Tick-borne diseases

What your dog can’t tell you but you should know

Ticks are much more than just nasty little arachnids: they are highly specialised biological vectors capable of transmitting bacteria and protozoa directly into your dog’s bloodstream. When feeding, they insert their mouthparts — the hypostome, a kind of serrated stiletto — and release saliva containing anaesthetic and immunomodulatory substances that block pain and inflammation. Thanks to this “biochemical cocktail,” they can feed for many hours undetected, while discreetly introducing Babesia, Ehrlichia, Borrelia, Rickettsia, or Hepatozoon into the animal’s body.

In citizen science studies conducted in northern Europe, it was found that more than 80% of the ticks sent to laboratories came from dogs and cats, and that approximately 17% of these ticks carried at least one pathogen such as Borrelia or Rickettsia. This confirms that companion animals act as veritable “biological vacuum cleaners” for ticks in the environment and as sentinels for the circulation of diseases in a region (Sormunen et al., 2025).

In Spain, a multicentre study involving 4,643 dogs from 111 clinics showed that more than one in five dogs (22.14%) was infected with at least one vector-borne disease (heartworm, leishmaniasis, anaplasmosis or ehrlichiosis), and that infection with Ehrlichia canis reached 4.26% of the animals analysed. Furthermore, co-infections are not a statistical rarity: a significant proportion of positive dogs were infected with two or even three pathogens at the same time, which complicates both the clinical picture and therapeutic decisions.

Over a ten-year period, 1,169 dogs and cats suspected of having haemoparasitosis were evaluated at a veterinary hospital in Portugal. A total of 3,791 tests were performed, and 437 animals tested positive for at least one of the four haemoparasites studied (Rickettsia conorii, Babesia canis, Ehrlichia spp., Haemobartonella spp.). The prevalence increased from 29.5% in the period 2015–2019 to 39.9% between 2020–2024, suggesting that the circulation of vectors and pathogens is increasing over time, not decreasing.

Globally, a systematic review and meta-analysis published in 2024 estimated that the worldwide prevalence of canine babesiosis is approximately 12%, with Europe leading at 20.7%, and that Babesia canis is the most prevalent species with a frequency of close to 21.6%. This level of circulation confirms that babesiosis should be considered one of the first suspicions when a dog with exposure to ticks presents with anaemia and systemic signs.

With this data in mind, it makes sense to rank tick-borne diseases from most dangerous and relevant to least frequent or least serious, starting with babesiosis and ending with hepatozoonosis, without forgetting that all deserve to be taken seriously from a clinical perspective.

Babesiosis: the silent destruction of red blood cells

In canine babesiosis, protozoa of the genus Babesia —especially Babesia canis in Europe— invade erythrocytes and destroy them from within, triggering haemolytic anaemia that can be fulminant. It is considered a disease with severe clinical impact, as it can progress to acute renal failure, neurological disorders or systemic inflammatory response syndrome.

The aforementioned global meta-analysis places the average prevalence of canine babesiosis at 0.120 (12%), with a European peak of 0.207 (20.7%) and a prevalence of Babesia canis close to 21.6%. These figures place babesiosis at the centre of the map of canine vector-borne diseases in Europe.

In Europe, transmission is mainly linked to ticks such as Dermacentor reticulatus and, in some areas, Rhipicephalus sanguineus. In Spain, DNA from Babesia canis and Hepatozoon canis has been detected in ticks collected from dogs in different regions, confirming the simultaneous circulation of several agents in the same vectors.

In a peri-urban park in the United Kingdom, dogs were monitored with GPS while walking and the risk of tick infestation was quantified. The study showed that the most important risk factor was not the exact route or duration of the walk, but the frequency: dogs that visited the park four or five times a week were significantly more likely to bring ticks home than those that visited once a month. This finding emphasises that repeated exposure, even in seemingly ‘controlled’ peri-urban environments, significantly increases the risk of babesiosis and other vector-borne diseases.

