Toxoplasma gondii in meat or meat products for consumption: Molecular diagnosis (PCR); Mouse bioassay.
Toxoplasmosis caused by a coccidia, Toxoplasma gondii, is a zoonosis of universal distribution, one of the most common parasitic protozoan infections worldwide.
This coccidia has two types of hosts, the final one with a sexual cycle, and one broker, only a schizogonic asexual development cycle without sexual reproduction cycle.
The definitive hosts are the felines, such as cats and other wild felines, where sexual forms are developed in a schizogonic and sporogonic cycle, which takes place in the cells of the intestinal mucosa, resulting in tachyzoites as part of schizogonic asexual cycle, that evolve macrogametocyte and microgametocyte (male and female, respectively), which upon reduction produce meiotic macrogametes and microgametes that fertilize, generating the zygote. The mature zygote and generates espoquiste initially containing two sporoblasts and finally two sporocysts each with four sporozoites. Oocysts contain within the esporoquistes with infective sporozoites. Felids excrete millions of oocysts in their feces that contaminate the environment.
Intermediate hosts, where the schizogonic asexual cycle develops, may be any warm - blooded animal, such as any mammalian species or birds that become infected by ingesting mature sporulated oocysts with oocysts, or tissue cysts in tissues mainly muscle or brain intermediate hosts. In the asexual cycle schizogonic, generated from intake oocysts from feces felids, sporozoites from oocysts through the intestinal wall and introduced into cells of various tissues, such as muscle cells, multiplying as very active trophozoites which in the case of Toxoplasma gondii, tachyzoites are called. Tachyzoites multiply and are released from the cells which have multiplied by binary fission, infecting new cells. When they are passing lag phase encyst in tissues, where they remain latently, denominating bradyzoites.
The human infections are often asymptomatic in immunocompetent individuals, considering that one third of the world 's population is infected with Toxoplasma gondii. However, infection can be severe in immunocompromised patients (infected with human immunodeficiency virus -VIH-., Organ transplant recipients, neoplastic, ...). In immunocompromised patients the infection can occur, but also the reactivation of tissue cysts bradyzoites dormant. Brain, eye or lymph node involvement, often the most frequent manifestations. Pregnant women represent a major risk because they can cross the placenta during the parasitemia of tachyzoites released from cells which have multiplied, and move to the embryonic or fetal tissue, with consequent damage generated by their multiplication in them, It is the cause of abortions, neonatal death or born with physical or mental impairment. The greatest risk for placental transmission occurs when a woman lacks antibodies (seronegative), shortly before pregnancy or early this because parasitemia can lead transplacental passage becomes infected.
People become infected when they eat raw or undercooked meat from infected animals, birds (turkeys, chickens, ...) or mammals (mainly pigs, sheep, cattle, ...), where the parasite is feasible, or when they eat food or water contaminated with oocysts (environmental or contaminated vegetables) from cat feces. The types of food, cooking methods, prevalence of infected cats, and animal husbandry methods represent elements that influence the degree of transmission to humans. For example, the current trend in some sectors to favor organic farms breeding pigs increases the prevalence of the infection with Toxoplasma gondii.
The boar (wild boar, Sus scrofa) is considered a good indicator of environmental contamination with oocysts, so several researchers have conducted studies of this intermediate host to determine the degree of parasitism in wild animals. Furthermore, in some European countries meat is consumed locally and distributed in the food chain.
Recommended tests for diagnosis:
Various methods have been used to determine the parasitism in various animal species. To be more affordable, detecting IgG antibodies, is the method that has been used either with serum samples from animals or juice samples obtained from meat of slaughtered animals for human consumption. They have been used many kinds of serological methods (passive agglutination, complement fixation, passive haemagglutination, immunofluorescence, ELISA, ...). These methods have the advantage that they are very affordable laboratories, but have the disadvantage of the need for the serum sample of animals, and sometimes there is no tissue although there parasitación detected antibodies.
The second method used by frequency has been bioassay using mice are inoculated intraperitoneally with digestive or prepared from meat of slaughtered animals homogenates, and in which subsequently the Toxoplama gondii tachyzoites are observed. This method, using mice and cats, is considered the reference method ( "Gold Standard") for detecting parasitism, but has the disadvantage of the need to use test animals, the time required, and not be useful for processing large number of samples.
Currently, they are imposing molecular methods (PCR in various ways: -nested- nested capture in real time, ...). These methods have the disadvantage that there may be different effects in different tissues and not found parasites in tissue being examined, but have the advantage over serology demonstrate the actual presence of the parasite, and to obviate the use of experimental animals with a consequent ethical benefit regarding the use of experimental animals and speed in obtaining results.