Understanding Toxoplasmosis in Cats: Symptoms and Treatment

1. Introduction

Toxoplasmosis is a parasitic infection caused by the organism named Toxoplasma gondii. It infects birds and mammals, including humans. Sometimes it can have severe and harmful effects on human health, primarily if the immune systems of these individuals are compromised. The objective of this chapter is to discuss this disease in cats. T. gondii is an obligate intracellular coccidian parasite belonging to the phylum Apicomplexa. Typically, the only definitive host for T. gondii is felids who ingest the bradyzoites that are contained in the tissues of chronically infected intermediate hosts. In the intestines of cats, sexual reproduction and oocyst spore formation occur, and thousands can be released in the feces of a single cat. Furthermore, the sporulated oocysts can remain infectious in the soil for several years, and this factor can be of crucial epidemiological significance. The age of cats and type of diet consumed can determine oocyst shedding. Although cats can become infected upon ingestion of sporulated oocysts, the more common routes of infection are consumption of raw or uncooked meats, placenta, or tissue. Especially for pet and farm cats, the implications of this from a public health perspective are significant. The disease in cats is considered self-limiting but can have quite diverse clinical signs, from hardly noticeable to life-threatening peracute cases.

1.1. What is Toxoplasmosis?

Toxoplasmosis is an infectious disease caused by a protozoan named Toxoplasma gondii. It is the only naturally occurring coccidian of cats able to infect all groups of vertebrates. T. gondii infection occurs through three well-known ways: 1) direct consumption of T. gondii oocysts shed by the definitive host cat through feces; 2) ingestion of tissue cysts in raw or undercooked meat from T. gondii-infected herbivores and omnivores; and 3) transplacental transmission from mother to fetus. Several intermediate hosts, including humans and wild felids, can be affected by T. gondii, sometimes resulting in dramatically different outcomes. T. gondii infection in immune-competent hosts is usually asymptomatic. However, in congenitally infected people and in immune-compromised patients, T. gondii infection may cause severe or deadly health conditions including abortion, stillbirth, retinochoroiditis, hydrocephalus, encephalitis, and death. Zoonotic T. gondii is widely present across the globe.

Toxoplasmosis is widespread because cats—the only definitive host—live in a variety of habitats on all continents except Antarctica. It is estimated that approximately one third of the global human population has been infected with the parasite; therefore, active signs or serological diagnostic tests may represent an important tool for the diagnosis and management of this disease. Domestic cats infected with T. gondii may produce hundreds of millions of infectious oocysts in their feces that may contaminate the environment and infect several other host species. This fact also highlights the complex biology of cats, and to be properly managed, cats need to be understood by their caregivers from a biological and ecological perspective. This review seeks to highlight the main and most significant aspects of Toxoplasmosis from the parasite standpoint in relation to the host: its relevant biological aspects, epidemiological data, transmission, diagnosis, prevention, and studies focusing on the Brazilian scenario.

1.2. Prevalence in Cats

Toxoplasma gondii is a common apicomplexan parasite that can infect almost all warm-blooded animals, but cats are its definitive host. A variety of studies have shown a high prevalence of T. gondii in cats worldwide, ranging from 3.9% to 42.7% in domestic cats and up to 54% or 82% in wild cats. There are many factors leading to the high prevalence in cats. Domestic cats in cities have less chance of hunting than wild cats, but their prevalence of T. gondii is still high. This is mainly due to environmental and lifestyle factors. Many domestic cats are very fond of living outdoors and have a diet supplemented by prey, which puts them at risk of disease. Outdoor living is also associated with poor hygienic practices. Feline species that spend more time outdoors or have increased access to hunting are more likely to come into contact with infected rodents, raw meat, or other sources of T. gondii. Street cats and mixed-breed cats have a higher prevalence, which is mainly related to active hunting. This may lead to a higher chance of being infected than those that stay at home. In addition, stray cats account for a high proportion of T. gondii prevalence. Communities may focus more on the adoption of unwanted mature cross-breeds, and the prevalence of T. gondii in older animals is higher.

