Seroepidemiology of Toxoplasma gondii Infection in Elderly People in a Northern Mexican City

Abstract

Whereas it is well-known that the seroprevalence of infection with Toxoplasma gondii increases with age, details of the seroepidemiology of infection in elderly people are largely unknown. Anti-T. gondii IgG and IgM antibody levels were determined in 483 subjects aged ≥60 years old in Durango City, Mexico, using enzyme-linked immunosorbent assays. Socio-demographic, clinical, and behavioral characteristics from each participant were also obtained. In total, 58 (12%) of 483 participants (mean age 70.35±6.63 years) had anti-T. gondii IgG antibodies, and 14 (2.9%) also had anti-T. gondii IgM antibodies. Seroprevalence of infection varied significantly with age, birthplace, and educational level. Seroprevalence increased with age, especially in women (p=0.01), and was higher in subjects born outside of Durango State than those born within Durango State (p=0.008). Seroprevalence was higher in men with up to 12 years of education than in men with more than 12 years of education (p=0.01). Multivariate analysis of behavioral data showed a positive association of T. gondii infection with the presence of cats in the neighborhood, as well as consumption of boar, pigeon, iguana, and armadillo meats, and chorizo. Gender-specific analysis showed the described associations in women but not in men. In contrast, consumption of beef showed a negative association with T. gondii infection. This is the first report of the seroprevalence and contributing factors for T. gondii infection in elderly people, and of an association of the consumption of armadillo and iguana meats with T. gondii infection. Our results will provide the basis for the design of optimal preventive measures against T. gondii infection.

Introduction

T he parasite Toxoplasma gondii causes infections all around the world (Dubey 2010). Major routes of T. gondii infection include ingestion of food or water that is contaminated with oocysts shed by cats, and eating undercooked or raw meat containing tissue cysts (Montoya and Liesenfeld 2004; Dubey 2010). Vertical transmission can also occur and may lead to congenital disease (Montoya and Liesenfeld 2004). Other routes of parasite transmission such as transplantation and blood transfusion are less common (Figueroa-Damian 1998; Wurzner 2004; Barsoum 2006). The course of infection with T. gondii depends on the immune response of the host. In immunocompetent humans, infection with T. gondii is usually asymptomatic, but in up to 10% of infected individuals symptomatic toxoplasmosis with cervical lymphadenopathy may develop, and ocular disease may be both an early and a late complication (Montoya and Liesenfeld 2004). In immunocompromised individuals, infection with T. gondii may lead to central nervous system disease (Montoya and Liesenfeld 2004, Walker and Zunt 2005). It is known that the prevalence of T. gondii infection increases with age (Cavalcante et al. 2006; Alvarado-Esquivel et al. 2008; Salahi-Moghaddam and Hafizi 2009). Aging is associated with impaired functioning of many organs and systems, including the immune system (Praczko and Kostka 2006; Crétel et al. 2010). Elderly people suffer from more frequent and severe infections than young people (Pawelec 2006; Ongrádi et al. 2009). Thus elderly people might have a higher risk for clinical toxoplasmosis than younger people. Risk factors associated with T. gondii infection have not been studied in detail in elderly people. Therefore, we sought to determine the seroprevalence of T. gondii infection in the elderly in Durango City, Mexico, and to identify characteristics of this population associated with T. gondii seropositivity.

Materials and Methods

Study design and study population

We performed a cross-sectional survey in elderly urban people in Durango City, Mexico, from November 2006 to March 2011. Inclusion criteria for the study were residence in Durango City and age 60 years and above. Participants were recruited into the study during visits to senior centers, a nursing home, a health center, and a hospital in Durango City. In all, 550 subjects were invited, of which 483 (87.8%) agreed to participate.

Ethical aspects

This study was approved by the Institutional Ethical Committee of the Instituto de Seguridad y Servicios Sociales de los Trabajadores del Estado (ISSSTE) in Durango City. The purpose and procedures of the study were explained to all participants, and written informed consent was obtained from each of them.

