Emergence of Brown Dog Tick–Transmitted Rocky Mountain Spotted Fever: Changing Epidemiology and Ecology
Rocky Mountain spotted fever (RMSF) is a dangerous tick-transmitted disease caused by Rickettsia rickettsii. It kills people and dogs in under-resourced rural communities throughout the Americas, in urban epidemics in southern Brazil and northern Mexico, and sporadically in North, Central, and South America. This disease is responsible for more fatalities than any other tick-borne disease in North America.1 Dogs also experience RMSF, and their risk closely mirrors that of the humans with whom they live.
Multiple ticks can transmit R rickettsii, including the brown dog tick (BDT; Rhipicephalus sanguineus sensu lato), American dog tick (Dermacentor similis/variabilis), Pacific Coast tick (Dermacentor occidentalis), and wood tick (Dermacentor andersoni). In Latin America, cases are also acquired from Amblyomma spp ticks. Each tick has its own typical host range; patterns of RMSF risk to people and dogs reflect the ecological limitations of the hosts and ticks.2 Immature Dermacentor spp prefer rodent hosts, leading to sylvatic disease and RMSF.3 Case fatality rates are 5% to 7% in the US, with the highest among children aged < 10 years.1
In contrast, RMSF cases associated with the BDT in southwestern US and northern Mexico are peridomestic and occurring in rapidly emerging epidemics. The BDT relies on dogs for hosts and can survive and reproduce rapidly in hot, dry climates.4 It thrives in and near homes and kennels, and it becomes far more abundant where there are many dogs, particularly those that roam in streets.5,6 In 2002, an epidemic of BDT-transmitted RMSF began among residents on tribal lands in Arizona; these epidemics in eastern Arizona have simmered but receive high-level intervention from state and federal health authorities.
In California, a recent 40-year review7 showed a near doubling in RMSF incidence and a shift to southern California and more cases in Latino people. Thirty of 53 reported cases (57%) were hospitalized, and 3 of 53 (6%) died. Cases in California and Arizona are often associated with travel to Mexico. There is now evidence of people in San Diego and Los Angeles Counties acquiring the infection locally, with no travel history.8 In late 2023, a health alert was issued by the CDC concerning severe and fatal RMSF in the US with exposure in Tecate, a border city in Mexico experiencing newly emerging RMSF.9
Rocky Mountain spotted fever was first diagnosed in Mexico in the early 1900s10,11 when cases associated with BDTs were sporadic and rural. Subsequently, there were decades of an apparent hiatus during a time of intense dichloro-diphenyl-trichloroethane use.11 Then, in the early 2000s, RMSF reemerged in northern Mexico with at least 9,152 human cases from 2009 to 2023 and case mortality from 10% to 50%.11–16 Epidemics occur in Baja California, Sonora, Chihuahua, Coahuila, and Nuevo Leon.17,18 These cases tend to occur where people are impoverished, community infrastructure (such as water and sanitation facilities) is precarious, and people have marginal access to medical care, sometimes with inadequate access to life-saving doxycycline.6,12,19,20
Data are lacking on the number of dogs infected, including how many progress to severe or fatal disease. Documenting active infection in dogs is uncommon, with a limited duration of bacteremia and acute infection in contrast with chronic canine tick-borne diseases such as monocytic ehrlichiosis.21,22 However, an epidemiology study23 conducted in shelters along the US-Mexico border found 2 of 239 dogs that were actively infected, which is a remarkably high prevalence. Between 14% and 42% of the dogs in the eastern US have antibodies to R rickettsii or cross-reacting rickettsiae,24,25 while seroprevalence ranges from 5% to 29% in California and Arizona23,26,27 and 59% to 81% in Mexican border states.6,19,20
Dogs as Sentinels for Human Disease
Dogs are the perfect One Health partner in the fight against RMSF. They are susceptible and suffer the same disease as people and, through their close association with humans, act as sentinels for the disease. They also contribute to the epidemiology of the disease, as they amplify R rickettsii. These ticks then can pass the infection on to other dogs or people or produce infected larval ticks through transovarial transmission.28 Experimental studies document rickettsemia of 3 to 13 days in dogs,28 the window of time in which ticks can acquire the infection. Due to huge infestation burdens, a single infected tick has the capacity to infect large numbers of dogs. Numerous epidemiological studies6,11,19,20,29 highlight that the presence of owned and unowned roaming dogs in neighborhoods is associated with increased human case rates, canine seroprevalence, and tick abundance.
