Anaplasmosis in Cats: What Happens After a Tick Bite

Anaplasmosis in cats is a tick-borne bacterial infection caused primarily by Anaplasma phagocytophilum, a pathogen that invades white blood cells and triggers fever, lethargy, and low platelet counts. European guidelines from the Advisory Board on Cat Diseases identify it as the most clinically significant rickettsial infection in cats, though it remains underdiagnosed partly because its symptoms look like so many other feline illnesses.¹ The infection is treatable, but both the disease and the standard drug carry risks worth understanding.

How Cats Pick Up the Infection

Anaplasma phagocytophilum spreads through the bite of Ixodes ticks, the same group of hard-bodied ticks that transmit Lyme disease in people and dogs.² Different Ixodes species dominate in different regions: I. ricinus in Europe, I. scapularis (the blacklegged or deer tick) in eastern North America, and I. pacificus on the U.S. west coast. Cats that spend time outdoors in wooded or brushy areas where these ticks live are at highest risk.

Transmission is not instant. Based on studies in humans and dogs, the tick needs to be attached and feeding for roughly 24 to 48 hours before the bacterium passes into the host.³ That delay is practically important: if you find and remove a tick from your cat within the first day, the chance of transmission drops significantly. Cats that groom themselves thoroughly may remove some ticks before the window closes, which could partly explain why confirmed feline cases are less common than in dogs, though that is hard to prove.

Which Anaplasma Species Infect Cats

A. phagocytophilum gets most of the veterinary attention because it is the species most clearly linked to clinical disease in cats. It targets neutrophils, a type of white blood cell, and causes what veterinarians sometimes call granulocytic anaplasmosis. But a second species, A. platys (and closely related strains), has also been detected in cats. In a molecular survey of cats in Rio de Janeiro, about 4% tested positive for A. platys by one PCR method, while none were positive for A. phagocytophilum.⁴

The distinction matters because the two species behave differently inside the body. A. phagocytophilum infects neutrophils, while A. platys-like strains in cats show a preference for platelets, more like what happens in dogs infected with A. platys.⁵ The clinical picture of A. platys infection in cats is still not well characterized, partly because molecular surveys that find positive cats often do not include detailed follow-up on symptoms. Most published case reports and clinical guidelines focus on A. phagocytophilum, so the rest of this article does the same unless noted otherwise.

What the Bacteria Do Inside the Cat

Once a tick delivers A. phagocytophilum into the cat’s bloodstream, the bacteria latch onto neutrophils using a molecule called P-selectin on the cell surface. The bacteria then slip inside the cell through endocytosis, essentially hijacking the neutrophil’s own intake machinery. Inside the cell, the organisms replicate within membrane-bound compartments and form clusters called morulae, which are sometimes visible under a microscope as dark-staining dots inside white blood cells.⁶

The bacterium is remarkably good at disabling its host cell. It shuts down the neutrophil’s ability to move toward infection sites, hampers the cell’s capacity to engulf other pathogens, suppresses the release of toxic oxygen molecules normally used to kill bacteria, and delays the programmed cell death that would ordinarily destroy an infected cell. In short, the pathogen turns the cat’s own immune cells into safe houses for replication while simultaneously weakening the immune response.⁷ This immune suppression helps explain why co-infections with other tick-borne or opportunistic pathogens can sometimes complicate the picture.

Signs and Symptoms to Watch For

The frustrating reality of feline anaplasmosis is that it looks like a lot of other things. The clinical signs are nonspecific: fever, lethargy, and reduced appetite are the hallmarks.⁸ A review pooling published cases found that roughly 94% of infected cats showed lethargy, 88% had elevated body temperature (ranging from about 39°C to nearly 42°C), and 76% showed appetite loss.⁹ A separate study of PCR-confirmed cases in Germany, Austria, and Switzerland reported similar numbers, with lethargy and fever each appearing in 83% of positive cats.¹⁰

Beyond the big three symptoms, some less expected signs turn up in a meaningful fraction of cats:

Painful limbs or abdomen: About 30% of cats in the literature review showed pain on palpation of the limbs or belly.
Conjunctivitis: Reddened, inflamed eyes appeared in roughly a third of reported cases.
Dehydration: Present in about 15% of cats.
Pale gums, breathing difficulty, or rapid heart rate: Each seen in around 9% of cases.

