2. Ticks and Tick-borne Diseases 12 (2021) 101788
2
outside of Texas. We conducted a descriptive epidemiologic and geo
spatial analysis of the reported data. Incidence was calculated for
infection across demographic groups using Texas data from the 2010 US
Census. A fisher’s exact test was used to compare categorical variables.
This study was approved by TxDSHS Institutional Review Board (IRB)
and considered exempt by the Baylor College of Medicine IRB.
3. Results
During 2008-2017, 101 cases of ehrlichiosis reported in Texas were
included in our analysis. The majority of cases were categorized as
probable (n=75, 74.3%) with only 26 confirmed (25.7%). Among the
confirmed cases, the most common confirmatory method was
E. chaffeensis specific PCR (n=24), with only 2 detected by seroconver
sion. An increasing trend in cases reported annually over the study
period was noted with a unique spike of cases identified in 2008
(Fig. 1A). A higher number of cases (n=27; 27%) occurred in this year as
compared to subsequent years with unique epidemiologic findings. As
such, we present the analysis of the 2008 cases and subsequent years
separately.
Cases Reported in 2008
The 27 cases reported in 2008 had a median age of 9 years old (range
2-86). Less than a third of cases were hospitalized (n=8; 30%). There
was no difference in the distribution between males and females, and the
majority of cases (78%) were Hispanic, white (Table 1). Cases were
reported from January to August with the peak in February (Fig. 1B).
Only 1 case was confirmed with the remaining 26 considered probable.
Almost all cases were likely autochthonous (n=23, 85%).
A geospatial analysis of the likely autochthonous cases reported in
2008 identified a high proportion occurring in rural areas of south
Texas, below 29 degrees latitude (n=22 out of 23, 96%; Fig. 2A). Webb
and surrounding counties along the Texas-Mexico border represented a
focus of disease (n=9). A second focus was noted on the Texas coast in
Nueces and bordering counties (n=10). The remaining cases were re
ported in Cameron County (n=2) and Matagorda County (n=1).
Cases Reported 2009-2017
The median age of the 74 cases reported from 2009 to 2017 was 56
(range 3-84). A large percentage of cases required hospitalization due to
their illness (n=49; 66%; Table 1). The majority of cases identified were
non-Hispanic, white with more men reported than women (n=44 and
n=30, respectively). Symptom onset typically occurred in warmer
months during April to September (Fig. 1C). The majority of cases
identified were likely autochthonous (n=45; 60.8%).
The geographic distribution of autochthonous cases reported during
2009 to 2017 indicate a shift from south Texas into the central and
eastern portions of the state. Foci of cases were reported around urban
centers of Dallas, Houston, Austin, and San Antonio (Fig. 2B).
Epidemiologic Comparisons between 2008 Cases and Subsequent Years
When compared to the 2009-2017 cases, the 2008 cases had a lower
median age and were less likely to be hospitalized (P<0.01).
Figure 1. Trend in ehrlichiosis cases reported over time in Texas, 2008-2017. (A) Number of cases reported per year in Texas during 2008 to 2017; (B) Epidemiologic
curve of cases reported in 2008 with cases reported in south Texas in black and cases reported outside of south Texas in red; and (C) Epidemiologic curve of cases
reported between 2009 and 2017
T.A. Erickson et al.
3. Ticks and Tick-borne Diseases 12 (2021) 101788
3
Autochthonous cases presenting in 2008 (n=22; 95.6%) were also more
likely to acquire their infections south of 29 degrees latitude when
compared to subsequent years (n=11; 24.4%; Fisher’s exact p<0.0001).
We also observed unique demographics in that Hispanic, white in
dividuals were more likely to be infected in the 2008 cases as compared
to subsequent years (P<0.01). Cases in 2008 were rarely confirmed
(n=1; 4%) as compared to 2009-2017 (n=25; 34%). Temporally, the
2008 case onset of illnesses peaked in non-summer months compared to
summer months in subsequent years.
4. Discussion
We observed a concerning increase in the incidence and changing
geographic distribution of ehrlichiosis cases reported in Texas. This
observation is mirrored in the traditionally high burden areas for ehr
lichiosis (Arkansas, Oklahoma, Missouri, Tennessee, and Kentucky)
(CDC, 2018; Mead et al., 2015). It is difficult to ascertain whether these
reported increases are the result of improved surveillance, true emer
gence, or a combination of the two. In south Texas the level of physician
awareness may be elevated due to local research and education on the
topic.
