Seroprevalence of feline leukemia virus,feline immunodeficiency virus and heartworm infection among owned cats in tropical Mexico

Abstract

Several infectious agents may be distributed within a healthy population of cats where diverse risk factors predispose them to come into contact with pathogens. Blood samples from 227 owned cats in Merida, Mexico, were collected with the objective of determining the seroprevalence and associated risk factors of feline leukemia virus (FeLV) and Dirofilaria immitis antigen, and feline immunodeficiency virus (FIV) antibody. Serological detection of FeLV and D immitis antigens, and FIV antibodies was performed using the commercial kit SNAP Feline Triple Test. The prevalence was found to be 7.5% for FeLV, 2.5% for FIV and 0% for D immitis. Adult cats were at a higher risk of coming into contact with FeLV (P <0.01) than younger cats. Owing to its low prevalence, a risk factor analysis was not performed for FIV. The prevalence of retroviral infections found in this study was low, but within the limits reported in the different geographical areas of the world. Cases of filariosis in the domestic cats of Merida, Mexico, may be absent or very low; however, the low sample size may have influenced these results.

Introduction

The feline lymphotropic retroviruses, feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV), are recognizable agents in domestic cats. These infections are associated with a variety of direct and indirect clinical signs that exhibit very complex biological characteristics, with potentially fatal implications. ¹ These domestic cat viruses can cross the species barrier with potentially devastating consequences to wild, large felids: for example, in an FeLV outbreak in free-ranging Florida panthers in early 2001, 23 panthers showed the classical clinical signs and five eventually died. ² Cats infected with FIV may not show clinical signs, but may develop an immunodeficiency syndrome, which predisposes them to clinical disease. FeLV is a more pathogenic virus and induces significant suppressive effects on bone marrow and the immune system. ³ Despite the importance of these retroviruses in domestic cats, information about its distribution at local and national level is scarce. However, feline heartworm (Dirofilaria immitis) is considered a severe and highly life-threatening disease.^4,5 However, few studies have shown the presence and/or prevalence of this agent in the different regions of the world. In the USA, a prevalence of 0.9–4.6% has been reported, which correlates with the prevalence of positive dogs in the same area. ⁶ This disease is found in dogs from Merida, the capital city of Yucatan, Mexico, with a prevalence of 8.3%; ⁷ therefore, a number of D immitis-infected cats is to be expected. The domestic cat population in the city of Merida has not been determined. However, pet cats are very common in the city, so their number is most likely similar or greater than that observed in dogs. ⁸ Cats in the city of Merida are characterized by freely roaming around the neighborhood, so they may even be fed and sheltered by neighbors. However, one family can be identified as the owner, vaccinating the cat against rabies and, in some cases, taking them to be sterilized. These populations are complex groups, competing for territory and breeding, and are exposed to various infectious agents, including retroviruses. More information is needed on the prevalence of infectious diseases in cats to understand the epidemiology of the different diseases in an area and define better prophylactic management and therapeutic measures for the different cat populations.

The purpose of this study was to determine the seroprevalence of FeLV and D immitis antigens, and FIV antibodies among owned cats from a tropical area in Mexico.

Materials and methods

Animals and study design

A cross-sectional study was performed in 227 domestic cats from the city of Merida, the capital of Yucatan State (19°30’ and 21°35’ north latitude, 87°30’ and 90°24’ west longitude). Healthy cats of both sexes older than 3 months and without a history of FeLV/FIV vaccination were selected by convenience from those admitted for surgical sterilization during spaying campaigns or those healthy cats admitted to private clinics around the city only for routine surgery. A clinical examination was performed on the cats to determine their health status. Blood samples were obtained by jugular venipuncture and serum was collected and stored at −20°C until testing.

Laboratory testing

For the serological detection of FeLV and D immitis antigens, and FIV antibodies, a commercial kit – SNAP Feline Triple Test (Idexx Laboratories) – was used. The kit reports 100% sensitivity and 98.6% specificity for FeLV, 93.5% sensitivity and 100% specificity for FIV, and 89.3% sensitivity and 99.5% specificity for D immitis.

