Herd Incidence Research Articles

Evaluating potential sources of bovine tuberculosis infection in a New Zealand cattle herd

In New Zealand, a major source of bovine tuberculosis (Mycobacterium bovis, Tb) infection in farmed cattle is thought to be transmission from infected wildlife, primarily the brushtail possum (Trichosurus vulpecula). At Hohotaka, central North Island, control work from 1988 to 1994 achieved a sustained mean reduction of 87.5% in the density of Tb‐infected possums. As expected, annual Tb incidence in local cattle herds consequently declined by a similar amount (83.4%). This decline was gradual, due to within‐herd transmission of residual infection and imperfect detection of infected animals. By fitting models to the observed rate of decline in Hohotaka Tb reactors, we are able to deduce the contributions to reactor rates of possums and cattle‐to‐cattle Tb transmission. Our results suggest that Tb transmission between cattle within the herd may have contributed around 20% to 32% of infections prior to the wildlife control programme. Nevertheless, our estimate for the rate of within‐herd transmission is well below that required to maintain cattle Tb in the absence of infectious wildlife. Our analysis supports the policy of wildlife control as the most effective method for reducing cattle Tb in areas such as Hohotaka.

Nutritional risk factors in Setaria anceps for milk fever incidence in dairy herds of the humid tropics

Setaria anceps is a pasture grass commonly utilized in dairy farms in the humid tropics. It is noted for its good dry matter yield and nutritional quality. However, a high incidence of milk fever among dairy cows grazing this pasture has been claimed by the Costa Rican farmers. We therefore collected and analyzed 48 samples of S. anceps in the Quesada district of San Carlos county in Costa Rica to determine the mineral composition and potential milk fever risk factors associated with consumption of this pasture. An equal number of samples of the pasture Cynodon nlemfuensis were taken as a control. This is the most common pasture of the area and a low incidence of milk fever has been related to its consumption. Forage samples were measured for Ca, P, Mg, K, Na, S and Cl and the Cation–Anion Balance (CAB, mEq kg −1 of DM) was calculated as [(Na ++K +)−(S 2−+Cl)]. Values for Ca, P, Mg, K, Na, S and Cl in S. anceps were, 0.25, 0.19, 0.24, 3.5, 0.55, 0.17 and 0.84% of dry matter and for C. nlemfuensis 0.31, 0.22, 0.24, 2.67, 0.04, 0.27 and 0.59. CAB was 861 mEq kg −1 of DM in S. anceps and 500 in C. nlemfuensis. The significantly ( ** P<0.01) higher CAB, K and Na values, and lower ( ** P<0.01) S values in S. anceps strongly suggest that its electrolyte composition and balance could be key nutritional risk factors increasing the susceptibility of dairy cows to milk fever incidence. Since average calcium consumption in the area is in the order of 45 g cow −1 day −1, cows grazing S. anceps should be offered low calcium diets (less 20 g cow −1 day −1) to reduce their susceptibility to milk fever.

Herd Prevalence and Incidence of Streptococcus agalactiae in the Dairy Industry of Prince Edward Island

Herd prevalence and incidence of mastitis caused by Streptococcus agalactiae was determined for dairy cattle on Prince Edward Island during December 1992 and June 1994. For each census, bulk tank milk samples from all dairy herds (n = 452) in the province were tested on two occasions, and the results were interpreted in parallel. The combined sensitivity of the testing protocol was estimated to be 91%.The confirmatory latex agglutination test had previously reported specificities approaching 100%. Therefore, the estimated specificity of the testing protocol was assumed to be 100%. The apparent prevalence of S. agalactiae in December 1992 and in June 1994 was 17.7 and 13.1%, respectively. Based on the characteristics of the test, the estimated true prevalence was 18.9% in December 1992 and 14.4% in June 1994. Infection with S. agalactiae was associated with elevated bulk tank somatic cell count (SCC) and elevated standard plate counts. Economic losses associated with S. agalactiae were attributed to production losses (associated with bulk tank SCC), milk quality penalties (associated with bulk tank SCC and standard plate count), and decreases in milk quality (associated with bulk tank SCC). For herds that had been negative for S. agalactiae in December 1992, evaluation in June 1994 yielded an incidence of new infections of 3.51 per 100 herds per year.

