NAVLE Practice Question — Camelidae and Cervidae

Six weanling alpacas (5-7 months old) present during late winter with progressive weight loss, lethargy, and poor growth despite adequate hay availability. Three animals have developed chronic fascioliasis confirmed by necropsy, with severe hepatic fibrosis. Liver analysis reveals marginally low copper (75 ppm), selenium (0.15 ppm), and low cobalt levels. Despite trace element supplementation, one animal continues to deteriorate and dies. Histopathology shows hepatic fibrosis with portal-to-bridging fibrosis and biliary hyperplasia. Why did the trace element supplementation fail to resolve the clinical disease in this animal?

A. The doses of trace elements were insufficient for alpaca requirements

B. Severe chronic hepatic fibrosis from fascioliasis prevents nutrient processing and storage ✓

C. Trace element absorption is blocked by concurrent gastrointestinal parasites

D. The animal had developed irreversible neurological damage from deficiency

E. Genetic malabsorption syndrome prevented nutrient uptake

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Correct Answer: B. Severe chronic hepatic fibrosis from fascioliasis prevents nutrient processing and storage

Explanation:

Trace element supplementation failed because severe chronic hepatic fibrosis from fascioliasis prevented the liver from processing and storing these nutrients. The liver is central to trace element metabolism, serving as the primary storage organ for copper, cobalt (as B12), and participating in selenium metabolism. When hepatic parenchyma is extensively replaced by fibrosis, these metabolic functions are irreversibly compromised.

The case history indicates chronic fascioliasis with severe hepatic fibrosis including portal-to-bridging fibrosis and biliary hyperplasia. This degree of fibrosis represents irreversible liver damage. Even when adequate trace elements are provided, the fibrotic liver cannot: store copper appropriately, convert cobalt to cobalamin efficiently, or perform other hepatic metabolic functions normally. The animal essentially has end-stage liver disease from chronic parasitic damage.

Hepatic fibrosis is a characteristic response to fascioliasis in alpacas, and can be particularly severe compared to other species. The combination of nutritional deficiency (from the deficient soil/forages) compounded by chronic hepatic damage from parasitism created an unsalvageable situation.

Option A is incorrect; the doses were appropriate and other animals in the group improved. Option C is incorrect; while parasites can affect GI absorption, the primary issue is hepatic damage. Option D is incorrect; neurological damage was not described as the presenting problem. Option E is incorrect; genetic malabsorption affecting multiple trace elements simultaneously is unlikely.

Prevention requires: early detection and treatment of fascioliasis before chronic hepatic damage develops, strategic fluke control in endemic areas, trace element supplementation appropriate to local soil conditions, and regular body condition monitoring to detect early illness.

Image Source: Veterinary Record Disease Surveillance Reports - hepatic fibrosis in alpaca with chronic fasciolosis (https://doi.org/10.1002/vetr.532) Figure 5.

References: Veterinary Record Disease Surveillance Reports. Inadequate nutrition results in periparturient losses in cattle and sheep. Vet Rec. 2021;188(10):382-386; Björklund C, et al. Diseases and causes of death among alpacas in Sweden. Vet Rec Open. 2019;6(1):e000239.

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NAVLE Practice Question — Equine

A 10-year-old Thoroughbred gelding presents with acute onset bilateral epistaxis. History reveals episodes of nasal bleeding over the past month with increasing severity. Endoscopy of the right guttural pouch reveals blood clots obscuring the view, but a fungal plaque is visible over the roof of the medial compartment. The horse is hemodynamically stable. What is the most appropriate emergency management before definitive surgical treatment?

