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Hematologic effects of short-term oral diethylcarbamazine treatment given to chronically feline leukemia virus-infected cats
Cancer Letters, 45 (1989) 183-187 Elsevier Scientific Publishers Ireland Ltd.
183
Hematologic effects of short-term oral diethylcarbamazine treatment given to chronically feline Ieukemia virus-infected cats L.W. Kitchen” and F.J. Matherb of Cancer Biology, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115 and of Infectious Diseases, Tulane University Medical Center, 1430 Tulane Avenue, New Orleans, LA 70112 and bDepartment of Biostatistics and Epidemiology, School of Public Health and Tropical Medicine, Tulane University, 1430 Tulane Avenue, New Orleans, LA 70112 (U.S.A.) “Department Division
(Received 5 October 1988) (Revision received 16 February 1989) (Accepted 17 February 1989)
Summary
Diethylcarbamazine (N,N-diethyl-4-methyl1 -piperazine carboxamide [DECl) is widely used, especially in tropical regions, to prevent and treat filariasis. The a&$/aria/ effect of this drug has been attributed to immunomodulation. Evidence is accumulating to indicate fhaf DEC may mitigate the course of feline leukemia virus injection (FeLV) in cats. Previous studies have suggested that continuous oral DEC treatment given shortly afier evidence of FeLV infection prevents or delays lymphopenia and prolongs survival. The present study focuses on the hematologic effects of one month oral DEC treatment given to adult chronically FeLV-injected cats and uninfected cats as compared to untreated FeLV-infected cats. Such treatment frequently resulted in abruptly lowered peripheral lymphocyte counts in chronically FeLV-infected cats. Further studies are warranted to evaluate whether administration of DEC could eliminate circulafing retroviral-infected
cells.
Correspondence to: L.W. Kitchen, Division of Infectious Diseases, Tulane University Medical Center, 1430 Tulane Avenue, New Orleans, LA 70112, U.S.A.
Keywords: diethylcarbamazine;
feline leukemia virus; AIDS; HIV; lymphocytes. Introduction
Increased total white blood cell counts have been noted following treatment of filarialinfected humans with diethylcarbamazine (DEC) [2]. Administration of DEC to FeLVinfected cats - one animal mode1 for AIDS has frequently resulted in decreased serum viral infectivity and increased serum antibody levels to feline oncornavirus-associated cell membrane antigen (FOCMA) 151. In addition, continuous oral administration of DEC to two kittens shortly after they first tested positive for FeLV leukocyte antigen resulted in a reduced rate of decline of circulating lymphocytes on a statistically significant basis and prolonged survival in comparison to two untreated littermates [6]. DEC has not been shown to prevent infection of susceptible cells with FeLV in vitro (L. Kitchen, unpublished data). The present study examines the hematologic effects of short-term (1 month) administration of DEC given to adult chronically FeLV-infected cats.
