Toxoplasmosis – a threat for animals and humans



Toxoplasmosis is a widely spreading zoonotic infection, a disease that can transfer to people from animals. Toxoplasma (T.) gondii, an intracellular protozoon of class Apicomplexa, is causative agent of toxoplasmosis which completes its life cycle in more than one species and cause infection in different species. It is classified into three genotypes on the basis of polymorphism of genes (types I, II, and III). T. gondii type-I is more aggressive in humans. It replicates in the intestine of the primary host (cats) and also gets sexual maturity there.


Cats are the most concerned source although some other species of animals like bovines, hares, small ruminants, poultry birds, horses and some ungulate species i.e. kangaroos, wild boars act as intermediate hosts. Young animals and animals which have vulnerable immune system are infected frequently. In pregnant ewes, stillbirths, abortions and neonatal deaths are observed. Adult small ruminants exhibit fever, difficult breathing and tremors. In poultry birds toxoplasmosis leads to loss of appetite, pallor, weakness and diarrhea. In calves, fever, swollen lymph nodes and nervous signs are also seen. In cats, fever, loss of weight, encephalitis and respiratory disorders are seen. In horses most prominent signs are paralysis, circling, in-coordination in walk and blindness.


As it is zoonotic infection so it can also transmit to humans and they act as an intermediate host of T. gondii. Mild or asymptomatic infection is exhibited by the healthy individuals. In pregnant women, toxoplasmosis infects the unborn fetus and cause abortions, still birth and damage to the fetus eyes, nervous system, skin, and ears. Infection may contract during pregnancy due to exposure of infection source. Pregnant women are more prone to infection because of weak immune system and their fetuses are also infected congenitally.


In late 1930 T. gondii was recognized. Firstly, Sabin isolated T. gondii in 1939 from humans. Methylene blue test was introduced by Sabin and Feldman in 1948. By this test sero-epidemiological studies were conducted in humans and other animal species for the determination of worldwide prevalence and distribution of toxoplasmosis. Infection may transfer to human by consumption of contaminated food and water, especially by consumption of raw/ uncooked meat of and unpasteurized milk or by direct contact with faeces of cats.


This infection is found all over the globe but incidence rate varies in different areas and different cultures. It is estimated that more than 30 per cent human population is affected in world and only 10-20 per cent patients show symptoms. Estimation of sero-prevalence for human is different in different geographical areas within one country, in different cultures and societies. The level of antibodies in blood was detected from 0-100 per cent in infected adult individuals of human population over last 30 years. Its pervasiveness is higher in those countries where cats are kept as pet animal.


Oocytes (resting stage of protozoa) of T. gondii are shed in the faces of felines which contaminate the food, drinking water and vegetation also. It is consumed by the warm blooded animals and humans which act as intermediate host where oocytes sporulate and transformed into infective stage (tachyzoites of 4-8 µm length and 2-3 µm width) of the protozoa in the suitable environment. They travel in blood and reach different body parts and transformed into bradyzoite cysts (5-10 µm diameter) present in skeletal muscles and neural tissues. If intermediate hosts meat (warm blooded animal) is consumed by humans it causes zoonotic infection or consumed by felines, protozoa again activated in the intestine of the felines.


Transmission of toxoplasmosis may occur through several potential routes between different species and within the species. Vertical transmission/congenital trans-placental transmission occurs by tachyzoites which produces distinct clinical picture. Horizontal transmission (lead to acute toxoplasmosis) occurs by ingestion of oocytes which are shed in faeces of cats, ingestion of cysts and tachyzoites.


The severity and symptoms of the infection are variable, depending upon routes of transmission. Acute toxoplasmosis is asymptomatic usually but immuno-compromised patients exhibit fever, depression, swollen lymph nodes, enlargement of spleen and liver but recover within few weeks. In congenital toxoplasmosis, signs and symptoms in neonates and fetuses are variable. It is characterized by inflammation of eye parts (retina and choroid), calcium and iron deposits in the brain and CSF accumulation in the brain but this triad is not always present. Small size skull and convulsions are seen. Mostly new born babies are asymptomatic but in later life, mental retardation, deafness, seizures and vision loss (ocular toxoplasmosis) are observed.


