Canine Vector-Borne Disease and Hemoparasite Prevalence in North America

Scanning electron micrograph of the bacteria Borrelia burgdorferi. Source: CDC 2011
Scanning electron micrograph of the bacteria Borrelia burgdorferi. Source: CDC 2011
March 3, 2023

Dr Lizzie Youens BSc(Hons) BVSc MRCVS 

It is mid summer and you are practicing in Chicago. A new canine patient who has never traveled out of the area presents with lethargy, fever, anorexia, and mildly enlarged lymph nodes. You start with an in-house CBC and see that all of the cell lines are low except for the neutrophils.

As you build your differential list with what you know thus far, is vector-borne disease on your radar?

In North America, regardless of your practice region, you may regularly encounter vector-borne disease. Depending on where you are within the US, the prevalence of one disease is likely higher than another.  When you are suspicious that your patient has a hemoparasite, do you reach for your microscope and blood smear?

Vector-borne disease is a growing problem across the United States, especially with high concentrations of tick-borne disease. While the climate and habitats vary across the country, it is well recognized that vector-borne disease has long become a global problem, not merely restricted to less temperate climates. Depending on where you practice in the world, the same disease like Anaplasma platys may present slightly differently or act more aggressively.  

Hemoparasitic disease is of concern for many species including humans. The zoonotic potential of many of these diseases means that this is a public health concern as well. Read on to learn more about canine blood parasites and what this means for dogs from cities to farms.

What is a canine hemoparasite?

Hemoparasites are defined as any organism which lives within the blood cells of a host animal. The term can cover multiple types of microorganism, from bacteria to protozoa to nematode and rickettsiae. One shared characteristic is that they are usually transferred between hosts via a vector such as a tick, fly, or other insect. 

Hemoparasites will not be visible during all stages of the disease. For instance, only 2/19 dogs will have prevalent hemoparasites in cases of Erlichia canis.  (Ettinger et. al, 2010)

What hemoparasites affect dogs?

There is a long list of organisms that can live in the blood cells of dogs. It would be an exhaustive process to go through every individual pathogen, but there are a group of common organisms transmitted by ticks and flies that commonly cause clinical disease to dogs in the US. 

These canine hemoparasites are most likely to cause disease, and are therefore the focus of a parasite exam in most veterinary settings, including a veterinary pathology laboratory.

A list of the most commonly encountered hemoparasites in the USA:

Anaplasma phagocytophilium

Anaplaysma platys

Babesia gibsoni

Babesia vogeli

Hepatozon ameriacanum

Hepatozoon canis

Bartonella henselae

Borrelia burgdorferi

Borrelia burgofria & mayonii

Borrelia hernsii & turicate

Dirofilaria immitis

Ehrlichia canis

Erlichia chaffeensis

Erlichia ewingii

Neorickettsia helminthoeca

How do you detect hemoparasites in dogs?

Due to the prevalence of both vectors and parasites, and the potential implications for both canine and human health, regular screening diagnostics for parasites are recommended by the AAHA during yearly wellness visits. This may include, depending on the species of patient, a complete blood count with pathology review, vector-borne disease ELISA testing, and a fecal ova and parasite exam. 

In most cases of patients presenting with classic hemoparasite clinical symptoms such as lethargy, fever, anorexia and varying hemorrhagic disorders, the clinician is likely to inquire about travel history, flea and tick preventative, and establish a diagnostic minimum database which includes a CBC (complete blood count), biochemistry profile, ELISA testing for vector-borne disease, fecal float, and direct smear and urinalysis. 

Based on the physical examination, clinical history, and diagnostic test results, the clinician can properly prioritize vector-borne disease in their differential list. Identification of morulae (bacterial clusters) within specific cells can help narrow which vector-borne disease to worry about. However, what would you do if you found a morulae in a neutrophil? Is it Ehrlicha or Anaplasma? 

Choosing the right test, based on the stage of disease is critical. There are many tests available to diagnose vector-borne diseases. This may seem exciting, but some tests will have higher sensitivity and specificity than others depending on the stage of the disease.

An ELISA (enzyme-linked immunoassay) test is commonly used as a first-line screening tool. This combo test includes an antigen test available for Dirofilaria immitis, and an antibody test for Erhlichia sp, Anaplasma sp. and Borrelia burgdorferi. IFA (indirect fluorescent antibody) is another common option that assesses for the presence of antibodies to a specific disease in a sample. Since these two tests look for antibodies, detection of the disease relies on the body’s immune response. If an Ab-based test is run too early in the disease stage, it will be negative, even though the disease is present. Practitioners must also recognize that this combo ELISA test has some cross-reactivity.

This is where PCR comes in. PCR tests identify segments of DNA specific to a particular disease. PCR is an excellent tool for diagnosing vector-borne diseases in the early stages as this is when bacteriemia is most likely. It also helps solve the problem of cross reactivity within some antibody tests. 

Other diagnostics can be helpful depending on which vector-borne disease elicits the highest concern. If salmon poisoning was a top differential, then a fecal float with sugar should be performed. Some organisms can be cultured, and it is important in these cases to provide a clinical history so the right culture media can be used.

Morula of Ehrlichia canis in the cytoplasm of a monocyte. Source: Texas Veterinary Medical Diagnostic Laboratory

Geographic prevalence

Getting a travel history for each canine patient is very important when there is concern for vector-borne disease. Knowing where your patient has been within the United States, relevant diseases in that region, and the incubation times for each disease can help understand if concerns may be valid.   

The Companion Animal Parasite Council (CAPC) has been compiling large datasets and prevalence maps of canine hemoparasites. Each year, the number of vector-borne disease cases rises. The CAPC is an excellent resource as it has a series of maps indicating the prevalence of various parasites in companion animals across the United States.

