Feline Drug Therapy Update

A study being performed at Colorado State University is testing feline drugs on specific conditions.

This column is based on abstracts presented at the 24th annual ACVIM Forum in Louisville (2006).

In recent years molecular biology has resulted in the reclassification of Hemobartonella felis as a Mycoplasma species. There are various forms of this organism; the one usually responsible for the typical clinical signs of anemia is Mycoplasma haemofelis. Less pathogenic is Mycoplasma haemominutum.1

Diagnosis of these organisms can be a challenge.2 Clinicians are always suspicious of this disease when a cat is anemic, especially with hemolytic anemia. In some cases the organism can be seen on a blood smear. It helps if these blood smears are done immediately; if stored too long in EDTA, the organisms can detach.

A diagnostic test that has become much more common and is highly reliable is PCR which detects DNA of the organism. In one experimental study, 100 percent were positive on PCR, yet only around 40 percent were positive on cytology.3

A variety of treatments have been used to treat cats with clinical signs and this infection. Common antibiotics include tetracycline or doxycycline. Generally treatment is continued for two to three weeks. Enrofloxacin and azithromycin have all been tried as well.

Though all of these antibiotics may be of benefit treating the clinical signs, none of them seems capable of resulting in a PCR negative state if testing is carried out over a longer period of time, though cats do tend to be PCR negative during antibiotic therapy.

Researchers from Colorado State University carried out a study to investigate if marbofloxacin can be effective for treating M. haemofelis.4

Previous research had shown that marbofloxacin decreased the number of M. haemominutum copies without clearing the infection.1

Twelve cats were infected with M. haemofelis. Blood was collected before and after infection for CBC and PCR assay. Six cats were treated with marbofloxacin (1.25 mg/lb PO daily for 14 days) once the PCV decreased below 30 percent or body temperature exceeded 102.5° F.

If cats were PCR positive on Day 7 of therapy they were treated for a total of 28 days. The treated cats had higher PCVs than the untreated cats. Five of the six were PCR positive on Day 7 and received antibiotics for 28 days.

Once treatment was completed, one untreated cat and two treated cats that had been previously positive were PCR negative.

One limitation to the study is that PCR assay was only carried out for six weeks, and some of the negative cats may have become positive later. As with other antibiotics, it appears that at the dose used, marbofloxacin can help with clinical disease, but clearing of the infection is unlikely.

Potassium Bromide

Epilepsy is a relatively uncommon problem in cats, though treatment seems to be less successful. Phenobarbital is the most commonly used drug and can be efficacious.

As with dogs, side effects can include neutropenia, sedation, polyuria, polydipsia and coagulation defects.

If response to phenobarbital is poor, diazepam is commonly added to the protocol. This, too, can have adverse effects in cats and severe liver disease has been reported in cats given oral diazepam. In dogs, potassium bromide has become a popular medication to treat seizure disorders, either as a sole agent or as an add-on to phenobarbital.

Potassium bromide has been used in cats.5 A dose of 30 mg/kg/day resulted in therapeutic blood levels within two weeks in experimental cats.

This report also retrospectively looked at the results of treating 17 cats with seizures. In seven of 15 cats where seizure activity was reported, seizures were well controlled. In eight cats, however, adverse side effects occurred. Coughing was most common, with six cats being affected.

One cat was euthanized because of the severity of the cough and lung changes noted radiographically. In two others bromide was discontinued because of the cough. The severe respiratory changes have been noted in other studies and abstracts.

Researchers from the Royal Veterinary College in the United Kingdom looked at the efficacy of bromide use in cats.6 Nine cats with epilepsy were studied. Bromide appeared to be efficacious in the treatment of seizures; the incidence decreased from around four per month to less than one per month on bromide.

Five of the cats had no seizures during the study period. Coughing developed in six of the nine cats after 8.2 + 2.1 months. This resulted in discontinuation of bromide in three cats, two owners elected to continue bromide in spite of the cough. One cat developed dermatitis that may have been associated with bromide use. Bromide was discontinued and the cat failed to respond to phenobarbital, resulting in its euthanasia.

This report shows that bromide appears to have efficacy for treating seizures in cats which is consistent with previous reports. The incidence of adverse side effects was relatively high and respiratory problems appear to be common.

Side effects can be severe enough to result in death. Nonetheless, bromide is a consideration in cats that are not responding to or cannot tolerate phenobarbital or diazepam. Close monitoring for any signs of respiratory problems is vital. Prompt discontinuation of bromide may result in resolution of respiratory signs.

Heparin

Thromboembolism is a rare occurrence in cats, other than in association with cardiomyopathy.

Aspirin, warfarin and heparin have been used in cats. Low molecular weight heparins (LMWHs) have become widely used in humans because they have many advantages over regular or unfractionated heparin.

LMWHs can be given less frequently, have a more predictable anticoagulant effect and are less likely to result in unwanted bleeding. Therapy with LMWH cannot be monitored with APTT as is common with heparin therapy; instead anti-Xa activity is the best way to monitor therapy with LMWHs.

In cats there have been few studies looking at the pharmacokinetics of LMWHs. A retrospective study reported on 57 cats given dalteparin (approximately 100 units/kg once or twice daily) for prolonged periods of time.7 The medication was well tolerated with few side effects. Bleeding was uncommon; of 43 cats with cardiomyopathy, eight went on to develop thromboembolism.

Researchers from the Western College of Veterinary Medicine in Saskatoon looked at the effects of dalteparin administration in eight cats.8 The cats received 100 U/kg twice daily subcutaneously for seven days.

A variety of coagulation parameters were assessed over the study period. Maximal anti-Xa activity was noted four hours after administration in of eight cats; in the other four cats anti-Xa activity was not detected.

Based upon these results, 100 U/kg of dalteparin twice daily is not adequate to achieve therapeutic blood levels as recommended in humans.

Anthony Carr, DVM, Dipl. ACVIM, is an associate professor at the Western College of Veterinary Medicine in Saskatoon, Saskatchewan, Canada.

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REFERENCES:

1. Tasker S, Caney S, Day MA et al. Effect of chronic feline immunodeficiency infection, and efficacy of marbofloxacin treatment, on 'Candidatus Mycoplasma haemominutum' infection. Microbes & Infection 8;653-61: 2006
2. Tasker S, Lappin MR. Haemobartonella felis: recent developments in diagnosis and therapy. J Feline Med Surg 4;3-11: 2002.
3. Westfall Ds, Jensen WA, et al. Inoculation of genotypes of Haemobartonella felis (California and Ohio variants) to induce infection in cats and the response to treatment with azithromycin. Am J Vet Res 62;687-691: 2001.
4. Ishak Am, Dowers KL, et al. Marbofloxacin for the treatment of experimentally induced Mycoplasma haemofelis infection in the cats. J Vet Intern Med 20;760:2006.
5. Boothe DM, George KL, Couch P. Disposition and clinical use of bromide in cats. JAVMA 221;1131-1135: 2002.
6. Volk HA, Chanlder KE, et al. New insights into efficacy and side effects of potassium bromide in epileptic cats. J Vet Intern Med 20;780: 2006.
7. Smith CE, Rozanski EA, et al. Use of low molecular weight heparin in cats: 57 cases (1999-2003). JAVMA 225;1237-1241: 2004.
8. Vargo C, Taylor S, Carr A. Determination of the effect of low molecular weight heparin administration on coagulation parameters in healthy cats. J Vet Intern Med 20;749:2006

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