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LASSA FEVER VIRUS ANTIBODIES IN MASTOMYS NATALENSIS CAUGHT IN DIFFERENT PARTS OF NIGERIA

A. FABIYI (1), B. DOBROKHOTOV (2), 0. TOMORI (1), A.A. ARATA (3)

1. Virus Research Laboratory, University of Ibadan, Nigeria.
2. WHO/Arbovirus Vector Research Unit, Enugu, Nigeria.
3. WHO/Vector Genetics and Bionomics, Division of Vector Biology and Control, Geneva, Switzerland.

INTRODUCTION

In 1972, Monath et al. (1) reported the isolation of Lassa virus from the tissues of Mastomys natalensis during an epidemic of Lassa fever in a community in Sierra Leone. The majority of the virus isolates were from rodents caught in homes occupied by Lassa fever patients. The role of this rodent as a reservoir was confirmed by studies in Nigeria (2), when animals were collected between November and December 1972, and March and April, 1973, during the early dry season and the beginning of the rainy season. There was no evidence of an epidemic of Lassa fever during the periods of rodent collection. Most of the epidemiological surveillance for Lassa have been through serological evaluation of antibodies in man; there are no known studies of antibodies in sera of Mastomys natalensis (Wolf, et al. (1977) in a recent article in the Bulletin of WHO 55(4): 441-444, reported on an arenavirus closely related to Lassa virus from Mastomys in south-east Africa.). The present report is based upon the results of serological evaluation of CF antibodies in sera of Mastomys natalensis collected in several parts of Nigeria.

MATERIALS AND METHODS

During studies on the ecological distribution of Mastomys natalensis carried out by one of us (Dobrokhotov), attached to the WHO Arbovirus Vector Research Unit stationed in Enugu, Anambra State of Nigeria, sera samples were taken. Areas surveyed included NerPanskshin, Pankshin in Plateau State and Azare village 15 km apart in Bauchi State; Kaduna in Kaduna State and Mamu River Forest Reserve near Enugu in Anambra State. The rodents were bled in the field and sera placed in refrigerated plastic vials with screw caps, and transported to the Virus Research Laboratory in Ibadan. This report is based on 104 Mastomys sera: of these 29 were from Ner-Pankshin, 28 from Pankshin, both in Plateau State; 22 were from Azare-Gadau and 25 from AzareDalli in Bauchi State. Other sera from Anambra 10 and Kaduna 15 states were negative.

Complement Fixation Test (CFT)

Sera were inactivated at 60ºC for 30 minutes in the same vials in which they were transported to the Laboratory. None of the vials was opened until after inactivation.

Antigents for the CF test were supplied by the Center for Disease Control (Atlanta, USA), as lyophilized betapropiolactone (BPL) inactivated Vero tissue culture propagated Lassa virus. The control antigen consisted of uninoculated Vero tissue culture cells treated similarly as the Lassa virus antigen.

The CFT was conducted according to the micro-method of Weinbren as modified by the Virus Research Laboratory (3). Initially, the one hour and overnight incubation periods were used, however, the overnight incubation consistently gave reproducible results, and was used in subsequent tests. All the sera positive in the screening tests, as well as those with anti-complementary reactions were later titrated.

RESULTS

The results of the Lassa virus CF antibody tests on Mastomys natalensis sera are shown in Table 1: 56 sera from other rodent species from the same localities: Tatera, Myomys, Rattus, Arvicathus, Lemniscomys, the hedgehog, Erinaceus, and shrew, Crocidura were negative for Lassa CF antibodies.

Each serum was tested at least three times, and the reciprocals of serum dilutions reported here represent the average of the titres of these tests.

The titres of positive Mastomys sera from Ner-Pankshin ranged from 1:4 to 1:256: eleven (37%) of 29 sera were positive, with 4 showing high CF titres (1:32 - 1:256). Ten of the total sera tested were anti-complementary.

