The American Journal of Tropical Medicine and Hygiene - Volume 6, Issue 4, July 1957

As indicated by the title, this article deals with the control of vectors. None of its content applies to the eradication of any vector species, which involves quite different criteria.

According to one dictionary, a vector is an “organism, usually an insect, which carries and transmits disease-causing organisms.” This definition is what has come to be known as the classic definition of a vector.

Some 20 years ago the number of arthropod species which had been incriminated in the transmission of various infections of man in this country, and hence were classic vectors, was rather lengthy. But from the standpoint of control, interest was chiefly centered on only four species, all insects. These were the house fly and three mosquito species: Aedes aegypti, Anopheles quadrimaculatus and Anopheles freeborni.

The vectorial status of these four species was clear-cut. A. quadrimaculatus not only could, but did, transmit malaria at the rate of hundreds of thousands of cases annually.

From June 1, 1955 to July 24, 1956 Anopheles gambiae were collected in villages in Marshall territory, coastal Liberia, using three collecting techniques: hand-catching mosquitoes on human bait in the open, trapping with human bait, and hand-catching mosquitoes found resting in the houses. All three methods yield valid population samples for estimating population size, and for studying both malaria and filariasis infectivity rates, demonstrating that gambiae feeds readily in the open and does not need to enter houses. The gambiae population is greatest during the rainy season from May to October, reaching peaks at the beginning and at the end of the rains, which are also the two periods of highest transmission. The gambiae population thus varies inversely with the temperature, which is lowest during the rains.

The oöcyst, sporozoite and total infection rates were found to yield the same epidemiologic pattern, the last being the most accurate and the easiest to interpret. The relationship between the sporozoite rate and the oöcyst rate can be used to estimate the average age of wild caught mosquitoes when the timing of the developmental cycle of Plasmodium in the mosquito is also known. The average age thus derived for the mosquitoes studied was 27 days. Species identification of oöcysts was possible in a third of the positive midguts; of these falciparum made up 82.4 per cent, and malariae and ovale each 8.8 per cent. Vivax was not found in wild gambiae. The disposition of the oöcysts in the midgut differed from that described in A. atroparvus and A. stephensi; the density was greatest in the middle third (45.5%), somewhat less in the anterior third (36.2%), and least in the posterior third (18.3%).

The overall rate of infection in A. gambiae with third stage bancroftian filariae (3.6%) was much higher than rates found by previous workers here, but is probably nearer the truth since it is based on many more dissections. Previous reports that A. melas is a more efficient vector of filariae than gambiae were based on small samples and are not convincing. Seasonal variation both in rate and in density of infection was noted, both increasing with the increase in the gambiae population at the beginning of the rainy season, both being lowest during the dry season. Geographically, rates of infection were found to be highest near the tidal rivers. becoming progressively lower inland, away from these rivers, and on higher ground.

Plasmodium malariae (USPHS Strain) has developed resistance to pyrimethamine (Daraprim®) where the amount of drug given was greater than that recommended for suppression.

Four experiments designed to test the efficiency of monthly suppressive doses of antimalarial drugs for the control of malaria in indigenous West Africans are reported. The results consistently showed that monthly pyrimethamine in 25 mgm. doses to adults and children over three years and 12.5 mgm. doses to children under three years, in the absence of mosquito control measures, was effective in markedly lowering parasite rates and reducing clinical malaria attacks for periods of from one to two years. They further demonstrated that if strains of P. falciparum resistant to pyrimethamine occur in this part of West Africa, they are much less prevalent than reports from East Africa would indicate for that area and do not interfere with malaria control under the conditions of the experiments described in this report.

The chimpanzee displays a natural resistance to the blood forms of Plasmodium ovale but allows multiplication and growth of this phase when splenectomized.

The tissue phase of P. ovale is able to grow and reproduce normally in chimpanzee liver and persists there for at least 39 days. The natural resistance of the chimpanzee to the blood phase is without effect upon the tissue phase of P. ovale.

P. ovale is unchanged on passage through chimpanzees and is able to infect man following two chimpanzee-mosquito passages.

Anopheles gambiae is established as an efficient vector of P. ovale.

Schizonts of P. ovale in liver pieces of a chimpanzee taken 7, 18 and 39 days after infection with sporozoites are described and discussed.

A peculiar type of degeneration has been observed in cultures of small race E. histolytica in the presence of penicillin G, yielding structures resembling B. hominis. Penicillin G was inhibitory to the propagation of cultures of two strains of small race E. histolytica in mixed bacterial flora in concentrations which had no effect on similar cultures of two large race strains.

