Redescription of Hammondia hammondi and its differentiation from Toxoplasma gondii J.P. Dubey * , C. Sreekumar United States Department of Agriculture, Beltsville Agricultural Research Center, Animal and Natural Resources Institute, Animal Parasitic Diseases Laboratory, Building 1001, BARC-East, Beltsville, MD 20705-2350, USA Received 24 February 2003; received in revised form 26 May 2003; accepted 30 May 2003 Abstract Hammondia hammondi is a protozoan parasite that, until 1975, was misidentified as Toxoplasma gondii. Recently, the validity of H. hammondi has been questioned. In this article, the authors redescribe the parasite and its life cycle, provide accession numbers to its specimens deposited in a museum, and distinguish it structurally and biologically from T. gondii. Hammondia hammondi was found to be structurally, biologically, and molecularly different from T. gondii. Published by Elsevier Ltd. on behalf of Australian Society for Parasitology Inc. Keywords: Hammondia hammondi; Toxoplasma gondii; Life cycle; Cats; Tachyzoites; Bradyzoites; Oocysts; Sporozoites 1. Introduction Domestic cats (Felis domesticus) and other felids are important in the epidemiology of Toxoplasma gondii infection because they are the only hosts known to excrete the environmentally resistant oocysts (Dubey and Beattie, 1988; Tenter et al., 2000). Toxoplasma gondii oocysts are morphologically similar to oocysts of another coccidian, Hammondia hammondi (Frenkel and Dubey, 1975). The distinction between H. hammondi and T. gondii is important from an epidemiological perspective because all isolates of T. gondii are potentially pathogenic for humans and animals whereas H. hammondi is not known to cause clinical disease in any naturally infected host. Recently, Mehlhorn and Heydorn (2000) and Heydorn and Mehlhorn (2001) questioned the validity of H. hammondi as an organism distinct from T. gondii. The objective of the present study was to redescribe H. hammondi, deposit specimens in a museum, and compare its biology with that of T. gondii. 2. Materials and methods 2.1. Parasite strains Oocysts of the H.H. 34 strain of H. hammondi were obtained from Riahi et al. (1995). This strain was originally isolated in 1989 by Dr J.K. Frenkel from a domestic cat from Kansas City, Kansas, USA, and has been maintained in our laboratory since 1995 by passage between mice and cats. It was chosen for the study as the original type strain (CR-4) is now lost and because more is known of the H.H. 34 strain than any other isolate of H. hammondi. The VEG strain of T. gondii (Dubey et al., 1996) was used as control. 2.2. Experimental animals Parasite-free cats from a closed colony were used; the history of this cat colony and its management were described previously (Dubey, 1995). Outbred Swiss Web- ster (SW) female albino mice were obtained from Taconic Farms, Germantown, New York. Gamma interferon gene knockout (KO) mice were obtained as reported by Dubey and Lindsay (1998). As KO mice lack the gene for the production of the cytokine gamma interferon, they are highly susceptible to T. gondii infection and were used to facilitate detection of low levels of infection. All animal 0020-7519/$30.00 Published by Elsevier Ltd. on behalf of Australian Society for Parasitology Inc. doi:10.1016/S0020-7519(03)00141-3 International Journal for Parasitology 33 (2003) 1437–1453 www.parasitology-online.com * Corresponding author. Tel.: þ 1-301-504-8128; fax: þ1-301-504-9222. E-mail address: jdubey@anri.barc.usda.gov (J.P. Dubey).