Clinically, dogs may present with high fever, very pale or yellowish mucous membranes (jaundice) and dark urine due to haemoglobinuria. Exercise intolerance is marked, and everyday tasks such as climbing stairs or playing for a few minutes cause immediate exhaustion. In fact, in many dogs living in rural areas, the first sign of piroplasmosis is not fever or anaemia, but rather that they “no longer want to play as before” or lag behind on walks. This subtle change in daily energy levels usually occurs several days before the urine darkens or the mucous membranes become pale.  In complicated cases, signs of acute renal failure and coagulation disorders may appear. In all these cases, specific treatment—usually with imidocarb dipropionate—is combined with fluid therapy and, if necessary, transfusions to sustain the animal until the parasitaemia is controlled.

Your vet advises – Piroplasmosis

If your dog has ever had piroplasmosis, it is advisable to be particularly strict about tick prevention for the rest of its life. Many dogs that have had babesiosis are left with some damage to their immune or renal systems, and a new infection, even a mild one, can cause them to become unwell much more easily than a dog that has never had it.

From the owner’s point of view, simple actions such as checking their coat after a walk, detecting and manually removing ticks, and observing any changes in the colour of their mucous membranes or urine are powerful tools for early diagnosis and improving the prognosis.

Ehrlichiosis: sabotaging the immune system

Canine ehrlichiosis is caused by Ehrlichia canis, an intracellular bacterium that invades monocytes and disrupts the functioning of the immune system. It is mainly transmitted by the brown dog tick, Rhipicephalus sanguineus, a species that is extremely well adapted to living with dogs, even indoors and in kennels. A classic study in Castile and León showed a seroprevalence of 19.2% of E. canis in dogs in that region, with an impressive 53.3% in the province of Salamanca, especially in country and guard dogs.

The most recent national study, involving 4,643 dogs, confirmed that 4.26% of the animals had antibodies to Ehrlichia canis and that vector-borne diseases are distributed throughout all of Spain’s autonomous communities.

The disease usually progresses in three stages. In the acute phase, fever, apathy, loss of appetite and thrombocytopenia (low platelet count) appear, manifesting as bruising, nosebleeds or small red spots on the mucous membranes. In fact, one of the signs that most surprises owners is the appearance of small red or purple spots on the gums or belly skin. These are not insignificant “pins and needles”: they are mini-haemorrhages caused by the drop in platelet count. When detected early, they often allow the disease to be diagnosed in its acute phase, before it becomes chronic. In the subclinical phase, the dog may appear completely normal while the bacteria hide in organs such as the spleen and bone marrow. In the chronic phase, ehrlichiosis can lead to severe anaemia, weight loss, uveitis, recurrent secondary infections and even organ failure.

Studies on preventive treatment compliance among dog owners have shown that, although most recognise the importance of protecting their animals throughout the year, in practice many apply the products for fewer months than recommended by their vets. In a study of 559 owners at 24 clinics, it was found that owners protected their dogs for an average of 1.4 months less per year than was recommended for optimal control. This gap between recommendation and actual practice contributes to the continued transmission of Ehrlichia and other pathogens.

Your vet advises you – Ehrlichiosis

In a dog that has had ehrlichiosis, any future surgery (dental cleaning, late sterilisation, removal of a lump, etc.) should be planned with a prior analysis that includes platelets. Even if the dog is clinically well, a low count can completely change the way it is anaesthetised and the type of monitoring it will undergo during the procedure. 

As for treatment, doxycycline for at least 28 days is the antibiotic of choice, and full adherence to the protocol is key. In chronic phases or with immune-mediated complications, it may be necessary to add transfusion support or immunomodulators, always with a very careful assessment of the risk-benefit balance. At home, checking beds, blankets, sofas and kennels, especially in spring and summer, helps to detect the presence of ticks waiting for a new host.

Borreliosis (Lyme disease): the joint phantom

Lyme disease in dogs is caused by bacteria of the Borrelia burgdorferi sensu lato complex, transmitted mainly by the Ixodes ricinus tick. It is known that effective transmission of the bacteria often requires the tick to remain attached for 24–48 hours.

A large seroprevalence study in European dogs reported Borrelia positivity rates of around 2–3% on average, with significant regional variations aligned with the distribution of Ixodes ricinus.

In dogs, borreliosis manifests as migratory lameness (the painful leg changes over time), low-grade fever, stiffness and joint discomfort, and even Borrelia-associated nephritis in some cases, due to deposits of immune complexes in the renal glomerulus. In humans, the same bacterium can cause the typical migratory target erythema, chronic fatigue and neurological disorders if not treated in the early stages.

