Feline toxoplasmosis
Author(s):
Christopher Fernandez-Prada...
Updated:
AUG 30, 2023
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Toxoplasmosis is now known to have a worldwide distribution, and although the cat is the sole definitive host, the parasite can have significant implications for both human and animal health.
Christopher Fernandez-Prada
DMV, MSc, PhD, Department of Pathology and Microbiology, University of Montreal, Canada
Dr. Fernandez-Prada is a veterinarian and molecular-parasitology researcher devoted to tackling zoonotic parasites. He holds an Associate Professor position at Université de Montréal (UdeM) – Veterinary College and is Director of the Animal Parasitology Diagnostic Lab at the university, and also serves as adjunct professor at the Faculty of Medicine of McGill University. His research is focused on the discovery of drug-resistance mechanisms, and the development of novel diagnostic tests and vaccines against protozoan parasites.
Victoria Wagner
DMV, MSc, Department of Pathology and Microbiology, University of Montreal, Canada
Dr. Wagner graduated from the UdeM Veterinary College in 2020, after which she completed a Masters in Veterinary Parasitology at the same institution in collaboration with the McGill University Health Centre, focusing on canine leishmaniasis. Dr. Wagner currently works in both small animal general practice and laboratory animal medicine, and has a specific interest in research and education pertaining to zoonotic parasites.
Key points
- Feline infection with Toxoplasma gondii is commonplace worldwide, but most cats remain asymptomatic.
- Human T. gondii infection can occur through ingestion of oocysts (e.g., food, soil and contaminated water) or tissue cysts (undercooked meat).
- The detection of T. gondii oocysts in cat feces as a stand-alone diagnostic test is not recommended due to the short shedding period and their similarity to other parasites.
- Oocysts require a minimum of 24 hours after shedding before becoming infective, so daily litter box cleaning is an effective strategy to prevent transmission in the home.
Introduction
The intracellular protozoan parasite Toxoplasma gondii can infect the majority of vertebrate animal species, and although felids serve as its sole definitive host, it is being increasingly recognized as a major threat in many ecosystems. Ingestion of the parasite by either the definitive or an intermediate host will typically culminate in either a chronic subclinical or asymptomatic infection; in susceptible or non-immunocompetent subjects, however, toxoplasmosis may lead to abortion and reproductive failure, or even organ failure and death (1). Nowadays toxoplasmosis is found worldwide; the prevalence of oocyst shedding varies from 0.7-41% of cats at any given time, depending on the country in question. In the USA the prevalence of positive T. gondii antibody titers in cats ranges from 14-100% (2). Whilst many humans are also infected (for example, approximately 40 million in the USA), most are asymptomatic, but some, especially those that are immunocompromised, may suffer from associated ocular or neurological problems (3).
Toxoplasma gondii life cycle
Toxoplasma gondii has a complex lifecycle, requiring a definitive host and an intermediate host to complete sexual and asexual cycles respectively (Figure 1). The parasite exists in 3 distinct forms – tachyzoite, bradyzoite, and sporozoite (in oocyst), all of which can be infectious to humans. Tachyzoites constitute the rapidly-dividing asexual stage of the parasite, and may be found in the tissues of any vertebrate host. Bradyzoites are also ubiquitous and will also be found in host tissues, but this life stage divides slowly and is encysted.
Figure 1. Life cycle of Toxoplasma gondii. Unsporulated oocysts are shed in the cat’s feces, which then take 1-5 days to sporulate in the environment before becoming infective. Intermediate hosts in nature (including mammals and birds) become infected after ingesting soil, water or plant material contaminated with oocysts. Cats become infected after consuming intermediate hosts (e.g., birds and rodents) harboring tissue cysts, or by direct ingestion of sporulated oocysts. Oocysts transform into tachyzoites shortly after ingestion; these localize in neural and muscle tissue and develop into tissue cyst bradyzoites. Humans become infected by ingesting undercooked meat containing tissue cysts or food, water, soil and other materials contaminated with oocysts from cat feces. Vertical transmission from mother to fetus and transmission through blood transfusion are also possible.
© Dr. Aida Minguez-Menendez (UdeM)/Redrawn by Sandrine Fontègne
Cats, the definitive host of T. gondii, become infected through ingestion of tissue cysts, for example when hunting and eating prey (Figure 2) or sporulated or infective oocysts (from soil, water, or plants contaminated with feline feces). The parasite can reproduce sexually in this species, and cats infected by bradyzoites or oocysts will begin to shed oocysts in their own feces 3-10 or 19-48 days following ingestion, respectively, and may continue to shed for up to 2 weeks (Figure 3) 2,4. Oocysts sporulate and become infective 1-5 days following excretion, and are extremely resistant in the environment.
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