Data analysis

We determined the association of only FeLV antigen status with the selected risk factors (ie, age, sex, free access to the street, number of cats be households, and body condition score). The prevalence of FIV was too low for statistical analysis, and the prevalence of D immitis was zero. The included risk factor variables were age (young <1 year or adult ≥1 year), sex (male or female), free access to the street (yes or no), number of cats by households (1 or >2, including the patient sampled) and the body condition score (BCS) of the cats. For the determination of the BCS, a modified method was employed that considered fat covering assessed by palpation of the ribs, lumbar vertebrae, pelvic bones and all body prominences. ⁹ The cats were scored as having a bad, regular or good BCS. Statistical analysis to determine the association of a positive cat with the studied variables was performed using a χ² test. The Yates or two-tailed Fisher corrected tests were used when low numbers of positive cases were found within a studied variable. The odds ratios and 95% confidence intervals were also calculated using the EpiInfo program. ¹⁰ The level of significance was set at P ≤0.05.

Results

The prevalence of infection in the cats was found to be 7.5% (17/227) for FeLV, 2.5% (5/227) for FIV and 0% for D immitis. An evaluation of the risk factors was only performed for FeLV. From the studied variables, only age (adult cats) was associated with higher risk to come in contact with FeLV than younger cats; no significant association was found with the other studied variables (Table 1). No association was found between the studied variables in the positive cases of FIV.

Table 1

Analysis of risk factor for feline leukemia virus in 227 domestic cats from Yucatan, Mexico

		Positive	Positive
	Total	n	%	OR	CI	P
Sex
Male	104	8	7.8	1.1	0.35–3.10	0.90
Female	123	9	7.3
Age
Young	110	3	2.7
Adult	117	14	16.4	0.2	0.05–0.08	0.01
BCS
Regular–poor	120	12	10.0	0.1	0.70–7.70	0.10
Good	107	5	4.7
Access to the streets
Yes	159	13	8.2	1.4	0.40–5.40	0.70 *
No	68	4	5.9
Cats/household
1	32	3	9.4
≥2	195	14	7.2	1.3	0.29–5.42	0.70 †

OR = odds ratio; CI = confidence interval; BCS = body condition score

*

Yates

†

Fisher

Discussion

Worldwide, FeLV and FIV are common retroviruses that predispose cats to important infectious diseases in vital organs and secondary opportunistic infections. ³ For the diagnostic approach of such viruses, a clinical examination and serological tests may be useful. However, a correct serological interpretation in survey studies, including in clinically healthy cats, as in the present study, requires caution, especially when a low prevalence is observed in a small sample size. Determining the stage of infection is difficult after initial contact with the viruses because this makes the serological interpretation difficult. FeLV infections can be regressive and eliminate all traces of infection, and in these cases viremia is absent or progressive (characterized by a persistent virema), and can make the cats susceptible to other diseases.^11,12 A high proportion (approximately 72%) of positive enzyme-linked immunosorbent assays (ELISAs) are likely to give false-positive results; however, as in the case of FeLV, routine screening is an excellent method for ruling out viremia because a false-negative result is likely to be obtained in only 1/1000 tests. ¹³ In the present study, infection status was not determined: seropositive animals may indicate previous contact with the agent, but the probability of a false-positive result is high. However, seronegative animals can rule out viremia.

Studies of retroviral infections in cats in Latin America are rarely conducted. In Costa Rica, out of 96 studied cats, 8.8% were seropositive for FIV and 16.7% for FeLV; from those that were FeLV seropositive, 64% were older than 1 year. ¹⁴ In Brazil, the prevalence of FeLV was 11.5% in 1094 cats examined. ¹⁵ The prevalence of FeLV can vary greatly in populations of healthy cats from other latitudes, such as North America (2.3%), ¹⁶ Australia (Sydney; 2%), ¹⁷ Norway (1.2%) ¹⁸ and Spain (15%). ¹⁹ In clinically ill cats, FeLV may reach seroprevalences of 18.0–30.4%.^19,20 These worldwide differences in prevalence rate may suggest a large variation in the level of supervision (access or not to outdoors and contact with other cats) and health care of the studied feline populations. The risk of viral transmission can increase or decrease depending on the level of supervision of the cats and contact with other cats. In the present study, no significant differences in the prevalence of FeLV infection were found in free-roaming cats, household cats or in households with more than one cat. It is expected that exposure to retroviruses is more likely to occur in free-roaming cats. ²¹ However, in the present study, the low prevalence found and the reduced sample size did not allow for statistical association. It was reported that an infection with FeLV is more common in young animals owing to their narrower intraspecific relations (bites and licking during playing), which favor the transmission of the virus. Unless immunosuppressed, the immune system in young cats is not different from those of adults, and the response to infection is similar. The older the cat, the longer the amount of time at risk to come into contact with the agent. Infected mothers would also be a major source of transmission, and transmission to kittens occurs easily by this route. ²² In the present study, the risk of contact with FeLV virus was higher with increasing age, as reported in Brazilian domestic cats. ¹⁵ However, not all cats in contact with the agent necessarily become seropositive. FeLV has no predilection for breed or sex, but there is a greater tendency for it to occur in males because of their street habits.^15,23