In Pankshin none of the Mastomys sera was positive, but 5 were anticomplementary. Mastomys sera from Azare-Gadau were all negative for antibodies to Lassa virus. One sera of 25 (4%) from Azare-Dalli was positive with a titre of 1:8.


DISCUSSION

In humans, generally the presence of CF antibodies in the circulating blood is an indication of a recent virus infection. In Lassa fever, however, CF antibodies could only be detected 20 or more days after the onset of the disease. There is also evidence of persistent antibodies four to five years after either infection or exposure to the virus (3).

TABLE 1
RESULT OF CF ANTIBODY STUDIES ON SERA OF MASTOMYS NATALENSIS FROM SEVERAL PARTS OF NIGERIA


CF TITRES X


Source

     

Number positive

         
 

Total


4


8


16


32


64


128


256

 

Plateau State:

                 

Ner-Pankshin


18(a)

 

22


3


2


1


0


1


37


Pankshin28(b)


0


0


0


0


0


0


0


0

 

Bauchi State:

                 

Azare-Gadau


22


0


0


0


0


0


0


0


0


Azare-Dalli


24


0


1


0


0


0


0


0


4


Total


92


2


3


3


2


1


0


1


11.5


a) includes 5 anti-complementary.

b) includes 5 anticomplementary.

x) reciprocals of serum dilutions, giving at least a 3+ fixation in CF tests.

In addition, infected human serum has been shown to have high concentration of the virus (4,5). Virus has been isolated from human blood 19 days after onset and from urine 32 days after onset (5,6,7,8). According to Walker et al. (9), experimental infection of neonatal mastomys did not cause any clinical disease or pathological lesions despite the presence of virus in the blood, lymphnodes, liver, spleen, lung, brain, urine and throat secretions throughout a 74 day study. Infected adult Mastomys also remained normal but had virus in many organs, the virus persisting for 103 days at the termination of the experiment.Lassa virus CF antibodies were also demonstrated in infected animals.

Serological surveys for Lassa virus CF antibodies have been carried out among hospital personnel in different parts of Nigeria. In the Plateau and Kaduna States covering Ner-Pankshin, Pankshin, Vom, Birkin Ladi, Jos and Zonkwa the highest positive rate (9.8%) occurred in the Ner-Pankshin and Pankshin areas, whereas other areas showed an average positive percentage of 5.3% only (10). Furthermore, epidemiological investigations of a Lassa fever outbreak which occurred in a road company camp in the Pankshin area in 1976 showed that 15.5% of those bled for antibody evaluation had Lassa antibodies. There was, however, no evidence of clinical Lassa virus infection during the period of investigation (11). On the other hand, during the 1974 Lassa outbreak in Onitsha, Anambra State, none of the human and animal sera tested was positive for Lassa CF antibodies, although 3 cases of Lassa fever infection were confirmed by virus isolation and/or antibody studies (12). Finally, out of a total of 8 Lassa virus strains isolated from tissue pools and blood specimens obtained from Mastomys natalensis, five strains were from Ner-Pankshin and two from Vom in the Plateau State and only one from the Bauchi area. These virus isolations correspond very closely with the antibody findings reported in this paper.

The Pankshin area of the Plateau State may be considered a highly endemic zone for Lassa fever in the state, as evidenced by the high level of antibodies in humans and Mastomys, as well as the high rate of virus isolation from Mastomys in that area.

There is no doubt that a combination of surveillance in man and in mastomys using serological and isolation methods coupled with study of ecological distribution of the rodents would yield valuable information delineating the distribution of Lassa fever disease in Nigeria. Such knowledge would lead to proper surveillance, and proper selection of study areas and the planning and execution of adequate and proper control measures.

Apparently, there are two different species within the "Mastomys natalensis" complex: one (dark colour) is largely commencal, while the other (brown colour) tends to be free ranging. It has also been confirmed that certain populations have 32 chromosomes and others 36, with no apparent hybrids. Studies are being carried out to try to correlate morphological and cytotaxonomic characters. At the time of these studies (end of rainy season) both forms occurred in houses. Antibodies have been found among both forms and among males and females. Antibody titres were higher in adult animals than in young ones.