0304~3835/89/$03.50 0 1989 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland
184
Materials and methods
A total of 35 cats were studied. Of these 35, 4 were 9-month-old littermate cats. All four were FeLV-naive cats that had been inoculated with identical doses of the Rickard strain of FeLV at approximately 6 weeks of age [9]. Two of these four animals (cats K and SU) were treated with DEC, and 2 were untreated (cats SA and P) . Cats K and SU were given 10 mg/kg of DEC orally as a single dose per day for 28 days. DEC treatment in the same dosage was also given for 30 days to 18 outbred cats that tested positive for FeLV leukocyte antigens and seven cats that tested negative for FeLV leukocyte antigen. Of these 18, 1 was 1 month pregnant when DEC treatment was begun and 17 were non-pregnant. In addition, six cats testing negative for FeLV leukocyte antigen given 1 ml LEUKOCELL IM were also studied. LEUKOCELL is a commercial FeLV vaccine manufactured by Norden Laboratories, Lincoln, Nebraska, U.S.A. Of these six, three cats were given 10 mg/kg per day of DEC orally for 2 weeks, and the remaining three cats were given vaccine without DEC medication. Tests for FeLV antigen in peripheral blood leukocytes were carried out by the Laboratory of Veterinary Oncology at the Memorial SloanKettering Cancer Center, New York City [3]. Blood samples for packed cell volume (PCV) and white blood cell counts were drawn at selected intervals (see Tables 1 and 2). The number of lymphocytes/mm3 peripheral blood was calculated by multiplying the proportion of lymphocytes observed on examination of 100 white blood cells in the peripheral blood smear by the total white blood cell count. Results
All four Rickard FeLV-inoculated cats tested positive for FeLV leukocyte antigens before and after 30 days treatment. Both DECtreated FeLV-Rickard cats developed transient lymphopenia (< 1500 lymphocytes/mm3 peripheral blood), whereas the untreated
FeLV-Rickard cats remained non-lymphopenic (Table 1). The treated FeLV cat with more profound lymphopenia after DEC treatment (cat K) also developed transient anemia (PCV = 23 at the end of 28 days of DEC treatment). The PCV of cat K tested in the normal range 6 days after cessation of DEC treatment. Other hematologic effects were not noted immediately following DEC treatment. The four cats were euthanatized for humane reasons when moribund. Untreated cat SA died with spinal lymphoma 62 days after the start of the trial. Untreated cat P died with thymic lymphoma after 259 days. Treated cat SU died with multifocal lymphoma/leukemia 92 days after the start of the trial, whereas treated cat K survived 384 days and died with thymic lymphoma. The two untreated cats survived an average
Table 1. Lymphocyte counts/mm3 peripheral blood/ packed cell volumes (PCV) in two adult Rickard FeLVinoculated cats (K, SU) treated with DEC and two untreated inoculated littermates (P, SA) . Time K (days) 0 28 34 59 80 129 i64 192 206 220 245 257 276
SU
P
SA
2560/33 730/23 52/24
2534/38 l&9/29
5075/32 7480/31
3820/37 1956/32
860/29
3565/39 1352/34
3720/38
7821/35
7*/39 1793/35 1x/33 S&2/36
2650/37 4203/36 5320/37 1811/33 3200/32 2004/35
1595/41.2
Cats K and SU were treated with DEC days l-28. Counts indicating lymphopenia (< 1500 lymphocytes/ mm3 blood) or indices corresponding to anemia (PCV < 24) are underlined.
of 161 days from the start of the trial, and the two treated cats survived an average of 238 days. Therefore, survival was prolonged in the treated group by an average of 77 days. Of 18 outbred naturally-infected adult cats testing positive for FeLV leukocyte antigens before and after receiving 10 mg/kg per day of oral DEC for 30 days, seven (39%) of the 18 tested lymphopenic before treatment and 11 (61%) of the 18 were lymphophenic after treatment. Twelve (67%) of the 18 cats were noted to have lower lymphocyte counts after DEC therapy as compared to pretreatment values. The six remaining cats had higher lymphocyte counts following DEC treatment. Of these six, the cat with the largest increase in lymphocyte counts (no. 18 in Table 2A) was pregnant and delivered 7 days before the posttreatment blood sample was drawn. The mean Table 2A.
Lymphocyte counts/mm3 peripheral blood before and after 30 days oral diethylcarbamazine treatment in 18 outbred FeLV-infected cats and 7 FeLVuninfected cats. FeLV-infected
cats
Cat.
Lymphocyte pre/post
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
4826/ 1076 1529/756 1908/ 1392 1430/359 -2624/ 1273 1921/m 1776/1074 5066/4836 6840/6204 3234/2950 2089/ 1592 1203/1120 -1035/ 1224 581/889 -825/1536 1560/2576 1116/1144 -a/2305
FeLV-uninfected count
cats
Cat.