These figures come from a pooled analysis of 33 to 34 cats described across multiple published reports.¹¹ That is not a large sample, and case reports tend to capture sicker animals, so the true clinical spectrum in the wider cat population could be broader or milder than what these numbers suggest.

Blood Work and Diagnosis

Thrombocytopenia, a drop in platelet count, is the most characteristic laboratory finding. It showed up in about 59% of cats in the literature review, though the true rate is murky because cat platelets clump easily during blood collection, which can make counts look falsely low.¹² Around a quarter of reported cats were anemic, and about 15% had low white blood cell counts, which is somewhat paradoxical for a bacterial infection where you might expect white cells to increase.¹³

Veterinarians can sometimes spot the infection on a standard blood smear by finding morulae, those dark clusters of bacteria inside neutrophils. In a study of fifteen cats in Italy, morulae were found in 1% to 21% of examined neutrophils, and the cats responded to treatment, confirming the diagnosis.¹⁴ Finding morulae is quick and cheap but insensitive: during early infection or at low bacterial loads, a smear can easily come back looking normal even when the cat is infected.

PCR testing on a blood sample is the most reliable confirmation method. It detects bacterial DNA directly and can distinguish between A. phagocytophilum and A. platys. Antibody tests (serology) exist and can show past exposure, but a positive antibody result does not necessarily mean the cat is currently sick. In areas where Ixodes ticks are common, many cats may have been exposed without ever developing symptoms.

Treatment and the Doxycycline Challenge

Doxycycline is the standard treatment. A typical dosing regimen is 5 mg/kg by mouth twice daily, or 10 mg/kg once daily, and most cats improve within a few days of starting the drug.¹⁵ The Italian case series noted that infected cats responded well to doxycycline, and it remains the first-line recommendation in European feline guidelines.¹⁶

The catch is that doxycycline carries a specific and well-documented risk in cats: esophageal damage. If a capsule or tablet lodges in the esophagus, it can cause ulceration and, eventually, scarring that narrows the esophagus (a stricture). A case series documented four cats that developed esophageal strictures after receiving doxycycline. All presented with regurgitation that began at variable times after starting the drug.¹⁷ Another report reinforced that giving doxycycline capsules to cats without adequate fluids increases the risk of esophageal ulceration and scarring.¹⁸

The practical takeaway: whenever you give a cat doxycycline by mouth, follow it immediately with a syringe of water (at least a few milliliters) or a small amount of food to ensure the pill reaches the stomach. Some veterinarians now prefer liquid formulations or compounded suspensions to reduce the risk altogether. If your cat starts regurgitating food during a course of doxycycline, stop the drug and contact your vet immediately.

For cats that cannot tolerate doxycycline, fluoroquinolones may be used as an alternative, though the evidence for their effectiveness specifically against anaplasmosis is thinner than for doxycycline.¹⁹ Your vet might also prescribe supportive care such as intravenous fluids for dehydrated cats or anti-nausea medication for those that have stopped eating.

Co-infections Complicate the Picture

Ticks are not clean syringes carrying one pathogen at a time. A single Ixodes tick can harbor multiple organisms simultaneously, and cats bitten in tick-heavy areas may pick up more than just A. phagocytophilum. The European ABCD guidelines specifically note that co-infections with other rickettsial organisms are possible in cats.²⁰ Lyme disease (Borrelia burgdorferi), babesiosis, and bartonellosis are all transmitted by the same or related tick species and could theoretically be acquired at the same time.

