A repetitive DNA probe for the sensitive detection of Fasciola hepatica infected snails

Epizootiologic studies on F. hepatica frequently use microscopic techniques for the detection of infected snails, however, the poor efficiency, sensitivity, and specificity associated with these techniques limit their usefulness. A DNA-based test for the identification of snails infected with larval stages of F. hepatica would solve these problems and enable a level of detection accuracy previously unavailable. We have cloned and sequenced a 124 bp fragment of repetitive DNA from F. hepatica which hybridizes specifically with DNA of F. hepatica but not with DNA of its snail intermediate hosts Fossaria cubensis and Pseudosuccinea columella, or with DNA of Fascioloides magna and Paramphistomum liorchis, ruminant trematodes which share the same intermediate host and same enzootic range as F. hepatica. Using this 124 bp fragment as a probe, infection in snails was detected immediately following miracidial penetration, thus a sensitivity equivalent to the minimum biologic unit of the parasite was achieved. This 124 bp repeated sequence belongs to a large family of 124 bp repeats that share a high level of sequence identity and constitute approximately 15% of the F. hepatica genome. We also report here the development of a quick and inexpensive DNA extraction protocol for use in field-collected snails. Thus, we have developed both a highly sensitive and specific DNA probe and a means to use the probe in a large epizootiologic study of F. hepatica where thousands of field-collected snails need to be assayed for infection.

Distribution of fasciolosis in Kansas, with results of experimental snail susceptibility studies

A total of 278 veterinarians throughout Kansas were sent mail-in survey forms asking specific questions relating to their experience with fasciolosis in their practice area. Replies were received from 178 (64%) veterinarians representing six practice types; one-third reported having seen cases of fasciolosis in their practice. The results of our survey indicate that the majority of the cattle diagnosed with liver fluke disease in Kansas are imported from other areas of the USA. However, in both central and southeastern regions of Kansas, some cattle that had never been out of the state were infected with Fasciola hepatica. Thus, these areas of Kansas should be considered endemic for liver fluke disease. Methods of diagnosis, types of operations, and improvements seen after treatment were also discussed. In order to ascertain the existence of one or more possible snail intermediate hosts within Kansas, five species of lymnaeid snails were collected from central and southeastern parts of the state and tested for their susceptibility to infection by Fasciola hepatica. The snails collected included Pseudosuccinea columella, Fossaria obrussa, Fossaria bulimoides, Fossaria parva and Fossaria dalli. Of these, Pseudosuccinea columella and Fossaria bulimoides proved susceptible to experimental infection by Fasciola hepatica. Metacercariae obtained from experimentally infected snails were used to infect both a weanling calf thereby completing the life cycle of the parasite. This report is the first to identify the existence of suitable snail intermediate hosts for Fasciola hepatica in Kansas.