Lymphocyte pre/post
1 2 3 4 5 6 7
1197/1551 1526/1704 2100/1656 1773/2290 7498/5115 4175/3108 11264/12768
corresponding to Values lymphopenia lymphocytes/mm3 blood) are underlined.
count
(< 1500
lymphocyte count of the 18 treated cats was 2244 before treatment and 1829 after treatment (Table 2A). Statistical evaluation to determine whether the lymphocyte values following DEC treatment were significantly lower compared to pretreatment values was done using two-tailed paired t-tests on the lymphocyte counts (expressed as log,,/mm3 blood) [7]. Lymphocyte counts were not significantly decreased following DEC treatment when data from all 18 outbred FeLV-infected cats in Table 2A were analyzed (P = 0.15). However, when the one pregnant cat was excluded, post-diethylcarbamazine lymphocyte counts in the remaining non-pregnant 17 cats were significantly lower than the initial counts (P = 0.045). No significant decrease in lymphocyte counts was noted following 30 days DEC treatment in seven FeLV-uninfected cats. One of the 18 outbred FeLV-infected cats was neutropenic (neutrophils < 2500/mm3 peripheral blood) before administration of DEC and 3 of the 18 cats were neutropenic after DEC was given. One of 10 FeLV-uninfected cats was neutropenic before treatment, and none were neutropenic following DEC treatment. Post-treatment anemia was noted in 1 of the 18 FeLV-infected outbred cats (pre-treatment PCV = PCV = 26 and the post-treatment 21). The mean PCV of these 18 cats was 36.6 pretreatment and 34.8 post-treatment. Anemia was not noted following DEC treatment given to 10 FeLV-uninfected cats. Of seven cats that tested negative for FeLV leukocyte antigens before and after 30 days DEC treatment, 1 was lymphopenic before treatment and none were lymphopenic after treatment. DEC administration did not result in notably decreased mean lymphocyte counts in 3 FeLV-vaccinated cats (Table 2B) The mean lymphocyte count of the 10 treated FeLVnegative cats was 3872 before treatment and 4755 after treatment. As was previously noted, the two DECtreated Rickard FeLV-inoculated cats in the present study survived an average of 77 days
186
Table 2B. Lymphocyte counts/mm3 peripheral blood before/2 weeks after 1 ml LEUKOCELL IM in six FeLV leukocyte antigen negative cats, three of which also received 2 weeks oral DEC treatment. Vaccinated/treated
Vaccinated/untreated
Cat
Cat
Lymphocyte counts pre/post
1 2 3
2600/2880 6480/4690 7750/6063
Lymphocyte counts
pre/post 1 2 3
2660/2340 4992/11695 1534/5320
IongeE> than
their two untreated littermates. There have been 10 deaths among the DECtreated 18 outbred FeLV-infected cats now followed for a total of 559 cat-months (since first documentation of FeLV leukocyte antigen in the cats’ circulating white blood cells). Two cats were lost to follow-up when they left their household after 14 months observation; these cats were not considered fatalities but months of observation until loss were included in the analysis. To date, 1.79 deaths per 100 catmonths observation have been noted. Discussion The results suggest that short-term oral DEC treatment may frequently result in transient lymphopenia in non-pregnant cats chronically infected with FeLV, but does not result in lymphopenia in FeLV-uninfected cats. Although the pregnant cat was excluded after analysis of the results, it is possible that parturition and associated stress may have resulted in higher lymphocyte counts in the pregnant cat in comparison to the remaining non-pregnant cats. We cannot exclude the possibility that the decrease in lymphocyte counts in the nonpregnant cats could be related to such factors as FeLV infection status, the existence of initially high counts in some cats, and cat-to-cat initial lymphocyte count variability. In order to confirm our hypothesis, the effect of DEC treatment on lymphocyte values of a larger