Dogs can serve as sentinels for RMSF risk when they acquire ticks, RMSF, or other coinfecting pathogens transmitted by BDTs. In fact, there are multiple instances of people finding very sick or even dead dogs shortly (weeks to months) before the onset of human cases in a region, and cases occurring in dogs immediately prior to or concurrently with their owners have been reported.30–32 However, although many regions require that human cases be reported to health officials, this would rarely be done for canine cases; rather, dogs are often managed as individual patients, so unfortunately little insight is gained into canine RMSF epidemiology and population trends.
Clinical RMSF in Dogs and People
There is considerable overlap in presentation and clinical signs of RMSF among dogs and people with similar associated hematological and clinical chemical findings and results of targeted diagnostic tests. However, there is the caveat that canine cases may not be as extensively evaluated as some human cases, and much data on the clinical course of disease in dogs are derived primarily from a small number of experimental infection studies and case reports.21,28,31,33 As an obligate intracellular bacterial pathogen that targets endothelial cells lining the blood vessels, R rickettsii contributes to systemic vasculitis and vascular leakage. Thus, any organ system may be affected and signs of disease are often nonspecific. The “spotted fever” moniker is derived from the rash that often occurs; in humans, it typically begins as a maculopapular rash that progresses to a petechial rash.34 Petechia and ecchymosis may also occur in dogs; however, in both species, rash is typically not seen for the first few days of the disease and its absence has limited utility in ruling out disease in the early time period when administration of antimicrobials is most important for saving lives.21,35
In both dogs and people, disease ranges from mild to severe to fatal, and early signs are often flu-like in nature.21,36 The range of disease severity may be related to infectious dose, specific R rickettsii bacterial strain, innate susceptibility, and comorbid conditions, as well as socioeconomic and health/veterinary care access factors. Affected people may report considerable pain in joints, the chest, the abdomen, and the head,37 and while these signs are difficult to assess in veterinary patients, lethargy and lameness are reported in many cases.35 Some dogs also present with anorexia, vomiting, and diarrhea.38 Severe cases may feature progression of oral, ocular, or dermatological petechiae; edema of the limbs, ears, face, and scrotum; orchitis; skin necrosis; vomiting and diarrhea; CNS signs including obtundation and seizures; conjunctivitis and ocular discharge; uveitis; and retinal or scleral hemorrhages.39,40
Diagnostic Considerations for RMSF in Dogs
Identifying RMSF as a possible differential is the most important criterion for electing to initiate RMSF treatment because diagnoses are complicated by nonspecific signs that may be consistent with other more common diseases that mimic RMSF (Figure 1).1 If there is any chance that a dog may have RMSF, treatment should be started immediately.1,41 Possible exposure to ticks can be difficult to ascertain. There are often no ticks on the dog at the time of presentation to a veterinary practice, and in the majority of cases, owners have not observed ticks on the dog.35 Recent travel to or residing in an epidemic or endemic region (eg, to northern Mexico or the southeastern US) by the patient or other dogs in the household is important. However, given that RMSF occurs sporadically across many states in the US,1 lack of apparently obvious history of tick exposure does not rule out RMSF; if there is no history of living in or traveling to the American southwest or northern Mexico, where peridomestic BDT-transmitted RMSF is suspected, possible exposure would be via sylvatic Dermacentor spp ticks. In that case, lifestyle such as living outdoors and activities such as hiking or hunting with the dog become more relevant historical details, as does time of year. In humans, most (although not all) cases of RMSF in the US occur from April through September.7,42 This seasonality is less apparent in northern Mexico for BDT-associated cases.29
Steps involved in the clinical approach to Rocky Mountain spotted fever. R. rickettsii = Rickettsia rickettsii.
