Antiviral activity of oral ultralow doses of antibodies to gamma-interferon: experimental study of influenza infection in mice]

Course intragastric administration of ultralow doses of human gamma-interferon antibodies (ULD anti-IFN-gamma) to intact mice resulted in an increase of endogenous IFN-gamma production by the animal lymphocytes. Oral prophylactic administration of ULD anti-IFN-gamma significantly lowered the influenza virus concentration in the animal lungs at the initial stage of the aerogenous infection: in 2 (p = 0.05) and 3 (p = 0.07) days after the contamination. The therapeutic antiviral effect of ULD anti-IFN-gamma in mice with influenza was evident from a significant decrease of the influenza virus concentration in the lungs of the animals on the 4th (p = 0.05) and 5th (p = 0.07) days after the contamination. The antiviral effect of ULD anti-IFN-gamma after the prophylactic and therapeutic use is likely provided by induction of endogenous IFN-gamma.     

Combined action of antiblastoma preparations on lysozyme activity in animal organs]

The lysozyme activity in the spleen, kidneys and lungs of the mice treated with neocid, sarcolysine or Tio-Fef in therapeutic doses increased or remained at the control level by the 5th or 10th day after the drug administration. The use of sarcolysine per se or in combination with neocid increased the activity of lysozyme in the spleen, kidneys and lungs during the whole period of the experiment as compared to the control. The values of the lysozyme activity in the spleen and lungs of the animals treated with neocid in combination with sarcolysine were higher for 5 days, and in all organs examined were higher by the 10the day as compared to the animals treated with neocid alone. Increased lysozyme activity in the spleen, kidneys and lungs was observed under the effect of neocid in combination with sarcolysine as compared to the lysozyme activity in mice treated with sarcolysine per se (assay on the 10th day). Decreased lysozyme activity was determined in the spleen, kidneys and lungs by the 5th day and in the kidneys by the 10th day in the mice treated with sarcolysine in combination with Tio-Tel and in the spleen and lungs by the 5th and 10th days in the animals treated with neocid in combination with sarcolysine or Tio-Tef as compared to the animals treated with sarcolysine. The lysozyme activity in the kidneys under the effect of sarcolysine combination with Tio-Tef was lower by the 5th days and higher by the 10th day as compared to that under the effect of Tio-Tef.     

Chemotherapeutic efficacy of ascofuranone in Trypanosoma vivax-infected mice without glycerol

Ascofuranone, an antibiotic isolated from Ascochyta visiae, showed trypanocidal activity in Trypanosoma vivax-infected mice. A single dose of 50 mg/kg ascofuranone effectively cured the mice without the help of glycerol. Repeated administrations of this drug further enhanced its chemotherapeutic effect. After two, three, and four consecutive days treatment, the doses needed to cure the infection decreased to 25, 12, and 6 mg/kg, so that the total doses administered were 50, 36 and 24 mg/kg, respectively. Ascofuranone (50 mg/kg) also had a prophylactic effect against T. vivax infection within the first two days after administration. This prophylactic activity diminished to 80% by day 3 and completely disappeared four days after administration. Of particular interest in this study was that ascofuranone had trypanocidal activity in T. vivax-infected mice in the absence of glycerol, whereas co-administration of glycerol or repeated administrations of this drug are needed for Trypanosoma brucei brucei infection. Our present results strongly suggest that ascofuranone is also an effective tool in chemotherapy against African trypanosomiasis in domestic animals.     

Effectiveness of topically administered neutralizing antibodies in experimental immunotherapy of respiratory syncytial virus infection in cotton rats.

Initial studies of the prophylactic effect of parenterally administered respiratory syncytial virus (RSV)-neutralizing antibodies in cotton rats indicated that virus replication in lung tissues was restricted when animals with preexisting antibody titers in serum of 1:100 or more (as measured by plaque reduction) were challenged intranasally with 10(4) PFU of virus. Subsequently, a therapeutic effect of parenterally administered RSV antibodies (present in human gamma globulin) was demonstrated in both cotton rats and owl monkeys. Parenteral inoculation of RSV-infected cotton rats or owl monkeys with purified human immunoglobulin licensed for intravenous administration in humans (IVIG) effected a 10(-1.7) to 10(-2.7) reduction in the level of pulmonary virus at the height of infection. Because of these encouraging results, we examined topical administration of IVIG to determine whether it was also effective and whether it offered an advantage over the parenteral route with regard to simplicity and the dose required for full therapeutic effect. IVIG (0.025 g/kg) administered topically by the intranasal route to anesthetized cotton rats at the height of RSV infection effected a 10(2.2)-fold reduction in viral titers of pulmonary tissues and a complete clearance of detectable virus in 92% of the animals within 24 h. In contrast, 4 g of IVIG per kg was required to produce a comparable therapeutic effect when the material was administered parenterally. Thus, the therapeutic effect of IVIG was 160 times greater by the topical route than by parenteral inoculation.     

