Malaria N° 9 - Subject N° 5
ANOPHELES - PLASMODIUM - RELATIONSHIPS; CONSEQUENCE FOR THE MALARIA TRANSMISSION
The inter-relationship between Anopheles and its vectors constitute a subset of relationships which link together the three malaria actors: Man, Plasmodium, and Anopheles. The transmission of malaria involves as much the transfer of the parasite from mosquito to man as from man to mosquito. The following will be discussed in succession, with the aim of illustrating a few particular points of the Anopheles-Plasmodium relationship and their impact on the transmission of malaria: the infected bite of on anopheles, factors modulating the infectivity of gametocytes in the case of for the vector and finally transmission relationships between transmission-morbidity-mortality.
The bite of an anopheles, carrying sporozoites within its salivary glands, involves three distinct but linked processes which are at the some time both distinct and related: the penetration and probing by the mouth parts into the skin of the buccal appendages (probing), the injection of saliva containing sporozoites and blood feeding. Recent discoveries, concerning this infected bite focus mainly on the following four points:
- (1) The number of sporozoites actually injected into the host is approximately ten, but can occasionally be several hundreds. This number is surprisingly small and is equivalent to about 1 % of the number of sporozoites contained within the salivary glands.
- (2) Sporozoites injected into the skin during probing seem to be the only ones capable of reaching the liver of the host sporozoites injected into venous blood during feeding have no effect on the parasitic cycle as they are immediately re-ingested by the mosquito in its blood feed.
- (3) The success of a single bite in infecting a non-immune host is never absolute; though still imprecisely evaluated, it may be of the order of one chance out of two.
- (4) The success of an anopheles bite biting behavior of anopheline with the presence of sporozoites within its salivary glands appears to be modified, according to the vector species and perhaps also, depending on to the plasmodial species, with an increased in the penetration time of probinges and/or an increased average in the mean number of host being bitten in order to achieve complete repletion; such changes augment the transmission of malaria.
Factors modulating gametocyte infectivity for the vector are multiple and once again up to now, relatively poorly understood. Six factors are described here:
- (1) Gametocyte density is probably the most obvious factor; the lowest sufficient gametocyte density is required for vector infectivity to have a reasonable weak probability to be infective for the vector of success, but this condition is not enough, as in some cases, high densities are clearly not infective.
- (2) The sickle cell anaemia status of the gametocyte carrier can increases the infecting
ability of these gametocytes.
- (3) The high proportion of male gametocytes also plays a role in favouring successful the chances of infection.
- (4) Serum factors involving immunity by limiting or blocking transmission, can play an important role.
- (5) The age of gametocytes is an important factor, which is also remains poorly documented. Very young gametocytes at the time when they appear in the peripheral circulation are non-infective or only poorly infective; likewise, several week old gametocytes also have reduced infectivity.
- Finally (6), the direct or indirect impact of medicines anti-malarial drugs can be of great importance. By way of example, chloroquine has a stimulating effect on the infectivity of gametocytes, the sulphadoxine-pyrimethamine combination has a sporonticidal effect. Factors studied which do not influence vector infectivity are: the sex of the gametocyte carriers, their blood group, their rhesus factor, their body temperature and finally the presence and the density of asexual blood borne parasites.
Transmission-morbidity-mortality relationships lie at the heart of the anti-anopheline control problematic. In an epidemic or Lower poor endemicity area it has been clearly shown that transmission must be the lowest possible. On the other hand, in areas of mid to high endemicity, the comparison of epidemiological situations, varying according to the level of transmission, provide solid arguments to show that the level of transmission level is not a determinant factor for overall malaria morbidity and malarial mortality overall. However, this transmission level is a determinant factor for the dynamics of immune acquisition morbidity and mortality: in area of high endemicity the youngest age groups are those which show the highest morbidity and mortality rates clearly more affected in areas of high transmission, whilst the consequences of malaria are distributed more equitably between the different age groups in the weaker transmission regions. Many anti-vector projects (for example with bednets impregnated with insecticide) have shown some efficacy and an improvement in the malaria situation, but it is likely that:
- (1) This improvement is only a short term effect (one or two years) and/or
- (2) That this improvement is not only due to the reduction in transmission, but also to other factors controlled or otherwise which involve case management and health education.