Veterinary Research Update
Veterinary Research Update

Dr. Clell V. Bagley, D.V.M.
USU Extension Veterinarian

    The following is a summary of three recent research reports related to dairy health.

Protection of Pregnant Cattle and Their Fetuses Against Infection with Bovine Viral Diarrhea Virus Type 1 by Use of a Modified-live Virus Vaccine

    The objective of this study was to determine efficacy of a vaccine containing modified-live bovine viral diarrhea virus (BVDV) type 1 for protecting pregnant cows and their fetuses against virulent heterologous BVDV type 1.

    This was a randomized controlled cohort study using 18 yearling beef heifers seronegative for BVDV and negative when tested for BVDV by virus isolation. Cattle were randomly assigned to control (unvaccinated) or vaccinated (12) groups. Vaccinated heifers were given a combination vaccine containing modified-live BVDV type 1 comprising a cytopathic (NADL) strain. All 18 heifers were then bred and challenge-exposed between 70 and 75 days of gestation with BVDV type 1, administered intranasally. Cattle were monitored, and infection status of offspring was determined after parturition. Antibody concentrations of vaccinated and control heifers were also monitored.

    All six calves from control heifers had positive results on multiple virus isolation tests and were considered persistently infected. In comparison, only two calves from vaccinated cows had positive results on virus isolation tests and were considered persistently infected. One vaccinated heifer aborted, but the fetus was not persistently infected, and the abortion was not attributed to BVDV infection.

    Analysis of these data indicated that a single dose of a modified-live NADL-derived BVDV type I vaccine will confer protection to dams and their fetuses against challenge-exposure to heterologous BVDV type 1 organisms.

Amer. Jrnl. Of Veterinary Research 59(11):1409-1413, Nov. 1998

Role of Horn Flies (Haematobia Irritans) in Staphylococcus aureus-Induced Mastitis in Dairy Heifers

    The objective of this study was to determine whether Staphylococcus aureus can colonize in horn flies and whether colonization is sufficiently persistent for transmission of the organism to cows by flies.

    Two Jersey heifers exposed to infected horn flies were studied. Staphylococcus aureus was allowed to colonize in horn flies, and duration of colonization was determined. Flies with colonized S. aureus were allowed to feed on teats of uninfected heifers to determine whether intramammary infection could be transmitted from fly to heifer. Scab material from naturally infected heifers was submitted for bacteriologic culture to determine whether S. aureus was present and whether scabs could serve as a possible source of S. aureus for flies.

    Staphylococcus aureus colonized in horn flies and remained for up to 96 hours after exposure. Exposure of teats of uninfected heifers to horn flies colonized with S. aureus resulted in intrammary infection in three and four exposed teats. Culture of scab material from teats of naturally infected heifers revealed high concentration of S. aureus (> 107 colony-forming units/mg), and flies without previously colonized S. aureus were allowed to feed on scabs; S. aureus colonized in them just as readily as it did in flies that had fed on experimentally infected blood.

     Horn flies are capable of transmitting S. aureus- induced intrammary infection to heifers, and scabs on teats are a potential source of S. aureus. Fly control on dairy cows in herds with known S. aureus problems is recommended as a method to help prevent these infections.

American Journal of Veterinary Research, Sept. 1998 Vol. 59, No. 9, pp. 1122-1124.

Timing of Insemination for Dairy Cows Identified in Estrus by a Radiotelemetric Estrus Detection System

    The optimal time of artificial insemination (AI) was determined from data for 2661 AI in 17 herds utilizing a radiotelemetric system for estrus detection that has the potential for continuous 24-h surveillance to monitor behavioral events associated with estrus. The system consisted of pressure-sensitive radio frequency transmitters affixed over the sacrum region of cows. The activation of the sensor sent a radiotelemetric signal to a microcomputer via a fixed antenna. Cow identification, date, time, and duration of each standing event were recorded in the software program provided with the system. Each farm selected a 3-h interval to AI for cows that were identified in estrus during the previous 24 h. Pregnancy status was determined from data for return to estrus and palpation of the uterus 35 to 75 d following AI. Standing events during estrus averaged (� SD) 8.5 � 6.6 per cow, and the number of events per estrus across herds averaged from 6.2 � 5.1 to 12.8 � 9.9 per cow.

    The duration of estrus ranged from 5.1 � 3.8 to 10.6 � 6.8 h across herds; the mean was 7.1 � 5.4 h. The interval from the first standing event to AI affected the probability of pregnancy; the highest conception rates for AI occurred between 4 and 12 h after the onset of standing activity. The probability of pregnancy was higher for cows > 100 d in milk, exhibiting > 2 standing events during estrus, and inseminated during March, April, or May.

    Previous studies have reported that, when onset of estrus is not known, once daily AI for cows observed in standing estrus can be used as effectively as the a.m.-p.m. guideline and results in no difference in resulting conception rate. Our results would suggest that, if onset of estrus is unknown, AI should be performed within 4 to 12 h of observation of estrus.

Journal of Dairy Science, July 1998, Vol. 81, No. 7, pp. 1874-1882. ©