Atypical Manifestations of Cat-Scratch Disease, United States, 2005–2014

Atypical disease is most common among children and leads to increased risk for hospitalization.

C at-scratch disease, a zoonotic bacterial infection, occurs worldwide and is caused by Bartonella henselae, a fastidious, intracellular gram-negative bacillus (1). Cats are the major reservoir of B. henselae and are infected by Ctenocephalides felis cat fleas. Although most cats infected with B. henselae are asymptomatic, signs such as fever and myocarditis might develop in some cats (2,3). Humans usually become infected through the scratches or bites of infected cats. B. henselae has also been shown to infect dogs (4), in some cases resulting in canine endocarditis (5)(6)(7). Although some human cases of cat-scratch disease have been linked to canine-human transmission (8)(9)(10)(11)(12), further research is needed to clarify the public health significance of B. henselae infection in dogs.
The true burden of cat-scratch disease in the United States is unknown because it is not a reportable condition; however, efforts have been made to estimate its incidence in the United States. In 1993, an analysis of hospital discharge data estimated a nationwide incidence of hospitalized cases of 0.77-0.86 cases/100,000 population annually (13). A subsequent study that examined a database of national health insurance claims during 2005-2013 found that incidence of cat-scratch disease in the United States was highest in southern states (6.4 cases/100,000 population) and in children 5-9 years of age (9.4 cases/100,000 population) (14).
Cat-scratch disease typically manifests as fever and an erythematous papule at the site of the cat scratch or bite, followed by lymphadenopathy in the regional lymph nodes that drain the area of inoculation (15). The papule usually appears 3-10 days after inoculation and can persist for several weeks, with regional lymphadenopathy developing 1-3 weeks postinoculation (1). From 80% to 95% of cases of catscratch disease are consistent with this typical presentation, and the remainder of cases manifest as atypical and more severe symptoms (16,17).
Atypical manifestations of cat-scratch disease can involve the eyes, nervous system, heart, liver, spleen, skin, or musculoskeletal system and might result in major illness (1,15). When cat-scratch disease involves the eye, the anterior compartment might be affected by Parinaud oculoglandular syndrome, and the posterior compartment might be affected by retinitis, retinochoroiditis, optic neuritis, uveitis, and vitritis (18)(19)(20). Nervous system involvement most commonly manifests as encephalopathy, but seizures, nerve palsies, neuritis, myelitis, and cerebellar ataxia have also been reported (21,22).
Endocarditis is more often seen in adults with cat-scratch disease than in children, although preexisting valvular disease puts children at increased risk for this complication (1). Bartonella infection can also cause abdominal pain and microabscesses in the liver and spleen (23), and in immunocompromised hosts can result in bacillary peliosis hepatis (24). In addition to the classic erythematous papule at the site of inoculation, erythema nodosum and bacillary angiomatosis are reported dermatologic manifestations of atypical infection (25,26). Osteolytic lesions, osteomyelitis, and arthritis have also been associated with cat-scratch disease (16,24,26). A study in 1998 found cat-scratch disease to be the third leading cause of prolonged fever of unknown origin in children, and a history of cat exposure was frequently absent (27).
Atypical manifestations of B. henselae infection can be severe, difficult to diagnose, and lead to lasting impairment. It is unclear why certain patients develop atypical cat-scratch disease, and little is known about its epidemiology. Improved understanding of atypical cat-scratch disease could lead to better recognition of cases by clinicians and inform efforts to understand the pathophysiology of this disease. The purpose of this study was to better characterize the rare and serious complications of this nonreportable zoonotic infection by using nationwide insurance claims data.