Socio-demographic, clinical, and behavioral data

We explored the socio-demographic, clinical, and behavioral characteristics of the participants with the aid of a standardized questionnaire. Socio-demographic data included age, gender, birthplace, place of residence, area of residence, educational level, socioeconomic status, and occupation. Clinical data included the presence of underlying diseases; the presence or history of lymphadenopathy; frequent presence of headaches; history of blood transfusion, transplantation, or surgery; and memory, reflex, hearing, and visual impairments. Behavioral data included animal contacts, contact with cat feces, traveling in Mexico and abroad, meat consumption (pork, beef, goat, sheep, boar, chicken, turkey, pigeon, rabbit, venison, squirrel, horse, opossum, or other), degree of meat cooking, consumption of unpasteurized milk, and dried or cured meat (ham, sausages, salami, or chorizo). The chorizo consumed in Durango is a red fresh sausage made mostly of raw pork mixed with raw chili pepper. However, chorizo made from turkey meat is increasingly being consumed. Consumption of unwashed raw vegetables, fruits, or untreated water, frequency of eating out of the home (at restaurants and fast food outlets), contact with soil (gardening or agriculture), and the types of floors at home were also included.

Serological examination for T. gondii antibodies

Serum samples were obtained by centrifugation of fresh whole blood from the study participants. Serum samples were kept frozen at –20°C until analysis. Serum samples were assayed by qualitative and quantitative methods for anti-T. gondii IgG antibodies with a commercially available enzyme immunoassay, the Toxoplasma IgG kit (International Immuno-Diagnostics, Foster City, CA). Anti-T. gondii IgG antibody levels were expressed as International Units (IU) per milliliter, and a result equal to or greater than 8 IU/mL was considered positive. In addition, sera positive for T. gondii IgG were further tested for anti-T. gondii IgM antibodies using a commercially available enzyme immunoassay the Toxoplasma IgM kit (International Immuno-Diagnostics). All tests were performed following the manufacturer's instructions.

Statistical analysis

The statistical analysis was performed with the aid of Epi Info version 3.5.1 and SPSS version 15.0 (SPSS Inc., Chicago, IL). For calculation of the sample size, we used a reference seroprevalence of 7.4% (Alvarado-Esquivel et al. 2007a) as the expected frequency of the factor under study, 50,000 as the size of the population from which the sample was selected, a worst acceptable result of 12.5%, and a confidence level of 95%. The result of the calculation was 426 subjects. We used the Pearson's chi-square test and the Fisher's exact test (when values were less than 5) for comparison of the frequencies among groups. Bivariate and multivariate analyses were used to assess the association between the characteristics of the subjects and T. gondii seropositivity. Analysis was performed of three groups: men, women, and the whole population. As a criterion for inclusion of variables in the multivariate analysis, we considered variables with a p value equal to or less than 0.20 obtained in the bivariate analysis to allow for potential confounding. Age-adjusted odds ratio (OR) and 95% confidence interval (CI) were calculated by a multivariate analysis using multiple, unconditional logistic regression. When a cell in the 2×2 contingency table had a value of zero, the OR was calculated by adding 0.5 to all table cells (Fleiss 1981). A p value<0.05 was considered statistically significant.

Results

Of the 483 participants, 58 (12%) had anti-T. gondii IgG antibodies, and 14 (2.9%) of them were also positive for anti-T. gondii IgM antibodies. Specific anti-T. gondii IgG antibody levels were obtained in 51 of the 58 anti-T. gondii IgG-positive participants: 15 (3.1%) had IgG levels between 8 and 99 IU/mL, 4 (0.8%) between 100 and 150 IU/mL, and 32 (6.6%) greater than 150 IU/mL.

General socio-demographic characteristics of the 483 persons studied are shown in Table 1. The mean age of the participants was 70.35±6.63 years (range 60–91 years). The frequency of infection increased with age, especially in women (p=0.01). Participants born in Mexican states other than Durango State showed a significantly higher seroprevalence of T. gondii infection than those born in Durango State (p=0.008). Prevalence of infection varied with educational level in male participants (p=0.01). Men with up to 12 years of education had a significantly higher frequency (p=0.01) of infection than those with more than 12 years of education (22/131 versus 1/41, respectively).

