Toxoplasmosis is a zoonotic disease which affects nearly all warm blooded animals and man. The disease runs a sub-clinical course and seldom causes overt clinical disease. It affects reproductive system, nervous system, skeletal muscles and eyes.
Distribution
Toxoplasmosis was for the first time discovered in 1908 by Nicolle and Manceax from North African rodent. The parasite is world wide in distribution but the incidence is more in warm, moist climates than cold, dry climates. In India, Krishnan and Lal (1933) recorded the disease for the first time from rabbits. Ray and Raghavachari (1941) demonstrated the organism from the internal organs of dog.
Aetiology
The disease is caused by parasite known as Toxoplasma gondii. Toxoplasma gondii are found in three forms: (i) Tachyzoites which are the rapidly multiplying forms found in the blood and body fluids (ii) cyst form which contains bradyzoites are slowly multiplying form and found in the body tissues and (iii) Sporulated oocyst voided in the faeces of cats. The parasite has got affinity for epithelial, reticulo-endothelial and blood cells. The organism can not be grown in artificial media but can be cultivated in tissue cultures. The organism could be grown in mono layer of lamb testicular cells.
Susceptible Hosts
Toxoplasmosis has been recorded in cattle, buffalo, sheep, goat, pig, dog, horse, cat and man as well as in rats.
Seroprevalence of toxoplasma antibodies in dog, pig, sheep, goat, cattle, buffalo, camel, monkey, pigeon and rodents in India has been reported. Rats have been considered as carrier of transmitter of T. gondii in cat and livestock and man.
Bradyzoites, tachyzoites and oocysts – all are infective to definitive and intermediate hosts.
Mode of Transmission
Toxoplasmosis has been reported to be transmitted by the following ways in man and animals.
(i) Through cat: The cat is the only definitive host of the parasite. Infected ats shed large number of oocysts in the faeces. Stray cats contaminate the soils around human habitations and thus play a vital role in the transmission of toxoplasmosis.
(ii) Meat and Meat products: Consumption of raw or undercooked meat or meat products containing the tissie cysts is an important source of toxoplasma infection. The cysts persist in the heart and brain for years and those meat remin as infectious to all species.
(iii) Congenital infection: Transplacental infection has been reported.
(iv) Other methods: Inhalation (aerosol) and ingestion of infected milk may transmit the infection. Infection has been traced to be transmitted through semen. Experimental transmission can be made by intramuscular or subcutaneous inoculation of infected materials. Man may acquire infection through:
- Ingestion of oocyst from food, meat, water.
- Placenta in uterus.
- Organ transplantation.
Clinical Findings
Cat: Fever, bilirubinemia, lymphadenitis, dyspnoea, anaemia, iritis, encephalitis and intestinal obstruction. Acute disease is characterized by anorexia, lethargy, pyrexia, dyspnoea and death.
Dog: Asymptomatic. Lesions involve lungs and central nervous system. Concurrent toxoplasmosis and distemper is common. Abortion and neo-natal mortality may ensue.
Pig: There is abortion and still births in sows and dyspnoea and wasting in young piglets.
Sheep and goat: Abortion is the main menifestation in ewes. In goats, acute infection is characterized by high rise of temperature, dyspnoea, diarrhoea, muscular tremors, paresis of hind quarters, erythraemia and anaemia.
Cattle and Buffalo: High rise of temperature and enlargement of lymph nodes are the important clinical features.
Human: Most cases remain asymptomatic. There is mild fever leading to encephalitis. Gets aggravated while accompanied with AIDS. There are symptoms of rash, lymphadenitis, chorioretinitis and abortion in female. Child may die or born with congenital cataract, hydrocephalus, microcephalus or anencephalous condition. These may be some after birth delayed manifestations characterised by congenital cataract, chorioretinitis, anaemia and mental retardation.
Diagnosis
Clinical signs are non-specific and the organism is difficult to demonstrate. Therefore, diagnosis in man and animals is accomplished by serological tests.
(a) Isolation of T. gondii: Parasites can be demonstrated from lymph fluid, placenta, cotyledons and muscles. It requires mouse inoculation. Isolations have been described by Cater (1969), Gupta (1979) and Chellapa (1987). Maintenance of T. gondii in the laboratory has been described by Devada and Anand in 1997.
(b) Methylene Blue Dye Test: Sabin and Feldman (1948) described this test for the first time. This test was subsequently standardized by Beverly and Beattie (1952). This test now a days is carried out in microtiter plates and can be judges by employing inverted microscope. This test has got limitations since a constant supply of living organism is needed as antigen and needs suitable human serum as heat labile accessory factor. Maintenance of positive mice is difficult and risky from human health point aspect.
(c) Complement fixation test: This test may be employed for the detection of toxoplasmosis of man and animals. This test is less sensitive compared to other tests and is rarely used now a days.
(d) Indirect haemagglutination test: This test can be successfully used for the diagnosis of toxoplasmosis. This test is sensitive as well as specific in nature. Commercial kits are available.
(e) Direct agglutination test: Modified form of this test as suggested by Desmonts and Remington (1980) is used. The test kit is commercially available. This test can be employed to test human and animal sera since it is not a species specific test.
(f) Latex agglutination test: The newly devised test is found to be superior using Toxoreagent. Toxoplasma antibody in animals and man can be satisfactorily detected by this test. A commercial latex kit is available. This can be used for the detection of ruminant toxoplasmosis.