Citation: Journal of the American Veterinary Medical Association 263, 3; 10.2460/javma.24.11.0756
Both CBC and clinical chemistry changes are nonspecific in RMSF. Early disease often features leukopenia or leukocytosis, anemia, and mild to severe regenerative thrombocytopenia; elevated liver enzymes and hypoalbuminemia may also occur.41 Rickettsia rickettsii is not expected on a peripheral blood smear, although this test helps detect Babesia, Anaplasma, and Ehrlichia.1,43 A positive PCR test confirms an active, current infection, although a negative PCR test does not rule out RMSF. Polymerase chain reaction testing can also be performed on blood, eschars, scabs, or punch biopsies. Many commercial laboratories offer PCR diagnostics for ticks as well.
Serological tests detect antibodies against rickettsiae and indicate past exposure. In-clinic screening for anaplasmosis and ehrlichiosis can be accomplished with the use of point-of-care tests, but such tests do not include spotted fever–group rickettsiae. As Anaplasma platys and Ehrlichia canisare both also transmitted by BDTs and coinfection with R rickettsiimay occur,23 Anaplasmaand Ehrlichiaseropositivity (either due to current or previous infection) does not rule out RMSF and may serve to highlight the dog’s exposure to BDTs. Of note, however, both the Anaplasmaand Ehrlichia wells on commonly used point-of-care tests detect exposure to multiple other Anaplasma and Ehrlichia pathogens (eg, Anaplasma phagocytophilum and Ehrlichia chaffeensis), which have highly dissimilar ecologies to RMSF, in addition to BDT-transmitted A platys and E canis .
A titer of 1:64 is a typical cutoff for interpreting a test to be negative or positive, although some laboratories use 1:80. A negative serology result early in infection is typical.21 Some literature suggests that a titer at the time of presentation ≥ 1:1,024 is likely to represent RMSF,41 although repeated exposure may increase titers. However, in addition to indicating past exposure, positive titers also occur because of serological cross-reaction among all members of the spotted fever group of rickettsiae.1,44 Depending on location and population, 5% to 60% of apparently healthy dogs may have positive titers.6,30 Persistent titers last for at least 180 days and possibly much longer.21,22 A paired titer helps differentiate between current and previous infection, because a single baseline titer has limited utility for inferring that clinical disease is associated with active RMSF.
The serological tests described earlier usually detect immunoglobulin G. Immunoglobulin M (IgM) could theoretically help differentiate between recent and previous exposure, as IgM antibodies rise earlier in the course of infection, and there are some guidelines for using IgM in humans. However, IgM serology has poor specificity and poor correlation with diagnosis relative to immunoglobulin G.45
To reiterate, given that time is of the essence to protect the life of the patient, it is imperative for clinicians to (1) be aware of whether ticks and/or RMSF occur in the areas where the patient lives and has traveled; (2) obtain good history of health and travel; (3) ask about ticks but not rule out RMSF if no ticks have been seen; and (4) initiate doxycycline treatment immediately if initial clinical and/or laboratory findings may be compatible with RMSF.
Treatment
Doxycycline is essential for treating RMSF and saves lives46; in contrast, most other antimicrobials lack efficacy for RMSF. One of the main reported contributors to the high human case fatality rate is failure of prompt diagnosis and initiation of tetracycline-class antimicrobials. The risk of fatality increases 2- to 3-fold if treatment is delayed > 5 days and 4-fold if delayed > 6 days.1,11 Doxycycline is given to dogs and people for 7 to 14 days.41 Given that R rickettsii –infected dogs are often coinfected with E canis and that this latter infection may be chronic, dogs may eventually require a more prolonged treatment with doxycycline for 28 days or longer.41,47 If doxycycline cannot be used, less optimal options include enrofloxacin or chloramphenicol.41
Much of the treatment of RMSF is clinically indicated supportive care, including fluids, analgesia, management of edema and shock, etc. Fluid treatment is indicated to improve perfusion, although caution is needed to avoid exacerbating tissue—especially cerebellar—edema. When anemia and/or thrombocytopenia are severe at presentation, differentiating between immune-mediated disease and infectious disease can be a diagnostic challenge. Nevertheless, anti-inflammatory doses of steroids do not potentiate severity of the infection in experimental infections, may help reduce tissue damage caused by R rickettsii , and may minimize immune-mediated complications but may prolong rickettsemia.41 Clinical signs begin to improve within 24 to 48 hours of starting antimicrobial therapy, if started early in the course of disease, and thrombocytopenia should begin to resolve concurrently.
