JAVMA, Vol 238, No. 6, March 15, 2011 Scientific Reports 731 SMALL ANIMALS I nterdog household aggression (sometimes described as sibling rivalry, whether or not the dogs involved are genetically related) is a distressing problem for dog own- ers and a common reason that owners seek behavioral advice or veterinary medical treatment. It is not uncom- mon for 1 or both dogs to require treatment for injuries or for owners to need medical attention for injuries re- ceived while interceding in a dog fight. Interdog house- hold aggression tends to result in more severe injuries than do fights between dogs from separate households. 1 The authors of several publications have examined the issue of interdog household aggression. 1–8 The most thorough description of the problem with treatment and outcome assessment was published in 1996 by Sherman et al. 1 The authors reported that typically 1 dog insti- gates fights and that often this is the younger of the 2 Interdog household aggression: 38 cases (2006–2007) Kathryn M. Wrubel, PhD; Alice A. Moon-Fanelli, PhD; Louise S. Maranda, DVM, PhD; Nicholas H. Dodman, BVMS, DACVB Objective—To analyze factors associated with interdog household aggression and deter- mine treatment outcomes. Design—Retrospective case series and survey. Animals—38 pairs of dogs with interdog household aggression. Each pair of dogs was considered 1 case. Procedures—Records of dogs with interdog household aggression that were examined during initial or follow-up consultations at a veterinary teaching hospital from December 5, 2006, to December 5, 2007, were analyzed for clinical features. Data regarding outcome, owner compliance, and efficacy of recommended treatments obtained by use of a follow- up survey were evaluated. Results—Most cases (30/38 [79%]) of interdog household aggression involved same-sex pairs; 26 of 38 (68%) cases involved 1 female or a pair of females. Instigators and recipients of aggression were clearly identified in 27 of 38 (71%) cases; most instigators were the younger of the pair (20/27 [74%]) or were newer additions to the household (19/27 [70%]). Fight-eliciting triggers included owner attention, food, excitement, and found items. Some dogs had risk factors for behavior problems such as a history of living in multiple households (21/51 [41%]), adoption after 12 weeks of age (20/51 [39%]), or being acquired from a shel- ter (17/51 [33%]). Effective treatment recommendations included implementing a so-called nothing-in-life-is-free program, giving 1 dog priority access to resources, and administering psychotropic medication. Frequency and severity of fighting were significantly reduced af- ter consultation. Owners reported a 69% overall improvement following treatment. Conclusions and Clinical Relevance—Most treatment strategies were considered effec- tive. Consistency and predictability of social interactions are essential in resolving interdog household aggression. (J Am Vet Med Assoc 2011;238:731–740) dogs or is a newer addition to the household. The study also found that fights most often occur between dogs of the same sex (usually pairs of females). Fight-eliciting triggers include physical resources, owner proximity or attention, and access to confined spaces; another com- mon trigger for fights is excitement, which may occur during greetings, play sessions, walks, or automobile rides. 1 In addition to actual fights, typical behaviors observed in cases of interdog household aggression in- clude mounting, blocking, standing over the other dog, posturing, staring, and vocalization. It has been postulated that interdog household ag- gression occurs when dogs attempt to establish or rees- tablish a hierarchy 2,5 or when a dominant-subordinate relationship is contested. 1 Fighting in interdog house- hold aggression cases often begins when circumstances are changing (eg, an older dog becomes sick or dies, a new dog is added to the household, or a younger dog becomes socially mature). Hierarchies are dynamic and are established by many factors including competition over resources that are valuable to 1 or both dogs, mem- From the Departments of Clinical Sciences (Wrubel, Moon-Fanelli, Dodman) and Environmental and Population Health (Maranda), Cum- mings School of Veterinary Medicine, Tufts University, North Grafton, MA 01581. Dr. Wrubel’s present address is InTown Veterinary Group Inc, Animal Behavior Services, Massachusetts Veterinary Referral Hos- pital, 20 Cabot Rd, Woburn, MA 01801. Dr. Moon-Fanelli’s present address is Animal Behavior Consultations LLC, Brooklyn Veterinary Hospital, 150 Hartford Rd Rt 6, Brooklyn, CT 06234. Dr. Maranda’s pres- ent address is Division of Clinical Research, University of Massachusetts Medical School, 55 Lake Ave N, Worcester, MA 01655. Address correspondence to Dr. Dodman (nicholas.dodman@tufts.edu). ABBREVIATIONS AKC American Kennel Club CI Confidence interval VAS Visual analogue scale