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Tick-Borne Diseases: A Real Threat to a Pet's Health

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Jane E. Sykes
Professor, University of California, Davis

Introduction

Ticks are capable of transmitting viruses, bacteria and protozoa to mammalian hosts, although tick-transmitted illness currently recognized in dogs and cats in the United States usually results from infection with bacterial and protozoal pathogens (Table 1). Heavy tick infestations can also cause anemia and substantial morbidity in kennel situations and some tick species (Dermacentor spp. in the United States) can cause tick paralysis. Ticks serve as reservoirs for infectious agents in the environment, which can be transmitted transovarially to successive generations of ticks, or transstadially between tick stages (i.e., larva to nymph to adult). Tick-borne diseases have restricted geographic distributions. This is because each pathogen is adapted to one or more specific tick species, and the geographic distribution of the pathogen generally reflects that of the transmitting tick species. Pets, especially dogs, act as sentinels for human exposure to tick-borne pathogens, and so owners of pets that have been diagnosed with tick-borne disease should be advised that they may also have been exposed to the pathogen. Because ticks can transmit multiple pathogens simultaneously, co-infections can occur and complicate the clinical picture.

In general, tick-borne disease is more commonly recognized in dogs than in cats. The exception is cytauxzoonosis, which occurs only in cats. Lack of known exposure to ticks should not be a reason to exclude tick-borne diseases from the differential diagnosis list, because frequently tick exposure goes unrecognized by pet owners. Infection with tick-transmitted pathogens leads either to illness or subclinical seroconversion. Because most tick-borne pathogens can cause subclinical infections, the results of tests that are based on antibody detection should be interpreted carefully.

Ticks of Importance in the United States

Hard ticks (Ixodidae) transmit most tick-transmitted diseases of importance to dogs and cats in the United States (Figures 1-6). Larvae and nymphs of the soft tick Otobius megnini (spinose ear tick) cause otic acariasis in dogs and cats, which is associated with acute otitis externa.

Approach to the Diagnosis of Tick-Transmitted Disease

Clinical signs of tick-transmitted disease in dogs and cats vary depending on the pathogen (Table 2). Fever, splenomegaly and thrombocytopenia are common non-specific findings in animals with tick-borne illness, and may be a reason for performing specific diagnostic tests for tick-transmitted pathogens. Tick-borne illness may be associated with secondary immune-mediated phenomena, such as polyarthritis and glomerulonephritis. Understanding the geographic distribution, pathogenesis and time-course of each tick-borne disease can help the clinician decide the best diagnostic test(s) to choose.

In some cases, the organism itself may be visualized in blood smears or tissues from sick animals (Figure 7). The most commonly used laboratory diagnostic tests for tick-borne illness are either serological tests that detect antibody, or PCR tests for pathogen DNA. Some diseases, such as borreliosis and chronic E. canis infection, can have long incubation periods and so antibody tests are already strongly positive at the onset of illness. An appreciable change in antibody titer may not occur in these situations, and it can be difficult to determine whether the animal's disease is truly related to the positive test result, or whether the test result just reflects previous exposure. The test result needs to be interpreted in light of the animal's clinical signs. Other diseases, such as granulocytic anaplasmosis, have short incubation periods and are acute diseases. For these diseases, antibody test results may be negative or at a low titer early in the course of illness, and documentation of seroconversion or positive PCR tests are required to prove recent or active infection, respectively. Because canine monocytic ehrlichiosis has acute, subclinical and chronic phases, the test of choice varies depending on the phase of disease. Acute disease diagnosis requires acute and convalescent phase serology and PCR, whereas chronic disease is more readily diagnosed using positive serology, consistent clinical signs, and response to treatment. PCR may be less sensitive in animals with chronic disease compared with animals that have acute disease.

Treatment and Prevention of Tick-Borne Infectious Diseases

Tick-transmitted bacterial diseases generally respond rapidly (within 24 to 48 hours) to treatment with doxycycline. Other differential diagnoses, including other tick-transmitted diseases or primary immune-mediated diseases, should be considered if the animal fails to respond to treatment as expected (Table 3).

