Ticks and Tick-Transmitted Diseases in Cats

Some cat owners believe that tick control is not necessary for their cat because they never see ticks on their pet. Risk perception is even lower for cats that live strictly indoors. However, while outdoor access is a risk factor for acquiring ticks, indoor-only cats are not always spared. Large-scale surveys of ticks recovered from companion animals showed that 3.4% to 4.2% of ticks submitted were reportedly removed from cats that lived strictly indoors.^1,2 Cats that live totally outside are at substantially increased risk for tick infestations, with 1 survey finding 18.7% of free-roaming cats in the central United States to be infested.³ Compared with dogs, cats do tend to have fewer ticks, with surveys finding that tick burdens on dogs ranged from 1 to 4765 ticks while cats hosted only 1 to 46 ticks.^1,2

The tick species most commonly found on cats are Ixodes scapularis (FIGURE 1), Amblyomma americanum (FIGURE 2), and Dermacentor variabilis (FIGURE 3); other Ixodes and Dermacentor species, Amblyomma maculatum, Rhipicephalus sanguineus (FIGURE 4), and Otobius megnini are less common.^1-3 The likelihood of recovering a particular species from a feline host is largely driven by the geographic range and seasonality of that species.

Figure 1. Ixodes scapularis.

Figure 2. Amblyomma americanum.

Figure 3. Dermacentor variabilis.

Figure 4. Rhipicephalus sanguineus.

Interestingly, R sanguineus is among the tick species most commonly recovered from dogs, has nationwide distribution in the United States, and is capable of surviving for long periods indoors, but it is rarely recovered from cats.

Tick-Transmitted Diseases in Cats

Although most tick species are competent vectors for multiple disease agents, not all tick species transmit all tick-borne infections, and each species typically transmits a relatively short list of infectious agents. Tick species identification, along with knowledge of the geographic range and seasonality of different species and which pathogens they transmit, may be extremely useful for evaluating patient risk for a particular infection and tailoring diagnostic strategy accordingly.

The vast majority of tick-borne infections are transmitted through blood feeding; therefore, the best method of protecting against these infections is preventing ticks from feeding or minimizing the time they spend feeding on a host. Tick control products that repel ticks from feeding, rapidly kill ticks, or impede feeding processes may help to prevent or reduce blood feeding, effectively blocking transmission of tick-borne infections. For example, collars containing imidacloprid and flumethrin have been shown to effectively prevent transmission of Cytauxzoon felis by preventing tick attachment.⁴ Topical application of selamectin plus sarolaner has also been shown to prevent transmission of C felis by killing attached ticks before transmission can occur.⁵ Most recently, it was demonstrated that topical application of selamectin plus sarolaner can prevent transmission of Borrelia burgdorferi, the causative agent of Lyme disease.⁶ Acaricidal products formulated for use on cats, many of which also include flea control and control of some endoparasites, are listed in TABLE 1.

Cytauxzoonosis

Cytauxzoonosis, caused by C felis, is the most clinically severe and notorious tick-transmitted disease of cats. C felis is a protozoan hemoparasite of domestic and wild felids that is transmitted primarily by A americanum and occasionally by D variabilis.^7,8 Infection is most common in the south central, southeastern, and mid-Atlantic regions of the United States.^9-11

Although C felis infection was once accepted as highly fatal, it is now known that a considerable number of apparently healthy cats in endemic areas harbor C felis.^10-12 Factors contributing to infection survival, such as the possibility of regional strain differences, have been investigated but are still not well described.¹³ Despite the possibility of survival, for cats that present to veterinarians with acute clinical disease, the mortality rate is 40% to 100% even with therapeutic intervention.^11,14

C felis infection occurs in 2 distinct stages. During the first, schizogenous stage, host macrophages are invaded by the parasite and become enlarged as the parasite asexually reproduces within them, eventually growing to mechanically occlude vessels.¹⁵ This is when acute clinical disease usually occurs, beginning 11 to 16 days postinfection (DPI) with fever, inappetence, depression, dehydration, and generalized pain, progressing to respiratory distress.^15,16 During the second, erythrocytic stage of infection, intraerythrocytic parasites become detectable by blood smear (around 18 DPI).¹⁶ As disease progresses, cats often become icteric, ataxic, hypothermic, and eventually moribund. If the cat survives acute infection, the erythrocytic stage becomes chronic, and it is assumed that persistence of the intraerythrocytic parasites may be lifelong.

Currently, there are not many diagnostic options for timely diagnosis of C felis infection. Therefore, diagnosis may be presumptive and confirmed after initiation of treatment. Polymerase chain reaction (PCR) testing of a blood sample is often regarded as the most sensitive antemortem diagnostic test, and some assays may permit diagnosis very early in the infection; however, it is only available through diagnostic laboratories.¹⁷ Blood smears can be performed more feasibly in-clinic, but onset of acute clinical signs often precedes detection of intraerythrocytic stages by this method. Rarely, schizonts may be recovered by blood smear or more commonly by fine-needle aspiration of the lungs, lymph nodes, or spleen; however, this is not always a reliable means of antemortem diagnosis.

There is no labeled treatment for C felis infection, but improvement in outcome has been achieved through using a combination of atovaquone and azithromycin or a combination of ponazuril and azithromycin along with heparin and intense supportive care.^14,18 Imidocarb dipropionate has been used historically, but there is evidence that it may be less effective compared to other treatments.¹⁴ Even with treatment, prognosis is guarded to poor.

