The 1996 outbreak of CCHF in Oudtshoorn created a new awareness of
this rare disease. At that time, we were given answers to some of
our questions, by
Prof. Bob Swanepoel of the
National Institute for
Virology in Johannesburg
who kindly supplied a unique insight into the nature of the 1996 outbreak.
"The world
distribution of CCHF virus coincides pretty well with the distribution of
Hyalomma ticks (Africa, eastern Europe and Asia). Locally,
Hyalomma species are called "bontpootluise". "Bont" means
multicoloured or variegated, and refers to the reddish-brown and white bands
on the legs of ticks of the genus Hyalomma.
Other ticks can transmit, but there seems to be a particular link with Hyalommas, and this has certain epidemiologic consequences, relating for instance to the host preferences of the immature and adult ticks (which differ from ticks of other genera) :
larvae and nymphs feed on small
mammals up to hare size
and ground-frequenting birds,
while adults prefer
large animals,
the larger the better.
How do human infections originate?
"Humans gain infection from tick bite (yes, the occasional
Hyalomma),
or from contact of infected fresh blood (or other tissues) with broken skin
- with the infected blood/tissues coming either from human patients (nosocomial
infections - needle sticks etc),
or other animals,' commonly sheep and cattle."
How does CCHF affect other animals?
"To our knowledge, only humans and newborn mice readily succumb to disease; other animals, including nonhuman primates, are either refractory or undergo mild infection, sometimes with transient viraemia - including sheep and cattle (our unpublished results).
Sheep and cattle are viraemic for up to about a week, and often exposure
to ticks and virus infection occurs at an early age. Farmers may castrate,
dehorn, stick in ear tags or immunize the young animals, and thus expose
themselves through getting infected blood onto broken skin.
Sometimes animals meet tick infestation for the first time late in life and
then succumb to tick-borne diseases of livestock (such as babesiosis or
anaplasmosis) at the same time that they happen to have first met CCHF virus,
and are thus viraemic at a time when they are treated, autopsied, or even
butchered by farmers, veterinarians or farm workers respectively. This
constitutes another type of incident leading to common source outbreaks."
"Most often the disease affects stockmen and other farm dwellers,
and townspeople only become infected when they visit the countryside and
get tick bite, or hunt and slaughter animals etc.
It is also enough to squash infected ticks with bare fingers - one does not
have to be bitten.
The only town dwellers who are regularly exposed to infection are slaughtermen
at abbatoirs - since they encounter fresh blood and other tissues
of livestock (commonly sheep and cattle) hundreds of times daily - sometimes
more than a thousand head of sheep or cattle a day, they must come across
animals in the short viraemic phase of infection fairly often, and we know
they get the disease more frequently than other people, and it seems, also
a relatively mild/silent infection with seroconversion.
All the same, there would probably be many more abbatoir infections if the
viraemia in livestock were more intense than it is - very low-titered in
comparison with Rift Valley fever for instance.
Ticks which detach from hides and skins at slaughterhouses, after their engorgement has been so rudely interrupted, will sometimes attach to whatever is available, and this constitutes another hazard for abbatoir workers.
What then of risk for the urban consumer of meat?
" In 17 years of looking at about 2000 cases of suspected viral haemorrhagic
fever, we have never found CCHF in a town dweller who did not have a history
of recent tick bite, or animal blood contact in the countryside or at an
abbatoir,
nor were we able to isolate virus from meat from experimentally infected
sheep killed in viraemia.
The virus is not very resistant to heat and pH extremes etc, and we assume
the fall in pH associated with "maturation" of meat by hanging of carcases
after slaughter, is sufficient to put paid to residual virus after "bleeding
out" of the animal at slaughter.
Can ostriches be a source of infection?
We previously failed to obtain ostriches to experiment on, and tested guinea fowl and chickens - which were fairly resistant to the virus - but they may be quite different to ostriches which, I think, many have a slightly different thermoregulatory capacity from other birds, for one thing."