Rocky Mountain Spotted Fever Prevention and Tick Management
Veterinarians in clinical practice can help families manage the risk of RMSF and other tick-borne diseases for both human and canine members of the family. Ideally, the veterinarian is knowledgeable about those ticks and diseases present in the region and in other areas to which the family may travel and can advise about measures people at risk should take, how to prevent tick bites, and how to eradicate ticks present in a home. Client education about the risk of ticks and tick-borne disease should happen at all visits, perhaps with literature showing maps of risk by disease and state in the US and Mexico, available from the Companion Animal Parasite Council and the CDC. When a dog is found to have ticks, a human family member reports being bitten by ticks, or a dog is suspected to have or diagnosed with a tick-borne disease, the veterinarian also is obligated to help explain further measures the family should take to protect themselves. When the ticks are BDTs or the suspected diagnosis in the dog is RMSF, it is essential that the family know the early signs of RMSF in people and how important it is to report any fever or other compatible human symptoms to their physician immediately. However, a dog with RMSF, as long as it doesn’t continue to have ticks, does not pose a direct health risk to family members or the treatment team (ie, transmission is via the ticks but not the dog). In contrast, exposure to infected tissues (such as ruptured engorged ticks or blood) via inoculation or through mucous membranes may lead to infection.
Veterinarians typically receive far more training about ticks than the general public and physicians, so they can serve as a resource about tick species, managing ticks on dogs, and managing BDT infestations in a home. They may be the first to detect tick infestations on dogs (eg, during physical examination of the dog) and should also share with pet owners the typical manifestation of BDT infestation in a home, empowering the pet owner to detect any ticks present. Identifying the tick species present is important for understanding whether there is RMSF-associated risk. A very useful tool for tick identification is a small magnifying glass attachment for a phone camera that can be purchased for $20 or less (Figure 2). Use of this tool facilitates direct identification or photo documentation to share with an expert. We suggest that the tick be removed from the pet with the use of forceps and placed on a piece of white paper. With the phone resting on a box approximately 10 cm high, well-lit photographs should be taken of both the dorsal and ventral aspects of the tick. Then, the ticks can be saved at room temperature in 70% ethanol in case confirmation or future testing is required. The presence of BDT indicates that there may be an indoor or peridomestic infestation posing an acute risk, while the identification of Dermacentor spp suggests that there may be a shared exposure risk in the outdoors. Finding other genera such as Ixodes or Amblyomma may convey additional, non-RMSF risk information to owners.
Example setup for use of a smartphone and clip-on magnifying glass attachment to take photos for tick identification (A). An unengorged female (B) and engorged male (C) brown dog tick are shown.
Citation: Journal of the American Veterinary Medical Association 263, 3; 10.2460/javma.24.11.0756
All dogs in homes where there is any risk of ticks should be on high-quality tick preventive medication, although the considerable cost of most preventive medications is a barrier to their use. Collars containing 10% imidacloprid and 4.5% flumethrin are marketed as able to control ticks for 8 months.48,49 Other orally and topically delivered acaricides labeled as being effective against BDT and Dermacentor spp include (but are not limited to) isoxazolines (fluralaner, lotilaner, sarolaner, alfoxolaner, etc), fipronil, and mixed fipronil products (eg, fipronil plus methoprene) or imidacloprid plus permethrin.50 It is important in the case of BDTs that all dogs be treated, even if they do not leave the home or property, whereas only those dogs that hike or undertake outdoor activities need preventives against Dermacentor spp. Families should understand that roaming untreated dogs coming into their yard can be a continual source of new BDTs, as can an adjacent home with an infestation.
Veterinarians can advise how to manage BDT infestations in a home, yard, and kennel but should know that there is no easy or durable fix. Brown dog ticks can hide in dirt, under debris, and even in cracks and in naturally porous stucco and concrete. They can reach enormous population densities in neighborhoods with abundant free-roaming or stray dogs.48,49,51 Any debris and clutter should be removed from all possibly infested areas prior to fumigation or spraying, and spraying should be done with a high-quality acaricide such as those containing deltamethrin. Respraying will be necessary every several months, and tick infestations may persist, although probably at lower densities. It is likely helpful to seal certain porous surfaces like dirt and stucco. Failures of BDT control result from neighbors not using acaricides in the home or on dogs19; reintroduction from infested roaming dogs; people using ineffective products including many shampoos, herbal remedies, and insecticides (ticks are not insects); and emerging acaricide resistance in BDTs, although those ticks currently at the US-Mexico border show little evidence of this problem.52
The Role of the Veterinarian in Public Health
Veterinarians may have opportunities to share epidemiological data with other One Health professionals, have valuable insights into herd health, and can help create healthy environments for dogs and people.