BVD Vaccination, some common sense approaches

Our knowledge of the bovine virus diarrhea virus is still changing, even though we have known about the virus for many years. One of the things we have learned is that the virus has the ability to mutate readily. The "Veterinary Medicine" BVD review from October 1990 gives a good reference although it is already becoming outdated. The incidence of infected herds appears to be on the rise. This is seen primarily in the dairy herds and purebred beef herds. The BVD viruses appear to have the ability to be tissue specific. Different strains may affect certain organs in the body giving rise to a whole host of syndromes caused by the BVD virus.

Climatic factors associated with the infection of herds of cattle with bluetongue viruses.

The incidence of bluetongue virus infection of 15 cattle herds in Queensland, Australia, was determined by a serum neutralization test. The maximum temperature (degree C), minimum temperature (degree C) and rainfall (mm) data were obtained from the meteorological recording stations closest to each herd. Using unweighted least-squares regression analysis, the best statistical model explaining the most variability in the herd incidence rate included the ratio between the maximum and minimum temperature recorded at both 1 month and 6 months preceding seroconversion, and rainfall recorded at both 2 months and 6 months preceding seroconversion. More than 90% of the variability in the incidence of bluetongue virus infection in the herds was explained by the model, a considerable improvement on previous models that used prevalence data. The prospective nature of the study also supports a strong causal relationship between climatic factors and the occurrence of infection in cattle herds.

Cow, farm, and ecologic risk factors of clinical vesicular stomatitis on Costa Rican dairy farms.

Twenty-two Costa Rican dairy herds were actively monitored for vesicular diseases between November 1991 and March 1992, in anticipation of the annual dry season occurrence of vesicular stomatitis (VS). Thirty-nine confirmed cases of VS were concentrated on nine farms, resulting in an average herd incidence rate of 11.1% of mature cattle for the affected farms, or 2.6% for all farms. Affected cattle were generally older (average age = 5.4 years), with seven-year-old cows having the highest age-specific incidence rate (6.5%). The New Jersey serotype of VS was diagnosed for all but four cases, primarily manifesting as teat vesicles. Cases had high acute and convalescent antibody titers for both major VS serotypes, New Jersey and Indiana, with significant titer increases after clinical disease. Screening and multiple variable regression analyses of 140 cow, farm, and ecologic variables revealed that clinical disease was significantly associated with cows being in lactation, and with higher acute antibody titers to serotype Indiana, but not with any predisposing diseases. Significant farm factors were the presence of poultry and a longer calving interval on the farm, but not higher rates of other diseases. According to a prior epidemiologic hypothesis, two ecologic factors were forced to be included into the farm model: the reported presence of sand flies, and a higher proportion of the farm being in forest land, with subsequent stepwise regression. The resulting model containing only the two forced variables was significant (P < 0.003) and correctly classified 78% of farms into the correct group, as compared with 79% for the final stepwise model.(ABSTRACT TRUNCATED AT 250 WORDS)

Infection of cattle with bluetongue viruses in Queensland, Australia: results of a sentinel herd study, 1990–1992

Between 1990 and 1992, 47 sentinel herds of 10–20 cattle each were established throughout Queensland, Australia to monitor bluetongue virus infection. Sixteen herds at 12 locations seroconverted to bluetongue viruses during the study. Herd incidence rates ranged from 0.0 to 3.45 seroconversions per cattle-year at risk. The mean incidence rate was 0.29 seroconversions per cattle-year at risk (95% confidence interval 0.23–0.36), and the median incidence rate was 0.32 seroconversions per cattle-year at risk (95% CI 0.0–0.54). Only seroconversions to bluetongue virus serotypes 1 and/or 21 were detected in the study; no evidence was found of infection with other serotypes previously isolated in Australia (3, 9, 15, 16, 20, 23). Seroconversion to serotype 1 occurred more frequently than seroconversion to serotype 21. Seroconversion generally occurred between April and July, being most common in May, but varied considerably within each of the 3 years of the study. This study confirmed the presence of only serotypes 1 and 21 in Queensland and demonstrated the sporadic and restricted nature of bluetongue virus infection in the Queensland cattle herd.

