# [Toxoplasmosis](/knowledge/parasites/pet-parasites/toxoplasmosis-feline-transmission-public-health-clinical-management) vs [Cat Scratch Fever](/knowledge/veterinary-medicine/zoonotic-disease-risk/cat-scratch-fever): Differential Diagnosis and Clinical Comparison

## Key Takeaways

- **Etiologic Distinction:** Toxoplasmosis is caused by the apicomplexan parasite *Toxoplasma gondii*, with cats as definitive hosts shedding oocysts, while cat scratch fever is caused by the bacterium *Bartonella henselae*, transmitted primarily via fleas and causing persistent bacteremia in cats.
- **Transmission Pathways:** *T. gondii* is transmitted to cats via ingestion of tissue cysts in prey or sporulated oocysts from the environment, whereas *B. henselae* transmission is almost exclusively flea-mediated, with indoor-only cats having a very low risk.
- **Clinical Manifestations in Cats:** *T. gondii* infection can lead to significant ocular, neurological, or respiratory signs, particularly in immunocompromised individuals, while *B. henselae* infection is often subclinical or associated with mild fever and transient lymphadenopathy.
- **Diagnostic Modalities:** Differentiating these diseases involves serology (IgM/IgG for *T. gondii*, IgG/IgA for *B. henselae*), PCR on blood or affected tissues, and in some cases, histopathology for *T. gondii*.
- **Therapeutic Approaches:** Toxoplasmosis is treated with clindamycin or trimethoprim-sulfonamide, while *B. henselae* infections are often managed with doxycycline or azithromycin, though treatment in asymptomatic cats is debated.
- **Zoonotic Significance:** Both pathogens are zoonotic; *T. gondii* poses risks to immunocompromised individuals and pregnant women, and *B. henselae* causes cat scratch disease in humans, emphasizing the importance of flea control and hygiene.

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## Introduction

[Toxoplasmosis](/knowledge/parasites/pet-parasites/toxoplasmosis-feline-transmission-public-health-clinical-management), caused by the obligate intracellular apicomplexan parasite *[Toxoplasma gondii](/knowledge/parasites/protozoa/toxoplasma-gondii-lifecycle-neurological-infection)*, and cat scratch fever, caused by the gram-negative facultative intracellular bacterium *[Bartonella henselae](/knowledge/bacteria/pet-bacteria/bartonella-henselae-cat-scratch-disease-clinical-zoonosis)*, are two distinct infectious diseases that share the domestic cat (*Felis catus*) as a primary reservoir host [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. Despite their overlapping feline host association and potential for zoonotic transmission, their etiologic agents, pathophysiologic mechanisms, clinical presentations, and diagnostic approaches differ fundamentally [<a href="#ref-3">3</a>]. This review provides an exhaustive comparison of these two infections from a veterinary clinical and diagnostic perspective, emphasizing differential diagnosis in feline medicine.

## Etiology and Life Cycle

*[Toxoplasma gondii](/knowledge/parasites/protozoa/toxoplasma-gondii-lifecycle-neurological-infection)* is a coccidian parasite belonging to the phylum Apicomplexa [<a href="#ref-1">1</a>]. Felids are the definitive hosts; only felids shed environmentally resistant oocysts in feces [<a href="#ref-1">1</a>]. The life cycle includes an enteroepithelial sexual phase in the feline small intestine, leading to oocyst shedding, and an extraintestinal asexual phase in intermediate hosts (all warm-blooded animals) where tachyzoites and later bradyzoites in tissue cysts develop [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. The parasite invades host cells actively using the apical complex and forms a parasitophorous vacuole that evades lysosomal fusion [<a href="#ref-4">4</a>].

*[Bartonella henselae](/knowledge/bacteria/pet-bacteria/bartonella-henselae-cat-scratch-disease-clinical-zoonosis)* is a hemotropic, fastidious, gram-negative bacterium belonging to the alpha-2 subgroup of Proteobacteria [<a href="#ref-2">2</a>, <a href="#ref-5">5</a>]. It is transmitted among cats primarily by the cat flea *Ctenocephalides felis* [<a href="#ref-2">2</a>]. After inoculation, the bacterium adheres to erythrocytes and endothelial cells, inducing intracellular invasion mediated by type IV secretion systems [<a href="#ref-5">5</a>]. Bacteremia can persist for months to years in immunocompetent cats, and the organism can be transmitted to other hosts via flea feces or bites [<a href="#ref-2">2</a>]. Unlike *T. gondii*, *B. henselae* does not have a sexual stage or environmental oocyst stage; its persistence relies on vector-borne transmission and reservoir host bacteremia [<a href="#ref-5">5</a>].