Table 1.

Socio-demographic Characteristics of the Study Subjects and Seroprevalence of Toxoplasma gondii Infection

	Men (n=174)				Women (n=309)				All (n=483)
		Prevalence of T. gondii infection				Prevalence of T. gondii infection				Prevalence of T. gondii infection
Characteristic	No. of subjects tested ^a	No.	%	p Value	No. of subjects tested ^a	No.	%	p Value	No. of subjects tested ^a	No.	%	p Value
Age (years)
60–69	74	11	14.9	0.64	154	19	12.3	0.01	228	30	13.2	0.07
70–79	85	9	10.6		126	10	7.9		211	19	9.0
80–89	14	3	21.4		28	5	17.9		42	8	19.0
90–99	1	0	0.0		1	1	100.0		2	1	50.0
Birthplace
Durango State	139	16	11.5	0.08	275	28	10.2	0.05	414	44	10.6	0.008
Other Mexican state	19	5	26.3		32	7	21.9		51	12	23.5
Place of residence
Always in Durango State	168	22	13.1	0.57	305	35	11.5	0.77	473	57	12.1	0.93
Ever in other Mexican state	6	1	16.7		3	0	0.0		9	1	11.1
Ever abroad	0	0	0.0		1	0	0.0		1	0	0.0
Area of residence
Urban	141	20	14.2	0.41	263	31	11.8	0.82	404	51	12.6	0.5
Suburban	15	1	6.7		26	2	7.7		41	3	7.3
Ever rural	7	0	0.0		18	2	11.1		25	2	8.0
Educational level
No education	13	2	15.4	0.01	36	3	8.3	0.92	49	5	10.2	0.16
1–6 years	85	11	12.9		139	16	11.5		224	27	12.1
7–12 years	33	9	27.3		54	7	13.0		87	16	18.4
>12 years	42	1	2.4		80	9	11.3		122	10	8.2
Socio-economic level
Low	43	4	9.3	0.22	60	9	15.0	0.43	103	13	12.6	0.94
Medium	111	16	14.4		232	24	10.3		343	40	11.7
High	2	1	50.0		5	0	0.0		7	1	14.3
Occupation
Non-laborer^b	3	0	0.0	0.65	165	17	10.3	0.65	168	17	10.1	0.34
Laborer^c	171	23	13.5		143	18	12.6		314	41	13.1

Participants with available data.

Non-laborer=no occupation, student, or housewife.

Laborer=employee, professional, business, agriculture, cattle raising, factory worker, construction worker, or other.

Concerning clinical data, there was no difference in the frequency of underlying disease; presence or history of lymphadenopathy; frequent presence of headaches; history of blood transfusion, transplantation, or surgery; and impairments of memory, reflex, hearing, and vision, among T. gondii-positive and T. gondii-negative subjects.

Results of the bivariate analysis of a selection of behavioral characteristics are shown in Table 2. Behavioral characteristics with a p value less than or equal to 0.20 in the bivariate analysis included cats in the neighborhood; cleaning cat feces; soil contact; travel abroad; consumption of beef and meats of boar, pigeon, rabbit, horse, iguana, and armadillo; and consumption of raw meat, sausage, and chorizo. Multivariate analysis of these variables showed a positive association of T. gondii infection with the presence of cats in the neighborhood, especially in women, consumption of boar and pigeon meat in women, and consumption of iguana and armadillo meat and chorizo in women and the whole population. In contrast, consumption of beef showed a negative association with T. gondii infection (Table 3).

Table 2.