Clinical Mastitis in Cows Treated with Sometribove (Recombinant Bovine Somatotropin) and Its Relationship to Milk Yield

Effect of sometribove (methionyl bovine somatotropin) on mastitis in 15 full lactation trials (914 cows) in Europe and the US and 70 short-term studies (2697 cows) in eight countries was investigated. In full lactation studies, sometribove (500 mg/2 wk) was given for 252 d, commencing 60 d postpartum. Although herds varied considerably, incidence of clinical mastitis within a herd was similar for cows receiving control and sometribove treatments. Relative risk analyses indicated no treatment effect, and percentage of mastitis during treatment was similar for control and sometribove groups. A positive linear relationship existed between peak milk yield and mastitis incidence (percentage of cows contracting mastitis or cases per 100 cow days); sometribove treatment did not alter this relationship. Increases in mastitis related to milk yield increase from sometribove or related to genetic selection were similar. When expressed per unit of milk, mastitis incidence declined slightly as milk yield increased; this relationship was not altered by sometribove. No effect on clinical mastitis was observed in 70 commercial herds utilizing sometribove for 84 d. However, effects were significant for stage of lactation and milk yield. Overall, studies represented a wide range of research and commercial situations demonstrating that sometribove had no effect on incidence of clinical mastitis during the lactation of treatment. Furthermore, sometribove did not alter typical relationships between milk yield or herd factors and incidence of clinical mastitis.

A practice-based survey of the frequency of Johne's disease in south west England.

The frequency of Johne's disease in cattle in south west England was estimated from data collected by telephone interviews with veterinarians and farmers. The response rate was 81.6 per cent. The disease frequency was expressed as the proportion of farms with clinical disease and the cumulative incidence in the infected herds. The proportion of farms affected was 1.0 per cent and the cumulative incidence on those farms was 1.9 per cent per year. Similar values were obtained when diagnosis by faecal examination, post mortem examination and histology was taken into consideration; 0.9 per cent of farms were affected and the cumulative incidence in the infected herds was 2.0 per cent per year. The survey was validated against three external reference points. There was good agreement between the use of vaccine and MAFF records, and the total number of holdings and census data. When the responses of the veterinarians were compared with those of farmers there was also good agreement on the use of vaccine (kappa = 77.8 per cent), the number of cases reported in the last year of diagnosis (r = 0.78) and the total number of cattle in the herds (r = 0.75). However, the results suggested that the total number of cattle holdings was overestimated and consequently the proportion of farms affected may have been underestimated.

Estimation of Herd Incidence of Infection with Bovine Virus Diarhoea Virus (BVDV) in Herds Previously without Animals Persistently Infected with BVDV

On a previous occasion, all animals in 9 herds had been bled for bovine virus diarrhoea virus (BVDV) and antibodies. No animals persistently infected (PI) with BVDV were detected. Three years later 10 animals in each herd were tested again. By this time 60 out of 90 previously seronegative animals had seroconverted. Seroconversions had occurred in 8 of the 9 herds corresponding to a incidence risk of herd infection of 0.52 per year. The incidence risk of seroconversions in individual animals was 0.31. Examination of young stock for antibodies and determination of antibody titer in bulk milk were good indicators for ongoing infections in the herds. The number of herd infections seemed to be higher than could be explained from purchase of PI animals.

