To the Editor:
Rickettsia typhi is the causal agent of murine typhus, a febrile illness affecting
humans worldwide (
1
). In Mexico, recent studies demonstrated a 14% prevalence of antibodies against typhus
group rickettsiae in healthy adult blood donors in Mexico City, and a recent nonfatal
case of endemic typhus was reported in Yucatan (
2
,
3
).
In May 2011, a 42-year-old woman and her 12-year-old son sought care at the clinical
service of the Autonomous University of Yucatan. They had malaise, headache, fever
(39°C), fibromyalgia, sore throat, and fatigue and an erythematous rash on the chest
that after 6 days spread to the abdomen and extremities.
Dengue fever was diagnosed, and the patients were treated empirically with acyclovir,
methanesulfonamide, N-(4-nitro-2-phenoxyphenyl) and clarithromycin. Dengue could not
be confirmed by laboratory testing.
Murine typhus was diagnosed on the basis of PCR amplification and immunofluorescent
assay for antibodies to R. typhi. Rickettsia species was determined by sequencing
of rickettsial genes. Three serum samples were collected from the woman (8, 12, and
16 days after illness onset) and 1 from the boy (8 days) in 3.8% sodium citrate as
anticoagulant, and DNA was extracted immediately by QIAamp DNA Blood Mini Kit (QIAGEN
Valencia, CA, USA) in accordance with the manufacturer’s instructions. Single-step
PCR amplification was performed by using genus-specific primers for the rickettsial
17-kDa protein and citrate synthase (gltA) genes as reported (
4
)
Sequences of the citrate synthase and 17-kDa PCR products were compared at the National
Center for Biotechnology Information BLAST software (
5
). Three PCR amplicons of both genes were fully sequenced and compared with sequences
in GenBank. The 17-kDa and citrate synthase fragment sequences (GenBank accession
nos. JX198507 and JX458814) showed 99% and 100% identity, respectively, with R. typhi
strain Wilmington strain (GenBank accession no. AE017197.1) (Table).
Table
Ig detection by immunofluorescent assay and molecular results by PCR and sequence
identity of the amplicons of
gltA and 17-kDa genes of Rickettsia spp. from 2 patients with murine typhus, Yucatan,
Mexico
Patient age, y/sex
Days after illness onset
Immunofluorescence assay
Rickettsia species*
R. typhi
R. rickettsii
PCR/sequence test
IgM
IgG
IgM
IgG
17 kDa
gltA
45/F
8
256
128
Neg
128
Pos
Pos
R. typhi
12
128
128
Neg
64
Pos
Pos
R. typhi
16
64
128
Neg
64
Neg
Neg
ND
12/M
8
128
128
Neg
64
Pos
Pos
R. typhi
*Identified species showed 99% identity with R. typhi Wilmington strain 17-kDa antigen
gene (GenBank accession no. AE017197.1) and 100% identity with R. typhi Wilmington
strain gltA gene (GenBank accession no. AE017197.1). ND, species not determined.
Immunofluorescent assay was performed by using R. rickettsii and R. typhi antigen
fixed on slides. We examined the serum samples for IgG and IgM, assessing reactivity
of γ chain–specific and μ heavy chain–specific secondary conjugates, respectively,
with rickettsial antigens. All 3 samples from the woman and the sample from the boy
contained antibodies to R. typhi (Table). Both patients were treated with 100 mg of
oral doxycycline 2×/day for 7 days (boy), and 10 days (woman); symptoms improved in
72 hours for the child. The woman’s symptoms resolved completely in 5 days.
Typhus has been endemic in Mexico since before the conquest period (
6
). Socioeconomic aspects play a major role in zoonotic diseases, such as rickettsioses,
especially in their distribution in urban and suburban areas because of factors such
as marginalized communities, animal breeding, education levels, poverty, and social
exclusion from health systems.
Overcrowding resulting from migration from rural areas to large urban centers contributes
to increased zoonoses in urban areas. Also contributing is the ecologic imbalance
of flora and fauna associated with deteriorating sanitary conditions in areas where
mammals involved in the cycle of R. typhi, such as rodents and opossum, may live in
the same habitat as humans and colonize backyards, waste deposit area, and areas around
the neighborhoods where they can find food. The concurrence and presence of mammals,
vectors, and humans may contribute to maintaining transmission of endemic typhus in
a reduced area, with the possibility to cause outbreaks (
7
,
8
). Housing conditions and culture, such as courtyards with vegetation and presence
of pets, in several suburban areas of Yucatán encourage close contact between humans
and possible reservoirs for several infectious diseases, such as mice, rats, opossums,
dogs and cats, and their ectoparasites (
9
).
Because housing and cultural conditions are similar in all countries of Latin America,
endemic typhus probably is transmitted in the same way: by coexistence with domestic
animals and close contact with wild reservoirs. Epidemiologic control should be closely
linked to education aimed at encouraging villagers to interrupt the cycle of transmission
and thus prevent the disease.
We have confirmed the presence of murine typhus in Yucatan. The finding of human cases
demonstrates the need to consider R. typhi infection in the differential diagnosis
of febrile illnesses considered endemic in Yucatan, such as dengue fever and leptospirosis
(
10
). Early and accurate diagnosis should enable physicians to treat this disease appropriately
and early in the clinical course to prevent increased illness and death.