group of FeLV-infected cats would need to be compared with the course of lymphocyte values of a similar group of untreated FeLVinfected cats. Our results suggest that DEC treatment in chronically FeLV-infected cats may prolong survival, although more precise studies are needed to confirm this. A historical control study involving a large number of cats in a single household (which computed cats lost to follow-up in a similar manner to the present study) noted 34.6 deaths/1000 cat-months (or 3.46 deaths/100 cat-months) which exceeds the death rate noted in the 18 DECtreated FeLV outbred cats in the present study [ 11. The small number of Rickard FeLV-inoculated cats in the present study does not permit statistical evaluation of the effect of DEC treatment on survival in this category. Since previous studies have suggested that continuous DEC treatment begun soon after evidence of FeLV infection reduces the rate of decline of peripheral lymphocyte counts, it is possible that DEC treatment may prevent spread of FeLV to uninfected lymphocytes, perhaps by decreasing the number of circulating FeLV-infected cells in addition to decreasing serum viral infectivity [5]. This hypothesis would be particularly compelling if a small number of circulating lymphocytes were infected with FeLV early in the course of disease, and if the number of infected circulating lymphocytes increased with duration of infection. There is some experimental data supporting this possibility [lo]. Although the transient anemia in cat K after DEC treatment could be related to opportunistic infection, the fact that the PCV tested in the normal range 6 days after DEC was discontinued suggests that the anemia was related to DEC treatment. FeLV has been shown to infect feline erythroid precursors as well as neutrophils and platelets, and elimination of some circulating red blood cells bearing FeLV antigens may have occurred following DEC therapy [lo]. Although short-term DEC treatment rarely results in neutropenia and has not been shown to result in thrombocytopenia, severe throm-
187
bocytopenia (and anemia) accompanied by active hematopoiesis in bone marrow have been noted after long-term DEC treatment in one FeLV-infected cat [6]. Since human immunodeficiency virus (HIV) infects a low proportion of circulating blood cells, DEC treatment may have less profound hematologic effects in HIV-infected humans than in FeLV-infected cats [4]. Nevertheless, it is worth exploring whether administration of DEC to HIV-infected persons could result in killing or elimination of HIV-infected cells, given the growing consensus that elimination of HIV-infected cells may be important in the control of HIV infection, particularly if this can be accomplished shortly after introduction of the virus into the body [8]. Since evidence from several laboratories suggests that antibody-dependent cellular cytotoxicity of filarial larvae is enhanced by DEC in vitro, whether DEC could eliminate HIV-infected cells by this mechanism warrants study [12]. Further, given widespread use of DEC to control filariasis in the developing world, it is important to determine whether this drug can alter peripheral cell counts and T-lymphocyte subsets in HIV-infected humans, because such alterations could affect both individual clinical management decisions and research results. DEC is orally bioavailable and some studies suggest that DEC is concentrated in brain tissue. Therefore the question of whether DEC could also be useful as an adjunct therapy to eliminate some categories of leukemic cells also warrants evaluation [ 111. Acknowledgments I thank Dr. W.D. Hardy, Jr. and his colleagues for analysis of the peripheral blood leukocytes of the cats in this study for FeLV antigens, Dr. J.C. Kitchen, Jr., for assistance with manuscript preparation and Dr. S. Cotter
and Dr. N. Larson for veterinary assistance. This investigation was supported by D. Lu, the Department of Medicine at Tulane University Medical Center, and the New Orleans Veterans Administration Center. References 1
2
3
4
5
6
7
8 9
10
11 12