Environmental and managerial determinants of somatic cell counts and clinical mastitis incidence in Ohio dairy herds

A stratified-random sample of 48 Ohio dairy herds participated in a 1-year disease monitoring survey to study herd management and environmental conditions affecting udder infection and clinical mastitis incidence. The mean monthly bulk-tank somatic cell count was used as an indicator of overall udder infection. Clinical incidence was determined by monthly on-farm interviews with the dairy producers. Management and environmental conditions were assessed by direct observation as well as by personal interview of dairy managers. The final multivariable analysis-of-variance model of log bulk-tank somatic cell count had an R 2 value of 0.43. Lower log bulk-tank somatic cell count was found in herds with hired milkers, a clean and dry cow exercise area, clean teats following milking and fewer milking cows. The number of months spent on pasture was also significant. The final model for clinical mastitis incidence had an R 2 value of 0.38. Less clinical mastitis was found on farms where straw bedding was used, pre-dip was not used, where there were fewer cows, fewer person-hours per cow were spent milking cows, a greater percentage of calvings occurred in the designated calving facility, and cows spent fewer months per year on pasture. Other potentially important disease determinants could not be included in the final models because of limited sample size relative to the model degrees of freedom (six each).

Epidemiological and Genetical Studies in Norwegian Pig Herds

Herd differences in disease incidence in 70 Norwegian pig herds were assessed by health card registrations. Such differences proved to be considerable. The incidence of the MMA-syndrome, mastitis, metritis and neonatal diarrhoea was higher in herds producing only weaners ("weaner herds") than in herds with combined production. The mean disease incidence in herds providing breeding stock for the Norwegian Pig Breeders' Association ("breeding herds") was at the same level as in ordinary combined production herds. There was no consistent relationship between changes in disease incidence and herd size, though the incidence of the MMA-syndrome was lower in the largest herds.

Outbreaks of border disease in goats induced by a pestivirus-contaminated orf vaccine, with virus transmission to sheep and cattle

Five herds with a total of 276 female goats experienced severe outbreaks of reproductive failure due to a pestivirus-contaminated experimental orf vaccine given early in the breeding season. The reproductive failures comprised barrenness in 42 goats, abortion in 53 and, in 118, the birth of dead or weak kids. The incidence of female goats with such failures was 82 per cent overall, herd incidence rates ranging from 79 to 96 per cent. No progeny showed characteristic signs of border disease (BD). Microscopic lesions in the CNS were moderate, mostly characterized by hypercellularity, cell disorganization and decreased myelin content. Non-cytopathic strains of pestivirus were demonstrated in tissue or serum from two weak-born and two apparently healthy kids. Neutralizing antibodies against a bovine pestivirus were found in all 254 goats examined about 4 months after vaccination. At the end of the breeding season, all kids were removed and 74 young kids were introduced from presumably normal herds. Approximately 8 months later, 64 (86 per cent) of the bought-in kids had pestivirus antibodies. Two years after the outbreaks, such antibodies were found in all but three of 127 vaccinated goats still alive, and another 2 years later, in all 22 vaccinated goats in one of the herds. Sheep also were kept on the same five farms and cattle on one. Sheep in two of the flocks showed reproductive failure probably caused by pestivirus transmitted from infected goat offspring. A pair of twin lambs showed nervous signs and brain lesions characteristic of BD, while 17 other lambs had a very low growth rate. Virus was not isolated from any lamb. The prevalence rates of ewes with pestivirus antibodies in the two affected flocks were 33 and 72 per cent, respectively. One of nine cows aborted a pestivirus-infected foetus, and all were antibody-positive.

The incidence, cost and factors associated with foot lameness in dairy cattle in south-western Victoria.

A survey of 73 dairy farms in south-western Victoria was conducted to assess the cost and mean herd incidence of foot lameness for the period from calving to the end of November, 1985, and to identify the herd, management and environmental factors associated with foot lameness. The mean herd size was 125 cows (range 82 to 220). Lameness occurred in 64 (88%) herds, and the mean herd incidence was 7.0% (range 0.0 to 30.9%). The main clinical signs associated with lameness were the presence of overworn and/or bruised soles, or stones lodged in the interdigital cleft. Factors associated with lameness were: property and herd size, age of cow, bail feeding, voluntary entry into the bails, and features of the farm track including its length, the presence of steep slopes, the type of surface material, presence and treatment of broken sections and maintenance including rolling history. The association of these factors with specific clinical signs was examined. The mean cost was estimated to be $42.90 per lame cow due to loss of production, treatment, the culling or death of lame cows, and extra man hours spent managing lame cows. It was concluded that the site, construction, maintenance and use of the farm track were of major importance to the incidence of lameness in herds in this area and recommendations for reducing lameness are made.

