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⭐ Radiographs — Thorax & Skeleton

Radiographic Evaluation of Infectious Disease in Dogs & Cats

Thoracic and skeletal radiographs are the first-line imaging modality for many infectious diseases. They stage heartworm severity, characterize pulmonary fungal patterns, identify discospondylitis, and reveal the skeletal manifestations of tick-borne and protozoal infections.

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Radiographic Evaluation at a Glance

Primary Use
Thoracic disease staging, pulmonary pattern recognition, and skeletal lesion characterization
Key Advantage
Rapid, widely available, no sedation required; ideal for serial monitoring of treatment response
Limitations
Cannot evaluate soft tissue detail, CNS, or early bone changes; consider MRI or ultrasound for complementary information
Common Indications
Coughing, dyspnea, spinal pain, lameness, heartworm staging, and treatment monitoring
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Parasitic Infections

Heartworm Disease

Dirofilaria immitis

Heartworm disease remains one of the most important parasitic infections in dogs and is increasingly recognized in cats. Radiographs are essential for staging disease severity, which directly guides treatment decisions. The American Heartworm Society classifies canine heartworm from Class 1 (asymptomatic) through Class 4 (caval syndrome). Rescued and relocated animals are at particular risk — Hurricane Katrina in 2005 resulted in widespread redistribution of heartworm-positive dogs across the United States.

ClassClinical SignsRadiographic Findings
Class 1 Asymptomatic or slight exercise intolerance Within normal limits or mild changes
Class 2 Coughing, exercise intolerance, fever Moderate disease: dilated pulmonary arteries, right heart enlargement, pneumonitis
Class 3 Ascites, dyspnea, coughing ± hemoptysis, weight loss, lethargy Severe: reverse "D" cardiomegaly, tortuous pulmonary arteries, pneumonitis/pneumonia
Class 4 Weakness/collapse (shock), all Class 3 signs Same as Class 3; echo shows adult worms in right heart and MPA

⚠ Important: Radiographic changes from heartworm disease do NOT fully resolve

Even after successful adulticide treatment, pulmonary arterial changes and lung parenchymal scarring can persist indefinitely. This is important when interpreting radiographs of patients with prior heartworm history.

Feline heartworm: Cats show enlargement, blunting and tortuosity of peripheral pulmonary arteries (especially right-sided), cardiomegaly with right ventricular enlargement, and patchy focal or diffuse pulmonary parenchymal changes. Two positive ELISA tests are recommended for confirmation.

Toxoplasmosis (Pulmonary Form)

Toxoplasma gondii

While toxoplasmosis is most commonly associated with CNS disease, the pulmonary form produces a distinctive radiographic pattern. Cats serve as the definitive host, and infection is most commonly seen in immunocompromised felines. Respiratory disease can be the primary or sole clinical manifestation.

Clinical Signs

  • Fever and lethargy
  • Weight loss and anorexia
  • Pneumonia and respiratory distress
  • CNS signs (concurrent)

Radiographic Findings

  • Diffuse nodular interstitial pattern
  • Alveolar infiltrates
  • Mild pleural effusion (less common)

Lungworms

Aelurostrongylus, Oslerus, Crenosoma, Angiostrongylus, and others

Lungworms are transmitted through ingestion of intermediate hosts (snails, rodents, earthworms, lizards). Multiple species affect dogs and cats, producing variable radiographic patterns from subtle bronchial thickening to severe mixed pulmonary disease. Diagnosis is confirmed through Baermann fecal testing or bronchoscopy with BAL.

Clinical Signs

  • Chronic cough (often dry or honking)
  • Wheezing and sneezing
  • Nasal discharge
  • Lethargy
  • Poor response to empiric antibiotics

Radiographic Findings

  • Bronchial pattern
  • Interstitial pattern
  • Alveolar disease
  • Mixed pulmonary disease
A young animal with chronic cough that is PCR-negative for common respiratory pathogens and unresponsive to doxycycline should raise suspicion for lungworm. Baermann fecal flotation is the recommended diagnostic test.

Hepatozoonosis

Hepatozoon americanum (Old World) / Hepatozoon canis (New World)

Hepatozoonosis is a tick-borne protozoal infection transmitted by the Gulf Coast tick (Amblyomma maculatum) and brown dog tick (Rhipicephalus sanguineus). Unlike most tick-borne diseases, transmission occurs when the dog ingests an infected tick rather than through the tick bite itself. H. americanum (despite the name, associated with the Gulf Coast tick) is more severe and often fatal within one year; H. canis is less severe and may be subclinical.