Chloroquine as used commonly in dermatologic practice has relatively low toxicity in spite of prolonged course of therapy. The most frequent reactions are minor disturbances such as anorexia, nausea, weight loss, nervousness and difficulty in accommodation. Less frequent reactions are maculo-papular drug eruptions and achromotrichia. The rarer complications are major drug eruptions and visceral reactions. Attention to minor reactions will often prevent more serious reactions later. Hydroxychloroquine (Plaquenil) is usually, but not always, more easily tolerated than chloroquine.

Administration of single doses of nitrofurazone of 50 to 150 mg./kg. I.M. to 32 guinea pigs each infected with one of 11 different strains of T. gambiense cured half the infections and suppressed the rest. The effectiveness of a given dose of nitrofurazone varied with the strain of T. gambiense; infections caused by some strains (E, K, X, or Z) were cured, while infections caused by other strains (M, Q, R, T, U, and Y) were only suppressed. Variable therapeutic results were obtained with infections caused by strain L. Nitrofurazone produced a better therapeutic response in guinea pigs infected with T. gambiense when the drug was administered about a month following the infection than when treatment was initiated three to five months following infection.

Single doses of nitrofurazone were apparently not so effective in subacute T. rhodesiense infections as in chronic T. gambiense infections in guinea pigs. Single doses of nitrofurazone, 50 to 150 mg./kg. I.M., administered to 30 guinea pigs each infected with one of seven different strains of T. rhodesiense were curative in 10 per cent, suppressive in 80 per cent and ineffective in 10 per cent of the animals.

Nitrofurazone, 5-nitro-2-furaldehyde semicarbazone (Furacin), a new trypanocidal drug, was used for the first time on human beings in the treatment of 32 patients suffering with Trypanosoma gambiense sleeping sickness, with encouraging therapeutic results. Administered orally, the drug is readily absorbed from the intestinal tract and is effective against trypanosomes in the circulating blood and in the cerebrospinal fluid. Oral doses of 2.1 to 12.5 mg./kg. t.i.d. for seven to thirty-six days were tolerated by the patients without any permanent ill effects. During therapy some cases experienced side effects of temporary nausea and vomiting, and muscular and articular pains in the legs, which subsided within a month following cessation of therapy.

Cures were effected with nitrofurazone alone in 2 of 3 adults without central nervous system involvement; 2 of 3 children with serious C.N.S. involvement were cured with a combination of nitrofurazone and Lomidine. A so-called “cocktail treatment” of nitrofurazone in conjunction with Bayer 205, Lomidine and Arsobal produced favorable prognoses in 9 of 20 hopeless relapsing cases refractory to all previous treatment. This combination was no more toxic than any of the drugs taken separately.

In treating African sleeping sickness, particularly chronic, drug-resistant cases with severe nervous involvement, it is recommended that nitrofurazone be used orally, either alone or in conjunction with other trypanocides.

Giardiasis, even when asymptomatic, is a public health problem in hospital personnel. With the aim of avoiding the occasional toxic effects of atabrine, amodiaquin was administered in a single dose of 0.6 gm. to 15 adults harboring Giardia lamblia, resulting in the cure of only 5, or 33 per cent.

The staff of the Gorgas Memorial Laboratory has been interested for a number of years in the vectors of yellow fever in Panamá. In the fall of 1949 an attempt was made to isolate virus from mosquitoes captured at two locations; Pacora, where human yellow fever deaths were recorded in November and December, 1948 and Buena Vista, where deaths occurred in August and September, 1949. From the end of September, 1949 to the onset of the dry season in early January, 1950, 2,723 mosquiotes were collected from the Pacora station and 1,672 from the Buena Vista station. However, no yellow fever virus was recovered (Rodaniche, 1951). Although no further human cases were recorded from eastern Panamá until the present year, there was reason to suspect the infection had remained endemic there. This view was supported by the finding of occasional juvenile monkeys with high titers of neutralizing antibodies against yellow fever in their blood (Rodaniche, in press).

In 1956 Rodaniche reported the first isolation of Ilhéus (Ilhéus encephalitis) virus in Central America and reviewed the literature. The virus was recovered from a pool of mosquitoes of the genus Psorophora, including principally P. ferox but also P. lutzii and P. varipes. In the present publication a brief report will be made of the isolation of this virus from Sabethes chloropterus captured in Guatemala in 1956. This finding is of interest because it represents the first isolation in Guatemala and the first from this species of mosquito.

