Coinfection with multiple tick-borne pathogens - Veterinary Medicine
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Coinfection with multiple tick-borne pathogens
It's not always cut-and-dried—a single tick bite delivering a single pathogen. Sometimes ticks carry more than one infective agent, and sometimes a pet has been bitten by multiple ticks carrying different microorganisms.So when should you suspect that your patient is infected with more than one tick-borne pathogen?



Several reports document coinfection with tick-borne pathogens in people.15-19 The true prevalence of coinfection in people is unknown, but the medical literature suggests that coinfection is infrequent.15,18 A number of reports describe coinfections in veterinary patients,1,20-25 but there have been few epidemiologic studies on the prevalence of coinfections in dogs.

In one serologic survey of 277 dogs in Rhode Island, 21% were seropositive for two or more tick-transmitted pathogens, including Borrelia burgdorferi, Bartonella vinsonii, E. canis, and Rickettsia species.26 Coinfection with as many as six different tick-borne pathogens has been documented in dogs.1 In a study of dogs in Thailand, more than half of a population of dogs seroreactive to E. canis also had antibodies to Bartonella vinsonii subsp berkhoffii,20 leading another group of researchers5 to suggest that dogs exposed to E. canis in the United States and Thailand are also at risk of coinfection with B. vinsonii subsp berkhoffii. In another study, Bartonella species DNA was detected by PCR testing in seven of 12 dogs naturally infected with Ehrlichia species; five of 12 of these dogs were also seropositive for antibodies to B. vinsonii.21 Finally, about 26% of mice surveyed at United States military installations and training sites in Korea were found to be infected with multiple tick-borne pathogens.27

Collectively, the studies of coinfections in people, dogs, and other species emphasize the fact that evidence of naturally occurring coinfections in animals can be found when efforts are made to look for them.


Susceptibility to coinfection may be affected by factors such as geography, travel, husbandry, host immune status, and the use of preventive drugs and prophylactic therapy. Knowing the common tick species and pathogens within a practice's geographic area, as well as a patient's travel history, is paramount if coinfection is suspected. Suspicion for coinfection should be high if a patient has had frequent exposure to vectors or reservoir hosts or if there is a high prevalence of disease in a practice area.

In one report, factors associated with a high rate of coinfection with Ehrlichia species, Anaplasma species, Babesia canis, Bartonella vinsonii, and Rickettsia rickettsii in a Walker hound kennel included cohabitation; frequent exposure to ticks, mice, and rodents in the vicinity of the kennel; and frequent travel to wooded areas.1

Coinfection should also be suspected if a patient is more clinically ill than would be expected based on infection with a single pathogen or if a patient has an atypical clinical presentation. Histopathologic examination of joints from mice experimentally coinfected with Borrelia burgdorferi and A. phagocytophilum showed more severe arthritis at two weeks after infection than did mice infected with B. burgdorferi alone, while mice solely infected with A. phagocytophilum did not develop arthritis.10 In people, worse clinicopathologic changes may result from concurrent borreliosis and babesiosis18,19 or concurrent borreliosis and A. phagocytophilum infection.10,18 Although there is not yet direct evidence that more severe clinical disease occurs in coinfected companion animals, these studies in other species may serve as a valuable precedent when evaluating veterinary patients.

Yet another situation in which to suspect coinfection is if a patient has either a delayed response or no response to appropriate therapy directed at a single pathogen. For example, in a patient with E. canis infection, if platelet counts do not increase within seven days of therapy, another mechanism for thrombocytopenia should be suspected, such as immune-mediated destruction or coinfection with Babesia or Bartonella species.28 In one report, a dog coinfected with Bartonella vinsonii subsp berkhoffii and Babesia canis had persistent thrombocytopenia until both infections had been treated.23

Another laboratory indicator supporting coinfection may be a positive antinuclear antibody (ANA) test result. One study demonstrated that dogs that were seroreactive to multiple tick-borne pathogens were more likely to have positive ANA results than were dogs from the same geographic region that were seroreactive to only one antigen.25


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