## Transmission and Epidemiology

[Toxoplasma gondii](/knowledge/parasites/protozoa/toxoplasma-gondii-lifecycle-neurological-infection) transmission in cats occurs primarily through ingestion of tissue cysts in raw or undercooked meat (especially rodents or birds) and, less commonly, through ingestion of sporulated oocysts from contaminated environments [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. Vertical transmission (transplacental) is rare in cats but can occur [<a href="#ref-1">1</a>]. Seroprevalence in cats varies widely (10-80%) depending on lifestyle; outdoor hunting cats have higher risk [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>].

[Bartonella henselae](/knowledge/bacteria/pet-bacteria/bartonella-henselae-cat-scratch-disease-clinical-zoonosis) transmission is almost exclusively flea-mediated [<a href="#ref-2">2</a>, <a href="#ref-5">5</a>]. Direct cat-to-cat transmission without fleas is inefficient [<a href="#ref-2">2</a>]. Prevalence of *B. henselae* bacteremia in cats ranges from 4% to 70%, with higher rates in young, stray, and flea-infested populations [<a href="#ref-2">2</a>, <a href="#ref-5">5</a>]. Unlike [toxoplasmosis](/knowledge/parasites/pet-parasites/toxoplasmosis-feline-transmission-public-health-clinical-management), indoor-only cats with no flea exposure are at very low risk [<a href="#ref-2">2</a>].

## Pathogenesis and Clinical Signs in Cats

### [Toxoplasma gondii](/knowledge/parasites/protozoa/toxoplasma-gondii-lifecycle-neurological-infection)

Following ingestion, sporozoites or bradyzoites penetrate the intestinal epithelium, differentiate into tachyzoites, and disseminate hematogenously to all tissues, particularly skeletal muscle, myocardium, central nervous system, and the eye [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. The feline immune response (both humoral and cell-mediated) usually limits tachyzoite replication and induces tissue cyst formation [<a href="#ref-4">4</a>]. In immunocompetent adult cats, infection is most often subclinical [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. Clinical disease occurs more commonly in kittens, immunosuppressed cats (e.g., feline leukemia virus or [feline immunodeficiency virus](/knowledge/viruses/pet-viruses/feline-immunodeficiency-virus) co-infected), or cats receiving immunosuppressive therapy (e.g., glucocorticoids) [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>].

Clinical [toxoplasmosis in cats](/knowledge/bacteria/pet-parasites/toxoplasmosis-cats-zoonotic-risk-pregnancy) can manifest as:
- Fever, lethargy, anorexia [<a href="#ref-1">1</a>]
- Ocular disease (anterior uveitis, chorioretinitis, optic neuritis) [<a href="#ref-4">4</a>]
- Neurologic signs: seizures, ataxia, circling, hyperesthesia, behavioral changes [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]
- Respiratory signs (pneumonia, dyspnea) due to pulmonary involvement [<a href="#ref-1">1</a>]
- Hepatic signs (icterus) and pancreatic involvement [<a href="#ref-4">4</a>]

### [Bartonella henselae](/knowledge/bacteria/pet-bacteria/bartonella-henselae-cat-scratch-disease-clinical-zoonosis)

The pathogenesis of *B. henselae* in cats is less well defined. The bacterium invades and multiplies within erythrocytes and endothelial cells, causing persistent intravascular infection [<a href="#ref-2">2</a>, <a href="#ref-5">5</a>]. Many infected cats show no clinical signs, and the role of *B. henselae* as a primary feline pathogen has been debated [<a href="#ref-2">2</a>]. However, experimental and clinical evidence associates this infection with:

- Fever of unknown origin [<a href="#ref-2">2</a>]
- Mild transient lymphadenopathy [<a href="#ref-2">2</a>]
- Chronic gingivitis/stomatitis in some studies [<a href="#ref-5">5</a>]
- Endocarditis (rarely) [<a href="#ref-2">2</a>]
- Possible association with uveitis (similar to [toxoplasmosis](/knowledge/parasites/pet-parasites/toxoplasmosis-feline-transmission-public-health-clinical-management) but less common) [<a href="#ref-5">5</a>]

The most significant impact of *B. henselae* is its zoonotic potential, causing [cat scratch disease](/knowledge/veterinary-medicine/bacterial-diseases/cat-scratch-disease) in humans [<a href="#ref-2">2</a>].

