In VBA, you can use the InStr function:

Dim strField1 As String
Dim strField2 As String

 strField1 = "Mouse Dog Cat Horse"
 strField2 = "cat"
 If InStr(strField1, strField2) > 0 Then
' strField2 appears in strField1
 Else
' strField2 does not appear in strField1
 End If

Of course, I'm assuming that your example is simpler than what you're
actually trying to do. That formula would also say that cat appears in the
string "Toss Scatters Strews". If all of your words are separated by spaces,
you could use

 If InStr(" " & strField1 & " ", " " & strField2 & " ") > 0 Then
' strField2 appears in strField1
 Else
' strField2 does not appear in strField1
 End If

Another approach would be to use the Split function to take the contents of
strField1 and put each word in an array:

Dim booFound As Boolean
Dim lngLoop As Long
Dim strField1 As String
Dim strField2 As String
Dim varWords As String

 strField1 = "Mouse Dog Cat Horse"
 varWords = Split(strField1, " ")
 strField2 = "cat"
 For lngLoop = LBound(varWords) To UBound(varWords)
   If varWords(lngLoop) = strField2 Then
     booFound = True
     Exit For
   End If
 Next lngLoop
 If booFound = True Then
' strField2 appears in strField1
 Else
' strField2 does not appear in strField1
 End If