COLLECTION OF MAMMALS AND ARTHROPODS DURING THE EPIDEMIC OF HAEMORRHAGIC FEVER IN ZAIRE
O.R.S.T.O.M., Institut Pasteur, B.P. 923, Bangui, Central African Empire
A survey team of the International Commission, stayed in the Yambuku epidemic area between November 1, and November 9, 1976. By then, the haemorrhagic fever outbreak was nearing its end.
One aim was to search for cases and convalescent patients.
The other purpose was to make a first investigation on the natural conditions susceptible to have favored the transmission and to collect mammals and arthropods for virus-isolation attempts. The hasty organization fixed by the circumstances for this ecological investigation makes the results unfortunately overmodest.
Prospected area. At that time, the mode of contamination of the first known
patient, who had made a travel in the forests of Mobaye-Bongo zone, but also received a perenteral injection at Yambuku Hospital a few days before his fatal illness, already appeared conjectural. Therefore, it was decided that the first ecological investigations would concern Yambuku, epicentre of the epidemic, and some other places within a radius of ten to twenty kilometers (Yalikonde, Yahombo, Yalikenga, Bolisa westward, Yamolembia, Yandongi and Yamunzwa eastward and part of Yamikeli on the Bumba-Yandongi road). All these inhabited places, except Bolisa, were more or less recently affected by the epidemic (last deaths on October 28 and November 3, respectively in Yahombo and Yamikeli).
- Culicidae :
The circumstances prohibited the organization of man-baited catches. Mosquitoes were caught by glass tubes from resting sites inside houses and in outbuildings. Five to ten minutes were given to each visited compound, which were inspected by night or in the early morning. The knock-down method was also used.
- Cimicidae :
Bedbugs were collected from the raphia slats of the beds or, after shaking, from the soil.
2. Mammals .
- Monkeys and mean-sized animals
Cartridges were distributed to some hunters and animals were shot in the nearby forest.
Trapping proved very disappointing, partly due to the lack of a sufficient number of traps (Chauvency's type). Therefore wild and domestic species were mainly hand caught by villagers to whom disposable gloves were supplied.
- other mammals :
They were generally shot. Seven bats were net caught.
Organ samples from a dead cow, sera from ten pigs and one cow were collected.
Virological sampling. Insects were pooled by species. Bedbugs, slightly anaesthetized, were introduced into Nunc tubes and put in dry ice.
As most mammals were dead when submitted, these were not bled but spleen, liver, kidney and heart were pooled in individual pools.
Entomological survey results.
The mosquito prevalence in the houses was surprisingly low. From 61 compounds inspected only 3 Anopheles funestus Giles, 2 Culex pipiens fatigans Wiedemann, 8 C. cinereus Theo. and 5 Mansonia africana (Theo.) were caught. The presence in the houses at night of C. cinereus, chiefly an ornithophilic species, results from the poultry frequently admitted to sleep there.
Knock-downs, carried out in six compounds, were negative.
No virus was recovered from this small number of mosquitoes.
All villagers testified that mosquito biting activity was then very low in and around the dwellings, also mosquito nets were not in use.
Aedes aegypti (L.) was worth special attention since Kunz and Hofmann had shown that Marburg virus, multiplies in it after intrathoracic injection although this does not prove A. aegypti transmits the disease in nature.
The prevalence of A. aegypti was assessed in Yandongi, the most urbanized place and therefore the most favorable for its multiplication. 28 compounds were inspected for larvae. Breteau index was 3.6; house and container indices were 2.3. These figures are low and, well under those generally observed during yellow fever epidemics. Other species found were Culex p. fatigans, C. cinereus and C. nebulosus Theo.
No insecticide had been applied during the last months in the ward selected for this inspection. Low A. aegypti indices are usual in wet equatorial countries of Africa, where domestic water storages are generally small.
Culicoides were copiously biting in the early morning and at the end of
Numerous bedbugs were found in many houses but not in all. They all belong to the species Cimex hemipterus (Fabricius).
Though many authors tried in the past to incriminate bedbugs in the transmission of various diseases, either through bites or through feces (1), all evidence was generally negative or inconclusive. However, recently, some selvatic species of bedbugs were shown to be vectors or reservoirs of arboviruses (Kaeng Khoi, Fort Morgan and Bijou Bridge viruses). Still more recently, in Senegal, Hepatitis B virus was detected in C. hemipterus collected from bedding and kept alive without a blood meal for 30 days (2).
In the Yambuku area,, 818 nymphs and adults of C. hemipterus were collected from Yalikonde, Yahombo, Yandongi and Yamonzwa in houses or wards more or less recently affected by the epidemic.