Clinical Signs

  • Fever and lethargy (waxing and waning)
  • Decreased appetite and weight loss
  • Muscle pain and weakness
  • Reluctance to move
  • Ocular and nasal discharge
  • H. americanum: usually fatal within 1 year

Radiographic Findings

  • Axial periosteal reaction (spine, pelvis)
  • Appendicular periosteal reaction (long bones)
  • Smooth to irregular periosteal new bone
  • Can mimic hypertrophic osteopathy
Treatment for H. americanum involves combination antiparasitic therapy (trimethoprim-sulfa, clindamycin, and a quinolone). The disease is never fully cured, and long-term management is required. Low albumin on bloodwork is a common concurrent finding.

Chagas Disease

Trypanosoma cruzi

Chagas disease is transmitted through the feces of the triatomine ("kissing") bug and is endemic throughout the Americas, including Texas. Over 60% of kissing bugs carry the parasite. Ingestion of the bug (rather than bite alone) results in higher parasite loads and 3x mortality. The disease produces progressive myocardial fibrosis and cardiac failure. Certain breeds show striking prevalence: English/French Bulldogs (54%), German Shepherds (30%), and Chihuahuas (29%).

Clinical Signs

  • Progressive heart failure (biventricular)
  • Arrhythmias (can occur with normal cardiac size)
  • Exercise intolerance and weakness
  • Ascites and respiratory distress
  • Collapse
  • May be subclinical for years

Radiographic Findings

  • Generalized cardiomegaly (biventricular)
  • Right heart failure pattern
  • Pulmonary venous congestion
  • Pleural effusion
  • Hepatomegaly and ascites

⚠ Arrhythmias despite normal radiographs

Chagas can produce life-threatening arrhythmias even when cardiac size and function appear normal on radiographs and echocardiography. Screening via ECG and serology is recommended for at-risk breeds in endemic areas. Overall prevalence in dogs is approximately 16.8%, similar to shelter populations (18%).

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Fungal Infections

Coccidioidomycosis (Valley Fever)

Coccidioides immitis / C. posadasii

Coccidioidomycosis is found in desert soils and released as airborne spores, primarily in the southwestern United States. Radiographs play a dual role: identifying pulmonary and skeletal disease at diagnosis, and monitoring response to antifungal therapy (typically fluconazole) over months of treatment.

Clinical Signs

  • Coughing (often dry, honking)
  • Fever and weakness
  • Lameness and spinal pain
  • Weight loss
  • Draining skin tracts (cutaneous form)

Radiographic Findings

  • Bone lesions resembling proliferative OSA
  • Interstitial disease to pulmonary nodules
  • Tracheobronchial lymphadenopathy
  • Multi-site skeletal involvement
Serology (MiraVista Coccidioides IgG EIA) confirms diagnosis. Serial radiographs after 3 months of fluconazole therapy can show dramatic improvement in both pulmonary and osseous lesions, guiding treatment duration.

Histoplasmosis

Histoplasma capsulatum

Histoplasmosis is found in soil contaminated with bat or bird droppings, predominantly in the Ohio and Mississippi River valleys. Cats are affected at least as often as dogs. Pulmonary disease is the most common radiographic presentation, but systemic spread can produce erosive joint disease, lymphadenopathy, and concurrent GI involvement (see Ultrasound guide). The disease can mimic asthma in cats and must be differentiated from other causes of diffuse interstitial or nodular lung disease.

Clinical Signs

  • Lethargy and weight loss
  • Anorexia and fever
  • Coughing and dyspnea
  • Lameness (erosive joint disease)
  • Uveitis (cats)
  • GI signs (diarrhea, weight loss)

Radiographic Findings

  • Diffuse interstitial disease
  • Nodular pattern (miliary or larger)
  • Alveolar pattern (less common)
  • Erosive joint disease (tarsal, carpal)
  • Lymphadenopathy (hilar/tracheobronchial)
In cats, histoplasmosis can present radiographically identical to feline asthma (bronchial pattern without hyperinflation). History of increased respiratory rate and anorexia in an endemic area should prompt histoplasma testing. Fluconazole treatment with serial radiographic monitoring is standard.

Blastomycosis

Blastomyces dermatitidis

Blastomycosis is found in sandy soil near water sources. The organism is inhaled as spores or enters through skin wounds. Dogs are more commonly affected than cats. The pulmonary form produces pyogranulomatous lung disease with characteristic miliary nodules. Cutaneous draining tracts, uveitis, and lymphadenopathy are important concurrent findings that support the diagnosis.

