2.2 Feline Common Clinical Diseases & Medicine
Key Takeaways
- Urethral obstruction leads to life-threatening hyperkalemia, managed using calcium gluconate for cardioprotection and insulin/dextrose to shift potassium.
- Alimentary lymphoma is distinguished from IBD via biopsies showing muscularis propria thickening and clonal T-cell receptor rearrangements (PARR).
- Feline ATE (saddle thrombus) presents with the 5 Ps (pain, paresis, pulselessness, pallor, poikilothermia) and poses a high risk of reperfusion hyperkalemia.
- Methimazole reversibly inhibits thyroid peroxidase, but may unmask chronic kidney disease post-treatment due to decreased GFR.
- Hepatic lipidosis requires aggressive enteral feeding tube placement, with cautious titration to avoid hypophosphatemic refeeding syndrome.
2.2 Feline Common Clinical Diseases & Medicine
Feline medicine requires an understanding of several common infectious, endocrine, neoplastic, and metabolic disorders. Mastery of their pathophysiology, clinical presentation, and specific diagnostics is essential for NAVLE preparation.
Lower Urinary Tract Disease & Urethral Obstruction
Feline lower urinary tract diseases are common clinical presentations. Feline Idiopathic Cystitis (FIC) is a sterile, inflammatory condition of the bladder. The pathophysiology involves an alteration in the bladder's glycosaminoglycan (GAG) protective lining, allowing noxious urine solutes to stimulate sensory C-fibers. This stimulation triggers the release of substance P, causing neurogenic inflammation, vasodilation, and mucosal hemorrhage. Stress is the primary driver of FIC episodes due to an uncoupling of the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis. Management is focused on Multimodal Environmental Modification (MEMO) (e.g., providing adequate resource distribution, vertical space, scratching posts, and synthetic pheromones), dietary changes (feeding canned food to reduce urine specific gravity to <1.025), and analgesia.
Urethral Obstruction (UO) is a life-threatening emergency primarily affecting male cats due to their narrow, long urethra. Obstructions are caused by urethral plugs (composed of struvite crystals and inflammatory proteins), uroliths, or urethral spasms. Complete obstruction leads to post-renal azotemia, metabolic acidosis, and progressive hyperkalemia. Hyperkalemia is cardiotoxic, raising the resting membrane potential of cardiac cells and decreasing excitability.
- ECG Abnormalities: Initial changes include tall, tented T-waves and a prolonged PR interval. As potassium levels rise, there is a loss of P-waves (atrial standstill) and widening of the QRS complexes, eventually progressing to a sine-wave pattern and ventricular fibrillation.
- Medical Management:
- Calcium Gluconate (10%): The first-line therapy for hyperkalemic cardiotoxicity. Administered slowly IV, it stabilizes the myocardial cell membrane by restoring the threshold potential. It does not lower serum potassium but protects the heart for 20 to 30 minutes.
- Regular Insulin and Dextrose: Regular insulin (0.5 U/kg IV) promotes the intracellular shift of potassium via the sodium-potassium ATPase pump. Dextrose (2 g per unit of insulin) is administered concurrently to prevent hypoglycemia.
- Sodium Bicarbonate: Promotes an intracellular potassium shift in exchange for hydrogen ions as it buffers metabolic acidosis.
- Decompressive Cystocentesis: Can be performed carefully if catheterization is delayed, to relieve backpressure and reduce systemic potassium.
Alimentary Lymphoma vs. Inflammatory Bowel Disease (IBD)
Gastrointestinal diseases in cats present with chronic vomiting, diarrhea, weight loss, and anorexia. Differentiating inflammatory bowel disease (IBD) from low-grade (small-cell) alimentary lymphoma is a diagnostic challenge.
| Feature | Inflammatory Bowel Disease (IBD) | Low-Grade Alimentary Lymphoma |
|---|---|---|
| Cell Population | Polyclonal infiltration (lymphocytes & plasma cells). | Monoclonal infiltration (small, mature T-lymphocytes). |
| Infiltration Depth | Restricted to the mucosal layer. | Extends into the submucosa and muscularis propria. |
| Ultrasound Findings | Diffuse mucosal thickening; layering is preserved. | Muscularis propria thickening; loss of layering. |
| Definitive Diagnosis | Polyclonal results on PARR. | Clonal T-cell receptor rearrangement on PARR. |
| Standard Treatment | Dietary modification, prednisolone. | Oral chlorambucil and oral prednisolone. |
| Median Survival Time | Good long-term prognosis (years). | 1.5 to 2 years. |
Full-thickness intestinal biopsies obtained via laparotomy or laparoscopy are the gold standard. Endoscopic biopsies are less invasive but may miss lesions restricted to the deeper layers. PCR for Antigen Receptor Rearrangement (PARR) is used to confirm clonality: a clonal (monoclonal) T-cell receptor rearrangement supports lymphoma, while a polyclonal rearrangement supports IBD. High-grade (large-cell) lymphoma requires multi-agent chemotherapy (such as the CHOP protocol) and has a poor prognosis, with median survival of 6 to 9 months.
Hypertrophic Cardiomyopathy (HCM) & Aortic Thromboembolism (ATE)
HCM is the most common heart disease in cats. It is characterized by concentric left ventricular hypertrophy leading to diastolic dysfunction (impaired filling). Breeds like the Maine Coon and Ragdoll carry an autosomal dominant mutation in the myosin-binding protein C (MYBPC3) gene. Left atrial enlargement occurs as pressure backs up from the stiff left ventricle.
