Lab Note 4: Introducing the parasites of interest
Muskrats are host to a variety of parasites that range from larger cestodes (tapeworms) to small Apicomplexans (single cellular, eukaryotic organisms). This Lab note is devoted to introducing the main parasites in muskrats we are interested in screening for: Hydatigera taenieaformis, Versteria rafei, and Toxoplasma gondii. But before we get into the weeds it may be helpful to cover some aspects about parasite life cycles.
Parasites generally exhibit direct or indirect lifecycles. Direct lifecycles only require the involvement of one host species. Indirect lifecycles involve multiple host species to complete. Most of our parasites of interest for this project carry out indirect lifecycles, however there is one exception that will be discussed below. Indirect lifecycles involve a definitive host (a host where the adult or sexually reproductive form of a parasite resides) and one or more intermediate hosts (host that harbor the larval and, in some cases, asexually reproducing forms or the parasite). There are other host types such as incidental hosts and paratenic hosts which represent a host that becomes infected but is not the “usual” preferred host(s), and a host where the parasite cannot develop but can persist, respectively.
Toxoplasma gondii is a protozoal parasite that has a very wide host range and poses significance to human and animal health. Toxoplasma gondii is well known for its use of felids (cats) as a definitive host. While the great variety of other hosts consist of intermediate and incidental hosts, those being endothermic (warm blooded) animals from mammals to birds (Hill and Dubey 2013) (Dubey 2010) (Torrey and Yolken 2013). The typical lifecycle of T. gondii moves between cats and the variety of intermediate hosts to complete the lifecycle. However, T. gondii can undergo a direct lifecycle as well and infect new felids without the need to pass through an intermediate host. Infection with T. gondii across the hosts can range in from subclinical infection to clinical disease (toxoplasmosis) and mortality. In muskrats, T. gondii is one of the more reported protozoan parasites, but they do not seem to be significantly affected by it (Ganoe et al. 2020). Though, muskrats do tend to display higher rates of infection relative to mammals and rodents in upland, drier environments.
Hydatigera taenieaformis and Versteria rafei are cestodes (tapeworm) that exhibit indirect life cycles. In the case of H. taenieaformis, canids and felids are considered the most common definitive hosts while V. rafei and the Versteria as a whole appear to use mustelids (mink, weasels, etc.) as definitive hosts. Both cestodes will utilize rodents as intermediate hosts where they encyst in visceral organs such as the liver until they are consumed by a definitive host and the larvae can develop into adults to begin the cycle again. H. taenieaformis and V. rafei have displayed the capacity to infect and cause disease in other animals and humans in their larval stage. Infection with H. taenieaformis in muskrats has been well documented and seems to rarely be associated with disease (Ganoe et al. 2020). Infection with V. rafei is far less common and has only been reported in three muskrats (Ganoe et al. 2021) one of those being a fatal infection (Niedtinghaus et al. 2022). While reports of V. rafei are far less common, which makes it harder to determine its threat to muskrats, The overall infection of muskrats as intermediate hosts by both cestodes seem to be more often associated with subclinical infection. If parasite burden is high enough to a level where the presence of encysted larvae could impair organ function, then it is possible for adverse health effects to appear.
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