Based on the data initially collected by CAPC, North America was split into four regions: Midwest, West, Northeast and Southeast.

D. immitis showed higher prevalence generally in the Southeast (2.9%), and lowest in the Northeast. Certain counties had results that were much higher than expected, including Summit County, Utah and Belknap County, New Hampshire. B. burgdorferi antibody results were highest in the Northeast (13.3%), and lowest in the Southeast and West. Lyme disease is known to be endemic in certain areas such as Virginia (9.7%) and West Virginia (3.5%). Antibodies to Ehrlichia were highest in the Southeast (3.2%), and lowest in the Northeast and Midwest. Prevalence of Erhlichia was found to be rising. Anaplasma spp. were highest in the Northeast (7.1%) and Midwest (3.9%).

Microfilariae of the canine heartworm, Dirofilaria immitis. Source: Adobe images

Factors Affecting Prevalence: City vs Rural

Factors influencing hemoparasite prevalence via tick vectors were found to include vector distribution, abundance and activity, host factors such as deer and small mammal populations, abiotic factors such as precipitation and temperature, habitat variation and social factors which influence the exposure of dogs to ticks, such as access to preventative care (Stich et al., 2014). 

Studies looking at vector factors such as distribution and abundance have found general trends support a higher incidence of tick-related disease in more rural areas compared to urban. (Duffy et al., 1994) 

Epidemiologists focused on Lyme disease have commented that this tick-borne disease appears to be widening its area of concern. Most Lyme infections historically are found in rural areas. However, in 2007, a review was done in the city of Chicago to assess the concentration of Lyme disease in the ticks in the area. Despite its urban landscape, the prevalence was so high that recommendations for the prophylactic use of doxycycline for human tick bite patients were announced. (Jobe et al., 2007). The introduction of ticks into urban areas is thought to be affected by wild bird populations (Hamer et al., 2012).


Treatment options regardless of diagnosis often revolve around tetracycline administration, specifically Doxycycline. Although some practitioners over the years have also relied on fluroquinolones, specifically Enrofloxiacin, there are some hemoparasitic diseases that do not respond as expected such as Ehrlichia canis. Due to an intrinsic gyrase-mediated resistance to this drug class, clinical improvement may be seen, but full resolution not achieved.(Ettinger et. al, 2010) Conversely, treatment for canine bartonellosis calls for combination therapy.(Ettinger et. al, 2010)  

After therapy, depending on which hemoparasite is present, clinical signs may or may not have resolved. Interestingly, cytopenias associated with the severe chronic form of E. canis may persist for several months. Rechecking CBC with pathology review is recommended for all patients 1 and 3 months post-conclusion of therapy.

Canine hemoparasites: conclusion

Understanding vector-borne disease as a veterinarian is essential as it affects both pet and human health. The widening geographic regions of disease and zoonotic potential make obtaining a definitive diagnosis via the available methods a worthy goal. Although certain regions may be known for certain types of vector-borne disease, it is clear that an open mind must be kept when trying to dig further into these cases. 

It may not have been clear at first, but our new patient in Chicago in fact was suffering from vector-borne disease. After performing an in-house ELISA, a positive result for Anaplasma sp. and for B. burgdorfi, the causative agent of Lyme disease, was obtained. These positive tests provided our definitive diagnosis of Anaplasmosis. Since it is not uncommon for co-infections to occur, diagnostics including a biochemistry, UA, and radiographs were performed. Since proteninuria was identified, a Lyme C6 Quantification was submitted. Therapy with Doxycycline was initiated. Lyme C6 quantification was >30u/mL confirming a diagnosis of Borreliosis. Gratefully, the patient was already on the right treatment. 


Ettinger, Edward C. Feldman. Textbook of Veterinary Internal Medicine : Diseases of the Dog and Cat. Philadelphia :W.B. Saunders Co., 2010.

Duffy, D., Clark, D., Campbell, S., Gurney, S. & Simon, N. (1994) ‘Landscape patterns of abundance of Ixodes scapularis on Shelter Island, New York.’ Journal of Medical Entomology 31(6)

Gotsch, S., Leschnik, M., Duscher, G. & Burghstaller, G. (2009) ‘Ticks and haemoparasites of dogs from Praia, Cape Verde’ Veterinary Parasitology 166(1-2) pp.171-174

Hamer, S., Goldberg, T., Kitron, U., Brawn, J., Anderson, T & Loss, S. (2012) ‘Wild birds and urban ecology of ticks and tick-borne pathogens, Chicago, Illinois, USA, 2005-2010’ Emerging Infectious Diseases Journal 18(10)

Jobe, D., Nelson, J., Adam, M. & Martin, S. (2007) ‘Lyme Disease in Urban Areas, Chicago.’ Emerging Infectious Diseases Journal 13(11)

Little, S., Beall, M., Bowman, D. & Stamaris, J. (2014) ‘Canine infection with Dirofilaria immitis, Borrelia burgdorferi, Erhlichia spp. and Anaplasma spp. in the United States, 2012-2012’ Parasites & Vectors 7(257)

Stich, R., Blagburn, B., Bowman, D., Carpenter, C., Cortinas, M., Ewing, S., Foley, D. & Foley, J. (2014) ‘Quantitative factors proposed to influence the prevalence of canine tick-borne disease agents in the United States.’ Parasites & Vectors 7(417)


Canine Vector-Borne Disease and Hemoparasite Prevalence in North America

Canine blood disease emanating from hemoparasites is mostly thought of as a rural problem, but what happens when dogs get sick in cities? Read on to learn more about rural versus urban prevalence of canine hemoparasites in the United States.

March 3, 2023
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