Methods
To identify potential cases of atypical cat-scratch disease, we conducted a retrospective analysis of Atypical manifestations that can be severe and difficult to diagnosis develop in 5%-20% of patients with catscratch disease. To clarify the epidemiology of atypical cat-scratch disease in the United States, we analyzed data from the 2005-2014 MarketScan national health insurance claims databases by using the International Classification of Diseases, 9th Revision, Clinical Modification, codes for cat-scratch disease and selected atypical manifestations: retinitis/neuroretinitis, conjunctivitis, neuritis, encephalitis, hepatosplenic disease, osteomyelitis, erythema nodosum, and endocarditis. Atypical cat-scratch disease accounted for 1.5% of all cases, resulting in an average annual incidence of 0.7 cases/100,000 population. Atypical cat-scratch disease was associated with increased risk for hospitalization (risk ratios 8.77, 95% CI 6.56-11.72) and occurred most often in female patients 10-14 years of age. Ocular (48.7%), hepatosplenic (24.6%), and neurologic (13.8%) manifestations were most common among patients. A more comprehensive understanding of atypical cat-scratch disease can improve patient diagnosis and potentially elucidate pathophysiology of the disease.
persons enrolled in the Truven Health MarketScan Commercial Claims and Encounters Database (Truven Health Analytics, https://www.ibm.com) during 2005-2014. The MarketScan Commercial Claims and Encounters Database includes persons <65 years of age covered by select employer-sponsored health insurance plans in all 50 states and contains administrative claims data on outpatient visits, inpatient admissions, and emergency department visits. Demographically, the MarketScan population generally mirrors the US population, with a slight overrepresentation of persons 50-59 years of age and a slight underrepresentation of persons 20-29 years of age (28).
Billing codes from outpatient, inpatient, and emergency department visits are assigned by either a clinician or billing specialist according to the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM), and procedures are captured as either ICD-9-CM codes, Current Procedure Terminology codes, or Healthcare Common Procedure Coding System codes. Because the International Classification of Diseases, 10th Revision, Clinical Modification, was not officially adopted in the United States until 2015, those codes were not included.
We identified cat-scratch disease cases by extracting all enrollee visit records during the study period with an ICD-9-CM code for cat-scratch disease (078.3). The first instance of a 078.3 diagnosis code in a patient record was considered the index event. Atypical manifestations of interest were selected for analysis if they had recorded precedent in the literature as a complication of cat-scratch disease and distinct, clearly discernable ICD-9-CM codes associated with the specific manifestation. Based on these criteria, the known complications of cat-scratch diseases included for analysis were endocarditis, osteomyelitis, erythema nodosum, conjunctivitis, retinitis/neuroretinitis, encephalitis, neuritis, and hepatosplenic disease.We included ICD-9-CM codes associated with optic neuritis in the retinitis/neuroretinitis category. Encounters with an ICD-9-CM code for cat-scratch disease and an accompanying diagnostic code to indicate the anatomic location of a wound or inoculation site for B. henselae were also flagged for analysis and were categorized as either head or neck region, arm or shoulder region, leg or hip region, or torso region. We compiled a detailed list of all ICD-9-CM codes used to identify atypical manifestations of cat-scratch disease (Appendix Table, https://wwwnc.cdc.gov/ EID/article/26/7/20-0034-App1.pdf).
We extracted insurance billing records of enrollees with ICD-9-CM codes for cat-scratch disease and selected manifestations at either the same encounter or within a 30-day window of one another. These records were evaluated along with previous and subsequent records by 2 independent reviewers (R.J.M. and C.A.N.) to ensure that the clinical picture was consistent with the coded atypical manifestation based on diagnosis codes, procedure codes, and provider types. If plausible alternative causes of the selected manifestation or likely coding errors were identified, we did not include the enrollee record as an atypical case. In cases of discordance, a third reviewer (Paul Mead) determined final categorization based on record review.
We included persons with an ICD-9-CM code for cat-scratch disease but without accompanying atypical manifestation as typical cases for comparison. Residence in a rural area was assigned if an enrollee did not reside in a metropolitan statistical area, as designated by the US Office of Management and Budget. Because previous research has identified increases in cat-scratch disease in late summer, fall, and January (13,14,29), we categorized month of onset as either late summer and fall, January, or all other months for analysis.
We performed descriptive and comparative statistical analyses by using JMP version 13.2.1 (https://www.jmp.com) and SAS version 9.3 (https://www.sas.com). We used Pearson χ 2 tests or Fisher exact tests for comparisons of categorical variables. To compare the conditional probability of having atypical cat-scratch disease across strata of potential variables of interest (e.g., sex, age category), we calculated risk ratios (RRs) and associated 95% CIs. Human subjects review at the Centers for Disease Control and Prevention determined that institutional review board approval was not required for this study.

