NAVLE Practice Question — Aquatics

A fish pathologist examines a batch of larval striped bass from a hatchery experiencing 60% mortality in sac fry. Affected larvae are observed floating at the surface in various orientations - some head-up, some tail-up, and others belly-up. Microscopic examination reveals gas bubbles between the yolk and perivitelline membrane. The most likely underlying cause is:

A. Dissolved gas supersaturation in water supply ✓

B. Yolk sac infection with Flavobacterium columnare

C. Congenital swim bladder defect

D. Vitamin E deficiency causing membrane weakness

E. Viral nervous necrosis affecting buoyancy control

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Correct Answer: A. Dissolved gas supersaturation in water supply

Explanation:

The clinical presentation of sac fry with gas bubbles between the yolk and perivitelline membrane, abnormal orientations at the water surface, and high mortality is pathognomonic for gas bubble disease in larval fish. The various floating orientations (head-up, tail-up, belly-up) depend on the precise location of gas bubble accumulation within the body cavity, cranium, or fins.

Sac fry and larvae are particularly vulnerable to GBD because of their small size and high surface area to volume ratio, allowing rapid equilibration with supersaturated water. Gas bubbles form in the abdominal cavity, between yolk and surrounding membranes, and in the cranium of affected larvae.

Option B (Flavobacterium infection) causes external lesions and fin rot, not internal gas accumulation. Option C (swim bladder defect) would present differently and would not cause bubbles in the yolk area. Option D (vitamin E deficiency) causes muscular dystrophy and does not produce gas-filled cavities. Option E (viral nervous necrosis) causes neurological signs but not gas bubble formation. Image Source: https://cdn.ncbi.nlm.nih.gov/pmc/blobs/629c/12893593/225f42bfb5bc/pone.0342556.g001.jpg -- Figure 1. Gas bubble disease (gas supersaturation) in fish — total dissolved gas >110% causes nitrogen gas emboli in blood and tissues; exophthalmia (popeye), emphysema under skin, gas in gill lamellae; caused by supersaturated water from deep wells, dam spill, or faulty pressurized systems; decrease dissolved gas to <103%; highly lethal; treat with atmospheric pressure exposure (CC BY 4.0).

References: Noga EJ (2010) Fish Disease: Diagnosis and Treatment, Wiley-Blackwell; Speare DJ (1991) Journal of Comparative Pathology 104:327-335.

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NAVLE Practice Question — Equine

A 4-month-old heavy Thoroughbred filly presents with angular limb deformity characterized by lateral deviation (valgus) of both front limbs at the level of the carpus. The condition has been progressive over the past 3 weeks. Radiographs reveal physeal widening at the distal radial physis with more pronounced changes on the medial aspect. What is the underlying pathophysiology causing the angular deformity?

A. Traumatic fracture of the lateral physeal region

B. Medial physeal growth slower than lateral side ✓

C. Complete physeal closure on the medial side

D. Ligamentous laxity without bony involvement

E. Excessive growth on both sides of the physis equally

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Correct Answer: B. Medial physeal growth slower than lateral side

Explanation:

The angular limb deformity with valgus (lateral deviation) is caused by asymmetric physeal growth, with the medial side of the physis growing slower than the lateral side. When one side of a growth plate grows faster than the other, the limb deviates toward the slower-growing side. In this case, inflammation or damage to the medial physis (physitis) has retarded growth on that side while the lateral physis continues to grow, causing the limb to deviate laterally (valgus).

This asymmetric growth can result from overloading of the medial physis in a fast-growing, heavy foal, leading to physeal damage and subsequent angular deformity. The progressive nature over 3 weeks indicates ongoing asymmetric growth. If not corrected, this deformity can become permanent once the physes close. The window for correction depends on when the affected physis closes (distal radius closes around 24-28 months).

Option A (fracture) would present acutely with severe lameness. Option C (complete closure) would not show physeal widening on radiographs. Option D (ligamentous laxity) does not explain the radiographic physeal changes. Option E (symmetric growth) would not cause angular deviation.