Effect of amphotericin B on the energy metabolism of tissue forms of Candida albicans]

Dehydrogenase activity of the tissue form cells of C. albicans during the infection process in albino mice with and without amphotericin B treatment was studied. The strength of the metabolic reactions resulting in accumulation of ATP was evident from the activity of 4 main enzymes, i.e. succinate dehydrogenase, lactate dehydrogenase, alcohol dehydrogenase and glucose-6-phosphate dehydrogenase. The enzymatic activity was determined by the tetrasol method based on formation of diphormazan. Investigation of the fungal cells 10 minutes after the infection showed that preliminary intravenous or intraperitoneal administration of amphotericin B did not change the activity of the tissue forms. The cytochemical characteristics of the fungal cells remained the same as that in the untreated animals. Six hours after infection of the animals treated with amphotericin B administered intravenously the fungus vegetation decreased from 52 to 38 per cent, while in the animals treated with amphotericin B administered intraperitoneally it was suppressed completely. Simultaneously the energy metabolism was also suppressed, the activity of alcohol dehydrogenase being suppressed most significantly. The activity of this enzyme in the cells of C. albicans isolated from the animals treated with the antibiotic administered intraperitoneally was 14 times lower than that in the cells of the culture isolated from the control animals.     

Experimental protective action of kanamycin, ampicillin and their combination with methyluracil and pyrogenal]

Efficacy of kanamycin, ampicillin and their combinations with methyluracyl and pyrogenal in experimental Coli infections was studied. The antibiotics were administered an hour after the infection. Methyluracyl and pyrogenal were used according to 2 schemes. Scheme No. I: the drug is used daily for 7 days in increasing doses, the last dose is administered 24 hours before the infection. Scheme No. 2: the drug is used once at the moment of the infection. The methyluracyl doses were: 0.5, 1.0, 2.5 mg and 5 mg and 5 mg per a mouse during the following 4 days. The pyrrogenal doses were: 5, 10, 15, 25, 30 and 35 minimum pyrogenic doses. 5 mg of methyluracyl and 35 minimum pyrogenic doses of pyrogenal were used according to scheme No. 2. The most pronounced increase in the efficacy of kanamycin, ampicillin and their combination was observed in the animals treated simultaneously with methyluracyl and pyrogenal according to scheme No. 1. The efficacy of kanamycin and ampicillin increased 3 and 2.68 times respectively. ED50 of kanamycin and ampicillin used in combination in the animals treated with methyluracyl and pyrogenal was lowered 4 and 2.9 times respectively as compared to that in the animal groups treated only with the antibiotic combination and 21 and 15.2 times respectively when the antibiotics were used alone. Sanation of the animal organs was also rather successful. A single administration of methyluracyl and pyrogenal simultaneously with the infection (scheme No. 2) had a lower effect on the efficacy.     

Initiation of antiretroviral therapy during chronic SIV infection leads to rapid reduction in viral loads and the level of T-cell immune response

In the present era of increasing resistance of human immunodeficiency virus (HIV) to antiviral drugs, exploration of adjunct therapies directed at immune responses in combination with antiretroviral drugs may be of value for the treatment of acquired immunodeficiency syndrome. In this study, we designed a model for immune therapy using SIVmac251 infection in rhesus macaques. We explored the outcomes of primary infection on viral loads and the resulting T-cell immune responses in primates. The SIV-infected rhesus macaque model exhibited features similar to those observed in HIV-1 infection of humans. Major histocompatibility complex (MHC) segregation with viral loads were found to associate with viral containment and hence the duration of the disease-free latency period. Thus a better understanding of the relative roles of MHC class I allele in control of viral replication may provide important information for prophylactic or therapeutic vaccine designs. Mamu-A01 is significantly associated with higher immune response and control of viral replication. This allele is frequent in rhesus macaques of Indian origin (22%). Interestingly, Mamu-B01 (26% animals) was associated with lower immune responses and higher viral loads. Another allele, A08 was also predominantly present in 37% of the animals in this study. We observed higher viral replication in individual SIV-infected rhesus monkeys that did not demonstrate strong cellular immune responses. The results are important for understanding SIV disease progression in different MHC Mamu alleles and also for improving the interpretation and quality of pre-clinical studies in rhesus monkeys.     

