INVESTIGADORES
BELDOMENICO Pablo Martin
congresos y reuniones científicas
Título:
Infection dynamics of Rickettsia parkeri in cattle in the Paraná River Delta, Argentina
Autor/es:
MONJE LD; COLOMBO, VC; LABRUNA, MB; ANTONIAZZI LR; GAMIETEA I; NAVA S; BELDOMENICO PM
Lugar:
La Plata
Reunión:
Congreso; III Congreso Panamericano de Zoonosis; 2014
Institución organizadora:
Asociación Argentina de Zoonosis
Resumen:
Introduction: The alpha-proteobacterium Rickettsia parkeri was first reported as
pathogenic in the United States in 2004. In Argentina, cases of human rickettsiosis caused
by R. parkeri were documented in the provinces of Buenos Aires, Entre Rios and Chaco.
The cases of R. parkeri infection reported in Argentina were predominantly concentrated
around the Paraná River Delta, where the tick implicated in its transmission is Amblyomma
triste. Previous studies conducted in the same region reported a prevalence of R. parkeri in
A. triste ranging from 8% to 20%. Adults of A. triste can use cattle as hosts. Moreover, it
has been reported that cattle are susceptible of infection with several members of SFG
rickettsiae. Additionally, it has been demonstrated that R. parkeri-infected Amblyomma
maculatum ticks are capable of transmitting this SFG rickettsiae to cattle. The Paraná River
Delta constitutes a vast human-domestic-wildlife interface where the risk of pathogen
transmission across species is substantial. In this region, farming beef cattle is on the rise,
gradually displaced from the Pampas by agriculture. As cows suffer frequent A. triste
infestations, they are at risk of becoming infected with R. parkeri and could act as amplifier
hosts of the disease. To contribute to our knowledge on the ecology of this pathogen in the
region, we investigated the dynamics of R. parkeri and its vector in a herd of beef cattle
that was followed for eighteen months.
Materials and Methods: The study was conducted in fields of an Experimental Station
belonging to INTA Delta, Campana (34°9.5′S, 58°51.8′W) in Buenos Aires Province,
Argentina. A herd of beef cattle consisting of 21 Aberdeen Angus cows grazing in a mixed
system consisting of natural pasture and Salicaceae plantations in INTA Delta were
repeatedly bled from the tail vein every five weeks from December 2010 to May 2012. In
parallel to blood collection, the left ear of each cow was thoroughly examined in search of
ticks. Simultaneously, questing adult ticks were collected from the vegetation by dragsampling.
Presence of R. parkeri antibodies was determined by an indirect
immunofluorescence assay using crude antigens derived from R. parkeri strain At24. The
presence of rickettsial DNA was assessed in all cows showing an event of seroconversion.
For this purpose, both, the sample that seroconverted and the sample previously obtained
from the same cow (seronegative) were analyzed. Blood DNA samples were analyzed by
real-time PCR targeting the rickettsial gene gltA. The integrity of the DNA extracted from
the blood was assessed using a real-time PCR that amplifies bovine 18sRNA gene.
Results: Ticks (A. triste adults) in the environment were found only from August to
February, with a peak in August. The count of female ticks (FF) on the left ear largely
reflected the seasonal pattern observed for questing ticks. Both counts were highly
correlated (Spearman?s Rho correlation coefficient= 0.813, p= 0.0002). The temporal
pattern of seroprevalence for R. parkeri matched that of tick exposure, showing a peak in
August. Seroprevalence was positively correlated with total count of questing ticks
(Spearman?s Rho coefficient=0.612; p=0.015) and mean count of FF on the left ear
(Spearman?s Rho coefficient=0.546; p=0.0353). At the individual level, a generalized linear
mixed model with a binary response (seropositivity) using ?cow ID? and ?trapping session?
as random effects showed that every female tick attached on the left ear increased the odds
of seropositivity in 37.7% (Odds Ratio=1.377). During the entire study, 2 of 21 bovines
were never seroreactive to the R. parkeri antigen, one cow presented R. parkeri-reactive
antibodies in all the samples and the rest of the herd presented at least one episode of
seroconversion, which in most cases was only transient. Serum endpoint titers against R.
parkeri antigen varied from 1:64 to 1:512. No presence of rickettsial DNA was detected by
gltA real-time PCR in the blood of cows showing events of seroconversion. All bovine
blood DNA samples were positive for 18sRNA gene real-time PCR.
Discussion: Rickettsia parkeri has previously been reported infecting A. triste ticks in
Argentina, Brazil and Uruguay. As it has been shown in the present study, A. triste
parasitizes cattle. A large proportion of the ticks at the study site (8-20%) are infected by R.
parkeri and the seasonality observed in adult A. triste ticks was from late winter to midspring.
We found no evidence of rickettsemia by PCR, but the high R. parkeri-infection
rate in A. triste in the study area, the common occurrence of this tick parasitizing this herd
and the identification of antibodies against R. parkeri antigen in 90% of the animals(19 out
of 21) are evidence that infection is taking place. Of the 19 seropositive cows, 8 (42%)
presented repeated periods of seropositivity, lasting more than three months, and up to 18
consecutive sampling sessions. The rest of the seropositive animals of the herd presented
short periods of seropositivity, which in some cases were recurrent. These intermittent
periods of seropositivity could be due to the low titers of R. parkeri-reactive antibodies that
are generated in cattle, but we cannot rule out the possibility of new episodes of R. parkeriinfection
produced by A. triste ticks feeding on these cows. The time of the year with more
animals presenting R. parkeri-reactive antibodies (from late winter to mid-spring) was
coincident with the peak in the abundance of adult A. triste ticks, which is evidence of new
infections following infestation by ticks. In this respect, an important statistical relationship
was observed between cattle seropositivity and the number of female ticks attached to it,
which indicated that each additional tick attached on the left ear of a cow incremented in
almost 40% the odds of seropositivity for this animal. Our data suggest that A. triste ticks
are capable of naturally transmitting R. parkeri to cattle. Nonetheless, the negative PCR
results indicate that either the length of the rickettsemic period may not be long, which
suggests that cattle might not be very important for the amplification of R. parkeri or the
rickettsemic levels in cattle blood may not be high enough to allow real-time PCR
detection, suggesting that R. parkeri replication in cattle might not be efficient.
Notwithstanding, a feasible role for cattle in the ecology of R. parkeri could be providing a
blood meal to a large number of A. triste adult ticks which could increase tick population.