Group-housed Cats Research Articles

Feline Panleukopenia Outbreaks and Risk Factors in Cats in Animal Shelters.

(1) Background: This study aimed to determine the risk factors for outbreaks of feline panleukopenia in shelters. (2) Methods: Four shelters (A−D) with 150 cats were included. Fecal samples were analyzed by parvovirus real-time polymerase chain reaction (qPCR), including culture and sequencing of qPCR-positive samples. Information on cats, husbandry, hygiene, and infection management was evaluated to determine risk factors for feline panleukopenia and parvovirus shedding by logistic regression. (3) Results: Feline panleukopenia occurred in 28.0% (42/150) of cats (0 in shelter D). Shedding was found in 48.7% (73/150) (A: 21/73; B: 29/73; C: 7/73; D: 16/73). Of 73 qPCR-positive fecal samples, 65.8% (48/73) were culture-positive; sequencing revealed feline panleukopenia virus (FPV) isolates in 34/48 samples and vaccine virus isolate in 14/48; canine parvovirus was not detected. Presence of feline panleukopenia was significantly more likely in cats from shelter A (p < 0.05), unvaccinated cats (p < 0.001), and young cats (4 weeks to 2 years; p = 0.008). Parvovirus shedding was significantly more common in young cats (p < 0.001), cats with feline panleukopenia (p = 0.033), and group-housed cats (p = 0.025). (4) Conclusions: Vaccination is the most important measure to reduce the risk of feline panleukopenia in shelters. Risk of parvovirus shedding is especially high in young, group-housed cats.

Changes in sociability of shelter cats

This study was conducted for the purpose of long-term monitoring of changes in the sociability of group-housed cats towards a familiar caregiver in a private no-kill shelter. The sociability of the monitored cat population was assessed at two-week intervals during one calendar year. A total of 158 animals were rated on a 5-point scale, the individual levels of which represented the level of sociability (1-very friendly cat to 5-very unfriendly cat). The evaluation was performed by visual observation of the cats' response to human approach and contact by one observer. At the first assessment, more than three quarters of the cats (81%) showed very friendly (score of 1) or friendly behavior (score of 2). Of the 88 cats that were evaluated at least twice and at the same time their stay in the shelter terminated during the monitoring period, 56 cats (63.6%) did not change their score (worsen or improve) during their stay in the shelter. Among the cats with an observed change, there was a permanently improved score in a significantly higher number of cats (P < 0.001) during the stay in the shelter than a permanent deterioration (26; 29.5% and 3; 3.4%, respectively). There was a temporary improvement or worsening of the score in 3 cats (3.4%). The improvement in the sociability score during the stay in the shelter mainly concerned cats whose behavior was rated as neutral (score of 3), unfriendly (score of 4) or very unfriendly (score of 5) at the first evaluation. The length of stay of cats in the shelter (LOS) correlated with the level of sociability of the cats during the first (rtau = 0.72, P < 0.001) and the last evaluation (rtau = 0.23, P = 0.007); however, the LOS itself did not predict the level of sociability (P > 0.05). The sociability level at the first assessment was found to be a predictor of the sociability level at the last assessment (P < 0.001). The results of our study suggest that during the stay in the shelter, the cats generally improve their sociability towards a familiar person. Although it appears that cats with lower levels of sociability remain in the shelter for a longer period of time, improvements may increase their adoption potential. Support for programs to increase the cat sociability is needed and should be addressed in further research.

Changes in Health Indicators of Welfare in Group-Housed Shelter Cats.

The aim of this study was to detect changes in health-oriented welfare indicators of shelter cats housed in a shelter by means of long-term monitoring of health indicators of welfare in a population of group-housed cats in a private, no-kill shelter in the Czech Republic. The cat population housed in a large group was monitored for 1 year. The data recording took place at 2-week intervals. A total of 220 cats were evaluated using a protocol containing eight health-oriented welfare indicators: body condition, third eye visibility, eye discharge, eye irritation, nose discharge, the presence of pathologically induced respiratory sounds, coat condition, and lameness and abnormal posture. The assessment was performed based on the observation by two trained evaluators. The cats' condition was rated on a 5-point scale for each indicator, where the optimal condition was represented by the score of 1 and a severe deterioration by the score of 5 for the given indicator, except for the body condition indicator, where the optimal condition was represented by the score of 3. A deterioration in the score in at least one of the indicators during the stay in the shelter was recorded in 52 (41.6%) of 125 cats that were assessed at least twice. The effect of the LOS, sex and age on the scores for each health indicator was examined via a linear mixed model analysis, as this method allows for handling of dependencies in the data of repeated measurements. The effect of predictors on the third eye visibility scores was not found. The age of cats predicted the coat condition and body condition scores. The LOS predicted scores of the abnormal posture and lameness indicator and scores of a composite index composed of indicators related to upper respiratory tract disease. The results suggest that despite the fact that some improvement of health was documented during the cats' stay in the shelter, there were a non-negligible number of animals experiencing a permanent or long-term deterioration in health. Efforts to minimize the undesirable factors contributing to the deterioration of well-being of cats during their stay in a shelter should be made.