Efficacy of beta-cyclodextrin against experimental cryptosporidiosis in neonatal lambs

The efficacy of beta-cyclodextrin against experimental Cryptosporidium parvum infection was evaluated in neonatal lambs. The animals were treated by oral administration of the drug at 1 g/kg of body weight during 3 consecutive days. Preventive treatment was started within 1 day of birth, and therapeutic treatment was initiated at the onset of diarrhea following confirmation of infection. Disease development and drug efficacy were evaluated by monitoring the presence or absence of diarrhea and oocyst shedding from birth until 30 days of age. Weight gains at 15 and 30 days of age were also recorded. Beta-cyclodextrin was highly effective as a prophylactic treatment; 1 animal did not acquire the infection, diarrhea was prevented in infected animals, and there was a considerable decrease in oocyst shedding. The therapeutic treatment was effective in decreasing the severity of diarrhea and the duration of oocyst shedding. The animals tolerated the drug well, and there was a significant increase in their body weights.     

Effects of multiple, chronic and early hashish exposure on mating behavior, nest-building and gestation in mice

1. The effects of hashish extract on social behavior were investigated in pairs of mice living together from mating to parturition. The drug was administered orally to both animals at a dose of 20 mg delta 9-tetrahydrocannabinol/kg three times a week, either (I) during the 3 weeks from mating to parturition, (II) during the 12 weeks from weaning to mating and parturition, or (III) only during the 3 weeks after weaning. 2. In the mating tests, an acute administration (I) caused a general sedation. Chronically treated animals (II) showed decreased sexual behavior and lower social investigations despite significantly more non-social activities. Early drug treatment (III) had no effect on mating behavior. In all three series of experiments the females had conceived by the next morning. 3. The nest-building behavior during pregnancy was suppressed after multiple (I) and chronic (II) drug administration. In early drug-treated animals (III) the transport of nesting material was only slowed down. Non-social activities were normal or increased in all three series. 4. Parturition was significantly delayed by one day after multiple (I) and chronic (II) drug administration. The birth weight was not affected, but the litter size was decreased after chronic drug treatment. Early drug administration (III) had no effects on these parameters. 5. In conclusion, whereas tolerance to the sedative effects of hashish developed very rapidly, the drug influences on social behavior were stable. Therefore, discussions on legalization of cannabis should pay attention to the drug effects on social behavior.     

Treatment with beta-cyclodextrin of natural Cryptosporidium parvum infections in lambs under field conditions.

Following the unexpected activity of the excipient beta-cyclodextrin against experimental infection by Cryptosporidium parvum in suckling mice, its efficacy in the prevention and treatment of natural infections in lambs was evaluated under field conditions. Fifty-three crossbred neonatal lambs were randomly selected for the study. Treatment consisted of oral administration of an aqueous suspension of beta-cyclodextrin at a dose of 500 mg/kg of body weight. To test prophylactic efficacy, the suspension was administered at 1, 2 and 3 days of age. To evaluate therapeutic efficacy, the suspension was administered on each of the 3 days following onset of diarrhoea. Infection was monitored by daily examination of faecal samples, from birth to 30 days. The criteria studied in evaluating efficacy were: oocyst shedding, the presence of diarrhoea, and weight gain at 15 and 30 days. In the group that received prophylactic treatment with beta-cyclodextrin, there were no mortalities and, compared with control lambs, there was a decrease in the number of animals infected, a longer prepatent period and notable reduction in the patent period and the duration of diarrhoea. Therapeutic treatment also reduced the patent period and the severity of diarrhoea. beta-cyclodextrin was well tolerated by all of the treated animals.     

Increasing the effectiveness of immunotherapy of rabies by using rifampicin in experimental studies]

The experiments showed that in a dose of 35-70 mg/kg rifampicin inhibited reproduction of the fixed rabies virus in the brain of infected animals. The drug had no inhibitory effect on synthesis of the virus-neutralizing antibodies after vaccination. Combination of rifampicin with antirabies gamma-globulin had a marked synergistic effect. The animal survival after the combination use amounted to 75-100 per cent and depended on the infective dose of the virus and the scheme of the drug administration. It was concluded that rifampicin might be used in complex therapy of rabies during the incubation period (along with gamma-globulin and the vaccine) for inhibiting virus reproduction at early infection stages.     

