Theoretical Population Biology 86 (2013) 50–61 Contents lists available at SciVerse ScienceDirect Theoretical Population Biology journal homepage: www.elsevier.com/locate/tpb Modeling effective transmission pathways and control of the world’s most successful parasite Matthew Turner a,b , Suzanne Lenhart b,c , Benjamin Rosenthal d , Xiaopeng Zhao a,c,∗ a Department of Mechanical, Aerospace, and Biomedical Engineering, University of Tennessee, Knoxville, TN, United States b Department of Mathematics, University of Tennessee, Knoxville, TN, United States c National Institute for Mathematical and Biological Synthesis, University of Tennessee, Knoxville, TN, United States d Animal Parasitic Disease Laboratory, US Department of Agriculture, Beltsville, MD 20705, United States article info Article history: Received 2 November 2012 Available online 24 April 2013 Keywords: Toxoplasma gondii Epidemic model Transmission mechanisms Differential equations abstract Toxoplasma gondii (T. gondii) is a single-celled, intracellular protozoan responsible for the disease toxoplasmosis. The parasite is prevalent worldwide, and it infects all warm-blooded vertebrates. Consumption of meats in which this parasite has encysted confers risk of infection to people and other animals, as does ingestion of water or foods contaminated with environmentally resistant oocysts excreted by cats. Vertical transmission (from mother to offspring) is also possible, leading to disease risk and contributing additional means of ensuring perpetuation of transmission. In this work, we adopt a differential equation model to investigate the effective transmission pathways of T. gondii, as well as potential control mechanisms. Detailed analyses are carried out to examine the significance of transmission routes, virulence, vertical transmission, parasite-induced changes in host behavior, and controls based on vaccination and harvesting. Modeling and analysis efforts may shed insights into understanding the complex life cycle of T. gondii. © 2013 Elsevier Inc. All rights reserved. 1. Introduction Toxoplasma gondii (T. gondii) is the world’s most successful known parasite. It is an obligate intracellular protozoan parasite, which is highly transmissible and can even alter a host’s behavior to increase its own transmission. T. gondii can be found anywhere in the world, and it infects all warm-blooded vertebrates, including mammals and birds. ‘‘We live in a sea of Toxoplasma’’, said Dr. J. P. Dubey when remarking on T. gondii’s wide geographical distribution and high prevalence across host species. T. gondii infects 30% of the world’s population. Infections in humans can cause life-threatening encephalitis in immunocompromised persons such as AIDS patients or recipients of organ transplants. Infection acquired during pregnancy may cause severe damage to the fetus. Toxoplasma is a leading pathogen for foodborne disease in the US. Each year, foodborne Toxoplasma results in $3 billion in direct medical costs and lost wages, and causes significant loss of quality life (Batz et al., 2011). Yet, the common pathways of contamination for this pathogen are poorly understood. T. gondii has a complex life cycle (Fig. 1). Felids (domestic cats and their relatives) are the only definitive host in which ∗ Corresponding author at: Department of Mechanical, Aerospace, and Biomedi- cal Engineering, University of Tennessee, Knoxville, TN, United States. E-mail address: xzhao9@utk.edu (X. Zhao). the parasite can reproduce sexually to generate oocysts. A large number of unsporulated oocysts are shed in the cat’s feces; however, oocyst shedding occurs for only 1–2 weeks. Within 1–5 days, oocysts in the environment sporulate and become infectious. All warm-blooded vertebrates can serve as intermediate hosts, which acquire infection through ingesting soil, water, or plants contaminated with oocysts. Parasites in an intermediate host undergo asexual replication. First, oocysts transform into a motile form (tachyzoites) shortly after ingestion. These then develop into relatively quiescent tissue cysts (bradyzoites), often localized in neural and muscle tissue. Although cats may be infected by ingesting sporulated oocysts, cats are far more susceptible to the tissue cysts encountered by consuming infected prey. Importantly, cats develop immunity to the parasites after primary infection, usually as post-weaned kittens. Such immunity may wane after a period of years in the absence of continual challenge (Dubey, 2009b), but all evidence suggests that adult cats rarely, if ever, experience subsequent rounds of oocyst shedding. Humans can be infected by ingesting undercooked meat containing tissue cysts or from food/water contaminated by sporulated oocysts. Humans can also be infected through blood transfusion or organ transplantation. In addition, infection can be transmitted from mother to fetus. In the human host, tissue cysts may remain throughout the life of the host and are most commonly found in skeletal muscle, myocardium, the brain, and eyes. 0040-5809/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.tpb.2013.04.001