Study Population and Incidence
During 2005-2014, the MarketScan database contained a median of 44,488,485 (range 16,159,068-53,131,420) enrollees each year. Of 14,824 cat-scratch disease cases identified from MarketScan during this period, 224 (1.5%) cases were classified as atypical ( Table 1). The average annual incidence of atypical cat-scratch disease diagnoses during the study period was 0.7 cases/100,000 population.
Atypical cat-scratch disease was most common among adults 15-49 years of age (47.3%), and patients with atypical cat-scratch disease were more likely to be hospitalized than those with typical manifestations (p<0.0001).
Nearly half of all patients with atypical catscratch disease were younger adults (15-49 years of age). When we compared older adults with younger adults, older adults (50-64 years of age) had a decreased risk for having atypical cat-scratch disease (RR 0.63, 95% CI 0.44-0.90) ( Table 1).

Seasonality
Atypical cat-scratch disease diagnoses increased from August through March, and diagnoses were concentrated during August-October (33.5% of diagnoses) and January-March (29.5% of diagnoses) (Figure 2), although neither diagnosis in late summer and fall or diagnosis in January were found to be risk factors for development of atypical catscratch disease (Table 1). Trends in atypical catscratch disease diagnoses were similar to trends in typical cat-scratch disease diagnoses. However, typical cat-scratch disease had less defined peak periods, and diagnoses decreased sharply after January.

Geographic Distribution and Residence in Rural Area
Most (57.6%) cases of atypical cat-scratch disease occurred in the southern region of the United States (57.6%), followed by the midwest (16.5%) and northeast (12.5%) regions ( Figure 3). The geographic distribution of atypical cases did not differ significantly from cases of typical cat-scratch disease.
Residence in a rural area was not a risk factor for development of atypical cat-scratch disease (RR 1.06, 95% CI 0.78-1.45). Also, the proportion of patients with atypical cat-scratch disease living in a rural area did not differ from the proportion of patients with typical cat-scratch disease living in a rural area (p = 0.70). Among persons with ocular (retinitis/neuroretinitis and conjunctivitis) manifestations, most diagnoses were made during August-October (31.2%) and January-March (35.8%). Among persons with neurologic (neuritis and encephalitis) manifestations, diagnoses were concentrated during October (22.6%). We observed no notable trends in seasonality of diagnoses for other manifestations of atypical cat-scratch disease (Figure 4). We also observed no differences in geographic distribution or rurality by manifestation of atypical cat-scratch disease.

Hospitalization of Atypical Case-Patients
Patients with atypical cat-scratch disease were more likely to be hospitalized than patients with typical cat-scratch disease (RR 8.77, 95% CI 6.56-11.72) (Table 1). Among patients with atypical cat-scratch disease, children <14 years of age accounted for 60.7% of hospitalizations and had an increased risk for hospitalization compared with adults 15-49 years of age (RR 2.34, 95% CI 1.44-3.79). A total of 57.1% of the hospitalizations occurred during August-November, and we found an overall increased risk for hospitalization during this period when compared with all other months, except for January (RR 1.88, 95% CI 1.15-3.05) ( Table 3).

Location of Wound
Information on wound location was available for only 10 (4.5%) patients with atypical cat-scratch disease. Among these persons, 2 with conjunctivitis, 1 with encephalitis, and 2 with hepatosplenic disease had a wound on the head or neck; 1 with osteomyelitis and 3 with hepatosplenic manifestations had a wound on the arm or shoulder; 1 with endocarditis had a wound on the leg or hip; and 1 with endocarditis had a wound on an unspecified limb.

