Compositions and methods for preventing hematophagous infestation of cattle are provided, directed at isolated proteins that disrupt platelet aggregation. Also provided are nucleotide sequences encoding the proteins. The exemplary haematollogen protein was isolated from the salivary glands of Haematobia irritans. The compositions are useful as veterinary vaccines in prevention of blood-feeding in cattle by the infesting horn fly and are also useful in treatment of thrombosis.