Francis, D.P., Essex, M., Jakowski, R.M., Cotter, SM., Lerer, T. J. and Hardy, W.D., Jr. (1980). Increased risk for lymphoma and glomerulonephritfs in a closed population of cats exposed to feline leukemia virus. Am. J. Epidemiol., 111,337-346. Goodman, L.S., Gilman, A.G., Rail, T.W. and Murad, F. (1985) The Pharmacological Basis of Therapeutics, MacMillan Publishing Company, New York, p. 1011. Hardy, W.D. Jr., Old, L.J., Hess, P.W., Essex, M. and Cotter, S. (1973). Horizontal transmission of feline leukemla virus. Nature, 244,266-269. Harper, M.E., Marselle, L.M., Gallo, R.C. and WongStaal, F. (1986). Detection of lymphocytes expressing human T-lymphotropic virus type III in lymph nodes and peripheral blood from infected individuals by in situ hybridization. Proc. Natl. Acad. Sci., 83,772-776. Kitchen, L.W. and Cotter, SM. (1988). Effect of diethylcarbamazine on serum antibody to feline oncomavirus-associated cell membrane antigen in feline leukemia viruscats. J. Clin. Lab. Immunol., 25,101-103. Kitchen, L.W., Mather, F.J. and Cotter, SM. Effect of continuous oral diethylcarbamastne treatment on lymphocyte counts of feline leukemia virus-infected cats. J. Clin. Lab. Immunol., in press. Kleinbaum, D.G. and Kupper, L.L. Appfied regression analysis and other multivariable methods (1978). Duxbury Press, Wadsworth Publishing Co., North S&rate, MA. Levy, J.A. (1988). The transmission of AIDS; the case of the infected cell. J. Am. Med. Assoc., 20,3037-3038. Rickard, C.G., Post, J.E., Noronha, F. and Barr, L.M. (1969). A transmissible &us-induced lymphocytfc leukemia of the cat. J. Natl. Cancer Inst., 42,987-1014. Rojko, J.L., Hoover, E.A., Mathes, L.E., Olsen, R. and Schaller, J.P. (1975). Pathogeneds of experimental feline leukemia virus infection. J. Natl. Cancer Inst., 63, 759765. Sass, M. (1976) Human Hlariasis: a global survey of epidemiology and control, University Park Press, Baltimore. Subrahmanyam, D. (1987) Antifilariafs and their mode of action, Ciba Foundation Symposium. Filariasis, John Wiley and Sons, Chichester, pp. 251-252.
183
Hematologic effects of short-term oral diethylcarbamazine treatment given to chronically feline Ieukemia virus-infected cats L.W. Kitchen” and F.J. Matherb of Cancer Biology, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115 and of Infectious Diseases, Tulane University Medical Center, 1430 Tulane Avenue, New Orleans, LA 70112 and bDepartment of Biostatistics and Epidemiology, School of Public Health and Tropical Medicine, Tulane University, 1430 Tulane Avenue, New Orleans, LA 70112 (U.S.A.) “Department Division
(Received 5 October 1988) (Revision received 16 February 1989) (Accepted 17 February 1989)
Summary
Diethylcarbamazine (N,N-diethyl-4-methyl1 -piperazine carboxamide [DECl) is widely used, especially in tropical regions, to prevent and treat filariasis. The a&$/aria/ effect of this drug has been attributed to immunomodulation. Evidence is accumulating to indicate fhaf DEC may mitigate the course of feline leukemia virus injection (FeLV) in cats. Previous studies have suggested that continuous oral DEC treatment given shortly afier evidence of FeLV infection prevents or delays lymphopenia and prolongs survival. The present study focuses on the hematologic effects of one month oral DEC treatment given to adult chronically FeLV-injected cats and uninfected cats as compared to untreated FeLV-infected cats. Such treatment frequently resulted in abruptly lowered peripheral lymphocyte counts in chronically FeLV-infected cats. Further studies are warranted to evaluate whether administration of DEC could eliminate circulafing retroviral-infected
cells.
Correspondence to: L.W. Kitchen, Division of Infectious Diseases, Tulane University Medical Center, 1430 Tulane Avenue, New Orleans, LA 70112, U.S.A.
Keywords: diethylcarbamazine;
feline leukemia virus; AIDS; HIV; lymphocytes. Introduction
Increased total white blood cell counts have been noted following treatment of filarialinfected humans with diethylcarbamazine (DEC) [2]. Administration of DEC to FeLVinfected cats - one animal mode1 for AIDS has frequently resulted in decreased serum viral infectivity and increased serum antibody levels to feline oncornavirus-associated cell membrane antigen (FOCMA) 151. In addition, continuous oral administration of DEC to two kittens shortly after they first tested positive for FeLV leukocyte antigen resulted in a reduced rate of decline of circulating lymphocytes on a statistically significant basis and prolonged survival in comparison to two untreated littermates [6]. DEC has not been shown to prevent infection of susceptible cells with FeLV in vitro (L. Kitchen, unpublished data). The present study examines the hematologic effects of short-term (1 month) administration of DEC given to adult chronically FeLV-infected cats.