Campaigns against BDTs and the diseases they spread require multiple-part approaches, including eradication of ticks from the environment, treatment of all dogs present, outreach and education of the public and medical practitioners, and management of the dog population, and many of these approaches are specifically suited to the veterinary skillset. Rarely is only a single home infested, so entire neighborhoods, villages, and sometimes cities are involved. An aggressive intervention campaign against RMSF in eastern Arizona provided all dogs in a 600-home community with long-acting tick collars, and environments were treated with acaricide monthly.48,49 Canine spay, neuter, and restraint to the homesite were promoted, yielding a reduction in infestation rates on dogs to 1% (compared to 64% in a nearby community with no intervention) and, for a time, no detectable ticks in the environment. It successfully reduced human case numbers but at a considerable cost, requiring ongoing intervention to suppress BDTs and RMSF. Likewise, an intervention campaign featuring education programs for physicians and the public, dog spay and neuter, treatment of houses with acaricide, and use of long-acting acaricidal collars on dogs saved lives in Sonora, albeit with rebounds in tick abundance.11 Because the problem recurs even after such intensive intervention, most high-risk communities are exceedingly disadvantaged economically, and the epidemic is now present in far larger cities such as Tijuana or Mexicali, it is unlikely that simply scaling up these comprehensive “wraparound” campaigns is even feasible.
Managing dog populations is a serious challenge but likely to contribute considerably to lowering community RMSF risk. Abundant free-roaming or stray dogs particularly are repeatedly demonstrated to be associated with high risk for BDTs, canine spotted fever–group rickettsial seropositivity, and human RMSF cases.5,48,51,53,54 While stray or unrestrained owned dogs tend to patrol only a relatively small area of houses or city blocks, those dogs in areas with more RMSF (high-risk areas) tend to visit more houses than in low-risk neighborhoods.55 Veterinarians can advise on restraint of dogs to the home while also helping promote reducing overall dog numbers, including through spay and neuter. Because dogs often do survive RMSF or cocirculating spotted fever–group rickettsial infections, older dogs help to constitute local herd immunity, suggesting that high turnover in the canine population due to birth of puppies and high death rates helps fuel the epidemic. For example, high-risk areas in Mexicali had higher proportions of younger dogs and higher female dog fecundity compared with lower-risk sites.55 Thus, spay and neuter campaigns and encouraging residents to keep fewer dogs and not allow them to roam can improve dogs’ quality of life, promote herd immunity, and save canine and human lives.
Finally, veterinarians are valuable One Health partners in the fight against RMSF. They can disseminate knowledge about risk and risk management. Being able to diagnose RMSF helps inform about the status of patients as well as the community. Canine seroprevalence studies help highlight regions where R rickettsii is circulating. For example, on Arizona tribal lands, there was high canine rickettsial seroprevalence of over 70%.5,53 Before the onset of the RMSF epidemic in Mexicali, owned dogs at veterinary clinics and stray dogs had a similar but high seroprevalence of about 60%,56 increasing to 85% in a pilot of symptomatic dogs in the high-risk Los Santorales neighborhood during the epidemic.57 Unfortunately, however, such information is often not available. Nevertheless, a veterinary practice can share data with local veterinary medical associations, local and state health departments, and vector agencies. Finally, at the policy level, they must advocate for public health investment, spay/neuter campaigns, and canine population and movement control. While RMSF is encountered less frequently in veterinary practice than other tickborne diseases, its severity and public health implications make its recognition and management extremely important.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
This research was funded in part by the Pacific Southwest Regional Center of Excellence for Vector-Borne Diseases, which is funded by the US CDC (Cooperative Agreement 1U01CK000516).
ORCID
L. Backus https://orcid.org/0000-0001-9363-5555
A. M. López-Pérez https://orcid.org/0000-0001-5726-7943
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