Introduction

EBL or also known as Bovine Viral Leukosis (BVL) is a persistent infectious disease caused by a retrovirus that belongs to the genus Deltaretrovirus and attacks cattle, mainly milk-producing animals^1,2. Cattle are the only species that is naturally infected with EBLV, although it is possible to experimentally infect sheep, goats, horses, deer, rabbits, rats, guinea pigs, cats, among others³.

The virus mainly affects B lymphocytes, it is transmitted horizontally and vertically, the first being the most important source of contagion⁴, which is mainly caused by arthropods such as horse flies and by the ingestion of colostrum from an infected cow. In addition, it can occur due to iatrogenic infection, which occurs through surgical instruments or sleeves contaminated with infected blood that remains as a residue of a rectal palpation⁵.

The disease does not spread rapidly between herds, however, within the affected herds, the seropositivity can be up to 80 %. The usual incubation period is 4 to 5 yr. Infection is rare in animals younger than 2 years and its maximum frequency occurs between 4 to 8 years of age³. Animals infected with the virus usually have no visible signs, however, exophthalmia is the most specific sign of the disease, in which degeneration of the retroocular tissue and internal structures of the eye occurs^6,7. Other characteristic symptoms of the disease are loss of appetite, weight loss, general weakness and, sometimes, neurological manifestations. Superficial lymph nodes may become swollen and may be felt under the skin or by rectal examination⁸.

It is a disease that affects milk production. Animals infected with EBLV have been shown to have a decrease in milk production ranging from 2.5 to 5 % compared to the herd^9-12. In addition to this, there is greater susceptibility to the appearance of other diseases such as mastitis, diarrhea and pneumonia⁷. While more than 20 countries have successfully eliminated EBLV through control programs, the prevalence of the virus can be up to 90 % in endemic areas such as Eastern Europe, South America, and several Asian countries¹³.

In Colombia the disease is considered notifiable (ICA Resolution 3714 of 2015)¹⁴. However, there is no program to prevent the spread of EBLV in cattle, which would help prevent the appearance of the disease, contributing to the mitigation of its economic impact¹⁵. In the case of EBL, animals positive for the virus and those with persistent lymphocytosis can only be diagnosed through laboratory techniques, while animals with lymphosarcoma can be easily diagnosed by the veterinarian in the field¹⁶.

In Boyacá, little research has been done on the disease⁸, which shows the lack of information about it in the area. In turn, in the municipality of Paipa, the seroprevalence of the virus and the relationship that exists with its manifestation in dairy production are unknown, a condition that is considered of great relevance, since this is a municipality with an agro-industrial economic profile with a strong livestock sector, and a marked trend towards growth. Considering the above, the objective of this research was to determine the seroprevalence of EBLV in the municipality of Paipa, Boyacá.

Material and methods

Results

A seroprevalence of 31.1 % was determined for EBLV (311/1000) in females from the municipality of Paipa. The Jersey breed was the one with the highest seroprevalence (39.2 %), followed by the Holstein, Ayrshire and Normande breeds, with 38.1, 36.7 and 11.3 % respectively. Regarding the age groups evaluated, it was determined that females between 3 and 4 years old have the highest seroprevalence (36.7 %), followed by individuals older than 4 years and the group of cattle between 2 and 3 yr old, with 23.4 and 21 % respectively.

Likewise, the breed and age of the sampled individuals presented significant differences (P≤0.05). Regarding age, the age group of 3 to 4 yr presented a statistically significant association (P=0.000; P≤0.05). With respect to the breed, the females of the Normande breed presented a significant association (P=0.000; P≤0.05), in addition to having a value of OR<1, it can be considered that these cattle have less susceptibility to present antibodies against EBLV (Table 1).

Of 651 pregnant females by natural mounting, 32.6 % (212) were seropositive for EBLV, while of 525 cows pregnant by artificial insemination, 30.8 % (162) had antibodies against the virus. However, none of these variables presented a statistically significant association with the appearance of the disease (P>0.05).

Regarding the milking characteristics implemented in the farms, of the females subjected to manual milking, 30.5 % (210/688) were seropositive for the disease, while, of the mechanically milked cattle, 31.7 % (91/446) presented antibodies against the virus. Finally, it should be noted that there was no statistical association between the seropositivity for the disease and the type of milking implemented (P≥0.05).

Discussion

There are several studies carried out on EBL at the national level. Prevalences of 24.9 % have been reported in the Andean region, 14.4 % for the Caribbean, 15.3 % in the Piedemonte Llanero and 1.5 % in Córdoba using the immunodiffusion technique²⁰. On the other hand, implementing the ELISA technique in Pasto, the prevalence found was 19.8 %²¹, 15 % in Yopal²², 13.5 % in Toca (Boyacá)⁸, 16.32 % and 16.07 % in Patía and Mercaderes (Cauca)²³. Finally, by molecular tests, the departmental distribution of the disease by animal and farm evaluated was established in Cundinamarca (69 and 90 %), Boyacá (71 and 94 %), Antioquia (73 and 100 %), Meta (85 and 100 %), Nariño (14 and 75 %) and Cesar (17 and 75 %)²⁴.