Development of existing scoring systems to assess behavioural coping in shelter cats

Assessing how cats cope with the housing and husbandry at shelters is an important part of maintaining good animal welfare. There are non-invasive methods to assess how cats cope with their environment. The aim of this study was to investigate the reliability of the behaviours used in an extended Stress Assessment protocol for cats to detect stress. Looking at which behaviours are salient and possible to observe accurately, and which correlate with time until adoption. The study was carried out at a non-governmental medium sized cat rescue shelter in Midwestern USA. The shelter had a no-kill policy with screening of cats before intake from county shelters. The observed cats were either group-housed in five rooms (n = 70) or singly housed in double cages (n = 13). Observations were carried out during both morning and afternoon sessions, during which two 1-min observations recorded if cats performed any of 85 behavioural elements (BEs). Time at shelter and if cats were declawed or not were collected from shelter records after the observations. Statistical analysis of the BEs that best predicted the total time at shelter was calculated using the Survival Analysis based on the Cox proportional hazards regression model using a stepwise regression analysis separately for each scoring. The median time at shelter for group-housed cats was 26 days (IQR = 6–54) and for single-housed cats 29 days (IQR = 7–97). In total, 24 % of the BEs (20 of 85) were never recorded, however there were significantly more BEs recorded in group-housed cats (63 BEs) than in single-housed cats (49 BEs, p < 0.05). The survival analysis found 16 unique BEs to predict “Short time at shelter” (14 BEs in group-housed, two in single-housed), 14 were positively correlated meaning that they increased the chance of early adoption and two were negative meaning that they decreased the chance of early adoption. The survival analysis also calculated “Long time at shelter” and found 14 unique BEs where 12 BEs were in group-housed cats and three BEs were in single-housed cats. Seven of these were positively correlated meaning that they decreased the chance of early adoption, whereas seven were negatively correlated meaning that they increased the chance of early adoption. The conclusion is that the extended Stress Assessment could be used to detect BEs indicating stress of cats at shelters, and that there are BEs that can predict shorter time at the shelter. However, further investigations could help reduce the number of BEs needed.

The Diagnosis of Feline Leukaemia Virus (FeLV) Infection in Owned and Group-Housed Rescue Cats in Australia

A field study was undertaken to (i) measure the prevalence of feline leukaemia virus (FeLV) exposure and FeLV infection in a cross-section of healthy Australian pet cats; and (ii) investigate the outcomes following natural FeLV exposure in two Australian rescue facilities. Group 1 (n = 440) consisted of healthy client-owned cats with outdoor access, predominantly from eastern Australia. Groups 2 (n = 38) and 3 (n = 51) consisted of a mixture of healthy and sick cats, group-housed in two separate rescue facilities in Sydney, Australia, tested following identification of index cases of FeLV infection in cats sourced from these facilities. Diagnostic testing for FeLV exposure/infection included p27 antigen testing using three different point-of-care FeLV kits and a laboratory-based ELISA, real-time polymerase chain reaction (qPCR) testing to detect FeLV proviral DNA in leukocytes, real-time reverse-transcription PCR (qRT-PCR) testing to detect FeLV RNA in plasma, and neutralising antibody (NAb) testing. Cats were classified as FeLV-uninfected (FeLV-unexposed and presumptively FeLV-abortive infections) or FeLV-infected (presumptively regressive and presumptively progressive infections). In Group 1, 370 FeLV-unexposed cats (370/440, 84%), 47 abortive infections (47/440, 11%), nine regressive infections (9/440, 2%), and two progressive infections (2/440, 0.5%) were identified, and 12 FeLV-uninfected cats (12/440, 3%) were unclassifiable as FeLV-unexposed or abortive infections due to insufficient samples available for NAb testing. In Groups 2 and 3, 31 FeLV-unexposed cats (31/89, 35%), eight abortive infections (8/89, 9%), 22 regressive infections (22/89; 25%), and 19 progressive infections (19/89; 21%) were discovered, and nine FeLV-uninfected cats (9/89; 10%) were unclassifiable due to insufficient samples available for NAb testing. One of the presumptively progressively-infected cats in Group 3 was likely a focal FeLV infection. Two other presumptively progressively-infected cats in Group 3 may have been classified as regressive infections with repeated testing, highlighting the difficulties associated with FeLV diagnosis when sampling cats at a single time point, even with results from a panel of FeLV tests. These results serve as a reminder to Australian veterinarians that the threat of FeLV to the general pet cat population remains high, thus vigilant FeLV testing, separate housing for FeLV-infected cats, and FeLV vaccination of at-risk cats is important, particularly in group-housed cats in shelters and rescue facilities, where outbreaks of FeLV infection can occur.