Because it takes ticks 24-48 hours to transmit an infectious disease, searching for, and promptly removing, ticks after exposure can prevent tick-transmitted infectious diseases. Unfortunately, nymphal ticks may be difficult to find, and these are the stage that commonly transmit disease. The use of tick preventatives such as amitraz, fipronil and permethrin can also help to prevent these diseases. Only fipronil is approved for use in cats. Control of ticks in the environment through habitat elimination around the home (removal of long grass and brush, leaf litter, trash, weeds etc), control of reservoir hosts such as rodents and deer, and treatment of the local environment with acaracides (such as for indoor infestations with R. sanguineus) should also be considered. Several vaccines are commercially available for prevention of canine Lyme borreliosis. It is currently unknown whether these can prevent Lyme nephropathy, the most serious consequence of infection.

Table 1. Important ticks that transmit disease to dogs and cats in the United States, and the pathogens transmitted (Ticks are pictured as below in Fig. 1 through 6)

Tick Species	Common Name	Pathogens Transmitted	Geographic Distribution
Ixodes pacificus	Western Blacklegged Tick	Borrelia burgdoferi Anaplasma phagocytophilum Bartonella spp*	Western US
Ixodes scapularis (Fig. 1 a and b)	Blacklegged Tick, Deer Tick	Borrelia burgdoferi Anaplasma phagocytophilum Bartonella spp*	Eastern and southeastern US, upper Midwest (MN, WI)
Rhipicephalus sanguineus (Fig. 2 a and b)	Brown Dog Tick	Ehrlichia canis Rickettsia rickettsii Mycoplasma haemocanis Bartonella spp* Babesia canis Hepatozoon canis	Entire US
Dermacentor andersoni (Fig. 3 a and b)	Rocky Mountain Wood Tick	Rickettsii rickettsii Coxiella burnetii Francisella tularensis Bartonella spp*	Northwest (Rocky Mountain states)
Dermacentor variabilis (Fig. 4 a and b)	American Dog Tick	Rickettsii rickettsii Bartonella spp* Francisella tularensis	Eastern half of the US, western CA
Amblyomma americanum (Fig. 5)	Lone Star Tick	Ehrlichia ewingii Cytauxzoon felis Francisella tularensis	Southeastern quarter of the US, including Midwestern states, now extending into northeast
Amblyomma maculatum (Fig. 6)	Gulf Coast Tick	Hepatozoon americanum	Southeastern and south-central (OK, KS, AK) US

*Bartonella DNA has been found in these ticks, but the extent to which tick transmission occurs is not well understood.

Figure 1a. Ixodes scapularis female	Figure 1b. Ixodes scapularis male
Figure 2a. Rhipicephalus sanguineus female	Figure 2b. Rhipicephalus sanguineus male
Figure 3a. Dermacentor andersoni female	Figure 3b. Dermacentor andersoni male
Figure 4a. Dermacentor variabilis female	Figure 4b. Dermacentor variabilis male
Figure 5. Amblyomma americanum	Figure 6. Amblyomma maculatum

Table 2. Clinical signs and diagnosis of selected tick-borne infectious diseases of dogs and cats

