Feline Cytauxzoonosis Study Guide

Overview and Clinical Importance

Cytauxzoonosis is a life-threatening, tick-borne protozoal disease of domestic and wild felids caused by Cytauxzoon felis. First described in Missouri in 1976, this disease has historically been considered uniformly fatal in domestic cats, with mortality rates approaching 100% without treatment. The disease is endemic to the south-central and southeastern United States, with geographic expansion correlating with the distribution of its primary tick vector, Amblyomma americanum (the Lone Star tick). Understanding this disease is critical for NAVLE success, as it represents a high-yield topic in feline emergency and infectious disease medicine.

Etiology and Taxonomy

Cytauxzoon felis is a tick-transmitted, obligate, hemoprotozoal piroplasmid pathogen belonging to the phylum Apicomplexa, order Piroplasmida, and family Theileriidae. The organism is closely related to Theileria species, which cause East Coast fever in cattle.

Taxonomic Classification

Life Cycle and Transmission

C. felis has a complex life cycle requiring both a tick (definitive host) and a felid (intermediate host). Understanding this cycle is essential for NAVLE questions on pathogenesis and prevention.

Tick Vectors

Primary Vector: Amblyomma americanum (Lone Star tick) is the primary vector. This aggressive, indiscriminate feeder is identifiable by the single white spot on the female's dorsal scutum. Secondary Vector: Dermacentor variabilis (American dog tick) has been experimentally shown to transmit the organism but is considered less significant in natural transmission.

Life Cycle Phases

Transmission Details

• Minimum transmission time: greater than 36 hours but less than or equal to 48 hours of tick attachment
• Transstadial transmission occurs (larva to nymph to adult) but NOT transovarial
• Only nymph and adult ticks transmit infection to felids
• No evidence of direct cat-to-cat transmission without tick vector
• Blood transfusion from carrier cats transmits piroplasms but does NOT cause clinical disease

Epidemiology

Geographic Distribution

Cytauxzoonosis is endemic to the south-central and southeastern United States, with confirmed cases in: Missouri, Arkansas, Oklahoma, Texas, Louisiana, Mississippi, Alabama, Georgia, Florida, Tennessee, Kentucky, Kansas, Nebraska, Iowa, North Carolina, South Carolina, Virginia, and expanding northward. The geographic range correlates with A. americanum distribution and is expanding with climate change.

Risk Factors

Reservoir Hosts

Bobcats (Lynx rufus): Primary natural reservoir; typically subclinical with persistent parasitemia (prevalence up to 79% in endemic areas). Domestic cats: Survivors of acute infection become chronic carriers and can serve as infection reservoirs. Carrier prevalence in domestic cats ranges from 0.3% to 25.8% depending on geographic location.

Pathophysiology

The pathogenesis of cytauxzoonosis involves two distinct phases, with the schizogenous (tissue) phase being responsible for clinical disease. Clinical signs typically appear 5-14 days post-infection (average 10-11 days).

Schizogenous Phase (Pathogenic)

• Sporozoites invade mononuclear phagocytes (monocytes/macrophages)
• Asexual replication produces schizonts measuring 25-250 micrometers in diameter
• Schizont-laden cells occlude vessels in liver, spleen, lungs, lymph nodes, and other organs
• Vascular occlusion causes hypoxic tissue injury and multi-organ dysfunction
• Proinflammatory cytokine release (TNF-alpha, IL-1beta, IL-6) causes systemic inflammation
• DIC develops secondary to endothelial damage and procoagulant state

Merogonous Phase (Erythrocytic)

• Merozoites released from ruptured schizonts invade erythrocytes
• Piroplasms (1-2 micrometers) visible as signet ring forms in RBCs
• Parasitemia typically 1-4% (up to 50% in severe cases)
• This phase is relatively innocuous and does not cause clinical disease
• Survivors remain chronically parasitemic (carrier state)

Clinical Signs

Clinical signs develop acutely approximately 11 days post-infection and progress rapidly. Death typically occurs within 2-3 days of presentation without treatment.

