Emerging diseases and solutions: Innovations in feline practice

I have been a veterinarian for 31 years. Although progress in feline medicine was steady during the first 10 years of my career, the last decade has been like a snowball rolling down a hill. Technological advances have revolutionized much of what we do. Digital imaging systems-including radiography, ultrasonography, and magnetic resonance imaging-are used in private practices or, at the minimum, are readily obtainable. Laser surgery units make some of the most difficult surgical procedures relatively easy. However, there are less spectacular advances that merit attention. In this article, I'll discuss some of the innovations that I've found most useful in my feline practice.

Gary D. Norsworthy, DVM, DABVP

Diagnosing diarrhea

Diarrhea is one of the most common and frustrating conditions that I see. It's challenging because it can have so many causes. Many of the causal diseases, such as giardiasis, are not easily iagnosed. Practitioners consider giardiasis common based on how often drugs are used to treat the disease, but giardiasis cannot be confirmed easily. Several test modalities are available, but it's often necessary to repeat those tests several times before a positive result occurs in an infected cat. Recently, an in-house test was introduced that detects

Giardia

-soluble antigen (SNAP Giardia-IDEXX). This antigen is found consistently in the stool of infected dogs and cats. Tests that detect antigens are much more sensitive than fecal examination tests for

Giardia

cysts, which are found only sporadically. The SNAP Giardia test is also specific for

Giardia

. I routinely use the

Giardia

antigen test on cats with diarrhea. It can also be used as part of a routine screening profile for outdoor cats. The zoonotic implications of this disease make its detection even more important.

Diagnosing and treating renal diseases

Chronic renal disease is one of the most common causes of death in senior cats. Although renal failure may be detected easily, it's not easy to treat; treatment is often unsuccessful when the clinical signs include anorexia, dehydration, and lethargy, and when the serum creatinine concentration is greater than 7.0 mg/dl. Renal insufficiency is the stage of chronic renal disease that precedes failure, often by many months.1 Cats with renal insufficiency have mildly elevated creatinine values, but may be asymptomatic or have very mild signs (mild polyuria and polydipsia, decreased appetite, and gradual weight loss). These cats have lost more than 75% of their renal function, but they can remain relatively healthy as long as appropriate measures are taken.As the disease progresses to renal failure, it's accompanied by marked polyuria and polydipsia, anorexia, marked weight loss, dehydration, and lethargy.

Figure 1. An Endo-Sof subcutaneous catheter implanted in a patient. (Photos Dr. Gary Norsworthy)

Screening senior cats for evidence of renal insufficiency is a rewarding undertaking. Common laboratory tests, such as creatinine and BUN concentrations and urine specific gravity, can help diagnose the condition. Treatment includes a renal disease diet, potassium supplementation, and benazepril administration. Renal diets contain reduced protein and phosphorus levels and are nonacidifying. Potassium gluconate supplementation (2 to 4 mEq/day) can improve renal function in many cats because cellular potassium depletion results in decreased appetite, body weight, and activity level. It can also decrease the glomerular filtration rate (GFR).2

The ACE inhibitor benazepril has been shown to slow the progression of renal disease and extend the lives of these patients.3 The dosage is 2.5 mg/cat once a day for cats weighing up to 5 kg. Cats weighing 5 kg or more are given 5 mg/cat once a day. I begin treatment with this drug as soon as renal insufficiency is diagnosed.

Figure 2. Systemic blood pressure can be determined with Doppler or oscillometric units using a peripheral vein.

Owners can administer subcutaneous fluids as the disease progresses. Fluids may be infused with a conventional intravenous (IV) administration set and needle or through the new subcutaneous fluid catheter (Endo-Sof subcutaneous catheter- Global Veterinary Products). The catheter is surgically implanted near the dorsal midline. A fitting that accepts a standard IV line is sutured to the skin (Figure 1). I recommend this catheter when owners or cats are needle averse. They're not tolerated by all cats, but can be extremely helpful, even life-saving, for others. A phosphate binder (PhosLo, 1/4 tablet or 167 mg b.i.d.), appetite stimulant (cyproheptadine, 2 mg b.i.d.), hypotensive agent (amlodipine, 0.6 to 1.0 mg once a day), and erythropoietin (100 U/kg one to three times per week) are prescribed on an as needed basis.

