RAT CONTROL IN A PLAGUE OUTBREAK IN MALTA BY S. A. BARNETT, Ministry of Food* Infestation Control, London (With Plates 1 and 2 and 2 Figures and 1 Map in the Text) The population of Malta itself is 243,500, of whom 1. INTRODUCTION 179,400 live in the cities, their suburbs or outskirts. Bubonic plague broke out in Malta in June 1945. At The density is approximately four persons per acre, the request of the Colonial Office the present writer or 2526 per square mile; this is three times that of spent 8 weeks in Malta during the months August to England. September advising the Government on methods The rat problem in Malta was thus primarily that for the destruction of rats. The present paper de- of a densely populated sub-tropical area. But there scribes the rodent population of Malta; the recent is also a rural problem, since the dry, shallow soil of distribution of plague and murine typhus; the Malta is intensively cultivated, with potatoes and control methods applied in Malta; and the results wheat as important crops. Citrus trees, olives, carobs of control. and prickly pear are generally distributed where the The sources of information have necessarily been ground is fertile. Fields are small and bordered by numerous. Much of the data has been supplied by rubble walls (PI. l,figs. 1 and2; PI. 2, fig. 3). There is officers of the Medical and Health Department of thus plenty of cover for rats even in the dry season. the Government of Malta. In particular should be Gozo is a smaller island than Malta, generally mentioned Dr A. C. Briffa, Acting Chief Government similar, but less densely populated and more Medical Officer at the time of the plague outbreak; agricultural. Prof. V. J. Mifsud, Analyst and Bacteriologist to the Government of Malta; Doctors J. Galea, J. Morana (b) Wild mammals and E. Stilon, Medical Officers of Health; and Wild mammals include Rattus norvegicus Berken- Mr P. Cauchi, Rodent Control Officer. Valuable in- hout, R. rattus L., Mils musculus L. (the house formation has also been provided by the Naval, • mouse), and Oryctolagus cunicuhus L. (the rabbit); Army and R.A.F. establishments in Malta. Miller (1912) also mentions two species of weasel, The plan for the systematic destruction of rats in but these were not observed. Weasels are mouse Malta •was carried out by these authorities with ex- eaters and, even if present, are not of significance ceptional thoroughness and vigour, and it is thought for rat control. that the experience of this campaign will be of Table 1 gives the figures for rats trapped in the interest to other health authorities faced with year beginning 1 October 1944. The dominant rat is similar problems. Rattus norvegicus, the common brown rat of Western Europe. In Malta it infests food stores, private 2. ECOLOGY dwellings, shops and sewers in the towns; tips and farms in the countryside. The piles of rubble in (a) General Valletta and the Three Cities^ due to bombing, were The geology and climate of Malta are described by believed to harbour rats, but it is doubtful if they Harrison & Hubbard (1945). The Maltese Islands, were important: the important question was not of which Malta itself and Gozo are the only im- whether shelter was available, but whether there portant ones, are entirely of sedimentary rock, with was food. For the reasons already given there is two layers of hard coralline limestone and one of shelter for R. norvegicus everywhere in Malta and soft Globigerina limestone. Almost all buildings Gozo. In the agricultural areas the only sites of are of soft limestone (PI. 1, figs. 1 and 2). External infestations which were clearly of hundreds of walls typically have outer and inner skins of stone, rats were refuse tips. Farm buildings and their with a filling of rubble and mortar. They are surroundings were usually infested but not very generally at least 2 ft. 6 in. thick. This type of heavily as a rule, at least during the dry season, structure provides rats with excellent nesting sites when these observations were made. The impression in the walls. The climate is that of the north gained was that rats were often in twos or threes, sub-tropical zone, with a mean annual rainfall of sometimes in tens or twenties, but rarely more. The 21-5 in. Most of this rain falls during the winter. heaviest infestations of farm buildings seen were in * Infestation Control has now been transferred to Ministry of Agriculture and Fisheries. Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.234, on 30 Sep 2021 at 01:06:22, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0022172400036019 S. A. BABNETT 11 Gozo. The main sites of burrows, apart from buildings 3 were refuse collectors, and a fourth was a son of and yards, were the bottoms of the rubble walls and one of the 3; 5 others were associated in work, or the bases of prickly pear trees (PI. 2, figs. 3 and 4). topographically, with refuse collection.* The other species of rat, R. rattus, the so-called By the end of 23 December 1945 there had been black or ship rat, is also found throughout Malta. 75 cases with 20 deaths. (Systematic rat de- Unlike R. norvegicus it nests above ground, and in struction as described below began in August.) buildings in Malta it behaved in the manner familiar Since then, only 5 cases have been diagnosed; on in Britain, often infesting roofs and upper floors and 2 February, 2 March and 3 at Zebbug in the 2nd week nesting in walls and other suitable cover. It was of June. The last 3 occurred during the week before reported to nest in trees in the countryside, but this Zebbug was due to be systematically disinfested. was not seen. The three main colour varieties are There had been no previous cases of plague there. all present in Malta, namely, R. rattus rattus L., The final figures were 80 cases, with 22 deaths, which is black (at least on the back); R. r. alex- giving a case mortality of 27-5 % (see Text-fig. 1). andrinus Geoffroy, grey-brown; and R. r.frugivorus The map shows the distribution of plague cases. Rafinesque (also called tectorum Savi), which has Where an individual lived and worked in different a tawny brown back and pale belly. Inland, nearly places he is recorded on the map at the place where all R. rattus are of the last type. other cases were identified. Thus, if a man worked Table 1. Rats examined in bacteriology laboratory, October 19i4r-September 1945 if. rattus R. rattus var. frugivorus R. norvegicus all varieties , %of Total Total 0/ Total o/ Total it. rattus a 6 (b x 100)/a c (c x 100)/a d (dxl00)/c From naval dockyard 9,884 8,865 89-7 1019 10-3 234 230 From all other sources 29,395 27,511 93-6 1884 6-4 1449 77-0 Total 39,279 36,376 92-6 2903 • 7-4 1683 57-9' The above figures were kindly supplied by Prof. V. J. Mifsud, Government Analyst and Bacteriologist. Nearly all the rats were obtained by trapping; Since the two species may react differently to trapping, and since the trapping was not done'at random, the figures cannot be taken to show the relative numbers of the two rat species,, except very approximately. in the dockyard (which was certainly a plague area)> 3. PLAGUE IN MALTA and-lived in an inland village where there was no Plague has been recorded in Malta in 1813, 1917, plague, his case would be recorded on the map as. 1936-7 (see Bernard, 1937), and 1945. Of the out- belonging to the dockyard. breaks in the present century that in 1917 was of There were no cases of plague in Gozo. only 8 cases. In 1936-7, in a period of 13 months, During the period under review the examination there were 33 confirmed cases with 12 deaths. It of rats brought in dead (from whatever cause) was- was observed in this outbreak that a significant continued."!" From June 1945 to June 1946 in- proportion of cases was among persons associated clusive, out of 22,902 examined, 659 were R. rattus. with refuse disposal. In 1945 the first definite case Of this total, 20 rats were diagnosed as infected (se& was diagnosed on 17 June, and was from Hamrun. It Table 2) and of these 15 were R. norvegicus. It will soon became evident that there were two main areas be noticed that this species is clearly implicated as. involved: a large one including Marsa (the com- an important vector of plague in this outbreak (see mercial port area), Msida, and part of Birkirkara; Barnett, 1946). Although there was evidence of a. and a small one centred at Bubaqra. As is usual widespread epizootic there was evidently a low inci- in such outbreaks (see e.g. Wu Lien-Teh, Chun, dence of infection; there were no reports of heavy Pollitzer & Wu 1936), the majority of infected mortality among rats which could be attributed to- persons were accustomed to walking about in bare plague. feet in filthy surroundings which provided harbour- age for fleas. In Tower Road, Bubaqra, in which * This information was kindly supplied by Dr J. most of the Bubaqra cases lived, there were three Morana, Medical Officer of Health. privately owned refuse heaps.