NAVLE Bovine Herd Health Questions and Answers â Questions and Answers
Question 1: A dairy manager is concerned about a rising bulk tank somatic cell count (SCC). Which of the following is the most effective and direct on-farm screening tool to identify individual cows with subclinical mastitis?
- Bulk tank culture
- California Mastitis Test (CMT) (Correct answer)
- Teat end scoring
- Direct microscopic somatic cell count (DMSCC)
Correct answer: California Mastitis Test (CMT)
The California Mastitis Test (CMT) is a simple, cow-side test that provides a semi-quantitative estimate of the SCC in milk from individual quarters, allowing for rapid identification of subclinical mastitis. Bulk tank culture identifies pathogens present in the herd's milk but does not pinpoint individual infected cows. Teat end scoring assesses the risk of infection but does not diagnose existing mastitis. DMSCC is a precise laboratory test, not a rapid on-farm screening tool for individual animals.
Question 2: A producer is implementing a control program for Bovine Viral Diarrhea Virus (BVDV). What is the most critical step to eliminate the primary reservoir of the virus within the herd?
- Vaccinating all adult cows annually with a modified-live virus (MLV) vaccine.
- Implementing strict fly control measures during summer months.
- Culling all animals that test positive on a BVDV antibody ELISA.
- Identifying and removing all persistently infected (PI) animals. (Correct answer)
Correct answer: Identifying and removing all persistently infected (PI) animals.
Persistently infected (PI) animals are the primary reservoir for BVDV, as they continuously shed large amounts of the virus throughout their lives. Identifying and removing these animals is the cornerstone of any successful BVDV eradication program. While vaccination is an important tool for herd immunity, it cannot eliminate the virus if PI animals remain. Culling based on antibody status would remove previously exposed and immune animals, not just the PI shedders. Fly control is not a primary method of BVDV transmission.
Question 3: A high-producing dairy cow, 3 days post-calving, presents with a head turned into her flank, muscle fasciculations, a dry muzzle, and a heart rate of 100 bpm with weak pulse quality. She is afebrile and unable to stand. What is the most likely diagnosis?
- Ketosis
- Hypocalcemia (Milk Fever) (Correct answer)
- Displaced abomasum
- Metritis
Correct answer: Hypocalcemia (Milk Fever)
The clinical signs describedâflaccid paralysis (inability to stand), S-shaped neck curve, tachycardia, and GI stasis (dry muzzle), occurring shortly after calvingâare classic for Parturient Paresis, or Hypocalcemia (Milk Fever). Ketosis typically presents with neurologic signs or anorexia but not flaccid paralysis. A displaced abomasum would present with a 'ping' on auscultation and percussion. Metritis would typically present with fever and a foul-smelling uterine discharge.
Question 4: A 4-year-old beef cow presents with a history of chronic, profuse, watery diarrhea and significant weight loss over several months despite maintaining a normal appetite. On rectal palpation, the rectal mucosa feels thickened and corrugated. What is the most likely causative agent?
- Mycobacterium avium subsp. paratuberculosis (Correct answer)
- Salmonella enterica serovar Typhimurium
- Eimeria spp.
- Bovine Viral Diarrhea Virus (BVDV)
Correct answer: Mycobacterium avium subsp. paratuberculosis
This classic presentation of chronic 'pipestream' diarrhea, severe weight loss with a good appetite ('wasting disease'), and thickened intestinal walls is pathognomonic for Johne's disease, caused by Mycobacterium avium subsp. paratuberculosis (MAP). While Salmonella and BVDV can cause diarrhea, the chronicity, good appetite, and specific rectal finding are highly suggestive of Johne's. Coccidiosis (Eimeria) is typically a disease of younger animals.
Question 5: Which of the following is the most critical component of a biosecurity program to prevent the introduction of infectious diseases into a closed beef cow-calf herd?
- Requiring all visitors to wear disposable plastic boots.
- Vaccinating the resident herd for all common regional diseases.
- Testing and quarantining all newly purchased animals. (Correct answer)
- Maintaining a double-fenced perimeter to prevent nose-to-nose contact.
Correct answer: Testing and quarantining all newly purchased animals.
The introduction of new animals poses the highest risk for introducing new pathogens into an established herd. A strict protocol of testing for key diseases and quarantining new animals for a minimum of 30 days is the most critical biosecurity measure. This allows for observation, acclimatization, and re-testing before commingling. While other measures like visitor protocols and boundary fences are important parts of a comprehensive biosecurity plan, they are secondary to managing the risk from new animal introductions.
Question 6: A beef herd experiences an 'abortion storm' in the last trimester of gestation. Histopathology of several aborted fetal brains reveals multifocal, nonsuppurative necrotizing encephalitis with associated protozoal cysts. What is the most likely etiologic agent?
- Bovine Herpesvirus 1 (IBR)
- Leptospira interrogans
- Neospora caninum (Correct answer)
- Brucella abortus
Correct answer: Neospora caninum
Neospora caninum is a leading protozoal cause of abortion in cattle, often occurring in outbreaks during the second and third trimesters. The pathognomonic lesion for neosporosis is multifocal, nonsuppurative necrotizing inflammation in the fetal brain and heart, often with identifiable protozoal cysts. IBR typically causes multifocal hepatic necrosis in the fetus, Leptospirosis causes interstitial nephritis, and Brucellosis causes severe placentitis.
NAVLE Bovine Herd Health Questions and Answers
A dairy manager is concerned about a rising bulk tank somatic cell count (SCC).
Which of the following is the most effective and direct on-farm screening tool to identify individual cows with subclinical mastitis?