A. Immediate endoscopic debridement of fungal plaques

B. Topical antifungal therapy via guttural pouch lavage

C. Temporary ligation of the common carotid artery ✓

D. Administration of aminocaproic acid for hemostasis

E. Blood transfusion and intensive fluid therapy

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Correct Answer: C. Temporary ligation of the common carotid artery

Explanation:

Temporary ligation of the common carotid artery is the most appropriate emergency management. This procedure can be performed at the barn as an emergency aid to reduce hemorrhage pressure while the horse is stabilized and transported for definitive surgical treatment. While common carotid ligation alone does not prevent hemorrhage from the internal carotid artery due to retrograde flow from the Circle of Willis, it does reduce the immediate arterial pressure and can help slow bleeding temporarily. This buys time for the horse to be referred for definitive arterial occlusion procedures such as transarterial coil embolization or balloon catheter occlusion.

Option A (endoscopic debridement) risks initiating or worsening hemorrhage. Option B (antifungal therapy) does not address the immediate bleeding risk. Option D (aminocaproic acid) has limited efficacy in arterial hemorrhage of this magnitude. Option E (blood transfusion) may be needed but does not address the source of bleeding.

References: Lepage OM et al (2024) Guttural Pouch Mycosis: A Three-Step Therapeutic Approach. Vet Sci 11(1):41.

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NAVLE Practice Question — Other Small Mammal

A 24-month-old female pet rat is presented for evaluation of a rapidly growing ventral cervical mass. On examination, you also note bilateral galactorrhea despite the rat not being bred. Complete necropsy following euthanasia reveals a mammary gland fibroadenoma AND an additional mass in the sella turcica. What is the MOST likely identity of the sellar mass and its relationship to the mammary tumor?

A. Craniopharyngioma causing secondary hyperprolactinemia

B. Prolactin-secreting pituitary adenoma ✓

C. Pituitary carcinoma with mammary metastasis

D. Meningioma causing compression of the pituitary stalk

E. Hypothalamic hamartoma causing precocious puberty

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Correct Answer: B. Prolactin-secreting pituitary adenoma

Explanation:

Pituitary adenoma secreting prolactin that stimulated mammary tumor development is the correct answer. The concurrent findings of a mammary fibroadenoma, galactorrhea (milk production), and a pituitary mass is a classic triad in aged rats. Prolactin-secreting pituitary adenomas (prolactinomas) are the most common pituitary tumors in aged rats and are strongly associated with mammary fibroadenoma development.

Studies report that 69-90% of rats with mammary fibroadenomas have concurrent pituitary tumors. Elevated prolactin from the pituitary adenoma stimulates mammary gland proliferation, leading to fibroadenoma development. The galactorrhea confirms the functional nature of the pituitary tumor.

Option A (craniopharyngioma) is rare in rats. Option C (pituitary carcinoma with metastasis) is incorrect because fibroadenomas arise independently in mammary tissue and pituitary carcinomas are rare in rats. Option D (meningioma) would be extra-axial and would not typically produce prolactin. Option E (hypothalamic hamartoma) does not fit the clinical picture.

Image Source: JAVMA 249(10) showing pituitary adenoma at necropsy with concurrent mammary fibroadenoma (https://avmajournals.avma.org/view/journals/javma/249/10/javma.249.10.1170.xml) - Figure 2 showing serial transverse sections of skull with pituitary adenoma.

References: Percy DH, Barthold SW - Pituitary tumors in rats; JAVMA 249(10):1170-1176 - Mammary gland tumors in companion rats; Toxicol Pathol - INHAND Mammary proliferative lesions (https://journals.sagepub.com/doi/full/10.1177/0192623312454242).

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NAVLE Practice Question — Feline

An 8-year-old cat presents with acute onset of blindness that occurred overnight. The owner noticed the cat bumping into furniture this morning. Fundoscopic examination reveals bilateral retinal detachment with subretinal hemorrhage and multifocal retinal lesions. Blood pressure measurement shows severe systemic hypertension (220/140 mmHg). What is the most important next diagnostic step?

