High pathogenicity of Cryptosporidium parvum IId subtypes in dairy calves

Zoonotic cryptosporidiosis is caused primarily by Cryptosporidium parvum . Within C. parvum , the IIa and IId zoonotic subtype families are the most prevalent. The IId subtype family has emerged in China in recent years, causing outbreaks of cryptosporidiosis in dairy calves. The majority of infection studies have been conducted with IIa subtypes, while the pathogenicity of IId subtypes remains poorly understood. In this study, two IId isolates (IIdA20G1-HLJ and IIdA20G1-HB) from dairy farms in China were used to infect neonatal dairy calves, with a IIa isolate (IIaA17G2R1-Waterborne) from the USA employed as a control. The present study investigated the clinical, parasitological, and pathological characteristics of infected calves. The results demonstrated significantly greater intensity and duration of oocyst shedding in IId-infected calves than in control calves. In addition, the IIdA20G1-HLJ isolate induced peak oocyst shedding of 4.3×10 7 oocysts per gram of feces (OPG) in calves, with oocyst shedding over 10 6 OPGs at 5–9 d post-infection. In contrast, the IIaA17G2R1-Waterbrone isolate induced a lower peak oocyst shedding with 8.7×10 6 OPGs, and oocyst shedding with over 10 6 OPGs occurred at 2–4 d post-infection. Furthermore, calves infected with the two IId isolates presented more severe clinical signs and 20–25% mortality. Calves recovered from the primary infection with the IIa or IId subtype were resistant to a secondary challenge with the heterologous subtype, suggesting the existence of cross-protection between the IIa and IId subtypes. The present data demonstrate the high pathogenicity of C. parvum IId subtypes in China and the potential cross-protective immunity between the IIa and IId subtypes in calves.