Seasonal shedding of multiple Cryptosporidium genotypes in California ground squirrels (Spermophilus beecheyi)

Abstract

Twelve percent of 853 California ground squirrels (Spermophilus beecheyi) from six different geographic locations in Kern County, Calif., were found to be shedding on average 44,482 oocysts g of feces(-1). The mean annual environmental loading rate of Cryptosporidium oocysts was 57,882 oocysts squirrel(-1) day(-1), with seasonal patterns of fecal shedding ranging from <10,000 oocysts squirrel(-1) day(-1) in fall, winter, and spring to levels of 2 x 10(5) oocysts squirrel(-1) day(-1) in summer. Juveniles were about twice as likely as adult squirrels to be infected and shed higher concentrations of oocysts than adults did, with particularly high levels of infection and shedding being found among juvenile male squirrels. Based on DNA sequencing of a portion of the 18S small-subunit rRNA gene, there existed three genotypes of Cryptosporidium species in these populations of squirrels (Sbey03a, Sbey03b, and Sbey03c; accession numbers AY462231 to AY462233, respectively). These unique DNA sequences were most closely related (96 to 97% homology) to porcine C. parvum (AF115377) and C. wrairi (AF115378). Inoculating BALB/c neonatal mice with up to 10,000 Sbey03b or Sbey03c fresh oocysts from different infected hosts did not produce detectable levels of infection, suggesting that this common genotype shed by California ground squirrels is not infectious for mice and may constitute a new species of Cryptosporidium.

FIG. 1.

Prevalence of C. parvum infection in California ground squirrels (S. beecheyi), stratified by month and age class.

FIG. 2.

Intensity of fecal shedding of C. parvum oocysts in California ground squirrels (S. beecheyi), stratified by month and age class.

FIG. 3.

Environmental loading of C. parvum by California ground squirrels (S. beecheyi), stratified by month and age class.

FIG. 4.

Polymorphic region of the 18S SSU rRNA gene demonstrating three genotypes of Cryptosporidium (Sbey03a, Sbey03b, and Sbey03c) isolated from California ground squirrels (S. beecheyi), compared to C. parvum bovine genotype A (California dairy calf isolate Btd03a), C. parvum porcine genotype I (AF115377), and C. wrairi (AF115378). A period signifies that the base is identical to that of the bovine genotype A reference; a dash signifies a base deletion with respect to the bovine genotype A reference.