sábado, 28 de setembro de 2013

Risk factors for pre-weaning BRD

While a great deal of study has taken place on bovine respiratory disease (BRD) in feedlots, BRD also sometimes affects young calves, and less is known about the disease complex at that production stage. During the recent American Association of Bovine Practitioners conference, veterinarians provided some insight into pre-weaning incidences of BRD.
Dr. Russ Daly, from South Dakota State University, said pre-weaning BRD outbreaks are non predictable, and when outbreaks occur, within-herd incidence can be high. And he says outbreaks occur even in herds where calves are well-vaccinated at branding or turnout. Fortunately, he says, most affected calves respond well to treatment and death loss can be kept to low levels with diligent monitoring and timely treatment.
Daly says the typical reaction to outbreaks among producers and their veterinarians is to fall into fire-fighting mode, just trying to gain control of the disease. While timely treatment is important, he also advises taking a systematic approach toward mitigating the disease – at the kitchen table rather than at the chute. He encourages veterinarians to sit down with their clients to compile information on the outbreak to gain understanding of how it occurred, resolve it and prevent future problems.
He suggests gathering information on the animals affected, their age at the time of infection, where they were located on the ranch and when possible, other information such as identification of sick calves’ dams, age of the dams and dystocia scores. Also discuss time events such as herd-management dates, group movements, introduction of other animals to the herd and weather events around the time of the onset of clinical signs. The veterinarian potentially can use this information to determine how the disease established and spread within the herd
BRD of course, can involve several different pathogens including bacteria and viruses, and Daly encourages veterinarians to employ diagnostic testing in live animals and post-mortem examinations for any calves that die from BRD.
In text above, we outlined some of the challenges associated with pre-weaning bovine respiratory disease (BRD) in beef calves. This article will describe some of the risk factors associated with pre-weaning BRD based on presentations from veterinarians at the 2013 American Association of Bovine Practitioners conference.
Dr. Dave Smith, from Mississippi State University, said risk factors and host-pathogen interactions in for BRD in young calves are not well understood, particularly as factors such as age, weather, dust and other pathogens can influence whether or not a calf becomes sick. An agent might be necessary to cause disease, Smith says, but it might not be sufficient to cause disease.
The pathogens associated with BRD are common in beef herds, but clinical cases of BRD are relatively rare, until some stressors in the system result in an outbreak.
Studies of BRD outbreaks in calves indicate the most common timeframe is around 120 days of age. Smith points out that antibodies from colostrum decline over time and are essentially gone by about 90 days of age and calves often experience a gap in immunity before active immunity builds to adequate levels. There is some scattered incidence of BRD in much younger calves, probably related to a failure in their passive immune system such as from lack of quality colostrum soon after birth.
Some studies have shown higher incidence of BRD in calves from second-calf females compared with those from older cows, but the results are inconclusive, Smith says. Steer calves seem to be more likely to contract BRD than heifers.
Dr. Amelia Woolums, from the University of Georgia, said that although BRD incidence in calves fluctuates widely between and within herds, it is the leading cause of calf deaths, accounting for up to 16 percent of the total. Twenty years of data from over 110,000 cattle at the U.S. Meat Animal Research Center (MARC) show annual incidence ranging from 3 to 24 percent, with a mean incidence of 11 percent.  
Based on the MARC data and several other datasets, Woolums lists these risk factors that appear to be associated with pre-weaning BRD:
  • Year of birth – Incidence varies widely from one year to the next
  • Location of a group of calves on an operation
  • Dystocia
  • Sex of the calf – higher incidence in males
  • Dam age – higher incidence in calves born to heifers
  • Operations that imported weaned steers
  • Herds that had visitors
  • Calves fed antibiotics to prevent BRD (Woolums speculates these operations might have initiated prevention measures after seeing a problem develop.0
  • Larger herds (Incidence generally increases with larger herd sizes.)
  • Longer calving seasons
  • Herds that introduce new calves
  • Creep feeding
  • Heat synchronization programs (Like creep feeding, these events congregate calves and increase contact.)
Consistent risk factors across the data sets include introduction of new cattle, visitors to the operation and management activities that confine or concentrate calves during the risk period.
Woolum adds that a survey of 61 veterinarians in six states asked respondents to select from a list of potential risk factors. The most common selections, in order of frequency, include:
  1. Weather
  2. Inadequate colostrum
  3. New cattle
  4. Failure to vaccinate calves
  5. Failure to vaccinate cows
  6. Calf diarrhea
  7. Presence of a calf persistently infected with BVDV

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