Bivariate Analysis of Selected Putative Risk Factors for Infection with Toxoplasma gondii in Elderly People

	Men (n=174)				Women (n=309)				All (n=483)
		Prevalence of T. gondii infection				Prevalence ofT. gondii infection				Prevalence ofT. gondii infection
Characteristic	No. of subjects tested ^a	No.	%	p Value	No. of subjects tested ^a	No.	%	p Value	No. of subjects tested ^a	No.	%	p Value
Cats at home
Yes	74	12	16.2	0.45	128	16	12.5	0.71	202	28	13.9	0.29
No	99	11	11.1		181	19	10.5		280	30	10.7
Cats in the neighborhood
Yes	117	18	15.4	0.33	206	30	14.6	0.02	323	48	14.9	0.007
No	57	5	8.8		101	5	5		158	10	6.3
Cleaning of cat feces
Yes	33	5	15.2	0.45	94	14	14.9	0.21	127	19	15.0	0.20
No	140	18	12.9		214	20	9.3		354	38	10.7
Traveled abroad
Yes	109	12	11	0.35	177	18	10.2	0.52	286	30	10.5	0.18
No	64	11	17.2		129	17	13.2		193	28	14.5
Beef consumption
Yes	167	21	12.6	0.23	299	32	10.7	0.09	466	53	11.4	0.04
No	7	2	28.6		10	3	30		17	5	29.4
Boar meat consumption
Yes	20	3	15	0.51	18	5	27.8	0.04	38	8	21.1	0.11
No	154	20	13		291	30	10.3		445	50	11.2
Chicken meat consumption
Yes	170	22	12.9	0.43	305	34	11.1	0.38	475	56	11.8	0.24
No	4	1	25		4	1	25		8	2	25.0
Pigeon meat consumption
Yes	22	1	4.5	0.20	22	7	31.8	0.006	44	8	18.2	0.16
No	118	16	13.6		202	19	9.4		320	35	10.9
Rabbit meat consumption
Yes	67	10	14.9	0.64	98	14	14.3	0.35	165	24	14.5	0.18
No	106	12	11.3		211	21	10		317	33	10.4
Venison consumption
Yes	93	13	14	0.75	116	16	13.8	0.38	209	29	13.9	0.22
No	80	9	11.3		193	19	9.8		273	28	10.3
Horse meat consumption
Yes	24	5	20.8	0.18	12	2	16.7	0.39	36	7	19.4	0.14
No	150	18	12		295	32	10.8		445	50	11.2
Iguana meat consumption
Yes	2	1	50	0.24	3	3	100	0.001	5	4	80.0	0.0008
No	172	22	12.8		306	32	10.5		478	54	11.3
Armadillo meat consumption
Yes	5	2	40	0.13	4	2	50	0.06	9	4	44.4	0.01
No	169	21	12.4		305	33	10.8		474	54	11.4
Degree of meat cooking
Raw	1	1	100	0.02	1	0	0	0.90	2	1	50.0	0.20
Undercooked	15	1	6.7		11	1	9.1		26	2	7.7
Well done	130	16	12.3		197	23	11.7		327	39	11.9
Sausage consumption
Yes	141	21	14.9	0.14	247	31	12.6	0.25	388	52	13.4	0.05
No	33	2	6.1		62	4	6.5		95	6	6.3
Chorizo consumption
Yes	128	18	14.1	0.80	235	32	13.6	0.04	363	50	13.8	0.04
No	45	5	11.1		73	3	4.1		118	8	6.8
Soil contact
Yes	116	12	10.3	0.17	234	26	11.1	0.99	350	38	10.9	0.20
No	58	11	19		75	9	12		133	20	15.0

Figures may not add up to the total because of missing values.

Table 3.

Multivariate Analysis of Selected Characteristics of the Study Subjects and Their Association with Toxoplasma gondii Infection

