Infectious Agent Differentials for Medical Problems
For most clinical problems or syndromes recognized in small animal practice, there are several infectious agents that may be the cause. Bacterial, fungal, parasitic, rickettsial, and viral agents frequently are encountered in small animal practice. Common infectious agents and the clinical problems with which they are associated are summarized in Tables 259-1 to 259-4. Recognizing the infectious agents that are most commonly associated with medical problems or syndromes allows the veterinary clinician first to rank the list of differential diagnoses using the signalment, history, and clinical examination findings. Diagnostic assays to aid in the diagnosis of small animal infectious diseases are plentiful and relatively inexpensive. However, starting with a syndromic approach allows the differential list to be narrowed and subsequently aids in selection of the optimal diagnostic assays for further evaluation of the case or choice of the most logical empiric therapeutic trials. In this chapter, the bacterial, fungal, parasitic, rickettsial, and viral agents associated with common medical problems are discussed briefly.
TABLE 259-1
Common Bacterial Agents Infecting Small Animals and Associated Primary Clinical Findings
| Agent | Common Problems and Syndromes |
| Anaerobes (including Actinomyces and Nocardia) | Abscesses, stomatitis, rarely pneumonia |
| Bacterial endocarditis (multiple bacteria) | Fever, cardiac murmur |
| Bartonella henselae | Fever, uveitis, hyperglobulinemia, lymphadenopathy, myocarditis, endocarditis |
| Bartonella vinsonii subsp. berkhoffii (dogs) | Fever, epistaxis (vasculitis), endocarditis, thrombocytopenia, hemolytic anemia, polyarthritis, pyogranulomatous inflammation |
| Borrelia burgdorferi (dogs) | Lameness (polyarthritis), proteinuria with azotemia |
| Bordetella bronchiseptica | Sneezing or coughing |
| Brucella canis (dogs) | Reproductive failure, orchitis, uveitis, hyperglobulinemia; zoonotic |
| Campylobacter spp. | Mixed bowel diarrhea with or without vomiting |
| Chlamydia felis (cats) | Conjunctivitis; potentially sneezing and nasal discharge |
| Clostridium perfringens | Large bowel diarrhea, polycythemia, occasionally vomiting |
| Discospondylitis (multiple bacteria) | Fever, lameness, pain at the site of infection |
| Helicobacter spp. | Vomiting, nausea, weight loss, no diarrhea |
| Leptospira spp. | Fever, vomiting (nephritis, hepatitis), thrombocytopenia |
| Mycoplasma spp. | Sneezing or coughing (upper respiratory tract disease in cats and canine infectious respiratory disease complex in dogs), lameness (polyarthritis), reproductive failure |
| Mycoplasma haemominutum (cats) | Pale mucous membranes from hemolytic anemia |
| Mycoplasma haemofelis (cats) | Pale mucous membranes from hemolytic anemia |
| Mycoplasma turicensis (cats) | Pale mucous membranes from hemolytic anemia |
| Mycoplasma haemocanis (dogs) | Subclinical infection except in immunosuppressed dogs and then pale mucous membranes from hemolytic anemia may occur |
| Mycoplasma hematoparvum (dogs) | Subclinical infections except in immunosuppressed dogs, which can develop pale mucous membranes from hemolytic anemia |
| Salmonella spp. | Mixed bowel diarrhea with or without vomiting |
| Streptococcus equi var. zooepidemicus (dogs) | Cough |
| Staphylococcus pseudintermedius | Pyoderma |
| Yersinia pestis | Fever, lymphadenopathy, rarely cough or dyspnea |
TABLE 259-2
Common Fungal and Rickettsial Agents Infecting Small Animals and Associated Primary Clinical Findings
TABLE 259-3
Common Parasitic Agents Infecting Small Animals and Associated Primary Clinical Findings
| Agent | Type | Common Problems and Syndromes |
| Aelurostrongylus abstrusus (cats) | Lungworm | Coughing or dyspnea from bronchitis |
| Ancylostoma caninum (dogs) | Hookworm | Diarrhea with blood, pale mucous membranes |
| Ancylostoma tubaeforme (cats) | Hookworm | Diarrhea with blood, pale mucous membranes |