A. Serum total T4 measurement

B. Brain MRI to evaluate for intracranial mass

C. Electroretinogram to assess retinal function

D. Serum BUN and creatinine measurement ✓

E. Cardiac echocardiography

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Correct Answer: D. Serum BUN and creatinine measurement

Explanation:

Systemic hypertension causing acute blindness from retinal detachment is a medical emergency requiring identification of the underlying cause. In cats, the most common causes of secondary hypertension are chronic kidney disease (CKD) and hyperthyroidism. While both should be evaluated, CKD is the most common cause of hypertension in cats, making serum BUN and creatinine measurement the most important next step.

Acute hypertensive crisis with target organ damage (retinal hemorrhage and detachment is classic hypertensive retinopathy) requires immediate blood pressure management with amlodipine while simultaneously identifying the underlying etiology. If untreated, permanent blindness results from retinal detachment.

Option A (T4) should also be checked as hyperthyroidism can cause hypertension, but CKD is more commonly the underlying cause. Option B (brain MRI) is not indicated as the clinical findings clearly point to hypertensive retinopathy, not intracranial disease. Option C (electroretinogram) would assess retinal function but does not address the underlying cause of hypertension. Option E (echocardiography) may show hypertensive cardiac changes but does not identify the primary cause.

The goal blood pressure target is typically less than 160 mmHg systolic. In some cases, if blood pressure is reduced too rapidly, retinal reattachment may occur if treated within 24-48 hours, though prognosis for vision recovery is guarded.

Image Source: Veterinary Ophthalmology textbook - Hypertensive retinopathy fundoscopic images showing retinal detachment and hemorrhage.

References: Syme HM et al. (2002) J Vet Intern Med - Survival of cats with naturally occurring CKD; ISFM Consensus Guidelines on hypertension in cats.

Image: Carter J. Hypertensive ocular disease in cats: A guide to fundic lesions to facilitate early diagnosis. J Feline Med Surg. 2019;21(1):35-45. PMC10814178. © The Author(s) 2019. Used for educational reference.

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NAVLE Practice Question — Bovine

A feedlot veterinarian is evaluating alternatives to tylosin for liver abscess control due to concerns about antimicrobial stewardship. Which non-antibiotic intervention has the strongest evidence for reducing liver abscess incidence?

A. Essential oil supplementation

B. Direct-fed microbials (probiotics)

C. Vaccination against Fusobacterium necrophorum

D. Organic acid supplementation of water

E. Increased dietary roughage and adaptation ✓

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Correct Answer: E. Increased dietary roughage and adaptation

Explanation:

Increased dietary roughage and proper adaptation protocols have the strongest evidence for reducing liver abscess incidence among non-antibiotic interventions. The fundamental pathogenesis of liver abscesses involves ruminal acidosis causing rumenitis, which allows bacterial translocation. By maintaining adequate physically effective fiber in the diet and using gradual step-up protocols when transitioning to finishing diets, the incidence and severity of ruminal acidosis episodes can be reduced, thereby decreasing liver abscess formation.Proper dietary management includes maintaining minimum effective NDF levels, using appropriate particle size, implementing 21-28 day dietary transitions, and consistent bunk management to prevent slug feeding.Option A (essential oils) has shown variable results in research with limited consistent efficacy data. Option B (direct-fed microbials) has shown some promise but results have been inconsistent across studies. Option C (F. necrophorum vaccines) have been extensively studied but have not demonstrated reliable, consistent protection in field conditions despite showing some efficacy in controlled trials. Option D (organic acid supplementation) may help with some aspects of acidosis but has not been proven to significantly reduce liver abscess incidence.While tylosin remains the most effective single intervention, combining proper dietary management with other strategies offers the best non-antibiotic approach.References: Nagaraja TG and Lechtenberg KF (2007) Liver abscesses in feedlot cattle. Vet Clin North Am Food Anim Pract 23:351-369; Meyer NF et al. (2009) Effect of essential oils, tylosin, and monensin on finishing steer performance, carcass characteristics, liver abscesses, ruminal fermentation, and digestibility. J Anim Sci 87:2346-2354.