All of this means that owning a cat can pose a potential risk of infection to cat owners. A questionnaire survey found that keeping an outdoor cat can significantly increase the risk of exposure to T. gondii. Exposure factors are multifactorial and also include feeding meat and fish to their cats, keeping and using a sandpit together with their cats, and to a lower extent gardening with their cats. Owning a cat was a risk factor for humans to have Toxoplasmosis. In light of this, it is necessary for breeders to understand the general situation of Toxoplasmosis in cats. On the one hand, reasonable health education for the general public can effectively reduce the number of patients with Toxoplasma infection, and on the other hand, it can provide a theoretical basis for the prevention of cats infected with T. gondii. As an obligate feline parasite, T. gondii requires cats to complete its life cycle, mainly in the epithelial cells of the small intestine or lymphatic tissue.

2. Symptoms of Toxoplasmosis in Cats

Toxoplasmosis manifests in cats through a wide range of symptoms, from no signs of illness at all to severe organ disease. Acutely ill cats primarily display respiratory and gastrointestinal symptoms, such as anorexia, weight loss, fever, dyspnea, and nasal discharge. Neurological symptoms such as ataxia, circling, and paresis or paralysis can also be observed. Cats in which the disease takes a more chronic course predominantly show the following symptoms: respiratory problems, jaundice, malaise, retinal scars, and enlarged lymph nodes and abdominal organs. Other symptoms that are sometimes observed in chronically infected cats are heart and liver problems. One of the main problems with diagnosing these symptoms is that they are very vague and can be indicative of a range of other conditions as well. This is why many owners of cats that are diagnosed with toxoplasmosis initially consult their veterinarian for totally different complaints.

Several symptoms can be indicative of acute or chronic toxoplasmosis infecting cats. However, as many other diseases occur that present with the same symptoms, it can be tricky to diagnose a cat as having toxoplasmosis. Although this disease can occur without any signs in the cat, it is essential that the illness is detected early and that treatment is started in a timely fashion. Especially if toxoplasmosis presents acutely and manifests with severe symptoms, cats can become very ill and can experience serious consequences due to the infection. In this section, only describe the symptoms that are common with acute and chronic toxoplasmosis in cats. Provide a brief description of each of the symptoms and what they could look like in cats. For example, a cat with toxoplasmosis might present with ataxia, be finicky with its food, and have a fever. It is also important to explain why recognizing acute and chronic symptoms is important for the diagnosis of toxoplasmosis.

2.1. Acute Symptoms

In cases of Toxoplasma gondii infection in cats, both kittens and adults tend to cause very acute symptoms. Symptoms are often sudden in onset. The cat may have a low-grade fever and become quite lethargic, losing interest in the food that used to be its favorite. Acute symptoms can progress to a higher fever, inappetence, and a cat that is very indifferent to its surroundings. The rate and degree with which your cat may worsen in terms of clinical signs cannot be accurately predicted. But in some severe cases, this descent can occur rapidly and necessitate emergency care. It is known that such symptoms occurred in confirmed cases of food-borne toxoplasmosis. The medical term for these types of clinical symptoms that are sudden in onset and worsen rapidly is called "virulent" symptoms. Certain cats that develop toxoplasmosis, especially kittens, can present increased severity of clinical signs, including clinical respiratory distress or in association with neurological symptoms like seizures.

So, for all these reasons, we advise pet owners to be diligently attentive to their cat's health and to seek the rapid evaluation of a veterinarian for any cat showing these symptoms, especially if the cat is indeed infected with toxoplasmosis. Cat owners are very observant, and it usually takes them very little time to notice that something just isn't quite right with their cat. This is precisely the time to avail your pet of the attention of a professional.

2.2. Chronic Symptoms

For some cats, a phase of chronic, intermittent, or chronic progressive illness may develop one to three weeks following acute infection. In other cases, the presence of Toxoplasma parasites in tissues may go unnoticed for months to years. These cats have "chocolate" cysts in the underlying vessels of their retinas. Chronic Toxoplasmosis in cats may also occur in a slow progression, with cats occasionally exhibiting weight loss, intermittent fever, and behavioral changes. Gradual retinal degeneration may occasionally cause these cats to bump into things in low light, another frequently presented symptom in slow progressing cases of Toxoplasmosis. In many cases of chronic infection, cats are highly resistant to Toxoplasma. In these cases, cats can live many years with the infection. However, in some cases, mature cats become more susceptible due to waning immunity.