Diagnosing and treating hypertension

When assessing cats with chronic renal disease, it's also important to monitor them for systemic hypertension. This disease is often associated with renal failure; however, it's not limited to this stage of renal disease. Retinal detachment, hyphema, cardiovascular accidents, and seizures may occur secondary to hypertension in cats with renal insufficiency.

Two major types of equipment are available for measuring blood pressure in cats. Inexpensive, dependable Doppler units detect systolic blood pressure and are widely used because they are simple to operate (Figure 2) (Vet-Dop-Vmed Technology;Model 811-B-Parks Medical). Oscillometric units are more expensive, but they determine systolic, diastolic, and mean blood pressure (Memoprint-S+B medVET; Cardell BP Monitors, Sharn Veterinary Inc.).Although some clinicians prefer to know all three values, the systolic measurement is adequate for determining hypertension in cats (and dogs).

Hypertensive cats should be treated with a hypotensive agent. ACE inhibitors may be used, but their hypotensive effect is not as efficient as the calcium channel blocker amlodipine (0.6 to 1.0 mg once a day). Administer the drug for two days, then reassess the patient's blood pressure. The desired systolic range is 130 to 150 mm Hg. Adjust the dosage according to the response. More than 90% of the time, systemic hypertension is secondary to either hyperthyroidism or chronic renal disease. Hypertensive patients should be tested for these diseases and treated appropriately.

Diagnosing and treating heart disease

As a veterinary school graduate in 1972, I was taught that heart disease was rare in cats. However, that was only because we didn't have the tools to detect it. A screening lead II electrocardiogram (ECG) is now part of my annual feline exam, as detailed in the article, "Healthier patients are an annual visit away,"

Initially, I used ECGs as public relations tools. By performing the ECG in front of clients and providing them with a portion of the tracing, I tried to demonstrate the important role of annual exams in detecting health problems. In addition, I wanted to show them how thorough I was in my examinations. After performing screening ECGs for a few months, I began to find heart abnormalities in cats more frequently than I anticipated. I find an ECG abnormality or an audible murmur on auscultation in about 1 out of 25 seemingly healthy cats. This forced me to better understand ECG interpretation-a skill that eluded me while I was in veterinary school. It also increased my appreciation of the value and limitations of ECGs. I don't expect to make a specific diagnosis from the ECG. Instead, I use it as a tool to identify cats with heart disease. I use the stethoscope in the same manner.

Figure 3a.

When I diagnose a cardiac abnormality with either auscultation or an ECG tracing, I inform the owner that heart disease is present and recommend thoracic radiographs and an echocardiogram. Radiographs allow me to assess the cardiac silhouette, lung fields, tracheal position, the pleural space, and the major vessels. However, they're not sensitive enough to diagnose early heart disease or specific heart diseases. Therefore, an echocardiogram is essential to make a conclusive diagnosis and determine effective treatment (Figures 3a, 3b, & 4).

Figure 3a & 3b. Lateral and dorsoventral thoracic radiographs of a 2-year old cat with a history of syncope. The cardiac silhouette is with in normal limits.

We are now able to treat heart disease with more success than in the past. We used to rely on digoxin, but its side effects and lack of efficacy in cats made treatment of heart failure disappointing. The mainstay of my treatment protocol for congestive heart failure in cats is benazepril (2.5 mg/cat once a day for cats weighing up to 5 kg; cats weighing 5 kg or more are given 5 mg/cat once a day). Not only is it more effective than digoxin in correcting heart failure, but it has been shown to cause thinning of the left ventricular wall in cats with early hypertrophic cardiomyopathy.4 It recently became available as a generic product, so its cost is reasonable. Spironolactone, an older drug previously used as a diuretic, is making a resurgence because it helps treat heart failure when used in subdiuretic doses (6.25 mg b.i.d.). Atenolol, a betablocker, is indicated when premature ventricular contractions or tachycardia is present. It should be given at 6.25 mg b.i.d. Furosemide (1.1 to 2.2 mg/kg b.i.d.) and topical nitroglycerin round out the list of cardiac drugs I typically use.

