Parasitology
Detection of Cryptosporidium arvum in shellfish
Dr Steve Wright
Figure 1. Mussel - Mytilus edulis.
Cryptosporidium parvum is a zoonotic protozoan intestinal parasite which causes diarrhoea in lambs and calves, and is responsible for severe diarrhoeic episodes in humans. The infective stage, the oocyst, is small, very robust and can survive exposure to most common disinfectants, including chlorine. Transmission is either direct, via the faecal-oral route, or indirect, via environmental contamination with human or livestock waste. As there is no effective drug therapy for this parasite, cryptosporidiosis can be life threatening for the immunocompromised and can severely debilitate the immunocompetent. However, the dehydration caused by acute diarrhoea can be alleviated by oral or intravenous fluids and electrolyte replacement.
Oocysts can survive for protracted periods in both fresh- and seawater, therefore having the potential to contaminate watercourses, potable waters and estuarine environments - the latter posing a threat of contamination to marine organisms destined for human consumption. Bivalve molluscs are known to harbour environmentally-derived enteropathogens of humans as a result of filtering large volumes of water, and concentrating the recovered particles. Eating raw or lightly cooked shellfish (especially oysters and mussels) is common practice in many countries, and has increased significantly the public health risk of enteric pathogen infections. Recently, C.parvum has been shown to concentrate in filter-feeding molluscs both naturally and experimentally.
This project, funded by the Food Standards Agency, focuses on the detection of C.parvum in shellfish, and the process involved in purging these animals of their contaminants. We have evaluated the various methodologies available for extraction and identification of C.parvum oocysts from environmental samples, and determined that immunomagnetic separation is the most suitable method for shellfish tissues. We are also investigating the time taken for shellfish to purge themselves of contaminants. For depuration to be effective, the molluscs must be immersed in tanks of clean seawater so that they rid themselves of contaminants naturally. A UV light source sterilises bacterial contaminants, but may be insufficiently powerful to kill protozoal oocysts. At present, a minimum of 42 hours is deemed sufficient to complete the process of depuration in industry, but previous studies have shown that for mussels to purge themselves completely of C. parvum, a longer depuration time may be required. We aim to investigate this and, if needed, manipulate the current protocol to suit the depuration of C.parvum. Once the depuration study is completed, a survey to screen shellfish production sites around the UK for C.parvum will commence.