Note “One Health” – Borreliosis (Lyme)

Dogs living in areas at risk of Lyme disease are not only patients, they are also “living sensors” of the environment. When dogs in an area begin to be diagnosed with borreliosis, interest in Lyme disease among the human population usually increases and information on appropriate clothing, correct tick removal and monitoring of target erythema in people improves. 

From the practical perspective of the owner, any walk through tall grass or woods in areas where Ixodes ricinus is present carries a risk. The best preventive tool is a combination of effective antiparasitic treatments and a manual check of the coat when returning home. When an attached tick is detected, it should be removed with a tick hook, grasping it as close to the skin as possible, pulling gently without twisting or using irritating products, and then disinfecting the area.

In cases of unexplained lameness that is not due to obvious trauma, especially if the dog has had ticks in recent weeks, it is worth discussing with your vet the possibility of serological testing for Lyme disease. Early treatment with doxycycline for several weeks usually greatly improves the prognosis.

Rickettsiosis: the fever that resembles “dog flu”

Rickettsiosis in dogs is caused by bacteria of the genus Rickettsia, with Rickettsia conorii being one of the most relevant in the Mediterranean area, also implicated in Mediterranean spotted fever in humans. These infections can cause high fever, anorexia, lethargy, lymphadenopathy (swollen lymph nodes) and, occasionally, skin lesions or haematological disorders such as thrombocytopenia and mild anaemia.

In Spain, different species of Rickettsia have been detected in ticks collected from dogs, indicating that these animals act as important hosts in the transmission cycle.

Although many cases in dogs may go unnoticed or be confused with “canine flu”, rickettsiosis takes on special importance when the animal lives with young children, elderly people or immunocompromised individuals, as the environment is shared and some rickettsiae are zoonotic. In practice, if a dog with a history of ticks has a fever lasting several days, noticeable apathy and low platelet counts in blood tests, it is reasonable for the vet to include rickettsiosis in the differential diagnosis and consider empirical treatment with doxycycline, which is also the standard therapy in humans.

Your vet advises you – Rickettsiosis

Although rickettsiosis in dogs is usually less noticeable than piroplasmosis or ehrlichiosis, in households with elderly people, young children or immunocompromised individuals, it is advisable to take it very seriously. Keeping your dog well protected against ticks is an indirect way of protecting your family too, especially in Mediterranean areas. 

For the owner, the most effective measure is still to limit contact with ticks as much as possible by using regular antiparasitic treatments, checking the dog’s coat and not allowing it to sleep in heavily infested areas such as cluttered sheds, damp storage rooms or piles of unchecked firewood.

Hepatozoonosis: the infection that enters through the mouth

Canine hepatozoonosis is mainly caused by Hepatozoon canis in the Mediterranean area. Unlike the previous diseases, dogs do not become infected through bites, but by ingesting an infected tick, either by biting themselves due to itching or by swallowing small parasitised prey. The parasite invades leukocytes and muscles, causing intermittent fever, weight loss, weakness and, in some cases, muscle pain.

In a molecular study conducted on stray dogs in south-eastern Spain, Hepatozoon canis was detected in 13% of dogs analysed by PCR, often in co-infection with other haemoparasites.

Although many infections may be subclinical, in weakened dogs or those co-infected with other vector-borne pathogens, hepatozoonosis can contribute to chronic inflammation, with anaemia, progressive loss of muscle mass and decreased exercise tolerance.

In cats, feline cytauxzoonosis is beginning to be described more frequently in Europe: a case-control study showed that many cats infected with Cytauxzoon spp. were subclinical, while a smaller percentage developed severe disease, reinforcing the role of cats as a possible silent reservoir for certain tick-borne haemoparasites.

To reduce the risk of hepatozoonosis, dogs should be prevented from eating small dead animals or hunting remains, and excessive licking or biting due to itching should be monitored in areas with ticks. Environmental hygiene, without the accumulation of organic debris that attracts small mammals, also helps to break the cycle.

Co-infections: when a single tick brings several problems

Field evidence confirms that ticks do not usually come alone, either figuratively or literally. In the Spanish study by Montoya-Alonso et al. (2020), it was observed that a significant proportion of positive dogs had co-infections, i.e. a combination of two or more vector pathogens.

