Serological evidence of Rickettsia in horses and survey of tick-borne agents in ticks from horses and wildlife in the state of Pernambuco, Brazil.

Overview

• This research investigated the presence of Rickettsia bacteria in horses and ticks from horses and wildlife in Pernambuco, Brazil.
• It involved serological testing on horses and molecular screening of ticks to understand tick-borne infections in the region.

Background

• Tick-borne diseases are common vector-borne infections transmitted by ticks, often involving bacteria or protozoa.
• Rickettsia spp. are a group of bacteria responsible for spotted fever group (SFG) rickettsioses, which are zoonotic diseases transmitted by ticks.
• Horses can serve as hosts for ticks and may be exposed to these pathogens, indicating the potential for local disease transmission.

Study Design and Methods

• Serosurvey using indirect immunofluorescence assay (IFA) was conducted on blood samples from 270 horses across 11 farms in the coastal Pernambuco region.
• IFA targeted antigens belonging to five different Rickettsia species to detect antibodies indicating past or current infection.
• Molecular analysis (PCR) was used to test horse blood DNA for Anaplasmataceae (a family of bacteria), Borrelia (bacteria causing Lyme-like disease), and other tick-borne pathogens.
• A total of 1,852 ticks were collected mainly from horses and were morphologically identified to species level.
• An additional 48 ticks were collected from 14 wild vertebrate species to survey possible reservoirs and vectors in wildlife.
• Molecular PCR screening was also performed on 25 tick specimens for DNA of tick-borne agents: Anaplasmataceae, Piroplasmida (protozoan parasites), Borrelia, Coxiella, and Rickettsia.

Key Results

• Serology: 60.7% (164/270) of horses showed reactivity to Rickettsia spp., indicating widespread exposure to spotted fever group rickettsiae across the farms. Seropositivity ranged from 10% up to 100% across individual farms.
• Molecular tests on horse blood were negative for targeted tick-borne bacteria, suggesting no active bloodstream infections detected at the time of sampling.
• Tick identification from horses: 1,850 ticks were Dermacentor nitens and only two were Rhipicephalus microplus.
• No rickettsial DNA was detected in ticks collected from horses, suggesting a lack of infection in this tick population or low bacterial loads.
• Ticks from wildlife included six species of Amblyomma and Ornithodoros mimon, reflecting diverse tick fauna associated with native mammals and reptiles.
• Molecular screening detected Rickettsia DNA in wildlife ticks: three Amblyomma nodosum adults harbored Rickettsia parkeri strain NOD, and two Amblyomma longirostre larvae contained Rickettsia amblyommatis DNA.
• Other tick-borne pathogen DNA was not amplified in wild ticks tested.

Interpretation and Significance

• The high seroprevalence in horses provides clear evidence of exposure to spotted fever group rickettsiae in the coastal region of Pernambuco.
• Despite serological evidence, active infection in horses was not confirmed by PCR, which might be due to intermittent or low-level bloodstream infections, or that horses have only been exposed without current bacteremia.
• The predominant horse-associated tick, Dermacentor nitens, showed no detectable rickettsial DNA, suggesting it may not play a major role as a vector for these agents in this location.
• The detection of Rickettsia parkeri and Rickettsia amblyommatis in ticks from wildlife highlights the presence and circulation of multiple SFG rickettsiae in the environment.
• Wildlife-associated ticks may act as reservoirs or vectors of these rickettsial pathogens, posing a risk for spillover to domestic animals and potentially humans.
• This study contributes to epidemiological understanding of tick-borne pathogens in Northeastern Brazil and supports the need for surveillance and control measures for tick-borne zoonoses in the region.