## Differential Diagnosis: Key Clinical and Laboratory Distinctions

Differentiating between [toxoplasmosis](/knowledge/parasites/pet-parasites/toxoplasmosis-feline-transmission-public-health-clinical-management) and bartonellosis in cats relies on history, physical examination, and targeted diagnostic testing. The following table summarizes comparative features.

| Feature | [Toxoplasma gondii](/knowledge/parasites/protozoa/toxoplasma-gondii-lifecycle-neurological-infection) | [Bartonella henselae](/knowledge/bacteria/pet-bacteria/bartonella-henselae-cat-scratch-disease-clinical-zoonosis) |
|-----|----------|-----------|
| Primary host(s) | Felids (definitive) | Cats (reservoir) |
| Transmission route | Ingestion of tissue cysts/oocysts | Flea vector (Ctenocephalides felis) |
| Typical clinical signs | Ocular (uveitis), neurologic, respiratory, fever | Fever, lymphadenopathy, stomatitis (often subclinical) |
| Zoonotic disease | Toxoplasmosis (congenital, immunocompromised) | Cat scratch disease (lymphadenopathy, fever) |
| Diagnostic gold standard | Serology (IgM/IgG), PCR, histopathology | Blood culture (fastidious), PCR, serology (IFA/EIA) |
| Molecular detection | PCR on tissue/blood/cerebrospinal fluid | PCR on blood, lymph node aspirate |
| Serology interpretation | Paired titers or single high IgM indicates active infection | High IgG or IgA may reflect exposure; PCR positive confirm bacteremia |
| Treatment of choice | Clindamycin, trimethoprim-sulfonamide | Doxycycline, azithromycin (controversial in healthy cats) |

## Diagnostic Approach

A stepwise diagnostic algorithm assists in differentiating these infections and other febrile/ocular/neurologic feline conditions.

```mermaid
flowchart TD
 A["Feline patient: fever, uveitis, neurologic signs"] --> B{"History: outdoor access?"}
 B -->|"Yes"| C{"Vector history?"}
 B -->|"No"| D["Low risk for fleas; consider toxoplasmosis"]
 C --> E["High flea exposure: test B. henselae"]
 C --> F["Raw meat/carcass exposure: test T. gondii"]
 E --> G["Blood PCR for B. henselae + serology"]
 F --> H["Serology: T. gondii IgM/IgG + PCR if available"]
 G --> I{"Results"}
 H --> I
 I -->|"B. henselae PCR+ or serology+"| J["Diagnose bartonellosis"]
 I -->|"T. gondii IgM+ or PCR+"| K["Diagnose toxoplasmosis"]
 I -->|"Both negative"| L["Consider other causes: FeLV, FIV, FIP, neoplasia"]
 J --> M["Treat with doxycycline or azithromycin"]
 K --> N["Treat with clindamycin"]
```

### Serology and Molecular Methods

For toxoplasmosis, the reference serologic method is the modified agglutination test (MAT) or indirect immunofluorescent assay (IFA) detecting IgG and IgM [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. IgM positivity or a four-fold rise in IgG over 2-4 weeks indicates recent or active infection [<a href="#ref-1">1</a>]. PCR can detect *T. gondii* DNA in aqueous humor, cerebrospinal fluid, bronchoalveolar lavage fluid, or tissue biopsies [<a href="#ref-4">4</a>]. Histopathology with immunostaining reveals tachyzoites or cysts [<a href="#ref-1">1</a>].

For bartonellosis, blood culture in lysis-centrifugation tubes or enriched chocolate agar is highly specific but requires prolonged incubation (up to 21 days) [<a href="#ref-2">2</a>, <a href="#ref-5">5</a>]. Serology by IFA or ELISA detecting IgG and IgA is widely used; high titers indicate recent infection but cannot confirm active bacteremia [<a href="#ref-2">2</a>]. PCR on whole blood or tissue aspirates is sensitive and rapid, especially in febrile or bacteremic cats [<a href="#ref-2">2</a>, <a href="#ref-5">5</a>].

### Cytology and Histopathology

In toxoplasmosis, cytologic examination of effusions, bronchoalveolar lavage, or fine needle aspirates of liver or lymph nodes may reveal crescent-shaped tachyzoites [<a href="#ref-1">1</a>]. Histologic sections can show tissue cysts in brain, muscle, or eye [<a href="#ref-4">4</a>].

In bartonellosis, cytology is rarely diagnostic; organisms are poorly visible in routine stains. Silver stains (e.g., Warthin-Starry) can demonstrate coccobacilli in tissue, but are not commonly performed [<a href="#ref-2">2</a>].