At the international level the prevalences are variable, after the implementation of the ELISA technique, values of 14.6 % have been established in Chile²⁵, 92.7 % and 46.38 % in Peru^1,26, 5.6 % in Ecuador²⁷, from 11 to 100 % in Thailand²⁸. It should be noted that the variation in the results can occur due to the number of animals sampled and the techniques implemented for the diagnosis of the disease. In addition to this, the form of transmission of the virus must be taken into account, which can occur through milk and objects that are contaminated with infected lymphocytes, so the sanitary and management practices implemented in each sampled herd could influence the transfer of the virus from one animal to another⁶.

With respect to breeds, in the present study, a greater seroprevalence was found in the Jersey breed. This differs from the results obtained by Romero et al in 2015²⁹, where this breed presented a prevalence of 11.9 %. Likewise, during this study, statistical significant differences were found between this variable and the appearance of antibodies against the virus, results that agree with those reported by Hernandez et al³⁰, who affirm that there is a strong dependence between the breed group and the seropositivity measured by ELISA (P<0.01). However, this does not coincide with what was reported by other researchers³¹, who determine that there is no association between the breed variable and the presence of EBL.

On the other hand, the Normande breed is less likely to have the disease compared to individuals of the other breeds, which indicates that it acts as a protective factor against EBLV. This could occur in the first place because individuals of this breed have breed characteristics that make them less susceptible to different pathologies and also, not being considered as a specific breed biotype for milk production, it is less likely to have the virus, since it should be taken into account that dairy breeds are more susceptible to the presence of the disease as reported by other studies^7,10,32.

In addition to the above, it is important to note that the study developed by Hernandez et al³⁰ indicates a strong breed effect on the dynamics of infection with EBLV, where Creole cattle such as Harton del Valle had a lower rate of infection with the virus, animals that became infected developed less lymphocytosis, had a higher immune response and maintained a lower proviral load compared to the Holstein breed.

Regarding age, there were statistical significant differences, which is consistent with what was stated by Betancur and Rodas³¹, who identified a higher frequency of infection depending on the age range. Likewise, Hernandez et al³⁰ established that the presence of EBLV depends on the age of the animal; females between 3 and 4 years of age had the highest seroprevalence, after the implementation of molecular techniques and ELISA, they determined that animals older than 4 years had higher percentages of infection, observing a marked reduction in prevalences in younger animals. In this way, it can be established that there is greater susceptibility to the virus as the age of the animal increases, which could be explained by the existence of accumulated exposure to the virus by maintaining contact with infected animals^31,33. In addition, Gutiérrez et al³⁴ indicate that passive immunity in young animals can alter the percentages of infection measured by ELISA in a herd with high prevalence of EBLV.

On the other hand, the risk factors that predispose to the presence of the disease may be given mainly by the management and practices that are carried out in each of the farms, this being an aspect to take into account to prevent the appearance of the disease and control its spread^8,22. Although in the present study no statistically significant association was found with the reproductive variables evaluated, it was established that there is a high seroprevalence of the virus in females who are pregnant by natural mounting (32.6 %). This is because horizontal transmission occurs mainly due to the presence of infected lymphocytes in biological fluids such as semen³⁵. In addition, Bonifas and Ulcuango²⁷ determined that direct mounting contributes to the spread of infection due to the use of EBL-positive bulls. In addition to this, rectal examination is a potential route of transmission of the virus, but transmission is related to other factors, such as the number of palpations with a common glove, the level of contamination of the glove with infected lymphocytes and the age of the animals³⁶.

When relating the disease to the type of milking of the farms, no statistically significant differences were found between this variable and the disease. However, 30.5 and 31.7 % of cattle subjected to manual and mechanical milking, respectively, were seropositive for EBLV. Previous studies have reported that in dairy farms, when having different types of milking, greater intervention is required during this process, facilitating the iatrogenic dissemination of viral particles through milk, equipment or hands of the operators, generating that the females are more prone to the appearance of the disease^5,37,38,39.

Conclusions and implications

High seroprevalence of EBLV was determined in bovine females from the municipality of Paipa (Boyacá), finding statistical association with the breed and age of the individuals evaluated. It is considered that the early diagnosis of the disease will allow the establishment of effective programs to control it, preventing the spread of the virus in the region. In addition to this, future research is required to compare the influences of each risk factor (type of dehorning, implementation of hypodermic needles, mode of implementation of palpation sleeves, size of the herds, among others) responsible for transmission within the herd, thus contributing to the probability that the disease will reach low epidemic proportions in the future.