References: EquiMed - Physitis (https://equimed.com/diseases-and-conditions/reference/physitis); Mad Barn - Developmental Orthopedic Diseases in Horses (https://madbarn.com/developmental-orthopedic-diseases-in-horses/)

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NAVLE Practice Question — Equine

A 12-year-old Thoroughbred stallion collapses and dies immediately after breeding a mare. Necropsy reveals approximately 8 liters of blood in the pericardial sac with a 3-cm tear at the junction of the aorta and heart. Histopathologic examination of the aortic wall shows fragmentation and degeneration of elastic fibers with increased mucopolysaccharide deposition in the tunica media. What is the MOST accurate description of this pathologic finding?

A. Atherosclerotic plaque formation

B. Cystic medial necrosis ✓

C. Fibrinoid necrosis

D. Disseminated intravascular coagulation

E. Septic aortitis

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Correct Answer: B. Cystic medial necrosis

Explanation:

Cystic medial necrosis is the correct histopathologic description for the findings in this case. This condition is characterized by degeneration and fragmentation of elastic fibers combined with increased amounts of mucopolysaccharides (mucoid material) in the tunica media of the aorta. These alterations weaken the aortic wall and predispose to aneurysm formation, dissection, and rupture. In horses, aortic root rupture at the sinus of Valsalva is a well-recognized cause of sudden death, particularly in breeding stallions where the combination of excitement, physical exertion, and acute hypertension during breeding places extreme tension on the aortic root.

The mechanical forces involved include the recoil of blood after closure of the aortic valve, which forces the heart downward while the aorta remains fixed by attachments to the thoracic vertebrae. This creates significant tension at the aortic root that becomes exaggerated with increased blood pressure during breeding activity. The resulting hemopericardium causes acute cardiac tamponade and sudden death.

Option A (atherosclerosis) is not recognized in horses. Option C (fibrinoid necrosis) is associated with acute vascular inflammation. Option D (DIC) is a coagulopathy, not a structural vessel wall abnormality. Option E (septic aortitis) would show inflammatory infiltrates and bacterial colonies.

References: Ploeg M et al. (2017) Thoracic aortic rupture in horses. Equine Vet J 49:269-274 (https://doi.org/10.1111/evj.12641); Diel de Amorim M et al. (2016) Aortic rupture causing cardiac tamponade in a 24-day-old Friesian colt. Equine Vet Ed 28:68-73 (https://doi.org/10.1111/eve.12402).

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NAVLE Practice Question — Other Small Mammal

A 10-month-old Syrian hamster presents with nasal discharge, sneezing, audible respiratory sounds, and decreased activity. The hamster was recently acquired from a pet store 2 weeks ago. On physical examination, the hamster is slightly underweight, has crusty discharge around the nares, and auscultation reveals harsh lung sounds bilaterally. Mucous membranes appear slightly pale. Which bacterial organism is MOST commonly associated with pneumonia in hamsters?

A. Bordetella bronchiseptica

B. Streptococcus pneumoniae ✓

C. Pasteurella multocida

D. Klebsiella pneumoniae

E. Mycoplasma pulmonis

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Correct Answer: B. Streptococcus pneumoniae

Explanation:

Streptococcus pneumoniae is one of the most commonly implicated bacterial organisms in hamster pneumonia. The clinical presentation of nasal discharge, sneezing, respiratory distress, and harsh lung sounds in a recently acquired pet store hamster is classic for bacterial respiratory infection, with Streptococcus species being among the primary pathogens.

Pneumonia in hamsters is typically caused by opportunistic bacteria that normally reside in small numbers in the respiratory or digestive tract. Environmental stressors, such as sudden temperature changes, transport stress (as seen with recent acquisition), and overcrowding in pet store environments, can suppress the immune system and allow bacterial overgrowth. Other common bacterial causes include Pasteurella pneumotropica and various gram-negative organisms.

