Experimental Transmission of African Swine Fever Virus by the Soft Tick Ornithodoros (Pavlovskyella) marocanus (Acari: Ixodoidea: Argasidae) R. G. E N D R I S 1 AND W. R. H E S S 2 Plum Island Animal Disease Center, USDA-ARS-NAA, P.O. Box 848, Greenport, New York 11944-0848

J. Med. Entomol. 29(4): 652-656 (1992) ABSTRACT A total of 1,600 Ornithodoros (Pavlovskyella) marocanus larvae were fed on 74 a pig with a viremia of 10 HAdso/ml of African swine fever virus (ASFV). Infected larvae were sampled daily for 15 d, and nymphs were sampled at least once per instar until they became adults. Initial titers of 10 448 HAdso per larva declined to 10 404 within 2 d. Larval titers reached a maximum of 10 60 HAd50 per larva 10 d after the infective blood meal. Nymphs of each instar were fed on a susceptible pig and in each case transmitted ASFV by bite. Virus titers for first to fourth instars ranged from 104 61 to 103-34 HAd50 per nymph. Transstadial survival occurred in subsequent first, second, third, and fourth instars with an 89% survival rate over 250 d. Approximately 30% of adult ticks that were infected as larvae remained infected and transmitted ASFV to susceptible pigs 588 d later. In addition, ASFV was recovered from the same adult ticks 655 d after the infective blood meal. KEY WORDS Arachnida, Ornithodoros (P.) marocanus, Arbovirus, African swine fever virus

I T HAS LONG BEEN SUSPECTED that an arthropod vector was involved in the transmission of African swine fever virus (ASFV) from indigenous wild pigs to domestic pigs in Africa (Montgomery 1921). Sanchez-Botija (1963) in Spain first showed that an arthropod could harbor the virus in the field. Swine died from infection with ASFV when they were reintroduced to farms where ASFV had killed pigs 6-12 mo earlier. Initially, this was attributed to the persistence of the virus in dried blood and contaminated soil. On one farm, swine kept in a pen had died from ASFV. Four months after their death, SanchezBotija recovered ASFV from argasid ticks thought to be Ornithodoros erraticus (Lucas) taken from this pen. This observation suggested that O. erraticus could have been infected by feeding on viremic pigs; O. erraticus could retain the virus for up to 4 mo; and that the ticks were able to transmit ASFV by bite to other pigs. These findings inspired workers in Africa (Plowright 1977) to examine Ornithodoros moubata Murray, a tick frequently found in warthog burrows, and led to extensive studies that clearly established ASFV as an arbovirus (Plowright et al. 1969a,b; 1970a,b; 1974). Sanchez-Botija (1982) later reported that when swine were reintroduced into premises that had not been occupied by pigs for up to 8 yr, they could be infected by the bites of ticks. 1 Merck Research Laboratories, P.O. Box 2000, Rahway, NJ. 07065-0900. 2 395 Gillette Drive, East Marion, N.Y. 11939.

The taxonomic status of the argasid species associated with ASFV in Spain and Portugal has been unclear. In all previous reports dealing with ASFV and its associated arthropod vector in Spain and Portugal, O. erraticus, a soft tick species widely distributed along the Mediterranean coast in northern Africa, has been the designated vector. However, a careful review of the literature revealed that the soft tick associated with ASFV in Spain and Portugal was O. marocanus Velu. Velu (1919) identified the soft tick from Spain and Portugal as O. marocanus and Delanoe (1922) concurred with his finding. ColasBelcour (1929) specifically compared O. marocanus from the Iberian peninsula and Morocco with O. erraticus from northern Africa and disputed the finding that O. marocanus and O. erraticus were distinct species. El Shoura (1987) recounted more recent observations on the three species that comprise the erraticus group: O. erraticus (small form, found in northern Africa from Egypt to Kenya), O. sonrai (intermediate form, found from Tunisia to Senegal), and O. marocanus (large form, found in Iberia and Morocco). Briefly, Baltazard etal. (1950), Hoogstraal et al. (1954), and El-Zaidy (1958) recognized the small and large forms of O. erraticus on a morphological basis. Wallis & Miller (1983) demonstrated by electrophoretic analysis of proteins that two of the three closely related species (O. erraticus and O. sonrai) in the erraticus group are distinct species. The original descriptions of O. erraticus (Lucas), O. marocanus Velu, and O. sonrai Sautet & Witkowski (1944) have been ac-

July 1992

ENDRIS & HESS: TRANSMISSION OF AFRICAN SWINE FEVER VIRUS

cepted generally and Hoogstraal (1985) reiterated the validity of O. marocanus as a distinct species. We accept this judgment and now regard the soft ticks associated with ASFV in Spain and Portugal as Ornithodoros (Pavlovskyella) marocanus. Although ASFV still persists on the Iberian peninsula, efforts have been made only recently to learn more about the biology of O. marocanus, its interaction with ASFV, and its role in the maintenance of the disease in Spain and Portugal (Caiado et al. 1988, Hess et al. 1989, Canals et al. 1990, Oleaga-Perez et al. 1990). The study reported here was begun in 1987 and was forced to end before the results were fully expressed. However, sufficient information was gathered to point out the important role of O. marocanus in the epidemiology of ASFV on the Iberian peninsula and the difficulties that must be overcome to eradicate this disease. The results of this study enable us to suggest a number of strategies that may be employed to rid farms of infected O. marocanus populations. Materials and Methods Methods for virus detection and titration from blood and ticks, infection of swine, sampling procedures for infected ticks, and procedures for the conduct of transmission trials were described by Endris et al. (1991). Virus. The virus isolate used throughout this study was supplied by Alexander Galo from an infected domestic pig in Portugal in 1986. The isolate was characterized as hemadsorbing and highly virulent for swine by the criteria of Pan & Hess (1984). The isolate had been passed once in swine and frozen in a spleen—blood suspension at -70°C for

Experimental transmission of African swine fever virus by the soft tick Ornithodoros (Pavlovskyella) marocanus (Acari: Ixodoidea: Argasidae).

A total of 1,600 Ornithodoros (Pavlovskyella) marcocanus larvae were fed on a pig with a viremia of 10(7.4) HAd50/ml of African swine fever virus (ASF...
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