	Men			Women			All
Characteristic	Adjusted odds ratio ^a	95% Confidence interval	p Value	Adjusted odds ratio ^a	95% Confidence interval	p Value	Adjusted odds ratio ^a	95% Confidence interval	p Value
Cats in the neighborhood	1.94	0.68,5.55	0.21	3.32	1.24,8.84	0.01	2.63	1.29,5.36	0.008
Cleaning of cat feces	1.21	0.41,3.55	0.72	1.69	0.81,3.52	0.15	1.46	0.80,2.64	0.21
Traveled abroad	0.55	0.22,1.35	0.19	0.73	0.35,1.48	0.38	0.66	0.38,1.15	0.14
Beef consumption	0.35	0.06,1.95	0.23	0.28	0.07,1.17	0.08	0.31	0.10,0.93	0.03
Boar meat consumption	1.17	0.31,4.40	0.8	3.31	1.10,9.96	0.03	2.08	0.90,4.81	0.08
Pigeon meat consumption	0.30	0.03,2.42	0.25	4.54	1.64,12.57	0.00	1.82	0.78,4.25	0.16
Rabbit meat consumption	1.27	0.51,3.19	0.60	1.49	0.72,3.08	0.27	1.42	0.81,2.51	0.21
Horse meat consumption	2.08	0.68,6.36	0.19	1.71	0.35,8.22	0.50	2.00	0.83,4.85	0.12
Iguana meat consumption	8.48	0.49,145.4	0.14	59.12^b	2.98,1170	0.001	35.4	3.82,327.9	0.002
Armadillo meat consumption	5.11	0.79,33.02	0.08	8.83	1.19,65.59	0.03	6.69	1.72,25.92	0.006
Raw meat consumption	22.08^b	0.86,562	0.12	2.61^b	0.10,65.35	0.88	6.76	0.40,112.2	0.18
Sausage consumption	2.62	0.58,11.83	0.21	2.12	0.72,6.28	0.17	2.31	0.96,5.56	0.06
Chorizo consumption	1.35	0.46,3.92	0.57	3.94	1.15,13.41	0.02	2.32	1.06,5.10	0.03
Soil contact	0.45	0.18,1.12	0.09	0.92	0.41,2.08	0.85	0.68	0.38,1.22	0.20

Adjusted by age.

These odds ratios were calculated by adding 0.5 to each cell of the 2×2 table.

Discussion

We found a relatively low (12%) seroprevalence of T. gondii infection in elderly people in Durango City, Mexico. We are not aware of other seroprevalence studies in elderly people; therefore we cannot compare our results with those of other elderly populations. The elderly people we studied had a lower seroprevalence than the estimated mean worldwide prevalence (one-third of the total population) (Montoya and Liesenfeld 2004, Dubey 2010), despite the fact that seroprevalence of T. gondii infection increases with age. On the other hand, the seroprevalence found in elderly people is higher than the 6.1% seroprevalence reported in the younger (mean age 37±16.1 years) general population in Durango City (Alvarado-Esquivel et al. 2011a).

The prevalence of T. gondii infection was significantly lower in persons born in Durango State than in those born in other Mexican states. The reason for this difference is not clear. However, this finding suggests that T. gondii infection may have occurred more frequently in Mexican states other than Durango. Differences in the type of environmental and behavioral characteristics of the populations might help explain the differences seen in seroprevalence. The prevalence of infection varied significantly with educational level in male participants. This may indicate that a low educational level is related with behavioral characteristics that may facilitate the transmission of T. gondii.

Concerning behavioral characteristics, it is noteworthy that there was an association between T. gondii seropositivity and consumption of armadillo and iguana meats. To the best of our knowledge, this is the first report of an epidemiology study that describes an association of T. gondii infection with consumption of armadillo and iguana meats in humans. Serological evidence of infection with T. gondii has been reported in armadillos (Burridge et al. 1979; Carme et al. 2002; da Silva et al. 2006,2008; Deem et al. 2009). In addition, T. gondii was isolated from two armadillos in Brazil (da Silva et al. 2006). Therefore, our results provide complementary information about T. gondii infection in humans through consumption of armadillo meat. We found an association of T. gondii infection with the consumption of iguana meat. Whereas it is well known that T. gondii infects warm-blooded animals, only a limited number of reports deal with T. gondii infection in cold-blooded animals. Stone and Manwell (1969) performed experimental T. gondii infections in a number of reptiles and observed that infections persisted for 7–8 days in red-eared turtles, chameleons, and a blue spiny lizard, while infections in skinks were fatal in a week or less. Worldwide information about T. gondii infection in poikilothermic animals was recently reviewed by Dubey (2010). Since then, Taghadosi and associates (2010) reported findings of anti-T. gondii IgM antibodies in 5 of 50 samples from Salmonidae by using an enzyme-linked immunosorbent assay. We cannot rule out that T. gondii might infect iguanas, and further research to elucidate whether iguanas can be hosts for T. gondii is needed. It is important to note that the number of iguana consumers was low in our study because iguanas do not live in the dry climate of Durango, and iguana meat is not currently consumed in Durango. Therefore, interpretation of this association in the limited number of iguana consumers should be made with care. In this study, questions about eating habits did not discriminate between regular and sporadic consumption. A number of factors including the amount, frequency, and degree of cooking of iguana meat should be investigated. Eating unusual meats (such as iguana or armadillo) tends to be remembered, and this fact may have positively influenced our results. Further research to clarify the association of T. gondii infection with consumption of iguana meat is needed.

