DOCUMENT 2 – Goal 1: SNAIL SURVEYS
MILKSA Second Quarterly Report:
Project: Impact of Fasciola hepatica on Dairy Production and Sustainable Management on
Selected Farms in South Africa
A standardised method was developed, and a comprehensive Fasciola
intermediate snail host survey conducted for each of six previously selected
marshy spots (potential snail habitats) on each of the farms. Of importance is
that: (i) The spots were previously selected on the strength of investigation of
practically all likely points of transmission of the parasite on each of the farms,
with the six most promising ones then selected in relation to the numbers of
snails recovered per spot; (ii) An urgent requirement was to enable the
conduction of surveys during inclement weather, i.e. principally rainfall and
excessive wind, over the course of the visits to the farms. A procedure was duly
developed and preparations made and partially tested in the face of some
relatively light rainfall during the past two visits. This was required in the light of
the fact that the surveys need to be conducted monthly and in order to save
costs, the flight reservations need to be done a few weeks before each trip; lastminute changes to the scheduled visits are very costly and there is only a small
window per month during which visits are opportune for the farmers as regards
potential clashes with routine farm activities.
Life cycle of trematodes, including Fasciola (liver fluke) and
paramphistomids (conical fluke): Irrespective of whether the trematodes are
situated in the liver or the rumen of their hosts, their eggs are voided in the faeces,
on pasture. While the eggs can survive for long periods in the faeces, they can only
develop further once they land in open water, in which the eggs hatch and the
parasites develop further, become infective to the cattle, emerge from the snails and
encyst on the pasture. And, finally the liver fluke damages the host by migration of
the young fluke through the liver tissue, and by sucking blood when adult in the bile
ducts of the cattle.
Method of snail survey: Of special relevance, is that all the sampling sites were
selected by JvR, who has a great deal of experience not only of selection of such
sites in general, but also on dairy farms in the region where the present project is
being conducted.
With the help of household sieves, more or less 8cm in diameter, mud is scooped up
from 10 sites per selected marshy pool per farm, and carefully sieved on a special
sieve welded and prepared by JAvW according to a plan provided by Dr Vic
Hamilton-Attwell, a snail expert from the University of Stellenbosch. As far as
possible the snails recovered in the process are counted and identified and returned
to their sites of origin, in order not to disturb the populations by monthly sampling.
However, in cases where numerous snails are encountered per site, some of them
are sampled for examination, aimed at determining whether or not they are infected
with either Fasciola sp. or paramphistomid species.
Snail species: The two species of snails potentially of importance for transmission
of liver fluke, are Lymnaea truncatula and Lymnaea columella (introduced half a
century ago from the USA). However, at present it is uncertain to what extent the
latter species is of importance for liver fluke transmission. In the case of the conical
fluke, the intermediate host is Bulinus tropicus.
In the absence of rain the snails progressively penetrate deep into the drying mud,
where they can survive for months until it rains sufficiently again to “revive” the
marshy conditions in which the snails thrive. In other words, rain or the absence
thereof can be expected to have a profound effect on the extent to which the snails
are recoverable from samples of mud.
The results of the snail surveys are presented below, in a series of graphs:
FARM (I)
RESULTS OF SNAIL SURVEY
Farm (I) - Snails All sources : L. truncatula
80
Klein
Totaal
60
40
20
0
Apr
Mei
Jun
Farm (I) - Snails All sources : L.columella
60
50
40
30
20
10
0
Klein
Totaal
Apr
Mei
Jun
Farm (I) Snails per source : L.truncatula
50
40
30
20
10
0
1
3
April
Mei
Junie
Farm (I) Snails per source : L. columella
60
50
40
30
20
10
0
1
2
3
4
5
April
Mei
Junie
6
FARM (II)
RESULTS OF SNAIL SURVEY
Farm (II) - Snails All sources : L.truncatula
30
25
20
15
10
5
0
Klein
Totaal
Apr
Mei
Jun
Farm (II) - Snails All sources: L. columella
25
20
15
10
5
0
Klein
Totaal
Apr
Mei
Jun
Farm (II) - Snails per source : L. truncatula
1
14
12
10
8
6
4
2
0
2
3
4
5
6
April
Mei
Junie
Farm (II) - Snails per source : L.columella
20
1
2
15
3
10
4
5
5
6
0
April
Mei
Junie
FARM (III)
RESULTS OF SNAIL SURVEY
Farm (III) - Snails All sources : L. truncatula
1
0.8
0.6
0.4
0.2
0
Klein
Totaal
Apr
Mei
Jun
Farm (III) - Snails All sources : L. columella
25
20
15
10
5
0
Klein
Totaal
Apr
Mei
Jun
Farm (III) Snails per source : L. truncatula
1
1
2
0.8
0.6
3
0.4
4
0.2
5
0
April
Mei
Junie
6
Farm (III) Snails per source : L.columella
12
1
10
2
8
3
6
4
4
5
2
6
0
April
Mei
Junie
FARM (IV)
RESULTS OF SNAIL SURVEY
Farm (IV) - Snails All sources: L. truncatula
150
Klein
Totaal
100
50
0
Apr
Mei
Jun
Farm (IV) - Snails All sources: L. columella
80
60
40
20
0
Klein
Totaal
Apr
Mei
Jun
Farm (IV) - Snails per source : L.truncatula
120
100
80
60
40
20
0
1
2
3
4
5
6
April
Mei
Junie
Farm (IV) - Snails per source : L.columella
70
60
50
40
30
20
10
0
1
2
3
4
5
6
April
Mei
Junie
Numbers of snails: The snail survey results are very preliminary, in that much
variation is to be expected from month to month, owing to ecological factors
mentioned above, as well as to factors such as trampling of the survey sites per
farm by cattle. In other words, because of the short duration of monthly sampling to
date, the present results cannot be regarded as of great significance as regards
seasonal cycling and prevalence to be expected over the course of the year. On the
other hand, it is clear that relatively large numbers of snails were recovered on
some of the farms, up to as many as close to 115 on a single sampling occasion,
from a single source (marshy spot) on one of the farms, over against no Lymnaea
truncatula snails (the principal intermediate host of Fasciola hepatica) recovered
from Farm III over the entire three months of sampling.
While the first two graphs per farm list the total numbers of snails of the two species
recovered per farm, the last two are more detailed, in that they list the results of
individual sampling sites, but to the exclusion of those sites from which no snails, or
only the occasional one, were recovered.
Of importance in the graphs is not only the large amount of variation between
farms, but also that between the different sampling spots per farm. Not only were
very few snails found on Farm III, but even on the farms where relatively large
numbers of snails were found, there were large differences in the numbers
encountered per snail surveillance spot.
Infection of snails with parasites: Investigative evaluation of a few snails
over the period for the presence of Fasciola (liver fluke) and paramphostomid
parasites (conical fluke, probably Cotylophoron sp.), was done for confirmation of
the genera/species of the parasites in the snails. After morphological
identification, samples were preserved for later DNA confirmation towards the
end of Phase 1 of the project. In order not to influence the epidemiological data
on the seasonal cycling of the intermediate hosts of the parasites, the vast
majority of the snails are returned to their sites of collection after they have been
counted; only small numbers are collected for evaluation of levels of infection,
and only from collection sites where relatively large numbers of the snails are
encountered at any given time.