Antiretroviral therapy during primary immunodeficiency virus infection can induce persistent suppression of virus load and protection from heterologous challenge in rhesus macaques

A limited period of chemotherapy during primary immunodeficiency virus infection might provide a long-term clinical benefit even if treatment is initiated at a time point when virus is already detectable in plasma. To evaluate this strategy, we infected rhesus macaques with the pathogenic simian/human immunodeficiency virus RT-SHIV and treated them with the antiretroviral drug (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for 8 weeks starting 7 or 14 days postinfection. PMPA treatment suppressed viral replication efficiently in all of the monkeys. After chemotherapy ended, virus replication rebounded and viral RNA in plasma reached levels comparable to that of the controls in four of the six monkeys. However, in the other two animals, virus loads peaked only moderately after withdrawal of the drug and then declined to low or even undetectable levels. These low levels of viremia remained stable for at least 31 weeks after cessation of therapy. At this time point, these two monkeys were challenged with SIV(8980) to evaluate whether the host responses which were able to keep RT-SHIV replication under control were also sufficient to protect against infection with a highly pathogenic heterologous virus. Both monkeys proved to be protected against the heterologous virus. In one of the two animals, low levels of SIV(8980) replication were detected. Thus, by chemotherapy during the acute phase of pathogenic virus replication, we could achieve not only persistent virus load suppression in two out of six monkeys but also protection from subsequent heterologous challenge. By this chemotherapeutic attenuation, the replication kinetics of attenuated viruses could be mimicked and a vaccination effect similar to that induced by live attenuated simian immunodeficiency virus vaccines was achieved.     

Vaccine-elicited memory cytotoxic T lymphocytes contribute to Mamu-A*01-associated control of simian/human immunodeficiency virus 89.6P replication in rhesus monkeys

The expression of particular major histocompatibility complex (MHC) class I alleles can influence the rate of disease progression following lentiviral infections. This effect is a presumed consequence of potent cytotoxic T-lymphocyte (CTL) responses that are restricted by these MHC class I molecules. The present studies have examined the impact of the MHC class I allele Mamu-A*01 on simian/human immunodeficiency virus 89.6P (SHIV-89.6P) infection in unvaccinated and vaccinated rhesus monkeys by exploring the contribution of dominant-epitope specific CTL in this setting. Expression of Mamu-A*01 in immunologically naive monkeys was not associated with improved control of viral replication, CD4+ T-lymphocyte loss, or survival. In contrast, Mamu-A*01+ monkeys that had received heterologous prime/boost immunizations prior to challenge maintained higher CD4+ T-lymphocyte levels and better control of SHIV-89.6P replication than Mamu-A*01- monkeys. This protection was associated with the evolution of high-frequency anamnestic CTL responses specific for a dominant Mamu-A*01-restricted Gag epitope following infection. These data indicate that specific MHC class I alleles can confer protection in the setting of a pathogenic SHIV infection by their ability to elicit memory CTL following vaccination.     

Chronic D1 and D2 dopaminomimetic treatment of MPTP-denervated monkeys: effects on basal ganglia GABA(A)/benzodiazepine receptor complex and GABA content.