Descriptive epidemiology of calfhood morbidity and mortality in New York holstein herds

A prospective cohort study was done of 1171 Holstein heifer calves born from July 1983 to 30 April 1985 and kept to be raised as replacements in 26 commercial dairy herds in the vicinity of Cornell University. The objectives were to examine the interrelationships of management with morbidity and mortality at both the individual and herd level, and the long-term effects of morbidity on survical. The study population, data collection and processing methods, and the descriptive epidemiology (e.g. age and temporal distributions) of calfhood morbidity, mortality and treatments are described. The clinical signs (as diagnosed by farmers on standardized check-off forms) studied were: scours/diarrhea within 14 days of birth and from 15 to 90 days of age; dull, listless, droopy ears, or of feed within 90 days of birth; cough, runny nose or eyes, or trouble breathing within 90 days or birth; death prior to 90 days of age. The crude incidence rates of scours within 14 days of birth, scours from 15 to 90 days of age, dullness, respiratory illness and death were 9.9, 5.2, 7.7, 7.4 and 3.5 per 100, respectively, for the period of study. The only seasonal effect on incidence was for scours within 14 days of birth (which was higher in winter compared to summer across both years of the study). The incidence rates of scours from 15 to 90 days of age, dullness and respiratory illness were higher in the first year of the study compared to the second year; there was no effect of season on these outcomes. The mortality rate within 90 days of birth was not related either to year of study or season. The median ages at occurence of scours within 14 days of birth, scours from 15 to 90 days of age, dullness, respiratory illness, and death were 6, 30, 16, 25 and 28 days, respectively. Risks of clinical signs were highest in the first week of life, while risk of death was highest in the third week. Calves with only scours within 14 days of birth or only scours from 15 to 90 days of age were treated primarily with oral electrolytes, oral antibiotics, and restriction of milk consumption. Calves with only dullness or only respiratory illness were treated primarily with intramuscular antibiotics. The median herd incidence rates of scours within 14 days og birth, scours from 15 to 90 days of age, dullness, respiratory illness and death were 5, 0, 0, 0 and 0 per 100, respectively, during the periof of the study.

Observations on the incidence of herds with non-visible lesioned tuberculin test reactors in south-west England.

The herd incidence of confirmed Mycobacterium bovis infection in cattle in the south-west of England has been approximately ten times that of the remainder of England and Wales; this greater incidence has been attributed to infection from badgers. The incidence of herds with only non-visible lesioned tuberculin test reactors, from which M. bovis was not isolated, has also remained higher in the south-west region. The incidences of these latter unconfirmed incidents were compared in parishes in the south-west region in which M. bovis in cattle had been confirmed, and those where M. bovis had not been confirmed, for the period 1979-83. This analysis was carried out both for those parishes in which herds had been subjected to annual tuberculin testing and for those subjected to biennial tuberculin testing. The incidence of unconfirmed incidents was significantly higher in parishes in which confirmed incidents had occurred, and this difference was found in both the annual and biennially tested parishes. The relative risks for the incidence of unconfirmed incidents in annually and biennially tested parishes were 1.89 and 2.56, respectively. The incidence of unconfirmed incidents in biennially tested parishes was lower than in annually tested parishes. The incidence of non-specific tuberculin test reactor herds was estimated from tuberculin test results in the eastern region of England during a period when tuberculosis was not confirmed in cattle. A comparison of this incidence and that of unconfirmed incidents in the south-west region suggests that approximately 70% of the unconfirmed incidents in the south-west were related to exposure to M. bovis. The results of the analyses indicate that unconfirmed incidents cannot be completely ignored in epidemiological analyses and studies of bovine tuberculosis in the problem areas of the south-west region of England.