The external clinical appearance in terms of the BCS may be associated with the presentation or predisposition of disease. A poor BCS may indicate a weakened immune system and the presentation of the clinical signs of disease. The prevalence of FIV and FeLV has been reported to be higher in ill cats (with clinical signs of weight loss) than in healthy cats.^23,24 Such an association, however, may not necessarily be causal, but the reservoir competence of the individual may be associated with their nutritional state. In the present study, no association of BCS with FeLV was found, but a tendency towards FeLV-infected status was seen in cats with a regular to bad BCS. The sample size and low prevalence may have influenced these results, which could become significant if a higher sample size was included.

Detection of the antibody is considered a demonstration of the persistence of FIV in the infected animal. ²⁵ False-positive cases could be detected in cats younger than 10 weeks old when FIV-positive dams may have passed specific antibodies in the colostrum; ²¹ thus, in the present study, as no history of FIV vaccination nor clinical signs of the disease were reported in the dams, only cats over the age of 3 months were considered. However, FIV maternal antibodies may persist for up to 6 months, so positive kittens should be retested after that time interval. ²⁶ No further tests were possible in the present study.

The worldwide prevalence of FIV may vary from 0% ²⁷ to 13%. ²⁸ This variation may result from differences in the study design, sample size and diagnostic test used, and it probably also results from the diverse levels of supervision and health care of the cat populations around the world, and the lower or higher risk of being in contact with the infectious agent. Although FIV vaccination may be a cause of the false-positive ELISA test results, none of the selected cats in the present study had been vaccinated.

Domestic cats can be victims of D immitis. However, the prevalence is 80–95% lower in cats than in dogs, even in the same endemic area. ²⁹ In a national survey in the USA, a 1.2% prevalence of feline filariosis was found. ⁶ However, in some areas of the USA, the range of the infection varied between 11.8% ³⁰ and 14.0%. ³¹ This range of infection is most likely owing to the variation in the predisposing epidemiological conditions of each area, in particular with the presence and density of mosquito vectors, and the prophylaxis measures of the population. There are no reports on the presence and prevalence of this feline heartworm in Mexico. In this study, despite having obtained a prevalence rate of 0%, the disease is not necessarily absent in the area, but may be have been influenced by biological and technical factors.

For this study, a commercial ELISA snap type Feline Triple Test (Idexx Laboratories) was used, which detects circulating antigens in the blood. Given that the adult female larvae are releasing the antigens in the cats and that the number of adult worms can be between one and three per cat, there was probably an insufficient amount of antigen released for a commercial kit to be able to deliver a positive result, or the infection may result from only adult male parasites, thus giving a false-negative result.^5,32 Cats in this study younger than 6 months could not have been positive, regardless of the prevalence, and were excluded from the data analysis concerning heartworm infection. Additionally, the small number of cats in the present study may have reduced the probability of finding positive cases.

In a study of 1840 cats examined post-mortem, only 0.8% had adult D immitis in the heart. The maximum number of worms found was three adult larvae in two cats, two adult worms in three cats and a single larva in 10 cats. ³³ It is possible that some of the larvae were immature females, and antigens are not produced until 6 months after the infection, when the female worms reach reproductive maturity. ³⁴ Therefore, the low number of study cats, the low burden of female larvae and the limitations of the kits may explain why many positive cases were not detected.

This serological study provides preliminary data about the presence and prevalence in this area of Mexico of these three common pathogens in apparently healthy cats. However, these data should be interpreted with caution; the data may be an underestimate because no sick cats were included as controls, and a small sample size was used. More research needs to be conducted with a larger number of cats to more precisely determine the associated factors, preferably with the use of molecular tools.

Conclusions

The prevalence of FeLV and FIV infections in the studied cat population was low, but within the limits usually found in different geographical areas of the world. However, the sample of cats in this study was not representative enough to draw reliable conclusions. Owing to the low prevalence found, there was not a well-defined risk factor demonstrating the infections’ epidemiology and distribution in the studied cats, especially with such a small sample size. This was a preliminary study that will be useful in giving a general panorama of the situation in an apparently healthy cat population; generation of a larger dataset is needed. However, filariosis in the domestic cats of Merida city may be absent or very low compared with other geographical areas. A larger sample size may be required to obtain a better view of the epidemiological situation of heartworm in cats from the studied area.