The effect of various chronic dopaminergic treatments in 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys on the brain gamma-aminobutyric acid type A (GABA(A)) /benzodiazepine receptor complex and GABA content was investigated in order to assess the GABAergic involvement in dopaminomimetic-induced dyskinesia. Three MPTP monkeys received for one month pulsatile administrations of the D1 dopamine (DA) receptor agonist SKF 82958 whereas three others received the same dose of SKF 82958 by continuous infusion. A long acting D2 DA receptor agonist, cabergoline, was given to another three animals. Untreated MPTP as well as naive control animals were also included. Pulsatile SKF 82958 relieved parkinsonian symptoms but was also associated with dyskinesia in two of the three animals whereas animals treated continuously with SKF 82958 remained as untreated MPTP monkeys. Chronic cabergoline administration improved motor response with no persistent dyskinesia. MPTP treatment induced a decrease of 3H-flunitrazepam binding in the medial anterior part of caudate-putamen and an increase in the internal segment of globus pallidus (GPi) which was in general unchanged by pulsatile or continuous SKF 82958 administration. Throughout the striatum, binding of 3H-flunitrazepam remained reduced in MPTP monkeys treated with cabergoline but was not significantly lower than untreated MPTP monkeys. Moreover, cabergoline treatment reversed the MPTP-induced increase in 3H-flunitrazepam binding in the GPi. GABA concentrations remained unchanged in the striatum, external segment of globus pallidus and GPi following MPTP denervation. Pulsatile but not continuous SKF 82958 administration decreased putamen GABA content whereas cabergoline treatment decreased caudate GABA. No alteration in GABA levels were observed in the GPe and GPi following the experimental treatments. These results suggest that: (1) D2-like receptor stimulation with cabergoline modulates GABA(A) receptor density in striatal subregions anatomically related to associative cortical afferent and (2) the absence of dyskinesia in dopaminomimetic-treated monkeys might be associated with the reversal of the MPTP-induced upregulation of the GABA(A)/benzodiazepine receptor complex in the Gpi.     

Possible modulation of N-methyl-D,L-aspartic acid induced prolactin release by testicular steroids in the adult male rhesus monkey

N-methyl-D,L-aspartic acid (NMA), an agonist of the neurotransmitter glutamate has been shown to acutely stimulate the release of prolactin (PRL) in intact rats and monkeys. To further investigate the role of neuroexcitatory amino acids in PRL secretion, the effects of NMA administration were examined on PRL release in long term orchidectomized adult rhesus monkeys, in both the absence and presence of testosterone. Intact and long term castrated adult male monkeys weighing between 8-13 kg, were implanted with a catheter via the saphenous vein for blood withdrawal and drug infusion. Blood samples were collected at 10 min intervals for 50 min before and 70 min after administration of the drug or vehicle. Plasma PRL concentrations were estimated using radioimmunoassay. Whereas a single iv injection of NMA (15 mg/kg BW) induced a prompt discharge of PRL in intact monkeys, an identical dose had surprisingly no effect on PRL secretion in orchidectomized animals. On the other hand, plasma PRL increases in response to a challenge dose of thyrotropin releasing hormone (TRH; 6 micrograms/kg BW, iv) were similar in magnitude in the two groups of monkeys. Testosterone replacement in orchidectomized animals by parenteral administration of testosterone enanthate (200 mg/wk) reinitiated the PRL responsiveness to acute NMA stimulation. These results indicate that N-methyl-D-aspartic acid (NMDA) dependent drive to PRL release in the adult male rhesus monkey may be overtly influenced by the sex steroid milieu.     

Metoclopramide,a dopamine antagonist,stimulates aldosterone secretion in rhesus monkeys but not in dogs or rabbits

This study was designed to investigate the role of dopamine in the control of aldosterone secretion in three frequently used laboratory animals. Five New Zealand rabbits, five mongrel dogs and five rhesus monkeys received metoclopramide (MCP) (200 μg/kg iv) and blood samples were collected at 0,5,15,30 and 45 minutes after drug administration. MCP had no effect on plasma aldosterone concentrations at any sampling time in the rabbits or dogs. However, MCP produced a rapid and marked increase in plasma aldosterone from 6.5±0.6 ng/dl to 18.1±2.8 ng/dl at 5 min. and a maximum level of 40.5±4.4 ng/dl at 10 min. after drug administration in the monkeys. MCP had no significant effect on plasma cortisol or plasma renin activity levels in the three species. Prolactin rose in the monkeys from 8.6±1.2 ng/ml to a maximum of 123.5±8.5 ng/ml at 15 min. after MCP. Administration of MCP resulted in a rise in plasma 18-hydroxycorticosterone in the monkeys from 12.5±1.4 ng/dl to a maximum concentration of 50.0±5.1 ng/dl 15 min. after drug administration. Plasma corticosterone, 11-deoxycorticosterone, and 18-hydroxydeoxycorticosterone were not altered by MCP. Although unlikely, it is possible that ketamine may have accounted for some of the changes in plasma aldosterone and 18-hydroxycorticosterone observed after metoclopramide in the monkeys. The findings suggest that dopamine modulates aldosterone biosynthesis in the monkey probably by regulating glomerulosa 18-hydroxylase activity.     