Anaplasmosis

Two Anaplasma species, Anaplasma phagocytophilum and Anaplasma platys, have been reported to infect cats. A platys infection has rarely been reported in cats in the United States,¹⁹ likely due to the purported vector, R sanguineus, rarely feeding on cats. Evidence of A phagocytophilum infection, however, is common in cats and overlaps in distribution with B burgdorferi infection due to sharing vector species. Currently, it seems that A phagocytophilum is more likely to cause clinical disease in cats than B burgdorferi.²⁰

Diagnosis of Anaplasma species infection relies heavily on serology, although PCR testing can be used to detect acute infections. Currently, patient-side serologic tests for common tick-transmitted infections are only approved for use in dogs, but they have demonstrated efficacy when used extra-label in other host species.^21,22 PCR detection of acute infection can precede seroconversion, and this should be considered in cats with consistent clinical signs and history of tick exposure but no serologic evidence of infection.²¹ Serologic evidence of Anaplasma species (presumably A phagocytophilum) and B burgdorferi coinfection is not uncommon in cats from endemic areas, and this can be a complicating factor when correlating clinical signs with serologic test results.

The most common clinical signs reported with acute A phagocytophilum infection in cats are lethargy, fever, and anorexia²³; however, cats can exhibit serologic evidence of infection in the absence of clinical signs. In addition to PCR and serologic testing, visualization of intracellular morulae may also be used for diagnosing Anaplasma infection, although this is likely to be a less sensitive diagnostic method than PCR testing. A platys manifests as intraplatelet inclusions, and A phagocytophilum morulae are primarily found in neutrophils. It is important to note that A phagocytophilum and Ehrlichia ewingii infect the same cell types and cannot be distinguished morphologically on a blood smear.

Treatment of feline anaplasmosis is largely extrapolated from canine protocols. Administration of doxycycline at 10 mg/kg PO q24h for 21 to 28 days has achieved resolution or marked improvement of clinical signs within 24 to 48 hours in cats with suspected A phagocytophilum infection.^23,24

Lyme Disease

B burgdorferi, the causative agent of Lyme disease, is well known for causing serious disease in some hosts and is the most common tick-transmitted infection in humans and dogs in the United States.^25,26 B burgdorferi is transmitted by I scapularis in the Northeast, upper Midwest, and mid-Atlantic regions and by Ixodes pacificus along the West Coast.²⁷

Cats, particularly those in Lyme-endemic areas, may demonstrate serologic evidence of infection, with a recent survey finding a prevalence of 13.9% (26/187) in feline serum samples from the Northeast submitted for vector-borne disease diagnostic testing.¹⁹ However, when interpreting serologic test results, it is paramount to consider what information serologic testing actually provides. Detection of antibodies indicates historical infection but does not indicate when infection occurred or whether infection currently persists. Therefore, no causal relationship implicitly exists between serologic findings and clinical status, even in cats that are antibody-positive for B burgdorferi and have clinical signs consistent with tick-transmitted disease.

In cats with concurrent clinical signs and serologic evidence of B burgdorferi infection, the most commonly reported clinical signs are fever, anorexia, fatigue, and lameness.^28,29 The American College of Veterinary Internal Medicine recommends that suspected cases of clinical B burgdorferi infection in cats be treated similarly to those in dogs, using doxycycline as first-choice therapy.³⁰ Doxycycline has previously shown to improve clinical signs in cats with suspected Lyme disease, although in these cases it could not be definitively ruled out that disease was not caused by another tetracycline-responsive illness, including A phagocytophilum.²⁸

Ehrlichiosis

Three species of Ehrlichia have been described infecting cats: Ehrlichia canis, E ewingii, and Ehrlichia chaffeensis. Ehrlichia infection is common in dogs and can be highly pathogenic; however, natural Ehrlichia infection in cats appears to be rare in the United States compared to other tick-transmitted infections.¹⁹ Reports of amplifying Ehrlichia species DNA from cats are uncommon; however, clinical signs coinciding with DNA amplification, including lethargy, fever, generalized pain, and epistaxis, have been reported in a few cases.^19,31 E canis is typically transmitted by R sanguineus, while E ewingii and E chaffeensis are typically transmitted by A americanum. Given the rarity of feline Ehrlichia infection, there is limited information on successful treatment of natural infection, although it is likely that the doxycycline regimen used for treating suspected anaplasmosis would be efficacious.^32,33

Tularemia

Francisella tularensis is an infectious agent with complex etiology, having many different possible transmission routes. Among these is tick transmission, which is most often associated with D variabilis and A americanum.³⁴ Tularemia is a relatively rare disease in the United States in both humans and domestic animals but is a significant zoonotic concern due to its many transmission routes and classification within the highest risk category for potential bioterrorism use.³⁵

Tularemia infections are most often reported from the south central and Midwest United States and, compared with dogs, cats appear to be more susceptible.^34,36 Clinical signs reported in cats range in severity, although the majority of feline infections appear to be diagnosed postmortem, with nonspecific necrotic lesions found on the liver, lungs, small intestine, lymph nodes, and spleen at necropsy.³⁷ Most importantly, a number of human tularemia infections have been linked to transmission by bite from cats.³⁸ Although tick transmission should not be underestimated, current evidence suggests that the majority of tularemia cases in cats and humans appear to be related to contact with infected rabbits.

Summary

Although cats are at lower risk for tick parasitism and high-burden infestations than dogs, all cats, even indoor-only cats, are at some risk for harboring ticks and, consequently, infection with tick-transmitted pathogens. Knowing the geographic range and vector capability of various tick species is essential for making diagnostic decisions or necessary reporting following recovery of ticks from pets. The best method for ensuring protection from tick-transmitted diseases is compliant, year-round use of feline-formulated acaricidal products along with managing lifestyle factors to minimize risk of tick exposure.