biotec at biotec at
Wed Dec 4 07:59:20 EST 1996


Cowdriosis and dermatophilosis are major diseases of
ruminants, associated with African Amblyomma ticks and thereby
major constraints on productivity particularly in large
regions of sub-Saharan Africa and many Caribbean islands. They
also constitute a real threat to Central, South, and possibly
North America. They are associated with African Amblyomma
ticks the vector of cowdriosis and associated with severe
dermatophilosis. A programme of research funded by the
European Union was set up, the objective of which was to
contribute to the successful control of both the diseases and
of the associated tick species. The immediate objective was to
increase general knowledge about them and to research on new
control methods.

The programme (TSD2A-115) was co-ordinated by Professor G.
Uilenberg, University of Utrecht, with collaborating
institutes in France, the United Kingdom, Chad, Senegal and
the Antilles.

In vitro culture of Cowdria ruminantium in bovine endothelial
cells led to:

1. An improved indirect fluorescent antibody test.

2. A competitive ELISA detecting antibodies directed against a
32-kilodalton Cowdria protein competing with a monoclonal
murine antibody against this protein.

3.   Development of an indirect ELISA.

4. Western blot using the above mentioned monoclonal antibody.

5. Purification of Cowdria DNA for a genomic library to study
the phylogeny of C. ruminantium.

Antibodies have been identified in bovine sera from many
Caribbean islands using the above tests but C. ruminantium has
only been isolated on Guadeloupe, Marie-Galante and Antigua.
Thus the validity of the serological tests will have to be

A long-term sub-clinical carrier state has been confirmed in
goats. This has important epidemiological implications with
regard to the spread of the disease.  Also confirmed is the
existence of important antigenic strain differences which may
be a cause of poor results with immunisation. However the
attenuation of the Senegal stock did induce solid homologous

A start was made to investigate inherent genetically
determined resistance to cowdriosis in Creole goats by doing
crossing tests to attempt to identify possible genetic
markers. In Mozambique the percentages of positive sera (IFA)
in goats and cattle were much higher in the south in where A.
hebraeum is the vector than in the north where A. variegatum
is predominant. A third American vector species was identified
in A.dissimile.

Phylogenetically Cowdria was most closely related to Ehrlichia
canis and E. chaffiensis. It is also related to Anaplasma
marginale though not to Chlamydia.

Despite some common antigenic determinants there was no cross
immunity with E. phagocytophila.
There also appears to be an inverse relationship between
infecting dose and incubation period.

Experimental and epidemiological study has shown:
1.Transtadial transmission of Dermatophilosis congolensis was
not achieved.

2. Infection with D. congolensis alone leads to high antibody
levels but is fatal in the presence of ticks.

3. The most important factors in the pathogenesis of disease
are the presence of  A. variegatum and the type of cattle.

4. After a flumethrin pour-on campaign in St. Lucia, clinical
disease has became rare while the number of seropositives have
remained high. Seropositive animals occur on all surveyed
Caribbean islands while disease is limited to tick infested

5. Cattle should be amenable to selection for resistance.
Creole cattle on Guadaloupe more resistant to disease than
creole cattle on Martinique.

The tick triggers off clinical disease in latently infected
animals although the mechanism is unknown. Ruminants are
unable to mount an immune response to the tick which may be
due to the tick causing immunodepression. Certainly tick
saliva can inhibit lymphocyte stimulation in vitro. Concurrent
malnutrition also adversely affects immune responses to
vaccines against dermatophilosis although it appears not to
affect antibody levels. A live vaccine and a killed vaccine
with Freunds adjuvant were more effective than an ISCOM
vaccine in the face of a high challenge although they were
similar with low challenge. It was also found that sunlight
increased susceptibility of non-pigmented skin.

Amblyomma variegatum has been introduced to at least 13
Caribbean islands between 1967 and 1988. The movement of
cattle but possibly also the recent arrival from Africa of the
cattle egret may have spread the tick. Dogs may also act as
reservoir hosts.

The non-fed nymph can survive for up to 11 months the nymphs
for 15 months and the adults for 23 months. Therefore an
eradication plan should last for at least 24 months. Longer
acting acaricides will be more efficient than short acting
ones due to the brief period the tick is on the host for. Male
ticks secrete pheromones which could be used in conjunction
with acaricides in traps or on animals. however these
substances are volatile and require the presence of CO2.

Entomaphagous nematodes have failed to control A. variegatum
although the presence of Boophilus annulatus has a marked
lethal effect on ovipositing females. The cattle egret and
domestic chickens may also play a role in tick control as has
the ant Solenopsis geminata.

However acquired resistance in ruminants is poor and
preliminary attempts at immunisation with intestinal tick
protein has been disappointing. Further work is indicated.

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