(g) Fluorescent antibody test: This test is found satisfactory in detecting specific antibodies in sera or body fluids as well as for identifying antigen. This test may assist in diagnosis of congenital or acute infection.
(h) Enzyme linked immunosorbent assay: This test can be employed for screening of toxoplasmosis in man and animals. This test can be performed using commercially available ELISA kit.
(i) IFA – IGM and ELISA methods: This test has been recommended for the detection of acute toxoplasmosis. Test kits containing IFA toxo-IGM and EIA Toxo-M kits are available commercially for the detection of human toxoplasmosis.
(j) DNA test: This is new test for the detection of toxoplasmosis in man and animals.
T. gondii infected tissues will contain Toxoplasma nucleic acids (DNA). This DNA can be identified by using Toxoplasma specific DNA by gel electrophoresis. Reactions can be visualized on the gels. PCR is also used for diagnosis.
Treatment
There is no satisfactory treatment. Drugs like pyremethamine and sulphonamides have been used with success. SDDS (diaminodiphenyl sulfone) @ 100 mg/kg body weight for 14 days is the most effective treatment. Drugs which have been used in man and animals are as follows:
| Drug | Dose | Host |
| Pyrimethmine | @ 1 mg/kg b. wt. orally for the first 3 days followed by 0.4 mg/kg b. wt. | Mice, Cat, Dog, Pig, Man |
| Sulphonamides (Sulphadiazine, Sulphadimidine, Sulphamerazine) | @ 100 mg/kg b. wt orally once. May be increased upto 4 g orally in divided doses. | |
| Trimethoprim and Sulphamthoxazole | 80 mg of TMP and 400 mg of sulphamethoxazole | Man |
| Spiramycin | 2-3 gm/day in divided doses | Man |
| Azithromycin | 2-4 gm/day | Man |
| Clindamycin | 150-300 mg/adult at 6 hrs inverval | Man; Dog |
| Monensin | 5 mg/kg b. wt | Sheep |
| S.D.D.S | 5 mg/kg b. wt5 mg/kg feed | Cat; Pig |
Control
Control programme may be aimed based on three methods: (a) Epidemiological control (b) Control through chemotherapy and (c) Control by immunization.
(a) Epidemiological control
(i) Avoidance of exposure to cat.
- Lowering of cat population in endemic zones.
- Cat should not be allowed to feed raw (uncooked) meat.
- Cat should not be allowed to eat meat of wild animals and birds.
- Cat’s faeces should be properly disposed through burning.
- Rodent population should not be controlled by eats but to be made through rodenticides.
- Young cat may be fed drug as preventive measure to supress the shedding of oocysts.
(ii) Prevention of exposure in man.
- Hands should be properly cleaned after handling of raw meat through soap.
- Pregnant lady should be discouraged to go for gardening and handle soil and raw meat.
- Protective gloves should be used while gardening or coming in contact with soil in areas contaminated with cat faeces.
- Marketed vegetables should be washed thoroughly before eating.
- Raw milk should not be consumed.
- Dead foetus membranes and discharges should not be handled through nacked hands.
- Flied, cockroaches must not be allowed to feed foods as they may transmit infected oocyst from cat faeces to animal and human food.
(b) Chemotherapeutical control
Drugs as shown in treatment schedule may be fed as a prophylactic measure at periodical intervals to arrest development of oocyst and their shedding through faeces.
(c) Control by immunization
No fruitful vaccine is available. Both killed and live vaccine have been used in ewes wih variable results.
A vaccine containing T. gondii tachyzoites has been advocated to control the disease in ewes in New Zealand.
Public Health Aspects
Toxoplasmosis in man can occur as pre-natal or acquired infection. Pre-natal or congenital infection damages the eye and central nervous system characterized by blindness and epileptiform convulsions.
Acquired infection is less severe and characterized by lymph adenopathy, chorioretinitis and general malaise.
Some considerations on epidemiology of toxoplasmosis:
- It is one of the most significant wide spread protozoan infection in animals like cat, dog, cattle, buffalo, sheep, goat, pig, camel, horse, birds, rabbits, rats, monkeys and wild animals.
- Seroprevalence of it is about 8% in sheep and goat and 2.5% in cats.
- Possibility of infection in man is more through contaminated oocyst, feed and water rather than consumption of meat. Since cooking at 70° C used to kill the oocyst.
- Pig is one of the important source of infection and it is because of consumption of pork in man.
- Seroprevalence of T. gondii in India is low as compared to Western countries.
- About 10% of prevalence has been detected in pigeons.
- Chance of transmission of the disease from eggs to man has been thought off.
Toxoplasmosis in Man
It can be classified in three categories:
(a) Congenital toxoplasmosis: While a women pick up the infection during her pregnancy the child may be born with certain abnormalities. Such abnormalities include impaired vision, retinochoroiditis, hydrocephalus, convulsions and intracerebral calcification.
(b) Acquired toxoplasmosis: There is malaise, fatigue and fever along with sore throat and headache in some cases. But, most important feature is lymphadenopathy. Most often cervical lymph nodes are affected.
(c) Sub-acute/Chronic form: This is being defined by Parija (2004). These may appear as encephalitis. The main manifestation being seizure which may be observed in 15-40% cases.