Discussion
Using US nationwide insurance claims data, we identified and characterized 224 atypical cases of cat-scratch disease during 2005-2014 and estimated an average annual incidence of 0.7 cases/100,000 population. Nearly half of all atypical cat-scratch disease cases had ocular manifestations, most of which were retinitis/neuroretinitis. Atypical catscratch disease was most prevalent among female patients 10-14 years of age, who most commonly had ocular manifestations.
Trends in hospitalizations of patients with catscratch disease highlight the severity of atypical catscratch disease compared with typical cat-scratch disease. Atypical cat-scratch disease appears to be particularly severe among children <14 years of age, who had an increased risk for hospitalization. Adults 50-64 years of age had the lowest risk for development of atypical cat-scratch disease and specifically ocular manifestations. Reasons that older adults might have complications associated with cat-scratch disease less often than other age groups are unclear and require further study.
Severity of cat-scratch disease in children has been previously documented. In a study conducted by Reynolds et al., ≈25% of hospitalizations of children for cat-scratch disease were caused by complications associated with atypical cat-scratch disease; neurologic and hepatosplenic complications were most common (30). Although children in our study were also particularly at risk for hepatosplenic disease, neurologic and hepatosplenic complications were associated with increased risk for hospitalization in our overall study population, indicating that these manifestations are particularly severe for all age groups. Encephalitis was the most common neurologic manifestation in our population, which was also consistent with findings of Reynolds et al., in which most hospitalizations of children for neurologic complications of cat-scratch disease were caused by encephalitis or encephalopathy (30). Thus, physicians should consider catscratch disease in patients who have encephalitis or new onset hepatosplenic abnormalities, especially children.  Previous studies have documented the highest rates of cat-scratch disease in late summer and fall and a separate peak often seen in January (13,14,29). One such study found that rates of B. henselae seropositivity among samples submitted to Mayo Clinic Laboratories over a 10-year period were highest during September-January, with the highest annual rates in January (29). Typical cat-scratch disease diagnoses in our study followed similar seasonal patterns to those previously reported. However, atypical cat-scratch disease appeared more concentrated during August-October and January-March. The reasons for this finding are unclear but might include delays in diagnosis of atypical cat-scratch disease. For example, patients who contract cat-scratch disease and had complications during January might not be given a diagnosis of atypical cat-scratch disease at that time because they do not show classic symptoms or their symptoms take time to develop and care-seeking is delayed.
Furthermore, a recent case series of ocular manifestations of cat-scratch disease reported that 9 of 10 patients had symptoms <3 months before showing development of ocular complications and that 3 patients had been originally given misdiagnoses of etiologies other than cat-scratch disease (31). Given that ocular manifestations of cat-scratch disease were most common in our study, increased diagnoses of atypical cat-scratch disease through March could be a sign of delayed diagnoses, particularly for manifestations that are less severe, such as those involving the eye.
Trends related to geographic distribution of cases did not differ between atypical and typical cat-scratch disease. Similar to findings from 3 previous studies that reported the highest incidences of cat-scratch disease in the southern United States (13,14,30), in our study, most typical (53.0%) and atypical (57.6%) cat-scratch disease cases occurred in patients residing in this region. In addition, a national survey of US healthcare providers found that those in the Pacific and southern regions of the United States were more likely to have been given a diagnosis of cat-scratch disease than in other regions (32). These findings are further supported by studies that have found higher average B. henselae seroprevalences and active bacteremia in pet cats from warmer, more humid climates, including the southern United States (33,34). Thus, healthcare providers in regions with climates that support flea abundance should be aware of the risk for cat-scratch disease and be able to recognize its atypical manifestations.
This study had several limitations. First, although MarketScan is a large database of insurance claims data from persons covered by employersponsored insurance, it is a convenience sample and may not accurately represent the characteristics of all persons in the United States. For example, trends we see in atypical cat-scratch disease by geographic region and rural residence might be biased by differences in coverage and access to care that are not accounted for here. Furthermore, MarketScan does not include data for adults >65 years of age, military personnel, uninsured persons, or Medicaid/Medicare enrollees. These specific populations might show varying degrees of cat-scratch disease severity or risk that are not captured in our results. In addition, because only persons <65 years of age are included in the database, the proportion of children who have cat-scratch disease might be artificially inflated. The number of patients who had atypical catscratch disease was small, especially when broken down by manifestation. Thus, it is difficult to draw conclusions regarding risk factors for specific manifestations of atypical cat-scratch disease and hospitalization within these groups.