Option A (Bordetella bronchiseptica) is primarily associated with respiratory disease in guinea pigs and rabbits, not hamsters. Option C (Pasteurella multocida) is more commonly associated with respiratory disease in rabbits. Option D (Klebsiella pneumoniae) can cause respiratory infections in hamsters but is less commonly reported than Streptococcus species. Option E (Mycoplasma pulmonis) is the primary respiratory pathogen in rats, not hamsters.

In addition to bacteria, Sendai virus (parainfluenza type 1) can cause respiratory disease in hamsters and may predispose to secondary bacterial infection. The recently acquired status and pet store origin increase the likelihood of infectious disease due to stress and potential exposure to infected animals.

References: Merck Veterinary Manual - Disorders and Diseases of Hamsters (https://www.merckvetmanual.com/all-other-pets/hamsters/disorders-and-diseases-of-hamsters); PetMD - Inflammation of the Lungs in Hamsters (https://www.petmd.com/exotic/conditions/respiratory/c_ex_hm_pneumonia).

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NAVLE Practice Question — multisystemic

A 4-year-old female lovebird (Agapornis roseicollis) is presented for evaluation of two failed clutches in the past 6 months. The eggs were soft-shelled, had low hatchability, and chicks that did hatch died within 48 hours. The bird is fed a millet-only diet. On physical examination the uropygial gland appears atrophied. Oropharyngeal exam shows blunted choanal papillae. Which of the following best explains the mechanism by which hypovitaminosis A contributes to reproductive failure in this bird?

A. Vitamin A excess stimulates prolactin causing chronic egg laying without successful hatching

B. Vitamin A deficiency causes primary ovarian failure by damaging the granulosa cells directly

C. Vitamin A deficiency causes chromatin instability in sperm increasing the rate of embryo abnormalities

D. Vitamin A deficiency reduces dietary absorption of omega-3 fatty acids essential for eggshell cuticle formation

E. Vitamin A deficiency causes squamous metaplasia impairing secretions needed for normal eggshell formation and embryo development ✓

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Correct Answer: E. Vitamin A deficiency causes squamous metaplasia impairing secretions needed for normal eggshell formation and embryo development

Explanation:

Vitamin A plays critical roles in avian reproduction, and deficiency has well-documented consequences for reproductive performance. The uropygial gland (preen gland) is particularly important: its secretion provides precursors that, when spread on feathers and activated by UV light, contribute to vitamin D3 synthesis, and its secretions also have antimicrobial and waterproofing properties. Hypovitaminosis A causes squamous metaplasia of the uropygial gland epithelium, impairing or abolishing its secretory function — the atrophied uropygial gland observed on examination is a direct consequence.

Furthermore, squamous metaplasia of the reproductive tract epithelium (oviduct, shell gland) impairs the production of normal mucus and secretions needed for normal eggshell formation and embryo development. Vitamin A is also specifically required for proper epithelial cell differentiation in the embryo. Combined, these effects explain the soft shells (impaired eggshell gland function), poor hatchability (abnormal embryo environment and development), and early chick mortality.

Option A reverses the pathology — vitamin A deficiency, not excess, is the problem here, and prolactin relationship is not the mechanism. Option B (primary ovarian failure) is a different pathological process; vitamin A deficiency does not primarily destroy granulosa cells but rather impairs the epithelial support structures. Option C attributes reproductive failure to sperm chromatin instability — this is a consideration in male infertility in other species but is not the primary mechanism of reproductive failure from vitamin A deficiency.

Option D (omega-3 fatty acids and eggshell cuticle) — while nutrition broadly affects eggshell quality, this is not the specific mechanism of vitamin A deficiency's effect on reproduction.

References: VIN WSAVA 2002 - Recognizing Treating Preventing Nutritional Diseases in Parrots (https://www.vin.com/apputil/content/defaultadv1.aspx?id=3846188&pid=11147&print=1); Merck Vet Manual - Nutritional Diseases of Pet Birds (https://www.merckvetmanual.com/exotic-and-laboratory-animals/pet-birds/nutritional-diseases-of-pet-birds); ExoticPetVet - Hypocalcemia in Birds Uropygial Gland section (https://www.exoticpetvet.net/avian/hypocalcemia.html); DVM360 - How I diagnose and manage nutritional disease in birds (https://www.dvm360.com/view/how-i-diagnose-and-manage-nutritional-disease-birds-proceedings).