As in the case of acute infection, toxins generated by T. gondii during bouts of proliferation in tissues may cause recurrent disease over time. Cats may live with only mild signs of illness for some time while suffering long-term damage to their eyes, heart, liver, and pancreas. They are also susceptible to reinfection and may transmit the disease to other animals. Veterinarians advocate regular veterinary check-ups for infected cats to detect these problems early. Regular checks may also facilitate the detection and treatment of infections inevitable as a result of the presence of oocysts in mulch-use beds. The slow onset and gradual progression of clinical signs in chronic Toxoplasmosis may make it difficult to notice changes unless a cat undergoes annual veterinary check-ups. Training the owner to recognize nuances in their cat’s behavior may lead to earlier detection and treatment, enhancing the cat’s quality of life.

3. Diagnosis and Testing

There are two main ways to approach a diagnosis of suspected toxoplasmosis in your cat. The first step that a vet will likely take is the performance of a physical examination that may identify signs of toxoplasmosis, the review of the health history of the cat, and an evaluation of any symptoms that might be present. Discovering if the cat has eaten raw or undercooked meat or whether the cat has had an opportunity to ingest prey will be an important part of the history-taking process. Following is a discussion of the typical presentation of disease and the most commonly associated clinical signs.

In-depth details of the diagnostic tests recognize that full clinical assessment and physical examination are essential parts of the diagnosis of these problems. Therefore, we encourage full discussions with a veterinarian regarding the relative costs and benefits of diagnosis, including those from patient evaluation prior to the decision to test. Subsequent flavor of the type of diagnostic methods that may be used is indicative of the sort of advice that can be attained with better examination of the clinical evidence.

A veterinarian will take advantage of laboratory tests for the definitive diagnosis of toxoplasmosis in cats. Several different serological assays have been described and compared for their ability to detect Toxoplasma-specific antibodies in cats. The primary tests for T. gondii that are available for detection are the sedimentation test, plant agglutination test, and the modified direct agglutination test, which is the clinical gold standard. The indirect enzyme-linked immunosorbent assay remains the most commonly used available test for circulating antibodies, although this test has limitations if the cat has other feline retroviruses or has been vaccinated. These tests measure the presence of specific antibodies in the cat's serum. The quantity of the specific antibody in the serum, and how this quantity changes over time, can be correlated with the progression of infection and indicate if the symptoms match these stages of infection.

3.1. Physical Examination

The physical examination is the first diagnostic step in every veterinary case. It is at that point where the veterinarian can gather observational data on the behavior and clinical signs apparent to the client. The veterinarian may also use this time to observe and make a behavioral diagnosis (i.e., is the cat acting nervous, is the cat dehydrated from vomiting and diarrhea). Eventually, the veterinarian will observe the mucous membranes to see if they are pale, check hydration status, capillary refill time, heart rate, and body temperature. In cats with clinical signs of toxoplasmosis, a thorough physical examination including checking for other causes of lethargy, fever, and weight loss needs to be conducted. A middle-aged young adult cat or kitten with a fever and classic neurological signs should be considered a strong suspect for toxoplasmosis. The veterinary care team also advises that a thorough examination of the chest, abdomen, and neurological status be performed to evaluate disseminated systemic toxoplasmosis.

Veterinarians note that performing a direct ophthalmic examination is often difficult or not possible in cats because of aggression or poor temperament. Based on the information presented, what could or would you do during the physical examination in the toxoplasmosis case in cats? Examination of the cardiovascular and respiratory systems is recommended as progressive toxoplasmosis may affect the heart and lungs. Conducting a neurologic examination should also be a part of the complete physical examination in a cat under suspicion of toxoplasmosis.

3.2. Laboratory Tests

The feasible diagnosis of Toxoplasmosis in the cat involves serology and/or laboratory analysis of the peripheral blood smears. The results of these tests should be taken in context with the physical examination findings. Serological results should always be interpreted by veterinarians. Antibody assays represent the most likely type of test used for companion animals. Currently, the most common tests for Toxoplasma IgM and IgG antibodies are enzyme-linked immunosorbent assays, as well as indirect fluorescent antibody tests, which are primarily used by commercial laboratories and academic institutions. A positive IgM antibody test suggests that the cat may be actively infected with Toxoplasmosis, but it may persist longer than 2-3 months.