Milk fever prevention update and vitamin D3 toxicity

Discussing problems of milk fever with practicing veterinarians today reveals that in some herds there are occasionally epidemics that occur for a variable period of time in some herds. The overall incidence in the U.S. is apparently down from the 8-9% of the 1960’s to about 6-7%. However, sales of milk fever treatments may be rather stable. This may also mean that the average milk fever cow may get 2 bottles per treatment.&#x0D; Today, in the large herds, where caloric and calcium intake are very closely regulated to minimize overfeeding of energy and calcium, the herd incidence of milk fever may be below 1%. The major question for the medium size farm is whether or not it is worthwhile to prevent milk fever if the management upset is too great. This is a real problem for dairymen on all legume forages.

An epidemiological study of abomasal bloat in young lambs

Abomasal bloat in young lambs in clinically characterized by a distension of the abdomen, especially on the right side. The significant finding at necropsy is a gas-filled abomasum. Abomasal bloat was studied over a 2-year period in 28 flocks located in Alvdal and Rendalen, two mountain valley communities in south-east Norway. The herd incidence rate varied from 0 to 20%. Abomasal bloat was most often seen 15–30 days after birth, and most cases occurred in the period just before and after the lambs were turned out to spring pasture. There was no significant difference in the incidence rate of abomasal bloatin single, twin and triplet lambs. The following factors were found to have a significant effect on the incidence rate: geographical area, type of floor, and interaction between the type of floor and the % grass silage in the roughage. Highest rates were found in flocks with built up litter floors and more than 25% silage. Substituting perforated floors for built up litter floors in 3 sheep houses reduced the incidence rate markedly. The aetiology of the disease is discussed. It is suggested that young lambs housed on built up litter with a wet consistency nibble hay contaminated with faeces. This can lead to an excessive gas-producing flora in the abomasum.

The incidence of Brucella infections in producer-retailer herds in North Lancashire from 1965 to 1972.

The results are presented of testing untreated producer-retailer herd milk samples for the presence of Brucella abortus during the period 1965-1972 in the North Lancashire region.There was a steady decline in the incidence of infected herds from 22% in 1965 to 12% in 1971. A sharp fall to 5% in 1972 suggests that the introduction of the Brucellosis Incentives Scheme and the eradication programme has helped to reduce the practice of selling brucella-infected cattle in the open market which was prevalent in the period 1965 to 1970.This practice of selling brucella-infected cattle may also be a prime factor in the changing pattern of distribution of the biotypes of B. abortus which was observed during the period 1965 to 1970.A comparison of the two areas in the region show that the incidence of herd infection was always greater in the area containing flying herds than in the area in which self-contained herds predominated.

Serodiagnosis in Q fever with special emphasis on the radioisotope precipitation test.

A study of persons associated with dairy herds serologically positive for Q fever was carried out in seven counties of central Ontario in 1964–65 to determine the extent of exposure of such persons to Q fever rickettsiae. Milk samples from 2206 dairy herds (= premises) were first tested by the capillary tube agglutination (CA) technique and 639 (29%) were found positive. This is double the herd incidence of Q fever found in a survey carried out in this area 5 years earlier.Of these 639 positive premises, 406 were visited in 1965 and blood obtained from 371 volunteers (contact population). Random serum samples of 862 adult persons from the seven counties were obtained from specimens submitted to the Peterborough laboratory for syphilis serology (general population). All human sera were tested and 43.1%found positive by the radioisotope precipitation (RIP) test, 34.6% by the complement fixation (CF) test, and only 3% by the capillary tube agglutination (CA) test. In contrast, the RIP test was positive in 2% of the sera of the general population, the CF test in 1.4%, and the CA test in only 0.4%.These results show that subclinical infection is common among persons having frequent contact with dairy cattle infected with Q fever. The study also demonstrated the high degree of sensitivity of the RIP test.