Pathogen (Disease)	Organism Morphology; Location	Major Clinical Abnormalities	Diagnosis
Borrelia burgdorferi (Lyme borreliosis)	Spirochete, connective tissues	Fever, polyarthritis, protein-losing nephropathy	Positive serology in conjunction with consistent clinical signs; PCR on synovial fluid
Anaplasma phagocytophilum (Granulocytic anaplasmosis)	Morulae; neutrophils	Fever, lethargy, lymphadenopathy, splenomegaly, uncommonly vomiting, CNS signs	Morulae (commonly seen); 4-fold seroconversion*; PCR on whole blood
Ehrlichia ewingii (Canine granulocytic ehrlichiosis)	Morulae; neutrophils	Fever, lethargy, polyarthritis, CNS signs	Morulae; PCR on whole blood
Ehrlichia canis (Canine monocytic ehrlichiosis)	Morulae; monocytes	Acute: fever, lethargy, lymphadenopathy, splenomegaly, polyarthritis, edema, CNS signs Chronic: pancytopenia, wasting, hemorrhages	Acute disease: Morulae, 4-fold seroconversion*; PCR on whole blood Chronic disease: positive serology in conjunction with consistent clinical signs
Rickettsia rickettsii (Rocky Mountain Spotted Fever)	Not visible with routine light microscopy; endothelial cells	Fever, lymphadenopathy, CNS and ocular signs, polyarthritis, cutaneous necrosis	4-fold seroconversion*; direct FA or PCR on skin biopsies (whole blood less sensitive). Infections with non-pathogenic Rickettsia species result in cross-reacting antibodies in serologic tests
Bartonella spp. (Bartonellosis)	Not visible with routine light microscopy; erythrocytes, endothelial cells	Endocarditis, granulomatous disease, peliosis hepatis, bacillary angiomatosis	ELISA or IFA serology (Bartonella species-specific); Bartonella culture at a specialized laboratory
Babesia spp. (Babesiosis)	Piroplasms; erythrocytes	Fever, lethargy, pallor, icterus, splenomegaly	Identification of piroplasms; IFA serology; PCR
Cytauxzoon felis (Cytauxzoonosis)	Piroplasms (erythrocyes) and schizonts (macrophages)	Fever, obtundation, tachypnea, pallor, icterus, shock, rapid death	Identification or piroplasms, schizonts in tissue aspirates; PCR
Hepatozoon americanum (American hepatozoonosis)	Gamonts (circulating neutrophils); cysts, meronts (skeletal muscle)	Fever, weight loss, muscle atrophy, generalized muscle and bone pain, osteoproliferative lesions, purulent ocular discharge, protein-losing nephropathy	Gamonts (rarely found); cysts and meronts in muscle biopsies; PCR on whole blood

*acute and convalescent phase serum samples tested using IFA serology
CNS, central nervous system; ELISA, enzyme linked immunosorbent assay; FA, fluorescent antibody testing; IFA, indirect fluorescent antibody; PCR, polymerase chain reaction

Table 3. Treatment of selected tick-borne infectious diseases of importance in dogs and cats

Disease	Duration	Treatment	Dose
Rocky Mountain Spotted Fever	2 weeks	Doxycycline	5 mg/kg PO q12h
Granulocytic anaplasmosis	2 weeks	Doxycycline	5 mg/kg PO q12h
Granulocytic ehrlichiosis	2 weeks	Doxycycline	5 mg/kg PO q12h
Lyme borreliosis	4 weeks	Doxycycline	5 mg/kg PO q12h
Canine monocytic ehrlichiosis	8 weeks	Doxycycline	5 mg/kg PO q12h
Tularemia	7-14 days	Gentamicin	5 mg/kg SC, IV, IM q24h
Bartonellosis	Unknown	Optimal treatment unknown; azithromycin, doxycycline, enrofloxacin have been suggested
Babesiosis (B. canis)	Repeat in 2 weeks	Imidocarb dipropionate	5-6.6 mg/kg IM once
Cytauxzoonosis	10 days	Atovaquone Azithromycin	15 mg/kg PO q8h 10 mg/kg PO q24h
Hepatozoonosis	2 weeks	Trimethoprim-sulfadiazine/ clindamycin	15 mg/kg PO q12h 10 mg/kg PO q8h
	Long-term	Pyrimethamine, then decoquinate	0.25 mg/kg PO q24h 10-20 mg/kg PO q12h

Figure 7. Morula of Anaplasma phagocytophilum in a neutrophil of a
dog with canine granulocytic anaplasmosis.

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References/Selected Reading:

Carrade DD, Foley JE, Borjesson DL, et al. Canine granulocytic anaplasmosis: a review. J Vet Intern Med 2009;23(6):1129-1141.

Guptill L. Bartonellosis. Vet Microbiol 2010;140(3-4):347-359.

Potter TM, Macintire DK. Hepatozoon americanum: an emerging disease in the south-central/southeastern United States. J Vet Emerg Crit Care (San Antonio) 2010;20(1):70-76.

ACKNOWLEGMENTS:

The images (Figures 5 & 6) of the ticks within come from the CDC and are copied for educational use as provided in their website:
http://www.cdc.gov/ticks/geographic_distribution.html

All other (Figures 1-4) tick pictures have been generously provided for publication by Dr. Dwight Bowman, Professor of Parasitology, College of Veterinary Medicine, Cornell University, Ithaca, NY