Comments on “Detection of Toxoplasma gondii in Raw Caprine,Ovine,Buffalo,Bovine,and Camel Milk Using Cell Cultivation,Cat Bioassay,Capture ELISA,and PCR Methods in Iran”

Dear Editor:

As you are aware, there is great concern regarding whether it is safe to consume raw (unpasteurized) milk (http://www.cdc.gov/foodsafety/rawmilk/raw-milk-index.html). Many Americans consume products labeled as “Only for pet consumption” or as participants in food cooperatives. Toxoplasmosis, caused by the protozoan Toxoplasma gondii, is one infection that has been associated with the consumption of raw milk (Riemann et al., 1975; Sacks et al., 1982; Skinner et al., 1990; Dubey and Jones, 2008; Jones et al., 2009).

Until recently, all published evidence indicated that viable T. gondii only occasionally occurs in the milk of goats and several other animals, reducing the likely contribution of raw milk to human exposure risk (reviewed in Dubey, 1986; Dubey and Beattie, 1988; Dubey, 2010). A recent report from Iran by Dehkordi et al. (2013) presented markedly different results. We have several concerns regarding this report.

Dehkordi et al. state that milk samples (250 mL) from 200 cattle, 164 buffalos, 185 sheep, 180 goats, and 160 camels were tested for T. gondii by various methods, and that viable T. gondii organisms were cultivated from 51 of 889 milk samples (8 bovine, 13 ovine, 18 caprine, 7 buffalo, and 5 camels). They also state that “Samples were collected under sterile hygienic conditions and were immediately transported at 4°C to the laboratory, and kept at −20°C until processed” (Dehkordi et al., 2013). However, how long the samples were frozen is not stated.

According to the authors, milk (200 samples) was filtered through a 3-μm filter to remove cells, and, after centrifugation, tachyzoites were enumerated in a hemocytometer and 103 tachyzoites per milliliter were inoculated onto Vero cells (Dehkordi et al., 2013). Viable T. gondii was cultivated from 51 of 889 milk samples. This result is unexpected because freezing milk should kill T. gondii (Kotula et al., 1991), and filtration should remove all host cells including intracellular T. gondii and would further reduce their occurrence in milk.

Milk samples (50 mL) from each of 51 animals were fed to cats over a period of 2–3 days. Feces of cats were examined for oocysts for 14 days, beginning 3 days after feeding milk. All 51 cats shed T. gondii oocysts. How these 51 animals were selected for bioassay in cats is not clear. Additionally, shedding of oocysts by each cat within 14 days of milk consumption indicates the presence of bradyzoites (Dubey, 2010) in milk, an unlikely event and therefore unexpected to occur in each instance.

Dehkordi et al. state that, by cell culture, T. gondii was cultivated from 51 milk samples and the same 51 samples were positive by bioassay in cats. However, such high specificity of cell cultivation is quite remarkable, and not recorded in prior literature using any tissue from asymptomatic animals (Dubey and Beattie, 1988; Dubey, 2010).

By polymerase chain reaction, T. gondii DNA was detected in 46 samples; by enzyme-linked immunoassay, 41 samples were positive. The authors speculated that arthropod transmission of T. gondii in areas of high humidity in Iran probably contributed to high prevalence of T. gondii. However, transmission of T. gondii by arthropods was discounted 5 decades ago when attempts to experimentally transmit T. gondii by arthropods were essentially unsuccessful (Dubey and Beattie, 1988).

It is worth mentioning that cattle and buffaloes are considered relatively resistant to infections with T. gondii (Dubey, 2010). The successful isolation of viable T. gondii from 7 of 164 samples of buffalo milk reported by Dehkordi et al. (2013) represents the first isolation of T. gondii from this host worldwide. The same comment applies to isolation of viable T. gondii from 8 of 200 milk samples from cattle. Since the 1950s, several authors have looked for T. gondii in the milk of experimentally inoculated cows, with essentially negative results (Dubey, 1986). The only positive isolation was made by Rommel and Breuning (1967), who inoculated 2058 mice with milk samples from three experimentally inoculated cows. One of the 2058 mice acquired toxoplasmosis; the positive milk sample had been collected 8 days after inoculation of the cow with T. gondii.

In our opinion, independent reproduction of such data (Dehkordi et al., 2013) would be needed to affirm the conclusion that various types of milk pose significant risk of toxoplasmosis exposure. Accurate assessment of risk is critical to prevention efforts for foodborne toxoplasmosis, an important source of infection.