Rickettsial infection in Cerro Largo, State of Rio Grande do Sul, Brazil

Sangioni, L.A.; Vogel, F.F.S.; Cadore, G.C.; Hilger, R.B.; Tonim, R.; Pacheco, R.C.; Ogrzewalska, M.; Labruna, M.B.

doi:10.1590/S0102-09352011000200035

COMMUNICATION

Rickettsial infection in Cerro Largo, State of Rio Grande do Sul, Brazil

Infecção rickettsial em Cerro Largo, Rio Grande do Sul

L.A. Sangioni^I; F.F.S. Vogel^I; G.C. Cadore^I; R.B. HilgeR^II; R. Tonim^II; R.C. Pacheco^III; M. Ogrzewalska^IV; M.B. Labruna^IV

^IUniversidade Federal de Santa Maria - Santa Maria, RS

^IISecretaria da Saúde - Cerro Largo, RS

^IIIUniversidade Federal do Mato Grosso - Cuiabá, MT

^IVUniversidade de São Paulo - São Paulo, SP

Keywords:Rickettsia, Brazilian spotted fever, sorology, Rio Grande do Sul

RESUMO

O presente estudo teve por objetivo avaliar, por imunofluorescência indireta, infecções ricketsiais em animais e humanos assintomáticos da área rural de Cerro Largo, RS. Dezesseis (51.6%) equinos, seis (22.3%) cães e oito (29.6%) humanos mostraram anticorpos reagentes (título <64) para Ricksettia rickettsii e para Ricksettia parkeri. Os estudos sorológicos indicaram que equinos, cães e humanos entraram em contato com rickettsias do grupo da febre maculosa nesse município. Dois cães mostraram títulos de R. parkeri no mínimo quatro vezes maior que os demais agentes ricketsiais testados e sugerindo que R. parkeri ou um genótipo próximo está circulando na região.

Palavras-chave:Rickettsia, febre maculosa, sorologia, Rio Grande do Sul

Bacteria of the genus Rickettsia are obligate intracellular organisms primarily associated with invertebrate hosts, such as ticks, mites, fleas, lice, leeches, and insects. A few Rickettsia species are capable of infecting vertebrates including humans, to whom they are vectored mostly by ticks (Weinert et al., 2009). In 2005, there was the first reported case of spotted fever group (SFG) rickettsiosis in Rio Grande do Sul, in a patient from Cerro Largo municipality who developed headache, arthralgia, generalized body aches, nasal secretion, and fever 20 days after a tick bite on his back. Since only one serum sample was collected from this patient (during the acute phase), which reacted positively (titer >64) against R. rickettsii antigens by indirect immunofluorescence assay (IFA), this case was regarded as probable. In 2007, there was the first laboratory-confirmed case of SFG rickettsiosis in Rio Grande do Sul, in another patient from the same area, who developed general symptoms similar to the first case, with no fatality. The case was serologically confirmed by IFA on paired serum samples using R. rickettsii antigens; acute serum was non-reactive whereas convalescent serum gave a titer of 128 (unpublished data from the Brazilian Ministry of Health). Based on these cases, an epidemiological study that evaluated rickettsial infection in domestic animals (considered to be sentinels for rickettsiosis) and asymptomatic humans was performed in the rural area of Cero Largo, where the two cases of SFG rickettsiosis had occurred.

The municipality of Cerro Largo has a population of 12,000 inhabitants and its economy is based on services, commerce, industry, and agropecuary (soy, wheat, corn, swine and dairy cattle). There are many small, family-employed farms. Often, these small properties raise horses and dogs in areas near native forests and ecological reserves. On the outskirts of the town, the horse and cart are common ways of transportation for many of the residents, who keep horses and dogs in natural fields that are infested with ticks.

In March 2007, blood samples were collected from 31 horses, 27 dogs, and 27 humans living in rural properties on the outskirts of Cerro Largo. Animal blood samples were collected by the jugular vein in order to obtain sera samples by centrifugation, which were stored at - 20°C until processed by serological analysis. All serum samples were individually processed by IFA using crude antigens of the following Rickettsia species that have been isolated from ticks in Brazil: R. rickettsii (strain Taiaçu) and R. parkeri (strain At24). If a serum reacted at the 1/64 dilution to any of these two antigens, its IgG titer was titrated from 1/64 to the endpoint dilution against both, and also against the following Rickettsia species: R. amblyommii (strain Ac37), R. rhipicephali (strain HJ5), and R. bellii (strain Mogi). Antigen preparation and IFA reactions were performed as previously described (Horta et al., 2004; Labruna et al., 2007). Serum showing for a Rickettsia species titer at least 4-fold higher than that observed for any other Rickettsia species was considered homologous to the first Rickettsia species or to a very closely related species, as previously determined (Horta et al., 2004; Labruna et al., 2007; Piranda et al., 2008; Saito et al. 2008).

