Human anti animal antibodies issues in blood testing
You do not need to be exposed directly to animals to get human anti-mouse (or human anti-animal) antibodies; you can also be exposed to animal products in various ways.  


  Circulating anti-animal antibodies can arise from iatrogenic and noniatrogenic causes. The former is the result of the normal response of the human immune system to an administered “foreign” protein antigen. Currently available diagnostic and pharmaceutical agents derived from an animal source are extensive and range from rodent immunoglobulins to hormones isolated from fish. In addition, some recombinant proteins are affinity purified on immobilized monoclonal mouse antibody columns, and the possibility exists for some of the mouse monoclonal antibody to detach and copurify with the protein


You can also get anti-animal antibodies from blood transfusions:


  Blood transfusion is also associated with an increased incidence of anti-animal antibodies. A study of 2829 participants in a population health survey revealed that 14.4% of the participants who had been transfused were anti-animal positive, compared with 10.4% of the participants who had never received a blood transfusion. This difference was presumably attributable to infusion of preexisting human anti-animal antibody or as a result of infusion of a foreign antigen present in the unit of blood 


or through vaccination:


  Vaccination against infectious diseases is another route by which animal protein antigens may be inadvertently presented to the immune system and trigger antibody formation. In the US, chick embryo or egg cultures are frequently used in vaccine production, and residual chicken protein may be present in vaccines, whereas in Europe, some vaccines contain rabbit serum, e.g., rubella vaccine in France, and multimicrobial vaccine (Bruschettini vaccine) in Italy 


or "unconventional therapies":


  The administration of unconventional therapies is also a route to immunization with animal protein. For example, a patient developed anti-rabbit antibodies following injections of “antireticulocytoxique”, which is a lyophilized serum obtained from rabbits injected with homogenates of human bone marrow and spleen and is intended as a tonic to improve senescence and to reduce fatigue and debilitation


Other routes:


  Noniatrogenic causes of anti-animal antibodies include maternal transfer across the placenta to the unborn child (37)(38), animal husbandry or the keeping of animals as pets (39), and the transfer of dietary antigens across the gut wall in conditions such as celiac disease (40)(41). Anti-animal antibodies are also more common in multiparous females (36), and a high incidence of human anti-animal antibodies has also been observed in association with certain disease states, e.g., idiopathic cardiomyopathy


All quotes from Human Anti-Animal Antibody Interferences in Immunological Assays, by Larry J. Kricka (1999).