Pathogenic characteristics of persistent feline enteric coronavirus infection in cats

Abstract

Feline coronaviruses (FCoV) comprise two biotypes: feline enteric coronaviruses (FECV) and feline infectious peritonitis viruses (FIPV). FECV is associated with asymptomatic persistent enteric infections, while FIPV causes feline infectious peritonitis (FIP), a usually fatal systemic disease in domestic cats and some wild Felidae. FIPV arises from FECV by mutation. FCoV also occur in two serotypes, I and II, of which the serotype I viruses are by far the most prevalent in the field. Yet, most of our knowledge about FCoV infections relates to serotype II viruses, particularly about the FIPV, mainly because type I viruses grow poorly in cell culture. Hence, the aim of the present work was the detailed study of the epidemiologically most relevant viruses, the avirulent serotype I viruses. Kittens were inoculated oronasally with different doses of two independent FECV field strains, UCD and RM. Persistent infection could be reproducibly established. The patterns of clinical symptoms, faecal virus shedding and seroconversion were monitored for up to 10 weeks revealing subtle but reproducible differences between the two viruses. Faecal virus, i.e. genomic RNA, was detected during persistent FECV infection only in the large intestine, downstream of the appendix, and could occasionally be observed also in the blood. The implications of our results, particularly our insights into the persistently infected state, are discussed.

Figure 1.

Faecal shedding of virus by cats included in infection experiments A2 and B2 after inoculation with FECV UCD (A) or FECV RM (B), as monitored by TaqMan^® RT-PCR. Means per group (n = 2). Note that RNA quantities below 3 × 10³ cannot be considered accurate (real quantities are probably lower) due to the non-linear relationship between input RNA and Ct value in the assay in the lower concentration range.

Figure 2.

Serum antibody titres, detected with ELISA, in cats included in infection experiment A2 after inoculation with FECV UCD. Means per group (n = 2).

Figure 3.

RIPA with serum from one representative cat inoculated with a 10⁻² diluted dose of FECV RM (experiment B2). A lysate of ³⁵S-methionine labelled FCWF cells infected with FIPV strain 79-1146 was used for the immunoprecipitations. MW: molecular weight markers, of which the molecular sizes (kDa) are indicated at the left; lane 1; ascitis 9912; lane 2: anti-RM plasma; lane 3–7: serum collected from the cat one day before inoculation and at days 13, 23, 43 and 50 after inoculation, respectively. Note the relatively poor recognition of the serotype II viral spike (S) protein by the serotype I sera.

Figure 4.

Body weight of FECV UCD-inoculated cats included in infection experiment A2. Cats 119 and 089 were inoculated with undiluted faecal extract, cats 129 and 091 with a 10⁻¹ dilution, cats 093 and 131 with a 10⁻² dilution, and cats 125 and 136 with a 10⁻³ dilution of the faecal extract. The upper figure shows an expanded view of the initial phase of the infection.

Figure 5.

Viral genomic RNA in blood. Two 17-weeks-old cats were inoculated with a 10⁻² dilution of FECV UCD stock. Genomic RNA was determined in blood cells by real-time RT-PCR.

Figure 6.

Virus in the intestinal tract during persistent FECV infection. Two cats inoculated with a 10⁻¹ dilution of FECV UCD stock were euthanized 64 and 68 days later while still shedding virus. Intestines were sectioned into 5-cm (from stomach to appendix) and 1-cm segments (downstream of appendix) and the viral RNA in faecal content and intestinal tissue was determined for each segment by real-time RT-PCR. Section numbers refer to the following parts of the gut. For cat 146: 1–10, duodenum; 10–156, jejunum and ileum; 157–166, colon; 167–177, rectum. For cat 150: 1–10, duodenum; 10–160, jejunum and ileum; 160–177, colon; 178–184, rectum.