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Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 26, No. 7, July 2020 Furthermore, misclassification could have occurred when ICD-9-CM codes were used to classify atypical cat-scratch disease for several reasons. ICD-9-CM codes are subject to error from the clinicians and billing specialists who enter them. In addition, we excluded records that fit our criteria for manifestations of atypical cat-scratch disease but lacked additional supporting information, which could have caused us to underestimate the true burden of atypical cat-scratch disease. Last, codes for some known atypical cat-scratch disease manifestations, such as pulmonary complications and thrombocytopenia, were excluded because of etiologic ambiguity in enrollee records.
In conclusion, our findings indicate that atypical cat-scratch disease in the United States follows trends similar to those for typical cat-scratch disease but is more prevalent and severe among children <14 years of age and is least likely to occur in older adults (50-64 years of age). In addition, differences in seasonality of diagnoses were seen, which might be an indication that diagnosis of atypical cat-scratch disease is often delayed. Ocular (retinitis/neuroretinitis and conjunctivitis) and hepatosplenic complications were the most common manifestations of atypical catscratch disease. Improved understanding of atypical cat-scratch disease might lead to better recognition of cases by clinicians, as well as inform efforts to clarify the pathophysiology of this disease. Description  36925  Better eye: moderate vision impairment; lesser eye: moderate vision impairment  3693  Unqualified visual loss, both eyes  36960 Profound impairment, one eye, impairment level not further specified 36961

ICD-9-CM code
One eye: total vision impairment; other eye: not specified 36962 One eye: total vision impairment; other eye: near-normal vision 36963 One eye: total vision impairment; other eye: normal vision 36964 One eye: near-total vision impairment; other eye: vision not specified 36965 One eye: near-total vision impairment; other eye: near-normal vision 36966 One eye: near-total vision impairment; other eye: normal vision 36967 One eye: profound vision impairment; other eye: vision not specified 36968 One eye: profound vision impairment; other eye: near-normal vision 36969 One eye: profound vision impairment; other eye: normal vision 36970 Moderate or severe impairment, one eye, impairment level not further specified 36971 One eye: severe vision impairment; other eye: vision not specified 36972 One eye: severe vision impairment; other eye: near-normal vision 36973 One eye: severe vision impairment; other eye: normal vision 36974 One eye: moderate vision impairment; other eye: vision not specified 36975 One eye: moderate vision impairment; other eye: near-normal vision 36976 One eye: moderate vision impairment; other eye: normal vision 3698 Unqualified visual loss, one eye 3699 Unspecified visual loss 37221 Angular blepharoconjunctivitis 37222 Contact blepharoconjunctivitis 37289 Other disorders of conjunctiva 3729 Unspecified disorder of conjunctiva 37700 Papilledema, unspecified 37702 Papilledema associated with decreased ocular pressure 37703 Papilledema associated with retinal disorder 37732 Retrobulbar neuritis (acute) 37742 Hemorrhage in optic nerve sheaths 37749 Other disorders of optic nerve 3688 Other specified visual disturbances 4387 Late effects of cerebrovascular disease, disturbances of vision 37239 Other conjunctivitis 3410 Neuromyelitis optica 3630 Focal chorioretinitis and focal retinochoroiditis (fill) 3631 Disseminated chorioretinitis and disseminated retinochoroiditis (fill) 3632 Other and unspecified forms of chorioretinitis and retinochoroiditis (fill) 3684 Visual field defects (fill) 3725 Conjunctival degenerations and deposits (fill) 3770 Papilledema (fill) 3771 Optic atrophy (fill) 3772 Other disorders of optic disc (fill) 3773 Optic neuritis (fill) 3790 Scleritis and episcleritis (fill) 3792 Disorder of vitreous body (fill) 3799 Unspecified disorder of eye and adnexa (fill) 36900 Profound