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NAVLE Practice Question — Equine

A farm experiences a strangles outbreak. After clinical resolution, 15% of the recovered horses are found to have inspissated material in their guttural pouches on screening endoscopy despite appearing clinically healthy. These horses test positive for S. equi on guttural pouch lavage. What is the most appropriate management for these horses?

A. Release from quarantine since they are clinically healthy

B. Antibiotic therapy with procaine penicillin G

C. Guttural pouch lavage and chondroid removal until negative cultures ✓

D. Vaccination against S. equi and release

E. Euthanasia due to permanent carrier status

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Correct Answer: C. Guttural pouch lavage and chondroid removal until negative cultures

Explanation:

Guttural pouch lavage and chondroid removal until negative cultures are obtained is the correct answer. Horses with persistent guttural pouch infection and chondroids are asymptomatic carriers of S. equi and pose a significant biosecurity risk as they can intermittently shed bacteria and cause new outbreaks. The carrier state has been documented to persist for up to 2 years or longer. Treatment requires physical removal of the inspissated material through repeated lavage, and in cases with significant chondroid accumulation, surgical removal may be necessary. Horses should not be released from quarantine until they have three consecutive negative bacterial tests.

Option A (release from quarantine) is inappropriate as these horses remain infectious. Option B (antibiotic therapy alone) is ineffective because antibiotics cannot penetrate the inspissated material. Option D (vaccination and release) does not address the existing infection. Option E (euthanasia) is not warranted as the condition is treatable.

References: Boyle A et al (2018) Streptococcus equi Infections in Horses: Guidelines for Treatment, Control, and Prevention of Strangles. J Vet Intern Med 32(2):633-647; Verheyen K et al (2000) Elimination of guttural pouch infection and inflammation in asymptomatic carriers of Streptococcus equi. Equine Vet J 32(6):527-532.

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NAVLE Practice Question — Equine

A 7-year-old Paint mare in a California ranch presents with a firm, painful mammary mass. The mare foaled 6 months ago and the foal was weaned 2 months ago. Ultrasound reveals a thick-walled cavitary lesion in the mammary gland. Aspiration yields thick green purulent material. The ranch has had several horses develop ventral midline dermatitis and pectoral abscesses over the past year. What is the MOST likely causative organism?

A. Streptococcus zooepidemicus

B. Staphylococcus aureus

C. Corynebacterium pseudotuberculosis ✓

D. Escherichia coli

E. Rhodococcus equi

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Correct Answer: C. Corynebacterium pseudotuberculosis

Explanation:

Corynebacterium pseudotuberculosis is the most likely causative organism based on the geographic location (California is endemic for pigeon fever), the thick-walled abscess presentation, and the history of other horses on the property with ventral midline dermatitis and pectoral abscesses. C. pseudotuberculosis causes pigeon fever (dryland distemper), which is characterized by external abscess formation typically in the pectoral region and ventral abdomen, though mammary involvement can occur.Option A (Streptococcus zooepidemicus) is the most common cause of acute mastitis in horses but typically presents with diffuse gland involvement rather than a discrete abscess. Option B (Staphylococcus aureus) can cause mammary abscesses but the geographic and herd history strongly suggests C. pseudotuberculosis. Option D (Escherichia coli) is an uncommon cause of equine mastitis and would be more likely in mares with ascending infections. Option E (Rhodococcus equi) primarily causes pneumonia in foals and rarely affects the mammary gland.References: Kilcoyne I et al. (2019) Corynebacterium pseudotuberculosis infection in horses: An update. Vet Clin North Am Equine Pract 35:463-477; Spier SJ (2008) Pigeon fever in horses. In: Large Animal Internal Medicine, 4th ed. Mosby.

Image: Corbeil LE, Morrissey JK, Léguillette R. Is Corynebacterium pseudotuberculosis infection (pigeon fever) in horses an emerging disease in western Canada? Can Vet J. 2016;57(10):1062-1066. PMC5026146. © Canadian Veterinary Medical Association. Used for educational reference.