| Babesia vogeli (dogs) | Piroplasm | Pale mucous membranes and weakness from hemolytic anemia in dogs |
| Babesia gibsoni (dogs) | Piroplasm | Pale mucous membranes and weakness from hemolytic anemia in dogs |
| Babesia conradae (dogs) | Piroplasm | Pale mucous membranes and weakness from hemolytic anemia in dogs |
| Baylisascaris procyonis (dogs) | Roundworm | Subclinical in dogs; zoonotic |
| Cryptosporidium spp. | Coccidian | Small bowel diarrhea and weight loss |
| Cytauxzoon felis (cats) | Piroplasm | Pale mucous membranes and weakness from hemolytic anemia in cats |
| Dipylidium caninum | Cestode | Subclinical or unthrifty |
| Dirofilaria immitis | Heartworm | Coughing or dyspnea from bronchitis or emboli, proteinuria |
| Echinococcus granulosa (dogs) | Cestode | Subclinical or unthrifty; zoonotic |
| Echinococcus multilocularis | Cestode | Subclinical or unthrifty; zoonotic |
| Oslerus osleri (dogs) | Nematode | Coughing or dyspnea from worms in trachea |
| Giardia spp. | Flagellate | Small bowel diarrhea and weight loss |
| Hepatozoon americanum (dogs) | Coccidian | Lameness due to muscle inflammation and periosteal bone inflammation |
| Isospora spp. | Coccidian | Large bowel diarrhea or subclinical |
| Leishmania spp. | Flagellate | Skin disease, weight loss, multisystemic signs, Walker hounds |
| Ollulanus tricuspis (cats) | Nematode | Vomiting in cats |
| Neospora caninum (dogs) | Coccidian | Abortion, stillbirth, ascending neuromuscular disease |
| Paragonimus kellicotti | Fluke | Cough |
| Physaloptera canis | Nematode | Vomiting |
| Pneumonyssoides caninum (dogs) | Mite | Serous nasal discharge and sneezing |
| Strongyloides stercoralis | Nematode | Mixed bowel diarrhea |
| Toxocara canis (dogs) | Roundworm | Vomiting, abdominal distention, failure to thrive |
| Toxocara cati (cats) | Roundworm | Vomiting, abdominal distention, failure to thrive |
| Toxascaris leonina | Roundworm | Vomiting, abdominal distention, failure to thrive |
| Trichuris vulpis (dogs) | Whipworm | Large bowel diarrhea, occasional vomiting |
| Taenia spp. | Cestode | Subclinical or unthrifty |
| Toxoplasma gondii | Coccidian | Fever, uveitis, lameness (muscle pain) |
| Tritrichomonas foetus | Flagellate | Large bowel diarrhea |
| Trypanosoma cruzi (dogs) | Flagellate | Ventricular arrhythmias, myocarditis |
TABLE 259-4
Common Viral Agents Infecting Small Animals and Associated Primary Clinical Findings
| Agent | Common Problems and Syndromes |
| Canine adenovirus 1 | Vomiting, icterus, diarrhea (canine infectious hepatitis) |
| Canine adenovirus 2 | Sneezing, nasal discharge, cough (part of CIRDC) |
| Canine coronavirus | Small bowel diarrhea in puppies |
| Canine distemper virus | Vomiting, diarrhea, sneezing, coughing, seizures, chorea, myoclonus, posterior uveitis |
| Canine herpesvirus | Vomiting, diarrhea, abdominal distention in fading puppies, possibly cough as part of CIRDC |
| Canine influenza virus | Cough, sneezing, nasal discharge (part of CIRDC) |
| Canine parvovirus | Vomiting, neutropenia, bloody diarrhea |
| Canine respiratory coronavirus | Cough, sneezing, nasal discharge (potentially part of CIRDC) |
| Feline calicivirus | Sneezing, stomatitis, vasculitis (virulent systemic strains) |
| Feline coronaviruses | Small bowel diarrhea, fever, uveitis, hyperglobulinemia, failure to thrive, central nervous system disease |
| Feline immunodeficiency virus | Fever, weight loss, multisystemic signs with involvement of most body systems |
| Feline herpesvirus 1 | Sneezing, nasal discharge, keratitis, conjunctivitis, rarely stomatitis |
| Feline leukemia virus | Multisystemic signs due to immunodeficiency and secondary invaders, cytopenias, aplastic anemia, lymphoma |
| Feline panleukopenia virus | Vomiting, bloody diarrhea, neutropenia |
| Canine parainfluenza virus | Cough, sneezing, nasal discharge (part of CIRDC) |
| Rabies virus | Neurologic signs; zoonotic |