Clinical Findings by System

Diagnosis

Laboratory Findings

Definitive Diagnostic Methods

Blood Smear Examination

Piroplasms (merozoites): 1-2 micrometer signet ring-shaped organisms within erythrocytes. Classic appearance is a small (less than 1 micrometer), peripherally placed, deep purple nucleus with pale blue cytoplasm extending in a crescent. May also appear as safety-pin, tetrad, or comma shapes. IMPORTANT: Up to 50% of cats are negative for piroplasms at initial presentation. Repeat smears 12-24 hours later may reveal organisms.

Schizonts (Koch's bodies): Large (25-60 micrometer, up to 250 micrometer) schizont-laden macrophages occasionally seen at feathered edge of blood smear. Visualization of schizonts is PATHOGNOMONIC for acute disease (not seen in chronic carriers).

Cytology of Tissue Aspirates

Fine-needle aspirates of spleen, lymph nodes, or liver may demonstrate schizonts before piroplasms are visible on blood smear. Splenic aspirates have highest sensitivity (77.1%) compared to lymph node (52.8%) or blood smear (41.7%). Use caution in coagulopathic patients.

PCR Testing

PCR is highly sensitive and specific; can detect infection 24 hours before clinical signs appear. Available through Vector-borne Disease Diagnostic Laboratory (NC State). Quantitative PCR (qPCR) may have prognostic value - lower parasitemia associated with improved survival.

Diagnostic Imaging Findings

• Thoracic radiography: Interstitial to alveolar pulmonary pattern; pleural effusion possible
• Abdominal radiography: Hepatomegaly, splenomegaly
• Abdominal ultrasound: Hepatosplenomegaly; lymphadenopathy; non-specific findings

Differential Diagnoses

Treatment

Treatment success depends on early diagnosis and aggressive intervention. Current combination therapy achieves approximately 60% survival rate when initiated promptly.

Antiprotozoal Therapy

Mechanism of Action: Atovaquone is a ubiquinone analog that inhibits the cytochrome b subunit of the parasite mitochondrial electron transport chain. Azithromycin inhibits protein translation at the mitochondrial ribosome level.

Supportive Care

• IV fluid therapy: Crystalloids for dehydration and hypotension; correct electrolyte imbalances
• Heparin: Low-dose heparin (50-100 IU/kg SC q8h) to prevent/treat DIC
• Blood transfusion: For severe anemia; use screened, type-matched donors
• Nutritional support: Assisted feeding if anorexic; consider feeding tube placement
• Pain management: Opioids as needed; NSAIDs generally avoided due to GI/renal concerns
• Oxygen therapy: For dyspneic patients; flow-by or oxygen cage
• Minimize stress/handling: Critical for patient survival; limit procedures to essential interventions

Ineffective Treatments

The following have shown limited or no efficacy: imidocarb dipropionate (inconsistent results, approximately 25% survival), parvaquone/buparvaquone (ineffective), and diminazene aceturate (ineffective and toxic).

Prognosis

Prevention

No vaccine is currently available. Prevention focuses on tick control and exposure reduction.

Tick Control Products with Proven Efficacy

Additional Prevention Strategies

• Keep cats indoors: Most effective prevention; eliminates tick exposure
• Manual tick removal: Daily inspection; remove ticks within 36 hours of attachment
• Year-round tick prevention: Essential in endemic areas for all cats with outdoor access
• Environmental management: Reduce brush and tall grass; limit access to wooded areas

Memory Aid

"BOBCAT FEVER" - Remember the key features:

B - Bobcats are reservoir hosts

O - Outdoor cats at risk

B - Blood smear shows signet ring piroplasms

C - Cytauxzoon felis is the causative agent

A - Atovaquone + Azithromycin is treatment of choice

T - Tick-transmitted (Lone Star tick primary vector)

F - FEVER is most consistent early sign (high then hypothermia = death)

E - Endemic to Southeast/South-central US

V - Vascular occlusion by schizonts causes disease

E - Emergency! 60% survival with treatment, near 100% fatal without

R - Rapid progression (death in 2-3 days without treatment)