0304~3835/89/$03.50 0 1989 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland
184
Materials and methods
A total of 35 cats were studied. Of these 35, 4 were 9-month-old littermate cats. All four were FeLV-naive cats that had been inoculated with identical doses of the Rickard strain of FeLV at approximately 6 weeks of age [9]. Two of these four animals (cats K and SU) were treated with DEC, and 2 were untreated (cats SA and P) . Cats K and SU were given 10 mg/kg of DEC orally as a single dose per day for 28 days. DEC treatment in the same dosage was also given for 30 days to 18 outbred cats that tested positive for FeLV leukocyte antigens and seven cats that tested negative for FeLV leukocyte antigen. Of these 18, 1 was 1 month pregnant when DEC treatment was begun and 17 were non-pregnant. In addition, six cats testing negative for FeLV leukocyte antigen given 1 ml LEUKOCELL IM were also studied. LEUKOCELL is a commercial FeLV vaccine manufactured by Norden Laboratories, Lincoln, Nebraska, U.S.A. Of these six, three cats were given 10 mg/kg per day of DEC orally for 2 weeks, and the remaining three cats were given vaccine without DEC medication. Tests for FeLV antigen in peripheral blood leukocytes were carried out by the Laboratory of Veterinary Oncology at the Memorial SloanKettering Cancer Center, New York City [3]. Blood samples for packed cell volume (PCV) and white blood cell counts were drawn at selected intervals (see Tables 1 and 2). The number of lymphocytes/mm3 peripheral blood was calculated by multiplying the proportion of lymphocytes observed on examination of 100 white blood cells in the peripheral blood smear by the total white blood cell count. Results
All four Rickard FeLV-inoculated cats tested positive for FeLV leukocyte antigens before and after 30 days treatment. Both DECtreated FeLV-Rickard cats developed transient lymphopenia (< 1500 lymphocytes/mm3 peripheral blood), whereas the untreated
FeLV-Rickard cats remained non-lymphopenic (Table 1). The treated FeLV cat with more profound lymphopenia after DEC treatment (cat K) also developed transient anemia (PCV = 23 at the end of 28 days of DEC treatment). The PCV of cat K tested in the normal range 6 days after cessation of DEC treatment. Other hematologic effects were not noted immediately following DEC treatment. The four cats were euthanatized for humane reasons when moribund. Untreated cat SA died with spinal lymphoma 62 days after the start of the trial. Untreated cat P died with thymic lymphoma after 259 days. Treated cat SU died with multifocal lymphoma/leukemia 92 days after the start of the trial, whereas treated cat K survived 384 days and died with thymic lymphoma. The two untreated cats survived an average
Table 1. Lymphocyte counts/mm3 peripheral blood/ packed cell volumes (PCV) in two adult Rickard FeLVinoculated cats (K, SU) treated with DEC and two untreated inoculated littermates (P, SA) . Time K (days) 0 28 34 59 80 129 i64 192 206 220 245 257 276
SU
P
SA
2560/33 730/23 52/24
2534/38 l&9/29
5075/32 7480/31
3820/37 1956/32
860/29
3565/39 1352/34
3720/38
7821/35
7*/39 1793/35 1x/33 S&2/36
2650/37 4203/36 5320/37 1811/33 3200/32 2004/35
1595/41.2
Cats K and SU were treated with DEC days l-28. Counts indicating lymphopenia (< 1500 lymphocytes/ mm3 blood) or indices corresponding to anemia (PCV < 24) are underlined.
of 161 days from the start of the trial, and the two treated cats survived an average of 238 days. Therefore, survival was prolonged in the treated group by an average of 77 days. Of 18 outbred naturally-infected adult cats testing positive for FeLV leukocyte antigens before and after receiving 10 mg/kg per day of oral DEC for 30 days, seven (39%) of the 18 tested lymphopenic before treatment and 11 (61%) of the 18 were lymphophenic after treatment. Twelve (67%) of the 18 cats were noted to have lower lymphocyte counts after DEC therapy as compared to pretreatment values. The six remaining cats had higher lymphocyte counts following DEC treatment. Of these six, the cat with the largest increase in lymphocyte counts (no. 18 in Table 2A) was pregnant and delivered 7 days before the posttreatment blood sample was drawn. The mean Table 2A.