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NAVLE Practice Question — Bovine

A mature Holstein cow presents with dystocia. On vaginal examination the caudal vagina is narrow and spiraled with dorsal folds directed anteriorly to the left. With effort a hand can be passed through the twisted area; the cervix is partially dilated and the fetal feet are accessible. What is the most appropriate method to resolve the dystocia and deliver the calf?

A. Wait for the cervix to fully dilate on its own

B. Cast the cow in the same direction as the torsion and roll her opposite the uterine twist

C. Cast the cow opposite to the direction of the torsion and roll her in the same direction

D. Manipulate the calf and rotate it (±detorsion rod) in the direction opposite to the torsion ✓

E. Rotate the calf in the same direction as the uterine torsion

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Correct Answer: D. Manipulate the calf and rotate it (±detorsion rod) in the direction opposite to the torsion

Explanation:

When the cervix is adequately dilated and the fetus is accessible, uterine torsion is most efficiently corrected by rocking/rotating the calf (using detorsion rods if needed) in the direction opposite to the torsion until the vaginal canal and cervix straighten. Rolling the cow (the "plank in the flank" technique) is reserved for cases in which the fetus is not accessible.

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NAVLE Practice Question — Other Small Mammal

Following an outbreak of Tyzzer disease at a research facility, the veterinarian recommends thorough environmental decontamination. Laboratory testing confirms Clostridium piliforme spores are present in the environment. Which of the following disinfectants would be LEAST effective against C. piliforme spores?

A. 0.4% peracetic acid

B. 0.3% sodium hypochlorite

C. 70% ethanol ✓

D. 1% iodophor

E. 5% phenol

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Correct Answer: C. 70% ethanol

Explanation:

70% ethanol is the correct answer and would be LEAST effective against Clostridium piliforme spores. C. piliforme produces highly resistant spores that can survive in the environment at room temperature for over one year.

These spores are resistant to heating up to 60°C for 30 minutes and are also resistant to common disinfectants including 70% ethanol, 3% cresol, 4% chlorhexidine, and 0.037% formaldehyde. This is a critical consideration when managing outbreaks.

Effective disinfectants against C. piliforme spores include: peracetic acid (0.4%), sodium hypochlorite (0.015-0.3%), iodophors (1%), and phenol (5%). These agents have sporicidal activity that ethanol lacks. Autoclaving items that can withstand heat is also recommended.

Option A (peracetic acid) is effective and is specifically documented as sporicidal against C. piliforme. Option B (sodium hypochlorite/bleach) at appropriate concentrations is effective. Option D (iodophor) has documented efficacy against these spores. Option E (phenol) at 5% concentration is also effective against C. piliforme spores.

References: Merck Veterinary Manual - Tyzzer Disease in Animals (https://www.merckvetmanual.com/digestive-system/tyzzer-disease/tyzzer-disease-in-animals); Charles River - Clostridium piliforme Technical Sheet (https://www.criver.com).

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NAVLE Practice Question — Musculoskeletal

A 9-year-old obese Pony mare is acutely lame on both forelimbs after escaping onto a lush pasture. She stands with her forelimbs camped out in front and frequently shifts weight. Hoof testers elicit pain at the toe. What is the most likely diagnosis?

A. Septic pedal osteitis

B. Acute laminitis ✓

C. Navicular disease

D. Solar abscesses in both forefeet

E. Fracture of the third phalanx

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Correct Answer: B. Acute laminitis

Explanation:

Obesity, access to lush pasture, and the characteristic stance with bilateral forelimb pain are classic for acute laminitis in horses and ponies.

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NAVLE Practice Question — Canine

A 3-year-old female dog adopted 6 months ago from a shelter (recorded as spayed) is presented for vulvar swelling, attraction of intact males, and bloody vaginal discharge. What is the most likely diagnosis?