If there is a high suspicion for Toxoplasmosis but the IgM antibody test is negative, the IgG test may be submitted one to two weeks later, which will confirm a negative or recent positive reaction, and this test is a sign that the patient is likely not infected. Serologic testing is most helpful when applied to a patient with clinical signs or abnormal physical examination findings. It is not indicated as appropriate or reliable as a "screening" test for Toxoplasmosis. In North America, the majority of stray cats are infected, and if a high seroprevalence rate is reported for adult cats in a particular region, it makes the test invalid for a suspected potential infection with the coccidium. It is also generally considered to have a negative predictive value. Not all cats that have antibodies are currently infected. Thus, a negative serology for Toxoplasmosis in a cat with severe neurologic disease is of little use, but a positive test is evidence that the cat may have Toxoplasmosis. There may be false positives or negatives for serological test results, but sometimes a positive Toxoplasma titer is the tip-off of a different infectious agent at play. Other methods of isolating Toxoplasma organisms directly or indirectly from the tissues can be utilized, but none are commonly employed due to such low efficacy. Many veterinarians will not biopsy nor attempt to get tissue samples, and for most of these histopathological examinations, they will not diagnose Toxoplasmosis. The PCR test has a lower diagnostic threshold but may be applied in difficult diagnostic cases.

4. Treatment Options

Treating toxoplasmosis in cats is important to help your cat recover from the illness. Veterinarians will tailor treatment to relieve symptoms, and the approach may depend on the severity of the disease, the presence of other complications, and other factors. Cats with mild symptoms of toxoplasmosis may need minimal treatment, but the majority of cats will need some form of supportive care. Cats that are severely ill due to toxoplasmosis may require hospitalization for more intensive treatment and monitoring.

It is important to understand that no treatment will cure toxoplasmosis. Cats who recover from acute toxoplasmosis may be left with tissue cysts for the rest of their lives, so the disease may not be entirely "cured" even if immediate symptoms subside. Fortunately, treatment can help many cats with toxoplasmosis recover and lead a poor quality of life. Treatment options for toxoplasmosis in cats vary depending on the specific disease symptoms and how severely the cat is affected. Because of these variations, cat owners should consult their veterinarian for recommendations before starting any treatment. Veterinarians may recommend one or more prescription medications to help pets with toxoplasmosis recover. Two antibiotics, clindamycin and trimethoprim-sulfa, are both often used to treat acute toxoplasmosis. Not all cats will respond to clindamycin or trimethoprim-sulfa. In addition to antibiotics, the FDA-approved medication ponazuril has been studied for the treatment of toxoplasmosis in cats. Although it was designed to treat the protozoan organism Neospora caninum, ponazuril also has a significant effect on Toxoplasma gondii and can help cats recover from infection. Off-label prescription drug options are also available; these target the protozoal organisms in particular in an attempt to kill them off throughout the body. Injections of clindamycin are sometimes given if the cat is not eating or has severe symptoms. Most of these injections require a visit to the veterinary clinic every 2-3 days. Symptoms such as fever can also be managed with non-prescription medication by your veterinarian to help alleviate the discomfort. Additional medications may be needed to control seizures if the disease affects the central nervous system. Inflammation, fever, and other associated symptoms can be managed through symptomatic treatment while the body itself fights the disease. Cats may need additional supportive care including nutrition and fluid therapy. Some cases of Toxoplasma infection in cats will resolve on their own without extraordinary intervention. In others, medical or surgical treatment may be needed to manage paralysis, immune system dysregulation, or secondary symptoms, such as seizures or abnormal eye movement. Like other infections, treatment of toxoplasmosis usually involves targeted, systemic medications to help the body rid itself of parasites. However, unlike treatment of many bacterial infections, Toxoplasma treatment is not generally "curative;" immunocompetent individuals often will live in balance with the infective organisms for the rest of their lives following initial treatment.

4.1. Medication

Some medications that your vet may use to treat toxoplasmosis in cats include:

Clindamycin: This is one of the most commonly prescribed antibiotics for toxoplasmosis. It is considered a first-line drug to treat this condition. Clindamycin works by interrupting the intracellular uptake of amino acids in T. gondii. This leads to the death of the parasite, thus resolving the disease. The drug is given orally as a liquid and is very easy to administer. It stops the parasite from spreading, and so it can be effective even if only given for a short time, although your vet may still recommend your cat be on medication for 3-4 weeks to ensure it has been killed. Possible side effects of clindamycin include vomiting, diarrhea, and a decreased lack of appetite. Your vet may advise a slower introduction of the drug if high doses are recommended or if your cat has been unwell.

