by FMD virus include artiodactyl mammals (even-toed ungulates, Figure
1) which include: Bovidae (cattle, zebus, domestic buffaloes, yaks),
sheep, goats, swine, all wild ruminants and suidae (pigs). Camelidae
(camels, dromedaries, llamas, vicunas) have low susceptibility. Cattle
and pigs are of most economic significance.
Species differ in the severity of clinical presentation (Table 1). There have been a few reports of infection in rodents and humans, but are of little clinical significance.
Figure 1: Artiodactyl limbs. From left, pig, deer and bovine.
Table 1: Overview of FMD host species and associated disease severity
1 All artiodactyl domestic species, including sheep, goats, buffalo and wild species, including deer, giraffe, hippos, camelids, antelope are susceptible. Exceptions are Old World camels which are resistant and South American camelids such as alpacas and llamas, that are mildly susceptible. African buffalo, may serve as a reservoir of FMDV and thus are the only wild species of epidemiological significance.
2Rats, mice, and guinea pigs can be infected experimentally.
3 Only a few human cases have been reported in spite of people's regular exposure to infected livestock. The cases that have occurred were due to close contact with infected animals, a virus in the laboratory or consumption of unpasteurized milk.
Transmission can be either direct
(contact via compromised skin or aerosol droplets or long-range airborne) or indirect
(fomites). Table 2 shows the experimental minimum infectious dose for the most affected species.
in animals that are in close contact are especially susceptible to
infection because all excretions and secretions from the infected
animal contain virus although shedding is most concentrated in
vesicular fluid (Table 3).
Table 2: Minimum Infectious doses for bovine and swine experimentally exposed to FMDV via direct routes .
Units are in TCID50 (bovine thyroid tissue culture 50% dose end-point estimates)
1Via common production practices shearing, de-worming and rounding up for lambing or for clinical examination, blood sampling and milking.
Table 3: Viral loads in bodily secretions and excretions
Like most other RNA viruses replication and virion assembly takes place within the cytoplasm of infected cells (Figure 2). In animals that inhale the virus, the primary site of replication is non-keratinized stratified
squamous mucosal epithelium of the pharynx (Table 4). However,
the virus migrate to secondary sites of replication (mainly in
keratinized epithelial cells) via the lymphatics where amplification
occurs. Thus, FMD virions are highest in number within the se cells
Figure 2: Site of viral replication in FMDV infected susceptible cells (BHK-38). a) Normal cell appearance. Nuclear DNA is blue and microtubule network is green. b) to d.) Nuclear DNA is blue and FMDV capsids are green. b) 1.5 hours post infection (hpi). Low levels of virus in the cytoplasm beside nucleus. c) 2.0 hpi. Accumulation of virus in the cytoplasm on one side of the nucleus. d) 2.5 hpi. Cytoplasm is full of virions. The cell is rounded and shows the cytopathic effect of FMDV.
Table 4: Sites of primary and secondary replication and sites of amplification
Table 5: Viral loads in host tissues
Immunity to FMD and protection from clinical disease is primarily mediated by circulating antibodies, and protection after recovery from infection or after immunization (active or passive) is closely correlated with the titres of circulating antibodies. Despite this, antibodies are ineffective in clearing virus from the pharynx of carrier ruminants and in preventing primary infection.
~50% of cattle, sheep and goats will become carriers. Cattle remain carriers <2 years (<1 year in sheep and goats). Vaccinated ruminants following natural exposure to infectious virus may become carriers. Carriers may present a potential for virus maintenance. Viral excretion by carrier animals is intermittent and declines progressively. Pre-disposing factors for the carrier state are the species of the animal and the strain of the virus.
is an acute disease with a short incubation period and peak viremia
occurs a few days after the onset of symptoms. The immune response is
quick and effective as antibody titres peak a few days after peak
viremia and viremia is cleared in about one week. However, some
individuals can become long term carriers (Table 6).
Table 6: Time course of FMD virus infection in artiodactyls.
1Depending on the infecting dose, susceptibility of the host, and strain of virus—in pigs that are highly stressed, it may be as short as 18 hr with some strains of FMD virus. Mean = 3-5 days in housed direct contact cattle 2 days in intensive sheep
2Onset of clinical symptoms
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