Effects of a nutrient-enriched water on water intake and indices of hydration in healthy domestic cats fed a dry kibble diet.

OBJECTIVE To evaluate the effects of drinking nutrient-enriched water (NW) on water intake and indices of hydration in healthy domestic cats fed a dry kibble diet ad libitum. ANIMALS 18 domestic shorthair cats. PROCEDURES Group-housed cats were assigned to tap water (TW; n = 9) or NW (9) groups. All cats received TW at baseline (days -7 to -1). No changes were made to the food-water regimen for the TW group. The NW group received NW instead of TW from days 0 through 10, then received TW and NW in separate bowls (days 11 through 56). Food intake was measured through day 10; liquid consumed by drinking was measured throughout the study. Blood and urine samples were collected at predetermined times for analyses; 48-hour urine collection (days 28 through 30 or 31 through 33) was performed to assess output volume and aid endogenous creatinine-based glomerular filtration rate (GFR) determination. Data were analyzed with linear mixed-effects models. RESULTS Baseline TW and calorie intake were similar between groups. The NW treatment was significantly associated with increased liquid consumption during the treatment phase. Mean urine output was significantly higher in the NW group (15.2 mL/kg/d) than in the TW group (10.3 mL/kg/d). Mean GFR (1.75 vs 1.87 mL/min/kg, respectively) did not differ between groups. Effects of treatment and time were each significant for urine specific gravity and osmolality and urine creatinine, phosphate, and urea nitrogen concentrations, with lower values for the NW group. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that consumption of the NW can increase liquid intake and improve measures of hydration in healthy cats. These effects may offer health benefits to some cats in need of greater water consumption.

Effect of a shelf-furnished screen on space utilisation and social behaviour of indoor group-housed cats (Felis silvestris catus)

The environment of the laboratory cat can be restrictive and may impact their welfare. Enrichment is often provided to alleviate welfare impacts but is seldom assessed or validated for efficacy. This study investigated the effect of novel room furniture (a screen) on the expression of agonistic and affiliative behaviours and space utilisation amongst colony-housed laboratory cats. Video footage of cats (N=29) housed in social rooms (N=4) was collected for 2 days before (baseline phase), 4 days during (test phase) and 2 days following (removal phase) introduction of the novel furniture. Space utilisation data were collected using scan sampling every 10min and analysed using a generalised linear mixed effects model and Tukey’s HSD test. Behavioural data were collected using continuous sampling for 3h a day in 6×30min episodes and analysed using a Poisson generalised mixed effects model.Significantly more agonistic events occurred before the morning feed compared to after feeding within all phases (pre-feed mean=0.227; post-feed mean=0.026; P<0.0001). However no significant differences were observed before the morning feed compared to after feeding between phases indicating that the screen had no effect on reducing pre-feed aggression at the morning feed. Agonistic behaviours occurred significantly less following the morning feed during the test phase when compared to the baseline phase (test post-feed mean=0.011; baseline post-feed mean=0.029; P=0.0342). Significant differences were also observed on removal of the screen with agonistic behaviour increasing above baseline at the afternoon pre-feed time point, possibly indicative of aggression due to frustration or a rebound effect (removal pre-feed mean=0.151; baseline pre-feed mean 0.048; P<0.0001). Affiliative interactions between phases were not significantly affected by screen presence. Given the ratio of the screen to existing shelving (0.58:0.42) a statistical significant proportion of cats were found to be on the screen in the test phase of the study (P<0.0001). This study suggests that exploiting the unused vertical space by the addition of stand-alone shelving should be considered a valuable resource for the cat by increasing useable space and reducing agonistic interactions.