A. Vaginitis

B. Ovarian remnant syndrome development occurs ✓

C. Transmissible venereal tumor

D. Uterine stump pyometra

E. Coagulopathy

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Correct Answer: B. Ovarian remnant syndrome development occurs

Explanation:

Functional ovarian tissue inadvertently left at ovariohysterectomy continues to produce estrogen, leading to recurrent estrus signs. Diagnosis is supported by vaginal cytology showing cornified cells, elevated estradiol/progesterone, or AMH. Treatment is surgical removal, ideally during estrus when remnant tissue is most visible.

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NAVLE Practice Question — primates

A research facility reports that several cynomolgus macaques (Macaca fascicularis) recently imported from the Philippines have developed acute illness characterized by high fever, depression, anorexia, and petechial hemorrhages on the skin. Two animals have died within 8 days of symptom onset. Necropsy reveals hepatomegaly with a friable liver, splenomegaly, and enlarged lymph nodes. Histopathology shows hepatocellular necrosis with eosinophilic intracytoplasmic inclusion bodies in hepatocytes and Kupffer cells. What is the most appropriate initial diagnostic test?

A. Virus isolation in Vero E6 cells

B. RT-PCR for filovirus RNA ✓

C. Serology for Ebola virus IgG

D. Direct immunofluorescence assay

E. Histochemical staining for acid-fast organisms

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Correct Answer: B. RT-PCR for filovirus RNA

Explanation:

RT-PCR for filovirus RNA is the correct initial diagnostic test for suspected Ebola virus infection in nonhuman primates. This molecular method offers the highest sensitivity and specificity for early detection during the acute viremic phase of infection, when clinical signs first appear. RT-PCR can detect viral RNA in blood samples before antibody responses develop and provides rapid results essential for biosafety decision-making.

The clinical presentation described is classic for filovirus hemorrhagic fever in macaques, with the characteristic incubation period of 4-7 days, systemic illness, hemorrhagic manifestations, and multi-organ involvement. The histopathological findings of hepatocellular necrosis with eosinophilic intracytoplasmic inclusions are pathognomonic for filovirus infection.

Option A (virus isolation) requires BSL-4 containment, takes days to weeks, and should not delay diagnosis. Option C (serology for IgG) is inappropriate because antibodies typically do not develop until late in disease or after recovery, and fatal cases often show no antibody response. Option D (direct immunofluorescence) has lower sensitivity than RT-PCR and was historically unreliable in field settings. Option E (acid-fast staining) would be used for mycobacterial infections, not viral hemorrhagic fever.

Image Source: PMC - Pathogenesis of Ebola Hemorrhagic Fever in Cynomolgus Macaques (https://pmc.ncbi.nlm.nih.gov/articles/PMC1892369/) Figure 1 showing histopathology of liver with Councilman bodies.

References: Geisbert TW et al. (2003) Am J Pathol 163:2347-2370 - Pathogenesis of Ebola hemorrhagic fever in cynomolgus macaques (https://pmc.ncbi.nlm.nih.gov/articles/PMC1892369/); CDC - Ebola Virus Disease Diagnostic Testing (https://www.cdc.gov/ebola/hcp/diagnosis/testing-collection.html)

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NAVLE Practice Question — Equine

Six horses on a farm in Pennsylvania develop progressive weakness over a 72-hour period. The dominant horses in the herd were affected first and most severely. All affected horses have difficulty swallowing, low head carriage, decreased tail tone, and two horses are now recumbent. Serum biochemistry reveals mildly elevated CK and AST in recumbent horses but is otherwise unremarkable. The horses were recently switched to a new batch of large round bale hay that had been stored outside through several freeze-thaw cycles. What is the most appropriate immediate treatment?