Multivariate analysis showed a positive association of T. gondii infection with the presence of cats in the neighborhood, especially in women, consumption of boar and pigeon meat in women, and consumption of chorizo in women and the whole population. Contact with cats has been associated with T. gondii infection in younger populations in Durango, including blood donors (Alvarado-Esquivel et al. 2007a); workers occupationally exposed to water, sewage, and soil (Alvarado-Esquivel et al. 2010a); and schizophrenic patients (Alvarado-Esquivel et al. 2011b). Serological evidence of T. gondii infection has also been found in cats in Durango (Alvarado-Esquivel et al. 2007b; Dubey et al. 2009), and viable T. gondii parasites have been isolated from cats in Durango (Dubey et al. 2009). Of note, an association of T. gondii infection with behavioral variables was found in women and the whole population, but not in men. This finding likely suggests different infection risk factors in women and men. However, it is possible that women more precisely report their behavioral characteristics than men, and this may help to identify risk factors. In addition, the sample size of men was smaller than that of women, and this could reduce the probability of finding statistically significant differences. Our findings indicate that studying variables separately according to gender may help to identify the routes of T. gondii infection in more detail. Differences in occupational activities might help explain the specific routes of infection in men and women. In Durango, home activities including cleaning (including backyards and streets that might contain parasite oocysts), and cooking (with exposure to raw meat that may contain tissue cysts), are mostly performed by women. Therefore, these activities may increase the risk of infection in women. Remarkably, infection was associated with consumption of meat from a number of wild animals (boar, pigeon, iguana, and armadillo). The association of T. gondii infection and the consumption of boar meat found in this study further confirms our previous observation in the younger general population in Durango City (Alvarado-Esquivel et al. 2011a). Pigeon meat consumption was associated with T. gondii infection in a rural population of Durango State (Alvarado-Esquivel et al. 2010b). However, this is the first time we found such association in an urban population. Pigeons in Durango were recently studied for the presence of T. gondii and viable T. gondii parasites were isolated (Alvarado-Esquivel et al. 2011c). Our findings support the increased risk of T. gondii infection seen with the consumption of pigeon meat. Similarly, the association of T. gondii infection with the consumption of chorizo found in this study further confirms our previous observation in a younger population consisting of workers occupationally exposed to water, sewage, and soil in Durango City (Alvarado-Esquivel et al. 2010a). Chorizo is made of pork, and serological evidence of T. gondii infection has been reported in pigs in Durango (Alvarado-Esquivel et al. 2011d). In contrast, T. gondii infection was negatively associated with beef consumption. This result indicates that beef consumption did not play an important role in T. gondii infection in the population studied.

This is the first report of the seroprevalence and contributing factors for T. gondii infection in an elderly population, and of an association of the consumption of armadillo and iguana meats with T. gondii infection. Our results will contribute to the design of optimal preventive measures against T. gondii infection.

Footnotes

Acknowledgment

The authors thank Fidela Rosas Meza for her valuable support in processing the questionnaires.

Author Disclosure Statement

No competing financial interests exist.

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