"Having said all that, we come to Oudtshoorn, the centre of an ostrich industry which sells (locally and abroad), feathers, skins and meat. An ostrich farmers' co-operative runs several slaughterhouses, and these resumed slaughtering (after an off season) on 21 October, 1996; (two of the abbatoirs are in Oudtshoorn). By pure coincidence a survey was undertaken at the largest of the abbatoirs of the tick burden (mainly Hyalomma truncatum) on birds coming in from various farms, and it was found that six consigmnets were heavily infested. This is the slaughterhouse where the outbreak occurred towards the end of October, 1996.
Antibody detection and PCR did a splendid job of rapid laboratory confirmation for us."
For a while, all of the abbatoirs were closed and by the end of the theoretical incubation period no further primary common source cases had been observed. There appear to have been no secondary (human-to-human cases).
What advice was given to ostrich farmers?
In November 1996, "I advised that farmers should have a section of
their feedlots double fenced, the inner fence to be small animal proof, and
that ostriches should be treated with a short half life pyrethroid acaricide
(non residual) 14 days before being sent for slaughter and placed in
the fenced off "quarantine" section for this period - they should then (
by analogy with sheep and cattle experiments) arrive at the slaughterhouse
tick- and virus- and acaricide-free, but we still need to do ostrich experiments.
Pyrethroids in any event have very rapid "knockdown" (lethal) effect on ticks,
and very low mammalian toxicity."
Also, I shall be visiting the area and, with Dr Burger, will visit farms
to collect ticks etc. From our sheep and cattle surveys we know that the
virus is everywhere, and while we are not trying to incriminate anybody,
it will be interesting to see if we can get virus isolates from ticks to
match up by nucleotide sequencing with any isolates from patients.
(The fact that most infections have been relatively benign is interesting
of itself)."
Bob Swanepoel,
November 1996.
Results of follow-up experiments
SUMMARY
Following the occurrence of an outbreak of Crimean-Congo haemorrhagic fever (CCHF) among workers at an ostrich abattoir in South Africa in 1996, nine susceptible young ostriches were infected subcutaneously with the virus in order to study the nature of the infection which they undergo. The ostriches developed viraemia which was demonstrable on days 1-4 following infection, with a maximum intensity of 4.0 log10 mouse intracerebral LD50/ml being recorded on day 2 in one of the birds. Virus was detectable in visceral organs such as spleen, liver and kidney up to day 5 post-inoculation, one day after, it could no longer be found in blood. No infective virus was detected in samples of muscle, but viral nucleic acid was detected by reverse transcription-polymerase chain reaction in muscle from a bird sacrificed on day 3 following infection.
It was concluded that the occurrence of infection from
ostriches at abattoirs could be prevented by keeping the birds free of ticks
for 14 days before slaughter.
Despite the apparent lack of evidence of disease in urban consumers of meat,
it remains unacceptable that CCHF infected animals should reach abattoirs
to pose a potential threat to workers and the public. Tick infested animals
pose an additional threat to abattoir workers since partially engorged ticks
tend to detach from their hosts after slaughter, or from the hides and skins
of the hosts, and may then attach to any humans in the vicinity.
From the publication "Experimental infection of ostriches with
Crimean-Congo haemorrhagic fever virus" by:
R. SWANEPOEL1, P.A.
LEMAN1, F.J. BURT1, J.
JARDINE1, D.J.
VERWOERD2, I.CAPUA3,
G.K. BRÜCKNER4 and W.P.
BURGER5
1. National Institute for Virology and Department of Virology,
University of the Witwatersrand, Private Bag X4,
Sandringham 2131, South Africa
2 .Onderstepoort Veterinary Institute, Onderstepoort 0110, South Africa
3. Istituto Zooprofilattico Sperimentale, Teramo, Italy
4. Directorate of Veterinary Public Health, Pretoria 0001, South Africa
5. Klein Karoo Co-operative Ltd, Oudtshoorn 6620, South
Africa
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CCHF diagnosis is done at the National Institute for Virology in Johannesburg.
Page prepared by Linda
Stannard, Div of Medical Virology, UCT.
Updated December,1999.