (n = 2), Yersinia sp. (n = 1), and Clostridium sp. (n = 1).

Discussion

It is well recognized that data generated by surveys completed by farmers are likely to give some information about the prevalence of mortality and common conditions but suffer from the risk of inaccurate diagnosis, whereas those from surveys of pathology laboratory submissions may suffer reporting bias and be unrepresentative of the general population. Since farmers may not have postmortem examinations performed on all mortalities, the proportions of diseases measured by pathologic surveys are often not the same as the proportions of disease occurring within the general population. For the purposes of this study, surveying pathology laboratories provided a broader database of disease etiologies and some support for the validity of disease etiologies reported by farmers, when they responded to the questionnaires.

Surveys of submissions to pathology laboratories, however, provide valuable information to farmers, veterinarians, and diagnostic pathologists. Although pathology survey results may not be truly representative, they do provide rough estimates of the importance of various diseases as causes of mortality. Especially for farmed deer species, such as WTD, in which published information about etiologies and occurrence of disease is sparse, pathology surveys identify, report, and alert farmers, veterinarians, and pathologists to diseases that previously they may not have been aware of. Focusing attention on diseases that have either been previously unreported or may be newly emerging within cervid populations allows interested groups to target efforts at prevention or control of these diseases at an early stage.

Trauma is recognized as a major cause of both morbidity and mortality in farmed wapiti and red deer (5,18–21). In a survey of 191 cervid farmers (5493 animals, of which 90.5% were WTD) in Michigan, a 1-year period prevalence rate of 1.9% and a case fatality rate of 52% of injured deer were recorded (15). Data from our survey (Table 2) show that almost a quarter of all deaths reported by farmers were due to trauma, most of which occurred during handling (Table 4); and that between 23% and 49% of deer injured during these procedures went on to die. Trauma was the most common primary diagnosis at pathology laboratories.

It is likely that the design of handling systems and inappropriate human actions made a significant contribution to these injuries; for example, on a farm where stockmanship was of a high order and a handling system had been designed with low-light sheds and a padded chute (22), only 2 handling-related deaths occurred over a period of 10 y. Many hundreds of handling events took place during this period (Friesen R. personal communication). It is incumbent upon WTD farmers to ensure that good handling systems are in place and properly trained individuals are working with the animals, because much of the responsibility for successful (or unsuccessful) outcomes rests with the humans involved (23). Good handling will increase profit margins by reducing both injuries and stress related disease.

When data for all ages of deer were combined, the second most common cause of death and morbidity reported by farmers was necrobacillosis. The data indicate that fawns are particularly susceptible. When unknown causes of death and morbidity are removed from calculations, this condition alone caused 38% of deaths and 56% of sickness in fawns.

Necrobacillosis is well known in both wild and captive deer populations, and it is a common disease condition in numerous other species (3,5,8,9,24). The disease has been reported to be of particular significance in farmed WTD (25). What is probably the first report of necrobacillosis in WTD appeared in the 1877 book The Antelope and Deer of North America by Caton (12), in which he states, “I have lost many Virginia deer with a swelling under the lower jaw. It commences two or three inches back of the chin, and finally swells out so as to involve the whole head below the eyes; sometimes it gathers in a sac of half an ounce of pus-like matter, one of which I opened, but the deer died.” He later goes on to say that if he detects it early enough and opens the lesion the “deer gets well at once.” A useful review of the history of this condition and its occurrence in WTD is provided in Wobeser et al (26).

It is likely that farmers of WTD have become all too familiar with necrobacillosis, which would account for the apparently low frequency of its diagnosis in pathology laboratories, as most farmers will recognize it and choose not to submit carcasses or samples, which will cost them money.

Tuberculosis and chronic wasting disease, 2 diseases of importance to WTD in North America, were not reported in the survey. The former has been documented in farmed deer in Michigan in 1997 (27), and the latter had not been reported in farmed WTD at the time that the survey was conducted. The number of farms on which either of these conditions has been reported in WTD is small, and as our survey return rate of 30% of 340 farms initially approached was only a fraction of the estimated 4000 plus WTD farms (>71 000 animals) in Canada and the USA, we had less than a 2.5% chance of receiving reports of either condition.

Some of the data from both surveys may not have provided accurate diagnoses, recall bias and diagnostic errors may have skewed the information. Nonetheless, the surveys provided a useful snapshot of WTD morbidity and mortality. If injuries could be reduced by improved handling methods, and if necrobacillosis could be better controlled through improved management and possible use of vaccines, profitability would rise and welfare concerns would be much reduced.

Data from both sets of questions about causes of disease provided a list of diseases ranked by importance that can be used by farmers and veterinarians as a list of “most likely causes” when they are faced with disease outbreaks on WTD deer farms. This list will allow farmers and veterinarians to be more selective of diagnostic tests and treatment programs when managing disease outbreaks. Making this list available to researchers studying cervid diseases will allow them to focus their efforts on diseases that are most common or economically significant on WTD farms.