Cryptosporidium parvumDose Response Experiments

General overview

Various species of Cryptosporidium infect most vertebrates. C. parvum infects cattle but can also infect humans; C. hominis appears to be restricted to humans, and began to be recognized in the early 2000s (Hunter 2005) [1]. The oocysts are the infective stage and are about 5 microns in size; they are excreted in feces and are transmitted to new hosts by the fecal-oral route. They are highly resistant to chlorine, but are vulnerable to ultraviolet light disinfection (AWWA 1999) [2]. The durability and infectiousness of the oocysts, as well as their documented ability to cause large outbreaks (Mac Kenzie et al., 1994) [3], means that control of Cryptosporidium is very important for drinking water treatment. Water treatment utilities should consider all surface water to be contaminated with oocysts (AWWA 1999) [2]. Effective control of Cryptosporidium is generally achieved in drinking water treatment through filtration yielding nonturbid water (<= 0.1 nephelometric turbidity unit) (AWWA 1999).

Cryptosporidiosis is a disease described by a self-limited watery diarrhea with an incubation period of 3 to 7 days (Miliotis & Bier 2003) [4]. Asymptomatic infections are also common in apparently healthy children and adults (Blaser 2002) [5]. However, the disease is particularly dangerous to people with HIV/AIDS because there is no effective treatment (Miliotis & Bier 2003) [4]. This can lead to lethal infections, or chronic disease lasting months or years that severely damages the gut.

Summary of Data

An experiment (DuPont et al., 1995) [6] from feeding an isolate from a calf (Iowa isolate) to human volunteers yields an exponential model with an ID50 of 165 oocysts (Teunis 1999) [7] also fitted a model to these data which is similar to the model presented here.

Messner et al. (2001) [8] described dose response model fits using the complete unpublished data set from DuPont et al., 1995 [6]:

• Reanalysis of stool samples from the above experiment (DuPont et al., 1995) [6] using flow cytometry revealed that 2 individuals thought to be uninfected were actually infected (Messner 2001) [8].
• Two other feeding studies (Okhuysen et al., 1999) [9] in human volunteers using different isolates yielded models with ID50s of 179 oocysts (UCP isolate, also from a calf) and 9 oocysts (TAMU isolate, from an infected veterinary student).

Messner et al. (2001) [8] fit the exponential model to these three datasets. This was appropriate for the Iowa and TAMU isolates, but the Beta-Poisson model provided a better fit than the exponential model for the UCP isolate. The UCP and TAMU datasets are smaller than the Iowa datasets.

Okhuysen et al. (2002) [10] also conducted a feeding study in adult humans using C. parvum originating from red deer (Moredun isolate).

Chappell et al. (1999) [10] also conducted a feeding study in humans using the Iowa isolate of C. parvum. Although the data were not published, they estimated an ID50 of 83 oocysts for volunteers lacking anti-C. parvum IgG, and an ID50 of 1,880 oocysts for volunteers who had anti-C. parvum IgG.

References