Increase in efficacy of antibiotic therapy of anthrax under experimental conditions]

The results of experimental therapy of antraxis infected mice with cefazoline (kefzol) and ampicillin incapsulated into liposomes are presented. Protective activity of the same free antibiotics combinated with amixine and leukinferone was evaluated also. Treatment with liposomal cefazoline enhanced mice survival upto 60 per cent, and life period upto 1.3 +/- 0.3 days. After liposomal ampicillin administration for 3 times the same indices were 60 per cent and 6.5 +/- 0.9 days, after 2 times administration--80 per cent and 14 +/- 1.8 days when compared to the groups of the animals treated with free antibiotics. It was shown that administration of ampicillin with amixine or with leukinferone provided enhanced mice survival upto 20 per cent, administration of cefazoline with leukinferone--upto 30 per cent.     

Histochemical response of mice to mistletoe lectin I (ML I)

Summary The acute toxicity of lectin ML I from the toxic drug, mistletoe, was demonstrated in previous experiments. Because the reason for this extremely high toxicity is not yet clear, mice were studied histochemically at different times after treatment with various doses of ML I, ML I A or ML I B chain separately, or recombinations of ML I A and ML I B. Various plasma membrane-associated hydrolases as well as Golgi apparatus-and endoplasmic reticulum-linked hydrolases, peroxisomal and extraperoxisomal oxidases, lysosomal hydrolases, mitochondrial dehydrogenases, the cytoskeletal proteins keratin and vimentin as well as iron, glycogen and lipids were analysed in all organs and tissues of female mice. Irrespective of the dose, a clear-cut response was only observed in the liver. After ML I treatment, glycogen disappeared completely from all hepatocytes, and this effect did not depend on the ML I-concentration and exposure time. The increase in activity of Golgiassociated thiamine pyrophosphatase in hepatocytes and of non-specific alkaline phosphatase in the sinusoidal endothelial cells depended on the applied ML I concentration and the time of treatment. Doses of 600 or 900 ng ML I/kg drastically increased the phosphatase activities. These clear-cut changes of glycogen and enzyme activities were not observed after administration of the ML I B chain alone, and less so when the mice were treated only with the ML I A chain, or were treated with a recombination of ML I A and ML I B even at concentrations higher than that of ML I. The glycogen as well as thiamine pyrophosphatase and non-specific alkaline phosphatase responses after ML I administration were highly reproducible. None of the other investigated cell constituents responded to treatment with ML I, irrespective of the organs, tissues and cells analysed. This was also true for mice treated with ML I concentrations higher than the LD₅₀, after which the animals died with 2–3 days. In conclusion, application of ML I induces clear-cut liver cell changes, which are not observed after treatment with individual constituents of the ML I molecule either singly or in recombination. However, these changes do not explain the deaths of the animals.Supported by WTZ (Vereinbarung über wissenschaftlich-technische Zusammenarbeit: Projekt 13)     

Antihypoxic activity of cytochrome c heme peptides

A comparative study of antihypoxic activity of five and two cytochrome c derivatives was performed during their single prophylactic administration on the model of acute hypobaric hypoxia (AHBH) and during rehabilitation period after AHBH, respectively. Antihypoxic efficiency of cytochrome c derivatives was shown to be dependent on doses, time of drug administration, and type of experimental animal resistance. The heme-nonapeptide of cytochrome c proved to be of maximum efficiency during prophylactic administration and rehabilitation period after AHBH.     

Studies on the Cambodian I strain of Plasmodium falciparum in Aotus monkeys

The Cambodian I strain of Plasmodium falciparum, originally from Kampuchea was adapted for development in three different types of Aotus monkeys. High-level parasitemias were readily produced in splenectomized Colombian A. trivirgatus griseimembra monkeys. Initially, only minimal parasitemias developed in A. t. trivirgatus monkeys from Colombia. However, in one animal, adaptation occurred and high-level parasitemias were obtained during the second recrudescence of the infection. Passage to other A. t. trivirgatus monkeys indicated that the parasite was well adapted for development in splenectomized animals; low to moderate parasitemias were still produced in intact animals. This line of the parasite produced high level parasitemias when inoculated into splenectomized Aotus monkeys from Peru. Infections in Anopheles freeborni mosquitoes were obtained as late as the 7th passage in A. t. griseimembra monkeys and as late as the 7th recrudescence of the infection in an individual monkey (348 days after inoculation). The sporogonic cycle was completed in An. freeborni mosquitoes, and one transmission to an A. t. griseimembra monkey via the bites of infected mosquitoes was obtained.