Large Animal Internal Medicine Flashcards

A 4-year-old Thoroughbred gelding is presented for a 2-week history of progressive, asymmetric ataxia and gluteal muscle atrophy. The horse is bright and alert with a normal appetite. Neurologic examination reveals proprioceptive deficits and weakness that are worse in the right hindlimb. What is the most likely causative agent of these clinical signs?

Answer: Sarcocystis neurona

The classic clinical presentation for Equine Protozoal Myeloencephalitis (EPM) is progressive, asymmetric neurological deficits, including ataxia and muscle atrophy. Sarcocystis neurona is the most common causative agent of EPM in North America. The other options cause different diseases: Equine herpesvirus-1 typically causes symmetric hindlimb ataxia, Clostridium tetani causes spastic paralysis (tetanus), and Streptococcus equi causes strangles.

A high-producing dairy cow that calved 24 hours ago is found in sternal recumbency with an S-shaped curve to her neck. She is afebrile and unable to rise. This presentation is most consistent with parturient paresis. What is the primary metabolic disturbance responsible for this condition?

Answer: Hypocalcemia

Parturient paresis, or milk fever, is caused by an acute drop in blood calcium levels (hypocalcemia). This occurs at the onset of lactation when the demand for calcium for colostrum and milk production suddenly and massively exceeds the body's ability to mobilize calcium from the diet and bone stores. The clinical signs of muscle weakness, recumbency, and flaccid paralysis are direct consequences of low extracellular calcium.

Which of the following dietary modifications is most effective in preventing obstructive urolithiasis in male small ruminants, particularly those on high-concentrate diets?

Answer: Increasing the dietary calcium-to-phosphorus ratio to at least 2:1

High-concentrate (grain) diets are often high in phosphorus and low in calcium, which predisposes to the formation of phosphatic (struvite) calculi. Ensuring the dietary calcium-to-phosphorus ratio is at least 2:1 helps prevent the formation of these stones by reducing the amount of phosphorus absorbed and excreted in the urine.

A 10-year-old mare is admitted for acute, profuse, watery diarrhea, depression, and fever. A complete blood count reveals a packed cell volume (PCV) of 60% and profound neutropenia with a degenerative left shift. What is the most likely cause of the neutropenia?

Answer: Sequestration of neutrophils into the inflamed colon and margination due to endotoxemia

In acute colitis (e.g., Salmonellosis), endotoxemia is a major feature. Endotoxins (LPS) from gram-negative bacteria cause widespread endothelial activation. This leads to neutrophils moving from the circulating pool to the marginated pool along blood vessel walls and migrating into the severely inflamed intestinal tissue. This rapid shift out of circulation results in a profound, acute neutropenia on the CBC.

A group of recently weaned beef calves develops signs of bovine respiratory disease (BRD) after transport. Several calves die, and necropsy reveals severe fibrinous pleuropneumonia. Which bacterial pathogen is most noted for producing a potent leukotoxin that is a key virulence factor in causing this specific pathology?

Answer: Mannheimia haemolytica

Mannheimia haemolytica is a primary bacterial agent in BRD that produces a potent leukotoxin (Lkt). This toxin specifically targets and destroys ruminant leukocytes, leading to massive tissue damage, inflammation, and the characteristic fibrinous pleuropneumonia seen in severe cases of 'shipping fever'.

An 18-year-old pony is presented with hirsutism, laminitis, and polyuria/polydipsia. Pituitary Pars Intermedia Dysfunction (PPID) is suspected. Which of the following is considered the most reliable and safe first-line diagnostic test for PPID, especially in early or equivocal cases?

Answer: Thyrotropin-releasing hormone (TRH) stimulation test measuring ACTH

The thyrotropin-releasing hormone (TRH) stimulation test, which measures the ACTH response to TRH administration, is considered a highly sensitive test for diagnosing PPID, particularly in early stages. It is generally preferred over the dexamethasone suppression test (DST) due to concerns that administering corticosteroids in the DST could potentially exacerbate or induce laminitis in at-risk animals. Baseline cortisol is unreliable due to normal fluctuations.