No virus was recovered.
Mammal survey results.
Antibody surveys on Marburg virus in Uganda (3) had pointed to Cercopithecus monkeys, especially C. aethiops (L.), to be involved in the natural cycle of the disease.
Efforts were made to collect specimens from these. However very few monkeys were shot, because the hunters' guns were generally locally made and our cartridges frequently returned intact though having been percussed.
Monkeys were abundant in the area and are frequently used as food.
The species we have registered are all chiefly forest-dwellers :
Cercopithecus ascanius schmidti Matschie, which seems to be common (a small troop was seen), C. nictitans (L.), C. pogonias grayi Fraser, C. neglectus Schlegel, Colobus guereza Rüppell.
Cercopithecus aethiops which is chiefly a savanna-dweller was not observed, 4 although this species is recorded from this part of Zaire by Schouteden (4)
No behaviour anomaly or abnormal mortality had been observed by the hunters among the forest monkeys during the last months.
5 Cercopithecus were collected : C. ascanius (2), C. nictitans, C. pogonias, C. neglectus.
No virus was recovered.
1. Domestic rodents
The black rat, Rattus rattus (L.), was the single domestic rodent. Mastomys natalensis (Smith) was not found in the family compounds. It seems that, according to a process ascertained in other parts of Africa (5,6) Mastomys has been evicted by the common rat.
R. rattus were caught in Yambuku catholic mission, Yahombo, Yalikenga and Yalikonde. They were especially numerous in the latter village. Rice corps frequently stocked in dwelling-rooms were very attractive for them. No abnormal mortality was noticed among these domestic rodents during the epidemic.
The organs of 30 R. rattus were grouped into pools.
No virus was obtained.
2. Wild rodents
On the whole, 88 wild rodents were caught in the nearby forest.
69 Praomys, among which P. tullbergi minor (Hatt) seems to be predominating, 5 Thamnomys rutilans (Peters), 1 Hylomyscus sp., 1 Lemniscomys striatus (L.), 1 Lophuromys a sikapusi (Tem.). Squirrels :
2 Funisciurus sp. and in addition, 6 unidentified specimens subsequently collected.
3 Graphiurus sp.
No virus was recovered.
Wild rodents are occasionally eaten by the villagers, especially the giant gambian rat (Cricetomys emini) (which does not appear in our collections), never the black rat.
3. Other mammals or small vertebrates
Likewise, no virus was obtained from some other wild animals: 2 duikers Cephalophus monticola (Thünberg), 1 giant bat Hypsignathus monstrousus (Allen), 1 lizard Agama sp. and the 7 unidentified net-caught bats.
Organs or sera of cows and pigs gave also negative results.
During the outbreak, the mosquito biting activity was low in and around the dwellings and A. aegypti was very scanty. This does not favour the role of an arthropod in the transmission. Culicoides, by their mere abundance, can be excluded as vectors.
No virus was recovered from more than 800 bedbugs collected. However, this does not exclude definitely the role of bedbugs in intra-family contaminations. Although monkeys are quite common in the Yambuku area forest, only 6 could be shot. From 147 mammals collected in the area, no virus was recovered. No abnormal mortality or behaviour anomaly was observed among the wild fauna and the domestic rodents during or just before the epidemic. However, there is a need for serum surveys for Ebola antibodies in wild animals.
I am indebted to Dr. F. Petter, from the Museum National d'Histoire Naturelle, Paris, for identification or taxonomical verification of some rodents.
1. Burton, G.J. (1963) Bedbugs in relation to transmission of human diseases. Pub]. Hlth. Rep., 78, 513-524.
2. Wills, W., Larouze, B., London, W.T., Millman, I., Werner, B.G., Ogston, W., Pourtaghva, M., Diallo, S., Blumberg, B.S. (1977) Hepatitis-B virus in bed bugs (Cimex from Senegal . Lancet, i i , 217-220.
3. Henderson, B.E., Kissling, R.E., Williams, M.C., Kafuko, G.W., Martin, M. (1971) Epidemiological studies in Uganda relating to the "Marburg" Agent. in Marburg Virus Disease, Martini, G.A. and Siegert, R., eds., SpringerVerlag Berlin, 166-176.
4. Schouteden, H. (1944) De zoogdieren van Belgisch-Congo en van Ruanda-UrUndi, 19 p. 40-42.
5. Rosevear, D.R. (1969) The rodents of West Africa. British Museum (Nat.Hist.), p. 416.
6. Isaacson, M. (1975) The ecology of Praomys (Mastomys) natalensis in Southern Africa. Bull. WHO, 52, 629-636.