Lymphocyte counts/mm3 peripheral blood before and after 30 days oral diethylcarbamazine treatment in 18 outbred FeLV-infected cats and 7 FeLVuninfected cats. FeLV-infected
cats
Cat.
Lymphocyte pre/post
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
4826/ 1076 1529/756 1908/ 1392 1430/359 -2624/ 1273 1921/m 1776/1074 5066/4836 6840/6204 3234/2950 2089/ 1592 1203/1120 -1035/ 1224 581/889 -825/1536 1560/2576 1116/1144 -a/2305
FeLV-uninfected count
cats
Cat.
Lymphocyte pre/post
1 2 3 4 5 6 7
1197/1551 1526/1704 2100/1656 1773/2290 7498/5115 4175/3108 11264/12768
corresponding to Values lymphopenia lymphocytes/mm3 blood) are underlined.
count
(< 1500
lymphocyte count of the 18 treated cats was 2244 before treatment and 1829 after treatment (Table 2A). Statistical evaluation to determine whether the lymphocyte values following DEC treatment were significantly lower compared to pretreatment values was done using two-tailed paired t-tests on the lymphocyte counts (expressed as log,,/mm3 blood) [7]. Lymphocyte counts were not significantly decreased following DEC treatment when data from all 18 outbred FeLV-infected cats in Table 2A were analyzed (P = 0.15). However, when the one pregnant cat was excluded, post-diethylcarbamazine lymphocyte counts in the remaining non-pregnant 17 cats were significantly lower than the initial counts (P = 0.045). No significant decrease in lymphocyte counts was noted following 30 days DEC treatment in seven FeLV-uninfected cats. One of the 18 outbred FeLV-infected cats was neutropenic (neutrophils < 2500/mm3 peripheral blood) before administration of DEC and 3 of the 18 cats were neutropenic after DEC was given. One of 10 FeLV-uninfected cats was neutropenic before treatment, and none were neutropenic following DEC treatment. Post-treatment anemia was noted in 1 of the 18 FeLV-infected outbred cats (pre-treatment PCV = PCV = 26 and the post-treatment 21). The mean PCV of these 18 cats was 36.6 pretreatment and 34.8 post-treatment. Anemia was not noted following DEC treatment given to 10 FeLV-uninfected cats. Of seven cats that tested negative for FeLV leukocyte antigens before and after 30 days DEC treatment, 1 was lymphopenic before treatment and none were lymphopenic after treatment. DEC administration did not result in notably decreased mean lymphocyte counts in 3 FeLV-vaccinated cats (Table 2B) The mean lymphocyte count of the 10 treated FeLVnegative cats was 3872 before treatment and 4755 after treatment. As was previously noted, the two DECtreated Rickard FeLV-inoculated cats in the present study survived an average of 77 days
186
Table 2B. Lymphocyte counts/mm3 peripheral blood before/2 weeks after 1 ml LEUKOCELL IM in six FeLV leukocyte antigen negative cats, three of which also received 2 weeks oral DEC treatment. Vaccinated/treated
Vaccinated/untreated
Cat
Cat
Lymphocyte counts pre/post
1 2 3
2600/2880 6480/4690 7750/6063
Lymphocyte counts
pre/post 1 2 3
2660/2340 4992/11695 1534/5320
IongeE> than
their two untreated littermates. There have been 10 deaths among the DECtreated 18 outbred FeLV-infected cats now followed for a total of 559 cat-months (since first documentation of FeLV leukocyte antigen in the cats’ circulating white blood cells). Two cats were lost to follow-up when they left their household after 14 months observation; these cats were not considered fatalities but months of observation until loss were included in the analysis. To date, 1.79 deaths per 100 catmonths observation have been noted. Discussion The results suggest that short-term oral DEC treatment may frequently result in transient lymphopenia in non-pregnant cats chronically infected with FeLV, but does not result in lymphopenia in FeLV-uninfected cats. Although the pregnant cat was excluded after analysis of the results, it is possible that parturition and associated stress may have resulted in higher lymphocyte counts in the pregnant cat in comparison to the remaining non-pregnant cats. We cannot exclude the possibility that the decrease in lymphocyte counts in the nonpregnant cats could be related to such factors as FeLV infection status, the existence of initially high counts in some cats, and cat-to-cat initial lymphocyte count variability. In order to confirm our hypothesis, the effect of DEC treatment on lymphocyte values of a larger