Pyrimethamine: In combination with clindamycin, pyrimethamine is very effective at treating toxoplasmosis in both cats and humans. It is also a first-line medication for this disease. Pyrimethamine causes deficiencies in B vitamins inside the parasite's cells, which ultimately leads to death. It is usually given at the same time as clindamycin every 12 hours to start, and then reduced to once every 24 hours for at least 3-4 weeks. Your vet may be reluctant to use pyrimethamine in some cases, such as if your cat has severe anemia, because the drug can contribute to this side effect. If pyrimethamine is prescribed, your vet should also recommend a supplement to protect your cat’s bone marrow. Ensure you do not exceed the recommended dose of pyrimethamine, as it can be toxic at high levels.

Folinic Acid: This is a type of B vitamin, given to prevent damage that pyrimethamine may cause to your cat's bone marrow. Generally, it is given at the same time as pyrimethamine and clindamycin, though an exact dose and duration could depend on your individual cat’s health. Always ask your vet for advice and do not change the dose recommended by your vet.

4.2. Supportive Care

Just as important as any medication will be the day-to-day care of your cat. Make sure that your cat is drinking water. If possible, add water to his or her food or consider giving your cat unflavored Pedialyte. Check the skin turgor around the neck for dehydration. If you establish that your cat is dehydrated, wrap a dry towel snugly around your cat, or provide a clean towel or blanket in a dim place for your cat to sleep in throughout the day. Cats can get stress-induced pressure even though it may appear very peaceful. In this quiet, darkened environment under a covered box or blanket, the body will try to heal itself. Keep the litter box, food, and water close enough so that it is not stressful for your cat to consume. Ensure he or she is at an optimal or near-optimal body temperature and that he or she is kept inside and protected against any weather extremes. The home environment should be as stress-free and devoid of any other diseases, bacteria, or viruses as possible. The home environment should be such as to promote healing every day by ensuring that the cat eats, is hydrated, and is not exposed to any other stressors during the recovery time, because an animal in a house with other cats who are also ill will slow down the rate of recovery.

A regular, convenient, and safe place for a cat to sleep is critical. Recovery for cats, as it is for us, is physical and mental, allowing major damage and stress time to heal. As with medications, supportive care is part of the treatment plan. The veterinarian has to be in control of adjusting medication or supportive care based on the response observed after each prescription if an animal is ill. Supportive care is a broad area of treatment for an ill pet in need of many supportive measures during illness or after an operation, just like someone who is an inpatient at the hospital. It always calls for medication and is generally supportive. It is mandatory in illness as it is not only medicine that keeps your cat healthy.

5. Prevention Strategies

Preventing infection in cats is an active area of research; however, the most effective prevention strategy is directed toward cat owners. Several measures are recommended to minimize the risk of T. gondii infection, some of which may be more practical for cats in the United States and some of which are more widely generalizable. The primary preventive measure is controlling outdoor access where possible. Outdoor access by cats is associated with a variable prevalence of T. gondii antibodies, so indoor-only cats are at a lower risk of exposure. Routine physical examination and appropriate vaccination for cats provide many benefits and also keep cats in good health since many cat illnesses can cause re-shedding of Toxoplasma gondii from tissue cysts. Control of infection in multi-cat households is also a priority, given the high pressure of oocysts in this environment.

There are several practical ways to minimize the risk of infection. Environmental decontamination measures can be applied to areas where cats are able to defecate. For cats that have access to the outside, providing one location for defecation may be impractical; however, minimizing contact with infected cat feces is beneficial. Taking a proactive approach by performing fecal PCR analysis in multi-cat environments can reduce the length of oocyst shedding in any positive cat, and confining positive cats can minimize re-contamination. These results have significant applications, particularly in high-risk environments such as shelters. Reducing oocyst production is an important veterinary public health strategy, and agents and medications have been in use for many years. Although some veterinary compounds can reduce oocyst production, the value of infection control using medications has yet to be determined. Client education is also paramount for veterinarians to reduce the likelihood of new cat infections and of T. gondii oocyst shedding in positive cats. Likewise, precautionary measures that target specific wildlife or rodents, which could predominate in a given location, may provide more effective disease control strategies.

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