Clinical Signs

  • Rapid breathing and audible respiratory effort
  • Coughing
  • Uveitis and blindness
  • Draining cutaneous tracts (especially extremities)
  • Fever, depression, weight loss
  • Lymphadenopathy

Radiographic Findings

  • Pulmonary nodules (miliary pattern)
  • Lymphadenopathy (hilar/tracheobronchial)
  • Osteomyelitis (appendicular skeleton)
  • Pyogranulomatous pulmonary disease
Diagnosis is often confirmed via lymph node FNA cytology, urine antigen testing, or skin biopsy from draining tracts. Thoracic radiographs and concurrent abdominal ultrasound help stage systemic disease.
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Bacterial Infections

Discospondylitis

Brucella canis, Staphylococcus aureus, Streptococcus, E. coli, fungal organisms

Discospondylitis is infection of the intervertebral disc and adjacent vertebral endplates. It arises from hematogenous spread (most common), migrating foreign bodies, or surgical contamination. Brucella canis is an important cause in intact and rescue dogs and carries zoonotic significance. Non-Brucella bacterial causes include S. aureus, Streptococcus, and E. coli, typically from hematogenous seeding (urinary tract, dental, skin infections).

Clinical Signs

  • Spinal pain (can be severe, intermittent)
  • Reluctance to run, jump, or climb
  • Weight loss, anorexia, depression
  • Fever
  • Nonspecific pain after surgery (TPLO, etc.)
  • Screaming in pain (acute episodes)

Radiographic Findings

  • Disc space collapse and narrowing
  • Endplate lysis (irregular margins)
  • Irregular bony spondylosis (bridging)
  • Spinal subluxation (severe cases)
  • May involve multiple disc spaces

⚠ Brucella canis: Zoonotic Concern

Brucella canis carries zoonotic risk, particularly for breeders and veterinary staff handling reproductive tissues. Rescue dogs, both neutered and intact, can be carriers. Any dog with discospondylitis should be tested for Brucella, and the organism should be reported to public health authorities if identified.

Serial radiographs every 4–8 weeks monitor response to antibiotic therapy. If neurological signs develop, MRI is indicated to evaluate for epidural empyema and spinal cord compression. See our MRI guide for advanced spinal imaging.
Quick Reference

Feline Infectious Lung Disease Patterns

Cats present unique diagnostic challenges because multiple infections produce overlapping pulmonary patterns. This summary helps differentiate the most common causes:

Radiographic Pattern by Disease

DiseasePrimary Pattern
HeartwormPatchy or diffuse interstitial
ToxoplasmosisDiffuse nodular interstitial with alveolar infiltrates
LungwormsBronchial, interstitial, mixed, or alveolar
HistoplasmosisDiffuse interstitial to nodular, ± alveolar
BlastomycosisNodular lung disease (miliary)

Need Radiographic Interpretation?

Submit radiographs for board-certified radiologist review through our teleradiology service, or schedule an appointment at any SVI location.

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Frequently Asked Questions

Radiographic Infectious Disease FAQ

Bone lesions from coccidiomycosis can closely resemble osteosarcoma (OSA) radiographically — both produce aggressive, proliferative bone reactions. However, fungal disease often involves multiple bones simultaneously and favors flat bones and the axial skeleton, while primary bone tumors are more typically solitary and in the metaphysis of long bones. A board-certified radiologist considers the pattern of involvement, geographic history, and clinical context. Serology and biopsy or FNA are usually needed for definitive differentiation.
For pulmonary and osseous fungal disease, radiographs are typically repeated every 3 months during treatment to assess response. Pulmonary nodules and bone lesions should show progressive improvement. Treatment (usually fluconazole) continues until both clinical signs and radiographic abnormalities have substantially resolved, often 6–12 months or longer. Your veterinarian may adjust this schedule based on your pet's individual response.
Radiographic changes from heartworm disease typically do not fully resolve, even after successful adulticide treatment. Pulmonary arterial enlargement, tortuosity, and parenchymal scarring can persist indefinitely. This is important for both the interpreting radiologist and the referring veterinarian to be aware of — prior heartworm history should always be communicated so that these residual changes are not misinterpreted as active disease.
Yes. Sage Veterinary Imaging offers teleradiology services — radiographs can be submitted digitally for board-certified radiologist interpretation. This is particularly valuable for infectious disease cases where pattern recognition requires specialized expertise. See our teleradiology page for submission details and turnaround times.