SUMMARY

A survey for Lassa virus complement fixing (CF) antibodies was carried out on the sera of rodents and shrews trapped from different parts of Nigeria. Eleven of 29 Mastomys natalensis (37%) trapped at Ner-Pankshin, Plateau State, and 1 of 25 (4%) trapped in Azare Dalli, Bauchi State, were positive for Lassa fever virus CF antibodies. All other small mammal sera were negative. The results of the antibody surveillance in these rodents correspond with the findings of previous virus isolation attempts in mastcmys and with human antibody surveillance in different parts of Nigeria.

ACKNOWLEDGEMENT

This study was supported in part by the Viral Diseases Unit and the Division of Vector Biology and Control of the World Health Organization and the Federal Ministry of Health, Nigeria.

The cooperation of staff of the WHO Arbovirus Vector Research Unit, Enugu (Drs. Y.H. Bang, A.B. Knudsen and D.N. Bown); Dr. E.A. Smith of the Directorate of the Public Health Service, Nigeria; and the WHO Regional Office for Africa (Brazzaville), is greatly appreciated.

REFERENCES
1. Monath, T.P., Newhouse, V.F., Kemp, G.E., Setzer, H.W., Cacciapuoti, A. (1974) Lassa virus isolation from Mastomys natalensis rodents during an epidemic in Sierra Leone, Science, 185: 263-265.
2. Wulff, H., Fabiyi, A., Monath, T.P. (1975) Recent isolations of Lassa Virus from Nigerian rodents, Bull. WHO, 52: 609-613.
3. Fabiyi, A., Tomori, 0. (1975) Use of complement fixation (CF) test in Lassa fever surveillance: Evidence for persistent CF antibodies, Bull. WHO, 52: 605-608.
4. Henderson, B.E., Gary Jr., G.W., Kissling, R.E., Frame, J.B., Carey, D.E. (1972) Lassa fever virological and serological studies, Trans. Roy. Soc. Trop. Med. Hyg., 66: 409-416.
5. Monath, T.P., Maher, M., Casals, J., Kissling, R.E., Cacciapuoti, A. (1974) Lassa fever in the Eastern Province of Sierra Leone II. Clinical and virological studies in selected hospitalized patients, Amer. J. Trop. Med. Hyg., 23: 1140-1149.
6. Carey, D.E., Kemp, G.E., White, H.A., Pinneo, L., Addy, R.F., Fom, A.L.M.D., Stroh, G., Casals, J., Henderson, B.E. (1972) Lassa fever epidemiological aspects of the,1970 epidemic, Jos, Nigeria, Trans. Roy. Soc. Trop. Med. Hyg., 66: 402-408.
7. Monath, T.P., Mertens, P.E., Patton, C.R., et al. (1973) A hospital epidemic of Lassa fever in Zorzor, Liberia, March-April, 1972, Amer. J. Trop. Med. Hyg., 22: 773-779.
8. Leifer, E., Goeke, D.J., Bourne, H. (1970) Lassa fever, a new virus disease from West Africa, II. Report of a laboratory-acquired infection treated with plasma from a person recently recovered from the disease, Amer. J. Trop. Med. Hyg., 19: 677-679.
9. Walker, D.H., Wulff, H., Lange, J.V., Murphy, F.A. (1975) Comparative pathology of Lassa virus infection in monkeys, guinea pigs, and mastomys natalensis, Bull. WHO, 52: 523-534.
10. Fabiyi, A., Tomori, 0., Pinneo, P. Lassa fever antibodies in hospital personnel in the Plateau State of Nigeria (In preparation).
11. Smith, E.A., Fabiyi, A., Kuteyi, O.E., Tomori, 0. Epidemiological aspect of the 1976 Pankshin Lassa fever outbreak (In preparation).
12. Bowen, G.S., Tomori, 0., Wulff, et al. (1975) Lassa fever in Onitsha, East Central State, Nigeria, 1974, Bull. WHO, 52: 599-604.

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