Nosocomial feline calicivirus-associated virulent systemic disease in a veterinary emergency and critical care unit in France.

Case series summaryIn October 2011, an abnormally large morbidity and mortality event was noted in the intensive care unit (ICU) of a veterinary school hospital in Nantes, France. Cats, and cats only, transferred from the emergency room presented with fever, ulcers on the tongue and cutaneous lesions around venepuncture or surgical incision sites, leading to suspicion of a feline calicivirus-associated virulent systemic disease confirmed with reverse transcriptase-polymerase chain reaction. A total of 14 cats were suspected. The clinical features and the origin of the contamination were described for each cat. The median length of incubation was 4.5 days. Fifty-seven percent of the cats were euthanased (8/14) and 21% died (3/14), with a combined mortality of 79% (11/14) – the highest ever reported. Median survival was 12 days. The recovery rate was 21% (3/14).Relevance and novel informationEight outbreaks have been reported, in veterinary clinics or in group-housed cats. The main unusual aspects of the present outbreak were: (1) the extreme flare-up of lesions at sites of skin breach, precluding any puncture/incision; (2) the suggested better survival rate at home than in hospital; and (3) the immediate control of the outbreak after recognition of the disease. Other striking but less unusual features of this outbreak were: (4) the increasing of the virulence of the calicivirus with the passage of time; and (5) the primary role that the caregivers’ hands played in the spread of the outbreak.

Long-term synchronized electrophysiological and behavioral wireless monitoring of freely moving animals

Parallel electrophysiological recording and behavioral monitoring of freely moving animals is essential for a better understanding of the neural mechanisms underlying behavior. In this paper we describe a novel wireless recording technique, which is capable of synchronously recording in vivo multichannel electrophysiological (LFP, MUA, EOG, EMG) and activity data (accelerometer, video) from freely moving cats. The method is based on the integration of commercially available components into a simple monitoring system and is complete with accelerometers and the needed signal processing tools. LFP activities of freely moving group-housed cats were recorded from multiple intracortical areas and from the hippocampus. EMG, EOG, accelerometer and video were simultaneously acquired with LFP activities 24-h a day for 3 months. These recordings confirm the possibility of using our wireless method for 24-h long-term monitoring of neurophysiological and behavioral data of freely moving experimental animals such as cats, ferrets, rabbits and other large animals.

Effects of single caging and cage size on behavior and stress level of domestic neutered cats housed in an animal shelter

Cats need a minimum amount of space even in animal shelters. In this study the effects of single caging and cage size on the behavior and stress level of domestic cats were investigated. Six neutered cats (2-15 years old) that had been housed in a group for at least 7 months were moved to three kinds of single cages (small, medium and large) by rotation on a Latin square design. They experienced each cage size for 6 days. Cats could use vertical dimensions when housed in a group room and the large cage. Behavioral observation was conducted for 3 h in the evening, and stress levels were assessed by urine cortisol-to-creatinine ratios. The amounts (estimated proportions) of time spent in locomotion and social/solitary play were lower even in large cages than in group housing (both P < 0.05). Conversely, the amount of time spent resting tended to increase when housed singly (P = 0.104). The urine cortisol-to-creatinine ratios of singly housed cats tended to be higher than that of group-housed cats (P = 0.086). The results indicate that cats become less active when they are housed singly in cages regardless of the cage size. Cats seem to feel no undue stress even in small cages if the stay is short.

Factors affecting faecal glucocorticoid levels in domestic cats(Felis catus): a pilot study with single and large multi-cat households

AbstractDomestic cats(Felis catus)are widely believed to be highly sensitive to the effects of social stress, especially when living in high density populations. Cats are capable of adapting to living in a group, but this will often require opportunities for escaping and hiding. In this pilot study, adrenocortical activity, as a valuable physiological indicator of arousal underpinning potential emotional stress, was evaluated through the measurement of mean faecal glucocorticoid metabolites (mGCM) in fourteen singly and sixteen group-housed cats. Living conditions and ratings of the owners’ quality of life (evaluated from self-reported questionnaires) were used as factors associated with faecal glucocorticoid levels of the cats. A direct association between the scores of owners’socialdimension of quality of life and the cats’ mGCM was found for single cats only, with higher owner social scores associated with higher cat mGCM. No significant differences in mGCM were found between singly versus group-living cats. This suggests that the under-explored factor of owner lifestyle could play an important role in domestic cats’ day-to-day levels of arousal, especially when kept as single pets.