The increased number of cases and unique clustering reported in
2008 likely represents an event outside the normal transmission patterns
for Ehrlichia sp. in Texas. In addition to an unusual spike in cases that
year, we observed unique epidemiologic characteristics in the 2008
cohort. Compared to subsequent years, these case-patients had a lower
median age, were less likely to be hospitalized, and were more likely to
acquire their infections south of 29 degrees latitude. We also observed
unique demographics in that Hispanic, white individuals were more
likely to be infected. This latter finding may be expected considering 22
of 23 autochthonous cases occurred in counties in south Texas, where
the proportion of Hispanic populations range from 65% to 96% of the
population (Census, 2010). While it might seem unusual that the case
counts peaked in non-summer months, this is common in south Texas,
which is considered subtropical, with warm weather and arthropod
vector activity potentially occurring year-round. With reported rickett
sial cases in south Texas, a bimodal distribution has been observed, with
two peaks observed in the summer months and from January to March
(Murray et al., 2017).
Of note, none of the 27 cases reported in 2008 were confirmed by
PCR. This coupled with unique epidemiologic characteristics of the
cluster of cases introduces the strong possibility of cross-reaction on
serologic testing with other Ehrlichia species (i.e., E. canis and E. ewingii).
This may explain the shift in temporality of infection and lower rates of
Table 1
Characteristics of ehrlichiosis case-patients reported in Texas, 2008-2017
Characteristics N (%) Ehrlichiosis 2008 (n=27) Incidence (per 1,000,000) Ehrlichiosis 2009-2017 (n=74) Incidence (per 1,000,000) Comparison†
Confirmed cases 1 (4%) 0.04 25 (34%) 0.99 P<0.01
Sex
Female 15 (56%) 1.18 30 (41%) 2.37 P=0.258
Male 12 (44%) 0.96 44 (59%) 3.53
Race
White, Non-Hispanic 4 (15%) 0.35 51 (69%) 4.47 P<0.01
White, Hispanic 21 (78%) 2.22 14 (19%) 1.48
Unknown or Other 2 (7%) 0.47 9 (12%) 2.10
Age
<5 years 5 (19%) 2.59 2 (3%) 1.04 P<0.01
5-19 years 11 (41%) 1.93 12 (16%) 2.11
20-39 years 4 (15%) 0.56 8 (11%) 1.11
40-64 years 4 (15%) 0.52 32 (43%) 4.14
≥65 years 3 (11%) 1.15 20 (27%) 7.69
Hospitalized 8 (30%) 49 (66%) P<0.01
Acquired in south Texas‡
21 (91%) 10 (22%) P<0.01
†
Fisher’s exact test.
‡
Analysis includes only locally acquired cases. A total of 23 locally acquired case-patients were reported in 2008 and 45 were reported between 2009 and 2017.
Figure 2. Geospatial distribution of autochthonous ehrlichiosis cases per 1,000,000 population reported in (A) 2008 and (B) 2009-2017 in Texas.
T.A. Erickson et al.
4. Ticks and Tick-borne Diseases 12 (2021) 101788
4
hospitalization identified in south Texas in 2008. While infection with
E. canis is considered rare and has not been reported in the United States,
it has been reported from regions where human exposure to the brown
dog tick is common (Perez et al., 2006).
This study has some noteworthy limitations. Only one quarter of the
cohort met the definition of a confirmed case, though this is common in
studies of intracellular bacterial disease in the state of Texas (Erickson
et al., 2017; Murray et al., 2017; Ruiz et al., 2020). For cases not
confirmed by PCR, we cannot rule out the potential for cross-reactivity
with other related organisms, such as Anaplasma spp. and other Ehrlichia
spp. (Ismail and McBride, 2017). Finally, passive surveillance systems
are inherently limited in their ability to capture all clinical and
non-clinical cases, leading to potential underrepresentation of the true
burden of disease. Despite these limitations, this study represents an
important epidemiologic investigation of ehrlichiosis in Texas.
5. Conclusion
Our study suggests that ehrlichiosis is present in Texas with the po
tential for outbreaks to occur. Our findings highlight the importance of
further investigation into transmission risk and patterns of disease by
researchers and public health partners. Active surveillance could allow
detection of similar events in real-time so that public health in
terventions, such as community awareness and tick prevention cam
paigns, can be implemented to reduce the burden of this significant
clinical disease.
Funding source
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
Author Contributions
TAE- Conceptualization, Formal analysis, Writing; BM- Data acqui
sition, Writing, review & editing; KOM- Writing, review & editing; SMG-
Conceptualization, Formal analysis, Writing
Declaration of Competing Interest
The authors have not conflict of interests to declare.