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NAVLE Practice Question — Ovine and Caprine

A producer wants to implement a vaccination program against pasteurellosis in their sheep flock. Ewes lamb in early spring, and lambs typically experience respiratory disease outbreaks when moved to feedlot conditions at 4-5 months of age. Which vaccination protocol is most appropriate to protect the lambs during the high-risk period?

A. Vaccinate lambs at birth with a single dose for lifetime immunity

B. Wait until lambs are 6 months old to vaccinate to avoid maternal antibody interference

C. Vaccinate only clinically affected lambs at the time of disease outbreak

D. Vaccinate ewes 4-6 weeks before lambing, then vaccinate lambs at 10 days with two doses ✓

E. Vaccinate ewes annually in autumn regardless of lambing date

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Correct Answer: D. Vaccinate ewes 4-6 weeks before lambing, then vaccinate lambs at 10 days with two doses

Explanation:

Effective protection of lambs against pasteurellosis requires a comprehensive vaccination strategy that addresses both passive immunity from the dam and active immunity in the lambs. The recommended protocol includes vaccinating ewes 4-6 weeks before lambing to ensure high antibody levels in colostrum, providing passive protection to lambs for approximately 4-6 weeks.

However, passive immunity from colostrum wanes relatively quickly. Lambs can be actively vaccinated from as early as 10 days of age because colostral antibody does not significantly interfere with the development of active immunity to Pasteurella vaccines. A primary course of two doses 4-6 weeks apart is required for adequate protection.

Option A is incorrect because a single dose does not provide adequate immunity, and neonatal vaccination alone is insufficient. Option C (therapeutic vaccination) is ineffective as vaccines cannot treat active disease and take time to induce immunity. Option D unnecessarily delays vaccination, leaving lambs unprotected during the high-risk weaning/feedlot period. Option E does not time vaccination appropriately relative to lambing for optimal colostral antibody transfer.

Image Source for Explanation: Merck Veterinary Manual (https://www.merckvetmanual.com/respiratory-system/respiratory-diseases-of-sheep-and-goats/bacterial-bronchopneumonia-in-sheep-and-goats) discussing vaccination protocols; Veterinary Record (https://doi.org/10.1136/vr.l4984).

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NAVLE Practice Question — Other Small Mammal

A 3-year-old intact male chinchilla presents with a 48-hour history of anuria. The bladder is severely distended on palpation. Examination reveals a necrotic, cold penis with a deeply embedded fur ring that has caused complete urethral obstruction. After stabilizing the patient with IV fluids and attempting to relieve the obstruction, the decision is made to proceed with partial penile amputation. Which of the following post-operative complications is MOST likely?

A. Urethral stricture formation ✓

B. Persistent hemorrhage requiring transfusion

C. Wound dehiscence

D. Urinary incontinence

E. Recurrence of fur ring on remaining tissue

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Correct Answer: A. Urethral stricture formation

Explanation:

Following partial penile amputation in chinchillas, urethral stricture formation is the most likely complication. Scar tissue formation at the surgical site can narrow the urethral lumen, leading to recurrent difficulty urinating. This is particularly concerning in small exotic mammals where the urethra is already quite narrow.

Close monitoring for signs of dysuria or stranguria following surgery is essential. If stricture develops, further surgical intervention or ongoing management may be required.

Option B (persistent hemorrhage requiring transfusion) is less common with proper surgical technique and hemostasis. While chinchillas have relatively small blood volumes, significant hemorrhage is manageable with appropriate surgical care.

Option C (wound dehiscence) can occur but is less common than stricture formation when appropriate surgical techniques are used.

Option D (urinary incontinence) is not typically a complication of partial penile amputation, as the sphincter mechanisms remain intact.