Co-infections matter clinically because they can make symptoms harder to interpret and treatment more complicated. A cat that is not responding as expected to doxycycline for suspected anaplasmosis might actually have a concurrent infection driving its symptoms. Veterinarians working in endemic areas increasingly test for a panel of tick-borne pathogens rather than a single organism, especially when blood work and clinical signs do not fit neatly into one diagnosis.

Tick Prevention as the First Line of Defense

Because there is no vaccine against feline anaplasmosis, keeping ticks off your cat is the only reliable way to prevent infection. The literature is emphatic on this point: cats should be protected with ectoparasiticides, particularly in areas where Ixodes ticks are endemic.²¹ Several topical products and collars are labeled for tick prevention in cats, but not all tick products safe for dogs are safe for cats. Permethrin-based products, widely used on dogs, are toxic to cats and should never be applied to them or used in a household where cats might groom a treated dog.

For outdoor cats or those in high-risk areas, monthly topical tick preventives containing ingredients like fipronil or fluralaner are common choices. Your veterinarian can recommend a product appropriate for your cat’s age, weight, and lifestyle. Daily tick checks are a useful complement, especially during peak tick season (spring through fall in temperate climates). Given the 24-to-48-hour transmission window, finding and removing ticks promptly adds a meaningful layer of protection on top of chemical prevention.

The Same Strains That Infect People

One finding that tends to catch cat owners off guard is that the A. phagocytophilum strains infecting cats appear to be the same ones that cause human granulocytic anaplasmosis. A study using three different molecular typing methods found high concordance: the strains in cats consistently matched those known to cause disease in humans, dogs, and horses.²² Phylogenetic analyses in Korean shelter cats have also shown that regional strain variation exists, with some genetic markers differing between isolates from Asia, Europe, and the Americas, but the overall pathogen is the same species across hosts.²³

This does not mean your cat can give you anaplasmosis directly. The bacterium passes through tick bites, not through cat saliva, scratches, or shared living space. But it does mean that a cat carrying the infection in a household signals that infected ticks are present in the local environment. If your indoor-outdoor cat tests positive for A. phagocytophilum, it is worth being more vigilant about checking yourself and other family members for ticks after spending time outdoors in the same area.

Why Feline Cases Are Probably Underreported

Compared to the volume of literature on canine and human anaplasmosis, feline cases are documented in relatively small numbers. The pooled literature review that compiled clinical data drew on only 33 to 34 cats across decades of publications.²⁴ Several factors likely contribute to the low count. Cats are less often tested for tick-borne diseases than dogs. Outdoor cats may recover from mild infections on their own without ever seeing a veterinarian. And because the signs overlap heavily with common feline illnesses like upper respiratory infections or inflammatory bowel disease, a veterinarian who is not specifically thinking about tick-borne disease might not order the right test.

Seroprevalence surveys, which measure antibody exposure rather than active disease, consistently show higher rates of past infection than the clinical case count would suggest. This gap implies that many cats encounter the pathogen and either fight it off with minimal symptoms or develop illness that gets attributed to something else. Veterinary awareness has been growing, particularly in Europe where several research groups have published case series and guidelines in recent years, but in practice the disease remains easy to miss.

When an Outdoor Cat Gets Sick After a Tick Bite

If your cat spends time outdoors, recently had a tick, and develops a fever along with lethargy or appetite loss within one to two weeks, anaplasmosis belongs on the short list of possibilities your vet should consider. Mentioning the tick exposure is helpful because many veterinarians will not default to testing for A. phagocytophilum unless tick contact is part of the history. A blood smear looking for morulae is a reasonable first step, and PCR testing can confirm or rule out the diagnosis if the smear is negative and suspicion remains high.

Treatment outcomes are generally good when the infection is caught and treated. Most cats improve within 24 to 48 hours of starting doxycycline, with fever dropping and appetite returning quickly. Long-term complications are uncommon in otherwise healthy cats, though cats with concurrent immune suppression from feline immunodeficiency virus or feline leukemia virus may have a harder time clearing the infection. Keeping up with tick prevention year-round, rather than only in summer, is the single most effective thing you can do to protect your cat from this and other tick-borne diseases.