References
CDC, 2018. National Notifiable Diseases Surveillance System. In: 2018 Annual Tables of
Infectious Disease Data. Retrieved from. https://www.cdc.gov/nndss/infectious-ta
bles.html (Accessed 07.01.2020).
Census, 2010. 2010 Census. Retrieved from https://www.census.gov/2010census/data/
(Accessed 07.01.2020).
Dumler, J.S., Madigan, J.E., Pusterla, N., Bakken, J.S., 2007. Ehrlichioses in humans:
epidemiology, clinical presentation, diagnosis, and treatment. Clin. Infect. Dis. 45
(Suppl 1), S45–S51. https://doi.org/10.1086/518146.
Erickson, T., da Silva, J., Nolan, M.S., Marquez, L., Munoz, F.M., Murray, K.O., 2017.
Newly Recognized Pediatric Cases of Typhus Group Rickettsiosis, Houston, Texas,
USA. Emerg. Infect. Dis. 23, 2068–2071. https://doi.org/10.3201/eid2312.170631.
Ismail, N., McBride, J.W., 2017. Tick-Borne Emerging Infections: Ehrlichiosis and
Anaplasmosis. Clin. Lab. Med. 37, 317–340. https://doi.org/10.1016/j.
cll.2017.01.006.
Long, S.W., Pound, J.M., Yu, X.J., 2004. Ehrlichia prevalence in Amblyomma americanum.
Central Texas. Emerg Infect Dis. 10, 1342–1343. https://doi.org/10.3201/
eid1007.03-0792.
Mead, P., Hinckley, A., Hook, S., Beard, C.B., 2015. TickNET-A Collaborative Public
Health Approach to Tickborne Disease Surveillance and Research. Emerg. Infect. Dis.
21, 1574–1577. https://doi.org/10.3201/eid2109.150301.
Mitchell, E.A., Williamson, P.C., Billingsley, P.M., Seals, J.P., Ferguson, E.E., Allen, M.S.,
2016. Frequency and Distribution of Rickettsiae, Borreliae, and Ehrlichiae Detected
in Human-Parasitizing Ticks, Texas, USA. Emerg. Infect. Dis. 22, 312–315. https://
doi.org/10.3201/eid2202/150469.
Modarelli, J.J., Borst, M.M., Piccione, J., Esteve-Gasent, M.D., 2019a. Molecular
identification of Ehrlichia ewingii in a polyarthritic Texas dog. Vet. Clin. Pathol. 48,
96–99. https://doi.org/10.1111/vcp.12690.
Modarelli, J.J., Tomecek, J.M., Piccione, J., Ferro, P.J., Esteve-Gasent, M.D., 2019b.
Molecular prevalence and ecoregion distribution of select tick-borne pathogens in
Texas dogs. Transbound Emerg Dis 66, 1291–1300. https://doi.org/10.1111/
tbed.13145.
Murray, K.O., Evert, N., Mayes, B., Fonken, E., Erickson, T., Garcia, M.N., Sidwa, T.,
2017. Typhus Group Rickettsiosis, Texas, USA, 2003-2013. Emerg. Infect. Dis. 23,
645–648. https://doi.org/10.3201/eid2304.160958.
Perez, M., Bodor, M., Zhang, C., Xiong, Q., Rikihisa, Y., 2006. Human infection with
Ehrlichia canis accompanied by clinical signs in Venezuela. Ann. N. Y. Acad. Sci.
1078, 110–117. https://doi.org/10.1196/annals.1374.016.
Ratnasamy, N., Everett, E.D., Roland, W.E., McDonald, G., Caldwell, C.W., 1996. Central
nervous system manifestations of human ehrlichiosis. Clin. Infect. Dis. 23, 314–319.
https://doi.org/10.1093/clinids/23.2.314.
Ruiz, K., Valcin, R., Keiser, P., Blanton, L.S., 2020. Rise in Murine Typhus in Galveston
County, Texas, USA, 2018. Emerg. Infect. Dis. 26, 1044–1046. https://doi.org/
10.3201/eid2605.191505.
Yabsley, M.J., Varela, A.S., Tate, C.M., Dugan, V.G., Stallknecht, D.E., Little, S.E.,
Davidson, W.R., 2002. Ehrlichia ewingii infection in white-tailed deer (Odocoileus
virginianus). Emerg. Infect. Dis. 8, 668–671. https://doi.org/10.3201/
eid0807.020018.
T.A. Erickson et al.