This study was approved by the Ethics Committee at the Universidade Federal de Santa Maria and is in a accordance with the CNS Resolution 196/96 (CEP 23081.008609/2007-54/CAAE: 0091.0.243.000-07).

A total of 16 (51.6%) horses, six (22.3%) dogs, and eight (29.6%) humans were shown to contain reactive antibodies (titer <64) to R. rickettsii and R. parkeri. These sera showed endpoint titers varying from 64 to 2,048 to at least one of the five rickettsial antigens tested by titration (Tables 1-3). No serum showed higher titer to a Rickettsia species other than those elicited to R. parkeri or/and R. rickettsii. No serum showed endpoint titer to R. rickettsii 4-fold higher than that to R. parkeri. Two canine sera (Table 2) showed titers to R. parkeri at least 4-fold higher than those to any of the other four antigens. The antibody titers in these two dogs were considered to have been stimulated by R. parkeri or a very closely related species. This result is in accordance with a recent epidemiological study performed in another part of the State of Rio Grande do Sul (Saito et al., 2008), in which at least 100 dogs showed serological evidence of infection by R. parkeri or a very close-related genotype, as indicated by titers anti-R. parkeri at least 4-fold higher than those against the same other four Rickettsia species tested in the present study.

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None of the serologically positive humans of the present study recalled recent febrile illness compatible with classic spotted fever (fever, headache, and rash), and also, none of them reported to have taken antibiotics to treat a possible rickettsiosis. However, rickettsiosis can manifest with a multiplicity of general symptoms (Parola et al., 2005), making it difficult to recall previous clinical rickettsiosis with certainty. These facts also indicate that the SFG agent infecting humans in Rio Grande do Sul is potentially not lethal, in contrast to Southeastern Brazil, where R. rickettsii is highly pathogenic with high fatality rates in untreated cases.

The serological results indicated that horses, dogs, and humans have been in contact with SFG rickettsiae in Cerro Largo municipality. Results from at least two dogs indicated that R. parkeri or a close-related genotype is circulating in the region, a condition also observed in other municipalities in the State of Rio Grande do Sul (Saito et al., 2008). R. parkeri and its closest known species (R. africae and R. sibirica) are recognized human pathogens in other countries where they induce a milder spotted fever with no lethality reported to date (Parola et al., 2005). These facts suggest that a different rickettsial agent, possibly related to R. parkeri, is the etiological agent of spotted fever in Southern Brazil (States of Rio Grande do Sul and Santa Catarina). Further studies are needed to isolate the rickettsial agent in order to determine its definitive taxonomic identification.

From 2006 to 2007, 20 ticks that were found attached or walking on humans in the rural area of Cerro Largo were collected by the local population and sent to the Laboratório Central do RS (LACEN), where they were all identified as adults of Amblyomma ovale (unpublished data), indicating that this species is the main human-biting tick in Cerro Largo. A. ovale is the main vector of Rickettsia sp. strain in Southeastern Brazil (Sabatini et al., 2010). Also, 17 of these ticks were tested by polymerase chain reaction targeting SFG rickettsial DNA, which was not found in any individual tick (unpublished data). Since these ticks represented only a small sample, further studies should encompass a larger sample of A. ovale, a potential vector of spotted fever in Southern Brazil.

ACKNOWLEDGMENTS

We are grateful to Secretaria de Saúde de Cerro Largo for logistic support and valuable help during field works. Thanks to Renata D'Avila Couto (Brazilian Ministry of Health) for providing information about laboratory-confirmed cases of BSF in Rio Grande do Sul. This work was financially supported by the Universidade Federal de Santa Maria (FIPE) and Universidade de São Paulo.

Recebido em 3 de dezembro de 2010

Aceito em 21 de dezembro de 2010

E mail: sangioni@smail.ufsm.br

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