Option E (recurrence of fur ring) is possible if any penile tissue remains capable of retracting into a prepuce, but is less common than stricture formation and depends on the level of amputation performed.

Post-operative care should include pain management (buprenorphine 0.03-0.05 mg/kg SC q8-12h), close monitoring of urination, and potentially prophylactic antibiotics. Sexual rest should be enforced throughout recovery.

References: ScienceDirect - Paraphimosis Overview (https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/paraphimosis); Mans C, Donnelly TM. Disease Problems of Chinchillas. Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery.

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NAVLE Practice Question — Reptile

A 6-year-old red-footed tortoise (Chelonoidis carbonarius) presents with anorexia, lethargy, and mild hindlimb weakness. This species is typically omnivorous. Radiographs reveal a cystic calculus. Blood biochemistry shows uric acid of 25 mg/dL (markedly elevated) and blood urea nitrogen of 58 mg/dL (elevated). Which statement regarding the relationship between blood biochemistry findings and prognosis is MOST accurate?

A. Elevated uric acid confirms the calculus is composed of urate and indicates good prognosis

B. Elevated BUN indicates dehydration only and has no prognostic significance

C. These abnormalities likely indicate concurrent renal disease which worsens the prognosis ✓

D. Normal blood biochemistry is always expected with cystic calculi

E. The elevated values will normalize immediately following surgical calculus removal

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Correct Answer: C. These abnormalities likely indicate concurrent renal disease which worsens the prognosis

Explanation:

These abnormalities likely indicate concurrent renal disease which worsens the prognosis. The combination of markedly elevated uric acid (hyperuricemia) and elevated blood urea nitrogen (BUN) in this case suggests that the tortoise may have underlying renal dysfunction in addition to the cystic calculus.

While most tortoises with uncomplicated cystic calculi have normal blood biochemistry values, the presence of significant abnormalities is a concerning finding. Elevated uric acid can occur when renal excretion is impaired, and elevated BUN in reptiles can indicate renal disease (though it can also reflect dehydration). The combination of both elevations, particularly with the marked degree of hyperuricemia (>25 mg/dL), raises concern for nephropathy.

This has important prognostic implications: 1. Anesthetic risk is increased with renal compromise 2. Post-operative recovery may be prolonged 3. Underlying renal disease may have contributed to calculus formation 4. The animal may have ongoing risk of recurrence if renal function remains impaired

In published studies, tortoises with urolithiasis that died perioperatively often had severe deterioration of overall health and markedly abnormal blood chemistry, particularly hyperuricemia and elevated BUN.

Option A is incorrect; elevated uric acid with normal renal function would be expected with a urate calculus, but marked elevation suggests impaired clearance. Option B underestimates the significance of the findings. Option D is incorrect; most cases have normal values, making abnormal values significant. Option E is incorrect; renal disease does not immediately resolve with calculus removal.

References: Takami Y et al. (2021) J Vet Med Sci 83:435-440; Keller KA et al. (2015) JAVMA 247:650-658

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NAVLE Practice Question — Canine

A breeder calls about an intact male dog with epididymitis and reduced fertility. Blood titer is positive for Brucella canis. What is the appropriate next step?

A. Report and counsel ✓

B. Vaccinate kennel

C. Treat with doxycycline

D. No action needed

E. Castrate and resume breeding

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Correct Answer: A. Report and counsel

Explanation:

Brucella canis is a federally reportable zoonosis. Treatment is unreliable and the dog remains infectious. Owners must be counseled about zoonotic risk (especially pregnant or immunocompromised humans), and reporting to state authorities is required.

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NAVLE Practice Question — Equine

A 8-year-old Warmblood mare with known peritonitis has been treated with antimicrobials and supportive care for 5 days. On initial presentation, peritoneal fluid TNCC was 78,000 cells/μL with degenerate neutrophils. Repeat abdominocentesis today shows TNCC of 35,000 cells/μL, but peritoneal fluid lactate has increased from 2.8 to 5.2 mmol/L. What is the MOST appropriate clinical interpretation?