Physical activity level of adult cats with varied feeding frequency

The prevalence of feline obesity is influenced by numerous factors, including inactivity and overconsumption of food. The objective of the present study was to evaluate the effect of feeding frequency on physical activity in adult cats. A total of twelve healthy adult cats were used in a cross-over study consisting of 32 d. In each of the two periods, six cats were fed either two meals or four meals daily. Throughout the study, cats were fed the same diet at amounts to maintain body weight and body condition score. Cats were individually housed 4 h/d at each scheduled feeding time, while for the other 20 h, cats were group-housed to allow for voluntary physical activity in the room with a 16 h light-8 h dark cycle. Voluntary activity levels were evaluated using Actical activity collars for seven consecutive days in each period. Daily average activity level for two-meal-fed cats (20.04 (SEM 2.19), activity counts/epoch (15 s)) was not different from four-meal-fed cats (20.14 (SEM 2.15), activity counts/epoch (15 s); P>0.05). In conclusion, when group-housed cats are fed to maintain their body weight and body condition score, varied feeding frequency between twice and four times daily may not affect activity levels.

P.1.d.015 Postponed influence of neonatal arginine-vasopressin and the arginine-vasopressin fragment Ac-D-MPRG administration to young white rats on behaviour

The effect of 6-hydroxydopamine (6-OHDA) injected into the cerebral ventricles on behaviour of singly- and group-housed cats was investigated. 6-OHDA in doses of 0.5, 1 and 2 mg was administered every morning for 5 to 8 days. In small doses 6-OHDA in singly- and group-housed cats evoked motor phenomena such as tremor, ataxia, rigidity, weakness and sometimes clonic-tunic convulsions. Occasionally restlessness, irritability and rage were observed. Large doses of 6-OHDA in group-housed cats, after a short latent period (2-3 days) produced aggression which intensified on subsequent injections, and thereafter, on repeated administrations, no longer occurred. The aggression consisted of restlessness, irritability, anger, rage, apprehension, threat, attack, fighting, flight and was initiated by the most restless cat, or by disturbing the animals, such as by moving the cage. When 6-OHDA no longer produced aggressive behaviour, motor changes such as tremor, ataxia, rigidity, walking on broad base, weakness with adynamia and clonic-tonic convulsions developed. These latter symptoms were produced by large doses of 6-OHDA in singly-housed cats. In these animals spontaneous signs of aggressive behaviour usually were not observed, although if handled they showed rage, snarling and hissing. When singly-housed cats were kept in the same cage with group-housed animals, the singly-housed cats usually became aggressive. It appears that hyperactivity induced aggression in 6- OHDA-treated cats.

Influence of indoor-cat group size and dominance rank on urinary cortisol levels

AbstractDomestic cats (Felis silvestris catus) are often housed indoors both singly and in groups. However, there is a lack of studies dealing with cat-cat relationships, group composition and effects of environmental parameters on the well-being of privately-owned cats. One way to index the effects of stressful situations is to measure glucocorticoid levels, as glucocorticoids are released from the adrenal cortex in response to stress-induced activation of the hypothalamic-pituitary-adrenal axis. Therefore, we investigated the influence of single and group housing on basal urinary cortisol levels of indoor-only domestic cats in private households, taking into account dominance status and environmental parameters. Urine samples were collected non-invasively by owners from six single-housed cats and six alpha-omega animal pairs of multi-cat households. Dominance status in group-housed cats was determined by competition test series. Additionally, we compared cortisol levels of privately-owned cats with those of shelter cats. Results showed that basal urinary cortisol levels of cats in private households are neither influenced by housing style (single cat vs multi cat) nor by individuals' dominance status. Correlations indicated a positive influence of human density, number of persons per household, and number of m2available to cats on basal urinary cortisol levels, whereas cat-related parameters such as number of cats per household, number of m2per cat, and number of persons per cat, did not have any significant influence on basal urinary cortisol levels. A comparison of basal urinary cortisol levels of privately-owned and shelter cats revealed no influence of location (private household, shelter) and group type (single, group [dominant or subordinate]) on basal urinary cortisol levels. This study is the first to investigate basal urinary cortisol levels of domestic cats in private households and an animal shelter considering housing style, dominance status, and environmental parameters.