Diagnosing and treating heartworm disease

Once thought to be a rare anomaly, feline heartworm infection is diagnosed with increasing frequency in most parts of the United States. Heartworm disease is an important cause of heart disease in cats. However, diagnosis and treatment are plagued with problems. Because the worm burden is so low in most affected cats, serum antigen tests often yield false negative results. Serum antibody tests lack the specificity to be reliable. Therefore, the typical diagnostic workup for feline heartworm disease includes serum antigen and antibody tests, thoracic radiographs, and echocardiograms.

Treatment is based on supportive care of the cat, usually with corticosteroids, until the adult worms die. The only approved drug for treating adult heartworms in dogs, melarsomine, is toxic in cats. Therefore, the best way to deal with feline heartworm disease is prevention. A chewable preventive product is available, but cats' finicky appetites often lead to noncompliance. Applying selamectin (Revolution-Pfizer Animal Health) monthly is a very practical way to prevent heartworm infection and control most external and internal parasites.

Figure 4. Echocardiogram of the cat in Figure 3 showing greatly thickened left ventricular walls associated with hypertrophic cardiomyopathy. Because the wall thickening is principally directed inward, severe disease was present even though the cat had a normal cardiac silhouette on radiography. Radiographs lack the sensitivity of echocar-diography in diagnosing this disease.

Advances in ultrasonography

Ultrasonography is a skill that many practitioners think is beyond their reach. Fewer than 10% of primary-care, small-animal hospitals own an ultrasound machine. However, like other skills practitioners possess, it can be mastered with determination. The major ultrasound companies offer hands on, off-site training courses to help practitioners become proficient with this technology. Several hours of personal instruction are needed to develop skills and build confidence. Most practitioners become quite proficient in routine scanning procedures after a few courses, as long as they practice with their own machines.

Most ultrasound units allow images to be saved. Images can then be e-mailed to veterinary consultants for interpretation. Using the specialists' feedback, practitioners can review the images on the ultrasound machine's hard drive to improve their interpretation abilities. Most practitioners who use telemedicine services do so regularly at first and then decrease their use as they become more proficient.

Conclusion

Feline medicine has never been more rewarding. Once mistakenly considered "small dogs,"cats have become a major part of the pet population and smallanimal practice. Improved healthcare, an indoor lifestyle, and more dedicated owners allow many cats to live 20 years or longer.They are the sole focus of my practice, but they also comprise a large portion of most small-animal practices.With increased understanding of feline diseases and new diagnostic and therapeutic products,we're able to give new meaning to the saying, "cats have nine lives."

References

1. Brown S.A.: Evaluation of chronic renal disease: A staged approach.

Comp. Cont. Ed. 21

(8):752-763; 1999.

2. DiBartola, S.P.; Rutgers, H.C.: Diseases of the kidneys. The Cat: Diseases and Clinical Management, 2nd Ed. (R.G. Sherding, ed.) Churchill Livingstone, London, England, 1994;pp 1711-1767.

3. Brown, S.A. et al.: Effects of the angiotensin converting enzyme inhibitor benazepril in cats with induced renal insufficiency. AJVR 62(3):375-383; 2001.

4. Amberger, C.N. et al.: Effects of benazepril in the treatment of feline hypertrophic cardiomyopathy: Results of a prospective, open-label,multi-center clinical trial. J Vet Cardiol 1(1):19-26; 1999.

Dr. Gary D. Norsworthy is a 1972 graduate of Texas A&M University. He practiced for two years at the Cat Clinic of Seattle,Washington. He practiced small-animal medicine from 1974 to 1999 in San Antonio,Texas, and then opened a feline-only practice in San Antonio in early 2000.

Dr. Norsworthy is also an accomplished author, editor, and lecturer.He has presented more than 300 lectures at veterinary conferences across the country.