group of FeLV-infected cats would need to be compared with the course of lymphocyte values of a similar group of untreated FeLVinfected cats. Our results suggest that DEC treatment in chronically FeLV-infected cats may prolong survival, although more precise studies are needed to confirm this. A historical control study involving a large number of cats in a single household (which computed cats lost to follow-up in a similar manner to the present study) noted 34.6 deaths/1000 cat-months (or 3.46 deaths/100 cat-months) which exceeds the death rate noted in the 18 DECtreated FeLV outbred cats in the present study [ 11. The small number of Rickard FeLV-inoculated cats in the present study does not permit statistical evaluation of the effect of DEC treatment on survival in this category. Since previous studies have suggested that continuous DEC treatment begun soon after evidence of FeLV infection reduces the rate of decline of peripheral lymphocyte counts, it is possible that DEC treatment may prevent spread of FeLV to uninfected lymphocytes, perhaps by decreasing the number of circulating FeLV-infected cells in addition to decreasing serum viral infectivity [5]. This hypothesis would be particularly compelling if a small number of circulating lymphocytes were infected with FeLV early in the course of disease, and if the number of infected circulating lymphocytes increased with duration of infection. There is some experimental data supporting this possibility [lo]. Although the transient anemia in cat K after DEC treatment could be related to opportunistic infection, the fact that the PCV tested in the normal range 6 days after DEC was discontinued suggests that the anemia was related to DEC treatment. FeLV has been shown to infect feline erythroid precursors as well as neutrophils and platelets, and elimination of some circulating red blood cells bearing FeLV antigens may have occurred following DEC therapy [lo]. Although short-term DEC treatment rarely results in neutropenia and has not been shown to result in thrombocytopenia, severe throm-
187
bocytopenia (and anemia) accompanied by active hematopoiesis in bone marrow have been noted after long-term DEC treatment in one FeLV-infected cat [6]. Since human immunodeficiency virus (HIV) infects a low proportion of circulating blood cells, DEC treatment may have less profound hematologic effects in HIV-infected humans than in FeLV-infected cats [4]. Nevertheless, it is worth exploring whether administration of DEC to HIV-infected persons could result in killing or elimination of HIV-infected cells, given the growing consensus that elimination of HIV-infected cells may be important in the control of HIV infection, particularly if this can be accomplished shortly after introduction of the virus into the body [8]. Since evidence from several laboratories suggests that antibody-dependent cellular cytotoxicity of filarial larvae is enhanced by DEC in vitro, whether DEC could eliminate HIV-infected cells by this mechanism warrants study [12]. Further, given widespread use of DEC to control filariasis in the developing world, it is important to determine whether this drug can alter peripheral cell counts and T-lymphocyte subsets in HIV-infected humans, because such alterations could affect both individual clinical management decisions and research results. DEC is orally bioavailable and some studies suggest that DEC is concentrated in brain tissue. Therefore the question of whether DEC could also be useful as an adjunct therapy to eliminate some categories of leukemic cells also warrants evaluation [ 111. Acknowledgments I thank Dr. W.D. Hardy, Jr. and his colleagues for analysis of the peripheral blood leukocytes of the cats in this study for FeLV antigens, Dr. J.C. Kitchen, Jr., for assistance with manuscript preparation and Dr. S. Cotter
and Dr. N. Larson for veterinary assistance. This investigation was supported by D. Lu, the Department of Medicine at Tulane University Medical Center, and the New Orleans Veterans Administration Center. References 1
2
3
4
5
6
7
8 9
10
11 12
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