Aortic Thromboembolism (ATE), or saddle thrombus, is a devastating sequela. Blood stasis in the dilated left atrium promotes thrombus formation. A fragment of this thrombus dislodges, travels down the aorta, and lodges at the aortic trifurcation, obstructing blood flow to the hind limbs.
- Clinical Presentation (The 5 Ps): Pain (vocalization and distress), Paresis or paralysis (bilateral hind limb motor loss), Pulselessness (absent femoral pulses), Pallor (pale or cyanotic footpads), and Poikilothermia (cold hind limbs).
- Reperfusion Injury: A major risk during therapy. When blood flow is restored to ischemic tissues, a sudden release of potassium, lactic acid, and reactive oxygen species enters the systemic circulation, causing severe systemic hyperkalemia, metabolic acidosis, and fatal arrhythmias. Clopidogrel and heparin are used to prevent further thrombus formation.
Feline Endocrine Disorders
Hyperthyroidism
The most common endocrine disease of senior cats, caused by benign multinodular adenomatous hyperplasia or adenoma (99%) of the thyroid gland.
- Clinical Signs: Weight loss despite polyphagia (ravenous appetite), hyperactivity, poor hair coat, vomiting, tachycardia, and a palpable thyroid slip.
- Treatment:
- Methimazole: Reversibly inhibits thyroid peroxidase. Side effects include GI upset, hepatotoxicity, bone marrow suppression (thrombocytopenia/leukopenia), and severe facial pruritus/excoriation.
- Radioactive Iodine (I-131): Curative gold standard; selectively destroys hyperfunctional thyroid tissue.
- Dietary Iodine Restriction (y/d): Restricts iodine necessary for thyroid hormone synthesis.
- Thyroidectomy: Surgical removal (risk of damaging parathyroid glands, causing hypocalcemia).
- Unmasking Chronic Kidney Disease (CKD): The hyperthyroid state increases cardiac output and GFR, which decreases serum creatinine. Resolving hyperthyroidism reduces GFR to normal, which often unmasks underlying CKD, leading to a rise in creatinine and BUN.
Diabetes Mellitus
Similar to human Type 2 diabetes, involving insulin resistance, obesity, and islet amyloidosis.
- Clinical Signs: PU/PD, polyphagia, weight loss, and a plantigrade stance (diabetic neuropathy).
- Diagnosis: Persistent hyperglycemia and glucosuria. Fructosamine is measured to rule out stress-induced hyperglycemia, as stress can transiently raise blood glucose to over 300 mg/dL.
- Diabetic Ketoacidosis (DKA): A crisis characterized by metabolic acidosis, ketonuria, vomiting, and dehydration. Treatment requires aggressive IV fluid resuscitation (typically using 0.9% NaCl or a balanced crystalloid), potassium and phosphorus supplementation, and regular insulin infusions.
Feline Infectious and Hepatic Disorders
- Feline Infectious Peritonitis (FIP): Caused by a mutation of Feline Enteric Coronavirus (FCoV). The mutated virus replicates in macrophages, leading to immune-mediated pyogranulomatous vasculitis. Wet FIP presents with high-protein, low-A:G ratio pleural or peritoneal effusions. Dry FIP causes granulomas in organs, leading to uveitis, chorioretinitis, and neurological signs. Treatment has been revolutionized by antiviral drugs like GS-441524 (a nucleoside analog).
- Feline Herpesvirus-1 (FHV-1): A DNA virus causing upper respiratory infection and ocular lesions, notably dendritic corneal ulcers (pathognomonic branch-like lesions). FHV-1 becomes latent in the trigeminal ganglion and recrudesces during periods of stress.
- Feline Hepatic Lipidosis: The most common liver disease in cats, triggered by prolonged anorexia in an obese cat. Rapid mobilization of peripheral fats exceeds the liver's capacity for beta-oxidation and VLDL export, causing severe intrahepatic lipid accumulation and cholestasis.
- Diagnostics: Characterized by a marked increase in alkaline phosphatase (ALP) with normal or minimally increased gamma-glutamyltransferase (GGT).
- Treatment: Enteral feeding via an esophagostomy or gastrostomy tube. Food must be introduced gradually to prevent refeeding syndrome, which is characterized by life-threatening hypophosphatemia and hypokalemia as insulin drives these electrolytes intracellularly.
An obese 6-year-old male neutered domestic shorthair cat is presented for acute-onset hind limb paralysis, vocalization, and extreme distress. On physical examination, the hind limbs are cool to the touch, the femoral pulses are absent bilaterally, and the footpads are cyanotic. Which of the following is the most common underlying cause for this condition?
A 12-year-old female spayed domestic longhair cat is diagnosed with hyperthyroidism. The owner is concerned about the potential side effects of methimazole. Which of the following adverse effects would require immediate and permanent discontinuation of the drug?
An obese 8-year-old female spayed cat is presented with a 2-week history of anorexia, lethargy, and vomiting. Physical examination reveals marked icterus, hepatomegaly, and severe muscle wasting. Laboratory analysis shows a severe elevation in alkaline phosphatase (ALP), normal gamma-glutamyltransferase (GGT), and hyperbilirubinemia. What is the cornerstone of successful therapy for this cat's condition?