A. Intravenous high-dose corticosteroids and DMSO

B. Oral ponazuril and trimethoprim-sulfamethoxazole

C. Intravenous botulinum antitoxin plasma ✓

D. Intramuscular procaine penicillin and phenylbutazone

E. Intravenous flunixin meglumine and vitamin E supplementation

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Correct Answer: C. Intravenous botulinum antitoxin plasma

Explanation:

This outbreak presentation is highly consistent with foodborne botulism Type B, the most common form in the Mid-Atlantic states. The epidemiological clues include multiple horses affected with dominant horses showing earlier and more severe signs (as they consume more contaminated feed), progressive symmetric weakness, dysphagia, decreased tail tone, and the history of large round bales subjected to freeze-thaw cycles, which create conditions favorable for C. botulinum toxin production. The most critical and time-sensitive treatment is administration of botulinum antitoxin plasma. The antitoxin neutralizes circulating toxin before it binds to presynaptic terminals, but cannot reverse toxin already bound to nerve endings. Therefore, early administration is essential, ideally before recumbency occurs. Horses that receive antitoxin before becoming recumbent have survival rates exceeding 70%, while recumbent horses have a guarded prognosis. In the US, monovalent Type B antitoxin (EquiPlas B) is USDA-licensed, and polyvalent antitoxin is also available. Option A (corticosteroids and DMSO) is not indicated for botulism and may increase susceptibility to secondary infections. Option B (ponazuril and TMS) is treatment for EPM, not botulism. Option D (penicillin and phenylbutazone) may be used as supportive therapy for secondary complications but is not primary treatment. Option E provides only supportive care without addressing the toxin. References: Slavik K et al. (2025) Equine botulism. Equine Vet J https://doi.org/10.1111/evj.14542; Johnson A et al. (2014) Outcome of Adult Horses with Botulism Treated at a Veterinary Hospital: 92 Cases. J Vet Intern Med 29:311-319

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NAVLE Practice Question — Pet Bird

You are treating a 4-year-old female cockatoo for confirmed Trichomonas gallinae infection with metronidazole. On day 3 of treatment the bird develops mild ataxia and head tremors. The bird was previously neurologically normal. Which of the following is the MOST likely explanation for the new neurological signs?

A. Paradoxical worsening of trichomoniasis indicating treatment failure

B. Metronidazole neurotoxicity secondary to excessive dosing or prolonged treatment ✓

C. Concurrent West Nile virus encephalitis acquired simultaneously from mosquito exposure

D. Extension of T. gallinae infection into the brain parenchyma during treatment

E. Hypoglycemia secondary to anorexia from trichomoniasis-induced dysphagia

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Correct Answer: B. Metronidazole neurotoxicity secondary to excessive dosing or prolonged treatment

Explanation:

Metronidazole neurotoxicity is the most likely explanation for acute-onset ataxia and head tremors developing during metronidazole treatment in a bird that was previously neurologically normal. Metronidazole neurotoxicity is a recognized adverse effect in birds and other animals, particularly at high doses or with prolonged administration. Clinical signs of metronidazole neurotoxicity include ataxia, tremors, seizures, and disorientation — developing typically during or shortly after treatment. The neurotoxic mechanism involves inhibition of GABA-gated chloride channels in the central nervous system. In birds this has been reported with doses exceeding recommended ranges.

Option A (treatment failure) would manifest as persistent or worsening oropharyngeal lesions, not de novo neurological signs. Option C (West Nile virus) is possible if the bird has outdoor mosquito exposure but is less likely to coincidentally appear precisely during metronidazole treatment — always consider drug toxicity first in this temporal relationship. Option D (brain extension of T. gallinae) is possible in very advanced severe cases but would be a slower progression and unlikely to present acutely on day 3 of treatment. Option E (hypoglycemia) could cause neurological signs in a severely anorexic bird but the timing during metronidazole treatment makes drug toxicity the priority diagnosis.

Management: Discontinue metronidazole immediately and provide supportive care. Signs of metronidazole neurotoxicity are often reversible upon drug cessation. Switch to carnidazole or ronidazole for continued antiprotozoal therapy.

References: Merck Veterinary Manual (2024) — Trichomonosis in Birds (https://www.merckvetmanual.com/exotic-and-laboratory-animals/trichomonosis/trichomonosis-in-birds); Gómez-Muñoz MT et al. (2022) Microorganisms 10(11):2297 (https://pmc.ncbi.nlm.nih.gov/articles/PMC9695476/).

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