## Treatment

[Toxoplasmosis in cats](/knowledge/parasites/pet-parasites/cat-toxoplasmosis-neonatal) is treated with:
- Clindamycin (10-12 mg/kg PO/IM q12h for 2-4 weeks) is the drug of choice [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>].
- Alternative: trimethoprim-sulfonamide (15 mg/kg PO q12h) or ponazuril (experimental) [<a href="#ref-4">4</a>].

Bartonellosis treatment remains controversial because most cats clear the bacteremia spontaneously. Therapy is generally reserved for sick cats or when [zoonotic risk](/knowledge/parasites/pet-parasites/zoonotic-risk-humans-get-parasites-from-pets) reduction is indicated:
- Doxycycline (10 mg/kg PO q24h for 4-6 weeks) is most frequently used [<a href="#ref-2">2</a>].
- Azithromycin (10 mg/kg PO q24h for 3-7 days, then q72h) also effective [<a href="#ref-2">2</a>].

Antimicrobial susceptibility testing is not routinely performed; resistance to beta-lactams is common [<a href="#ref-5">5</a>].

## Prevention and Zoonotic Considerations

Prevention of toxoplasmosis centers on preventing hunting (keep cats indoors), avoiding raw meat feeding, and daily scooping of litter boxes to prevent oocyst sporulation [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. Because oocysts require 1-5 days to sporulate, immediate removal eliminates infectivity [<a href="#ref-1">1</a>].

Prevention of bartonellosis requires rigorous flea control using veterinary-approved topical adulticides and environmental management [<a href="#ref-2">2</a>]. Because bacteremic cats can transmit *B. henselae* to humans via scratches or bites, declawing is not recommended; instead, nail trimming and avoiding rough play are suggested [<a href="#ref-2">2</a>].

Both infections are reportable zoonoses in some jurisdictions, although mandatory reporting often applies only to human cases [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. Veterinary practitioners play a central role in educating owners about risk mitigation.

## Conclusions

Toxoplasmosis and cat scratch fever differ fundamentally in their causative agents, transmission routes, clinical manifestations, and diagnostic methods. [Toxoplasmosis in cats](/knowledge/parasites/pet-parasites/cat-toxoplasmosis-pregnancy-infant-risk) is a parasitic condition with a defined set of clinical syndromes (ocular, neurologic, respiratory) and a well-established treatment protocol. Cat scratch fever, caused by *[Bartonella henselae](/knowledge/bacteria/pet-bacteria/bartonella-henselae)*, is a vector-borne bacterial infection that is often subclinical in cats but represents a significant zoonotic concern. Differential diagnosis relies on a thorough history of lifestyle and flea exposure, combined with appropriate serologic and molecular testing. A systematic diagnostic algorithm, as presented here, assists clinicians in distinguishing these infections from each other and from other febrile or inflammatory feline conditions.

## References

<a id="ref-1"></a>[<a href="#ref-1">1</a>] Dubey JP. Toxoplasmosis of Animals and Humans. 3rd ed. CRC Press; 2020. (Standard textbook; general reference for *Toxoplasma gondii* life cycle, clinical signs, diagnosis, treatment in cats)

<a id="ref-2"></a>[<a href="#ref-2">2</a>] Breitschwerdt EB, Kordick DL. Bartonella species. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th ed. Saunders; 2012:562-575. (General reference for *Bartonella henselae* pathogenesis, epidemiology, diagnosis, treatment in cats)

<a id="ref-3"></a>[<a href="#ref-3">3</a>] Sykes JE. Felidae as reservoirs of bacterial and parasitic zoonoses. In: Sykes JE, ed. Canine and Feline Infectious Diseases. Elsevier; 2014:724-736. (General comparative reference on feline zoonoses)

<a id="ref-4"></a>[<a href="#ref-4">4</a>] Lappin MR. Toxoplasmosis. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th ed. Saunders; 2012:788-804. (General reference for [feline toxoplasmosis](/knowledge/parasites/pet-parasites/cat-toxoplasmosis-symptoms-in-humans) clinical signs, diagnostics, management)

<a id="ref-5"></a>[<a href="#ref-5">5</a>] Guptill L. Bartonellosis. In: Miller E, ed. Feline Practice: Integrating Medicine and Well-Being. Wiley-Blackwell; 2012:245-258. (General reference for *Bartonella* in cats, pathogenesis, diagnostic methods)

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