In addition, DNA from Babesia canis, Hepatozoon canis and other piroplasms was detected simultaneously in ticks collected throughout the peninsula, demonstrating that a single tick can act as an ‘infectious cocktail’.

Practical note – Hepatozoonosis and co-infections

Hepatozoonosis alone sometimes causes very subtle symptoms; the condition becomes more complicated when it is accompanied by another tick-borne disease (such as babesiosis or ehrlichiosis). A dog that “does not fully recover” from piroplasmosis, despite having received the correct treatment, may be hiding underlying hepatozoonosis, which explains its chronic weakness. 

In clinical practice, this means that a dog with fever, anaemia, bleeding and lameness may be suffering from babesiosis and ehrlichiosis at the same time, or a combination of babesiosis with hepatozoonosis and leishmaniasis, making the clinical picture more serious and recovery slower. Therefore, in dogs with complex systemic signs and clear exposure to ticks, it makes a lot of sense to request expanded diagnostic profiles rather than looking for a single agent.

Comprehensive prevention: small routines, big impact

The best way to protect your dog against babesiosis, ehrlichiosis, borreliosis, rickettsiosis and hepatozoonosis is not a single miracle cure, but a set of simple habits maintained over time. The European ESCCAP guidelines remind us that ticks such as Ixodes ricinus, Dermacentor reticulatus and Rhipicephalus sanguineus are widely distributed throughout Europe — including Spain — and that prevention should be considered throughout the year, adapting the protocol to the area and the dog’s lifestyle.

Applying effective external antiparasitics at the exact recommended frequency, checking the coat after walks and keeping the environment (garden, kennels, doghouses) clean and clear of weeds are decisions that, when combined, drastically reduce the likelihood of your dog developing any of these diseases. Turning the check-up into a pleasant ritual — a moment of petting and quiet exploration when you get home — not only strengthens the bond, but can also be the difference between detecting a tick in time or discovering a serious illness months later.

References and links for consultation

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https://onlinelibrary.wiley.com/doi/full/10.1002/vms3.1427

Matijatko V. et al. (2012). Canine babesiosis in Europe: how many diseases? Trends in Parasitology 28(3):99–105.

https://www.sciencedirect.com/science/article/abs/pii/S1471492211001991

Montoya-Alonso J.A. et al. (2020). Current Distribution of Selected Vector-Borne Diseases in Dogs in Spain. Frontiers in Veterinary Science 7:564429.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7643126/

Sáinz A. et al. (1996). Seroprevalence of canine ehrlichiosis in Castilla-León (north-west Spain). Preventive Veterinary Medicine 29(1):1–8.

https://www.sciencedirect.com/science/article/abs/pii/S0167587796010604

Estrada-Peña A. et al. (2017). Species of ticks and carried pathogens in owned dogs in Spain: results of a one-year national survey.

https://pubmed.ncbi.nlm.nih.gov/28188085/

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https://link.springer.com/article/10.1007/s00436-022-07705-2

VISAVET (UCM). Lyme disease – Disease information.

https://www.visavet.es/infequus/en/lyme-disease.php

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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9169295/

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ESCCAP (2024). Guideline 5: Control of vector-borne diseases in dogs and cats in Europe.

https://www.esccap.org/uploads/docs/32ir16g1_0775_ESCCAP_Guideline_GL5_20241203_1p.pdf

CAPC – Companion Animal Parasite Council.

https://capcvet.org

Sormunen J.J. et al. (2025). Ticks and Tick-Borne Pathogens Encountered by Dogs and Cats: A North European Perspective.

https://pmc.ncbi.nlm.nih.gov/articles/PMC12158588/

Calouro C. et al. (2025). Tick-Borne Pathogens in Companion Animals and Zoonotic Risk in Portugal: A One Health Surveillance Approach.

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https://pmc.ncbi.nlm.nih.gov/articles/PMC3895857/

Lavan R. et al. (2017). Assessment of dog owner adherence to veterinarians’ flea and tick prevention recommendations in the United States.

https://pmc.ncbi.nlm.nih.gov/articles/PMC5460448/

Carli E. et al. (2022). Clinicopathological findings and risk factors associated with Cytauxzoon spp. infection in cats.

https://www.frontiersin.org/articles/10.3389/fvets.2022.976173/full