The mosquitoes were captured by Drs. J. Boshell and G. Bevier of the Pan American Sanitary Bureau in the Motagua valley of Guatemala during the 1956 outbreak of jungle yellow fever there, and forwarded to the Gorgas Memorial Laboratory where they were classified. The principle objective of the investigation was the isolation of yellow fever virus, and the recovery of the Ilhéus virus was an incidental finding.

St. Louis encephalitis virus has been isolated from three ground pool-breeding species of mosquitoes collected in Trinidadian forests. Identity was established by comparison with a known strain of St. Louis virus and other related viruses. These represent the first strains of St. Louis virus known to be isolated from mosquitoes outside of the continental United States.

St. Louis encephalitis virus has been recovered from the serum of a nestling bird in Trinidad, B.W.I. This is the first time this agent has been recovered from a local, nonmigratory bird south of the continental limits of the United States.

Sera from 155 children in a new housing project near Jerusalem were examined for antibodies against the three types of poliomyelitis. From the age of two to three years onwards more than 50 per cent of the sera showed antibodies against all three types of poliomyelitis. In the age group 6–7 years 95 per cent of the sera contained antibodies against all three types. Antibodies were more frequent in children of families where crowding and a low degree of cleanliness were present. Under the conditions observed no influence of the economical status of the family was apparent.

Fifty-seven blood sera obtained from life-long residents of Eastern Saudi Arabia were tested in tissue culture for neutralizing antibodies against poliomyelitis viruses, Types I, II, and III. Of the 57 sera tested, 55 were positive against one or more types. Antibodies to Type I were detected in 75%, against Type II, 77% and against Type III, 70%. It seems probable that all three types of poliomyelitis are endemic in the area and that exposure occurs at an early age.

Large numbers of Xenopsylla vexabilis hawaiiensis were needed to carry out tests of its efficiency as a plague vector. Efficient culture of the Hawaiian flea was accomplished in white enameled buckets containing a housed host rat together with a larval substrate consisting of sand and powdered dog pellets (Friskies). The pupae were kept at a temperature of from 23° to 27°C. and a relative humidity of from 93 per cent to 100 per cent. These conditions differed from those required by Xenopsylla cheopis, which was maintained separately at lower temperature and humidity. The pupae were sifted from the sand approximately every 3 weeks for maximum production.

In nature, X. v. hawaiiensis was found to breed in rat nests which consisted mainly of dried grass leaves. This evidence, together with the results of laboratory culture, suggests that the fleas probably do not breed on the surface of the ground, nor do the larvae require green grass as formerly postulated.

By examining the labor populations, recruited from different parts of the country, of several rubber plantations and a mining company located in areas where transmission of Schistosoma haematobium has never been known to occur, it has been possible to determine the general distribution and comparative intensity of this infection throughout Liberia. The coastal half of the country was found to be free of infection as was the entire Eastern Province and much of the Western Province. The highest infection rates were centered in the upper part of the Central Province between the St. Paul and St. John rivers; groups from this region showed infection rates above 50 per cent.

During 1953 a single fecal sample from each of over 10,000 Puerto Rican school children was examined for helminth eggs, especially those of S. mansoni. The following average percentage rates of infection were found: S. mansoni, 10.0; hookworm (Necator and Ancylostoma), 17.1; Trichuris trichiura, 92.6; Ascaris lumbricoides, 20.3; and Strongyloides stercoralis, 0.4. The incidence of S. mansoni in boys and girls was 12.5 and 8.1 per cent, respectively. This difference was significant at odds greater than 99:1. A high degree of correlation (r = 0.89) was found between the age of the boys and their rate of S. mansoni infection, while with the girls, these factors were not correlated (r = 0.38).

In general, the abundance of the snail vector (Australorbis glabratus) was related directly to the incidence of human schistosomiasis. There was little or no agreement between S. mansoni infection rates in snails and those in human beings at a given time in a particular locality. The epidemiological aspects of these findings are discussed in the light of results obtained from a study of human beings, snails and S. mansoni infections in three stream communities and from other studies in selected regions in Puerto Rico.

The specificity of a complement-fixation test for schistosomiasis, using as antigen a buffered saline extract of ether-extracted adult worms, was evaluated in other helminthic infections. No serologic reactions were observed with the sera of 39 nematode-infected natives of Trinidad, an island whose natives have never been found to have schistosomiasis. The sera of only two of 69 nematode-infected Puerto Ricans without overt evidence of schistosomiasis were reactive. Two of 8 Japanese infected with P. westermani reacted in the test for schistosomiasis.