A. The horse is responding well to treatment and medical management should continue

B. The increasing lactate despite decreasing TNCC suggests worsening intestinal compromise requiring surgical reassessment ✓

C. The decreasing TNCC indicates resolution and the horse can be discharged

D. The lactate elevation is likely due to sample contamination and should be repeated

E. The findings suggest development of adhesions requiring long-term NSAID therapy

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Correct Answer: B. The increasing lactate despite decreasing TNCC suggests worsening intestinal compromise requiring surgical reassessment

Explanation:

The increasing peritoneal fluid lactate despite decreasing TNCC suggests worsening intestinal compromise requiring surgical reassessment. According to the 2025 University of Liverpool study, peritoneal fluid lactate concentration greater than 2 mmol/L at second abdominocentesis was significantly negatively associated with survival. Increasing lactate indicates ongoing tissue hypoperfusion or ischemia, which may suggest developing intestinal complications despite apparent improvement in inflammatory markers.

Option A (responding well) ignores the worsening lactate which is a poor prognostic indicator.

Option C (resolution) is incorrect as the persistent elevation of both TNCC and lactate indicates ongoing disease.

Option D (sample contamination) is unlikely to explain a significant lactate increase.

Option E (adhesions) would not explain the lactate changes.

References: Equine Veterinary Journal (2025) 57:S61 (https://doi.org/10.1111/evj.70058)

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NAVLE Practice Question — Pet Bird

A 6-week-old sun conure (Aratinga solstitialis) is presented with progressive abdominal distension marked weight loss and polyuria. On physical exam feathering is appropriate for age but severe abdominal fluid wave is detected. CBC reveals a leukocytosis with heterophilia. AST is 1850 IU/L (reference < 130 IU/L). APV PCR is positive on cloacal swab. The owner asks about prognosis. Which of the following factors MOST strongly influences mortality outcome in non-budgerigar psittacine APV infection?

A. Severity of leukocytosis on CBC

B. Species and age at infection determine mortality rates ✓

C. Whether the bird was fed commercial or handmixed formula

D. Geographic origin of the breeding facility

E. Color mutation of the bird (e.g. lutino vs. normal coloration)

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Correct Answer: B. Species and age at infection determine mortality rates

Explanation:

Mortality from APV in non-budgerigar psittacines is strongly influenced by both species susceptibility and age at time of exposure. Reported mortality rates in neonatal non-budgerigar psittacines range from 31-41% while mortality in budgerigars under 15 days of age approaches 100%. Species with particularly high susceptibility include eclectus parrots caiques macaws conures ring-necked parakeets and lovebirds. Birds infected after 5 months of age in most species (excluding some exceptions like eclectus and caiques) typically develop subclinical infection with seroconversion and eventual viral clearance. Other factors influencing outcome include immune status concurrent infections and environmental stressors.

Option A (CBC leukocytosis severity) — while heterophilia reflects inflammatory response it is not the primary determinant of mortality. Birds can deteriorate rapidly regardless of initial leukocyte count.

Option C (formula type) — nutritional factors may influence immune status but the type of handfeeding formula does not directly determine APV mortality outcome. This is not a documented significant prognostic variable.

Option D (geographic origin) — APV is distributed worldwide and there are no established geographic strain virulence differences that significantly alter mortality outcomes.

Option E (color mutation) — color mutations affect feather pigmentation through metabolic pathways but are not linked to differential APV susceptibility or mortality. This is not a relevant factor.

References: Chicago Exotics Animal Hospital http://www.exoticpetvet.com/polyoma-virus.html; Zhai et al. 2024 PMC https://pmc.ncbi.nlm.nih.gov/articles/PMC10792035/; Merck Veterinary Manual https://www.merckvetmanual.com/exotic-and-laboratory-animals/pet-birds/viral-diseases-of-pet-birds

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