In pregnant women it leads to miscarriage/still abortion frequently. The female patients who contract the toxoplasmosis before pregnancy do not transfer it to the fetus. Risk of congenital transmission is less (10-25 per cent) during first trimester and more severe during the first trimester of the pregnancy. In third trimester, risk of congenital transmission is more (60-90 per cent) with less severity. In European countries, where incidence is higher, screening is done on monthly basis or on trimester basis. Screening is not recommended in countries with low incidence rate of toxoplasmosis. T. gondii acts as opportunistic pathogen in AIDS patients. Severe encephalitis has been observed in 40 per cent AIDS patients throughout the world.


It is diagnosed by clinical manifestations and serological tests e.g. EIA and IFA but they have their own specificity and sensitivity. The level of immunoglobulins especially IgG and IgM are high in serum within first two weeks of acute infection. If only IgG is detected in serum it determines that infection has occurred 6-12 months before. Test for IgM for toxoplasma may give false positive results but for confirmation an additional assay is IgG avidity test is performed. High IgG avidity determines that infection has occurred 3-5 months prior to testing. For diagnosis of congenital transmission of toxoplasmosis in utero testing is also done.


Affected pregnant women and neonates can be treated but cant be abolished because cysts always remain within the muscles where medicine cant penetrate. The drug of choice for toxoplasmosis is spiramycin (a macrolide antibiotic) because its adverse effect is only mild gastrointestinal symptoms and safe to pregnant women. Sulfonamides and pyrimethamine can be used but not in pregnant women because Sulfonamides cause jaundice to fetus and pyrimethamine is opponent of the folic acid. Babies suffering from congenital toxoplasmosis can be treated with sulfonamide and pyrimethamine. Chemotherapy can reduce the severity of the infection only but do not have any impact on the rate of congenital transmission of infection.


Different precautionary measures can be adopted to save the humans from toxoplasmosis, especially for pregnant women. By adopting the following measures toxoplasmosis can be controlled in animals and humans.


Keep meat producing animals inside the farms and do not graze them on pastures to avoid oocytes transmission.


Feed young animals with pasteurized milk to prohibit horizontal transmission of toxoplasmosis.


Prohibit the access of pets to the poultry farms and feed store houses.


Do not give undercooked/raw meat to cats, give them only well cooked food or commercially available canned food.


If cat is domesticated by you then daily alter the litter box because T.gondii becomes activated until 1-5 days of shedding.


Keep cats indoor to protect them from acquired infections.


Do not dispose the cats litter in the vegetation to protect other herbivorous intermediate hosts.


Avoid direct contact to cats litter and kittens.


Properly cook the meat to safe temperature at least 165°F and allow meat to rest for three minutes at least before carving and consuming.


Freeze meat before use for few days at sub-zero temperature to reduce occurrence of infection.


Thoroughly wash and peel the fruits and vegetables before use.


Do not get new cat and avoid direct contact to stray cats during pregnancy.


Always use treated water.


In Pakistan there is less dedication for pet keeping but due to modernization now it is becoming a trend to rear cats as pet animals. Similarly, the modification in the cooking method of Pakistan, variation in eating habits and promotion of Chinese food are also increasing the intimidation of toxoplasmosis in human. In Pakistan the incidence rate of toxoplasmosis in humans was very low but now it is increasing day by day. As number of feral dogs and cats are increasing which escalating the threats of toxoplasmosis. There are many organizations which are working for control and managing of free range animals. PAWS is a non-funding organization which carries out rescuing, adoption and treatment of free range animals. SPCA is another organization, which was functional in Karachi till 1980 but now is working only in Lahore in the premises of University of animal Sciences, Lahore.


In Pakistan the prevalence of toxoplasmosis in Southern Punjab is about 57.12 per cent, mostly rural females and butchers are infected with toxoplasmosis, screened by LAT (Latex agglutination test), “toxoplasmosis latex” kit which is commercially available. According to a study conducted by Agha Khan University, Karachi, the prevalence of toxoplasmosis in humans is about 37 per cent and prevalence of T.gondii in male is 40 per cent and in female 34 per cent. While according to the study of UVAS, Lahore, 22 per cent butchers are infected with T.gondii and screening was done by LAT and for confirmation PCR was used.


The writers are associated with the Department of Parasitology, University of Agriculture, Faisalabad, Pakistan. They can be reached at <dr.sarwattahira2953@gmail.com>

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