Impact of time-limited feeding and dietary carbohydrate content on weight loss in group-housed cats

Twenty-four adult cats were transitioned to time-limited feeding and randomized to either a dry low carbohydrate diet (LC) or a dry reduced energy diet (HC). In Trial 1 the LC and HC groups received equal amounts of food (by weight) for 13 weeks. Both groups consumed all food offered, hence the LC group received more energy/day than the HC group. In Trial 2 all cats were fed the LC diet for 12 weeks, but each group received the energy that the opposite group had received in Trial 1. In Trial 1 only the overweight HC cats (body condition score> 6/9) experienced a significant change in body weight (-0.52 +/- 0.08 kg). In Trial 2, LC/Low Calorie overweight cats lost 0.62 +/- 0.10 kg, whereas, the LC/High Calorie normal weight cats gained 0.68 +/- 0.05 kg. In conclusion, body condition and energy intake but not type of diet influenced weight in this cohort of group-housed cats.

Cats as a Risk for Transmission of Antimicrobial Drug-resistantSalmonella

To determine whether cats were a risk for transmission of Salmonella to humans, we evaluated the excretion of Salmonella by pet cats. Rectal-swab specimens were taken from 278 healthy house cats, from 58 cats that died of disease, and from 35 group-housed cats. Group-housed cats were kept in one room with three cat trays and a common water and feed tray. Eighteen (51.4%) of 35 group-housed cats, 5 (8.6%) of 58 diseased cats (5/58), and 1 (0.36%) of 278 healthy house cats excreted Salmonella. Salmonella isolates were of serotypes Typhimurium, Enteritidis, Bovismorbificans and 4:i:-. Acquired antimicrobial resistance was found in serotype Typhimurium (resistance to ampicillin, chloramphenicol, and tetracycline; to ampicillin; and to chloramphenicol) and 4:i:- strains (resistance to ampicillin, chloramphenicol, sulfonamides, trimethoprim, and sulfamethoxazole/trimethoprim). Cats that excrete Salmonella can pose a public health hazard to people who are highly susceptible to Salmonella, such as children, the elderly, and immunocompromised persons.

Comparison of behavioral changes in cats treated with intracerebroventricular 6-hydroxydopamine and reserpine

In singly- and group-housed cats, an intraventricular injection of 6-hydroxydopamine (6-OHDA) in doses up to 1.0 mg, after a latent period of 1 to 3 days, evoked motor responses including tremor, ataxia, rigidity, weakness with adynamia and clonic-tonic convulsions. However, the intraventricular administration of 6-OHDA in a dose of 2.0 mg in group-housed cats, also after a latent period of 1 to 3 days, caused aggression, a restlessness, irritability, rage, fear, threat, attack, fighting and flight. These responses were accompanied by autonomic signs of mydriasis and dyspnoea and motor changes including tremor, ataxia, rigidity, weakness with adynamia and clinic-tonic convulsions. In the singly-housed cat only the latter motor phenomena were observed after the higher dose. Intraventricular injection of reserpine (0.5-1.0 mg) in both singly- and group-housed cats produced catalepsy, sedation, miosis, ptosis, defecation and micturition as well as motor responses of tremor, rigidity and akinesia. It is concluded that although 6-OHDA and reserpine evoke different behavioral effects, the motor changes are similar.

6-Hydroxydopamine and Aggression in Cats

The effect of 6-hydroxydopamine (6-OHDA) injected into the cerebral ventricles on behaviour of singly- and group-housed cats was investigated. 6-OHDA in doses of 0.5, 1 and 2 mg was administered every morning for 5 to 8 days. In small doses 6-OHDA in singly- and group-housed cats evoked motor phenomena such as tremor, ataxia, rigidity, weakness and sometimes clonic-tonic convulsions. Occasionally restlessness, irritability and rage were observed. Large doses of 6-OHDA in group-housed cats, after a short latent period (2-3 days) produced aggression which intensified on subsequent injections, and thereafter, on repeated administrations, no longer occurred. The aggression consisted of restlessness, irritability, anger, rage, apprehension, threat, attack, fighting, flight and crying. Of autonomic phenomena mydriasis, dyspnea and sometimes piloerection were observed. The aggression was initiated by the most restless cat, or by disturbing the animals, such as by moving the cage. When 6-OHDA no longer produced aggressive behaviour, motor changes such as tremor, ataxia, rigidity, walking on broad base, weakness with adynamia and clonic-tonic convulsions developed. These latter symptoms were produced by large doses of 6-OHDA in singly-housed cats. In these animals spontaneous signs of aggressive behaviour usually were not observed, although if handled they showed rage, snarling and hissing. When singly-housed cats were kept in the same cage with group-housed animals, the singly-housed cats usually became aggressive. It appears that hyperactivity induced aggression in 6-OHDA-treated cats.